Metal ion vs. log βMY values.
\r\n\tCongenital hearing loss means hearing loss that is present at birth. I have managed children with hearing loss for many years, and the most touching thing is the light that blooms on the face while the hearing-impaired child heard his mother's voice at first time. The scene of "happy tears" impressed me so much. To hear the voice that has not been heard is so pleasant, as if this ordinary listening experience is a supreme listening enjoyment.
\r\n\r\n\tAge-related hearing loss means a progressive loss of ability to hear high frequencies with aging, also known as presbycusis. Among them are the influence of internal and external factors such as genes, drugs and noise exposure. The studies pointed out that the brain stimulation of the hearing-impaired person is greatly reduced compared with subjects with normal hearing. The connection of auditory cortex and other brain areas has declined a lot, which is probably one of the important causes of dementia or even depression in the elderly.
\r\n\r\n\tNoise-induced hearing loss is hearing impairment resulting from exposure to loud sound. There is actually continuous and endless noise in many workplaces, which may cause chronic and cumulative damage. Some young people often work hard but easily neglect to protect themselves. In addition, in recent years, entertainment noise (such as nightclubs, concerts, and personal listening devices) has caused hearing impairment in young people. These should be avoidable and preventable.
\r\n\r\n\tHearing Science is the study of impaired auditory perception, the technologies and other rehabilitation strategies for persons with hearing loss. Public health has been defined as "the science and art of preventing disease", improving quality of life through organized efforts. To avoid the “epidemic” of hearing loss, it is necessary to promote early screening, use hearing protection, and change public attitudes toward noise.
\r\n\r\n\tBased on these concepts, the book incorporates updated developments as well as future perspectives in the ever-expanding field of hearing loss. Besides, it is also a great reference for audiologists, otolaryngologists, neurologists, specialists in public health, basic and clinical researchers.
",isbn:"978-1-83968-678-8",printIsbn:"978-1-83968-677-1",pdfIsbn:"978-1-83968-679-5",doi:null,price:0,priceEur:0,priceUsd:0,slug:null,numberOfPages:0,isOpenForSubmission:!1,hash:"a4b7dbb02ba00e7412422cd5dbffa029",bookSignature:"Dr. Tang-Chuan Wang",publishedDate:null,coverURL:"https://cdn.intechopen.com/books/images_new/10529.jpg",keywords:"Hidden Hearing Loss, Plasticity, Electrophysiology, Otoacoustic Emission, Newborn Hearing Screening, Genetics, Aging, Hearing Aids, Noise Exposure, Occupational Hearing Loss, Epidemiology, Prevention",numberOfDownloads:null,numberOfWosCitations:0,numberOfCrossrefCitations:null,numberOfDimensionsCitations:null,numberOfTotalCitations:null,isAvailableForWebshopOrdering:!0,dateEndFirstStepPublish:"September 3rd 2020",dateEndSecondStepPublish:"October 1st 2020",dateEndThirdStepPublish:"November 30th 2020",dateEndFourthStepPublish:"February 18th 2021",dateEndFifthStepPublish:"April 19th 2021",remainingDaysToSecondStep:"3 months",secondStepPassed:!0,currentStepOfPublishingProcess:4,editedByType:null,kuFlag:!1,biosketch:"Dr. Tang-Chuan Wang is an excellent otolaryngologist-head and neck surgeon in Taiwan; a research scholar of Harvard Medical School and University of Iowa Hospitals. He worked in the Hospital of the University of Pennsylvania, Boston Children's Hospital, and Massachusetts Eye and Ear. Due to his contribution to biomedical engineering, he was invited into the executive committee of HIWIN-CMU Joint R & D Center in Taiwan.",coeditorOneBiosketch:null,coeditorTwoBiosketch:null,coeditorThreeBiosketch:null,coeditorFourBiosketch:null,coeditorFiveBiosketch:null,editors:[{id:"201262",title:"Dr.",name:"Tang-Chuan",middleName:null,surname:"Wang",slug:"tang-chuan-wang",fullName:"Tang-Chuan Wang",profilePictureURL:"https://mts.intechopen.com/storage/users/201262/images/system/201262.gif",biography:'Dr. Tang-Chuan Wang is an excellent otolaryngologist – head and neck surgeon in Taiwan. He is also a research scholar of Harvard Medical School and University of Iowa Hospitals. During his substantial experience, he worked in Hospital of the University of Pennsylvania, Boston Children\'s Hospital and Massachusetts Eye and Ear. Besides, he is not only working hard on clinical & basic medicine but also launching out into public health in Taiwan. In recent years, he devotes himself to innovation. He always says that "in theoretical or practical aspects, no innovation is a step backward". Due to his contribution to biomedical engineering, he was invited into executive committee of HIWIN-CMU Joint R & D Center in Taiwan.',institutionString:"China Medical University Hospital",position:null,outsideEditionCount:0,totalCites:0,totalAuthoredChapters:"2",totalChapterViews:"0",totalEditedBooks:"2",institution:{name:"China Medical University Hospital",institutionURL:null,country:{name:"Taiwan"}}}],coeditorOne:null,coeditorTwo:null,coeditorThree:null,coeditorFour:null,coeditorFive:null,topics:[{id:"16",title:"Medicine",slug:"medicine"}],chapters:null,productType:{id:"1",title:"Edited Volume",chapterContentType:"chapter",authoredCaption:"Edited by"},personalPublishingAssistant:{id:"252211",firstName:"Sara",lastName:"Debeuc",middleName:null,title:"Ms.",imageUrl:"https://mts.intechopen.com/storage/users/252211/images/7239_n.png",email:"sara.d@intechopen.com",biography:"As an Author Service Manager my responsibilities include monitoring and facilitating all publishing activities for authors and editors. From chapter submission and review, to approval and revision, copyediting and design, until final publication, I work closely with authors and editors to ensure a simple and easy publishing process. I maintain constant and effective communication with authors, editors and reviewers, which allows for a level of personal support that enables contributors to fully commit and concentrate on the chapters they are writing, editing, or reviewing. I assist authors in the preparation of their full chapter submissions and track important deadlines and ensure they are met. I help to coordinate internal processes such as linguistic review, and monitor the technical aspects of the process. As an ASM I am also involved in the acquisition of 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"}}]},chapter:{item:{type:"chapter",id:"41892",title:"Extracellular Electron Transfer in in situ Petroleum Hydrocarbon Bioremediation",doi:"10.5772/53290",slug:"extracellular-electron-transfer-in-in-situ-petroleum-hydrocarbon-bioremediation",body:'Anthropogenic contamination of soil, subsurface and surface waters and atmosphere with toxic organic chemicals is an environmental issue of world wide concern. While energy and goods production from fossil hydrocarbon sources is one of the driving factors of global economy, the often adverse environmental effects of exploration, production, transport and processing of crude and shale oils, tar sands, coal and natural gas seldom come into the focus of attention.
Chemically, the term hydrocarbons encompasses a large variety of compounds. Saturated and unsaturated structures composed of hydrogen and carbon make up the largest fraction of organic compounds derived from fossil sources, including crude oil, its derivatives and distillates as well tar oils originating from coal gasification. These are often referred to as petroleum hydrocarbons (Cermak, 2010), also encompassing mono- and polycyclic aromatic structures of environmental and toxicological concern (Loibner et al., 2004).
In the US alone, between 5,000 and 7,000 spills of hazardous material occur every year (Scholz et al., 1998). Although the US-EPA’s Spill Prevention Control and Countermeasure Program for inland oil spills has reduced the amount of undesired hydrocarbon release to less than 1% of the total volume of oil handled each year (US-EPA, 2012), considerable surface areas and contaminated ground water bodies remain to be decontaminated. Microbial, physical and chemical remediation strategies are available.
For soil and water bodies contaminated with organic pollutants, bioremediation refers to the engineered exploitation of an ecosystem’s intrinsic capacity for the attenuation of adverse biogeochemical influences. In in situ remediation soil, water and contamination remain in place[1] -. These processes are driven by a large variety of microorganisms, including bacteria, fungi and archaea (microbial bioremediation) and may also involve plants (phytoremediation). The targeted addition of enriched or modified microorganisms (bioaugmentation) is justified in case the resident microbial population is incapable or incapacitated to perform substantial contaminant transformation or, more importantly, mineralisation. In contrast, the naturally present microbial population at a long-term contaminated site is expected to have the potential metabolic capacity for the degradation of aromatic and aliphatic petroleum hydrocarbons under both aerobic and anaerobic conditions (Widdel and Rabus, 2001). In biostimulation efforts, growth factors are administered to the contaminated area, including electron acceptors or donors for biochemical contaminant oxidation or reduction, respectively, and / or of micro-and macronutrients, vitamins and trace elements and others (Bartha, 1986; Cerniglia, 1992). A variety of strategies is in application or under development.
Implementing in situ bioremediation is an intricate task for designing and executive engineers, while decreasing the burden to economy and environment in lessening the need for soil excavation. Some strategies of physical, chemical and also microbial remediation involve harsh and cost-intensive measures effecting substantial changes to the biogeochemical conditions in the treated environments. Thus, the optimization of existing strategies and the development of novel approaches to deal with environmental pollution in sustainable ways is required both from economic and environmental viewpoints.
This review is dedicated to elucidating mechanisms for extracellular electron transfer to geogenic electron acceptors, which can potentially be coupled to oxidative hydrocarbon detoxification. An increase in the accessibility of naturally present electron accepting compounds to hydrocarbon-degrading bacteria could substantially improve bioremediation efficacy of anaerobic petroleum hydrocarbon contaminated aquifers.
Petroleum hydrocarbons such as normal, branched and cycloalkanes and aromatic hydrocarbons are degradable in situ via biochemical oxidation both under aerobic and anaerobic conditions (DeLaune et al., 1980), provided the degradative activity is not inhibited by a lack of nutrients (nitrogen, phosphor, potassium), electron acceptors, trace elements and moisture, of if pH, temperature, salinity and contaminant concentrations are outside certain boundaries (Bartha, 1986; DeLaune et al., 1980). In contrast, hydrocarbons carrying strongly electron-withdrawing substituents are preferentially degraded via contaminant reduction, such as sequential reductive dechlorination for chlorinated solvents (Maymo-Gatell et al., 1999).
A large range of microorganisms have the metabolic capability of oxidative hydrocarbon degradation using a variety of terminal electron acceptors (TEA) in the vadose and saturated zones of the subsurface (Heider et al., 1998).
In the range of naturally occurring TEAs, molecular oxygen yields the largest amount of energy in terms of ATP production and is associated with high degradation rates with aerobic microbial communities. Molecular oxygen, however, has a low aqueous solubility (up to 10 mg/L). Fast oxygen depletion in microbial contaminant oxidation processes is opposed to slow diffusive replenishing in the groundwater. Thus, petroleum hydrocarbon contaminated aquifers are often depleted of molecular oxygen, while other, energetically less favourable TEA are reduced more slowly. Oxygen limitation is thus manifested in low natural contaminant attenuation rates, and anaerobic conditions are often encountered at biologically active contaminated sites.
This has prompted the development of strategies for the stimulation of anaerobic hydrocarbon degradation. One approach is the addition of naturally occurring, well-soluble alternative terminal electron acceptors such as nitrate and sulphate (Hasinger & Scherr et al., 2012; Bregnard et al., 1998). Hydrocarbon oxidation coupled to nitrate reduction, however, yields approximately one tenth of the biochemical energy of aerobic degradation per mole hydrocarbon, and sulphate reduction about 1%. Both terminal electron accepting processes are associated with significantly lower degradation rates compared to aerobic degradation (Widdel and Rabus, 2001). The use of well soluble and less chemically reactive alternative electron acceptors, however, is connected to an increase in influence radii when introduced into the aquifer (Hasinger & Scherr et al., 2012).
Considerable amounts of alternative TEA, however, are required for aquifer remediation. On a theoretical mass basis, around 5 g of nitrate and sulphate are needed for the anaerobic mineralisation of 1 g of n-hexadecane, a mid-weight petroleum hydrocarbon, although lower demands were observed in practice (Hasinger & Scherr et al., 2012), are required. In addition, possible by-products of alternative TEA consumption, including products from incomplete nitrate reduction and sulphide precipitation, can render the implementation of these strategies under field conditions an intricate task.
Despite their poor aqueous solubility[1] -, and thus originally somewhat surprising, naturally abundant solid-phase minerals are known to play a quantitatively important role in subsurface microbial reduction and oxidation[1] - processes (Weber et al., 2006). Poorly soluble alternative\n\t\t\t\t\tTEA refers mainly to iron (Fe) and manganese (Mn) minerals in their tri- and tetravalent oxidation states (Lovley et al., 2004). Their oxides, hydroxides and oxyhydroxides are amongst the most abundant redox-active elements on the earth’s surface.
This comes surprising since solubility in water is deemed a prerequisite for chemicals to participate in microbial transformation processes. The participation of poorly soluble minerals in environmental redox-processes, however, is made possible by a variety of bacterial strategies to access and transfer electrons to (and from) poorly soluble mineral surfaces. These strategies include the secretion of chelators, production of electrically conductive protein structures and the participation of redox active natural organic moieties, or electron shuttles.
These processes are summarized as mechanisms for extracellular electron transfer, and are the subject of this review.
Poorly soluble, naturally abundant alternative electron acceptors have the potential to be incorporated into efficient bioremediation strategies. The abundance of reducible iron and manganese minerals in the subsurface, may, provided these minerals can be involved into microbial hydrocarbon oxidation, alleviate the need to artificially introduce electron acceptors, such as molecular oxygen, nitrate and sulphate.
There is evidence that iron and manganese respiration can be coupled to anaerobic contaminant oxidation of monoaromatic compounds (Kunapuli et al., 2008; Lovley et al., 1989). Under field conditions, however, pronounced iron reduction coupled to the oxidation of higher molecular weight hydrocarbons is seldom reported (Aichberger et al., 2007). These data, however, indicate that hydrocarbon oxidation coupled to solid-state mineral respiration is possible, however at limited rates. The rates of Mn and Fe respiration, on the other hand, can be significantly increased via the enhancement of extracellular transport mechanisms. Geobacter sulfurreducens, a model dissimilatory metal reducing bacterium (DMRB) transfers electrons 27 times faster to a transient electron storage, an electron shuttle or mediator, than to iron hydroxide (Jiang and Kappler, 2008). The introduction of such electron transfer facilitators is one of the key issues where extracellular electron transport mechanisms could be incorporated into petroleum hydrocarbon bioremediation. Extracellular electron shuttles are reversibly reduced and oxidized in many subsequent cycles. Thus, only substoichiometric amounts would theoretically be required to achieve substantially increased degradation rates.
Provided a comprehensive understanding of the underlying mechanisms, efficient bioremediation strategies may be devised to enhance extracellular electron transfer from polluting organic substances to poorly soluble mineral surfaces, adjoined by oxidative pollutant transformation and detoxification. The mechanisms of electron shuttling and other extracellular electron transfer strategies devised by DRMBs, thermodynamic considerations and microorganisms involved are reviewed in Chapters 3 through 8.
In summary, possible strategies to enhance anaerobic petroleum hydrocarbon degradation using naturally occurring, poorly soluble TEA include the addition of electron shuttling compounds (ES) of natural or anthropogenic origin, as was demonstrated for the anaerobic oxidation of dichloroethene (Scherr et al., 2011) before, to contaminated aquifers. The addition or targeted stimulation of microorganisms equipped with the metabolic capability to transfer electrons to mineral surfaces, via secretion of chelators and microbial ES or expression of conductive protein appendages, such as Geobacter or Shewanella, represents another approach.
A tentative collection of bioremediation strategies incorporating extracellular electron transfer for petroleum hydrocarbon contaminated aquifer remediation is provided in Chapter 9.
Different reactions can be mediated between participants of subsurface redox-processes via extracellular electron transfer, involving microbes as well as soluble and insoluble, organic and inorganic electron acceptors and donors. Extracellular electron transport is required in case the electron donor, electron acceptor or both can not be taken up by the bacterial cell, be it due to poor solubility, or if a direct contact between the microbe and electron sources or sinks is not possible, e.g. due to occlusion in pore spaces too small for microorganisms to enter. Possible extracellular pathways include, besides direct contact between cell and mineral surfaces:
Secretion of chelators for, e.g. Fe(III) and Mn(IV) minerals
Electron shuttling, i.e. reversible electron uptake and release by redox-active organic compounds excreted by microbes or in the natural background
Expression of electrically conductive, pilus-like bacterial assemblages, also termed nano-wires
These will be discussed in more detail in Chapter 5.
Electrons may be transported extracellularly between any of the parties participating in redox-processes: (i) microbes and poorly soluble bulk (ii) sinks and (iii) sources of electrons. Where either electron donor or acceptor are well soluble, usually no extracellular transport pathway is required. Figure 1 shows four possible flow paths or routes between the parties. Where extracellular transport is required, is indicated by solid lines. Dashed lines represent pathways for well-soluble participants.
Electron route A: electron donor to microbe
Via route A electrons are transported from an electron source to a microbe, a typical half-reaction pathway for the oxidation of zero-valent iron (ZVI). In case soluble polluting and non-polluting organic compounds (petroleum hydrocarbons, acetate and others) yield electrons, no extracellular pathway is required (dashed lines).
Route B: microbe to electron acceptor
In the oxidation of petroleum hydrocarbons coupled to iron or manganese reduction, the microbe gains energy from donating the electron obtained directly from the pollutant donor (dashed line to green microbe) to the energetically favourable, poorly soluble terminal electron acceptor (route B).
In case the terminal, inorganic electron acceptor is insoluble, as in the case for Fe(III) and Mn(IV) minerals, and/or no direct contact between cellular and electron accepting surface can be established, electron transfer rates can significantly be increased via extracellular electron transport vehicles (Jiang and Kappler, 2008). This can be aided by the production of electrically conductive pilus-like assemblages, also termed microbial nanowires (Reguera et al., 2005), electron shuttles or metal chelating agents chelators, as depicted in Figure 1.
Electron routes in biogeochemical redox-processes: extracellular electron transport (A) from an electron source (donor) to microbe, (B) from microbe to electron sink (acceptor), (C) between microbes and (D) abiotic source/sink exchange. Solid lines indicate extracellular electron transport, dashed lines represent direct transport, usually via uptake of donor or acceptor. HS = humic substances, VC = vinyl chloride.
Route C: electron transport between microbes
Shuttling of electrons between microbes is represented by route C. Electron shuttles have been hypothesized to syntrophically link diverse organisms in nature (Lovley et al., 2004). This may occur, for example, between outer and inner layers of a biofilm (Lies et al., 2005). Intraspecies electron transfer has been observed with a naphthoquinone moiety as shuttle in the gastrointestinal tract (Yamazaki et al., 1999); its functionality in the subsurface has not been investigated yet.
Route D: electron transfer between abiotic parties
Moreover, electron shuttles may participate in purely abiotic electron transfer reactions between electron donors and acceptors (route D in Figure 1).
Route D symbolises the abiotic reduction of metals by microbially reduced humic substances (HS), in case the HS-oxidation process is literally terminal. In case of microbial ’regeneration’, i.e. re-oxidation of HS, it serves as an electron shuttle in route B. Route D also describes abiotic chlorophenol degradation via ZVI, mediated by the presence of humic and fulvic acids (Kang and Choi, 2009), or shuttle-mediated abiotic reductive dechlorination mechanisms (Lee and Batchelor, 2002). This route also refers to the ability of natural organic matter to shuttle between oxidized and reduced forms of mercury that has only recently been recognized (Gu et al., 2011; Zheng et al., 2012).
Electrons need to be associated to a vector to participate in reactions beyond the immediate bacterial surface, i.e for travelling distances exceeding 0.01 μm (Gorby et al., 2006; Gray and Winkler, 1996). Soluble electron acceptors commonly used for petroleum hydrocarbon degradation include oxygen, nitrate and sulphate. These compounds can easily diffuse into the cell where they are reduced. Unlike these, the prevalent forms of iron and manganese in the environment are insoluble and remain extracellular, rendering the direct contact of cell and mineral surfaces an apparent necessity for respiration. This is contrasted by the ability of a number of microorganisms for dissimilatory reduction of undissolved mineral oxy(hydr)oxides, entailing electron transfer over distances of 50 μm and up. As an example the reduction of physically trapped or occluded iron(III) crystals was observed (Lies et al., 2005; Nevin and Lovley, 2002b). Different strategies enabling for long-distance electron transfer have been identified; they will be discussed in Chapter 5.
Microbes capable of dissimilatory metal reduction (DRMB) are phylogenetically dispersed throughout the Bacteria and Archaea; the most prominent members belong to Geobacter, Shewanella and Geothrix (see Chapter 7).
Iron, comprising approximately 3.5% per mass of the earth’s crust (Martin and Meybeck, 1979; Reimann and de Caritat, 2012) is the most abundant redox-active metal in the modern environment. While the main iron fraction is located in silica structures, approximately one third of iron atoms are present as amorphous or crystalline oxides in surface rocks (Canfield, 1997). At a circumneutral pH, iron is present primarily as quasi-insoluble minerals in di-valent ferrous (Fe(II), e.g. wüstite) trivalent ferric (Fe(III), e.g. goethite) or mixed-valence (Fe(II,II), e.g. magnetite) oxides, hydroxides and oxihydroxides (Scheffer and Schachtschabel, 2002).
Quasi-insoluble refers here to log K values of around -40 for goethite, ferric hydroxide and hematite (Morel and Hering, 1993). In the face of the poor solubility of iron oxides, it may come surprising that iron redox cycling in soil and sedimentary environments is now assumed to be dominated by microbial action (Arrieta and Grez, 1971; Chaudhuri et al., 2001; Weber et al., 2006). In fact, Fe(III) represents one of the most important terminal electron acceptors for microbes under anaerobic conditions (Lovley et al., 2004; Weber et al., 2006).
Iron, in its various oxidized or reduced states, has a high versatility in energy-creating processes in oxygen-rich and suboxic environments, as an electron donor for iron-oxidizing microbes under oxic and anoxic conditions as well as a terminal electron acceptor under anoxic conditions, and also influences other biogeochemical cycles. Microbial iron redox-cycling, also termed the iron redox wheel, is mechanistically connected to carbon, nitrogen, phosphorous and sulphur redox cycling (Li et al., 2012).
While poorly crystalline oxides appear to be readily metabolized (Phillips et al., 1993), the microbial utilization of highly crystalline oxides as electron acceptors in a (real) soil or sediment environment appears to be thermodynamically unfavourable (Thamdrup, 2000).
Mineral-bound manganese is significantly less abundant than iron in our environment, contributing approximately 0.07% by mass to surface rocks (Martin and Meybeck, 1979; Reimann and de Caritat, 2012).
The main fraction of manganese in the earth’s surface, roughly 75%, is present as oxides (Canfield, 1997) with the remainder located in silicates. Minerals contain varying amounts of both Mn(III) and Mn(IV), with poorly crystalline manganite, vernadite and birnessite minerals as most abundant minerals (Friedl et al., 1997; Zhu et al., 2012). Similarly to iron cycling, biotic manganese oxidation and reduction are commonly encountered processes in soils and sediments. It appears that the mechanisms for manganese respiration and oxidation are similar to those for iron, as most organisms that can respire Fe can also respire Mn and vice versa (Lovley et al., 2004; Shi et al., 2007).
Following reduction, both Fe(II) and Mn(II) are re-oxidized biologically or chemically rather quickly. The thermodynamics of Fe(II) regeneration appear more favourable than those for Mn(II) oxidation. Adding to this, Mn(II) has higher aqueous solubility than Fe(II), rendering microbe-driven post-depositional redistribution by reductive dissolution and transport from anoxic to oxic interfaces, where precipitation takes place, a main mechanism governing manganese localization (Thamdrup, 2000). Every atom may participate in subsequent reduction / oxidation cycles as many as 100 times (Thamdrup, 2000).
Besides iron and manganese, also polluting metals can be involved into bacterially mediated oxidation and reduction processes. These approaches can be used for bioremediation and the facilitated recovery of metals from waste water streams due to precipitation (Lovley and Phillips, 1992).
Uranium reduction
Some microbes can also reduce uranium (Cologgi et al., 2011; Lovley and Phillips, 1992; Lovley et al., 1991) at rates comparable to those of Fe(III) reduction (Lovley and Phillips, 1992). In contrast to manganese and iron, uranium precipitates during reduction from its hexavalent to its tetravalent form. The simultaneous precipitation of uranium with the reduction of Fe(III) has been observed for Geobacter sulfurreducens (Cologgi et al., 2011), leading to the assumption that similar mechanisms are governing both processes.
Reduction of other elements by DRMB
Similarly, Shewanella has been observed to precipitate reduced chrome (Cr(III)), likely involving the same enzymes as for Fe(III) reduction (Belchik et al., 2011). It can be hypothesised that the expression of extracellular electron transport pathways for soluble, oxidized electron acceptors precipitating during reduction, such as chrome and uranium was intended to keep the precipitates from forming inside the cells.
Dissimilatory iron-reducing bacteria were also found to participate in the reduction of technetium (Istok et al., 2004), cobalt, gold (Kashefi et al., 2001) and other metals, metalloids and radionuclides (Lloyd, 2003) as well as graphene oxide reduction (Jiao et al., 2011). These abilities may be used for the recovery or removal of metal resources from waste water streams or in situ remediation.
The ability to grow on these metals as sole electron acceptors has not been adequately demonstrated. Reducible iron and manganese minerals, however, will quantitatively dominate in most contaminated environments over heavy metals (Lovley et al., 2004) as terminal electron acceptors.
In this chapter, mechanisms devised by DMRB for accessing poorly soluble electron acceptors are reviewed. Most of our understanding of these processes is derived from investigations of dissimilatory iron and manganese reduction under anaerobic conditions. Geobacter and Shewanella are considered as model organisms in this respect (cf. Chapter 7).
In the course of microbial energy generation, electrons are transferred from an electron source to a sink, usually inside a cell. DMRB, in contrast, are faced with the problem of how to effectively access an electron acceptor that can not diffuse to the cell. In case the electron donor or acceptor do not permeate through the membrane, as in the case of insoluble metals, or where the presence of redox-products inside the cell is not desired, such as for uranium precipitates, cells can rely on active processes to access poorly soluble TEAs or utilize extracellular electron shuttles. These are naturally present or anthropogenically administered compounds that have the potential to facilitate extracellular electron transfer.
Several mechanisms to overcome the physiological challenges for the dissimilatory reduction of poorly soluble electron acceptors have been identified.
Direct electron transport
One route for extracellular electron transfer is the direct transport of electrons between the cellular envelope and electron accepting mineral surfaces, however requiring direct contact between cell and mineral surfaces. In these reactions, electrons have been found to travel over distances of 10 to 15 Å (Kerisit et al., 2007).
This direct transfer is mediated by outer-membrane cytochromes directly to the mineral’s surface, and are typical of Gram-negative bacteria such as Geobacter and Shewanella (Hernandez and Newman, 2001). Geobacter and also Shewanella are representatives of a family of ferric iron reducers that were found in a variety of anaerobic environments (more about them in Chapter 7). The involved multihaem c-type cytochromes are essential electron-transferring proteins, rendering the journey of respiratory proteins from trough the cell to the outer membrane possible (Myers and Myers, 1992; Shi et al., 2007) and forming a respiratory chain extended beyond the cell’s physiological periphery.
Directly nano-wired\n\t\t\t\t
Both Shewanella and Geobacter were observed to form pilus-like assemblages extending to the mineral surface when grown on insoluble electron acceptors, representing a special case of direct transport.
Geobacter was reported to produce monolateral pili to access iron and manganese oxides physically, i.e. via chemotaxis (Childers et al., 2002). Mineral reductases are located on the outer cell membrane but also on electrically conductive pilus-like assemblages prominently termed nano-wires (see Figure 1). These conductive protein nanostructures are ‘wiring’ the cell and the mineral phase, enabling for electron transport via a physical extension of the cell, thus outsourcing direct contact between cell and mineral surface to thin conductive appendages. Evidence for the production of nano-wires has been reported for both Shewanella (Gorby et al., 2006) and Geobacter (Reguera et al., 2005), however with distinct molecular mechanisms for the electron transport.
Different strategies are required for iron reducing microorganisms lacking conductive surface cytochromes, requiring for alternative strategies for extracellular electron transfer.
Electron shuttles are small molecules capable of undergoing repeated reduction and oxidation processes. An electron shuttle can be an organic or inorganic compound that is reversibly redox active and has the right redox potential, i.e. poised between the E0’ of the reductant and the oxidant. It serves as an electron acceptor and, once reduced, can itself transfer electrons to other organic or inorganic electron acceptors, whereupon it becomes re-oxidized. Such a shuttle provides a mechanism for an indirect reduction or oxidation process. In principle, a single shuttle molecule could cycle thousands of times and, thus, have a significant effect on the turnover of the terminal acceptor in a given environment.
Extracellular electron shuttles may be produced by microorganisms, originate from the humification of plant and animal matter or be anthropogenically added. While most electron shuttles are organic, heterocyclic aromatic structures, theoretically also inorganic, nonenzymatic compounds such as sulphide and reduced uranium, and organic compounds equipped with sulfhydryl groups (Nevin and Lovley, 2000) may serve as electron shuttles themselves.
In Figure 2 the redox-active groups of frequently encountered microbial, natural and anthropogenic electron shuttles are displayed. Heterocyclic aromatic compounds with redox-active functional groups, with di-ketone moieties in quinones, and nitrogen for phenazines and viologen, are common structures. Isoalloxazine represents the reactive structure in flavins.
Main redox-active structures found in extracellular electron shuttles
In case the electron acceptor is physically detached from the cell, e.g. oxides occluded in pores, direct contact is not possible for a large fraction of bacteria, e.g. in biofilms, or the microbe lacks the enzymatic toolkit to respire electron acceptors, electrons are encouraged to take alternative routes through extracellular space by using transport facilities provided by the microorganism itself, such as electron shuttles and / or chelating agents.
The production of electron transport mediators is known for many microbes. Beside melanin, produced by Shewanella alga (Turick et al., 2002), phenazine derivatives (Hernandez et al., 2004), flavins (von Canstein et al., 2008), quinone-type structures (Nevin and Lovley, 2002a) and possibly also siderophores (Fennessey et al., 2010; Kouzuma et al., 2012) were shown to enhance anaerobic ferric iron respiration.
Phenazine and its derivatives
Phenazines are small redox-active molecules secreted by bacteria and functionally similar to those of anthraquinones. They are, due to their antibiotic effect, believed to be part of the organisms’ biological warfare arsenal (Chin-A-Woeng et al., 2003; Hernandez and Newman, 2001). They are able to reductively dissolve iron and manganese (Hernandez et al., 2004; Wang and Newman, 2008), and are applied in microbial fuel cells (Pham et al., 2008). Phenazines are, however, of toxicological concern, such as pyocyanin, a blue pigment excreted by Pseudomonas aeruginosa, rendering the application of phenazines in environmental biotechnology an intricate task.
Flavins
Flavins have a midpoint potential of -0.2 to -0.25 V and can reduce most iron oxides and soluble forms of ferric iron, contributing to metal acquisition in plants and yeasts (Marsili et al., 2008). They have been proven to mediate the reduction of iron hydroxides inaccessible to microbes, via penetration through pores too small to allow for microbial passage (Nevin and Lovley, 2002b). Their isoalloxazine group acts as metal chelator.
The production of iron-solubilizing ligands has first been observed for Shewanella\n\t\t\t\t\t\tputrefaciens (Taillefert et al., 2007), which were shortly afterwards simultaneously identified as flavins by von Canstein and Marsili (Marsili et al., 2008; von Canstein et al., 2008). Also other γ-proteobacteria and Marinobacter are able to produce flavins, which may be used or pirated by other organisms directly (Hirano et al., 2003; von Canstein et al., 2008) and from flavin-coated surfaces (Marsili et al., 2008). The role of flavins in electron shuttling has recently been reviewed (Brutinel and Gralnick, 2012).
Side note: flavin or not flavin
Flavins were identified to have a major role in iron respiration of Shewanella oneidensis. In this context, both riboflavin (RF, Vitamin B2) and flavin mononucleotide (FMN) were detected in iron-respiring Shewanella cultures. Flavins are now well-studied (Brutinel and Gralnick, 2012; Marsili et al., 2008; von Canstein et al., 2008) examples of endogenously synthesized electron transporting compounds deployed to donate respiratory electrons to a distant mineral phase. Here, electrons are conveyed to extracellular flavins via cytochromes that are re-oxidized upon direct contact to Fe(III), resulting in increased electron transfer rates than at direct contact to the mineral surface (Brutinel and Gralnick, 2012).
Experimental evidence for the functionality of flavin-mediated enhanced iron respiration was manifested in the presence of flavins in the supernatant of Shewanella reducing poorly soluble Fe(III). A stoichiometric ratio of 1 (FMN oxidized) to 2 (Fe(II) produced) was noted (von Canstein et al., 2008). The nature of the force bringing these compounds into the outer cell space remains disputed in that FMN and RF may either be actively secreted (Marsili et al., 2008; von Canstein et al., 2008) or merely be released during cell lysis as part of the common intracellular enzymatic mix. In this context, the former hypothesis is corroborated by different concentrations and species of intra- and extracellular flavins detected in the supernatant and the proportionality of extracellular flavin concentrations to live rather than dead cell density (von Canstein et al., 2008). These arguments were, in turn, challenged recently (Richter et al., 2012).
Siderophores
Siderophores are low molecular weight compounds produced by bacteria, fungi and plants when exposed to low-iron stress conditions, and are very strong Fe(III) chelating agents. Most of them contain catechol or hydroxamate groups as iron-chelating groups (Neilands, 1995). In forming Fe(III)-siderophore complexes, they are scavenging Fe(III) from the environment, rendering it more accessible for bacterial uptake. Its role in increasing metal respiration is, to our knowledge, uncertain.
Although siderophores were found to catalyze the mineralisation of carbon tetrachloride, it is not known whether microorganisms can gain energy from the process (Lee et al., 1999). Recent research suggests indirect effects of siderophores on respiration in increasing iron uptake, which is in turn required for the synthesis of cytochromes for manganese respiration (Kouzuma et al., 2012), while others indicate no effect of siderophore-related iron solubilisation on its respiration (Fennessey et al., 2010). It appears likely that iron-containing shuttles participate in iron solubilizing mechanisms, where the cation remains with the redox-active centre, thus corresponding to permanent rather than transient iron complexation.
For a large variety of naturally occurring redox-active compounds, at least some functionality as extracellular electron shuttles was described (Guo et al., 2012; O’Loughlin, 2008; Van der Zee and Cervantes, 2009; Watanabe et al., 2009; Wolf et al., 2009). Main functional structures are depicted in Figure 2. Frequently encountered shuttles encompass the following:
Quinones, including
anthraquinones, their sulfonates, carboxylic acids and chlorinated moieties
juglone,
lawsone,
menadione and other naphthoquinones
ubiquinone, coenzyme Q10
Humic substances
Phenazines cores, including
neutral red
methylene blue
Vitamine cores, including
corrin
isoalloxazine
Indigo sulfonates and other derivatives
Viologens, including
methyl viologen
benzyl viologen
Cysteine and other thiol-containing molecules
Amongst these, the role of quinones and humic substances are discussed in more detail below.
Structure\n\t\t\t\t\t
Chemically, quinones are fully conjugated cyclic di-ketone structures derived from the oxidation of aromatic compounds, where an even number of –CH= groups is converted into –C(=O) groups. This is adjoined by a re-arrangement of double bonds and connected to the loss of aromaticity. Quinones may also be considered as products from polyphenol reduction (Figure 3). Quinones are stable throughout a number of repeated oxidation and reduction processes. In their redox-cycling, one- and two-electron reduction reactions lead to the formation of free semiquinone radicals and hydroquinones, as displayed in Figure 3 (Michaelis and Schubert, 1938).
Occurrence\n\t\t\t\t\t
Quinones are abundant structures in nature, participating in a wide variety of reactions in pro- and eukaryotic organisms. Membrane-bound ubiquinones and menaquinones shuttle electrons between respiratory protein complexes in bacteria. Phylloquinone participates in the electron transfer chain of in photosynthesis. Other quinones may have antimicrobial (Lown, 1983; Zhou et al., 2012) or even toxic (Shang et al., 2012) effects, such as juglone. Isoalloxazine, the reactive structure in flavins, is also a quinone (Muller et al., 1981).
Role of quinones in electron shuttling
The hydroquinone moiety chemically reduces Fe(III) to Fe(II) while it is simultaneously oxidized to a quinone (Scott et al., 1998). In turn, it is reduced back to hydroquinone by microbes deriving electrons from hydrocarbon oxidation and other processes. This is a type B reaction (Figure 1), with the electron donor being a hydrocarbon and the acceptor an insoluble mineral, such as a Fe(III) or Mn(IV) mineral. Although the energy gain from quinone reduction is lower than from direct reduction of Fe(III) minerals, the use of extracellular shuttles enables for significantly increased reduction rates, i.e. a higher bacterial energy gain per unit time (Jiang and Kappler, 2008).
Simple quinoid structures: 1,4- or para-benzoquinone (left), its semiquinone (center) and its hydroquinone (right)
The manifestation of a type A route would reflect electron shuttling from an organic electron donor to a halogenated or nitrous contaminant.
In summary, electron shuttles participate in facilitating the electron transfer from the microbe towards the electron acceptor in the type B reaction and from electron donor to microbe for sulphide, reduced metals or the cathode for a type A reaction; in both contexts, quinones are often encountered.
Iron-reducing microorganisms are able to grow on extracellular quinones as the sole source of energy. Humic substance-derived quinones are possible the most important natural source of extracellular shuttles for Fe(III) reducers (Lovley et al., 1996a; Scott et al., 1998).
In many studies, anthraquinone disulfonate (AQDS) was used as a model humic substance (Aulenta et al., 2010; Bhushan et al., 2006; Collins and Picardal, 1999; Kwon and Finneran, 2006; Kwon and Finneran, 2008; Scherr et al., 2011) to study extracellular electron transfer in the reduction of halogenated and nitrous organic compounds.
Humic substances (HS) are a heterogeneous family of dark-coloured, biologically largely refractory compounds originating from the incomplete biodegradation of plant, animal and microbial debris. They account for up to 80% of soil and sedimentary organic carbon (Schnitzer, 1989). Humic substances consist of structurally diverse macromolecules, containing a variety of functional groups including hydroxy- and carboxygroups as well as aromatic and aliphatic structures that can be resolved using infrared and magnetic resonance spectroscopy, amongst others (Ehlers and Loibner, 2006; Nguyen et al., 1991).
On a more general scale, the classification of humic substances is commonly performed based on isolation procedures rather than on molecular characteristics, usually by their acid (in)solubility, with brown to black humic acids (HA) defined as insoluble at a pH <2, with yellow-coloured fulvic acids (FA) completely soluble and humins insoluble both under all pH conditions (Scheffer and Schachtschabel, 2002).
Multiple interactions occur between the organic and inorganic soil matter fractions, including encapsulation (Baldock and Skjemstad, 2000), formation of adducts with organic contaminants (Karickhoff et al., 1979), chelation of metals (Arrieta and Grez, 1971; Gu et al., 2011; Lovley et al., 1996b), and as terminal and subterminal electron acceptors for abiotic and biotic reduction processes (Benz et al., 1998; Jiang and Kappler, 2008; McCarthy and Jimenez, 1985), amongst others. The multiple functions of humic substances have been proposed to be incorporated in different remediation technologies for contaminated sites, including bioremediation, reactive barriers and in situ immobilization (Perminova and Hatfield, 2005).
In terms of contributing to extracellular electron transfer enhancing contaminant transfer, humic substances play multiple roles.
Humic acids as chelators\n\t\t\t\t\t
Humic acids were found to increase the concentration of soluble Fe(III) by forming chelates (Lovley et al., 1996b). Chelated Fe(III) has a higher redox potential, rendering Fe(III) reduction more thermodynamically favourable (Thamdrup, 2000) and rendering the cation more accessible to microbes for reduction (Dobbin et al., 1995). It is, however, not clear whether extracellular chelated Fe(III) can be used by the cell directly via surface contact or absorption by the periplasm is required prior to reduction (Haas and Dichristina, 2002; Lovley et al., 1996b).
Terminal electron acceptors versus electron shuttles
Although humic substances are rather refractory to microbial breakdown, microorganisms interact with a variety of their functional groups, including the donation of electrons to reactive humic moieties. The ability to reduce humic substances and iron by bacteria and archaea appears to be closely linked (Benz et al., 1998; Lovley et al., 1998). The nature of electron transfer to humic or fulvic acids may be terminal or subterminal. In the latter case, microbially reduced humic matter abiotically transfers electrons to the Fe(III) surface, being re-oxidized themselves in the process (Lovley et al., 1996a; Lovley et al., 1996b) and thus function as natural, biogenic extracellular electron shuttles (see Figure 1). Reduced humic matter can also be „pirated“ by other microorganisms (Lovley et al., 1999), i.e. those that did not create them (route C in Figure 1) and be used as electron donors for microbially mediated reactions (route A).
Originally, the effect of humic substances in promoting biodegradation of organic pollutants was assumed to be dominated by chelation (Lovley et al., 1996b). Humic substances, however, are only weak chelators of iron. The role of humified organic matter as electron shuttle was first raised in 1989 (Tratnyek and Macalady, 1989) and is also referred to as the Tratnyek-Macalady hypothesis (Sposito, 2011). In fact, the function of fulvic acids of different origin can be explained as an initial contribution by electron shuttling, further enhanced by iron complexation in latter stages of contact (Royer et al., 2002).
Quality and origin of humic matter\n\t\t\t\t\t
Different humic substance fractions and origins have different effects on the rate and extent of extracellular electron shuttling. While there might be no effect at all by neither humic nor fulvic acids (O’Loughlin, 2008), humic acids, on a mass unit base, were found to stimulate iron reduction more strongly stronger than fulvic acids, also in environmentally relevant concentrations, due to thermodynamic mechanisms (Wolf et al., 2009). On the other hand, aquatic humic substances appear to have a higher electron accepting capacity than those derived from terrestrial environments (Royer et al., 2002; Scott et al., 1998). On a general basis, the shuttling capacity of humic matter is determined mainly by with its aromaticity (Aeschbacher et al., 2010; Chen et al., 2003), where carbohydrate fractions were found to be less redox-active than polyphenolic fractions (Chen et al., 2003)
Soil organic matter aromaticity is also one of the main functional groups in determining sorption of petroleum hydrocarbons (Ehlers et al., 2010; Perminova et al., 1999). Surface-adsorbed and freely dissolved humic matter were found to exhibit similar shuttling activities (Wolf et al., 2009).
While the role of humic substances in soil and sediment electron shuttling is unchallenged, the nature of functional structures in humic or fulvic acids conveying the electron shuttling capacity to humic substances is not fully elucidated yet.
Profound influence of humic quinone moieties in conveying shuttling properties to humic substances has been attested by numerous studies (Aeschbacher et al., 2010; Jiang and Kappler, 2008; Kang and Choi, 2009; Perminova et al., 2005; Scott et al., 1998). Quinone moieties such as AQDS are often used as practical and standardized surrogates to model the effect of humic substances in extracellular electron transfer, while representing only the humics’ redox properties, since AQDS can not chelate metals. The relevance of quinones, however, can be demonstrated by comparing shuttling efficacy of humic acid and pure AQDS. Two gram per litre of humic acids had a similar stimulating effect on the reduction of poorly crystalline ferric iron as approximately 40 mg / L AQDS (Lovley et al., 1996a).
In the search for other redox-active groups in humic substances, one model proposed three distinct sites participating in the redox activity of humic and fulvic acids (Ratasuk and Nanny, 2007), including two quinone structures with different electronegative properties plus one non-quinone structure. Evidence was supplied in that humic substances artificially depleted of their aromatic structures were not entirely depleted of their reducing capacity. In a different study, the humic substances’ content of free radicals accounted for only a small fraction of the observed electron equivalents in shuttling between iron and iodine moieties (Struyk and Sposito, 2001).
These studies corroborate the possibility that also non-quinone structures participate in conveying redox-activity to natural organic matter. The nature of these compounds, however, is unknown, while there is general agreement points towards that quinone moieties are the main, however not only redox-active groups in humic substances.
Further understanding can be expected by the computational and procedural determination of quinone redox properties (Aeschbacher et al., 2010; Cape et al., 2006; Guo et al., 2012).
Depending on the stability of the electron shuttling compound, it may undergo multiple cycles of electron take up and release. Thus, concentrations of redox-active moieties in the environment theoretically need to be substoichiometric in comparison to the amount of electron donating hydrocarbon and acceptor.
The oxidation state of a mediator itself also, at least theoretically, plays a role in its efficacy. Thus, an electron shuttle is effective at certain ratios of oxidized : reduced between 1:100 and 100:1 (Meckstroth et al., 1981), although in many studies it is attempted to completely reduce shuttles using harsh methods ahead of experimental use.
Practically, concentrations of pure quinones generally between 0.1 to 100 micromoles are commonly used for shuttling experiments. For some compounds, however, higher concentrations are inhibitory due to antimicrobial effects (Lown, 1983; Wolf et al., 2009). For some of the tested substances tested in a recent study (Wolf et al., 2009), a linear correlation between the normalized reaction rates and the logarithm of their concentrations was observed (humic and fulvic acid and AQDS), while others showed no concentration dependence (carminic acid and alizarin).
There has been criticism that concentrations of humic and fulvic acids commonly employed for experimental shuttling studies would far exceed those relevant under natural conditions, which are in the milligram per litre scale (Wolf et al., 2009), amounting to between 5 and 25 mg C/L (Curtis and Reinhard, 1994; Jiang and Kappler, 2008; O’Loughlin, 2008).
For humic and fulvic acids, enhancing effects on Fe(III) reduction attributed to electron shuttling were observed for concentrations of dissolved fulvic acids as low as 0.61 mg/L and of humic acids of 0.025 mg/L, i.e. at concentrations which do occur under natural conditions (Wolf et al., 2009) such as in pore waters of sediments rich in organic matter.
The ability to shuttle electrons between different organic and inorganic participants in the subsurface is largely dependent on the energies driving the extracellular electron transfer processes in simultaneous oxidation and reduction processes.
The relation of electrochemical potentials of electron donor, shuttle and electron acceptor determine the amount of energy, in terms of ATP generation or free energy (ΔG0′), a microorganism may harvest from the electron transfer. A minimum energy yield of -20kJ/mol is required for ATP production (Schink, 1997). This determines the lower threshold amount of energy required to be transformed in the process of microbial electron transfer from donor to shuttle to allow for ATP synthesis in the cells (Wolf et al., 2009). This first step depends on the potential of the electron donor or, more precisely, of its redox couple, and the shuttle.
In the subsequent, rate-limiting step of electron transport from the shuttle to the electron acceptor, the difference voltage between the mediator and the acceptor couple should be as negative as possible (Wolf et al., 2009). From a theoretical viewpoint, a mediator, assuming a concentration of 1M/L, can efficiently mediate between compounds with a E0’ range of +/- 118 mV of its own midpoint potential (Meckstroth et al., 1981).
These relations should be regarded in the selection of an electron shuttle. Several publications are dedicated to the description of the redox-properties of a wide range of mediators (Bird and Kuhn, 1981; Fultz and Durst, 1982; Meckstroth et al., 1981).
The location and type of the mediator’s functional groups influences its potential. As an example, quinones equipped with an increasing number electron donating groups (e.g. –CH3) become less easily reduced. Redox-potentials (all E0’) decrease from 280 mV for the unalkylated para-benzoquinone to 180 mV for 2,5-p-benzoquinone and to 5mV for the fully alkylated congener (2,3,5,6-tetramethyl-p-benzoquione). On the other hand, electron-withdrawing sulphone groups facilitate reduction, as is reflected by an increasing potential along with increasing sulphonation for anthraquinone (AQ). Here, an increase of the compounds’ potential from -225 mV for AQ-2-sulphonate by almost 40 mV to -184 mV for AQ-2,6-disulphonate is connected to the addition of one sulphone group (Fultz and Durst, 1982). The location of electron donating and accepting groups in respect to the keto-groups also determines the shuttles’ stability in recurring redox-processes (Watanabe et al., 2009).
In practice, however, the translation of thermodynamic principles to complex biogeochemical interactions can seldom be expected to be straightforward. In this case, small-scale inductive effects do not manifest themselves in decisively different shuttling efficacies, however a good correlation of shuttle electrochemical potential to Fe(II) production can be observed (O’Loughlin, 2008; Wolf et al., 2009). Mediation of ferrihydrite reduction with formate as electron donor was found to be most effective for a quinone range with potentials (all E0’) of between -137 mV (2-hydroxynaphtoquinone) to -225 mV for AQS as mentioned above (Wolf et al., 2009). These potentials are closely related to those of flavins excreted by Shewanella, around -215 mV (Marsili et al., 2008; von Canstein et al., 2008). Thermodynamic issues of extracellular electron transfer are discussed elsewhere in more detail (O’Loughlin, 2008; Watanabe et al., 2009; Wolf et al., 2009).
Dissimilatory iron (III) reducing microorganisms are phylogenetically diverse and can be found within the bacteria and archaeal domains. They are able to gain energy from the reduction of Fe(III) on the cost of the oxidation of organic substances or molecular hydrogen. Behind their rather modest designation as iron-reducers the ability to participate in a great variety of biogeochemical processes is hiding. Mechanisms required for iron reduction also negotiate interactions with other organic and inorganic chemical species in the subsurface. Thus, the influence of these organisms and their metabolic products extends far beyond biogeochemical carbon and iron cycling, but also in dissolution of inorganic polluting and non polluting metals, transition metals, micro- and macronutrients and of organic pollutants. Phylogenetically distinct DMRB have different mechanisms to access insoluble electron acceptors. It appears that the ability to reduce these acceptors has evolved in evolution several times (Lovley et al., 2004).
Amongst the variety of involved microorganisms in the terrestrial and aquatic environment, the selection of microbial study objects is somewhat biased by their cultivability as pure cultures under laboratory conditions (Amann et al., 1995). The large diversity of DMRB shall not be underestimated by the selection of few cultivable organisms, which represent less than one percent of total bacterial cell counts in soil and sediments (Jones, 1977; Torsvik et al., 1990).
Beside numerous other organisms capable of dissimilatory metal and transition-metal reduction, including Geothrix (Coates et al., 1999) and reservoir-borne bacteria (Greene et al., 1997; Greene et al., 2009), the most well studied DMRB belong to Geobacter and Shewanella species.
Geobacter are a genus of δ-proteobacteria. They are gram-negative and chemoautotrophic strict anaerobes and were found to dominate iron reducing populations in hydrocarbon-polluted environments over other known Fe(III) reducers such as Shewanella, Geothrix and Variovorax. The latter appear to be unable to couple Fe(III) reduction to pollutant oxidation (Rooney-Varga et al., 1999; Snoeyenbos-West et al., 2000).
Thus, Geobacter species appear to be the primary agents in Fe(III) and Mn(IV) reduction coupled to the oxidation of organic compounds in anoxic terrestrial environments.
The first description of the Geobacter species was published by D. Lovley in 1987 (Lovley et al., 1987) after isolating them from a Potomac river sediment. The species G. metallireducens and sulfurreducens were described in 1993 (Lovley et al., 1993) and 1994 (Caccavo et al., 1994), respectively. Geobacter sulfurreducens can not, in contrast to G. metallireducens, reduce Mn(IV), and can not use alcohols or aromatic compounds as electron donors.
Geobacter can use AQDS as extracellular electron shuttle to increase electron transfer to poorly soluble electron acceptors, and can also precipitate Uranium (VI) (Cologgi et al., 2011). G. metallireducens is chemotactic towards the soluble reduction products (Fe(II) and Mn(II)) of iron and manganese minerals (Childers et al., 2002)).
Nano-wiring and charging\n\t\t\t\t
Geobacter form pilus-like appendages, so-called bacterial nanowires (Reguera et al., 2005), when grown on insoluble Fe(III) and Mn(IV) oxides, to transfer electron to distant and insoluble electron acceptors and electrodes. These pili are electrically conductive. This is independent from c-type cytochromes, which are, in contrast, responsible for terminal connections with the electron accepting surfaces. They are also responsible for microbial ‘capacitation’, which allows Geobacter to extracellularly store electrical charge in times of scarceness of electron acceptors (Lovley et al., 2011).
Members of Shewanella, originally isolated from dairy products and introduced as Achromobacter\n\t\t\t\t\tputrefaciens by Derby & Hammer in 1931, are gram-negative and belong to the class of γ-proteobacteria (Venkateswaran et al., 1999).
They are facultative anaerobic bacteria equipped with a high respiratory versatility, amongst others capable of using oxygen, nitrate, volatile fatty acids and Fe(III), Mn(IV), As(V) and U(VI) as electron sinks (Bencheikh-Latmani et al., 2005; Cruz-García et al., 2007; Lim et al., 2008; Lovley et al., 2004; Myers and Nealson, 1988; Venkateswaran et al., 1999; von Canstein et al., 2008), and supplemented by evidence for Cr(VI) respiration (Bencheikh-Latmani et al., 2005). Extracellular respiration by Shewanella\n\t\t\t\t\toneidensis has also been noted to comprise graphene oxide (Jiao et al., 2011).
Flavins, electron shuttles and nano-wires\n\t\t\t\t
The versatility in using electroactive surfaces to donate electrons is reflected in the use of anodes (Logan, 2009), and also the ability to produce electrically conductive nanowires, similar to Geobacter, themselves (Gorby et al., 2006).
Shewanella are known to excrete quinones (Newman and Kolter, 2000) that carry electrons from the cell surface to the electron acceptor that is located at a distance from the cell. Excreted flavins chelate Fe(III), rendering it more accessible to the cells (von Canstein et al., 2008), thus promoting Fe(III) reduction at a distance.
A variety of easily degradable electron donors can be used for the anaerobic reduction of Fe(III) and Mn(IV), including short chain organic acids, alcohols or sugars (Lovley et al., 1996a). Iron- and manganese reducers are also known to dwell in petroleum reservoirs (Greene et al., 1997; Greene et al., 2009). This suggests that the metabolic pathways for the reduction of poorly soluble electron acceptors are resistant to exposure to potential inhibitors of microbial activity; this is of potential concern since low molecular weight hydrocarbons are known to act as disruptors of biological membranes.
In fact, poorly soluble electron acceptors can also be used for the anaerobic oxidation of less readily degradable organic compounds, including petroleum hydrocarbons. Several studies documented the occurrence of microbial hydrocarbon degradation coupled to the reduction of Fe(III) and Mn(IV) as sole electron acceptors with aquifer and sedimentary materials. Biodegradation of petroleum hydrocarbons under iron- and manganese reducing conditions has been noted primarily for low molecular weight aromatic compounds. This includes benzene (Kunapuli et al., 2008), toluene (Coates et al., 1999; Edwards and Grbic-Galic, 1994; Langenhoff et al., 1997; Lee et al., 2012), ethylbenzene (Siegert et al., 2011) and o-xylene (Edwards and Grbic-Galic, 1994) as well as phenol (Lovley et al., 1989), naphthalene and also liquid alkanes (Siegert et al., 2011).
It can be assumed that natural mechanisms, including chemotaxis, expression of electrically conductive pilus-like assemblages, extracellular chelators and molecular electron shuttles contribute to the occurrence of these processes.
The enhancement of naturally occurring iron and manganese reduction coupled to hydrocarbon oxidation is a promising strategy for petroleum hydrocarbon bioremediation (Lovley, 2011). There is, to our knowledge, only a limited amount of studies where such a strategy was followed. Humic substances and quinones were successfully applied to stimulate the degradation of a variety of aromatic compounds by Cervantes and co-authors, including the increase of toluene oxidation coupled to manganese respiration (Cervantes et al., 2001). Degradation of the fuel additive methyl tert-butyl ether (MTBE) under iron-reducing conditions was stimulated using humic substances (Finneran and Lovley, 2001) as well as quinones for anaerobic toluene oxidation (Evans, 2000). Iron chelation with humic substances increased anaerobic benzene oxidation (Lovley et al., 1996b), however a contribution by electron shutting was not discerned.
In the light of the large variety of potential applications of these processes for petroleum hydrocarbon bioremediation, more research tackling upon a broader variety of priority pollutants will be performed.
The application of electron shuttles as amendments in a variety of biotechnological processes has gained increased attention in the past 20 years. Extracellular electron shuttles have, for example, been shown to enhance the reductive degradation of a large variety of organic contaminants, including azo- and nitro- compounds such as dyes and explosives, of chlorinated hydrocarbons, and to induce the precipitation of metals such as uranium and chromium (reviewed by Field et al., 2000; Hernandez and Newman, 2001; Van der Zee and Cervantes, 2009; Watanabe et al., 2009). Recent developments include the application of novel electron shuttles and the continued investigation to predicting the redox-properties the most complex natural redox mediators, humic substances (Aeschbacher et al., 2010; Kumagai et al., 2012). The exploitation of extracellular electron transfer in anaerobic petroleum hydrocarbon bioremediation is, in contrast, explored to a lesser extent.
The limited availability of electron acceptors constraining oxidative hydrocarbon biodegradation in situ can be circumvented by a variety of measures, including biosparging, the introduction of oxygen releasing compounds or the use of naturally occurring, well soluble alternative electron acceptors such as nitrate or sulphate.
Moreover, the abundance of solid-phase, however microbially accessible electron acceptors in the subsurface offers the potential for a significant decrease in the amount of reagents to be introduced into an aquifer, provided solid-phase electron acceptors can be efficiently involved in oxidative contaminant biodegradation.
The enhancement of microbially mediated mechanisms to enhance the accessibility of Fe(III) and Mn(IV) can be applied in a variety of engineered approaches for the biodegradation of aquifers contaminated with fuel hydrocarbons, tar oils and other contaminants susceptible to oxidative biodegradation.
The list of possible approaches includes the following:
Injection of electron shuttles carefully designed by type and concentration, into contaminated aquifers, to increase the available pool of native or resident electron acceptors such as Fe(III) and Mn(IV) for microbial hydrocarbon oxidation.
Addition of electron shuttles to increase the electron accepting function of pre-existing engineered subsurface structures, such as permeable reactive barriers, gates and reactive zones.
Use of natural or engineered materials slowly releasing electron shuttles by leaching, such as humic substances.
Engineered approaches to increase the density and activity of microbial communities with the intrinsic capability to reduce poorly soluble native sources of electron acceptors, including but not limited to Geobacter species.
These considerations shall mediate research and development of an ever broader variety of potential opportunities to enhance naturally occurring oxidative contaminant degradation under suboxic conditions. The range of available tools for dealing with hydrocarbon contaminated environments may be extended by an additional instrument, the utilization of extracellular electron transport mechanisms designed by nature.
ATPadenosine-5’-triphosphate
AQDSanthraquinone disulfonate
Crchrome
DMRBdissimilatory metal reducing bacteria
ESelectron shuttle
FAfulvic acids
Fe iron
FMNflavin mononucleotide
HAhumic acids
HShumic substances
Mnmanganese
RFriboflavin
TEAterminal electron acceptor
ZVIzero-valent iron
Financial support for this publication was provided by the Austrian Federal Ministry of Agriculture, Forestry, Environment and Water Management and the Government of Upper Austria, Directorate for Environment and Water Management, Division for Environmental Protection.
Stability constant of the formation of metal complexes is used to measure interaction strength of reagents. From this process, metal ion and ligand interaction formed the two types of metal complexes; one is supramolecular complexes known as host-guest complexes [1] and the other is anion-containing complexes. In the solution it provides and calculates the required information about the concentration of metal complexes.
Solubility, light, absorption conductance, partitioning behavior, conductance, and chemical reactivity are the complex characteristics which are different from their components. It is determined by various numerical and graphical methods which calculate the equilibrium constants. This is based on or related to a quantity, and this is called the complex formation function.
During the displacement process at the time of metal complex formation, some ions disappear and form a bonding between metal ions and ligands. It may be considered due to displacement of a proton from a ligand species or ions or molecules causing a drop in the pH values of the solution [2]. Irving and Rossotti developed a technique for the calculation of stability constant, and it is called potentiometric technique.
To determine the stability constant, Bjerrum has used a very simple method, and that is metal salt solubility method. For the studies of a larger different variety of polycarboxylic acid-, oxime-, phenol-containing metal complexes, Martel and Calvin used the potentiometric technique for calculating the stability constant. Those ligands [3, 4] which are uncharged are also examined, and their stability constant calculations are determined by the limitations inherent in the ligand solubility method. The limitations of the metal salt solubility method and the result of solubility methods are compared with this. M-L, MLM, and (M3) L are some types of examples of metal-ligand bonding. One thing is common, and that is these entire types metal complexes all have one ligand.
The solubility method can only usefully be applied to studies of such complexes, and it is best applied for ML; in such types of system, only ML is formed. Jacqueline Gonzalez and his co-worker propose to explore the coordination chemistry of calcium complexes. Jacqueline and et al. followed this technique for evaluate the as partial model of the manganese-calcium cluster and spectrophotometric studies of metal complexes, i.e., they were carried calcium(II)-1,4-butanediamine in acetonitrile and calcium(II)-1,2-ethylendiamine, calcium(II)-1,3-propanediamine by them.
Spectrophotometric programming of HypSpec and received data allows the determination of the formation of solubility constants. The logarithmic values, log β110 = 5.25 for calcium(II)-1,3-propanediamine, log β110 = 4.072 for calcium(II)-1,4-butanediamine, and log β110 = 4.69 for calcium(II)-1,2-ethylendiamine, are obtained for the formation constants [5]. The structure of Cimetidine and histamine H2-receptor is a chelating agent. Syed Ahmad Tirmizi has examined Ni(II) cimetidine complex spectrophotometrically and found an absorption peak maximum of 622 nm with respect to different temperatures.
Syed Ahmad Tirmizi have been used to taken 1:2 ratio of metal and cimetidine compound for the formation of metal complex and this satisfied by molar ratio data. The data, 1.40–2.4 × 108, was calculated using the continuous variation method and stability constant at room temperature, and by using the mole ratio method, this value at 40°C was 1.24–2.4 × 108. In the formation of lead(II) metal complexes with 1-(aminomethyl) cyclohexene, Thanavelan et al. found the formation of their binary and ternary complexes. Glycine,
Using the stability constant method, these ternary complexes were found out, and using the parameters such as Δ log K and log X, these ternary complex data were compared with binary complex. The potentiometric technique at room temperature (25°C) was used in the investigation of some binary complex formations by Abdelatty Mohamed Radalla. These binary complexes are formed with 3D transition metal ions like Cu2+, Ni2+, Co2+, and Zn2+ and gallic acid’s importance as a ligand and 0.10 mol dm−3 of NaNO3. Such types of aliphatic dicarboxylic acids are very important biologically. Many acid-base characters and the nature of using metal complexes have been investigated and discussed time to time by researchers [7].
The above acids (gallic and aliphatic dicarboxylic acid) were taken to determine the acidity constants. For the purpose of determining the stability constant, binary and ternary complexes were carried in the aqueous medium using the experimental conditions as stated above. The potentiometric pH-metric titration curves are inferred for the binary complexes and ternary complexes at different ratios, and formation of ternary metal complex formation was in a stepwise manner that provided an easy way to calculate stability constants for the formation of metal complexes.
The values of Δ log K, percentage of relative stabilization (% R. S.), and log X were evaluated and discussed. Now it provides the outline about the various complex species for the formation of different solvents, and using the concentration distribution, these complexes were evaluated and discussed. The conductivity measurements have ascertained for the mode of ternary chelating complexes.
A study by Kathrina and Pekar suggests that pH plays an important role in the formation of metal complexes. When epigallocatechin gallate and gallic acid combine with copper(II) to form metal complexes, the pH changes its speculation. We have been able to determine its pH in frozen and fluid state with the help of multifrequency EPR spectroscopy [8]. With the help of this spectroscopy, it is able to detect that each polyphenol exhibits the formation of three different mononuclear species. If the pH ranges 4–8 for di- or polymeric complex of Cu(II), then it conjectures such metal complexes. It is only at alkaline pH values.
The line width in fluid solutions by molecular motion exhibits an incomplete average of the parameters of anisotropy spin Hamilton. If the complexes are different, then their rotational correlation times for this also vary. The analysis of the LyCEP anisotropy of the fluid solution spectra is performed using the parameters determined by the simulation of the rigid boundary spectra. Its result suggests that pH increases its value by affecting its molecular mass. It is a polyphenol ligand complex with copper, showing the coordination of an increasing number of its molecules or increasing participation of polyphenol dimers used as ligands in the copper coordination region.
The study by Vishenkova and his co-worker [8] provides the investigation of electrochemical properties of triphenylmethane dyes using a voltammetric method with constant-current potential sweep. Malachite green (MG) and basic fuchsin (BF) have been chosen as representatives of the triphenylmethane dyes [9]. The electrochemical behavior of MG and BF on the surface of a mercury film electrode depending on pH, the nature of background electrolyte, and scan rate of potential sweep has been investigated.
Using a voltammetric method with a constant-current potential sweep examines the electrical properties of triphenylmethane dye. In order to find out the solution of MG and BF, certain registration conditions have been prescribed for it, which have proved to be quite useful. The reduction peak for the currents of MG and BF has demonstrated that it increases linearly with respect to their concentration as 9.0 × 10−5–7.0 × 10−3 mol/dm3 for MG and 6.0 × 10−5–8.0 × 10−3 mol/dm3 for BF and correlation coefficients of these values are 0.9987 for MG and 0.9961 for BF [10].
5.0 × 10−5 and 2.0 × 10−5 mol/dm3 are the values used as the detection limit of MG and BF, respectively. Stability constants are a very useful technique whose size is huge. Due to its usefulness, it has acquired an umbrella right in the fields of chemistry, biology, and medicine. No science subject is untouched by this. Stability constants of metal complexes are widely used in the various areas like pharmaceuticals as well as biological processes, separation techniques, analytical processes, etc. In the presented chapter, we have tried to explain this in detail by focusing our attention on the applications and solutions of stability of metal complexes in solution.
Stability or formation or binding constant is the type of equilibrium constant used for the formation of metal complexes in the solution. Acutely, stability constant is applicable to measure the strength of interactions between the ligands and metal ions that are involved in complex formation in the solution [11]. A generally these 1-4 equations are expressed as the following ways:
Thus
K1, K2, K3, … Kn are the equilibrium constants and these are also called stepwise stability constants. The formation of the metal-ligand-n complex may also be expressed as equilibrium constants by the following steps:
The parameters K and β are related together, and these are expressed in the following example:
Now the numerator and denominator are multiplied together with the use of [metal-ligand] [metal-ligand2], and after the rearranging we get the following equation:
Now we expressed it as the following:
From the above relation, it is clear that the overall stability constant βn is equal to the product of the successive (i.e., stepwise) stability constants, K1, K2, K3,…Kn. This in other words means that the value of stability constants for a given complex is actually made up of a number of stepwise stability constants. The term stability is used without qualification to mean that the complex exists under a suitable condition and that it is possible to store the complex for an appreciable amount of time. The term stability is commonly used because coordination compounds are stable in one reagent but dissociate or dissolve in the presence of another regent. It is also possible that the term stability can be referred as an action of heat or light or compound. The stability of complex [13] is expressed qualitatively in terms of thermodynamic stability and kinetic stability.
In a chemical reaction, chemical equilibrium is a state in which the concentration of reactants and products does not change over time. Often this condition occurs when the speed of forward reaction becomes the same as the speed of reverse reaction. It is worth noting that the velocities of the forward and backward reaction are not zero at this stage but are equal.
If hydrogen and iodine are kept together in molecular proportions in a closed process vessel at high temperature (500°C), the following action begins:
In this activity, hydrogen iodide is formed by combining hydrogen and iodine, and the amount of hydrogen iodide increases with time. In contrast to this action, if the pure hydrogen iodide gas is heated to 500°C in the reaction, the compound is dissolved by reverse action, which causes hydrogen iodide to dissolve into hydrogen and iodine, and the ratio of these products increases over time. This is expressed in the following reaction:
For the formation of metal chelates, the thermodynamic technique provides a very significant information. Thermodynamics is a very useful technique in distinguishing between enthalpic effects and entropic effects. The bond strengths are totally effected by enthalpic effect, and this does not make any difference in the whole solution in order/disorder. Based on thermodynamics the chelate effect below can be best explained. The change of standard Gibbs free energy for equilibrium constant is response:
Where:
R = gas constant
T = absolute temperature
At 25°C,
ΔG = (− 5.708 kJ mol−1) · log β.
The enthalpy term creates free energy, i.e.,
For metal complexes, thermodynamic stability and kinetic stability are two interpretations of the stability constant in the solution. If reaction moves from reactants to products, it refers to a change in its energy as shown in the above equation. But for the reactivity, kinetic stability is responsible for this system, and this refers to ligand species [14].
Stable and unstable are thermodynamic terms, while labile and inert are kinetic terms. As a rule of thumb, those complexes which react completely within about 1 minute at 25°C are considered labile, and those complexes which take longer time than this to react are considered inert. [Ni(CN)4]2− is thermodynamically stable but kinetically inert because it rapidly exchanges ligands.
The metal complexes [Co(NH3)6]3+ and such types of other complexes are kinetically inert, but these are thermodynamically unstable. We may expect the complex to decompose in the presence of acid immediately because the complex is thermodynamically unstable. The rate is of the order of 1025 for the decomposition in acidic solution. Hence, it is thermodynamically unstable. However, nothing happens to the complex when it is kept in acidic solution for several days. While considering the stability of a complex, always the condition must be specified. Under what condition, the complex which is stable or unstable must be specified such as acidic and also basic condition, temperature, reactant, etc.
A complex may be stable with respect to a particular condition but with respect to another. In brief, a stable complex need not be inert and similarly, and an unstable complex need not be labile. It is the measure of extent of formation or transformation of complex under a given set of conditions at equilibrium [15].
Thermodynamic stability has an important role in determining the bond strength between metal ligands. Some complexes are stable, but as soon as they are introduced into aqueous solution, it is seen that these complexes have an effect on stability and fall apart. For an example, we take the [Co (SCN)4]2+ complex. The ion bond of this complex is very weak and breaks down quickly to form other compounds. But when [Fe(CN)6]3− is dissolved in water, it does not test Fe3+ by any sensitive reagent, which shows that this complex is more stable in aqueous solution. So it is indicated that thermodynamic stability deals with metal-ligand bond energy, stability constant, and other thermodynamic parameters.
This example also suggests that thermodynamic stability refers to the stability and instability of complexes. The measurement of the extent to which one type of species is converted to another species can be determined by thermodynamic stability until equilibrium is achieved. For example, tetracyanonickelate is a thermodynamically stable and kinetic labile complex. But the example of hexa-amine cobalt(III) cation is just the opposite:
Thermodynamics is used to express the difference between stability and inertia. For the stable complex, large positive free energies have been obtained from ΔG0 reaction. The ΔH0, standard enthalpy change for this reaction, is related to the equilibrium constant, βn, by the well thermodynamic equation:
For similar complexes of various ions of the same charge of a particular transition series and particular ligand, ΔS0 values would not differ substantially, and hence a change in ΔH0 value would be related to change in βn values. So the order of values of ΔH0 is also the order of the βn value.
Kinetic stability is referred to the rate of reaction between the metal ions and ligand proceeds at equilibrium or used for the formation of metal complexes. To take a decision for kinetic stability of any complexes, time is a factor which plays an important role for this. It deals between the rate of reaction and what is the mechanism of this metal complex reaction.
As we discuss above in thermodynamic stability, kinetic stability is referred for the complexes at which complex is inert or labile. The term “inert” was used by Tube for the thermally stable complex and for reactive complexes the term ‘labile’ used [16]. The naturally occurring chlorophyll is the example of polydentate ligand. This complex is extremely inert due to exchange of Mg2+ ion in the aqueous media.
The nature of central atom of metal complexes, dimension, its degree of oxidation, electronic structure of these complexes, and so many other properties of complexes are affected by the stability constant. Some of the following factors described are as follows.
In the coordination chemistry, metal complexes are formed by the interaction between metal ions and ligands. For these type of compounds, metal ions are the coordination center, and the ligand or complexing agents are oriented surrounding it. These metal ions mostly are the transition elements. For the determination of stability constant, some important characteristics of these metal complexes may be as given below.
Ligands are oriented around the central metal ions in the metal complexes. The sizes of these metal ions determine the number of ligand species that will be attached or ordinated (dative covalent) in the bond formation. If the sizes of these metal ions are increased, the stability of coordination compound defiantly decreased. Zn(II) metal ions are the central atoms in their complexes, and due to their lower size (0.74A°) as compared to Cd(II) size (0.97A°), metal ions are formed more stable.
Hence, Al3+ ion has the greatest nuclear charge, but its size is the smallest, and the ion N3− has the smallest nuclear charge, and its size is the largest [17]. Inert atoms like neon do not participate in the formation of the covalent or ionic compound, and these atoms are not included in isoelectronic series; hence, it is not easy to measure the radius of this type of atoms.
The properties of stability depend on the size of the metal ion used in the complexes and the total charge thereon. If the size of these metal ions is small and the total charge is high, then their complexes will be more stable. That is, their ratio will depend on the charge/radius. This can be demonstrated through the following reaction:
An ionic charge is the electric charge of an ion which is formed by the gain (negative charge) or loss (positive charge) of one or more electrons from an atom or group of atoms. If we talk about the stability of the coordination compounds, we find that the total charge of their central metal ions affects their stability, so when we change their charge, their stability in a range of constant can be determined by propagating of error [18]. If the charge of the central metal ion is high and the size is small, the stability of the compound is high:
In general, the most stable coordination bonds can cause smaller and highly charged rations to form more stable coordination compounds.
When an electron pair attracts a central ion toward itself, a strong stability complex is formed, and this is due to electron donation from ligand → metal ion. This donation process is increasing the bond stability of metal complexes exerted the polarizing effect on certain metal ions. Li+, Na+, Mg2+, Ca2+, Al3+, etc. are such type of metal cation which is not able to attract so strongly from a highly electronegative containing stable complexes, and these atoms are O, N, F, Au, Hg, Ag, Pd, Pt, and Pb. Such type of ligands that contains P, S, As, Br and I atom are formed stable complex because these accepts electron from M → π-bonding. Hg2+, Pb2+, Cd2+, and Bi3+ metal ions are also electronegative ions which form insoluble salts of metal sulfide which are insoluble in aqueous medium.
Volatile ligands may be lost at higher temperature. This is exemplified by the loss of water by hydrates and ammonia:
The transformation of certain coordination compounds from one to another is shown as follows:
A ligand is an ion or small molecule that binds to a metal atom (in chemistry) or to a biomolecule (in biochemistry) to form a complex, such as the iron-cyanide coordination complex Prussian blue or the iron-containing blood-protein hemoglobin. The ligands are arranged in spectrochemical series which are based on the order of their field strength. It is not possible to form the entire series by studying complexes with a single metal ion; the series has been developed by overlapping different sequences obtained from spectroscopic studies [19]. The order of common ligands according to their increasing ligand field strength is
The above spectrochemical series help us to for determination of strength of ligands. The left last ligand is as weaker ligand. These weaker ligand cannot forcible binding the 3d electron and resultant outer octahedral complexes formed. It is as-
Increasing the oxidation number the value of Δ increased.
Δ increases from top to bottom.
However, when we consider the metal ion, the following two useful trends are observed:
Δ increases with increasing oxidation number.
Δ increases down a group. For the determination of stability constant, the nature of the ligand plays an important role.
The following factors described the nature of ligands.
The size and charge are two factors that affect the production of metal complexes. The less charges and small sizes of ligands are more favorable for less stable bond formation with metal and ligand. But if this condition just opposite the product of metal and ligand will be a more stable compound. So, less nuclear charge and more size= less stable complex whereas if more nuclear charge and small in size= less stable complex. We take fluoride as an example because due to their smaller size than other halide and their highest electro negativity than the other halides formed more stable complexes. So, fluoride ion complexes are more stable than the other halides:
As compared to S2− ion, O22− ions formed more stable complexes.
It is suggested by Calvin and Wilson that the metal complexes will be more stable if the basic character or strength of ligands is higher. It means that the donating power of ligands to central metal ions is high [20].
It means that the donating power of ligands to central metal ions is high. In the case of complex formation of aliphatic diamines and aromatic diamines, the stable complex is formed by aliphatic diamines, while an unstable coordination complex is formed with aromatic diamines. So, from the above discussion, we find that the stability will be grater if the e-donation power is greater.
Thus it is clear that greater basic power of electron-donating species will form always a stable complex. NH3, CN−, and F− behaved as ligands and formed stable complexes; on the other hand, these are more basic in nature.
We know that if the concentration of coordination group is higher, these coordination compounds will exist in the water as solution. It is noted that greater coordinating tendency show the water molecules than the coordinating group which is originally present. SCN− (thiocynate) ions are present in higher concentration; with the Co2+ metal ion, it formed a blue-colored complex which is stable in state, but on dilution of water medium, a pink color is generated in place of blue, or blue color complex is destroyed by [Co(H2O)6]2+, and now if we added further SCN−, the pink color will not appear:
Now it is clear that H2O and SCN− are in competition for the formation of Co(II) metal-containing complex compound. In the case of tetra-amine cupric sulfate metal complex, ammonia acts as a donor atom or ligand. If the concentration of NH3 is lower in the reaction, copper hydroxide is formed but at higher concentration formed tetra-amine cupric sulfate as in the following reaction:
For a metal ion, chelating ligand is enhanced and affinity it and this is known as chelate effect and compared it with non-chelating and monodentate ligand or the multidentate ligand is acts as chelating agent. Ethylenediamine is a simple chelating agent (Figure 1).
Structure of ethylenediamine.
Due to the bidentate nature of ethylenediamine, it forms two bonds with metal ion or central atom. Water forms a complex with Ni(II) metal ion, but due to its monodentate nature, it is not a chelating ligand (Figures 2 and 3).
Structure of chelating configuration of ethylenediamine ligand.
Structure of chelate with three ethylenediamine ligands.
The dentate cheater of ligand provides bonding strength to the metal ion or central atom, and as the number of dentate increased, the tightness also increased. This phenomenon is known as chelating effect, whereas the formation of metal complexes with these chelating ligands is called chelation:
or
Some factors are of much importance for chelation as follows.
The sizes of the chelating ring are increased as well as the stability of metal complex decreased. According to Schwarzenbach, connecting bridges form the chelating rings. The elongated ring predominates when long bridges connect to the ligand to form a long ring. It is usually observed that an increased a chelate ring size leads to a decrease in complex stability.
He interpreted this statement. The entropy of complex will be change if the size of chelating ring is increased, i.e., second donor atom is allowed by the chelating ring. As the size of chelating ring increased, the stability should be increased with entropy effect. Four-membered ring compounds are unstable, whereas five-membered are more stable. So the chelating ring increased its size and the stability of the formed metal complexes.
The number of chelating rings also decides the stability of complexes. Non-chelating metal compounds are less stable than chelating compounds. These numbers increase the thermodynamic volume, and this is also known as an entropy term. In recent years ligands capable of occupying as many as six coordination positions on a single metal ion have been described. The studies on the formation constants of coordination compounds with these ligands have been reported. The numbers of ligand or chelating agents are affecting the stability of metal complexes so as these numbers go up and down, the stability will also vary with it.
For the Ni(II) complexes with ethylenediamine as chelating agent, its log K1 value is 7.9 and if chelating agents are trine and penten, then the log K1 values are 7.9 and 19.3, respectively. If the metal ion change Zn is used in place of Ni (II), then the values of log K1 for ethylenediamine, trine, and penten are 6.0, 12.1, and 16.2, respectively. The log βMY values of metal ions are given in Table 1.
Metal ion | log βMY (25°C, I = 0.1 M) |
---|---|
Ca2+ | 11.2 |
Cu2+ | 19.8 |
Fe3+ | 24.9 |
Metal ion vs. log βMY values.
Ni(NH3)62+ is an octahedral metal complex, and at 25 °C its log β6 value is 8.3, but Ni(ethylenediamine)32+ complex is also octahedral in geometry, with 18.4 as the value of log β6. The calculated stability value of Ni(ethylenediamine)32+ 1010 times is more stable because three rings are formed as chelating rings by ethylenediamine as compared to no such ring is formed. Ethylenediaminetetraacetate (EDTA) is a hexadentate ligand that usually formed stable metal complexes due to its chelating power.
A special effect in molecules is when the atoms occupy space. This is called steric effect. Energy is needed to bring these atoms closer to each other. These electrons run away from near atoms. There can be many ways of generating it. We know the repulsion between valence electrons as the steric effect which increases the energy of the current system [21]. Favorable or unfavorable any response is created.
For example, if the static effect is greater than that of a product in a metal complex formation process, then the static increase would favor this reaction. But if the case is opposite, the skepticism will be toward retardation.
This effect will mainly depend on the conformational states, and the minimum steric interaction theory can also be considered. The effect of secondary steric is seen on receptor binding produced by an alternative such as:
Reduced access to a critical group.
Stick barrier.
Electronic resonance substitution bond by repulsion.
Population of a conformer changes due to active shielding effect.
The macrocyclic effect is exactly like the image of the chelate effect. It means the principle of both is the same. But the macrocyclic effect suggests cyclic deformation of the ligand. Macrocyclic ligands are more tainted than chelating agents. Rather, their compounds are more stable due to their cyclically constrained constriction. It requires some entropy in the body to react with the metal ion. For example, for a tetradentate cyclic ligand, we can use heme-B which forms a metal complex using Fe+2 ions in biological systems (Figure 4).
Structure of hemoglobin is the biological complex compound which contains Fe(II) metal ion.
The n-dentate chelating agents play an important role for the formation of more stable metal complexes as compared to n-unidentate ligands. But the n-dentate macrocyclic ligand gives more stable environment in the metal complexes as compared to open-chain ligands. This change is very favorable for entropy (ΔS) and enthalpy (ΔH) change.
There are so many parameters to determination of formation constants or stability constant in solution for all types of chelating agents. These numerous parameters or techniques are refractive index, conductance, temperature, distribution coefficients, refractive index, nuclear magnetic resonance volume changes, and optical activity.
Solubility products are helpful and used for the insoluble salt that metal ions formed and complexes which are also formed by metal ions and are more soluble. The formation constant is observed in presence of donor atoms by measuring increased solubility.
To determine the solubility constant, it involves the distribution of the ligands or any complex species; metal ions are present in two immiscible solvents like water and carbon tetrachloride, benzene, etc.
In this method metal ions or ligands are present in solution and on exchanger. A solid polymers containing with positive and negative ions are ion exchange resins. These are insoluble in nature. This technique is helpful to determine the metal ions in resin phase, liquid phase, or even in radioactive metal. This method is also helpful to determine the polarizing effect of metal ions on the stability of ligands like Cu(II) and Zn(II) with amino acid complex formation.
At the equilibrium free metal and ions are present in the solution, and using the different electrometric techniques as described determines its stability constant.
This method is based upon the titration method or follows its principle. A stranded acid-base solution used as titrate and which is titrated, it may be strong base or strong acid follows as potentiometrically. The concentration of solution using 103− M does not decomposed during the reaction process, and this method is useful for protonated and nonprotonated ligands.
This is the graphic method used to determine the stability constant in producing metal complex formation by plotting a polarograph between the absences of substances and the presence of substances. During the complex formation, the presence of metal ions produced a shift in the half-wave potential in the solution.
If a complex is relatively slow to form and also decomposes at measurable rate, it is possible, in favorable situations, to determine the equilibrium constant.
This involves the study of the equilibrium constant of slow complex formation reactions. The use of tracer technique is extremely useful for determining the concentrations of dissociation products of the coordination compound.
This method is based on the study of the effect of an equilibrium concentration of some ions on the function at a definite organ of a living organism. The equilibrium concentration of the ion studied may be determined by the action of this organ in systems with complex formation.
The solution of 25 ml is adopted by preparing at the 1.0 × 10−5 M ligand or 1.0 × 10−5 M concentration and 1.0 × 10−5 M for the metal ion:
The solutions containing the metal ions were considered both at a pH sufficiently high to give almost complete complexation and at a pH value selected in order to obtain an equilibrium system of ligand and complexes.
In order to avoid modification of the spectral behavior of the ligand due to pH variations, it has been verified that the range of pH considered in all cases does not affect absorbance values. Use the collected pH values adopted for the determinations as well as selected wavelengths. The ionic strengths calculated from the composition of solutions allowed activity coefficient corrections. Absorbance values were determined at wavelengths in the range 430–700 nm, every 2 nm.
For a successive metal complex formation, use this method. If ligand is protonate and the produced complex has maximum number of donate atoms of ligands, a selective light is absorbed by this complex, while for determination of stability constant, it is just known about the composition of formed species.
Bjerrum (1941) used the method stepwise addition of the ligands to coordination sphere for the formation of complex. So, complex metal–ligand-n forms as the following steps [22]. The equilibrium constants, K1, K2, K3, … Kn are called stepwise stability constants. The formation of the complex metal-ligandn may also be expressed by the following steps and equilibrium constants.
Where:
M = central metal cation
L = monodentate ligand
N = maximum coordination number for the metal ion M for the ligand
If a complex ion is slow to reach equilibrium, it is often possible to apply the method of isotopic dilution to determine the equilibrium concentration of one or more of the species. Most often radioactive isotopes are used.
This method was extensively used by Werner and others to study metal complexes. In the case of a series of complexes of Co(III) and Pt(IV), Werner assigned the correct formulae on the basis of their molar conductance values measured in freshly prepared dilute solutions. In some cases, the conductance of the solution increased with time due to a chemical change, e.g.,
It is concluded that the information presented is very important to determine the stability constant of the ligand metal complexes. Some methods like spectrophotometric method, Bjerrum’s method, distribution method, ion exchange method, electrometric techniques, and potentiometric method have a huge contribution in quantitative analysis by easily finding the stability constants of metal complexes in aqueous solutions.
All the authors thank the Library of University of Delhi for reference books, journals, etc. which helped us a lot in reviewing the chapter.
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