Reported high-performance passive hybrid coatings: composition, substrate, thickness, impedance modulus at low frequency |Zlf |, lifetime, and solution.
\\n\\n
Released this past November, the list is based on data collected from the Web of Science and highlights some of the world’s most influential scientific minds by naming the researchers whose publications over the previous decade have included a high number of Highly Cited Papers placing them among the top 1% most-cited.
\\n\\nWe wish to congratulate all of the researchers named and especially our authors on this amazing accomplishment! We are happy and proud to share in their success!
\\n"}]',published:!0,mainMedia:null},components:[{type:"htmlEditorComponent",content:'IntechOpen is proud to announce that 179 of our authors have made the Clarivate™ Highly Cited Researchers List for 2020, ranking them among the top 1% most-cited.
\n\nThroughout the years, the list has named a total of 252 IntechOpen authors as Highly Cited. Of those researchers, 69 have been featured on the list multiple times.
\n\n\n\nReleased this past November, the list is based on data collected from the Web of Science and highlights some of the world’s most influential scientific minds by naming the researchers whose publications over the previous decade have included a high number of Highly Cited Papers placing them among the top 1% most-cited.
\n\nWe wish to congratulate all of the researchers named and especially our authors on this amazing accomplishment! We are happy and proud to share in their success!
\n'}],latestNews:[{slug:"intechopen-authors-included-in-the-highly-cited-researchers-list-for-2020-20210121",title:"IntechOpen Authors Included in the Highly Cited Researchers List for 2020"},{slug:"intechopen-maintains-position-as-the-world-s-largest-oa-book-publisher-20201218",title:"IntechOpen Maintains Position as the World’s Largest OA Book Publisher"},{slug:"all-intechopen-books-available-on-perlego-20201215",title:"All IntechOpen Books Available on Perlego"},{slug:"oiv-awards-recognizes-intechopen-s-editors-20201127",title:"OIV Awards Recognizes IntechOpen's Editors"},{slug:"intechopen-joins-crossref-s-initiative-for-open-abstracts-i4oa-to-boost-the-discovery-of-research-20201005",title:"IntechOpen joins Crossref's Initiative for Open Abstracts (I4OA) to Boost the Discovery of Research"},{slug:"intechopen-hits-milestone-5-000-open-access-books-published-20200908",title:"IntechOpen hits milestone: 5,000 Open Access books published!"},{slug:"intechopen-books-hosted-on-the-mathworks-book-program-20200819",title:"IntechOpen Books Hosted on the MathWorks Book Program"},{slug:"intechopen-s-chapter-awarded-the-guenther-von-pannewitz-preis-2020-20200715",title:"IntechOpen's Chapter Awarded the Günther-von-Pannewitz-Preis 2020"}]},book:{item:{type:"book",id:"7514",leadTitle:null,fullTitle:"Biofuels - Challenges and opportunities",title:"Biofuels",subtitle:"Challenges and opportunities",reviewType:"peer-reviewed",abstract:"Amongst concerns about climate change, energy security decline and depletion of fossil fuels, this book explores the high importance of and interests in alternative energy resources. 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Alteration of different protein kinases can result in remarkable changes in these processes. Moreover, these protein kinases are frequently recognized as oncogenic and can be crucial for the survival and spread of cancer cells. Because of the fundamental role of protein kinases in cell biology and their function in numerous sarcomas and cancers, an intensive search for new kinase inhibitors in academia and industries has been enduring for the last two decades. Protein kinase has become the most imperative and commercial class of drug target which is attracting pharmaceutical industries to spend 30 % of their current research investments only in developing kinase inhibitors for various therapeutic implications. This is exemplified by the fact that 75 drugs targeting protein kinase have been clinically approved to date. More than 100 kinase inhibitors are in the final stages of development and likely to be approved in the coming years. 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His major area of research interests are drug-design, polymer-drug conjugates for targeted delivery to cancerous cells and CNS, antimalarial therapeutic agents, natural product chemistry and green chemistry approaches for chemical synthesis. Dr. Singh has over 15 years of teaching experience and guided 01 PhD and 18 PG students. He is currently guiding 02 PhDs and 01 M. Pharmacy students. He has published more than 60 peer-reviewed SCI-indexed scientific research papers of total impact factor more than 100 in various Chemistry and Pharmacy Journals including the European Journal of Medicinal Chemistry, Biorganic Chemistry, Biomedicine and Pharmacotherapy, European Journal of Pharmaceutical Sciences, Mini-Reviews in Medicinal Chemistry, Pharmaceutical Research, Medicinal Chemistry, Bentham, J. Enzyme Inh. Med Chem. (Informa Healthcare), Res. Chem. Intermediate (Springer), Arabian Journal of Chemistry, etc as main or corresponding author. He currently serves on Editorial Advisory Board Member of 09 Peer-reviewed International Journals including SCI indexed MRMC and PeerJ. He has received Publon Award 2016, 2017 for top reviewer and 'Publon Excellent Peer Reviewer Award” for outstanding reviewing more than 150 research papers of different International Journals of ACS, RSC, Springer, Elsevier, Dove, Informa, and Bentham of Impact Factor varies from 1.0 to 9.5. He has also to his credit 46 National and International Conference Abstracts, 2 Book, 5 Best Paper Presentation Awards, 1 Travel grants to attend Int. Conf. and 5 Research Projects funded by Govt. Agencies, India. He is also on the panel of International Reviewer for reviewing Research Proposal for Royal Society Grants. He is also serving as PUBLON ACADEMY MENTOR and BENTHAM BRAND AMBASSADOR.",institutionString:"Punjab Technical University",position:null,outsideEditionCount:0,totalCites:0,totalAuthoredChapters:"0",totalChapterViews:"0",totalEditedBooks:"0",institution:{name:"Punjab Technical University",institutionURL:null,country:{name:"India"}}}],coeditorOne:null,coeditorTwo:null,coeditorThree:null,coeditorFour:null,coeditorFive:null,topics:[{id:"6",title:"Biochemistry, Genetics and Molecular Biology",slug:"biochemistry-genetics-and-molecular-biology"}],chapters:null,productType:{id:"1",title:"Edited Volume",chapterContentType:"chapter",authoredCaption:"Edited by"},personalPublishingAssistant:{id:"194667",firstName:"Marijana",lastName:"Francetic",middleName:null,title:"Ms.",imageUrl:"https://mts.intechopen.com/storage/users/194667/images/4752_n.jpg",email:"marijana@intechopen.com",biography:"As an Author Service Manager my responsibilities include monitoring and facilitating all publishing activities for authors and editors. 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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:"71406",title:"Organic-Inorganic Hybrid Coatings for Active and Passive Corrosion Protection",doi:"10.5772/intechopen.91464",slug:"organic-inorganic-hybrid-coatings-for-active-and-passive-corrosion-protection",body:'The development of efficient production techniques of metallic alloys in the late twentieth century formed the basis for the boom of construction, transportation, energy, packaging, and electronic industries [1, 2]. Although extensively used, unprotected alloys are subject to a constant oxidation process, either in contact with humid atmosphere, aqueous, or soil systems, turning them into natural ore [1, 3]. One common approach to prevent or at least delay metallic corrosion is the application of organic coatings as a physical diffusion barrier. A barrier coating in the form of a dense insulating layer acts as a quasi-ideal capacitor, which inhibits the contact of corrosive species, such as the electrolyte, with the substrate. Barrier coatings based on acrylate, epoxy, and polyurethane are extensively employed in automotive, aviation, and marine industries but also in electronics, decoration, food, and beverage items. However, if these coating systems are not designed to withstand aggressive environments, they fail, causing corrosion of the underlying alloy. Therefore, conventional high-efficiency coatings are prepared in the form of multilayers combining a chromate conversion layer (0.1–0.2 μm) with a primer (commonly an epoxy layer of about 15–25 μm loaded with corrosion inhibitors) and an organic topcoat, mostly in the form of a 50–100-μm-thick polyurethane, which provide barrier, decorative, hydrophobic, and UV-resistant characteristics [4, 5].
Chromates, widely employed as conversion layer in the aerospace industry, provide effective protection because the excess of non-reduced ions produces an extremely corrosion-resistant film composed of mixed Cr and Al oxides [6]. However, hexavalent chromium causes occupational health problems due to carcinogenic and mutagenic effects [7], which led to an intense search for high-performance chromium-free coatings able to protect passively and actively metal surfaces in harsh environments. To achieve this goal, the alternative material must provide (i) a dense passive barrier with a very low permeation rate and after failure (ii) actively inhibit corrosive processes with similar efficiency as the self-healing ability of chromate anions. Considering this challenge, considerable efforts have been spent by the scientific community to develop alternatives to chromium conversion coatings, and some encouraging results have been already achieved.
One promising class of coating systems, developed in the last decade, which fulfill the cited criteria, is organic-inorganic nanocomposites based on conventional acrylic, epoxy, and polyurethane materials combined with ceramic nanofillers, such as silica, ceria, zirconia, etc. These hybrid materials have demonstrated excellent barrier property, providing long-term protection for steel and aluminum alloys [3, 8, 9, 10]. The superiority of hybrid systems compared to purely polymeric phases comes from a tailored nanostructure achieved by proper amounts of inorganic nanofillers within the organic matrix and the careful tuning of synthesis conditions, resulting in a dense and homogeneous nanocomposite that acts as an efficient diffusion barrier, limiting the water uptake and diffusion of ionic species to a very low rate [11]. The inorganic nodes have the important role to densify the structure by anchoring covalently the polymeric chain segments through a cross-linking agent and to improve the adhesion at the coating/metal interface through covalent bonding [12, 13, 14]. The function of the polymeric phase is to provide minimum internal stress and porosity of the hybrid network, hermetically sealing the structure [3, 11, 12]. Hence, the key factor for the stability of the material under adverse conditions is the covalent conjugation between the two phases through the coupling molecule that has functional similarity to both parts. The presence of this organic-inorganic interphase overcomes the limitations of organic systems such as lack of thermal and mechanical stability, poor adhesion, and presence of voids due to material swelling [15, 16, 17, 18].
Nevertheless, even the best barrier fails after some time due to the permeation of water and aggressive species under weathering or mechanical damage. Smart coatings are an emerging technology to bypass the limitations of passive systems, providing a repair response mechanism after failure [5]. Several approaches have been developed in the search for chromium-free organic coatings using organic and inorganic additives for extrinsic self-healing strategies. These types of coatings are able to respond after local damage through reactions triggered by variations in pH, temperature, presence of water, mechanical damage, and UV [3, 4, 19]. In practice, however, some studied coating systems, especially those based on inhibitor-filled micro- and nano-containers, imply increasing costs, which are hardly accepted by the industry. Some recent studies reporting on incorporation of simple additives, such as PANI [20], TiO2 [21], ZnO [22], lithium [23, 24], ZrO2 [9, 25], cerium/ceria [26, 27, 28, 29], poly(2-butylaniline) (P2BA) [30], polydopamine (PDA) [31], 2-mercaptobenzimidazole (MBI) [32], and tannins [33], among others, have shown promising results toward the development of chromium-free active coatings. This chapter comprises an overview of efficient passive organic-inorganic coatings and their modified form for active protection of metal surfaces. Emphasis is given on inorganic additives that meet the current demand for nontoxic low-cost substances, such as cerium nanoparticles and lithium ions in hybrid matrices.
Organic-inorganic coatings are usually prepared by combining the polymerization of the organic phase using an anionic, cationic, thermal, or photo initiator with the sol-gel route of hydrolysis and condensation of an inorganic compound in the form of silicon, zirconium, aluminum, and titanium alkoxides [34]. In the next step, the hybrid sol is applied on the metal surface by spray, spin, or dip coating methods, and then the obtained film is dried and cured prior to the structural, thermal, mechanical, and electrochemical analysis.
Epoxy [35, 36, 37, 38] and polyurethanes-based [21, 39, 40] hybrid coatings present excellent protection against metallic corrosion; however recent research has shown that equal or even better results in terms of corrosion resistance and durability can be achieved with much thinner layers, based on acrylic or epoxy hybrids. Especially for poly(methyl methacrylate) (PMMA)-silica coatings, remarkable results have been reported when applied on carbon steel and aluminum alloys. For instance, using electrochemical impedance spectroscopy (EIS) assays, Hammer et al. reported that PMMA-silica coatings with thicknesses between 1.5 and 3 μm are able to withstand adverse conditions without failure for 18 days immersed in 3.5% NaCl, while maintaining the low-frequency impedance modulus (|Zlf|) in the GΩ cm2 range, a value about five orders of magnitude higher than that of bare steel [41]. Analyzing the structural properties of this material, dos Santos et al. showed that optimizing the inorganic solvent proportion (ethanol/H2O ratio) results in higher connectivity of the silica phase, leading to an improvement of the corrosion protection (|Zlf| > 1 GΩ cm2) and durability up to 6 months in 3.5% NaCl and more than 3 months in saline/acid solution [12].
Excellent passive corrosion protection of carbon steel was recently reported for some micrometer-thick PMMA-silica coatings in a study where the ratio of thermal initiator (benzoyl peroxide (BPO)) to monomer (methyl methacrylate (MMA)) was varied in the range of 0.01–0.1 (B001, B005, B010 samples) [11]. The authors found that this parameter strongly influences the hybrid structure in terms of polymerization efficiency, leading to an improved anticorrosion performance. From Raman spectra (Figure 1a), it can be observed that as the BPO fraction increased, the C〓C band at 1640 cm−1 disappeared, indicating that the larger number of radicals provided by BPO promotes a more efficient polymerization of MMA. This result is supported by thermogravimetry, carried out under N2 atmosphere to analyze the decomposition stages of the hybrid structure (Figure 1b). The first derivative of the thermogravimetric (TG) curves (Figure 1c) shows that the PMMA phase has three major degradation events, involving the rupture of head-to-head segments (stacking defects) at about 240°C (T1), breaking of unsaturated chain ends at ~300°C (T2), and a random breaking of head-tail segments at ~400°C (T3). The differential thermogravimetric (DTG) curves clearly show that the stacking defects (T1 event) are strongly suppressed for higher BPO/MMA ratios, evidencing improved polymerization. The residual mass of about 20 wt% found at 800°C corresponds to the nominal fraction of the silica phase in the hybrid.
(a) Raman spectra, (b) TG curves and (c) DTG curves of PMMA-silica hybrids prepared using BPO/MMA ratios of 0.01, 0.05, and 0.10 (reproduced with permission from Elsevier [11]).
In this study it was shown that the combination of adequate proportions between reactive groups of organic monomer (MMA), silica precursor (tetraethoxysilane (TEOS)), and coupling agent (3-(trimethoxysilyl)propyl methacrylate (MPTS)) leads to the formation of a homogeneous and defect-free structure (Figure 2a). According to atomic force microscopy (AFM), optical, and UV–vis results (Figure 2b–e), the prepared coatings are transparent (80–90% transmittance) and very smooth (RMS roughness <1 nm), expected features for effective barrier coatings. For increasing amounts of the thermal initiator, the red shift of the absorption edge (Figure 2e) may be related to increased absorption of BPO in the UVA spectral range. Furthermore, adhesion pull-off tests evidenced a strong adherence of the coatings to the steel substrate, reaching values up to 26.3 MPa for the B001 sample, while 14.8 MPa, 8.9 MPa, and 6.7 MPa were determined for the B005, B010 and pure PMMA, respectively.
(a) Representation of the molecular structure of the PMMA-silica hybrid precursors, showing the reactive groups of MMA and silicon alkoxides; (b) 3D AFM topography image of the B001 coating deposited on carbon steel; (c) representative images of the coated carbon steel; (d) unsupported hybrid; (e) UV–vis transmittance spectra of the unsupported hybrids (reproduced with permission from Elsevier [11]).
To evaluate the anticorrosion performance of coated and uncoated steel, EIS measurements were performed in duplicate after immersion of the coatings in neutral 3.5% NaCl solution at regular intervals until a significant drop in the impedance modulus was observed, representing the lifespan of the coating. After 3 h of immersion, all hybrid coatings showed a quasi-ideal capacitive behavior over almost the entire frequency range, contrasting with the poor performance of pure PMMA and bare steel (Figure 3a).
Bode plots recorded (a) after 3 h immersion in 3.5% NaCl, for PMMA-silica coatings on carbon steel prepared at different BPO/MMA ratios, pure PMMA (BPO/MMA = 0.01) and bare steel and (b) for different immersion times of the B010 sample. The symbols represent the experimental data, and the solid lines are fits obtained using the electrical equivalent circuit (EEC) model (reproduced with permission from Elsevier [11]).
The comparison with pure PMMA coating shows clearly the crucial role of the silica phase in increasing corrosion resistance, after 1 day of immersion, from a low-frequency impedance modulus of 200 kΩ cm2 (pure PMMA) to more than 5.0 GΩ cm2 (PMMA-silica coatings). Moreover, the BPO increase led to a considerable lifespan extension for the B010 coating (9.1 μm thick) reaching 583 days with a nearly unchanged electrochemical response (Figure 3b), compared to the lifetime of 40–50 days, observed for the B005 and B001 coatings (5.0 and 2.8 μm thick, respectively), and only 1 day of the PMMA film. This anticorrosion performance, associated with an improved structure (Figure 1), is comparable to that of the best performing anticorrosive coatings reported so far [8, 9, 12, 20, 21, 39, 42]. Results obtained for high-performance passive barriers based on organic-inorganic coatings reported by several research groups are summarized in Table 1.
Coating | Substrate | Thickness (μm) | |Zlf| (GΩ cm2)lifetime (days)solution | Ref. |
---|---|---|---|---|
PMMA-MPTS-TEOS | A1010 carbon steel | 2.3–9.7 | ~5, 583, 3.5% NaCl | [11] |
PMMA-MPTS-TEOS | AA2024 | ~3 | ~50, >560, 3.5% NaCl | [3] |
PMMA-MPTS-TEOS | A1010 carbon steel | 1.5–2 | ~5, 196, 3.5% NaCl | [12] |
Acrylic resin-SiO2 | Mild steel | 75 | ~10, 90, 3.5% NaCl | [43] |
PMMA-MPTS-TEOS-CNTs or GO | A1010 carbon steel | 3–6 | ~3, 211, 3.5% NaCl | [42] |
Epoxy-zinc phosphate-iron oxide | Cold rolled low carbon steel | 55–140 | ~450/60, 220/405, 3% NaCl | [8] |
Epoxy-PANI | Mild steel | 20 | 10–100, 120, 3.5% NaCl | [20] |
Epoxy-GO-P2BA | Q235 carbon steel | 20 | 10, 80, 3.5% NaCl | [30] |
PU-PANI | Mild Steel | ~60 | 10, 58, 3.5% NaCl | [40] |
PU-ZrO2-SiO2 | Carbon steel | 40–55 | ~100, 226, 3.5% NaCl | [9] |
PU-MMT | Carbon steel | ~40 | ~10, 225, 3.5% NaCl | [39] |
PU-PS-PLA-MBT | AA2024 | 152 | ~0.1, 83, 3.5% NaCl | [44] |
PU-TEOS-TIP-ZRP | AA2024 | 203 | ~10, 100, 3.5% NaCl | [45] |
Reported high-performance passive hybrid coatings: composition, substrate, thickness, impedance modulus at low frequency |Zlf |, lifetime, and solution.
PMMA, poly(methyl methacrylate); MPTS, 3-(trimethoxysilyl)propyl methacrylate; TEOS, tetraethoxysilane; CNTs, carbon nanotubes; GO, graphene oxide; PANI, polyaniline; MMT, montmorillonite; P2BA, poly(2-butylaniline); PU, polyurethane; PS, polysiloxane; PLA, polylactic acid; MBT, 2-mercaptobenzothiazole; TIP, titanium(IV) isopropoxide; ZRP, zirconium(IV) propoxide.
Additional information on the barrier property of the B010 sample was obtained by a cross-sectional analysis using scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM/EDX) before and after 583 days of immersion. Figure 4a shows an essentially unaffected morphology, free of pores and defects, after 583 days of immersion. Further evidence comes from the EDS profiles recorded along normal axis of the coating, shown in Figure 4b. The spectrum of the immersed sample shows no chlorine signal, which would indicate the presence of Cl− ions in the bulk, thus confirming the excellent barrier properties of this hybrid nanocomposite.
(a) Cross-sectional SEM images of the B010 coating, before (left) and after (right) 583 days of immersion in 3.5% NaCl solution, including details of the coating/steel interface. The layers visible in the images are due to the three dips applied by dip coating; (b) integrated EDX cross-sectional spectra, before and after 583 days of immersion (reproduced with permission of Elsevier [11]).
From these results, it can be concluded that the quantity of the thermal initiator plays a crucial role in terms of the connectivity of the organic phase, resulting in a remarkable improvement of the barrier property for the B010 coating (BPO/MMA = 0.1). The increase of the impedance modulus of more than five orders of magnitude and the long durability of this coatings, compared to the pure PMMA film, highlights the crucial role of the silica phase and the importance of an efficient polymerization. Considering the excellent performance achieved by PMMA-silica coatings without any additives, this material can be considered as a promising alternative for conventional primer systems for the protection of steel surfaces.
The second requirement for an effective chromium-free coating relies on active anticorrosion protection. Among corrosion inhibitors, it was demonstrated that the incorporation of lithium salts improves corrosion protection of epoxy coatings. As possible mechanisms, Visser et al. have proposed that the release of lithium ions from the epoxy matrix forms in the defective zone a Li/Al layer (hydrotalcite class) by a conversion process [46]. In recent work, lithium carbonate (Li2CO3) was for the first time added (500–2000 ppm) into the PMMA-silica system, and the results revealed a dual beneficial effect of lithium on the structure and self-healing ability of the coatings on the AA7075 aluminum alloy [24].
The structural analysis of the coatings showed that the changes of the small-angle X-ray scattering (SAXS) profiles imply significant modifications in the nanostructure of the silica phase for higher lithium loadings (Figure 5a). The profiles of Li0 (0 ppm) and Li05 (500 ppm) samples present a correlation peak, indicative of a concentrated set of silica domains with an average spacing of 3–4 nm (Figure 5b). However, for further addition of lithium (Li1 and Li2), the correlation peak vanishes due to the formation of a more diluted set of larger silica clusters. For these profiles, the average size of the silica nanoparticles (Rg) can be calculated according to the Guinier-Porod model [47, 48], revealing for the Li1 sample (1000 ppm) a gyration radius of about 1 nm and close to 3 nm for Li2 (2000 ppm).
(a) SAXS intensity profiles of the hybrids fitted according to the Guinier-Porod model (green lines); (b) structural representation of Li0 and Li05 PMMA-silica hybrids; (c) thermogravimetric curves and (d) differential thermogravimetric curves of PMMA-silica hybrids prepared with Li2CO3 concentrations of 0, 500, 1000, and 2000 ppm (reprinted (adapted) with permission from [24]. Copyright (2020) American Chemical Society).
Thermogravimetric measurements were used to access information on the polymerization efficacy of PMMA and the thermal stability of the material. The results reveal that the samples exhibited a thermal stability up to ~240°C (onset degradation temperature at 5% mass loss, Ts (Figure 5c)), while degradation events of the TG derivative curve (Figure 5d) showed that the presence of lithium suppresses head-to-head stacking defects (T1) and unsaturated PMMA chain ends (T2). These findings indicate a more efficient polymerization with higher Li content, which is an essential feature for an effective diffusion barrier.
The films with thickness between 4 μm and 6 μm present excellent adhesion to AA7075 substrate reaching values up to 28 MPa for coatings with higher Li loading [24]. The improved adhesion for lithium-rich coatings might be related to aluminum enrichment near the coating/substrate interface, suggesting a diffusion of Al from the alloy into the coating during the thermal treatment [24].
Besides the beneficial effects of lithium on the structural properties, Li-modified PMMA-silica hybrids yielded coatings with longer durability under immersion in 3.5% NaCl solution. This is evident from the time dependence of the open circuit potential (EOCP) and the low-frequency impedance modulus (|Zlf|), recorded by EIS during 310 days of immersion (Figure 6). For lithium-containing coatings, the time dependence of |Zlf| (Figure 6b) revealed a very interesting effect of coating regeneration, attributed to the chemical activity of the lithium ions. The data show several important features of the lithium activity for different Li loadings: (i) for all samples the time dependence shows a clear correlation between |Zlf| and EOCP (Figure 6a); (ii) after local failure of the coating, an impedance drop occurred, followed by a gradual recovery of about two orders of magnitude within 40 days (Li05), 28 days (Li1), and 20 days (Li2), indicating a faster regeneration process with increasing Li content; and (iii) the higher lithium concentration of Li2 coating delayed the appearance of localized corrosion compared to Li1 and Li05 samples. Furthermore, although the initial value of |Zlf| was smaller for Li1 and Li2 coatings, after 300 days of immersion, they showed a higher impedance value, an effect that can be related to their improved structural characteristics, as discussed before.
(a) Time evolution of EOCP and (b) |Zlf | for coatings modified with different amounts of lithium during 310 days of immersion in 3.5% NaCl solution. The inset shows in more detail the first impedance modulus recovery (99 days) for the Li1 sample (reprinted (adapted) with permission from [24]. Copyright (2020) American Chemical Society).
Representatively, the time evolution of the Bode plots obtained by EIS for the Li1 coating is displayed in Figure 7, together with the fitted curves (3 h, 99 days, and 126 days) using electrical equivalent circuits (EEC), shown in Figure 7a. Initially, Li1 presents an impedance modulus up to six orders of magnitude higher than bare aluminum alloy. As a consequence of the coating permeation by the electrolyte, the formation of conductive percolation paths leads after 99 days to the appearance of localized corrosion spots (pits), causing a decay of the impedance modulus at medium and low frequencies, indicative for failure of the coating and the beginning corrosion process at the coating/metal interface [11]. Nevertheless, after 126 days of immersion, the coating was spontaneously restored, increasing |Zlf| by two orders of magnitude. Next, the appearance of a second pit caused a new decay after 154 days, and thereafter the impedance modulus has recovered again after 183 days, remaining stable until 311 days of immersion.
(a) Electrical equivalent circuits used to fit (black lines) the EIS data of Li1 coating after (b) 3 h, 99 days, and 126 days of immersion in 3.5% NaCl solution and (c) time evolution of the bode plots after 154, 183, and 311 days of immersion (reprinted (adapted) with permission from [24]. Copyright (2020) American Chemical Society).
The self-healing process of the first recovery event was analyzed by fitting the EIS data using EEC containing a electrolyte resistance in series with two time constants, the first representing the upper water uptake layer and the second the inner intact/restored layer, and by adding a third time constant to simulate the corrosion process at the coating/metal interface (Figure 7a). Accordingly, the high-frequency data are generally attributed to phenomena occurring at the coating/electrolyte interface (R1/CPE1), the medium frequencies refer to the inner layer of the coating (R2/CPE2), and the low frequency is related to charge transfer resistance (Rct) and double-layer capacitance (Cdl) at the coating/substrate interface (R3/CPE3). The electrochemical parameters, extracted by the fitting procedure, are shown in Table 2. After 99 days, a strong decay of R1 and R2 can be observed followed by a sharp increase after 126 days, from 0.20 MΩ cm2 to 1.68 MΩ cm2 and 0.02 MΩ cm2 to 9.03 GΩ cm2, respectively. As expected, an inverse behavior can be observed for the Q values of the CPE parameter, related to the coating capacitance.
Li1 | 3 h | 99 days | 126 days | |||
---|---|---|---|---|---|---|
χ2 | 6.4 × 10−3 | — | 4.5 × 10−3 | — | 5.2 × 10−3 | — |
Rs (Ω cm2) | 52.8 | — | 42.4 | — | 23.0 | — |
R1 (MΩ cm2) | 17.1 | (12.8)* | 0.20 | (2.0) | 1.68 | (2.0) |
Q1 (nΩ−1 cm−2 sn) | 0.46 | (2.6) | 0.55 | (4.1) | 0.66 | (2.4) |
n1 | 0.97 | (0.2) | 0.97 | (0.3) | 0.95 | (0.2) |
R2 (GΩ cm2) | 52.2 | (3.8) | 0.02 | (1.1) | 9.03 | (3.6) |
Q2 (nΩ−1 cm−2 sn) | 0.21 | (5.6) | 17.0 | (1.8) | 3.85 | (0.7) |
n2 | 0.70 | (1.6) | 0.77 | (0.5) | 0.88 | (0.2) |
R3 (GΩ cm2) | 0.03 | (14.6) | ||||
Q3 (nΩ−1 cm−2 sn) | 2723 | (5.1) | ||||
n3 | 0.75 | (4.9) |
Electrochemical parameters derived by fitting of the EIS data using electrical equivalent circuits of Figure 7a for Li1 sample after 3 h, 99 days, and 126 days of immersion in NaCl 3.5%.
The values in brackets correspond to the error (%) of each parameter.
To obtain more information on the Li-induced self-healing mechanism, surface analysis by time-of-flight secondary ion mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS), and SEM were performed. Surface maps recorded by SIMS showed that the pit in the center of the immersion area (Figure 8b) contains only two small spots of lithium (red), surrounded by aluminum corrosion products (blue), and PMMA (green) containing a very low Li concentration (Figure 8a). In comparison, a higher Li surface concentration was detected outside the immersed area (Figure 8c), which indicates a Li+ leaching process from the coating surface, evidencing its high mobility. Furthermore, the cross-sectional image obtained by SEM (Figure 8f) shows a conical shape of the 70-μm-deep pit, surrounded by cathodic debris as a consequence of redox reactions. The cross-sectional SIMS maps of the pit revealed that its center is mainly filled with AlOHx corrosion products (Figure 8d) and that the top contains a lithium layer (Figure 8c), a clear evidence of the high mobility of lithium ions toward corrosive sites. XPS analysis performed in the top and center zones of the pit (Figure 8f) confirmed the SIMS findings, revealing a high Li concentration of 31 at.% and 22 at.%, respectively, and the presence of lithium oxide and lithium-containing aluminum oxide interphases (Figure 8g).
(a) ToF-SIMS map of the Li1 coating overlay of Li+ (red), C2H3O2+ (green), and Al+ (blue); (b) optical micrograph displaying the immersed area delimited by a blue dashed circle; (c) normalized ToF-SIMS map of Li+ (yellow) at the edge of the immersed zone; normalized ToF-SIMS maps of (d) AlOH+ (yellow) and (e) Li+ (yellow), recorded in top and center zones of Figure 8f; (f) SEM cross-sectional view of the pit of the Li1 sample obtained after 310 days of immersion in 3.5% NaCl solution; and (g) fitted XPS O 1 s and Li 1 s spectra taken at the top and center of the pit. For SEM analysis, sputtered Au was used to improve surface conductivity (reprinted (adapted) with permission from [24]. Copyright (2020) American Chemical Society).
The reversible process observed for lithium-containing coatings is related to redox reactions taking place at the coating/substrate interface, as illustrated in Figure 9. As soon as water, oxygen, and chloride ions reach the substrate, Cl− ions form a complex with aluminum producing soluble Al compounds, which cause the pit acidification/propagation. Simultaneously, at the top of the defect, Al(OH)x begins to precipitate at higher pH (reaction driven by hydroxyl ions, a product of carbonate action), leading to the formation of aluminum oxides. Finally, the curing activity of lithium within the pit is triggered at higher pH by the formation of Li+ intercalated aluminum oxide phase with a highly passive character [16, 49, 50].
Proposed mechanism for the formation of a lithium-induced protective barrier layer (reprinted (adapted) with permission from [24]. Copyright (2020) American Chemical Society).
More rigorous tests of lithium self-healing activity were conducted by salt spray tests [24]. SEM images in Figure 10a and b show that after 7 days of testing, the Li2 sample was almost completely covered by a film, whereas the Li0 sample presents abundant corrosion products in the scratch track. EIS measurements performed after testing revealed for the Li2 coating an increase of the impedance modulus of one decade, while for the Li-free coating, a drop of impedance was observed. A confirmation of Li+ activity in the corrosion zones was obtained by the overlay of Li+ (red), AlOH+ (green) and C2H3O2+ (yellow) SIMS map (Figure 10c) showing that lithium ions were preferentially leached from the walls of the scratch to active corrosion zones. The leached Li ions initiated a precipitation reaction of aluminum oxides, resulting in the formation of a protective layer within the scratch track that resulted in a significant reduction of the corrosion rate [46, 51].
SEM images of the scratched film recorded after 7 days of salt spray test and the corresponding EIS impedance modulus profiles after 1 day and 7 days for (a) Li0 and (b) Li2 coating on Al7075 substrate; (c) ToF-SIMS map of the Li2 sample showing the overlay map of Li+ (red), AlOH+ (green), and C2H3O2+ (yellow), after 7 days salt spray test (reprinted (adapted) with permission from [24]. Copyright (2020) American Chemical Society).
Structural, surface and electrochemical characterization showed that lithium ions were successfully incorporated into PMMA-silica coatings promoting a beneficial effect on the hybrid structure as well as smart corrosion inhibition. Based on the data of different surface analysis techniques, a self-healing mechanism was proposed that describes a lithium ion-induced formation of a protective layer of redox reaction products that block the corrosion process not only in localized defects (pits) but also in artificially damaged zones, thus extending considerably the service time of the PMMA-silica coating.
Recently, it was shown that organic-inorganic hybrids based on PMMA-cerium oxide are very promising materials for protective coating due to their excellent corrosion resistance, active self-healing ability, and environmental compatibility [29]. This nanocomposite material was prepared by the radical polymerization of methyl methacrylate and 2-hydroxyethyl methacrylate (HEMA), using benzoyl peroxide as thermal initiator, combined with the sol-gel synthesis of cerium oxide nanoparticles using Ce(NO3)3.6H2O salt and LiOH. The hybrid solutions were used to deposit films on A1020 carbon steel by dip coating, yielding coatings with a thickness of 10 μm. Three coating formulations were evaluated with the following molar proportions, 1Ce:2HEMA:20MMA, 1Ce:2HEMA:25MMA, and 1Ce:2HEMA:30MMA, keeping the BPO/MMA molar ratio equal to 0.05. The HEMA molecule is formed by a methacrylate group that reacts with the organic phase (PMMA) and a hydroxyl terminal group that can be linked to the hydroxyl surface groups of the cerium oxide particles, thus acting as coupling agent and imparting excellent homogeneity and compatibility between the organic and inorganic phases (Figure 11a).
(a) Schematic representation of the PMMA-cerium oxide coating deposited on carbon steel; (b) HRTEM image of the hybrid; (c) electrochemical impedance spectroscopy plots of the 1Ce:2HEMA:25MMA coating immersed in 3.5% NaCl solution; and (d) electrical equivalent circuit used to fit the EIS data (reproduced with permission from Elsevier [29]).
Structural characterization performed by XPS, SAXS, and high-resolution transmission electron microscopy (HRTEM) revealed the formation of CeO2 and Ce2O3 nanoparticles with an average size of 2.5 nm and the homogenous distribution of these particles in the PMMA matrix through covalent bonds with the HEMA molecule (Figure 11b) [29]. The coatings deposited on carbon steel were transparent and free of pores and presented low surface roughness (<1.6 nm), extracted from AFM topography images. In addition, thermogravimetric analysis showed that the PMMA-cerium oxide hybrids have thermal stability up to 220°C and confirmed the nominal inorganic mass fraction for the samples, varying between 22 wt% (1Ce:2HEMA:20MMA) and 10 wt% (1Ce:2HEMA:30MMA).
Careful tuning of the molar ratio between the reagents yielded coatings with notable anticorrosive performance, assessed by electrochemical impedance spectroscopy in aggressive saline environment [29]. The 1Ce:2HEMA:20MMA sample exhibited low corrosion resistance, with impedance modulus at low frequency |Zlf| of 10 kΩ cm2 after 1 day of immersion in 3.5% NaCl solution, only one order of magnitude higher than the uncoated carbon steel. A small increase of the polymeric content to 1Ce:2HEMA:25MMA, led to a sharp rise of the impedance modulus to 290 GΩ cm2, which remained almost unchanged after more than 6 months in saline solution (Figure 11c). A further increase of the MMA amount to 1Ce:2HEMA:30MMA resulted in an intermediate performance of the coating, with |Zlf| of 95 GΩ cm2 and durability of 5 months. The corrosion resistance achieved for the 1Ce:2HEMA:25MMA sample is comparable to that of the best anticorrosive coatings reported so far [22, 26, 27, 36, 43, 52, 53, 54], however, with the advantage of using nontoxic solvent or precursor. Several interesting results reported for high-performance organic-inorganic coatings with active corrosion protection are summarized in Table 3.
Coating | Substrate | Thickness (μm) | |Zlf| (GΩ cm2) 1st failure (days) solution | Ref. |
---|---|---|---|---|
PMMA-MPTS-TEOS-Ce(IV) | A1010 carbon steel | ~2 | ~1, 304, 3.5% NaCl | [26] |
PMMA-MPTS-TEOS-Li | AA7075 | 4–6 | ~1, 142, 3.5% NaCl | [24] |
PMMA-HEMA-CeO2 | A1010 carbon steel | 10 | ~290, 189, 3.5% NaCl | [29] |
PMMA-MPTS-TEOS-Ce | Mild steel | 26 | ~10, 362, 3.5% NaCl | [55] |
Epoxy-HT-BZ | Carbon steel | 35–45 | ~10, >60, 0.5 M NaCl | [56] |
PVB-p-GAMo | Mild steel | 47–53 | ~1, >21, 3.5% NaCl | [57] |
Epoxy-MMT-Ce(III) | Carbon steel | 50–70 | ~0.1, >100, 3.5% NaCl | [58] |
Epoxy-PBH-GO | Carbon steel | — | ~0.1, 40, 3.5% NaCl | [59] |
Epoxy-APS-BS | AA2024 | 300 | ~1, >350, 0.5 M NaCl | [60] |
Reported active hybrid coatings loaded with organic and/or inorganic corrosion inhibitors: composition, substrate, thickness, impedance modulus at low frequency |Zlf |, time interval until the first failure event occurs, and solution.
PMMA, poly(methyl methacrylate); MPTS, 3-(trimethoxysilyl)propyl methacrylate; TEOS, tetraethoxysilane; Li, lithium; GO, graphene oxide; HT, hydrotalcites; BZ, benzoate; PVB, polyvinyl butyral; p-GAMo, porous organosilica with ion molybdate; MMT, montmorillonite; PBH, polydopamine benzotriazole loaded halloysite nanotubes (HNTs); APS, (3-aminopropyl)trimethoxysilane; BS, bis[3-(triethoxysilyl)propyl]tetrasulfide.
The electrical equivalent circuit shown in Figure 11d was used to fit the EIS data of the 1Ce:2HEMA:25MMA sample, allowing a deeper insight into the behavior of the electrochemical system. The circuit is composed of a solution resistance (Rs) in series with two time constants (R1/CPE1 and R2/CPE2) related to coating resistance and capacitance of the near-surface region and that of the inner layer close to the coating/steel interface, respectively. During the immersion period of 189 days, the coating resistance of the bulk presented values up to 1 TΩ cm2 and coating capacitance of less than 0.1 nF cm−2, characteristics of an extremely efficient anti-corrosion barrier arising from the dense and highly insulating cross-linked PMMA-cerium oxide structure [29].
Moreover, a detailed analysis of scratched and immersed coatings performed by EIS, XPS, and SEM (Figure 12) evidenced that Ce ions act as self-healing agents, by formation of insoluble cerium oxide and hydroxide species by reactions between leached cerium ions and hydroxyl groups in the scratch track, inhibiting the progression of the corrosion process and consequently enhancing the coating lifetime [29].
(a) EIS plot of the scratched PMMA-cerium oxide coating immersed in 3.5% NaCl solution; (b) XPS O 1s spectra and (c) SEM image after immersion in 3.5% NaCl solution for 24 h (reproduced with permission from Elsevier [29]).
In summary, PMMA-cerium oxide coatings deposited on carbon steel combine high corrosion resistance, durability, self-healing property, low-cost, small-thickness, and eco-friendliness, thus representing a very promising alternative to conventional anticorrosive coatings for the protection of steel components.
Advances in the search for effective substituents of chromates claim long-term protection through a dense barrier coating associated with an active response in case of damage. In this chapter, we have shown that acrylic nanocomposites efficiently prevent corrosion for 583 days by applying a thin (10 μm), adherent (up to 26 MPa), and transparent (80–90% transmission) layer of PMMA-silica on carbon steel. Furthermore, it was demonstrated that by incorporation of corrosion inhibitors, such as lithium ions and cerium oxide nanoparticles, a self-healing ability of hybrid coatings can be achieved, based on different mechanisms: lithium ions induce the formation of a passive layer in the corroded zone composed of Li+ intercalated aluminum oxide phases, whereas cerium nanoparticles liberate cerium ions that react with water to form insoluble oxides and hydroxides in the affected zone, thus blocking the progress of the corrosion process of the AA7075 alloy and carbon steel, respectively. The efficient passive and active protection of metallic surfaces makes acrylic hybrid coatings potential candidates for a chromate-free future.
The authors would like to acknowledge the financial support of funding agencies, namely, Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) [grants 424133/2016-4, 307905/2018-7, 421081/2016-3], Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) [grants 465593/2014-3 and 88887.136401/2017-2100, Finance Code 001], and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) [grants 2015/11907-2, 2015/09342-7, 2014/12182-9]. We also thank the National Laboratory of Synchrotron Light Source (LNLS) for SAXS beamline access and LNNano for the HRTEM images, the Hercules program under grant agreement ZW13_07 for ToF-SIMS measurements, and the Electrochemical and the Surface Engineering Group (SURF, Brussels/Belgium) for the SEM and EDX analysis.
The authors declare no conflict of interest.
Due to the vital role of water for humanity, it is necessary to improve and maintain its quality. Environmental and global changes especially industrial wastes and domestic and agricultural activities are the main water pollution source. Worldwide, several water resources even underground water resources are contaminated, and they are not a suitable quality for drinking. Because of the rising living standards, growing world population, unconscious water consumption, and urbanization lead to increasing water supply costs. In most cases, as it contains different and large number of pollutants, wastewater lead to ecosystem hazards for being released around without being processed. So a few decades later, the world could face a major problem with freshwater supply [1]. In the past, very little financial resources have been allocated for wastewater because water supply received more priority than wastewater treatment (WWT). But, because of the increasing rapid population growth and trends in urbanization, WWT plays an important role in human life. Recently, because of the impact of sewage contamination of groundwater, rivers, and lakes, the growing awareness of wastewater treatment is now receiving greater attention from researchers and environmentalists. Research study results revealed that WWT, which is managed appropriately, has a large share in the growing economy when water resources treatment and supply are done in an appropriate manner [2, 3]. Safe, reliable, and sustainable treated WWT strategies have a vital role because of several challenges including adoption of low-cost WWT technologies. To prevent the spread of diseases, WWT systems are crucial, and they should have high levels of hygienic standards for reuse in agricultural and other areas. Lack of WWT can lead to environmental pollution, and it may cause a hazardous effect for the health of humans. To improve global health and to prevent spread of disease, reliable collection and treatment of wastewater are very important. Wastewater treatment and their reuse need innovative and appropriate technologies. Recently, WWT technologies including electrochemical technologies have regained their importance worldwide. In some cases, the electrochemical mechanism for metal recovery is very simple. These technologies have reached comparably with other technologies in terms of cost and efficiency [4]. Economic issues besides environmental and social aspects must be considered when choosing the most appropriate WWT method [5, 6]. All scientists and environmentalists desire widespread recognition of the need to implement more sustainable WWT techniques. Wastewater treatment technologies follow two main approaches: first is the development of a single indicator integrating different criteria and second is the development of a set of multidisciplinary indicators [7, 8]. When large volumes of treated wastewater contain low concentrations of chemical constituent discharge-receiving water body, it may still lead to water quality problem. Discharges from industrial activities have been identified as one of the major sources of aquatic pollution in industrialized countries. After 1990, to remove toxic pollutants in wastewater, scientists focused on persistent organic pollutants including PCBs, PAHs, and especially heavy metals due to destructive effects [9, 10]. People’s anxieties also increase because of pollutions caused by heavy metals. Pollutions caused by heavy metals spread into the aqueous systems from many industries such as metal plating and smelters, eluents from plastics, mining, and textile industries [11]. Toxic heavy metals including mercury and chromium are discharged to the environment, and unfortunately they cannot biodegrade in nature [12, 13]. Heavy metals can be traveled through the food chain via bioaccumulation, the increase of heavy metals in human body causes some major diseases like brain, pancreas, and heart diseases, and they can lead to wide spread capillary damage and gastrointestinal irritation besides possibly necrotic changes in some tissue [14]. Even at low concentrations, heavy metals can cause serious toxic and harmful effects on the organism and the environment. The World Health Organization (WHO) limited heavy metal concentrations. Such as in drinking water, maximum acceptable limit of copper concentration is offered as 1.5 mg L−1, when the limit concentrations of metals containing hazardous waste are different [15, 16]. Ion exchange, extraction, membrane filtration, and chemical precipitation especially adsorption techniques have been applied to remove heavy metals; on the other hand, generally adsorption technique is one of the most chosen method because of its simplicity, nontoxicity, cost-effectiveness, and local availability to remove toxic heavy metals from aqueous medium [12, 17, 18, 19]. In addition, heavy metal removal from different samples by natural adsorbents using adsorption is in the most appropriate technique, and the use of natural adsorbents has been the preferred choice for many researchers [20, 21]. In large number of studies, activated carbon, carbon nanotubes, clays, nanosized metal oxides, zeolites, and various biosorbents were used. However, statistical and optimization research using RSM with CCD or Box-Behnken design about heavy metal removal under various physicochemical parameters is restricted and very rare. Although numerous studies are in literature about heavy metal removal sorption using different materials, there are very little studies with the application of WWT using methodological approach. Classical and conventional methods cannot depict all factor combinations, which affect the experiment. At the same time, these methods take a lot of time to experiment for the determination of the optimum levels. Limitations can be eliminated using a statistical experimental design, which is optimizing all the effecting parameters collectively. In order for modeling of process parameters, RSM that contains a small number of experiments is widely used in various processes especially in adsorption [22]. Experimental design technique is a suitable tool for developing, improving, and optimizing process and multifactor experiments. It researches the common relationship between various factors for the most favorable conditions of the process, which helps to determine the interactions among optimized parameters [22, 23]. The primary target of RSM is to detect the optimum operational conditions for the system or to detect a region that compensates the operating specifications. The aim of this study was to present heavy metal removal from wastewater using RSM as a statistical technique. After discussion of wastewater treatment techniques as detail, several heavy metal removal methods from industrial wastewater will be presented.
There are two aims of wastewater treatment: firstly to purify wastewater without harming the public health and/or causing other nuisance and secondly to gain energy, nutrients, water, and other valuable resources from wastewater during purification steps.
Contaminated waters contain (Figure 1) various pollutants such as nutrients, various chemical compounds, and numerous pathogenic microorganisms besides toxic compounds. Inorganic solids, organic solids, and pathogenic microorganisms along with metals constitute a significant part of wastewater. While inorganic solids include salt, sediment, soil, and especially metals, organic solids contain food wastes, paper, and another household waste material. During WWT step, the removal of primarily organic particles especially suspended solids is vital prior to discharge to the environment. The proteins, lipids and carbohydrates are biodegradable components of wastewater. Biodegradable components contain carbon, and they can be converted to carbon dioxide. If these biodegradable organics are not removed from the wastewater, oxygen demand will exert in the receiving watercourse. Biochemical oxygen demand (BOD) or chemical oxygen demand (COD) is typical measures of organic matter. BOD is the most widely used parameter to quantify organic pollution of water. BOD is the measurement of the dissolved oxygen that is used by microbes in the chemical oxidation of organic matter.
Typical wastewater composition.
It is important to understand the nature of water pollutants because wastewaters contain a large number of pollutants; however, toxicity is observed when the acceptable limits are exceeded. Wastewater contents depend on industrial, agricultural, and municipal wastewater. There are various water pollutants in nature, and they can be categorized as microbiological, radioactive, particulate, organic, and inorganic chemical contaminants. Harmful microbes such as viruses, fungi, bacteria, algae, plankton, and other microorganisms are basic components of bio-pollution in the water. These microorganisms may be responsible for various diseases. Organic toxic pollutants include many insecticides such as dichlorodiphenyltrichloroethane, herbicides, and other pollutants were manufactured for use in various industries. However, heavy metals are the most common inorganic water pollutants. Microbiological, radioactive, particulate, organic, and inorganic chemical water contaminants remain either in suspended, colloidal, or in solvated form.
Because of the increasing population and rapid pollution of water resources, WWT and reuse are an important issue. The efficient use of existing water resources and treatment of polluted water resources with affordable and cheap technologies have been the focus of scientists. WWTs are needed for three reasons; these are water source reduction, WWT, and recycling. Recently, during purification step, while primary treatment includes preliminary physical and chemical purification processes, secondary treatment depends on biochemical decomposition of organic solids to inorganic or stable organic solids. Finally, after the third step called tertiary treatment processes, wastewater is converted into good-quality water, and it can be used for drinking or medicinal supplies. At the end of this step, almost all of the pollutants (up to 99%) can be removed from water. To producing good-quality and safe water, all these three processes should be combined together. Otherwise, it will not be possible to obtain safe water from the wastewater. Many advanced methods and techniques have been used for the recycle of safe water from wastewater, but economic and effective water treatment is still a serious problem. Treatment of wastewater and recycling technologies have been classified (Figure 2), and it is carried out in three stages. They are:
Primary treatment methods
Secondary treatment methods
Tertiary treatment methods
Wastewater treatment and recycling methods.
These methods are briefly described below.
In order to remove organic matter and suspended solids from wastewater by means of physical operations, for example, sedimentation and gravity separation, they are done in primary treatment stage. Preliminary treatment, which is described as preparation for secondary treatment, is in fact intended to produce a liquid waste suitable for biological treatment.
Screening separation method is used to remove solid wastes from wastewater. It is the process where suspended and floating materials including wood, paper, kitchen refuse, pieces of cloth, cork, hair, fibers, and fecal solids are removed from wastewater. In a WWT, screening is generally used as the first operation step. For this purpose, various size screens are used, and their size is selected as per the requirement. Finer particles such as sand and small pebbles can be eliminated by using screening separation method.
About 0.1–0.5 mm pore size is used in filtration separation method, water is passed through a medium having fine pores, and the filtration process is completed. Various membranes and filters, for example, cartridges, can frequently be used to remove suspended solids, greases, oils, and bacteria from the wastewater. The main purpose of filtration separation method is to separate the small solids and remove oil (they can be reduced up to 99%). Filtered water is used for many purposes such as ion exchange, adsorption, or membrane separation processes. In pharmaceutical and biotechnological industries, to the production of pure water, filtration separation method has become the main focus as promising separation tool for WWT. The used membrane has a key role due to selectivity, low fouling, and performance stability for long-term operation in the filtration separation method. Because of these advantages, this method and its performance are becoming more and more important. In addition, it is one of the important enrichment techniques for trace heavy metal ions along with simplicity and rapidity of the procedure. For all these reasons, many scientists have focused on this subject to develop and use alternative and effective membranes [24, 25].
This method is provided for separating components of a fluid or solid particles, but it is used especially for suspend solid from wastewater. Various types of centrifugal machines have been used to remove suspended noncolloidal solids in the centrifugal separation method. To separate solids from wastewater, centrifugal devices with various sizes are used. Density of suspended solids is the most important parameter when separating solid materials by centrifugation. In addition, oils and greases can be reduced and separated during application of centrifugal separation method.
Sedimentation and gravity separation method are based on the removal of suspended solids, grits, and silts from aqueous media. Suspended solid materials settle down to the bottom of the tank under the influence of gravity; this event may vary depending on solid size and density. Some chemicals can sometimes be added to accelerate sedimentation process. Although this method can reduce suspended solids only up to 60%, purification of wastes is a very useful separation application. Water treatment in this technique can be used in many areas such as water for membrane filtration processes and ion exchange method. It is generally applied out prior to conventional treatment.
Coagulation processes are a particularly effective cleaning method for containing oil-in-water emulsions such as sea, lakes, and rivers besides most industrial wastes contain especially oil or petroleum. After sedimentation and gravity separation method, if there are non-settleable solids in wastewater, this is called processing coagulation with the addition of certain chemicals to precipitate these non-settleable solids and non-precipitating deposits. There are some natural coagulants such as aluminum salts, iron materials, alum, starch, and activated silica and also some polymers that can be used as coagulants. In this process, the most important controlling factors are contact time, temperature, and pH. In addition, during biological treatment processes, to remove microbes and any organics in the water, some certain coagulants can be added. Coagulation processes play an important role in recycling and removing pollutants from wastewater.
In order to remove suspended solid including oils, greases, biological solids, and other solids from wastewater, flotation separation method is used. In these processes, suspended solids are removed by adhering them with either air or gas. Various chemicals like alum and activated silica are used to successfully apply the flotation process to wastewater because they help flotation separation method. For paper and refinery industries, flotation separation method is an effective method for WWT because suspended solids that oil and grease is can easily be removed (up to 75–99%) by these processes. Recently, to separate mixed plastic is too difficult using gravity separation; therefore, for WWT and recycling purposes, plastic flotation method has been used as effectively [26].
Secondary treatment techniques have been used to remove soluble and insoluble pollutants from wastewater as biological. The main objective of this process is to convert the organic and inorganic solids into fluorinated residues that are finely divided and dissolved in the wastewater and to remove of soluble and colloidal organics and suspended solids besides reducing BOD and COD through biological process. When water has a high microbe concentration like bacterial and fungal strains, secondary treatment techniques should be selected for treatment because organic matter is converted into other products via these microbes; besides, they detoxify toxic inorganic matter. After this process is applied to wastewater, toxic organic and inorganic substances can be removed [27].
In biological treatment processes, organic matter can be biodegradable by aerobic and facultative bacteria. Aerobic processes depend on temperature, the oxygen amount and availability of oxygen, and the biological activities of the bacteria. If bacterial growth is accelerated by adding some chemicals to the medium, the organic pollutant oxidation rate as biological will also be increased. Aerobic treatment techniques are the most effective method for removing suspended, volatile, and dissolved organics, nitrates, and phosphates besides BOD and COD. Because of the production of a huge amount of biosolids, aerobic treatment techniques have a big disadvantage; however, the biodegradable organic amount can be reduced substantially (up to 90%) using this method.
Anaerobic decomposition, called putrefaction, occurs when free dissolved oxygen is not present in wastewater, and this process is called as anaerobic treatment technique. In this treatment technique, organic matters convert into other organics including sulfur and carbon by anaerobic and facultative bacteria. There are two metabolic phases named acidogenic phase and methanogenic phase in the anaerobic separation technique. Some gases such as methane, hydrogen sulfide, ammonia, and nitrogen can be released. To reduce the biological load of wastewater, this method is very vital [1].
For the production of safe water that people can consume, tertiary water treatment techniques are very important, and they should be applied to wastewater. In this last step, wastewater is subjected to final treatment using some vital techniques, and they are briefly summarized below.
The distillation method is based on the principle that the water is evaporated to the boiling point and the steam is distilled by cooling. After this process, purified water can be obtained free from impurities up to 99% in addition to wastewater is also freed from the volatile pollution. The obtained water by the distillation method is usable in levels of laboratory applications and medicinal preparations. In addition, to prepare potable water from the sea, distillation separation method is an effective tool.
The crystallization method, which is based on the increasing principle of the concentrations of pollutants up to the crystallization point, is an effective method for obtaining quality water. Crystallization technique is useful to remove high concentrations of total dissolved solids including soluble organics and inorganics from wastewater, and it can be created either by mixing some solvents or by evaporation. This process is generally used for wastewater released to the environment from paper and dying industries. In addition, crystallization can be used for pH control because of other constituents including sulfite bicarbonate [1].
When compared to other techniques, evaporation separation method is a natural process and suitable method but only for small wastewater volumes due to its high-energy consumption. However, this technique has some problems such as pollution, calcification, and foaming that have occurred in the presence of suspended solids and carbonates in the wastewater. Thus, to increase the evaporation rate and to reduce energy consumption, vacuum evaporation step can be used. Under natural conditions, water surface molecules escape from the surface, and they generally collected pure water. Recently, to recycle water process, mechanical evaporators and sometimes vacuum evaporation have also been used. Using evaporation separation technique is effective for the removal of pollutants including organic and inorganic compounds, but some volatile organic compounds may recirculate into the water during the evaporation phase. Evaporation treatment technique is applicated to various industry wastewaters like pharmaceutical, petroleum, and fertilizer industries. The obtained water from evaporation treatment technique has been used for different purposes including cooling in towers and boilers [28].
Solvent extraction separation method is an important tool to dissolve pollutants from wastewater using various organic solvents like phosphoric acid. Acetone, methanol, hexane, ethanol, and acetonitrile are the most commonly used organic solvents. In this technique, some organic solvents are added to the wastewater to facilitate contaminant removal. The technique is very effective to remove oils, greases, and various organics. However, the process is often used for extraction and separation of heavy metals like lead, cobalt, and chromium using extraction and separation techniques from various industrial wastewater and effluents [29].
To remove various toxic and hazardous chemicals especially endocrine-disrupting chemicals from wastewater, chemical oxidation techniques are preferred, and it is a promising technology for the treatment of wastewaters containing pharmaceuticals products. Organic compounds that are oxidized by oxidation of readily degradable species such as alcohols and carboxylic acids are the main components of this process [30]. Ozone, hydrogen peroxide, and Fenton’s reagent are commonly used as chemical oxidation reagent. The chemical oxidation rate depends on some variables such as the presence of catalyst, temperature, and pH. Also, pollutants and nature of oxidants identify the rate of chemical oxidation. Various organic pollutants including hydrocarbons, dyes, and phenols can be removed from wastewater using chemical oxidation treatment technique. Recently, there has been a continuously increasing worldwide concern for the development of alternative wastewater reuse and recycling methods. Single oxidation separation method can sometimes be inadequate for the total decomposition of organic contaminants in wastewater. This requires advanced oxidation processes, which involve the use of more than one oxidation process at the same time [31]. Summarize, advanced oxidation process has big advantage because in this process all organic contaminants can be commonly oxidized to carbon dioxide form.
The precipitation method based on the principle that the solubility of the contaminants is reduced and the precipitates which are converted into the solid form are easily separated from the water surface is an effective method for removing metal ions and various organic contaminants from wastewater. Chemical precipitation is a physicochemical process and a very flexible approach to various pollutant removals and can be applied at several stages during wastewater treatment. In industrial applications, precipitation has been the most common technology for metals [32]. In this process, to reduce solubility of the dissolved pollutants, it can be carried out either by lowering the temperature of the water or by adding some chemicals like sodium bicarbonates and ferric chloride, but chemical addition is not preferred because it increases the cost. Common applications of precipitation separation method are wastewater treatment from chromium and nickel plating industries and water recycling besides water softening and removal phosphate from water.
Ion exchange technique provides advantages due to it being technologically simple and enables efficient removal of even traces of impurities from solutions, high treatment capacity, high-removal efficiency, and fast kinetics when compared other usual methods. It can be applicable to various industrial wastewaters to remove hazardous materials. Ion exchange treatment technique depends on toxic or undesirable ions, which are replaced with others ions. There are two types of ion exchangers, which can be classified as cation and anion exchangers. Ion exchangers are natural or synthetic resins with active sites on their surface. Synthetic resins are widely preferred because of their effectiveness in removing heavy metals from wastewater [33]. In order to remove hazardous ions from wastewater, some resins including zeolites, sodium silicates, and acrylic and metha-acrylic resins are used as the most common. Reversible process and low-energy requirements are the most important advantages of this method. Using this method, organic and inorganic pollutants can be reduced about by 95%, but pretreatment may be needed if the wastewater contains oil or grease.
Recently, from the industrial sources, a large amount of oily wastewaters has been generated. The most serious pollutants are oil-in-water emulsions because of treatment cost and ineffective of using treatment methods [34]. Using micro-filtration, a suspended solid pollutant that is a particle size from 0.04 to 1 mm can be removed. Microfiltration separation technique has been widely used to remove macromolecules, emulsion droplets, suspended particles, and microorganisms from various industrial fields including food, pharmaceutical, biotechnological, and petrochemical. In the last decade, membrane separations have been developed using various organic/inorganic membranes like ceramic membranes. It is becoming a promising technology for industrial processes and is utilized currently for oil field-produced WWT. When compared to traditional treatment methods, they have some advantages including high oil removal efficiency, low-energy cost, and compact design. Perhaps the most important advantage is that it does not require any chemicals. Some materials such as cellulose, fiberglass, and cotton can be used as filters in filtration method. Recently, several researchers focused on the new inorganic membrane development, for example, natural mineral-based ceramic membranes, carbon membrane, and zeolite membrane [35].
As membrane technology has been developed, membrane filtration mechanism became a feasible option for wastewaters. Reverse osmosis treatment technique that is called as hyperfiltration is the wastewater purification system that relies on the membranes’ development technology. Using membrane filtration mechanism has shown results of very high efficiency in the filtration of wastewater. According to various studies from literature, when it is used, removal percentage has been achieved as at least 99.9% for COD, total organic carbon, suspended solids, coliforms, and pathogens. To achieve the required filtration, various membranes including cellulose, polyether, and polyamide are used in this process. In this process, the most important parameter is free energy, and other considerable parameters can be identified as pressure, pH, and operation time. To remove the soluble pollutants which contain macro- and microlevel nonpolar, ionic and toxic materials from the wastewater reverse osmosis is a very suitable separation technique. Reverse osmosis treatment technique is the most economic process because the water obtained from this process is of ultrapure water. It can be used in pharmacy and medicines because it can remove various microbes, bacteria, and viruses at high percentages (up to 99.99%) when compared other techniques [36].
Electrolysis method based on the redox reaction principle can be expressed as the separation and deposition of the dissolving materials on an electrode surface. During electrolysis separation method, metal ions are deposited on the electrode and separated from the wastewater. In the last decade, electrochemical oxidation methods have been an increasing interest because they can be applicable to WWT. In this process, various electrodes and anodes such as iron electrode, boron-doped diamond electrode, PbO2 electrode, and graphite electrode [37, 38] have been used to remove different pollutants from wastewater.
To remove various ions and other pollutants which have serious impact on the environment from wastewater, several methods have been used. Electrodialysis technique may be one of the most effective methods among these techniques because of recent progress in membrane technology. Electrodialysis, which is a membrane separation technology, depends on an electric potential difference, which is used to drive ion migration toward oppositely charged electrodes. In this process, under the influence of electric current, water-soluble ions pass through the membranes that are made of ion exchange material [39]. Certain factors, for example, nature of pollutants, applied current amount, temperature, and pH, must be kept in mind to remove dissolved solids. This method has been used to produce potable water from brackish water and for water source reduction [40].
Adsorption separation method is an attractive process because it can be easily applied to WWT, which includes efficiency and flexibility. When it is compared with other treatment methods, it appears superior than others. Some factors that affect adsorption efficiency including the type of adsorbents, pollutant concentration, adsorbent particle size, pH, contact time, and temperature are very important for this process. A pretreatment may be needed to successfully apply the adsorption technique to wastewater because of the presence of suspended particles and oils. To remove pollutants especially heavy metals from wastewater, various adsorbents such as activated carbons from different materials [41, 42], Astragalus [19], carbon nanotubes [43], and a large number of biosorbents [44] have been used by different studies in the literature. However, novel and effective adsorbents with local availability besides economic suitability are still needed. Adsorption technique has two main problems: the first is the regeneration of columns and column life used as an adsorbent and the second is the management of the exhausted adsorbent.
Nowadays, because of rapid technological development especially in developing countries, environmental pollution is a serious problem for the ecosystem because wastewaters contaminated with toxic heavy metals are discharged directly or indirectly into the environment. Unlike most organic contaminants, heavy metals including As, Hg, and Cr are hazardous due to its nonbiodegradable nature [33, 45]. Thus, to protect the people and the environment, these hazardous ions should be removed from wastewater [46]. For example, while industrial wastewaters which contain Cr ions range from 0.5 to 270 mg L−1, inland surface water tolerance limits 0.1 mg L−1, and potable water Cr level should not exceed 0.05 mg L−1 according to various health organization such as the WHO and EPA [47, 48]. To remove heavy metal ions from wastewater, many conventional techniques such as membrane filtration, reverse osmosis, ion exchange, chemical precipitation, electrodialysis, electrochemical treatment, and adsorption have been employed. While most of these methods suffer from operational costs for the treatment process and high capital, the adsorption method is better than the other methods due to its flexibility in design, simplicity of operation, and facile handling, and it is considered more efficient and economical [45, 49]. Since the dynamic characteristics of the adsorption process are complex, it is essential to have optimum working conditions in order to achieve optimum pollution removal efficiency. Process optimization is crucial to determine design parameters value, which is achieving the optimal obtained response level. The RSM is one of the most used methods because of its developing, improving, and optimizing of the processes especially in the presence of complex interactions. It is also used to determine the ideal points of independent variables that are effective under optimum conditions and to evaluate the interactions of these variables [50]. Its greatest advantage is the decreased experimental trial number required to interpret multiple parameters. Therefore, RSM optimization process contains three main steps: (a) appropriate experimental design selection, (b) model coefficient estimation using analysis of variance (ANOVA), and (c) model validation based on prediction and experimental runs of the process response validation of the final model [51]. This experimental design method for an adsorption process is more practical than other approaches because it allows for the opportunity to monitor and interpret interactions between variables and to describe the overall effect of the parameters on the process. The RSM has been successfully used; in addition, its greatest applications have been in industrial research [52].
There are numerous studies, and different results were obtained using various adsorbents reported such as by Anupama et al. [53]. They used a CCD with RSM for removing Cr(VI) from aqueous medium [53]. They investigated the effect of some parameters including pH and temperature on adsorption, and the optimum pH, time, and adsorbent dose were found to be 2.32, 25.76 min, and 1.79 g L−1. Also various adsorption kinetic models and isotherms were compared to find fit model. Jain et al. [54] studied Cr(VI) removal from aqueous solution using Box-Behnken model with combined RSM approach by chemically treated Helianthus annuus flowers. They investigated three effective factors for Cr(VI) removal. It was reported that the optimum pH, adsorbent dose, and initial concentration of Cr(VI) were found to be 2.0, 5.0 g L−1, and 40 mg L−1, respectively [54]. In an another study [55], Box-Behnken design has been applied to evaluate operating variables interaction for Cr (VI), Ni (II), and Zn (II) ions adsorption on Bacillus brevis. They carried out a total of 17 experiments and used a quadratic model. Based on this model, it was reported that the regression equation coefficients were calculated, and the data fitted to a second-order polynomial equation for these metal ions removal with immobilized on B. brevis. According to another study, to evaluate and optimize Cr ions, adsorption on activated carbon experimental conditions using RSM as an efficient approach for predictive model building was performed by Sahu et al. [56]. A full factorial CCD was employed, and based on ANOVA, a high coefficient (R2 = 0.928) was obtained. In addition, satisfactory prediction of second-order regression model was derived. According to optimized process parameters, Cr(VI) removal percentage was obtained higher than 89% [56]. Kaplan Ince et al. [57] studied a batch experimental system for removal Pb(II) using clay, and optimized experimental approach was applied to some alcoholic beverages including beer and wine samples. Various effective parameters were investigated using a Box-Behnken experimental design methodology and RSM. They reported that the optimal conditions used for Pb(II) removal were pH of 5, contact time of 31 minutes, 75 mg for adsorbent dosage, and 100 rpm for agitation speed. Based on these results, maximum Pb(II) ion removal was calculated as 120 mg g−1 from aqueous medium using an ETAAS [57]. Balan et al. (2009) examined the efficiency of Cd(II) removal from aqueous solutions using sphagnum moss peat as biosorbent. They carried out a CCD for experimental design to evaluate an analysis of results and to optimize process parameters including the pH of solution, biosorbent dosage, and Cd(II) initial concentration. The optimum values of experimental parameters were obtained as 4.72 for pH, 14.7 g L−1 for biosorbent amount, and 13.64 mg Cd L−1 for initial concentration of Cd(II) [58]. In another study, removal of Cr(VI) from simulated wastewater using RSM was examined by Bhatti et al. [59]. They investigated the performance of a laboratory scale electrocoagulation system for the removal of Cr(VI) using Al-Al electrodes. They obtained an interaction between voltage × time and amperage × time coefficient of determination as 0.8873 and 0.9270, respectively. For the optimization of process variables including pH, voltage, and treatment time, the RSM was used. Prediction model results were validated through laboratory scale batch experiments [59]. In another similar study, to remove arsenic from contaminated water by arsenite, an electrocoagulation method with stainless steel electrode was used. A response surface methodology approach was performed to optimize significant process variables such as treatment time and solution pH. They obtained pH as 5.2, treatment time ¼ 20 min for 10, and 55–100 mg L−1 of initial arsenic concentration. It was stated that the waste elimination with electrocoagulation is a sustainable treatment technology with quick start-up, shorter treatment time, and minimum sludge generation [60]. An alginate-coated chitosan nanoparticle was carried out for heavy metal removal from industrial effluents by Esmaeili and Khoshnevisan [61]. To optimize the process of biomass for heavy metal removal from synthetic and industrial effluents containing nickel, an RSM approach was performed. Under optimum experimental conditions, which they obtained as a dose of 0.3 g biomass, pH of 3, 70 mg L−1 of initial concentration nickel, and 30 min contact time, maximum removal efficiency of biomass was found as 94.48% [61]. The Cd removal from wastewater and simulated aqueous solution was examined by Iqbal et al. [62] using a polyurethane material as adsorbent. The effect of operating parameters including adsorbent dosage, pH of solution, and metal ion concentration was modeled by RSM combined with CCD. Experimental runs and independent variables optimum values for Cd adsorption were obtained as 305 mg L−1 Cd ion initial concentration, pH 4.9, contact time 932 min, and adsorbent dose 1.3 g for polyurethane material. Based on the experimental results, to predict the response with good accuracy and reliability, it was mentioned that the RSM proved to be the best statistical model [62]. Ince and Kaplan Ince [63] examined the removal of Cr from industrial wastewater using RSM combined with CCD besides investigated as an efficient approach for examining predictive model building and optimization. To predictive regression models and optimize experimental variables, statistical design was modeled. The experimental parameters such as pH and agitation speed were selected for optimization. They obtained ideal Cr ion removal conditions as pH of 5.0, contact time 23.0 minutes, adsorbent dosage of 69.4 mg, and agitation speed of 135 rpm. The Cr removal efficiency was found at 23.16 mg g−1. Also, significant independent parameters and their interactions were verified by means of the ANOVA. The proposed adsorption process was applied to various industrial wastewaters. It was stated that a CCD method was identified to yield a maximum Cr ion removal of 99% [63].
The choice of method to be used in the treatment of water/wastewater depends on the wastewater type and its composition besides the economic aspect. For example, high-grade contaminated water containing solid waste and poor color must be subjected to tertiary water treatment after primary and secondary water treatment processes. If the water does not contain any solids and is contaminated by other contaminants including inorganic, organic, and biological pollutants, the application of the tertiary treatment technique is sufficient. While surface waters are often polluted by organic, inorganic, and biologic pollutants, secondary and tertiary methods of treatment are needed in the treatment of these waters, and only tertiary methods of treatment should be used since groundwater is exposed to hazardous metal ions and anion pollution. The present study summarized removing heavy metal ions in various industrial wastewaters exposed to heavy metal pollution and was focused on optimizing the removal method and determining optimum experimental conditions.
The authors declare that they have no conflicts of interest in the research.
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