The solubility of salts of alkaline earth metals at the temperature of 20 °C.
Chapter 1: "Permanent Maxillary and Mandibular Incisors"\n
Chapter 2: "The Permanent Maxillary and Mandibular Premolar Teeth"\n
Chapter 3: "Dental Anatomical Features and Caries: A Relationship to be Investigated"\n
Chapter 4: "Anatomy Applied to Block Anaesthesia"\n
Chapter 5: "Treatment Considerations for Missing Teeth"\n
Chapter 6: "Anatomical and Functional Restoration of the Compromised Occlusion: From Theory to Materials"\n
Chapter 7: "Evaluation of the Anatomy of the Lower First Premolar"\n
Chapter 8: "A Comparative Study of the Validity and Reproducibility of Mesiodistal Tooth Size and Dental Arch with the iTero Intraoral Scanner and the Traditional Method"\n
Chapter 9: "Identification of Lower Central Incisors"\n
The book is aimed toward dentists and can also be well used in education and research.',isbn:"978-1-78923-511-1",printIsbn:"978-1-78923-510-4",pdfIsbn:"978-1-83881-247-8",doi:"10.5772/65542",price:119,priceEur:129,priceUsd:155,slug:"dental-anatomy",numberOfPages:204,isOpenForSubmission:!1,isInWos:null,hash:"445cd419d97f339f2b6514c742e6b050",bookSignature:"Bağdagül Helvacioğlu Kivanç",publishedDate:"August 1st 2018",coverURL:"https://cdn.intechopen.com/books/images_new/5814.jpg",numberOfDownloads:7278,numberOfWosCitations:0,numberOfCrossrefCitations:1,numberOfDimensionsCitations:3,hasAltmetrics:0,numberOfTotalCitations:4,isAvailableForWebshopOrdering:!0,dateEndFirstStepPublish:"October 4th 2016",dateEndSecondStepPublish:"October 25th 2016",dateEndThirdStepPublish:"July 16th 2017",dateEndFourthStepPublish:"August 16th 2017",dateEndFifthStepPublish:"October 16th 2017",currentStepOfPublishingProcess:5,indexedIn:"1,2,3,4,5,6",editedByType:"Edited by",kuFlag:!1,editors:[{id:"178570",title:"Dr.",name:"Bağdagül",middleName:null,surname:"Helvacıoğlu Kıvanç",slug:"bagdagul-helvacioglu-kivanc",fullName:"Bağdagül Helvacıoğlu Kıvanç",profilePictureURL:"https://mts.intechopen.com/storage/users/178570/images/7646_n.jpg",biography:"Bağdagül Helvacıoğlu Kıvanç is a dentist, a teacher, a researcher and a scientist in the field of Endodontics. She was born in Zonguldak, Turkey, on February 14, 1974; she is married and has two children. She graduated in 1997 from the Ankara University, Faculty of Dentistry, Ankara, Turkey. She aquired her PhD in 2004 from the Gazi University, Faculty of Dentistry, Department of Endodontics, Ankara, Turkey, and she is still an associate professor at the same department. She has published numerous articles and a book chapter in the areas of Operative Dentistry, Esthetic Dentistry and Endodontics. 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Vaz-Leal",slug:"francisco-j.-vaz-leal",email:"fjvazleal@gmail.com",position:null,institution:null},{id:"188719",title:"Dr.",name:"María Cristina",middleName:null,surname:"Álvarez Mateos",fullName:"María Cristina Álvarez Mateos",slug:"maria-cristina-alvarez-mateos",email:"cristinaalvarezmateos@gmail.com",position:null,institution:null},{id:"195142",title:"Dr.",name:"Laura",middleName:null,surname:"Rodríguez Santos",fullName:"Laura Rodríguez Santos",slug:"laura-rodriguez-santos",email:"laura@unex.es",position:null,institution:null},{id:"195143",title:"Dr.",name:"María I",middleName:null,surname:"Ramos Fuentes",fullName:"María I Ramos Fuentes",slug:"maria-i-ramos-fuentes",email:"miramos@unex.es",position:null,institution:null}]},book:{id:"5372",title:"Eating Disorders",subtitle:"A Paradigm of the Biopsychosocial Model of Illness",fullTitle:"Eating Disorders - A Paradigm of the Biopsychosocial Model of Illness",slug:"eating-disorders-a-paradigm-of-the-biopsychosocial-model-of-illness",publishedDate:"February 1st 2017",bookSignature:"Ignacio Jauregui-Lobera",coverURL:"https://cdn.intechopen.com/books/images_new/5372.jpg",licenceType:"CC BY 3.0",editedByType:"Edited by",editors:[{id:"55769",title:"Prof.",name:"Ignacio",middleName:null,surname:"Jáuregui Lobera",slug:"ignacio-jauregui-lobera",fullName:"Ignacio Jáuregui Lobera"}],productType:{id:"1",title:"Edited Volume",chapterContentType:"chapter",authoredCaption:"Edited by"}}},ofsBook:{item:{type:"book",id:"9200",leadTitle:null,title:"Polymer Degradation",subtitle:null,reviewType:"peer-reviewed",abstract:"
\r\n\tPolymers are a new age material with wide applications in every part of the universe. As the backbone of the polymer is mainly composed of carbon, hydrogen, and oxygen their disposal after use is a universal problem unless it is biodegradable. Degradation is the scission of polymer backbone by breaking of various bonds and formation of copolymers. The degradation rates can be controlled by various parameters. The degradation results in loss of material from bulk due to polymer erosion. Also, different types of degradation brings the change in polymer properties due to its surroundings during their application as particular polymer/its composites. There are advantages and disadvantages of degradation as polymer composites can be used as protective material against the harmful environment. On the other hand, effective methods can be searched for non-biodegradable polymers. A database can be formed whenever such materials have industrial applications.
",isbn:null,printIsbn:"979-953-307-X-X",pdfIsbn:null,doi:null,price:0,priceEur:0,priceUsd:0,slug:null,numberOfPages:0,isOpenForSubmission:!1,hash:"10a018a7b05cf7b0b60648e17f09e974",bookSignature:"Dr. Pratima Parashar Pandey",publishedDate:null,coverURL:"https://cdn.intechopen.com/books/images_new/9200.jpg",keywords:"Polymers, Mechanism, Decomposition, Protective Material, Permeation Resistance, Hydrogenation, Degradation, Double Bond, Chain Scisson, Penetration, Swelling, Pyrolysis",numberOfDownloads:null,numberOfWosCitations:0,numberOfCrossrefCitations:null,numberOfDimensionsCitations:null,numberOfTotalCitations:null,isAvailableForWebshopOrdering:!0,dateEndFirstStepPublish:"March 18th 2019",dateEndSecondStepPublish:"September 20th 2019",dateEndThirdStepPublish:"November 19th 2019",dateEndFourthStepPublish:"February 7th 2020",dateEndFifthStepPublish:"April 7th 2020",remainingDaysToSecondStep:"a year",secondStepPassed:!0,currentStepOfPublishingProcess:5,editedByType:null,kuFlag:!1,biosketch:null,coeditorOneBiosketch:null,coeditorTwoBiosketch:null,coeditorThreeBiosketch:null,coeditorFourBiosketch:null,coeditorFiveBiosketch:null,editors:[{id:"142089",title:"Dr.",name:"Pratima",middleName:null,surname:"Parashar Pandey",slug:"pratima-parashar-pandey",fullName:"Pratima Parashar Pandey",profilePictureURL:"https://mts.intechopen.com/storage/users/142089/images/system/142089.png",biography:"Dr. Pratima Parashar Pandey is an Academician and Scientist in the field of Materials Science and Nanotechnology since last twenty five years. Earlier, she was in the field of polymer blends for twenty years and has published about fourteen papers in cited journals. Since, last ten years, she is in the field of metal nano polymer composites and has eleven research papers in SCI journals. She has written two chapters one, ‘Silver particulate films on softened polymer composite’ in the book ‘Applications of Calorimetry in a Wide Context - Differential Scanning Calorimetry, Isothermal Titration Calorimetry and Microcalorimetry’ Other, ‘Nano Biomaterials in Antimicrobial Therapy’ in a book ‘Recent Biopolymers’ published both in InTechOpen Publication. She has been reviewer, technical programme committee member and invited speakers for many international conferences.",institutionString:"CET Opto",position:null,outsideEditionCount:0,totalCites:0,totalAuthoredChapters:"3",totalChapterViews:"0",totalEditedBooks:"1",institution:{name:"CET Opto (China)",institutionURL:null,country:{name:"China"}}}],coeditorOne:null,coeditorTwo:null,coeditorThree:null,coeditorFour:null,coeditorFive:null,topics:[{id:"8",title:"Chemistry",slug:"chemistry"}],chapters:null,productType:{id:"1",title:"Edited Volume",chapterContentType:"chapter",authoredCaption:"Edited by"},personalPublishingAssistant:{id:"297736",firstName:"Katarina",lastName:"Paušić",middleName:null,title:"Ms.",imageUrl:"https://mts.intechopen.com/storage/users/297736/images/8495_n.jpg",email:"katarina.p@intechopen.com",biography:null}},relatedBooks:[{type:"book",id:"6574",title:"Pyridine",subtitle:null,isOpenForSubmission:!1,hash:"5f51507105e28f22bb8240225d781043",slug:"pyridine",bookSignature:"Pratima Parashar Pandey",coverURL:"https://cdn.intechopen.com/books/images_new/6574.jpg",editedByType:"Edited by",editors:[{id:"142089",title:"Dr.",name:"Pratima",surname:"Parashar Pandey",slug:"pratima-parashar-pandey",fullName:"Pratima Parashar Pandey"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"1591",title:"Infrared Spectroscopy",subtitle:"Materials Science, Engineering and Technology",isOpenForSubmission:!1,hash:"99b4b7b71a8caeb693ed762b40b017f4",slug:"infrared-spectroscopy-materials-science-engineering-and-technology",bookSignature:"Theophile Theophanides",coverURL:"https://cdn.intechopen.com/books/images_new/1591.jpg",editedByType:"Edited by",editors:[{id:"37194",title:"Dr.",name:"Theophanides",surname:"Theophile",slug:"theophanides-theophile",fullName:"Theophanides Theophile"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"3092",title:"Anopheles mosquitoes",subtitle:"New insights into malaria vectors",isOpenForSubmission:!1,hash:"c9e622485316d5e296288bf24d2b0d64",slug:"anopheles-mosquitoes-new-insights-into-malaria-vectors",bookSignature:"Sylvie Manguin",coverURL:"https://cdn.intechopen.com/books/images_new/3092.jpg",editedByType:"Edited by",editors:[{id:"50017",title:"Prof.",name:"Sylvie",surname:"Manguin",slug:"sylvie-manguin",fullName:"Sylvie Manguin"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"3161",title:"Frontiers in Guided Wave Optics and Optoelectronics",subtitle:null,isOpenForSubmission:!1,hash:"deb44e9c99f82bbce1083abea743146c",slug:"frontiers-in-guided-wave-optics-and-optoelectronics",bookSignature:"Bishnu Pal",coverURL:"https://cdn.intechopen.com/books/images_new/3161.jpg",editedByType:"Edited by",editors:[{id:"4782",title:"Prof.",name:"Bishnu",surname:"Pal",slug:"bishnu-pal",fullName:"Bishnu Pal"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"72",title:"Ionic Liquids",subtitle:"Theory, Properties, New Approaches",isOpenForSubmission:!1,hash:"d94ffa3cfa10505e3b1d676d46fcd3f5",slug:"ionic-liquids-theory-properties-new-approaches",bookSignature:"Alexander Kokorin",coverURL:"https://cdn.intechopen.com/books/images_new/72.jpg",editedByType:"Edited by",editors:[{id:"19816",title:"Prof.",name:"Alexander",surname:"Kokorin",slug:"alexander-kokorin",fullName:"Alexander Kokorin"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"1373",title:"Ionic Liquids",subtitle:"Applications and Perspectives",isOpenForSubmission:!1,hash:"5e9ae5ae9167cde4b344e499a792c41c",slug:"ionic-liquids-applications-and-perspectives",bookSignature:"Alexander Kokorin",coverURL:"https://cdn.intechopen.com/books/images_new/1373.jpg",editedByType:"Edited by",editors:[{id:"19816",title:"Prof.",name:"Alexander",surname:"Kokorin",slug:"alexander-kokorin",fullName:"Alexander Kokorin"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"57",title:"Physics and Applications of Graphene",subtitle:"Experiments",isOpenForSubmission:!1,hash:"0e6622a71cf4f02f45bfdd5691e1189a",slug:"physics-and-applications-of-graphene-experiments",bookSignature:"Sergey Mikhailov",coverURL:"https://cdn.intechopen.com/books/images_new/57.jpg",editedByType:"Edited by",editors:[{id:"16042",title:"Dr.",name:"Sergey",surname:"Mikhailov",slug:"sergey-mikhailov",fullName:"Sergey Mikhailov"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"371",title:"Abiotic Stress in Plants",subtitle:"Mechanisms and Adaptations",isOpenForSubmission:!1,hash:"588466f487e307619849d72389178a74",slug:"abiotic-stress-in-plants-mechanisms-and-adaptations",bookSignature:"Arun Shanker and B. Venkateswarlu",coverURL:"https://cdn.intechopen.com/books/images_new/371.jpg",editedByType:"Edited by",editors:[{id:"58592",title:"Dr.",name:"Arun",surname:"Shanker",slug:"arun-shanker",fullName:"Arun Shanker"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"878",title:"Phytochemicals",subtitle:"A Global Perspective of Their Role in Nutrition and Health",isOpenForSubmission:!1,hash:"ec77671f63975ef2d16192897deb6835",slug:"phytochemicals-a-global-perspective-of-their-role-in-nutrition-and-health",bookSignature:"Venketeshwer Rao",coverURL:"https://cdn.intechopen.com/books/images_new/878.jpg",editedByType:"Edited by",editors:[{id:"82663",title:"Dr.",name:"Venketeshwer",surname:"Rao",slug:"venketeshwer-rao",fullName:"Venketeshwer Rao"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"4816",title:"Face Recognition",subtitle:null,isOpenForSubmission:!1,hash:"146063b5359146b7718ea86bad47c8eb",slug:"face_recognition",bookSignature:"Kresimir Delac and Mislav Grgic",coverURL:"https://cdn.intechopen.com/books/images_new/4816.jpg",editedByType:"Edited by",editors:[{id:"528",title:"Dr.",name:"Kresimir",surname:"Delac",slug:"kresimir-delac",fullName:"Kresimir Delac"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}}]},chapter:{item:{type:"chapter",id:"46748",title:"Raw Materials for Production of SrAC",doi:"10.5772/58606",slug:"raw-materials-for-production-of-srac",body:'For the synthesis of strontium aluminate cement it is necessary to find the proper source of strontium (SrO) and aluminium oxide (Al2O3).
Two major strontium minerals are its carbonate, strontianite (SrCO3) and more abundant sulfate mineral celestite (SrSO4). William Cruickshank in 1787 and Adair Crawford in 1790 independently detected strontium in the strontianite mineral, small quantities of which are associated with calcium and barium minerals. They determined that the strontianite was an entirely new mineral and was different from barite and other barium minerals known in those times. In 1808, Sir Humphry Davy isolated strontium by the electrolysis of a mixture of moist strontium hydroxide or chloride with mercuric oxide, using mercury cathode. The element was named after the town Strontian in Scotland where the mineral strontianite was found [91].
The strontium oxide (SrO) is the first substantial component of strontium aluminate clinker. Therefore, the strontium carbonate (SrCO3) is the most appropriate input material for the synthesis of strontium aluminate clinker. In nature SrCO3 occurs as rare orthorhombic mineral strontianite[1] - (space group Pcmn) and together with isostructural minerals aragonite (CaCO3), witherite (BaCO3) and cerussite (PbCO3) it belongs to anhydrous carbonates from the group of aragonite[1] - [92,93].
The structure of strontianite (Fig.1(a)) is based on isolated [CO3]2-triangles which are placed in layers perpendicular to c-axis. The layer has two structural planes where [CO3]2-ions are oriented in the opposite direction. Cations with the coordination number of 9 are placed between these layers.
Structure of strontianite consists of isolated [CO3]2-triangles arranged into layers with Sr2+ions in the space between layers.
Natural and artificially synthesized binary (aragonites up to 14 mol. % Sr [94], strontianites up to 27 % Ca [94], witherites [94], baritocalcites [95]) or ternary solid-solutions (alstonites [94]) of these carbonates are intensively studied in order to elucidate the mechanism of their formation, their structure, the thermodynamic stability and the luminescence properties.
Calcium carbonate minerals include considerable amount of strontium from seawater as they precipitate. It stands to reason that the solid-solutions of strontianite with calcite and aragonite (CaxSr1-xCO3) are the most explored. There is an immiscibility gap in the range 0.12 (aragonites) < x < 0.87 (strontianites) under ambient conditions, which disappears at the temperature of ~107 °C [92,94,96,97-100].
Therefore natural sources of SrCO3 are rare and have no industrial importance, strontium carbonate as well as other compounds such as strontium nitrate, strontium oxide and chloride are prepared from the orthorhombic mineral celestite[1] - (SrSO4, space group Pnma with cell unit parameters a=8.359 Å, b=5.352 Å, c=6.686 Å and Z=4) using the techniques described in Chapter 2.1.1. The structure of celestite consists of isolated [SO4]2-tetrahedrons and Sr2+ions (Fig.2).
Structure of celestite (a) and distribution of large celestite deposits in the world (b).
Celestite together with isostructural barite (BaSO4) and anglesite (PbSO4) belong to anhydrous sulfates from the group of barite[1] -. Similarly to the solid solutions of carbonates mentioned above, also celestite and barite (BaSO4) coexist in the marine environment with significant fractions of Sr and Ba in solid solutions. Therefore it is better to identify barite suspended in seawater as the strontian barite (SrxBa1-xSO4) [101].
The second substantial component of strontium aluminate cement is aluminium oxide (Al2O3). The most stable crystalline form of Al2O3 is the polymorphic modification of hexagonal corundum (α-Al2O3) from the R
Pure aluminium oxide is relatively rare, but single crystals of gemstones such as sapphire (colorless) or ruby (red due to the content of chromium) can be found in nature [424]. Industrial production of Al2O3 is based on the Bayer process of bauxite. The main part of produced alumina is used in metal industry for the production of aluminium by Hall-Heroult Process [102-105].
The application of Al2O3 in ceramics includes the production of alumina porcelain and alumina oxide ceramics, ZTA (Zirconia Toughened Alumina) ceramics and the applications such as electroceramics, construction ceramics, shaped and unshaped refractory products, abrasive materials, etc [106-112]. From the point of view of the volume of production, polycrystalline alumina is the most frequently used material as ceramics for the structural applications. However, in comparison with for example, silicon nitride (Chapter 6), where the influence of various additives on the microstructure and properties is well characterized and understood, alumina remains the material with many unknown factors yet to be revealed. Alumina based materials can be roughly divided into three groups [424]:
Solid-state sintered aluminas: enable to prepare nanocrystalline materials with excellent mechanical properties and well-sintered ceramics being transparent to visible light [113,114].
\n\t\t\t\t\t\tLiquid-phase sintered aluminas (LPS): are substantial part of industrially produced alumina-based materials. Silica, alkali oxides and oxides of alkali earth metals are used as sintering additives [115-117].
\n\t\t\t\t\t\tAlumina-based composites: ZTA and alumina based nanocomposites with non-oxide phases such as SiC or TiC [118-123].
The preparation of Al2O3 mono-crystals is based on Verneuil process consisting in the flame fusion in high temperature region from 1500 to 2500 °C [124-127]. Bauxite (Fig.3) is also used for the production of calcium aluminate cements [128] or is calcined and used as opening material for the refractory products [129-131].
SEM image of calcined bauxite grain.
In order to obtain good quality in abrasive, refractory and pottery products, the content of impurities should be reduced. Chemical processes include the pyrochemical techniques, acid leaching methods or reductive dissolution alternatives. The pyrochemical techniques involve the treatment of bauxite at high temperature with gases such as H2, Cl2 or anhydrous HCl [132,133]. The acid leaching methods are based on the application of strong inorganic acids such as HCl or H2SO4 [134-137].
A serious problem with these techniques is that leaching of iron is often accompanied by substantial co-dissolution of aluminium hydroxides, particularly during the treatment of gibssitic and boehmitic ores. Selective dissolution of iron can be obtained applying mild reducing conditions. In this case the dissolution of Fe(III) oxides takes place via the reduction of ferric iron to the divalent state. It is widely accepted that biological mechanisms are often involved in the mobilization of iron in natural systems. For the particular case of bauxites the biological activity of iron reducing microorganisms is most probably involved in the generation of gray-colored iron depleted bauxites [138,139].
Since the production of alumina from bauxite ores consumes large amount of caustic soda, and generates large amount of “red mud” slurry waste, the alternative processes for the production of aluminium and aluminoalloys via carbothermic reduction of bauxite ores was investigated. The reduction sequence of metal oxides in bauxite ores is iron oxides then silica and titania and then alumina (Fig.4). Metallic iron is formed at the temperatures below 1100 °C. At 1200 °C or above the ferroalloy phase with silicon and aluminium is formed. Carbides of titanium, silicon and aluminium were formed by the carbothermal reduction. The metals were formed and dissolved in the ferroalloy phase, which after saturation, was segregated as metal carbides distributed inside the alloy phase as inclusions or around the alloy particles [140,141].
Process suggested for simultaneous recovery of iron, aluminium and titanium from red mud [146].
The utilization of Bayer’s process residues in the cement production is also studied. Previous works proposed a method of treating red mud with saturated Ca(OH)2 solution followed by 3% H2SO4, in order to remove Na. After heating, the treated material is suggested for the application in cement manufacturing. The major parts of red mud are hematite and alumina-rich phases (Fig.7), participating in the production of hydraulic crystal phases C3A and C4AF. Fe-rich waste could be then used for the production of sulphate resistant cements [142]. Other option includes the applications such as catalysts and adsorbents, ceramics, coatings and pigments, waste water and gas treatment, recovery of major and minor metals [143-146].
Bayer suggested that [143]: “Red, iron-containing residue, that occurs after digestion, settles well and, with sufficient practice, can be filtered and washed. Due to its high iron content and low aluminium oxide content, it can be, in an appropriate manner, treated or melted with other iron ores to iron”. The concept of bauxite residue as an iron resource was tested by a number of workers over the intervening 120 years, however, the “appropriate manner” of treatment remains elusive [144].
The aluminium gels, salts (sulphates, nitrates or chlorides) or alkoxides and advanced ceramic fabrication techniques can be applied for the preparation of high purity products (please referee to Chapter 9). Bauxite is the mixture of aluminium hydroxides and oxyhydroxides such as boehmite, diaspore and gibbsite, with varying content of admixture minerals. Goethite, lepidocrocite, hematite, magnetite, kaolinite, chlorites, calcite, anatase, phosphastes, etc are the major ones [148].
Bauxite, as the primary source of aluminum, represents a typical accumulation of weathered continental crust [147,148]. Bauxites are usually considered to be of three major genetic types [149-152]:
Lateric bauxites (sometimes called equatorial) are formed from weathered primary aluminosilicate rocks in equatorial climates comprising ∼90% of the world\'s exploitable bauxite reserves. Lateritic bauxite is generally formed by in-situ lateritization, therefore, the most important factors in determining the extent and grade of it are thought to be the parent rock composition, climate, topography, drainage, groundwater chemistry and movement, location of water table, microbial activity, and the duration of weathering processes.
Sedimentary bauxites are primarily formed by the accumulation of lateritic bauxite deposits during the mechanical transportation of surface flows. In addition, the consequent weathering and transfer of Al and Fe play substantial roles in bauxitization, which not only supports the formation of bauxite from kaolin clays but also refines the primary clastic ores.
Karst bauxites are named for their confinement to karst zones with karstified or karstifying carbonate rocks. Karst-type deposits originate from a variety of different materials, depending on the source area.
Distribution of superlarge bauxite deposits worldwide [152].
Each genetic group of bauxite experienced the separation of aluminum (Al) and silicon (Si) by the accumulation of Al, and the removal of Si, alkali metals, and rare earth elements from parent rock (sediment) during its weathering [148].
Bauxite deposits (Fig.5) form mainly at ambient pressure and temperature on the (sub)surface of continents. Abundant bioavailable irons, nutrient elements, sulfurs, and organic carbons make bauxite suitable for microorganisms to inhabit so they become rare geological sites that can preserve records of microbiological activity on the surface of continents under strong weathering effects. The microorganism activities can produce a family of minerals with special morphologies and stable isotope compositions. Bauxite deposits were studied in detail because of their economic value. They play an important role in the study of paleoclimate and paleogeography of continents because they contain scarce records of weathering and evolution of continental surfaces [148].
Chemical industry consumes over 95 % mined celestite for the conversion to other strontium compounds. The main admixtures in celestite ores are calcite (CaCO3), gypsum (CaSO4 2H2O), quartz (SiO2) and clay minerals. The gravity separation techniques and the flotation[1] - are mostly used for the separation of those admixtures due to high efficiency and low operating costs. Moreover, the process does not require the usage of other chemicals for the purification and has low environmental impact. On the other hand, the efficiency of these techniques for the preparation of celestite concentrate depends on the texture of ore as well as the type and quantity of associated impurities [153-155]. The particle size is other most important factor. Extremely fine particle sizes must be achieved by grinding in order to release celestite and calcite [156]. The difference in grindabilities makes it possible to separate celestite from gypsum by differential grinding [157].
The shear flocculation[1] - of fine celestite suspension with sodium dodecyl sulfate (SDS, C12H25SO4Na) or with anionic alkyl succinate surfactant can be performed in broad pH range (3 – 11) but the highest efficiency is reached at pH 7. Increasing concentration of surfactant has positive effect on the course of process. The most common inorganic dispersants used are sodium silicate, sodium phosphate and sodium polyphosphate. The investigation of mutual influence of additives shows that sodium silicate strongly prevents celestite with sodium dodecyl sulfate from shear flocculation, but the dispersive effect of SDS is low when anionic alkyl succinate surfactant is used. In the presence of sodium polyphosphate, the shear flocculation of celestite suspension increases slowly for both surfactants. The similar increase can also be observed for sodium phosphate in the presence of SDS. However, sodium phosphate dispersed the celestite suspension in the presence of anionic alkyl succinate surfactant [158]. Sodiumoleate (cis-9-Octadecenoic acid sodium salt) and tallow amine acetate (TAA) were more effective for celestite suspensions in the pH ranges 7–11 and 6–10, respectively [159].
The surface of celestite becomes hydrophobic by the adsorption of dodecyl sulfate on the surface. Sodium dodecyl sulfate is also effective for the flotation of celestite in the solution free of carbonate species over the broad pH range of 3-11. The surface transformation of celestite to strontium carbonate which takes place at pH ≥ 7.8 causes that the zeta potential of celestite begins to be more negative and subsequently resembles that of strontium carbonate. Sulfate ions are exchanged by carbonate ions in the celestite crystal lattice, so CO32- and HCO3- species are probably responsible for the negative increase in zeta potential. The surface transformation of celestite to strontium carbonate has no effect on floatability up to the pH of 10. Once the pH is higher than 10, the concentration of CO32- and HCO3- species in aqueous solution is very intrinsic and the decrease of floatability is probably caused by the absorption of these species on carbonated surface of celestite [155].
The coagulation and flocculation characteristics of celestite by inorganic salts, such as CaCl2, MgCl2 and AlCl3, indicate that magnesium ion was more effective on the celestite suspension than calcium and aluminum ions at high pH levels. The effect varied significantly depending on the concentration. While calcium and magnesium ions were not effective for the suspension below neutral pH, aluminum ion caused the stabilization of the celestite suspension at these pH levels [160].
In general, the aggregation of fine particles can be achieved by neutralizing the electrical charge of interacting particles by coagulation, or flocculation can be carried out by crosslinking the particles with polymolecules [161]. The pH of isoelectric point of celestite determined by the hindered settling technique is 2.6 [160].
There are two basic processes to produce SrCO3 from SrSO4 [162]:
Pyro-hydrometallurgical process or black ash method;
Hydro metallurgical process.
The “pyro-hydrometallurgical process or black ash\n\t\t\t\t\tmethod” is first of them. Celestite is carbothermically reduced to water soluble sulphide (SrS), which is next dissolved in hot water[1] - The first solid-state reaction during the carbothermic reduction takes place at up to 400 °C [163]:
After the formation of surface layer of the product the further progress of reaction 1 is inhibited. Formed carbon dioxide diffuses through the layer and reacts with celestite according to the following reaction:
Carbon dioxide diffuses further out of reaction zone and generates more CO according to the Boudouard reaction if the temperature is ≥ 720 °C:
That means that direct reaction of celestite with carbon (Eq.1) has little importance and SrSO4 can be transformed to SrS at the temperature higher than equilibrium of Boudouard reaction. The important factor of the process 2 is the reduction potential of gas phase given by the partial pressure ratio of pCO/pCO2. It was also observed that the rate of carbothermic reduction significantly increases if celestite concentrate and carbon are milled together. The temperatures in the range from 1100 to 1300 °C with the excess of metallurgical grade coke are necessary to produce water-soluble strontium sulfide.
The dissolution of strontium sulfide in hot water can be expressed by the following heterogeneous reaction [164]:
Eq.4 shows that the pH of leaching solution increases from almost neutral to the value of 11.5 – 12.5 as the concentration of OH- ions increases. Extremely high pH values (pH > 14) should be avoided in order to prevent the system from the precipitation of strontium hydroxide[1] - :
The value of equilibrium constant K at 25 °C is 3.55 10-29, i.e. log\n\t\t\t\t\tK=-28.45. Therefore, the concentration of Sr(OH)2 in leaching solution (Fig.6) can be expressed as:
That means that leaching of SrS must be carried out in relatively low alkaline medium in order to ensure high concentration of strontium in the solution. The solubility of strontium hydroxide is enhanced by increased temperature. Therefore leaching and precipitation of SrCO3 at higher temperatures mean that the formation of Sr(OH)2 precipitates is reduced.
On the other hand, leaching at pH < 7 generates hydrogen sulphide gas:
The generation of hydrogen sulphide gas takes place in early stages of leaching when the pH of slurry is relatively low.
Introducing the carbon dioxide gas or carbonating agent such as soda ash leads to the precipitation of strontium carbonate from supersaturated solution (Eq.13). The sequence of reaction steps includes the dissolution of carbon dioxide in solution and in situ formation of carbonic acid (H2CO3, Eq.8), the dissociation of H2CO3 (Eq.9 with the equilibrium constant (K´) given by Eq.10), the dissociation of bicarbonate species (Eq.11 with the equilibrium constant (K´´) given by Eq.12) and the precipitation of strontium carbonate (Eq.13 with the ion product (P) given by Eq.14) [163-165].
The solubility of strontium carbonate is 5.6 10-10 at the temperature of 25 °C and decreases to 1.32 10-10 at the temperature of 100 °C. The hydrolysis reaction leads to alkaline character of aqueous solution of SrCO3.
Influence of pH on the equilibrium concentration of Sr2+ and SrOH+ ions in solution.
Eqs.8-14 show that one mole of gaseous CO2 is required for the precipitation of each mole of SrCO3. The concentration of CO32- ions in leaching solution for given pH is expressed by the following law:
If the pH of leaching solution is higher than 7, H+ ions formed by the reaction 11 neutralizing OH- anions are released during leaching of SrS (Eq.4).
In general, the black ash method is concluded to be more the more economical than other alternatives [165].
The second technique for the preparation of strontium carbonate is the direct conversion method or hydrometallurgical method. Strontium carbonate is prepared by introducing SrSO4 powder into hot solution of Na2CO3, where the following conversion process takes place:
or better:
The effect of experimental conditions on the process includes the influence of temperature, solid to liquid ratio, particle size, stirring rate, Na2CO3 : SrSO4 molar ratio, etc. The conversion rate of celestite to strontium carbonate increases with temperature up to 70 °C [165-170]. Prepared carbonate or sulphide is further converted to other strontium salts [91].
It is also possible to use ammonium carbonate ((NH4)2CO3)[1] - and bicarbonate (NH4HCO3) instead of soda ash for the conversion [165,171-174]:
(in boiling mixture)
There is also an alternative in mechanochemical synthesis, where the mixture of SrSO4 and NH4HCO3 is intensively milled. The soluble ammonium sulphate is next removed by leaching of the product in water [165].
Moreover lots of special techniques for the preparation of SrCO3 were described in current literature. These methods include the preparation of strontium carbonate via solid-state decomposition route from inorganic precursor [746]. Simple solution techniques [175], solvothermal synthesis [176-178], refluxing method [188] hydrothermal synthesis [179-182], ultrasonic method or sonochemical-assisted synthesis [183,184], microwave assisted synthesis [185,186] and mechanochemical synthesis [168,187] were described. Depending on applied preparation technique, the strontium carbonate particles of different shape can be prepared, such as spheres, rods, whiskers and ellipsoids, needles, flowers, ribbons, wires, etc. [188].
The solubility of strontium salts is mostly either higher or lower than for corresponding calcium and barium salts (Table 1).
\n\t\t\t\tCations\n\t\t\t | \n\t\t\t\n\t\t\t\tCa2+\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tSr2+\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tBa2+\n\t\t\t\t\n\t\t\t | \n\t\t
Anions | \n\t\t\tSolubility [g∙100 cm-3] | \n\t\t||
OH- | \n\t\t\t0.160 | \n\t\t\t0.810 | \n\t\t\t3.890 | \n\t\t
F- | \n\t\t\t0.0017 | \n\t\t\t0.0175 | \n\t\t\t0.1600 | \n\t\t
Cl- | \n\t\t\t74.5 | \n\t\t\t53.1 | \n\t\t\t36.2 | \n\t\t
NO3\n\t\t\t\t2-\n\t\t\t | \n\t\t\t128.8 | \n\t\t\t70.4 | \n\t\t\t9.10 | \n\t\t
CO3\n\t\t\t\t2-\n\t\t\t | \n\t\t\t0.0014 (25 °C) | \n\t\t\t0.00155 (25 °C) | \n\t\t\t0.0022 (18 °C) | \n\t\t
SO4\n\t\t\t\t2-\n\t\t\t | \n\t\t\t0.20 | \n\t\t\t0.0148 | \n\t\t\t0.00023 | \n\t\t
C2O4\n\t\t\t\t2-\n\t\t\t | \n\t\t\t0.00058 | \n\t\t\t0.0048 | \n\t\t\t0.0125 | \n\t\t
CrO4\n\t\t\t\t2-\n\t\t\t | \n\t\t\t2.25 | \n\t\t\t0.204 | \n\t\t\t0.00037 | \n\t\t
The solubility of salts of alkaline earth metals at the temperature of 20 °C.
Metallic strontium can be prepared by the electrolysis of mixed melt of strontium chloride and potassium chloride in a graphite crucible using iron rod as cathode. The upper cathode space is cooled and metallic strontium collects around cooled cathode and forms a stick. Metallic strontium can also be prepared by thermal reduction of its oxide with aluminum. Strontium oxide-aluminum mixture is heated at high temperature in vacuum. Strontium is collected by the distillation in vacuum. Strontium is also a reducing agent. It reduces oxides and halides of metals at elevated temperatures to the metallic form.
Strontium is also obtained by the reduction of its amalgam, hydride, and other salts. Amalgam is heated and the mercury is separated by the distillation. If hydride is used, it is heated at 1 000°C in vacuum for the decomposition and removal of hydrogen. Such thermal reductions yield high–purity metal which, when exposed to air, oxidizes to SrO. The metal is pyrophoric, both SrO and SrO2 (strontium peroxide) are formed via ignition in air. When heated with chlorine gas or bromine vapor, strontium burns brightly, forming its halides (SrCl2 or SrBr2). When heated with sulfur, it forms sulfide (SrS) [91].
Strontium reacts vigorously with water and hydrochloric acid forming hydroxide Sr(OH)2 or chloride (SrCl2) with liberation of hydrogen [91]:
When heated under hydrogen it forms ionic hydride (SrH2), a stable crystalline salt. Heating metallic Sr in a stream of nitrogen above 380°C forms nitride (Sr3N2).
Pure alumina, which is required for the production of aluminum by the Hall process, is made by the Bayer process [91]. The Bayer process was developed in 1887 by Carl Josef Bayer (1847-1904). It is the method for industrial production of aluminium oxide from bauxite. This method replaces earlier techniques developed by Henri Étienne Sainte-Claire Deville (1818-1881). Fine milled bauxite powder is leached in the solution of sodium hydroxide in autoclave under the temperature range from 160 to 250 °C and the pressure from 0.4 to 0.8 MPa. The basic components of bauxite are dissolved and soluble salts according to the following reaction scheme are formed [189]:
Significant amount of impurities which remains in the dissolved solid rest, so called “red mud” (Fig.7), is next separated from the solution by filtration[1] -. The IEP values vary with red mud ranging from 6.35 to 8.70 [347]. The liquid filtrate is then diluted so that the concentration of Al2O3 in the solution reaches the value of 150 kg Al2O3∙m-3 and the nuclei of Al(OH)3 are introduced [190].
The dilution means decreasing the pH of alkaline solution[1] - and the precipitation of aluminium hydroxide. Precipitated gibbsite (Eq.27), which is the main product of Bayer process, is washed and calcined to Al2O3 in the rotary kiln (Eq.28).
Composition of red mud [145].
The purity of prepared aluminium oxide is about 99.5 % and Na2O is the main admixture in the product.
The flowing diagram of the process is shown in Fig.9. For the applications where high content of α-Al2O3 is necessary, the mineralization is accelerated by AlF3 (Eq.29). Several micrometer sized plate-like corundum crystals are formed.
Tricalcium aluminate hexahydrate (hydrogarnet) is used as a filter aid during the purification of sodium aluminate liquors. Furthermore, C3AH6 reduces the TiO2 content of precipitated gibbsite and the formation of hydrogarnet at high-temperature (250 °C) leaching minimizes the soda content of red-mud waste [191,192].
Fig.8 reveals that aluminium hydroxide can precipitate from the solution by introducing the carbon dioxide gas. The process can be expressed by the following reaction scheme:
Bayer process of the production of alumina.
The ultrasound [193] and the addition of organics such as methanol [194] and crown ether [195] intensify the nucleation and crystallization of sodium aluminate solution, which has the potential to enhance the throughput of a Bayer process. On the contrary, polyols [196], oleic acid [197] and alditols of hydroxycarboxili acids [198] inhibit the gibbsite precipitation from seeded sodium aluminate liquors.
The treatment and utilization of red mud waste are major challenge for the alumina industry. The main environmental risks associated with bauxite residue are related to high pH and alkalinity and minor and trace amounts of heavy metals and radionuclides. Many efforts are being globally made to find suitable applications for red mud so that the alumina industry may end up with no residue [199].
The possible applications of red mud include [199-204]:
Building/construction materials such as bricks, stabilized blocks, light weight aggregates and low density foamed products.
In cement industry as cements, special cements, additives to cements, mortars, construction concretes, repairs of roads, pavements, dykes.
Colouring agents for paint works for ground floors of industrial and other buildings.
Foamed paper in wood pulp and paper industry.
Reinforced red mud polymer products, ceramic/refractory products.
In metallurgical industries, as raw material in iron and steel industry as a sinter aid (binder) for iron ores, flux in steel making, etc.
Micro-fertilizer and a neutralizer of pesticides in agriculture.
Extracting rare-earth metals and alumo-ferric coagulants as technical raw materials.
Special use as inorganic chemicals, adsorbents, etc.
Aluminium oxide of high purity and high specific surface area can be prepared by thermal decomposition of alum (NH4Al(SO4)2⋅12 H2O)[1] -. Pure ammonium alum crystal is colorless and transparent and belongs to cubic crystal system. Melting temperature of ammonium alum crystal is 94.5 °C with the phase transition enthalpy of 122.2 kJ mol-1. [205-209]. NH4Al(SO4)2∙12 H2O is widely applied in industries and in water treatment [210,211]. Recently, ammonium alum is used as a promising material for Raman laser converters with a large frequency shift [212], for ferroelectricity [213] and phase transitions for storing energy absorbed by solar collectors [214,215], as the catalyst [216,217] and for rubidium recovery from the processing of zinnwaldite [218].
Ammonium aluminum sulfate (AAlSD[1] -) dodecahydrate undergoes the phase transitions at 58 K and 71 K on cooling and heating, respectively. At room temperature NH4Al(SO4)2∙12 H2O crystals have a cubic structure and belong to the space group with four molecules per unit cell with the lattice parameters a=12.242 Å. The structural phase transition mechanism is related to the hydrogen-bond transfer involving the breakage of weak part of the hydrogen bond [208].
The process of alum derived synthesis of alumina often produces nanosized powders consisting of amorphous or transition aluminas (Chapter 4.1). The thermal decomposition of aluminium alum can be described according to the following reaction scheme [219]:
The preparation of submicrometer-grained aluminas requires well-defined pure nanopowders which have many exploitable characteristics, such as low-temperature sinterability, greater chemical reactivity and enhanced plasticity. Therefore, whole range of methods was developed for the preparation of nanopowders with desired properties. These can be roughly devided to [424]:
High temperature/ flame/ laser synthesis: the method usually comprises the injection of suitable gaseous or liquid aluminium-containing precursors (e.g. aluminum tri-sec-butoxide) into the source of intensive heat (flame, laser or plasma), where the precursor decomposes and converts into oxide. In most cases, the transient aluminas are formed. Therefore further high-temperature treatment is necessary in order to obtain α-Al2O3.
Chemical method including the sol-gel process: obviously utilizes the low-and medium-temperature decomposition of inorganic aluminium salts and hydroxides or metal-organic compounds of aluminium. Typical precursors include aluminium nitrate and hydroxides.
Mechanically assisted synthesis: the method is based on high-energy milling of coarser-grained powder. In this case, the minimum particle size is limited to approximately 40 nm.
Other options include the combustion synthesis [220-224], the spray pyrolysis of aerosol of nitrate or other aluminum salts [225-227], the sol-gel process [228,229], the emulsion synthesis [230,231], etc.
Strontium aluminate is formed via solid-state reaction of equimolar amount of aluminium oxide with strontium oxide:
With regard to the molar weight ratios of SrO/Al2O3=1.016 and SrO/Fe2O3=0.65, the proper amount of SrO should be calculated as follows:
The mass ratio of used SrO and the theoretical amount calculated according to Eq.34 should be termed as the “Saturation Degree” or “Strontium saturation factor” of clinker by strontium oxide (SDSrO):
Analogically to ordinary Portland cement, the hydraulic module (MH) and the alumina module (MA) of strontium aluminate clinker should be defined:
To use the strontium aluminate cements for the production of refractory materials the value of SDSrO < 100 and low content of Fe2O3 are required. These compositions ensure that the first eutectic melt is formed at the temperature of 1760 °C instead of 1505 °C for the cement with SDSrO > 100.
The composition of mixture of raw materials is calculated from required composition of clinker according to the following relations:
The preparation of raw meal using only two components is a simple process. The example could be strontium carbonate with 98.4 % SrCO3 and alumina which does not contain any strontium carbonate. The analysing techniques are described in Chapter 2.4. From Eq.34 it can be read that equimolar mixture of SrCO3 and Al2O3 (xAl2O3=0.5) should be prepared. That means that xSrCO3=0.5 and xAl2O3=1 – xSrCO3=0.5. The molar ratio can be recalculated to the weight ratio as follows[1] -:
From the relationships introduced above we can calculate:
The amount of raw mixture constituent can be then calculated:
The raw meal contains 60.1 % of strontium carbonate and 39.9 % of alumina, i.e. both components are mixed in the weight ratio of 1.5 : 1.
The preparation of raw meal of given value of hydraulic module (Eq.37) is demonstrated in this chapter. As an example the raw meal for the strontium aluminate clinker with MH=0.98 will be prepared. There are two raw materials with the composition given in Table 2.
\n\t\t\t\tRaw material\n\t\t\t | \n\t\t\t\n\t\t\t\tSrO\n\t\t\t | \n\t\t\t\n\t\t\t\tAl2O3\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tFe2O3\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tZ =Al2O3+Fe2O3\n\t\t\t\t\n\t\t\t | \n\t\t|
SrCO3\n\t\t\t | \n\t\t\t1 | \n\t\t\t98.4 | \n\t\t\t0 | \n\t\t\t0.2 | \n\t\t\t0.2 | \n\t\t
Calcined bauxite | \n\t\t\t2 | \n\t\t\t0 | \n\t\t\t98.2 | \n\t\t\t1.6 | \n\t\t\t99.8 | \n\t\t
Composition of two raw materials for the preparation of raw meal.
Hence we have two equations:
where:
From the relationships introduced above we can calculate:
From the substitution of 48 by 50 and 51 the following relationship results:
Now it is possible to check, if the calculated results are correct:
\n\t\t\t\tRaw material\n\t\t\t | \n\t\t\t\n\t\t\t\tComposition\n\t\t\t | \n\t\t\twi\n\t\t\t\t [%] | \n\t\t\t\n\t\t\t\tRaw meal \n\t\t\t | \n\t\t||||
\n\t\t\t\tSrO\n\t\t\t | \n\t\t\t\n\t\t\t\tAl2O3\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tFe2O3\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tSrO\n\t\t\t | \n\t\t\t\n\t\t\t\tAl2O3\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tFe2O3\n\t\t\t\t\n\t\t\t | \n\t\t||
1 | \n\t\t\t98.4 | \n\t\t\t0 | \n\t\t\t0.2 | \n\t\t\t49.9 | \n\t\t\t48,3 | \n\t\t\t0 | \n\t\t\t0,1 | \n\t\t
2 | \n\t\t\t0 | \n\t\t\t98.2 | \n\t\t\t1.6 | \n\t\t\t50.1 | \n\t\t\t0 | \n\t\t\t49,2 | \n\t\t\t0,8 | \n\t\t
Total | \n\t\t\t100.0 | \n\t\t\t49,1 | \n\t\t\t49,2 | \n\t\t\t0,9 | \n\t\t|||
MH\n\t\t\t | \n\t\t\t49,1/ (49.2 + 0,9) = 0.98 | \n\t\t
Another case is that the mixture requires the preparation via mixing of three raw materials, e.g. we have strontium carbonate, calcined bauxite and corundum. Corundum is necessary to keep the value of hydraulic module (MH)=0.98 and alumina module (MA)=57. The composition of raw materials is listed in Table 3.
\n\t\t\t\tRaw material\n\t\t\t | \n\t\t\t\n\t\t\t\tSrO\n\t\t\t | \n\t\t\t\n\t\t\t\tAl2O3\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tFe2O3\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tZ =Al2O3+Fe2O3\n\t\t\t\t\n\t\t\t | \n\t\t|
SrCO3\n\t\t\t | \n\t\t\t1 | \n\t\t\t98.4 | \n\t\t\t0 | \n\t\t\t0.2 | \n\t\t\t0.2 | \n\t\t
Calcined bauxite | \n\t\t\t2 | \n\t\t\t0 | \n\t\t\t98.2 | \n\t\t\t1.6 | \n\t\t\t99.8 | \n\t\t
Corundum | \n\t\t\t3 | \n\t\t\t0 | \n\t\t\t99.5 | \n\t\t\t0.1 | \n\t\t\t99.6 | \n\t\t
Composition of three raw materials for the preparation of raw meal.
Where:
From the relationships introduced above we can calculate:
The substitution of equations 53-55 by Eq.58-63 yields to:
The value of determinant D can be calculated as follows:
The composition of raw meal is then:
Now it is possible, as previously, to check, if the calculated results are correct:
\n\t\t\t\tRaw material\n\t\t\t | \n\t\t\t\n\t\t\t\tComposition\n\t\t\t | \n\t\t\twi\n\t\t\t\t [%] | \n\t\t\t\n\t\t\t\tRaw meal \n\t\t\t | \n\t\t||||
\n\t\t\t\tSrO\n\t\t\t | \n\t\t\t\n\t\t\t\tAl2O3\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tFe2O3\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tSrO\n\t\t\t | \n\t\t\t\n\t\t\t\tAl2O3\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tFe2O3\n\t\t\t\t\n\t\t\t | \n\t\t||
1 | \n\t\t\t98.4 | \n\t\t\t0 | \n\t\t\t0.2 | \n\t\t\t49.9 | \n\t\t\t49.10 | \n\t\t\t0 | \n\t\t\t0.10 | \n\t\t
2 | \n\t\t\t0 | \n\t\t\t98.2 | \n\t\t\t1.6 | \n\t\t\t47.6 | \n\t\t\t0 | \n\t\t\t46.74 | \n\t\t\t0.76 | \n\t\t
3 | \n\t\t\t0 | \n\t\t\t99.5 | \n\t\t\t0.1 | \n\t\t\t2.5 | \n\t\t\t0 | \n\t\t\t2.50 | \n\t\t\t0 | \n\t\t
Total | \n\t\t\t100 | \n\t\t\t49.10 | \n\t\t\t49.24 | \n\t\t\t0.86 | \n\t\t|||
MH\n\t\t\t | \n\t\t\t49.10/ (49.24 + 0.86) = 0.98 | \n\t\t||||||
MA\n\t\t\t | \n\t\t\t49.24/ 0.86 = 57 | \n\t\t
The method of calculation of raw meal for the preparation of strontium aluminate clinker with given Saturation Degree (SDSrO) and alumina module (MA) is described. As an example the mixture with SDSrO=0.95 and MA=60 using three raw materials with the composition listed in Table 4 will be prepared.
\n\t\t\t\tRaw material\n\t\t\t | \n\t\t\t\n\t\t\t\tSrO\n\t\t\t | \n\t\t\t\n\t\t\t\tAl2O3\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tFe2O3\n\t\t\t\t\n\t\t\t | \n\t\t|
SrCO3\n\t\t\t | \n\t\t\t1 | \n\t\t\t97.3 | \n\t\t\t0 | \n\t\t\t0.3 | \n\t\t
Calcined bauxite | \n\t\t\t2 | \n\t\t\t0 | \n\t\t\t96.5 | \n\t\t\t2.6 | \n\t\t
Corundum | \n\t\t\t3 | \n\t\t\t0 | \n\t\t\t99.7 | \n\t\t\t0.1 | \n\t\t
Composition of three raw materials for the preparation of raw meal.
The composition of raw meal results from the solution of the following set of three equations:
where
From the relationships introduced above we can calculate:
The value of determinant D can be calculated as follows:
The composition of raw meal is then:
Now it is possible, as previously, to check, if the calculated results are correct:
\n\t\t\t\tRaw material\n\t\t\t | \n\t\t\t\n\t\t\t\tComposition\n\t\t\t | \n\t\t\twi\n\t\t\t\t [%] | \n\t\t\t\n\t\t\t\tRaw meal\n\t\t\t | \n\t\t||||
\n\t\t\t\tSrO\n\t\t\t | \n\t\t\t\n\t\t\t\tAl2O3\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tFe2O3\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tSrO\n\t\t\t | \n\t\t\t\n\t\t\t\tAl2O3\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tFe2O3\n\t\t\t\t\n\t\t\t | \n\t\t||
1 | \n\t\t\t97.3 | \n\t\t\t0 | \n\t\t\t0.3 | \n\t\t\t49.8 | \n\t\t\t48.48 | \n\t\t\t0 | \n\t\t\t0,15 | \n\t\t
2 | \n\t\t\t0 | \n\t\t\t96.5 | \n\t\t\t2.6 | \n\t\t\t24.8 | \n\t\t\t0 | \n\t\t\t23.97 | \n\t\t\t0,65 | \n\t\t
3 | \n\t\t\t0 | \n\t\t\t99.7 | \n\t\t\t0.1 | \n\t\t\t25.3 | \n\t\t\t0 | \n\t\t\t25,27 | \n\t\t\t0,03 | \n\t\t
Total | \n\t\t\t100 | \n\t\t\t48,48 | \n\t\t\t49,23 | \n\t\t\t0,82 | \n\t\t|||
MH\n\t\t\t | \n\t\t\t48.48/ (49.23 + 0.82) = 0.97 | \n\t\t||||||
MA\n\t\t\t | \n\t\t\t49.23/ 0.82 = 60 | \n\t\t
Using four raw materials for the preparation of raw meal where other parameter can be used for calculation. That can be useful for the preparation of cement with exceeding substitution of alumina. For example, the Saturation Degree of clinker by strontium oxide and alumina module can be redefined as follows:
Therefore a new type of module can be applied:
This is the case of raw meal the preparation of strontium aluminate clinker with following parameters SDSrO*=0.90, MA*=58 and MF=10. The composition of raw materials is listed in Table 5.
\n\t\t\t\tRaw material\n\t\t\t | \n\t\t\t\n\t\t\t\tSrO\n\t\t\t | \n\t\t\t\n\t\t\t\tAl2O3\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tFe2O3\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tCr2O3\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tY=Fe2O3+Cr2O3\n\t\t\t\t\n\t\t\t | \n\t\t|
SrCO3\n\t\t\t | \n\t\t\t1 | \n\t\t\t97.3 | \n\t\t\t0 | \n\t\t\t0.3 | \n\t\t\t0 | \n\t\t\t0.3 | \n\t\t
Calcined bauxite | \n\t\t\t2 | \n\t\t\t0 | \n\t\t\t96.5 | \n\t\t\t2.6 | \n\t\t\t0 | \n\t\t\t2.6 | \n\t\t
Corundum | \n\t\t\t3 | \n\t\t\t0 | \n\t\t\t99.7 | \n\t\t\t0.1 | \n\t\t\t0 | \n\t\t\t0.11 | \n\t\t
Cr2O3\n\t\t\t | \n\t\t\t4 | \n\t\t\t0 | \n\t\t\t2.8 | \n\t\t\t1.5 | \n\t\t\t32.6 | \n\t\t\t34.1 | \n\t\t
Composition of four raw materials for the preparation of raw meal.
The composition of raw meal results from the solution of the following set of four equations:
Where:
From the relationships introduced above we can calculate:
From the relationships introduced above we can further calculate:
The solution based on the Sauruss law is time consuming without specialized software. Nevertheless, it is possible to use the calculation according to Table 6. This solution is based on the Gauss inversion method. The symbols i and k denote the line and column of matrix for mathematical operation according to given rule, e.g. 2a1 is the second line and the first column member of matrix. The solution consists of the following steps:
Inserting the coefficients from the left side of Eqs.106-109 into proper line (1-4) of Table 6. For example the coefficient a11 from Eq.106 should be written in the first line and first column; the coefficient a32 belongs to the third line and second column, etc.
The column I contains the sum of members 1-4 for given line.
The column II contains the coefficients from the right side of Eqs.106-109 for the line from 1 to 4. Other lines refer to the results of mathematical operation defined in the column rule.
The operation on line 6 (-2a1 ∙ 5+2) means:-97.13 ∙ 1.00+97.13=0, where 2a1 is the coefficient related to the second line and first column. The operation 1 : 1a1 means that all numbers in the first line are divided by given term.
The last four lines of column II provide the solution for the composition of raw meal.
\n\t\t\t\tRule\n\t\t\t | \n\t\t\t\n\t\t\t\tik\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tMatrix\n\t\t\t | \n\t\t\t\n\t\t\t\tTest: 4\n\t\t\t | \n\t\t\t\n\t\t\t | \n\t\t\t\tTest:\n\t\t\t | \n\t\t|||
\n\t\t\t\tΣi\n\t\t\t | \n\t\t\t\n\t\t\t\tib\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tI+II.\n\t\t\t | \n\t\t||||||
1 | \n\t\t\t2 | \n\t\t\t3 | \n\t\t\t4 | \n\t\t\tI | \n\t\t\tII | \n\t\t\tIII | \n\t\t||
i ak\n\t\t\t | \n\t\t\t1 | \n\t\t\t1.00 | \n\t\t\t1.00 | \n\t\t\t1.00 | \n\t\t\t1.00 | \n\t\t\t4.00 | \n\t\t\t100.00 | \n\t\t\t104.00 | \n\t\t
2 | \n\t\t\t97.13 | \n\t\t\t-89.76 | \n\t\t\t-91.22 | \n\t\t\t-23.39 | \n\t\t\t-107.25 | \n\t\t\t0.00 | \n\t\t\t-107.25 | \n\t\t|
3 | \n\t\t\t-17.40 | \n\t\t\t-54.30 | \n\t\t\t93.90 | \n\t\t\t-1975.0 | \n\t\t\t-1952.8 | \n\t\t\t0.00 | \n\t\t\t-1952.8 | \n\t\t|
4 | \n\t\t\t0.30 | \n\t\t\t2.60 | \n\t\t\t0.10 | \n\t\t\t-324.50 | \n\t\t\t-321.50 | \n\t\t\t0.00 | \n\t\t\t-321.50 | \n\t\t|
1 :1a1 XX | \n\t\t\t5 | \n\t\t\t1.00 | \n\t\t\t1.00 | \n\t\t\t1.00 | \n\t\t\t1.00 | \n\t\t\t4.00 | \n\t\t\t100.00 | \n\t\t\t104.00 | \n\t\t
-2a1 ∙ 5 + 2 | \n\t\t\t6 | \n\t\t\t0.00 | \n\t\t\t-186.89 | \n\t\t\t-188.35 | \n\t\t\t-120.51 | \n\t\t\t-495.75 | \n\t\t\t-9712.45 | \n\t\t\t-10208.2 | \n\t\t
-3a1 ∙ 5 + 3 | \n\t\t\t7 | \n\t\t\t0.00 | \n\t\t\t-36.90 | \n\t\t\t111.30 | \n\t\t\t-1957.6 | \n\t\t\t-1883.2 | \n\t\t\t1740.00 | \n\t\t\t-143.2 | \n\t\t
-4a1 ∙ 5 + 4 | \n\t\t\t8 | \n\t\t\t0.00 | \n\t\t\t2.30 | \n\t\t\t-0.20 | \n\t\t\t-324.80 | \n\t\t\t-322.70 | \n\t\t\t-30.00 | \n\t\t\t-352.7 | \n\t\t
-5a2 ∙ 10 + 5 | \n\t\t\t9 | \n\t\t\t1.00 | \n\t\t\t0.00 | \n\t\t\t-0.01 | \n\t\t\t0.36 | \n\t\t\t-1.35 | \n\t\t\t48.03 | \n\t\t\t49.38 | \n\t\t
6 : 6a2 XX | \n\t\t\t10 | \n\t\t\t0.00 | \n\t\t\t1.00 | \n\t\t\t1.01 | \n\t\t\t0.64 | \n\t\t\t2.65 | \n\t\t\t51.97 | \n\t\t\t54.62 | \n\t\t
-7a2 ∙ 10 + 7 | \n\t\t\t11 | \n\t\t\t0.00 | \n\t\t\t0.00 | \n\t\t\t148.49 | \n\t\t\t-1933.8 | \n\t\t\t-1785.32 | \n\t\t\t3657.70 | \n\t\t\t1872.38 | \n\t\t
-8a2 ∙ 10 + 8 | \n\t\t\t12 | \n\t\t\t0.00 | \n\t\t\t0.00 | \n\t\t\t-2.52 | \n\t\t\t-326.28 | \n\t\t\t-328.80 | \n\t\t\t-149.53 | \n\t\t\t-478.33 | \n\t\t
-9a3 ∙ 15 + 9 | \n\t\t\t13 | \n\t\t\t1.00 | \n\t\t\t0.00 | \n\t\t\t0.00 | \n\t\t\t0.25 | \n\t\t\t1.25 | \n\t\t\t48.22 | \n\t\t\t49.48 | \n\t\t
-10a3 ∙ 15 + 10 | \n\t\t\t14 | \n\t\t\t0.00 | \n\t\t\t1.00 | \n\t\t\t0.00 | \n\t\t\t17.77 | \n\t\t\t14.77 | \n\t\t\t27.14 | \n\t\t\t41.91 | \n\t\t
11 : 11a3 XX | \n\t\t\t15 | \n\t\t\t0.00 | \n\t\t\t0.00 | \n\t\t\t1.00 | \n\t\t\t-13.02 | \n\t\t\t-12.02 | \n\t\t\t24.63 | \n\t\t\t12.61 | \n\t\t
-12a3 ∙ 15 + 12 | \n\t\t\t16 | \n\t\t\t0.00 | \n\t\t\t0.00 | \n\t\t\t0.00 | \n\t\t\t-359.08 | \n\t\t\t-359.08 | \n\t\t\t-87.51 | \n\t\t\t-446.58 | \n\t\t
-13a4 ∙ 20 + 13 | \n\t\t\t17 | \n\t\t\t1.00 | \n\t\t\t0.00 | \n\t\t\t0.00 | \n\t\t\t0.00 | \n\t\t\t1.00 | \n\t\t\t48.16 | \n\t\t\t49.16 | \n\t\t
-14a4 ∙ 20 + 14 | \n\t\t\t18 | \n\t\t\t0.00 | \n\t\t\t1.00 | \n\t\t\t0.00 | \n\t\t\t0.00 | \n\t\t\t1.00 | \n\t\t\t23.79 | \n\t\t\t24.79 | \n\t\t
-15a4 ∙ 20 + 15 | \n\t\t\t19 | \n\t\t\t0.00 | \n\t\t\t0.00 | \n\t\t\t1.00 | \n\t\t\t0.00 | \n\t\t\t1.00 | \n\t\t\t27.81 | \n\t\t\t28.81 | \n\t\t
16 : 16a4 XX | \n\t\t\t20 | \n\t\t\t0.00 | \n\t\t\t0.00 | \n\t\t\t0.00 | \n\t\t\t0.00 | \n\t\t\t1.00 | \n\t\t\t0.24 | \n\t\t\t1.24 | \n\t\t
Numerical solution for raw meal four raw materials.
Now it is possible, as previously, to check, if the calculated results are correct:
\n\t\t\t\tRaw material\n\t\t\t | \n\t\t\t\n\t\t\t\tComposition\n\t\t\t | \n\t\t\twi\n\t\t\t\t [%] | \n\t\t\t\n\t\t\t\tRaw meal \n\t\t\t | \n\t\t||||||
\n\t\t\t\tSrO\n\t\t\t | \n\t\t\t\n\t\t\t\tAl2O3\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tFe2O3\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tCr2O3\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tSrO\n\t\t\t | \n\t\t\t\n\t\t\t\tAl2O3\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tFe2O3\n\t\t\t\t\n\t\t\t | \n\t\t\t\n\t\t\t\tCr2O3\n\t\t\t\t\n\t\t\t | \n\t\t||
1 | \n\t\t\t97.3 | \n\t\t\t0 | \n\t\t\t0.3 | \n\t\t\t0 | \n\t\t\t48.16 | \n\t\t\t46.86 | \n\t\t\t0 | \n\t\t\t0.14 | \n\t\t\t0 | \n\t\t
2 | \n\t\t\t0 | \n\t\t\t96.5 | \n\t\t\t2.6 | \n\t\t\t0 | \n\t\t\t23.76 | \n\t\t\t0 | \n\t\t\t22.96 | \n\t\t\t0.62 | \n\t\t\t0 | \n\t\t
3 | \n\t\t\t0 | \n\t\t\t99.7 | \n\t\t\t0.1 | \n\t\t\t0 | \n\t\t\t27.81 | \n\t\t\t0 | \n\t\t\t27.72 | \n\t\t\t0.03 | \n\t\t\t0 | \n\t\t
4 | \n\t\t\t0 | \n\t\t\t2.8 | \n\t\t\t1.5 | \n\t\t\t32.6 | \n\t\t\t0.24 | \n\t\t\t0 | \n\t\t\t0.01 | \n\t\t\t0 | \n\t\t\t0.08 | \n\t\t
Total | \n\t\t\t100.00 | \n\t\t\t46.86 | \n\t\t\t50.69 | \n\t\t\t0.79 | \n\t\t\t0.08 | \n\t\t||||
SD*\n\t\t\t\tSrO\n\t\t\t | \n\t\t\t46.86/ (1.016∙50.69 + 0.65∙0.79 + 0.68∙0.08) = 0.90 | \n\t\t||||||||
MA\n\t\t\t\t*\n\t\t\t | \n\t\t\t50.69/ (0.79 + 0.08) = 58.0 | \n\t\t||||||||
MFe\n\t\t\t | \n\t\t\t0.79/ 0.08 = 10.0 | \n\t\t
It is also possible to define another kind of module using V2O3, Ti2O3 or their mixture with Fe2O3 and to calculate the raw meal composition by the same way. This calculation certainly requires the redefinition of Saturation Degree of clinker from strontium oxide to applied system.
The calculation mentioned above requires the analysis of raw materials to calculate the correction of working mixture composition. Some techniques applicable for the analysis of raw materials or cements are presented in this chapter. Chemical analysis, microscopy, XRD and other methods of examination should be carried out on the same, representative sample of material [7].
The EDTA disodium salt (Ethylenediaminetetraacetic acid disodium salt dehydrate, Na2H2Y⋅2H2O) titration is the most common technique used for the determination of strontium oxide in the strontium carbonate:
with the stability constant of complex:
The formation of stable complex during the assessment requires the pH ≥ 10 (the same pH range as for Mg2+and Ba2+).
In compliance with the current ASTM Standard Test Methods for chemical analysis of hydraulic cement (C 114) [241], strontium (usually present in Portland cement as minor constituent), is led to precipitate (Table 1) with calcium (CaC2O4) as oxalate (SrC2O4, monoclinic, space group P21/n [242,243]) and next it is subsequently titrated and calculated as CaO, or alternative correction of CaO for SrO is made, if the SrO content is known. Therefore, the development of a new, direct, sensitive and accurate method for the determination of strontium as minor constituent in cement is of upmost importance [244].
Strontium oxalate exists in two different forms [245]:
Neutral strontium oxalate hydrate, SrC2O4\n\t\t\t\t\t\t\txH2O.
Acid salt of strontium oxalate, SrC2O4\n\t\t\t\t\t\t\tyH2C2O4 xH2O.
Depending on the concentration of oxalic acid and ammonium oxalate as precipitating agents, both forms can be obtained. At sufficiently low pH, the stoichiometric compound SrC2O4⋅ ½H2C2O4⋅H2O is formed. The morphologies of precipitated particles (bi-pyramids, rods, peanuts, spheres, etc.) depend on the experimental conditions such as pH, temperature, ageing time and concentration of additives [246].
The structure of acidic strontium oxalate is shown in Fig.9(a). Oxalate and hydrogen oxalate anions are present in such a way that each asymmetric unit contains exactly one molecule with the structural formula Sr(HC2O4)⋅1/2(C2O4)H2O instead of Sr(C2O4)⋅1/2(H2C2O4)⋅H2O. Similarly to other known strontium oxalates, strontium is eight-fold coordinated by oxygen. In this coordination sphere, both, oxalate and hydrogen oxalate anions act once as bidentate and once as monodentate. Two remaining positions are occupied by H2O molecules. The SrO8 polyhedron can be described as distorted bicapped trigonal prism, with O7…O2…O5…Ow3 forming the square face. These polyhedrons are connected to each other only by edge sharing it to form one-dimensional chains along the c-axis [242].
Strontium oxalate: Structure of SrO8 polyhedron (a) and connection of polyhedrons via shared edge (b) according to [243].
The shared edges are O4…O4´ and Ow3…Ow3´(Fig.9 (b)), which means that H2O acts as bridging ligand between two strontium atoms. This is in contrast to all other Sr oxalates, where H2O is also coordinated to Sr, but without any bridging function. In the ac plane, the polyhedra chains are connected by the C2O42- groups, while in the bc plane the connection is made by the HC2O4- groups. In addition, there is the possibility to form intrachain (Dw2…O5 and D3…O2 along the bc plane) as well as interchain (Dw1…O6 along the ac plane) hydrogen bridges, which give the whole network an extra stability. Until now the four types of acid strontium oxalates are known, the type 2 and 4 are conformers (Fig.10) with calculated energy difference of ~6.69 kJ mol-1 [242,243].
Coordination types of acid strontium oxalate: type 4 (a) and type 2 (b).
Many analytical techniques were suggested for the determination of strontium in the cement matrix [238,247-250], i.e. under conditions including high concentration of calcium in the sample, based on complicated separation techniques of low selectivity. The atomic absorption spectrometric method can be used for the determination of calcium, magnesium and strontium in soils [236] but the assessment requires the removal of the silicon.
Derivative spectrophotometry is analytical technique combining high selectivity [251-255] and sensitivity [244,256-258]. The accuracy of assessment depends on the shape of normal absorption spectra of analyte and interfering substances, as well as on the instrumental parameters and the applied technique of measurement, e.g. peak-to-trough or zero-crossing [259-261]. Salinas et al. [262] developed the derivative spectrophotometric method for resolving binary mixtures when the spectra of components are overlapped. The method uses the first derivation of the spectra. The concentration of other component is then determined from the calibration graph. Later, the method was extended to the resolution of ternary mixtures in combination with zero-crossing method [263].
The determination of strontium and simultaneous determination of strontium oxide, magnesium oxide and calcium oxide content in Portland cement by derivative ratio spectrophotometry uses alizarin Complexone (alizarin-3-methylamine-N, N-diacetic acid, AC) as one of the most common reagents used for the spectrophotometric determination of metal ions. The AC reagent yields five colored acid–base forms in the solutions of pH ∼3.2–10.5: H4L, H3L−, H2L2−, HL3− and L4−, exhibiting the absorption maxima at 270, 335, 423, 525, and 580 nm, respectively. Distinct isosbestic points are observed for the particular acid–base equilibrium. The formation of SrL2-complex with liberation of one proton occurs in pH range from 7 to 10 [262]:
The determination of strontium as SrL2− complex was possible in the presence of Li+, Na+, K+, Cs+, Cd2+, Al3+, Fe3+, Mo6+, SO42−, SO32−, NO3−, Cl−, Br−, I− and PO43− (20.0 mg); Co2+,Ni2+, Pb+, Cr3+, Ti4+, C2O42− and CO32− (1.0 mg). Investigated ions Ca2+, Mg2+, Mn2+and Zn2+interfered seriously, even when present in amounts higher than 0.1 mg. The interference due to Mn and Zn was eliminated by the addition of ammonium hydroxide, and that of Ca and Mg was overcome by using the derivative ratio zero crossing method. Using the proposed method, it is possible to determine Sr, Mg, and Ca simultaneously in mixtures containing 1.5-18 μg⋅cm-3 of strontium, 0.5-5.0 μg⋅cm-3 of magnesium and 1.0-8.0 μg⋅cm-3 of calcium [262].
In many disciplines of science it is important to be able to determine the source of material or to characterize its transportation history. The chemical composition has been used extensively to determine the source of materials by fingerprinting the chemical composition of the material to be identified and comparing it to the chemical composition of potential sources. This approach has been used extensively for major elements as well as for trace elements [264,265].
Forensic isotope geochemistry relies on subtle differences in isotopic abundance of element to characterize particular material. These different isotopic abundances give rise to unique isotopic composition that will identify the material come from particular region. Many rocks composed of different minerals have distinctive isotopic compositions and their unique composition can be used to fingerprint them. This distinctive rock/mineral composition usually arises from the decay of radiogenic elements e.g., 87Rb to 87Sr; the transuranic elements to 208Pb, 207Pb, 206Pb; 147Sm to 143Nd [264].
Combined chemical and Sr isotopic analysis may provide the geochemical fingerprints from raw materials, which can be used to identify them in concrete. For successful chemical fingerprinting of cement in concrete, it is important to leach cement without significantly attacking the aggregate, but this can be minimized by using slightly alkaline or neutral EDTA as solvent in preference to weak mineral acids such as HNO3. Combined chemical and Sr isotopic analysis of commonly used New Zealand cements showed that they contain characteristic fingerprints, which may be used to identify them in concrete of unknown origin. Although cements have typically 87Sr/86Sr values similar to their mid-Tertiary limestone source rocks (0.7078 – 0.7085) most are easily distinguishable when their 87Sr/86Sr values are plotted against Ca/Sr [266].
Different forms of alumina may be identified by x-ray diffraction analysis [91]. Classical, wet analysis gives inaccurate results for Al2O3 unless the effects of P2O5 and TiO2 are not taken into account [7].
The standard test methods for the chemical analysis of hydraulic cement are specified by ASTM C 114-13. The chemical analysis of hydraulic cement is specified by ASTM C 114-88 standard. The mineralogy of cement cannot be determined from the chemical composition because the thermodynamic equilibrium usually is not reached during the production process. The phase (mineralogical) composition of strontium aluminate cement can be in principle determined by the same methods as for aluminous cements:
Selective dissolution [269];
Electron Microscopy [7,269];
Reflected Light Microscopy [7,269];
Quantitative X-Ray Diffraction Analysis (QXDA) [7,267-270,599],
Due to recent developments in cement clinker engineering, the optimization of chemical substitutions in the main clinker phases offers a promising approach to improve both reactivity and grindability of clinkers. Thus, the monitoring of chemistry of phases may become a part of the quality control at cement plants, along with usual measurements of the abundance of mineralogical phases [270].
The chemical reactions which take place after mixing cement with water are generally more complex than simple conversion of anhydrous compounds into the corresponding hydrates. The mixtures of cement with water, where the hydration reactions, setting and hardening take place are termed as pastes [271], while the hardened material can be termed as cement stone or hardened cement stone. The water to cement ratio (w/c) refers to the proportion by mass that is related to water and cement used for the preparation of cement paste [12,271].
The value water-to-cement (w/c) ratio is one of the most fundamental parameters in concrete mixture proportioning. The w/c ratio has a significant influence on most properties of hardened concrete, in particular on strength and durability due to its relationship with the amount of residual space i.e. capillary porosity, in the cement stone. Since the w/c ratio is an indication of quality of concrete mix, the situations often arise in which it is desirable to determine the original w/c ratio of particular concrete some time after it has hardened. This often happens when the disputes suspecting the noncompliance with the mix specification arise. The determination of the w/c ratio is also important for the quality control during the concrete production and for general quality assurance purposes [273-277].
Unfortunately, once concrete has set, it is very difficult to ascertain the exact amounts of cement and water which were originally added during batching. At any moment after setting, the hardened cement stone can be considered to consist of four main phases [273]:
Rest of unreacted cement;
Crystalline and semi-crystalline hydration products including their intrinsic gel pores;
Capillary pores;
Air voids.
The solid hydration products occupy a greater volume than the volume of reacted cement (Fig.11), but slightly smaller volume than the sum of volumes of cement and water due to chemical shrinkage [273,278]. Chemical shrinkage associated with hydration of OPC and AC is about 5 and 10 – 12 ml per 100 g of cement, respectively [12].
Proportion of main phases in hardened cement stone [273].
The methodology for the estimation of initial cement content, water content and water/cement ratio of hardened cement-based materials by electron microscopy was developed by Wong and Buenfeld [273] and Sahu at al. [275]. The acoustic-ultrasonic approach for non-destructive determination of w/c ratio was described by Philippidis and Aggelis [274]. Betcher at al. [277] published the method using 2.45 GHz microwave radiation which can be conveniently and accurately used for the on-site determination of the water-to-cement (w/c) ratio in a batch of fresh rapid-setting concrete.
Grinding occurs at the beginning and at the end of cement making process [279]. In recent years, the matrix model and the kinetic model, which were suggested by investigators, are used in laboratories and industrial areas. The kinetic model, which is an alternative approach, considers the combination as a continuous process in which the rate of breakage of particle size is proportional to the mass of particles of that size. The analysis of size reduction in tumbling ball mills using the concepts of specific rate of breakage and primary daughter fragment distribution has received considerable attention in the last years [280,320].
To optimize the cement grinding, the standard Bond grinding calculations [281] can be used as well as the modeling and simulation techniques based on the population balance model (PBM) [284,285]. The mill power draw prediction can be carried out using the Morrell power model for tumbling mills [279,282].
The Bonds equation describes the specific power required to reduce the feed from specified feed F80 to the product with specified P80 [279,281]:
where Wm is the mill specific motor output power (kWh⋅t-1), Wi is the Bond ball mill work index (kWh⋅t-1) P80 is the sieve size passing 80 % of the mill product (μm), F80 is the sieve size passing 80 % of the mill feed (μm). It was found in the crushing area that there are significant differences between the real plant data and the Bond calculations and therefore the empirical corrections were introduced. The following modified Bond equation was proposed for crushing [283]:
where Wc is the energy consumed for crushing the clinker (kWh t-1), Wi is the Bond ball mill work index (kWh t-1), Pc is the sieve size passing 80% of clinker after crushing (μm), Fc is the sieve size passing 80% of clinker before crushing (μm) and A is the empirical coefficient, which depends on clinker and crusher properties.
Based on the above considerations for crushing and grinding, the energy consumption for the clinker pre-crushing and ball milling can be estimated using the following Bond based model:
Since the pre-crushing product size Pc is equal to the mill feed size F80 then [279]:
where D is the interior mill diameter and Rr=F80/P80.
The basis of the population balance model for modeling the two-compartment ball mill is the perfect mixing ball mill model. This model considers a ball mill or a section of it as a perfectly stirring tank. Then the process can be described in the terms of transport through the mill and breakage within the mill. Because the mill or section of it is perfectly mixing a discharge rate, di, for each size fraction is an important variable for defining the product [279,284,285]:
Steady state operation conditions can be described by the following relation:
The substitution of si by pi/di leads to the equation:
where fi is the feed rate of size fraction [t⋅h-1], pi is the product flow of size fraction [t⋅h-1], aij is the mass fraction of particle of size that appears at size i after breakage, ri is the breakage rate of particle size i [h-1], si is the amount of size i particles inside the mill [t], di is the discharge rate of particle size i [h-1]. If the breakage distribution function is known, the calibration of the model to a ball mill involves the calculation of r/d values using the feed and product size distribution obtained under known operating conditions. Where the size distribution of the mill content is available, the breakage and discharge rates can be calculated separately.
The model consists of two important parameters, the breakage function (aij) that describes the material characteristics and the breakage/discharge rate function (ri/di) which defines the machine characteristics and can be calculated when the feed and product size distributions are known and the breakage function is available. The air classifier controls the final product quality. Therefore, the air classifier has a crucial role in the circuit and a strong attention is paid regarding the design and operation of the air classifier. The classification action is modeled using the efficiency curve approach. The effect of the classifier design and of operational parameters on the efficiency is complicated and the works proceed to improve the current models [279].
The consumption of steel grinding media plays an important role in the economics of grinding and as a consequence also in the overall processing of a large variety of ores. The cost associated with grinding media is chiefly determined by two factors; the price and the wear performance of the grinding media. The mass losses of grinding media can be attributed to three basic mechanisms; the abrasion, the impact and the corrosion. These mechanisms can be simultaneously active in given grinding environment, leading to complex interactions [286].
The action of grinding media within the rotating mill not only crushes the existing clinker particles, but also sharply compresses them, which in fact leads to the formation of electrostatic surface charges of opposed polarity. The cement particles agglomerate as a result of the forces of attraction acting on them. Consequently, the cement particle agglomeration reduces the efficiency of mill. The extent of agglomeration depends on [287]:
The specific characteristics of materials to be ground;
The operating parameters of mill;
The efficiency and distribution of grinding media;
The fineness of cement particles;
The internal operating conditions of mill (humidity, temperature, ventilation, condition of armor plating, etc.).
Additives, such as water, organic liquids and some inorganic electrolytes are used to reduce the surface free energy of the material being ground with a view to improve the grinding efficiency.
In the grinding process, a variety of grinding aids are used. There are aliphatic amines such as triethylenetetramine (TETA), tetraethylenepentamine (TEPA) and aminealcohols such as diethanolamine (DEA), triethanolamine (TEA) and triisopropanolamine (TIPA). Glycol compounds are represented such as ethyleneglycol (EG), diethyleneglycol (DEG). In addition, there are more complex compounds such as aminoethylethanolamine (AEEA) and hydroxyethyl diethylenetriamine (HEDETA). Phenol and phenol-derivates are also used as grinding aids, as well as other compounds, such as amine acetate, higher polyamines and their hydroxyethyl derivates [287,322].
Assuming that a grinding mill is equivalent to a chemical reactor with a first-order phenomenological rate of reaction kinetics [288], the rate of decrease in particle size during the batch grinding of brittle material in ball mill can be described by the first-order equation. The breakage rate of such material was expressed in literature as [289]:
where Si is the specific rate of breakage of feed size i and wi,t is the mass fraction of total charge. For single component (i=1) Eq.121 can be rewritten as:
The value S1 for different particle sizes can be estimated by performing the same experiment with uniform-sized material. Different values of S1 versus the size can then be plotted on log–log plot to give a straight line if all the sizes follow the first-order law of grinding kinetics.
The primary breakage distribution (Bi,j) is also defined in an empirical form in literature as [280,290,319,320]:
where Bi,j is the mass fraction of primary breakage products, xi is the largest size, and the parameters φ, γ and β define the size distribution of the material being ground. When plotting the size versus Bi,j on log paper, the slope of the lower part of the curve gives the value of γ, the slope of the upper part of the curve gives the value of β, and φ is the intercept [319].
The spheroidal aggregate of particles is called a granule, ball, pellet, or an agglomerate. The nucleation, compaction, size enlargement, and spheroidization of pellets take place in the course of balling and granulation and related agglomeration processes [291]. The granulation converts fine powder and/or sprayable liquids (e.g. suspensions, solutions or melts) into granular solid products with more desirable physical and/or chemical properties than the original feed material. This size enlargement technique constitutes a key process in many industries such as the pharmaceutical, food, ore processing and fertilizers ones. Particularly, the granulation process has clear advantages regarding the storage, handling and transportation of the final product [292,293].
The new king of agglomeration technology is binder-less granulation, where the original cohesiveness of powder material is utilized to arrange them into granules. The strength of product granules can be much weaker. However, if the product granules are just an intermediate product in a larger process, such weakness has significant advantages. In many material-forming processes, the boundary between granules remains even after shaping due to unnecessary strength of granules. With weaker granules, the density of green bodies produced by the application of the same pressure as for conventional granules can be much higher. In many cases, the binder removal cannot be done completely leaving possible defects caused by carbonacious pyrolysis residues. Weaker granules can also be advantageous in pharmaceutical processes depending on the purpose of granulation [294].
Using large pellets for the processing of strontium aluminate clinker (Fig.31 in Chapter 4.5) may change the behaviour during thermal treatment as well as some properties of the product due to increasing influence of partial pressure of carbon dioxide on the thermal decomposition of strontium carbonate. The material forming the diffusion barrier as the reaction zone is shifted from the surface into the deeper zones of pellet. Increasing partial pressure of carbon dioxide slows down the rate of thermal decomposition of strontium carbonate and increases the temperature required for the thermal decomposition (please see the discussion in Chapter 4.2) and temporary lack of SrO in the reaction zone. Therefore, the influence of large pellets on prepared strontium aluminate clinker is similar to the usage of mixture with lower saturation degree (discussed in Chapter 4).
Highlights
Gut dysbiosis, inflammation, and increased intestinal permeability are synergistically contributed to the pathogenesis of hepatocellular carcinoma.
Previous studies in animal models suggest that targeting the gut-liver axis can inhibit HCC development.
Targeting the gut-liver axis with probiotics, antibiotics, FMT, TLR4 antagonists, FXR agonists, and natural compounds could be the promising strategies for HCC prevention.
Hepatocellular carcinoma (HCC) is a heterogeneous type of tumor that is likely to develop on the background of an inflammatory milieu in patients with advanced liver disease. It is the third leading cause of cancer death globally and is more prevalent in men than in women [1]. Over the past two decades, there is increasing evidence from studies suggesting a causal link between gut microbiota in the progression of HCC. Normal commensal gut microbiota acts as an important source of energy and is pivotal to host metabolism and innate immunity [2]. Not unsurprisingly therefore, alteration to gut microbial composition has been linked to the promotion of chronic inflammatory bowel disease (IBD) via local effects. However, activation of such inflammatory effects can have a broader response across all organ beds such as the liver, kidney, brain, heart, and the hematopoietic system and have been strongly associated with carcinogenesis [3]. Anatomical considerations provide us with a logical understanding on why gut microbiosis may have such an impact on disease development, especially in the liver. Since the liver is anatomically connected to the intestine via the portal vein, it is the first organ to receive nutrient-rich blood and also the first target of gut microbiota. Furthermore, the liver can elicit an inflammatory response through microbe-associated molecular patterns (MAMPs) and pattern recognition receptors (PRRs). Though translocation of gut microbiota from the intestinal lumen to the systemic circulation is counterchecked by multilayer intestinal epithelium, any change in its integrity can initiate inflammation and contribute to fibrosis and thus chronic liver disease (CLD) progression and thus a precursor to HCC development, which is itself usually only seen in the context of cirrhosis, the most advanced form of CLD [4]. In this chapter, we summarize the available literature on the key role of gut microbiome in HCC pathogenesis and novel therapeutic approaches developed to target these processes.
\nThe gut microbiota resides in the gastrointestinal (GI) tract. The human gut harbors complex and diverse microbial community of 100 trillion microorganisms with more than 2000 distinct species of bacteria, in addition to fungi protozoans and viruses. These microorganisms are collectively called gut microbiota, which comprises of commensals, beneficial microbiota, and opportunistic pathogens residing in what is a complex and dense microenvironment. Immediately after birth commensal bacteria colonize the intestine and predominantly comprise Proteobacteria, Lactobacillus, and Actinobacteria, but as we mature into adults Bacteroidetes and Firmicutes species predominate [5]. The composition of microbiota also varies from the small intestine to the distal colon, due largely to the effects of nutrient availability, intestinal pH, and motility. Moreover the overall composition of the microbial community in the gut is further individualized by any alteration in our diet, age, lifestyle, disease, and also medication exposure [6]. A symbiotic relationship exists between gut microbiota and the human host, which are critical to our maintenance of health. For example, gut microbiota are involved in the metabolism of bile acids, synthesis of vitamins, digestion of complex polysaccharides, and production of short-chain fatty acids (SCFAs) [2]. SCFAs are a vital source of energy for enterocytes, which are integral in maintaining gut barrier integrity. In addition, gut microbiota are also involved in the development of local and innate immunity providing defense against not only the pathogenic invasion but also systemic infection [7]. Experimental studies from rodents and humans have demonstrated that the gut microbiota is involved in the progression of HCC by increasing LPS-mediated pro-inflammatory microenvironment in the liver.
\nGut microbiota are known to influence multiple extraintestinal organs; however the importance of the gut-liver axis has understandably received greater attention in recent years. The gut and liver share anatomy from the embryonic phase, with bidirectional interaction through the portal vein. The symbiotic relationship between the gut microbiota and the liver is modulated by the nutrition, immune, metabolic, and neuroendocrine crosstalk between them and thus shapes human health and disease [8]. Functionally, gut and liver coordination influences our physiology. The liver receives 70% of the blood supply from the gut via the portal circulation. The nutrient-rich blood from the gut is effectively processed by the liver and delivered to systemic circulation for normal body growth. In turn, the liver synthesizes bile acids (BAs) and other mediators, like IgA, which influence intestinal microbial composition and barrier integrity, thereby maintaining intestinal homeostasis [8]. Bile acids are involved in energy homeostasis by regulating the metabolism of glucose and lipids and also help in conjugation and detoxification process as well as maintenance of intact intestinal epithelia. Bile acids also regulate microbial composition via antimicrobial peptides production; in turn, microbiota influences the bile acid pool in the intestine as they are involved in secondary bile acid production [9]. IgA secreted from the liver and intestine prevents growth and invasion of pathogenic bacteria to maintain normal gut-liver homeostasis [7]. In normal physiological conditions, translocation of gut bacteria and their metabolites is tightly regulated by the intestinal epithelial barrier, and if any reaches the liver, it is eliminated by hepatic Kupffer cells. Any breach in barrier integrity resulting from intestinal inflammation allows microbiota to pass through the portal vein to potentially trigger hepatic immune cells to enact an inflammatory response (from hepatic stellate cells and Kupffer cells), which may result in necrotic inflammation and hepatic fibrosis contributing to worsening fibrosis and thus liver disease progression [10]. Accumulating evidence suggests gut dysbiosis, bacterial endotoxin, and increased intestinal permeability are hallmark features of CLD and positively correlate with disease severity. These factors play a crucial role in the pathogenesis of not only CLD but have also been shown to promote HCC through various mechanisms (Figure 1).
\nAn overview of homeostatic and impaired gut-liver axis in HCC progression. (A) In homeostatic condition gut lumen contains normal gut flora which is restricted by tightly closed intestinal epithelial cells to prevent its translocation to the liver. (B) Increased bacterial overgrowth in gut lumen and increased intestinal permeability promotes HCC progression through binding of LPS to toll-like receptor 4 (TLR4) which are present on Kupffer cells, hepatocytes, and HSC to elicit the release of pro-inflammatory cytokines and activation of proliferative and anti-apoptotic signals. Prebiotics, probiotics, synbiotics, FMT, and polyphenols can be used to restore eubiosis, while the use of antibiotics can potentially eliminate pathogenic bacteria and endotoxin release. FXR agonists can attenuate intestinal permeability and prevent bacterial translocation. TLR4 antagonists prevent binding of LPS to TLR4 and suppress cancer-promoting signals.
Dysbiosis defines any change in the typical gut microbial composition found in health. Several lines of evidence suggest that gut bacterial dysbiosis is a pathogenic factor in the progression of HCC whatever the trigger for CLD (e.g., alcohol, nonalcoholic steatohepatitis (NASH), viral hepatitis, etc.). The role of gut dysbiosis in the propagation of progressive CLD is likely triggered by the formation of microbial metabolites such as LPS, bacterial DNA, and deoxycholic acid, which causes chronic inflammation in the portal circulation and thus the liver. In cirrhotic patients overgrowth of the pathogenic bacteria such as Enterobacteriaceae, Veillonellaceae, and Streptococcaceae and decreased Lachnospiraceae have been observed to correlate with the child-turcotte-pugh (CTP) score, clinically used to assess the severity of cirrhosis [11]. Moreover, in cirrhotic patients studies have found an increase in both the oral and gut levels of the same microbial species suggesting invasion from the mouth to intestine [12]. It is therefore postulated oral bacterial overgrowth may have a profound effect on intestinal bacterial communities and thus CLD pathogenesis and HCC development. This concept is supported by a recent study that showed a high level of oral microbiota Oribacterium and Fusobacterium in HCC patients [13]. The alteration in gut microbiota composition in HCC is summarized in Table 1.
\nAuthors | \nSample type | \nChanges in fecal microbiota composition | \nClinical implication in HCC | \n
---|---|---|---|
Ren et al. [14] | \nFeces | \nKlebsiella i | \n\n
| \n
Ponziani et al. [15] | \nFeces | \nBacteroides ↑ Ruminococcaceae ↑ Enterococcus ↑ Phascolarctobacterium ↑ Oscillospira ↑ Bifidobacterium ↓ Blautia ↓ | \n\n
| \n
Grat et al. [16] | \nFeces | \nEscherichia coli ↑ | \n\n
| \n
Huang et al. [18] | \nHCC liver biopsy | \nHelicobacter pylori ↑ | \n\n
| \n
Zhang et al. [19] | \nFeces | \nEscherichia coli ↑ Atopobium cluster ↓ Prevotella ↓ Bacteroides ↑ Lactobacillus spp. ↑ Bifidobacterium spp. ↑ Enterococcus spp. ↓ | \n\n
| \n
Yoshimoto et al. [38] | \nFeces | \nClostridium cluster XI and XIVa ↑ | \n\n
| \n
Gut microbiota dysbiosis in HCC.
Microbial gene signatures that relate to energy production, nickel/iron transport, and amino acid transport appear to be altered in HCC patients when compared to healthy controls [13]. Moreover, when compared to cirrhotic patients, fecal samples from HCC patients have shown increased growth of phylum Actinobacteria and 13 genera including Gemmiger and Parabacteroides [14]. They also found a decrease in butyrate-producing bacteria and an increase in LPS-producing bacteria in HCC patients when compared to healthy controls [14]. A study conducted by Ponziani et al. in NAFLD-related HCC showed increased fecal Bacteroides and Ruminococcaceae, whereas reduced Akkermansia and Bifidobacterium were negatively correlated with intestinal inflammatory marker fecal calprotectin level [15]. This study also showed increased intestinal permeability in these patients accompanied by an increase in plasma level of IL8, IL13, CCL3, CCL4, and CCL5, showing the evidence that alteration in gut microbiota profile is associated with systemic inflammation that may contribute to HCC pathogenesis [15]. In another study, E. coli growth in fecal samples was significantly elevated in HCC patients compared to matched cirrhotic patients [16]. Interestingly, inoculation of AFB1 and/or Helicobacter hepaticus in Helicobacter-free C3H/HeN mice was associated with HCC progression [17]. This observation may suggest that neither direct bacterial translocation nor hepatocyte injury is necessary for HCC development. In clinical studies utilizing liver HCC tumor biopsy tissue, some authors report the presence of Helicobacter ssp. DNA, whereas other investigators failed to correlate the presence of Helicobacter ssp. DNA with HCC progression [18]. In DEN-treated rats, fecal and cecal samples show an increase of pathogenic bacterial species like E. coli, Atopobium, Collinsella, Eggerthella, and Corynebacterium; in contrast there was a decline in the numbers of beneficial bacteria like Lactobacillus spp., Bifidobacterium spp., and Enterococcus spp. [19]. Although the exact mechanism by which gut microbiota promotes HCC has not been firmly established, studies in murine models indicated LPS-TLR4 axis plays a crucial role in the progression of HCC [19, 20]. Zhang et al. suggest that gut dysbiosis merely promotes HCC by increasing LPS levels and that conversely probiotics may suppresses tumor growth [19]. Similarly, Dapito et al. propose that gut microbiota only has a role in the promotion of HCC rather than its initiation [20]. The ability of pathogenic bacteria to disrupt TJs protein thereby increasing intestinal permeability has also been postulated as another mechanism by which microbiota may promote CLD and HCC [21]. However further preclinical and clinical studies are needed to establish the causal link between gut microbiota and HCC progression and to further delineate the molecular mechanisms involved.
\nHCC is arising in an inflammatory environment of the CLD, and therefore, neutralizing inflammation with anti-inflammatory agents may reduce the incidence and recurrence of HCC. Much attention has been focused on the potential involvement of the toll-like receptors (TLR) signaling pathway in the development of liver inflammation and associated HCC progression. Gut-derived endotoxin initiates the innate immune system such as TLRs, which recognize bacterial products and are predominantly expressed throughout the gut-liver axis. In addition, TLR4 plays a central role through LPS (a component of Gram-negative bacteria)-induced hepatic inflammation, while TLR 2 senses component of Gram-positive bacteria such as peptidoglycan [22]. In this context, Yu et al. identified that increased activation of the TLR 4-LPS axis correlated with intestinal permeability in DEN-induced acute liver failure (ALF), which directly regulates pro-survival molecules and enhances hepatocyte proliferation [23]. Interestingly, in mice models the use of antibiotics and/or TLR4 genetic ablation prevented tumor growth and multiplicity [19, 23]. Dapito et al. showed a close link between gut microbiota and LPS-TLR 4 axis in HCC progression in a chimeric mice model [20]. This study also showed in DEN-CCl4 treated mice (with histological CLD) that low-dose LPS treatment triggered TLR4 activation and increased rate of tumor formation, whereas gut sterilization prevented HCC progression rather than regression of the established tumor [20]. Taken together these studies would therefore suggest that gut microbiota may not have a role in HCC initiation but may instead have a tumor-promoting effect through TLRs signaling pathways [20].
\nIn respect of HCC promotion, multiple downstream targets of the LPS-TLR4 axis have been identified in both in vitro and in vivo studies. HSCs, Kupffer cells, and hepatocytes show TLR4 expression and thus are sensitive to LPS challenge [24]. Dapito et al. showed that TLR4 activation in HSCs, hepatocytes, and non-bone-marrow-derived resident cells promotes hepatocarcinogenesis by upregulating epiregulin (hepatomitogen) and inhibiting cleaved caspase 3 via NF-κB activation [20]. Moreover, Yu et al. showed hepatic Kupffer cells as the chief target for LPS-induced TLR4 activation by increasing pro-inflammatory cytokines such as TNF-α and IL-6 production [23]. Similarly, in vitro studies have shown evidence of LPS-TLR4-promoted HCC cell proliferation via NF-κB, MAPK, and STAT3 mediated signaling pathways [24]. LPS-TLR4 has also been shown to promote epithelial-to-mesenchymal transition (EMT) in HCC by upregulating NF-κB- and JNK/MAPK-mediated expression, while NF-κB and JNK/MAPK signaling blockade inhibited EMT occurrence [25]. Similarly, LPS-TLR4 axis is also known to enhance angiogenesis in HCC mice model via production of pro-angiogenic factors by HSCs in tumor stroma [26]. However further studies incorporating TLR4 deletion are needed to better understand its role in hepatocyte proliferation and distinguish paracrine signaling from HSCs and Kupffer cells in HCC progression.
\nCommensals and opportunistic pathogens are kept in check within the intestinal lumen by a single layer of intestinal epithelial cells (IEC) which spans almost 400 m2 in surface area [27]. The gut barrier is highly dynamic in nature in which IEC is capable of self-renewal every 4–7 days with constant changes in intestinal luminal content. IEC are predominantly composed of absorptive enterocytes, which have metabolic and digestive functions. It also has secretory functions enacted by cell types such as enteroendocrine, goblet, and paneth cells which are specialized for maintaining digestive, immune, and epithelial barrier function [27]. Enteroendocrine cells connect the central and enteric neuroendocrine system via the secretion of various digestive hormones like gastrin, cholecystokinin, incretin, etc. The highly glycosylated mucins secreted by goblet cells form the first line of defense against microbial invasion and when compromised may predispose to disease as is evident in Mucin 2-deficient mice which are susceptible to colitis and inflammation-induced colorectal cancer [28, 29]. The intestinal barrier function is further strengthened by antimicrobial peptides (AMPs) including defensin, cathelicidin, and lysozyme [27]. These AMPs disrupt bacterial cell membranes and prevent adherence to gut mucosa.
\nApart from IEC, the gut barrier is primarily maintained by tight junction (TJs) components (e.g., claudin, occludin, zonula occludens, and other junctional adhesion molecules (JAMs)) preserving intact epithelia which in turn regulate the paracellular movement of solutes, water, and other nutrients while restricting the entry of bacteria from the lumen to systemic circulation [30]. The mechanism of increased intestinal permeability is poorly understood. Growing evidence suggests that inflammation and TJ protein disruption are two of the key players driving increased intestinal permeability. In our previous study, we found increased systemic ZO-1 level in HCC patients reflecting increased intestinal permeability [31]. Moreover, plasma ZO-1 level was positively correlated with the inflammatory marker hs-CRP and with disease severity, suggesting inflammation drives intestinal permeability associated with HCC progression [31]. Bacterial overgrowth leads to increased production of pro-inflammatory cytokines such as TNF-α, IL-6, and IL-1β, which is mainly mediated through the TLR4-NF-kB signaling pathway, thereby promoting intestinal inflammation and HCC progression [19]. These pro-inflammatory cytokines have a direct effect on TJ proteins like claudin and ZO-1, leading to enhance intestinal permeability [32]. Furthermore, in both in vivo and in vitro models, LPS dose dependently increases intestinal permeability via upregulating TLR4-mediated CD14 expression in enterocytes [33]. Similarly, NLRP6-deficient mice show altered microbiota and enhanced colonic inflammation through the chemokine (C-C motif) ligand CCL5 [34]. This results in increased intestinal permeability to microbial products and thus increases hepatic inflammation and progression from NAFLD to NASH [34]. Moreover, enteric pathogens such as Escherichia coli and Clostridium difficile are increased in CLD and are capable of increased intestinal permeability by modulating TJ integrity [11, 21].
\nBile acids are another key player for maintaining gut barrier function by promoting intestinal epithelial cell proliferation and microbiota composition [35]. There is clear evidence that bile acids have both direct antimicrobial effect and an indirect effect through FXR-induced AMPs and thus control growth and adhesion of intestinal bacteria. In fact, decreased bile acids pool in the intestine is associated with bacterial overgrowth and inflammation [36]. The study by Kakiyama et al. showed that cirrhosis reduced bile acids entering the intestine causing bacterial dysbiosis by reducing beneficial bacteria such as Gram-positive Blautia and Ruminococcaceae and increasing pathogenic bacteria like Enterobacteriaceae [37]. In a NASH-induced CLD/HCC mouse model, increased Gram-positive Clostridium clusters (XI and XVIa) were positively correlated with increased serum deoxycholic acid (DCA) [38]. Notably, Clostridium clusters are capable of synthesis of secondary bile acid DCA via 7α-dehydroxylation of primary bile acids. DCA is a DNA-damaging agent and a known pro-carcinogen shown to affect various cancer signaling pathways. In this context, a study by Yoshimoto et al. revealed DCA activates a senescent-associated secretory phenotype in HSCs, thereby producing various pro-inflammatory and pro-tumorigenic factors promoting HCC development in mice, while antibiotic treatment and/or blocking DCA production prevented HCC development [38]. Similarly, in a HCC mouse model induced by steatohepatitis-inducing high-fat diet (STHD-01), increased hepatic and fecal bile acids concentrations were observed [39]. In this model, DCA activated mTOR and promoted HCC development. However, following antibiotic treatment, there was a decrease in HCC progression suggesting an interrelationship between BA metabolism, gut microbiota, and HCC development [39].
\nBile acids maintain homeostatic IEC proliferation via EGFR- and FXR-dependent pathways, which helps the continuous regeneration of enterocytes and maintain intact epithelia [40]. Several studies demonstrate that the intestinal bile acid pool also regulates TJ protein distribution and expression. In Caco-2 cell monolayers, incubation with dihydroxy bile acids decreased transepithelial resistance (TER) and was accompanied by increased phosphorylation and redistribution of occludin [41]. In human colonic biopsies, DCA induces Cr-EDTA permeability altering TER and increasing translocation of E. coli. Several in vivo studies have also elucidated the role of bile acids in the regulation of TJ permeability [42]. In HFD mice increased intestinal bile acid pool was associated with increased intestinal permeability with decreased expression of TJ proteins ZO-2 and JAM-A [43]. Similarly, in bile duct ligated (BDL) rats where intestinal BA delivery was prevented, there was increased bacterial translocation and increased intestinal permeability with decreased expression of claudin-1 and occludin. Conversely, the above effects were ameliorated by FXR activation [44]. These studies highlighted the protective role of FXR in the maintenance of intestinal barrier integrity; however, it is unclear whether these effects were from direct activation of FXR on TJ proteins or indirect effects from altered mucosal immune cells. In addition to FXR, another bile acid receptor TGR5 also modulates barrier permeability and TJ protein expression. In TGR5 null mice, increased intestinal permeability due to alteration of TJ protein expression develops colitis [45]. Therefore, the quantity and composition of BA pool in the intestine represent an important factor in the regulation of gut microbial community and gut barrier integrity.
\nTraditionally, HCC is cured based on the available treatment options such as surgical treatment, chemotherapy, and local ablation therapy; however, patients are facing many problems including the poor hepatic reserve [46]. Furthermore, the possible therapeutic interventions targeting the gut-liver axis in HCC are summarized in Table 2.
\nAuthors | \nIntervention class | \nMedication | \nDesired effect in HCC | \n
---|---|---|---|
Zong et al. [53] | \nPrebiotics | \nLactulose | \n\n
| \n
Zhang et al. [19] | \nProbiotics | \nVSL#3 | \n\n
| \n
El-Nezami et al. [63] | \nProbiotics | \nLactobacillus rhamnosus LC 705 and Propionibacterium freudenreichii subsp. shermani | \n\n
| \n
Li et al. [46] | \nProbiotics | \nProhep | \n\n
| \n
Kumar et al. [62] | \nProbiotics | \nCombination of probiotic fermented milk and chlorophyllin | \n\n
| \n
Yoshimoto et al. [38] | \nAntibiotics | \n4Abx (ampicillin, metronidazole, vancomycin, neomycin) | \n\n
| \n
Abdel-Hamid et al. [67] | \nAntibiotics | \nClarithromycin and azithromycin | \n\n
| \n
Dapito et al. [20] | \nAntibiotics | \nRifaximin | \n\n
| \n
Nguyen et al. [93] | \nTLR4 antagonists | \nTAK-242 | \n\n
| \n
Nkontchou et al. [117] | \nNonselective β-blockers | \nPropranolol | \n\n
| \n
Chang et al. [118] | \nNonselective β-blockers | \nPropranolol | \n\n
| \n
Bishayee et al. [121] | \nNatural compound | \nResveratrol | \n\n
| \n
Ji et al. [124] | \nNatural compound | \nQuercetin | \n\n
| \n
Teng et al. [125] | \nNatural compound | \nCurcumin | \n\n
| \n
Therapeutic intervention targeting the gut-liver axis in HCC.
Prebiotics are non-absorbant and nondigestible food ingredients which promote growth or activity of beneficial bacteria like Bifidobacteria and Lactobacilli and inhibit the growth of potentially pathogenic bacteria [47]. Currently, prebiotics like lactulose, lactitol, fructo-oligosaccharides, and galacto-oligosaccharides are commercially available [48]. Synthetic disaccharides like lactulose and lactitol are extensively used for the treatment of hepatic encephalopathy in CLD patients as ammonia detoxifying agents [48]. Also, these disaccharides are metabolized by colonic bacteria to produce lactic acid and acetic acid due to which pH in the gut lumen is decreased [49]. Low pH enhances the growth of non-urease-producing lactobacilli and inhibits pathogenic urease-producing bacteria [50]. Chen et al. showed that in chronic viral hepatitis patients, lactitol administration significantly decreased plasma endotoxin levels and increased the growth of beneficial Lactobacilli and Bifidobacteria species [51]. In contrast, Bajaj et al. reported lactulose administration in patients with HE did not improve dysbiosis and increased growth of Gram-negative bacteria such as Enterobacteriaceae and Bacteroidaceae [52]. This indicates the pattern of gut microbiota abundance is the major determinants of disease severity [52]. In HCC patients administration of lactulose (30 mL/day) for 14 days significantly reduced ALT and bilirubin levels, while antioxidant enzyme SOD, anti-inflammatory markers IFN-γ and IL-4, and blood cells CD4(+)/CD8(+) were found to be increased suggesting its ability to reduce inflammation and restore oxidation/antioxidant system imbalance [53]. Similarly in partial hepatectomized rats, administration of lactulose induces liver regeneration by inducing hydrogen and abrogating oxidative stress (heme oxygenase-1, SOD-2) and inflammation (IL-6 and TNF-α) [54]. Moreover, mixed diets of galacto-oligosaccharides and fructo-oligosaccharides to infants were increased the growth of Bifidobacterium; however, this formulation has not been tried in patients with liver disease [55].
\nProbiotics are live microorganisms which when administered in adequate amounts confer a health benefit for the host [56]. Probiotic supplementation was also shown to restore intestinal dysbiosis in CLD patients [48]. Furthermore, mice treated with probiotic such as VSL#3 significantly reduces Clostridium spp. and modified gut microbiota [57]. In cirrhotic patients, VSL#3 supplementation for nearly 6 months was shown to reduce the risk of hospitalization and improved CTP and MELD score [58]. Similarly, in NASH-associated obese children, treatment with VSL#3 over the period of 4 months reduces fatty liver and improved lipid profile and insulin sensitivity [59]. The other probiotics such as Lactobacillus salivarius LI01 and Pediococcus pentosaceus LI05 reduced inflammation, protected the intestinal barrier, prevented bacterial translocation, restored eubiosis, and attenuated hepatic fibrosis in CCl4-induced cirrhotic rats [60]. Similarly, Lactobacillus GG (LGG) supplemented with standard diet in cirrhosis patients show significantly reduced blood endotoxemia and TNF-α, thereby restoring eubiosis [61]. However, in HCC, probiotic usage is meager, and only a few studies have identified the therapeutic potential. VSL#3 (contains three strains of Bifidobacteria, four strains of Lactobacilli, and one strain of Streptococcus thermophilus) treatment to DEN-induced rat hepatocarcinogenesis has shown to attenuate HCC progression, reduce tumor number and multiplicity, ameliorate hepatic and intestinal inflammation, and thus restore gut dysbiosis [19]. Li et al. identified that subcutaneous administration of Prohep (a novel probiotic mixture of Lactobacillus rhamnosus GG, Escherichia coli Nissle 1917, and heat-inactivated VSL#3) reduced the tumor size and HCC growth [46]. Prohep improves beneficial bacteria such as Prevotella and Oscillibacter and control tissue inflammation as evidenced by decreased T helper 17 cells in the gut, thereby attenuating the progression of HCC [46]. Moreover, in aflatoxin B1-induced HCC rats, treatment with probiotic fermented milk and chlorophyllin significantly reduced tumor incidence by decreasing the expression of cyclin D1, bcl-2, and c-myc proto-oncogenes [62]. Similarly, aflatoxin-induced HCC patients treated with dietary supplementation of probiotics (using viable Lactobacillus rhamnosus LC 705 and Propionibacterium freudenreichii subsp. shermani) decreased the urinary excretion of aflatoxin-DNA adduct (AFB-N7 guanine) and improved HCC [63]. Thus, probiotic supplementation could be beneficial to cirrhotic patients with the potential to progress to HCC.
\nSynbiotics are combined form of prebiotics and probiotics, which contains four fermentable fibers (symbiotic 2000) and four freeze-dried non-urease-producing lactic acid bacteria. Synbiotic administration to cirrhotic patients results in decreased plasma endotoxin and ammonia levels and increased fecal Lactobacillus spp. [64]. Interestingly 50% of these patients have improved child-turcotte-pugh score compared to placebo [64]. Moreover, in NAFLD patients synbiotic supplementation has shown to have beneficial effects by improving lipid profile, glucose homeostasis, hepatic marker enzymes, and inflammatory markers [65]. Synbiotic (FloraGuard) administration also has a protective role in alcohol-induced liver injury in rats [66]. In addition, the synbiotic treatment restored ethanol-induced intestinal permeability and increased the growth of beneficial bacteria Bifidobacterium spp. and Lactobacillus spp. [66]. Currently, studies are lacking for the use of synbiotics in chronic liver diseases or HCC prevention.
\nSeveral studies have postulated that antibiotic treatment may cause gut microbiota dysbiosis. It may represent effective strategies to prevent the tumor-promoting gut microbiota, its metabolites DCA, and pro-inflammatory signal inducer LPS which all have a role in the progression of CLD and HCC. In DEN-CCl4- and DMBA-HFD-induced HCC mice model, the oral antibiotics ampicillin, metronidazole, neomycin, and vancomycin significantly reduced the tumor number and size [20, 38]. In addition, this antibiotic combination also reduced the liver fibrosis and improved liver histology in cirrhosis. Moreover, the effectiveness of antibiotics administration was enhanced when given at late-stage HCC in mice rather than earlier stage [20]. In another study conducted in DEN-induced HCC rats, clarithromycin and azithromycin suppressed HCC progression through extrinsic and intrinsic apoptotic pathways, whereas erythromycin aggravated HCC and did not show antitumorigenic effect [67]. Vancomycin is an antibiotic used to treat a bacterial infection, which effectively prevented the mouse model of HCC; however, long-term administration to CLD patients develops potential side effects [38]. Many studies have concluded that norfloxacin and rifaximin treatments to cirrhotic patients have beneficial effects [68]. In a double-blind placebo-controlled clinical trial, long-term oral administration of norfloxacin to cirrhotic patients markedly reduces Gram-negative bacteria in the fecal matters and lowers the spontaneous bacterial peritonitis (SBP) [69]. Furthermore, CCl4-induced cirrhotic animals showed decreased SBP and inflammation following norfloxacin treatment [70]. Norfloxacin was identified as a promising antibiotic in regulating gut microbiota overgrowth and prevention of BT in both cirrhotic humans and rodents; indeed its effect on HCC patients remains unidentified. Rifaximin is a broad spectrum oral antibiotic having potential bactericidal activity against aerobic and anaerobic Gram-negative bacteria [71]. It is an excellent choice of drug to cure HE in advanced cirrhotic patients [72]. The study conducted by Vlachogiannakos et al. showed treatment with rifaximin for 4 weeks significantly decreased portal pressure and LPS levels in decompensated cirrhotic patients [73]. Long-term treatment with rifaximin reduces SBP occurrence, variceal bleeding, HRS, and HE with an overall improvement in survival rate in cirrhotic patients [74]. Similarly, in murine DEN-CCl4-induced HCC mouse model, rifaximin treatment was shown to ameliorate HCC progression [20]. Although rifaximin is clinically used for prevention of HE and other complications in cirrhotic patients, its role in HCC prevention in humans is further warranted.
\nFecal microbiota transplantation is currently being used for the treatment of Clostridium difficile infection [75]. The enteric dysbiosis was restored to normal gut flora following FMT from healthy donor to Clostridium difficile-infected patients [75]. Moreover, in mice with gut dysbiosis induced by antibiotics and chemotherapy, treatments were reversed by FMT [76]. In experimental cirrhosis with hepatic encephalopathy, FMT improves liver function and HE grade by limiting inflammation and improving tight junction integrity [77]. In this context, Bajaj et al. reported that FMT in cirrhotic patients with recurrent HE improves cognition, restores dysbiosis, and improves MELD score compared to standard care in those patients [78]. A number of clinical trials are ongoing in patients with NASH and cirrhosis for the efficacy of FMT [79, 80]. Vrieze et al. reported in patients with severe metabolic syndrome that treatment with FMT from a healthy donor improves liver biochemistry, peripheral insulin resistance, and restoration of eubiosis [81]. Furthermore, in alcoholic hepatitis patients, FMT treatment significantly reduced liver disease severity and HE occurrence [82]. FMT recipients also showed an increase in beneficial bacteria like Actinobacteria and Bifidobacterium longum and decrease in Pseudomonas and E. coli with an increase in bile secretion [82]. Collectively these findings indicated that FMT may restore gut dysbiosis and reduce complications in cirrhotic patients and thus attenuate HCC development. However, scarce literature supporting the beneficial effect of FMT and long-term study is needed to prove permanent colonization in altered gut microbiota environment in cirrhotic patients. Moreover, there is a chance of inducing viral infection and transmission of other pathogens through FMT which may have deleterious effect and immunosuppression in advanced liver disease patients [83, 84].
\nNumerous studies have shown that LPS-TLR4 is a key inflammatory pathway in the progression of CLD and has a tumor-promoting effect on HCC [20, 23, 85]. Therefore blocking this pathway might represent a promising treatment approach in controlling cirrhosis and HCC progression. Several TLR4 antagonists have been developed toward controlling LPS-activated TLR4-mediated inflammatory responses which include polymyxin B [86], glycolipids interfering CD14-LPS interaction [87], eritoran [88], resatorvid (TAK242) [89], and thalidomide [90]. In BDL-induced cirrhotic rats, intravenous administration of TAK-242 significantly reduces plasma transaminases and inflammatory cytokines [91]. It also ameliorates acetaminophen-induced acute liver failure [92]. Similarly, in transgenic HCC mice (HepPten−), TAK-242 treatment for 28 days significantly reduced tumor burden and ameliorated HCC progression [93]. Eritoran tetrasodium protects against liver ischemia/reperfusion injury by inhibiting inflammatory response induced by high-mobility group box protein B1 (HMGB1) [94]. Similarly in D-galactosamine- and LPS-induced acute liver failure, treatment with eritoran significantly reduces inflammation and hepatic marker enzymes [95]. Eritoran treatment also decreases proliferation and induces apoptosis in tumor cells in a chemically induced mouse model of colorectal carcinoma [96]. Although resatorvid and eritoran showed the beneficial effect in improving the survival of murine sepsis model, it failed to do so in patients with severe sepsis [97]. Several TLRs have been upregulated in HCC [98]. In addition, TLRs are also abundantly expressed on immune cells which recognize various pathogens such as HBV which upon activation induces an innate immune response [99]. All the TLRs are activated by two independent pathways: MyD88-dependent (except TLR3) and MyD88-independent (TLR3 and TLR4) pathway [100]. Activation of TLR3-TRIF signaling pathway leads to apoptotic activity independently and, in turn, also activates IRF3 transcription factor to produce interferons [100]. TLR3 agonist BM-06 (synthetic dsRNA) significantly inhibited HCC cell proliferation, increased apoptosis, and decreased cell invasion and migration with increased antiviral IFN level [101]. Activation of TLR9 leads to phosphorylation of NF-κB with increased production of pro-inflammatory cytokine TNF-α, IL-6, and IL-10 [102]. Mohamed et al. showed that inhibition of TLR7 and TLR9 with antagonist IRS-954 or chloroquine significantly reduces HCC cell proliferation, angiogenesis with increased apoptosis [103]. This was further supported by tumor xenograft and DEN-induced rat HCC model in which chloroquine treatment reduces HCC incidence [103]. Although growing evidence shows TLRs as an important mediator of HCC progression, the molecular mechanism for disease progression is not completely understood. Therefore, further research needs to be done for the use of TLR agonist or antagonist as a drug target for HCC prevention since TLRs are also involved in both cancer-promoting mechanism and immune-modulator which is responsible for an innate immune response against tumor cells and HBV and HCV infection [99, 104, 105, 106].
\nBacterial overgrowth due to impaired gut motility has been reported in cirrhotic patients [107]. Cisapride, a prokinetic drug, has shown beneficial effects not only by regulating intestinal motility but also inhibiting bacterial overgrowth and preventing bacterial translocation in both rodent models and liver cirrhotic patients [108, 109]. Cisapride in combination with norfloxacin significantly reduces the incidence of SBP in high-risk cirrhotic patients [110]. Although the mechanism of impaired gastrointestinal motility in cirrhotic patients is unclear, increased adrenergic activity may be responsible for the altered motility.
\nNonselective β-blockers are prevalently used in decompensated chronic liver disease patients to reduce morbidity and mortality. It is used as bleeding prophylaxis in cirrhotic patients with esophageal varices. NSBB also antagonizes β-adrenoceptors. The β-adrenergic receptor pathway is involved in maintaining normal physiological functions. The catecholamines such as epinephrine and norepinephrine are the key ligands for β-adrenoceptors (β1 and β2). Furthermore, increased expression of β-adrenoceptors was observed in HCC cell membranes compared to healthy liver cells; however, the mechanisms remain unclear [111]. Catecholamines exhibit pro-carcinogenic effects in gastric, pancreatic, and breast cancer, which is antagonized by NSBB. Its beneficial effects to reduce the risk of HCC have also been identified by the very recent observational and experimental trials [112]. Moreover, in ovarian and breast cancer patients, NSBB treatment was shown to reduce cancer formation and growth. In cirrhotic patients, increased catecholamines results in disease severity, and thus, NSBB treatment may be potent to inhibit carcinogenesis in cirrhosis [113]. A recent study by Leithead et al. showed NSBB is safe and may confer benefit in patients with ascites complicating the end-stage liver disease [114]. In addition, Reiberger et al. observed that NSBB treatment ameliorates intestinal permeability and reduces BT in cirrhotic patients [115]. In a recent study, Wang et al. showed propranolol induces apoptosis and suppresses proliferation of liver cancer cells [116]. Of note, long-term treatment with propranolol in patients with HCV-related cirrhosis reduces the incidence of HCC [117]. Similarly, in patients with unresectable/metastatic HCC cohort, propranolol treatment significantly reduces mortality risk and improved overall survival suggesting β-blockers might be another therapeutic approach in HCC prevention [118].
\nTreatment with natural compounds and food ingredients like polyphenols represents another therapeutic approach in the restoration of eubiosis, modulation of gut microbiota, reduction of inflammation, prevention of cirrhosis, and thus the progression of HCC. Many experimental data have shown evidence that flavonoids are proficient to alter gut microbial composition and restoration of eubiosis in chronic liver diseases. Proanthocyanidin improves beneficial microbiota Bacteroides such as Lactobacillus spp. and Bifidobacterium spp. composition and reduces intestinal inflammation and oxidative damage, thereby attenuating experimental colon cancer. Resveratrol, a flavonoid, is shown to modulate intestinal microbiota with a profound increase in Bifidobacterium spp. and Lactobacillus spp. and reduce systemic and colonic inflammation in rats [119]. Resveratrol also shown to have anticancer activity in HCC cell lines; inhibit proliferation, viability, invasion, and metastasis; and induce apoptosis [120]. Its anticancer property was also studied in DEN-induced hepatocarcinogenesis [121]. We found resveratrol treatment to cirrhotic mice attenuated systemic inflammation and ammonia levels and altered neuronal TJ proteins, thereby preventing secondary complications such as HE in cirrhosis [122]. Quercetin, a bioactive flavonoid, was shown to inhibit human HCC cell proliferation, migration, and invasion and trigger apoptosis both in vivo and in vitro [123]. Its profound antitumor effect was also shown in xenograft and DEN-induced HCC rodent models [124]. Curcumin, a powerful antioxidant, has a wide range of bioactive properties. Previous studies have shown evidence that curcumin has HCC chemoprevention in preclinical models as well as patients with HCC [125, 126]. Moreover, nimbolide from the leaf of the neem tree (Azadirachta indica) is another potential natural compound having antioxidant, anti-inflammatory, antibacterial, antiviral, and anticancer properties [127]. In vitro study in HCC cell line (HepG2) shows that nimbolide induces cell apoptosis by abrogating NF-κB and Wnt signaling pathway [128]. Currently, our lab is focusing on anticancer effects of nimbolide and its molecular mechanisms in an experimental hepatocarcinogenesis. Of note, these natural compounds have a wide variety of biological activities on gut microbiota and may preserve gut barrier integrity, microbial metabolites, TJ integrity, and mucosal immunology. Indeed, further human studies are warranted to see the effect of natural compounds on gut microbial modulation and prevention of HCC.
\nGut epithelial barrier acts as a fence for translocation of gut microbiota and its metabolite into the systemic circulation which is the major driving factor for CLD progression and HCC development [129]. Therefore, primarily targeting or restoring the gut epithelial barrier is an interesting therapeutic approach in HCC pathogenesis. Compelling evidence has shown that targeting gut microbiota (restoring eubiosis) directly or receptor-mediated pharmacological intervention using TLR4 antagonist or FXR agonist might improve epithelial barrier function [130, 131]. FXR is a BA receptor which is widely expressed throughout the gut-liver axis. Decreased BA is associated with intestinal bacterial overgrowth, increased gut permeability, and bacterial translocation in rodents [38]. FXR controls hepatic inflammation, promotes liver regeneration, and suppresses HCC formation mainly through enterokine FGF19 [132]. FXR null mice have an intestinal barrier dysfunction and high occurrence of HCC, whereas reactivation of FXR inhibited HCC through FGF15-cyp7a1 axis [133]. In this context, FXR agonist obeticholic acid (OCA) prevents gut barrier dysfunction and BT in cholestatic rats [131]. Furthermore, in CCl4-induced cirrhotic rats, OCA treatment significantly reduces BT and inhibits intestinal inflammation by restoring intestinal TJ proteins such as ZO-1 and occludin and antimicrobial peptides [134]. OCA treatment also improves hepatic inflammation and decreased portal pressure in BDL cirrhotic rats [135]. Consequently, OCA treatment may prevent HCC by limiting intestinal inflammation and improving gut barrier dysfunction in advanced liver disease.
\nIncreased TNF-α production in mesenteric lymph nodes by monocyte is the major factor responsible for increased intestinal permeability in cirrhosis and HCC [19, 136]. TNF-α decreases ZO-1 expression through NF-κB and MLCK activation [137]. TNF-α also downregulated occludin expression in Caco-2 enterocyte by targeting PI3k/Akt signaling [138]. In BDL rats, treatment with infliximab (IFX, a monoclonal antibody against TNF) significantly reduced portal pressure and attenuated inflammation [139]. Therefore modulating intestinal TJs protein with anti-TNF-α therapy may restore intestinal integrity. However, due to the immunosuppressive activity of TNF inhibitors, it may lead to systemic infection in cirrhosis patients. Hence, detailed knowledge for local inhibition is required for improvement in gut barrier dysfunction without affecting innate immune response.
\nRetinoic acid can modulate TJs proteins. In a mice model of colitis characterized by gut permeability, treatment with retinoic acid enhances barrier function by upregulating TJs proteins claudin-1, claudin-4, and ZO-1 [140]. However, the effect of retinoic acid in the preservation of intestinal integrity has not been tested in cirrhosis and HCC. Modulation of the epithelial barrier with probiotics seems to be beneficial. In this regard, Zhang et al. showed probiotics VSL#3 (combination of S. thermophilus, four Lactobacillus species, and three Bifidobacterium species) treatment restores intestinal permeability and dysbiosis and prevented HCC progression in rats [19]. Similarly, probiotics like E. coli Nissle1971 (ECN) was also shown to enhance intestinal TJs integrity by upregulating ZO-1 expression in the mouse model of DSS-induced colitis [141].
\nIn addition, treatment with red wine polyphenol promoted barrier function by significantly increasing mRNA expression of TJ proteins occludin, claudin-5, and ZO-1 in cytokine-stimulated HT-29 colon epithelial cells [142]. Resveratrol also preserves TJ barrier integrity and diminishes intestinal permeability by upregulating occludin, ZO-1, and claudin-1expression and thus abrogating intestinal inflammation and oxidative stress both in vivo and in vitro [143]. Similarly in NAFLD mice model, treatment with resveratrol enhances barrier function by increasing mRNA expression of TJ proteins ZO-1, occludin, and claudin-1 in the intestinal mucosa [144]. Curcumin also modulates intestinal barrier integrity and attenuates paracellular permeability and organization of TJs [145]. Thus, targeting TJ proteins, which maintain intact intestinal epithelia, is another area of therapeutic approach for the control of intestinal permeability in cirrhosis and HCC.
\nHCC is the end-stage liver disease, which mostly develops on the background of cirrhosis. As discussed above, the gut-liver axis plays a significant role in the progression of CLD and ultimately HCC and would be a potentially significant therapeutic target in the prevention of HCC. There have been several preclinical and human studies demonstrating an association between gut dysbiosis and HCC progression. However, from the animal studies, it is unclear whether gut microbiota initiates HCC or acts with other precipitating factors like chronic inflammation in the progression of HCC. Modulation of gut dysbiosis with prebiotics and probiotics, FMT, or preventing bacterial overgrowth with antibiotics may therefore prevent HCC. Moreover, in cirrhotic patients these interventions appeared to prevent secondary complications and improved survival. We may therefore speculate that the above interventions might prevent HCC development in high-risk cirrhotic patients. Moreover, we should consider trialing these therapies in HCC patients with unresectable tumors, which might improve survival time and secondary complications. Furthermore interventions that restore intestinal barrier integrity may prevent gut BT and may consider another line of therapy in HCC.
\nIn conclusion, there is growing evidence to suggest gut microbiota may play a significant role in the progression of CLD and thus HCC, which is likely to involve multiple pathways ranging from gut dysbiosis, endotoxemia, inflammation, loss of TJ integrity, and intestinal permeability. It is therefore suggested that the use of agents that have the potential to target microbial dysbiosis and restore intestinal epithelial barrier integrity may prevent bacterial translocation and ultimately delay HCC progression. However, whether bacterial overgrowth and/or intestinal permeability act independently or synergistically as causal pathogenic factors to influence the inflammatory milieu so closely associated with HCC progression remains unclear. Further research is therefore warranted to better understand the molecular pathways involved and guide the development of novel therapeutic interventions that can be taken to clinical trial to limit CLD/HCC progression through the targeting of dysbiosis and its effect on inflammation and intestinal permeability.
\nThe authors declare that they have no conflict of interest.
\nALT | alanine transaminase |
AMPs | antimicrobial peptides |
AST | aspartate transaminase |
BAs | bile acids |
BDL | bile duct ligation |
CCL | chemokine ligand |
CCl4 | carbon tetrachloride |
CD | cluster of differentiation |
CLD | chronic liver disease |
CTP | child-turcotte-pugh |
DCA | deoxycholic acid |
DEN | diethylnitrosamine |
DMBA | 2,4-dimethoxybenzaldehyde |
ERK | extracellular signal-regulated kinase |
FMT | fecal microbiota transplantation |
HCC | hepatocellular carcinoma |
HFD | high-fat diet |
HSCs | hepatic stellate cells |
IBD | inflammatory bowel disease |
IEC | intestinal epithelial cell |
IL | interleukin |
JNK | c-Jun N-terminal kinase |
LBP | lipopolysaccharide binding protein |
LPS | lipopolysaccharides |
MAMPs | microbe-associated molecular patterns |
MAPK | mitogen-activated protein kinase |
MELD | model for end-stage liver disease |
NAFLD | nonalcoholic fatty liver disease |
NASH | nonalcoholic steatohepatitis |
NF-κB | nuclear factor kappa-light-chain-enhancer of activated B cells |
NLRP | nucleotide-binding oligomerization domain, leucine-rich repeat- and pyrin domain-containing |
NSBB | nonselective beta-blockers |
OCA | obeticholic acid |
PRRs | pattern recognition receptor |
SCFAs | short-chain fatty acids |
STHD | steatohepatitis-inducing high-fat diet |
TB | total bilirubin |
TGR5 | takeda G-protein-coupled receptor 5 |
TLR | toll-like receptors |
TNF-α | tumor necrosis factor alpha |
TJs | tight junctions |
WHO | World Health Organization |
ZO | zonula occluden |
"Open access contributes to scientific excellence and integrity. It opens up research results to wider analysis. It allows research results to be reused for new discoveries. And it enables the multi-disciplinary research that is needed to solve global 21st century problems. Open access connects science with society. It allows the public to engage with research. To go behind the headlines. And look at the scientific evidence. And it enables policy makers to draw on innovative solutions to societal challenges".
\n\nCarlos Moedas, the European Commissioner for Research Science and Innovation at the STM Annual Frankfurt Conference, October 2016.
",metaTitle:"About Open Access",metaDescription:"Open access contributes to scientific excellence and integrity. It opens up research results to wider analysis. It allows research results to be reused for new discoveries. And it enables the multi-disciplinary research that is needed to solve global 21st century problems. Open access connects science with society. It allows the public to engage with research. To go behind the headlines. And look at the scientific evidence. And it enables policy makers to draw on innovative solutions to societal challenges.\n\nCarlos Moedas, the European Commissioner for Research Science and Innovation at the STM Annual Frankfurt Conference, October 2016.",metaKeywords:null,canonicalURL:"about-open-access",contentRaw:'[{"type":"htmlEditorComponent","content":"The Open Access publishing movement started in the early 2000s when academic leaders from around the world participated in the formation of the Budapest Initiative. They developed recommendations for an Open Access publishing process, “which has worked for the past decade to provide the public with unrestricted, free access to scholarly research—much of which is publicly funded. Making the research publicly available to everyone—free of charge and without most copyright and licensing restrictions—will accelerate scientific research efforts and allow authors to reach a larger number of readers” (reference: http://www.budapestopenaccessinitiative.org)
\\n\\nIntechOpen’s co-founders, both scientists themselves, created the company while undertaking research in robotics at Vienna University. Their goal was to spread research freely “for scientists, by scientists’ to the rest of the world via the Open Access publishing model. The company soon became a signatory of the Budapest Initiative, which currently has more than 1000 supporting organizations worldwide, ranging from universities to funders.
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\\n\\nBook chapters published in edited volumes are distributed under the Creative Commons Attribution 3.0 Unported License (CC BY 3.0). IntechOpen upholds a very flexible Copyright Policy. There is no copyright transfer to the publisher and Authors retain exclusive copyright to their work. All Monographs/Compacts are distributed under the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0). Read more
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\\n\\nIntechOpen is committed to ensuring the long-term preservation and the availability of all scholarly research we publish. We employ a variety of means to enable us to deliver on our commitments to the scientific community. Apart from preservation by the Croatian National Library (for publications prior to April 18, 2018) and the British Library (for publications after April 18, 2018), our entire catalogue is preserved in the CLOCKSS archive.
\\n"}]'},components:[{type:"htmlEditorComponent",content:'The Open Access publishing movement started in the early 2000s when academic leaders from around the world participated in the formation of the Budapest Initiative. They developed recommendations for an Open Access publishing process, “which has worked for the past decade to provide the public with unrestricted, free access to scholarly research—much of which is publicly funded. Making the research publicly available to everyone—free of charge and without most copyright and licensing restrictions—will accelerate scientific research efforts and allow authors to reach a larger number of readers” (reference: http://www.budapestopenaccessinitiative.org)
\n\nIntechOpen’s co-founders, both scientists themselves, created the company while undertaking research in robotics at Vienna University. Their goal was to spread research freely “for scientists, by scientists’ to the rest of the world via the Open Access publishing model. The company soon became a signatory of the Budapest Initiative, which currently has more than 1000 supporting organizations worldwide, ranging from universities to funders.
\n\nAt IntechOpen today, we are still as committed to working with organizations and people who care about scientific discovery, to putting the academic needs of the scientific community first, and to providing an Open Access environment where scientists can maximize their contribution to scientific advancement. By opening up access to the world’s scientific research articles and book chapters, we aim to facilitate greater opportunity for collaboration, scientific discovery and progress. We subscribe wholeheartedly to the Open Access definition:
\n\n“By “open access” to [peer-reviewed research literature], we mean its free availability on the public internet, permitting any users to read, download, copy, distribute, print, search, or link to the full texts of these articles, crawl them for indexing, pass them as data to software, or use them for any other lawful purpose, without financial, legal, or technical barriers other than those inseparable from gaining access to the internet itself. The only constraint on reproduction and distribution, and the only role for copyright in this domain, should be to give authors control over the integrity of their work and the right to be properly acknowledged and cited” (reference: http://www.budapestopenaccessinitiative.org)
\n\nOAI-PMH
\n\nAs a firm believer in the wider dissemination of knowledge, IntechOpen supports the Open Access Initiative Protocol for Metadata Harvesting (OAI-PMH Version 2.0). Read more
\n\nLicense
\n\nBook chapters published in edited volumes are distributed under the Creative Commons Attribution 3.0 Unported License (CC BY 3.0). IntechOpen upholds a very flexible Copyright Policy. There is no copyright transfer to the publisher and Authors retain exclusive copyright to their work. All Monographs/Compacts are distributed under the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0). Read more
\n\nPeer Review Policies
\n\nAll scientific works are Peer Reviewed prior to publishing. Read more
\n\nOA Publishing Fees
\n\nThe Open Access publishing model employed by IntechOpen eliminates subscription charges and pay-per-view fees, enabling readers to access research at no cost. In order to sustain operations and keep our publications freely accessible we levy an Open Access Publishing Fee for manuscripts, which helps us cover the costs of editorial work and the production of books. Read more
\n\nDigital Archiving Policy
\n\nIntechOpen is committed to ensuring the long-term preservation and the availability of all scholarly research we publish. We employ a variety of means to enable us to deliver on our commitments to the scientific community. Apart from preservation by the Croatian National Library (for publications prior to April 18, 2018) and the British Library (for publications after April 18, 2018), our entire catalogue is preserved in the CLOCKSS archive.
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I am also a member of the team in charge for the supervision of Ph.D. students in the fields of development of silicon based planar waveguide sensor devices, study of inelastic electron tunnelling in planar tunnelling nanostructures for sensing applications and development of organotellurium(IV) compounds for semiconductor applications. I am a specialist in data analysis techniques and nanosurface structure. 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