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| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
1 Key Laboratory of Biogeology and Environmental Geology of Ministry of Education, China University of Geosciences, Wuhan, 430074, China
2 Geomicrobiology Laboratory, State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing, 100083, China
3 Department of Geology, Miami University, Oxford, Ohio 45056, U.S.A.
4 Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520, U.S.A.
5 Department of Earth System Sciences, Yonsei University, Seoul, Korea
6 Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, U.S.A.
Correspondence: * E-mail: dongh{at}muohio.edu
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ABSTRACT |
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 Top Abstract IntroductionMicroorganisms used in...Clay minerals used in...Rate and extent of...Relative reducibility of Fe(iii)...Mechanisms of microbial...Mineral transformations as a...Field demonstration of microbial...Application of reduced clay...Future perspectivesAcknowledgmentsReferences cited |
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Key Words: Bacteria • illite • mechanism • microbial Fe(III) reduction • nontronite • smectite
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INTRODUCTION |
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TopAbstractIntroductionMicroorganisms used in...Clay minerals used in...Rate and extent of...Relative reducibility of Fe(iii)...Mechanisms of microbial...Mineral transformations as a...Field demonstration of microbial...Application of reduced clay...Future perspectivesAcknowledgmentsReferences cited |
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The structural ferric iron in clay minerals can be reduced either chemically or biologically. Many different chemical reductants have been used in the past, but the most commonly used agent is dithionite (Stucki 2006). Biological reductants are bacteria, including dissimilatory iron-reducing prokaryotes (DIRP) and sulfate-reducing bacteria (SRB). A wide variety of DIRP have been used to reduce ferric iron in clay minerals, including mesophilic, thermophilic, and hyperthermophilic prokaryotes. Likewise, multiple clay minerals have been used for microbial-reduction studies, including smectite, nontronite (an iron-rich smectite variety), illite, mixed-layer illite/smectite, chlorite, vermiculite, palygorskite, and their various mixtures. All of these clay minerals are reducible by microorganisms under various conditions with smectite (nontronite) being the most reducible and illite the least.
The kinetics of Fe(III) reduction in clay minerals is measured by wet chemistry, and their subsequent physico-chemical changes are typically characterized by X-ray diffraction, scanning and transmission electron microscopy, Mössbauer spectroscopy, Fourier transform infrared spectroscopy (FTIR), UV-vis spectroscopy, and synchrotron-based techniques. Reduced smectites (nontronites) are reactive towards various organic and inorganic contaminants. Degradable organic contaminants include pesticides, solvents, explosives, and nitroaromatic and polychlorinated compounds. Inorganic contaminants include heavy metals such as Cr(VI), U(VI), and Tc(VII).
Despite significant efforts, current understanding of mechanisms of microbial reduction of ferric iron in clay minerals is still limited. Whereas some studies have presented evidence for a solid-state reduction mechanism (Kashefi et al. 2008; Lee et al. 2006; Stucki and Kostka 2006), others argue that the clay mineral structure partially dissolves as a result of clay mineral-microbe interactions (Dong et al. 2003a; Furukawa and O’Reilly 2007; Jaisi et al. 2007b, 2008a; Kim et al. 2004; Li et al. 2004; O’Reilly et al. 2005, 2006; Zhang et al. 2007a, 2007b). Below we review the current status of research on microbe-clay mineral interactions and certain applications of reduced clay minerals in environmental remediation of contaminated groundwater and soils.
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MICROORGANISMS USED IN BIOREDUCTION STUDIES |
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TopAbstractIntroductionMicroorganisms used in...Clay minerals used in...Rate and extent of...Relative reducibility of Fe(iii)...Mechanisms of microbial...Mineral transformations as a...Field demonstration of microbial...Application of reduced clay...Future perspectivesAcknowledgmentsReferences cited |
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). Among these are mesophilic Shewanella oneidensis from anoxic sediments of Lake Oneida, New York (Cervini-Silva et al. 2003; Dong et al. 2003a; Furukawa and O’Reilly 2007; Kostka et al. 1996, 1999a, 1999b, 2002; Lee et al. 2006; O’Reilly et al. 2005, 2006; Xu et al. 2001); Shewanella putrefaciens CN32 from a subsurface core sample (250 m beneath the surface) obtained from the Morrison Formation (Dong et al. 2003b; Jaisi 2007; Jaisi et al. 2005, 2007a, 2007b, 2007c, 2008a; Seabaugh et al. 2006; Zhang et al. 2007b); Geobacter metallireducens from freshwater sediments of the Potomac River, Maryland (Kostka et al. 1999b; Lovley et al. 1998); Pseudomonas spp. from wheat rhizosphere soils (Ernstsen et al. 1998; Gates et al. 1993, 1998; Kostka et al. 1999b), Bacillus spp. and certain enrichment cultures from source clay mineral SWa-1 and rice paddy soils in China (Gates et al. 1998; Kostka et al. 1999b, 2002; Stucki et al. 1987; Wu et al. 1988). Shelobolina et al. (2003) described a new strain of Desulfitobacterium frappieri from subsurface smectite bedding of the Twiggs Clay Formation of Late Eocene Age, Georgia, that is capable of reducing structural Fe(III) in ferruginous smectite (SWa-1). Li et al. (2004) was the first to study microbial reduction of Fe(III) in nontronite by a sulfate-reducing bacterium (Desulfovibrio spp. strain G-11 isolated from an enrichment of rumen fluid). A thermophilic bacterium from the Chinese Continental Scientific Drilling Project has been shown to have the ability to reduce ferric iron in clay minerals (Zhang et al. 2007a; Zhang 2006). More recently, several thermophilic and hyperthermophilic microorganisms from hydrothermal springs of Yellowstone National Park and the Pacific and Indian Oceans including Geothermobacter ehrlichii, Geogemma pacifica, Ferroglobus pacificus, Georgemma barossii, Georgemma indica, Ferroglobus indicus, and undescribed archaeal strains have been shown to reduce structural Fe(III) in smectite SWa-1 (Kashefi et al. 2008). In summary, these studies have demonstrated that microorganisms capable of reducing structural Fe(III) in clay minerals are usually facultatively or obligately anaerobic and they widely occur in soils, sediments, sedimentary rocks, and hydrothermal environments.
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CLAY MINERALS USED IN BIOREDUCTION STUDIES |
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TopAbstractIntroductionMicroorganisms used in...Clay minerals used in...Rate and extent of...Relative reducibility of Fe(iii)...Mechanisms of microbial...Mineral transformations as a...Field demonstration of microbial...Application of reduced clay...Future perspectivesAcknowledgmentsReferences cited |
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). Recent studies have expanded to illite and chlorite (Table 1), mixed-layer illite-smectite and rectorite (Dong et al. 2009a), palykorskite (Dong et al. 2009a), vermiculite (Komlos et al. 2007), natural soils and sediments (Favre et al. 2002, 2006; Komlos et al. 2007; Stucki et al. 2007), and loess materials (Bishop et al. submitted).
Among all of the minerals studied, expandable smectite (nontronite or montmorillonite) is the most reducible by bacteria with the fastest rate and the highest extent of biological reduction (1–90% reduction), whereas illite is the least reducible (1–25%) (Table 1). The amount and the crystal-chemical environment of structural Fe(III) as well as the layer charge of clay mineral structures are some important factors that control the extent of reduction (Jaisi et al. 2007b). Layer charge is related to layer expandability, which may affect the electron transfer process. In general, the more expandable nontronite structure (i.e., low layer charge) should allow easier access of electron donor and shuttling compounds to the Fe(III) centers in the structure, which may result in a higher extent and faster rate of Fe(III) reduction. Indeed, our previous study (Seabaugh et al. 2006) concluded that the different extent of bioreduction between Fithian and Muloorina illites was related to layer charge. The correlation between relative reducibility and layer expandability is further strengthened by our latest data that shows that the rate and extent of mixed-layer illite/smectite and rectorite (ordered R1 I-S) are between those for smectite and illite (Dong et al. 2009b).
Chlorite is another clay mineral that contains a significant amount of iron in the structure. It has a general formula of (Mg, Fe)3(Si, Al)4O10(OH)2(Mg, Fe)3(OH)6 and the structural iron can be ferric or ferrous. Apparently due to the non-expandable nature of the layers, the octahedral Fe(III) within the TOT layer of ripidolite (chlorite) CCA-2 (Brandt et al. 2003) is bio-reducible by Shewanella putreficiens CN32 to a limited extent (<10%) (Jaisi et al. 2007b). This reduction was significantly enhanced in the presence of an electron shuttling compound, AQDS, similar to the case of microbial reduction of illite (Dong et al. 2003b). These lines of evidence suggest that the octahedral Fe(III) in non-expandable structures of clay minerals (such as illite and chlorite) is bioreducible, but to a limited extent, and that this bioreduction is enhanced by the presence of electron shuttling compounds.
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RATE AND EXTENT OF MICROBIAL REDUCTION OF FE(III) IN CLAY MINERALS |
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TopAbstractIntroductionMicroorganisms used in...Clay minerals used in...Rate and extent of...Relative reducibility of Fe(iii)...Mechanisms of microbial...Mineral transformations as a...Field demonstration of microbial...Application of reduced clay...Future perspectivesAcknowledgmentsReferences cited |
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) reveals that the extent and rate of microbial reduction of Fe(III) in clay minerals vary depending on experimental conditions such as the type of microorganisms and clay minerals, microbe/clay mineral concentration ratio, ferric iron content and layer charge, clay particle size (surface area), interlayer composition of clay minerals, presence or absence of electron shuttle, solution chemistry (pH, aqueous chemical composition), and temperature. A close examination of published data reveals the following general observations: (1) different microorganisms and clay minerals have major impacts on the extent and rate of bioreduction (Table 1); (2) the extent of bioreduction increases with increasing total Fe content of the clay minerals (Ernstsen et al. 1998; Gates et al. 1998), but decreases with increasing levels of initial Fe(II) present prior to bioreduction (Ernstsen et al. 1998); (3) decreasing clay particle size and increasing surface area increases the extent and rate of bioreduction (Ernstsen et al. 1998; Jaisi et al. 2007a); (4) lower layer charge of clay minerals favors bioreduction (Dong et al. 2009b; Seabaugh et al. 2006); (5) increasing microbial/clay mineral ratio increases the rate and extent of bioreduction (Jaisi et al. 2007c; Kostka et al. 1999a); (6) electron shuttling components enhance the extent and rate of bioreduction (Table 1); (7) solution chemistry (such as pH, presence or absence of Al and K) influences the bioreduction kinetics (Zhang et al. 2007a); (8) presence of organic matter in the interlayer appears to decrease the extent and rate of bioreduction (Zhang et al. 2007b); and (9) temperature affects bioreduction in different ways (Kashefi et al. 2008; Zhang et al. 2007a). Because of these limiting experimental conditions, the extent of bioreduction is often far from completion (i.e., 100%) (Dong et al. 2003a, 2003b; Gates et al. 1993, 1998; Jaisi et al. 2005; Kim et al. 2004; Kostka et al. 1996, 1999a, 1999b; Li et al. 2004; O’Reilly et al. 2006; Shelobolina et al. 2003; Stucki 2006). For a given system, the incomplete reduction of structural Fe(III) in iron-rich smectite/nontronite has been ascribed to multiple effects: (1) inhibition by cell- and clay mineral-sorbed Fe(II) that is released from the structure as a result of bioreduction; (2) inhibition due to accumulation of solid phase reduction products on reactive clay mineral surfaces; and (3) energetics of the system (Jaisi et al. 2007b). The inhibition effects have also been observed for iron oxides in batch experiments (Roden and Urrutia 1999, 2002; Roden and Wetzel 2002) and can be removed or minimized by: (1) dynamic flow and (2) addition of fresh cells or clay minerals (Jaisi et al. 2007b; Urrutia et al. 1998). Removal of ferrous iron, a product of microbial reduction of Fe(III) in iron oxides, can promote microbial reduction of crystalline iron oxide to completion (100% extent of reduction) (Roden and Urrutia 1999). However, in the case of microbial reduction of Fe(III) in nontronite, dynamic flow in reactors (O’Reilly et al. 2006) did not significantly increase the extent of bioreduction (7%) as compared with static batch systems (4%). Clearly more research is needed in this area to confirm these results.
The removal or minimization of the inhibition effects by addition of fresh cells is effective for both iron oxides (Urrutia et al. 1998) and iron-rich smectites/nontronite (Fig. 1) (Jaisi et al. 2007b). Whereas the addition of fresh cells introduces uncoated cell surfaces, it also changes the energetics of the system. Thus, the combined effects of increased energy in the system and clean surfaces from freshly added cells may have been responsible for the increased extent of bioreduction for both nontronite (Jaisi et al. 2007b) (Fig. 1) and iron oxides (Urrutia et al. 1998). It is conceivable that the extent of bioreduction may eventually reach 100%, if enough fresh cells are added in multiple batches.
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RELATIVE REDUCIBILITY OF FE(iii) OXIDES AND CLAY MINERALS |
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TopAbstractIntroductionMicroorganisms used in...Clay minerals used in...Rate and extent of...Relative reducibility of Fe(iii)...Mechanisms of microbial...Mineral transformations as a...Field demonstration of microbial...Application of reduced clay...Future perspectivesAcknowledgmentsReferences cited |
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MECHANISMS OF MICROBIAL REDUCTION OF CLAY MINERALS |
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TopAbstractIntroductionMicroorganisms used in...Clay minerals used in...Rate and extent of...Relative reducibility of Fe(iii)...Mechanisms of microbial...Mineral transformations as a...Field demonstration of microbial...Application of reduced clay...Future perspectivesAcknowledgmentsReferences cited |
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In contrast, other studies have presented evidence for microbial dissolution of nontronite (Dong et al. 2003a; Furukawa and O’Reilly 2007; Jaisi et al. 2007b; Kim et al. 2004; Li et al. 2004; O’Reilly et al. 2005, 2006; Zhang et al. 2007a, 2007b). A most recent study (Vorhies and Gaines 2009) presented mineral phase relationship and stable isotope data from Cambrian mudstones suggesting that a microbially driven clay mineral dissolution reaction releases a significant fraction of Si and Fe. These studies have largely used scanning and transmission electron microscopy (SEM and TEM), along with X-ray diffraction (XRD), to directly observe dissolution textures of smectites and the formation of biogenic products (Si, siderite, vivianite, and illite) (Fig. 2). Irreversible changes of cation exchange capacity and specific surface area upon reduction and re-oxidation of pure smectites (Kostka et al. 1999b; Shen and Stucki 1994) also imply a certain degree of dissolution. In addition, reductive dissolution of clay minerals as a result of microbial activities has been implied in laboratory and field studies of natural soils (Favre et al. 2002; Kirk et al. 2003). Many reasons for the apparent inconsistency in the mechanisms of microbial reduction of Fe(III) in smectites (dissolution vs. solid-state) are discussed below.
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Second, the mechanism of reduction may be dictated by the extent of Fe(III) reduction. If the extent of dissolution is small, electron microscopy may show dissolution products that may be undetectable by other methods (such as infrared). Even if there is dissolution of nontronite and formation of biogenic Si (a major dissolution product), it may be difficult to detect it by infrared spectroscopy because the Si-O vibration bands in biogenic Si and quartz may be similar to those in residual nontronite. Our most recent evidence suggests that when the extent of reduction in nontronite (NAu-2) reaches ~30%, the nontronite structure apparently becomes less stable or even becomes partly X-ray amorphous as evidenced by nearly complete extraction of Fe(II) from reduced nontronite by 0.5 N HCl, <100% retention of Fe(II) in the structure, and significant broadening of the 001 peak in the XRD pattern of reduced NAu-2 (Fig. 3) (Jaisi et al. 2008a). If the biological reduction continues beyond this extent, partial dissolution of the smectite/nontronite structure is to be expected. The breakdown of the structure would release Fe(II), which becomes available for partitioning into surface complexation sites, interlayer sites, and aqueous solution (Jaisi et al. 2008a). The extent of dissolution depends on the extent of reduction and reaches ~35% when the extent of reduction is at ~70% (Jaisi et al. 2008a). Clearly, this extent of dissolution is significant and may account for formation of biogenic precipitates (silica, illite, vivianite, and siderite) (Dong et al. 2003a).
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Fourth, the concentration and type of microorganisms may play an important role in determining the mechanism of bioreduction of Fe(III) in clay minerals. The majority of published work in support of the dissolution mechanism of Fe(III) reduction was performed using high cell concentrations of Shewanella spp. Under such conditions, many metabolic by-products may influence solution chemistry and possibly result in clay mineral dissolution.
The effects of different microorganisms on Fe(III) reduction mechanisms can be illustrated by comparing Geobacter vs. Shewanella species. The species of Geobacter cannot produce any electron-shuttling compounds (Nevin and Lovley 2002) and thus require a direct contact between the bacterium and the clay mineral in order for bioreduction to take place (Fig. 4). In such a case, the electron transfer pathway may be parallel to the (001) layers. Whereas electron transfer to the Fe(III) centers near the clay edge may be readily accomplished, it may be difficult to transfer electrons to the Fe(III) centers in the middle of the clay packet. Thus, the extent of bioreduction may be limited and the clay mineral may largely remain in solid state as a result of bioreduction. Indeed, the extent of reduction of structural Fe(III) in clay minerals by Geobacter sp. is generally lower that that achieved by other microbes (Table 1) (Shelobolina et al. 2003). This parallel-layer electron transfer would result in a fixed number of Fe2+-O-Fe3+ intervalence electron transitions regardless of the extent of bioreduction, thus displaying a constant intensity of this transition band in optical absorption spectra (Lear and Stucki 1987). The limited capacity of parallel-layer electron transfer can be alleviated by electron-shuttling compounds. Indeed, when humic substances or its analog, AQDS, are added along with the bacteria, the extent of bioreduction of smectite SWa-1 by Geobacter metallireducens is significantly enhanced (Lovley et al. 1998). One mechanism by which humics or AQDS stimulate reduction of structural Fe(III) in smectite is that microbially reduced humics might be able to access Fe(III) within the smectite structure that microorganisms cannot access.
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). In this case, the rate and extent of bioreduction should be much higher than the parallel-layer electron transfer only, with a probability of partially dissolving the clay mineral structure. Indeed, Shewanella species can reduce smectite to a much higher extent than Geobacter species (Kostka et al. 1999b; Shelobolina et al. 2003) (Table 1). The addition of electron shuttling compound, AQDS, significantly enhances the extent of reduction (Dong et al. 2003a; Jaisi et al. 2005), suggesting that bioreduction may still be limited by the availability of such shuttling compounds. If the electron transfer chain is both parallel and perpendicular to the basal plane of the smectite structure, these dual (or multiple) electron transfer pathways should result in an increasing number of Fe2+-O-Fe3+ intervalence electron transfer transitions with an increasing extent of bioreduction (Komadel et al. 2006). Therefore, optical spectrophotometry may be an attractive way to determine the electron transfer mechanisms by different bacteria (Komadel et al. 2006). This hypothesis remains to be tested by future studies. In addition to the ability of producing electron-shuttling compounds, certain microbes can produce cell surface polymers that may play important roles in bioreduction. For example, Kashefi et al. (2008) observed that the highest amounts of iron in smectite SWa-1 were solubilized by bacterial and archaeal strains that produced copious extracellular polymeric substances (EPS). Acidic EPS materials have been shown to bind cations such as Fe3+ and other trace metals (Decho 1990; Nichols et al. 2005; Sutherland 2001). Thus, EPS may be functionally similar to certain chelators in dissolving the clay mineral structure (Kostka et al. 1999a).
In summary, one or several of these factors may be important in determining the dissolution vs. solid-state bioreduction mechanism. We acknowledge that these two mechanisms represent two extreme cases and that reality might lie somewhere between these mechanisms. For example, Anastacio et al. (2008) observed that the final structure of the reduced-reoxidized smectite contained more defects than the original clay, suggesting some change to the structure as a result of reduction-reoxidation cycle. Likewise, Kukkadapu et al. (2006) observed that biogenic Fe(II) largely remained in the illite structure, but the illite became more soluble in weak acid than the original mineral, suggesting some change to the illite structure.
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MINERAL TRANSFORMATIONS AS A RESULT OF MICROBIAL REDUCTION OF CLAY MINERALS |
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TopAbstractIntroductionMicroorganisms used in...Clay minerals used in...Rate and extent of...Relative reducibility of Fe(iii)...Mechanisms of microbial...Mineral transformations as a...Field demonstration of microbial...Application of reduced clay...Future perspectivesAcknowledgmentsReferences cited |
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Under the circumstances of clay mineral dissolution as a result of microbial reduction of structural Fe(III), several new or altered minerals can possibly form, including biogenic silica (Dong et al. 2003a; Furukawa and O’Reilly 2007; Li et al. 2004; O’Reilly et al. 2005; Zhang et al. 2007a), smectite (Dong et al. 2003a; Zhang et al. 2007a), illite (Kim et al. 2004; Zhang et al. 2007b), vivianite (Dong et al. 2003a), siderite (Kim et al. 2004; O’Reilly et al. 2005; Zhang et al. 2007b), Fe-sulfides [when sulfate-reducing bacteria are used to reduce Fe(III) in nontronite] (Li et al. 2004), calcite (Li et al. 2004), and various forms of amorphous materials (Dong et al. 2003a; O’Reilly et al. 2005). Although these minerals can also form inorganically, biogenic minerals are often smaller and more uniform in particle size (O’Reilly et al. 2005; Zhang et al. 2007a) and have fewer compositional impurities than chemically formed counterparts (Carvallo et al. 2008). These differences in size and chemistry may be responsible for differences in their properties. For example, studies have shown that Fe-oxide and As-sulfides synthesized abiotically and biotically show different electrical and photoconductive properties (Lee et al. 2007) and rates of heterogeneous catalytic efficiency (Jung et al. 2007), but equivalent research has not been conducted for biogenic clay minerals.
In addition to these unique functional properties, biogenic minerals may have high solubility and reactivity, owing to their small size and large surface area. If present in large quantities, these highly soluble and reactive nanoparticles would play important roles in biogeochemical cycling of life-essential elements in natural environments. For example, it is well known that plant uptake of silica plays a major role in the terrestrial silica cycle (Raven 2003), but the total global rate of silica release from weathering of silicate minerals is only a small fraction of the uptake by plants (Berner and Berner 1996; Kump et al. 2000). This disparity between the rates of supply of new silica and its potential uptake by plants should lead silica to behave like other rock-derived nutrient elements such as potassium (K) and phosphorus (P) that are strongly recycled between biomass and the upper soil and litter layers. Although many previous studies have indicated that high concentrations of silica can occur in specific and rather unique environments, such as geothermal springs, hydrothermal vents, and volcanic ash deposits (Konhauser 2006), this reservoir of silica is generally not available to plants and other life forms. Thus, release of silica from microbial reduction of Fe(III) in clay minerals (Fig. 5) may potentially play an important role in the geochemical cycle of silica in soils and sediments. Indeed, a recent study (Vorhies and Gaines 2009) demonstrated microbial liberation of silica and iron from solid clay minerals in shallow marine sediments. This release is important in providing marine organisms with important nutrients.
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). The microbially promoted smectiteillite reaction can take place at room temperature and one atmosphere within two weeks (Kim et al. 2004). This reaction typically requires conditions of 300–350 °C, 100 MPa, and 4–5 months in the absence of microbial activity. Our recent studies (Zhang et al. 2007b) have demonstrated a strong catalytic effect on the smectite to illite reaction of organic matter intercalated in the interlayer of the smectite structure. Even at diagenetically relevant temperatures, the smectite to illite reaction can be catalyzed by thermophilic bacteria (Zhang et al. 2007a).
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Another study (Huggett and Cuadros 2005) investigated low-temperature illitization of smectite in the late Eocene and early Oligocene of the Isle of Wight (Hampshire basin), U.K. The authors conclude that the smectite to illite transition occurs through Fe reduction in octahedral sites leading to increased layer charge coupled with K fixation. The driving mechanism for the irreversible Fe(III) reduction in the smectite structure is wetting (reducing) and drying (oxidizing) cycles in gley soil, in which reoxidation of reduced Fe is never complete. A more recent study similarly demonstrates a smectite to illite reaction in a field setting that is promoted by wetting-drying cycles (Stanjek and Marchel 2008). In these studies, although the role of bacteria in the smectite to illite reaction was not specifically investigated, it certainly remains a strong possibility.
A recent study (Vorhies and Gaines 2009) investigated mineral textural relationships in carbonate concretions found within illite-bearing shales of the Middle Cambrian Wheeler Formation of the House Range, Utah. Textural, mineralogical, and oxygen isotope evidence all support microbial reduction of structural Fe(III) and dissolution of clay minerals. As a result of microbial dissolution, silica and iron are released into pore water, and under favorable conditions, biogenic minerals such as authigenic quartz, calcite, pyrite, and illite are formed. These results provide a field example where microbial reduction and dissolution of clay minerals can lead to neoformation of illite.
Recognition of biogenic minerals in the rock record is important as they can be used as bio-signatures to infer past biological processes. Unfortunately, this is a difficult process, as many characteristics of biogenic minerals may be the same as or similar to those of chemically formed counterparts. Although laboratory-synthesized Fe-oxide (Jung et al. 2007) and arsenic sulfide (Lee et al. 2007) minerals have shown some important differences between biogenic and abiogenic minerals, apparently because of unique biological processes involved, these differences may be difficult to extrapolate to naturally formed minerals. Clearly this represents an important area of future research.
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FIELD DEMONSTRATION OF MICROBIAL REDUCTION OF FE(III) IN CLAY MINERALS |
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TopAbstractIntroductionMicroorganisms used in...Clay minerals used in...Rate and extent of...Relative reducibility of Fe(iii)...Mechanisms of microbial...Mineral transformations as a...Field demonstration of microbial...Application of reduced clay...Future perspectivesAcknowledgmentsReferences cited |
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More recent studies of microbial reduction of Fe(III) in clay minerals and iron oxides have been conducted in the context of bioremediation (Anastacio et al. 2008; Komlos et al. 2007, 2008; Mohanty et al. 2008; Stucki et al. 2007). These studies are conclusive in demonstrating that Fe(III) in both iron oxides and silicate clay minerals is reducible by microorganisms under in situ field conditions. For example, in a saprolitic soil from the U.S. Department of Energy Field Research Center near Oak Ridge, Tennessee, electron donors (glucose and ethanol) were injected to biologically stimulate iron-reducing bacterial activity and thus to promote heavy metal immobilization (U, Tc, and Cr) (Stucki et al. 2007). As a result of such in situ biostimulation, major changes in iron mineralogy were observed, including significant decreases in iron oxide content and increases in clay mineral Fe(II) content.
In contrast to recent interests in iron redox chemistry in soils and terrestrial sediments, there have been many earlier studies of iron redox chemistry in marine sediments. Lyle (1983) was one of the first to study down-core concentrations of dissolved nitrate, Mn, and Fe in pore waters within the color transition zones of marine sediments, and based on laboratory-demonstrated reversibility of color changes, the author hypothesized that the color change was caused by in situ reduction of Fe(III) in smectites in the sediments. Based on Mössbauer spectroscopy studies, Konig et al. (1997) quantified the fraction of the bulk sediment iron that experiences reduction through a typical tan-green color transition in marine sediments, and reached a similar conclusion that the tan-green color transition in pelagic sediments is due to in situ transition of Fe(III) to Fe(II) in smectites. Similarly, based on detailed Mössbauer spectroscopy studies, Drodt and colleagues (Drodt et al. 1997, 1998) observed that a significant part of the clay mineral iron is redox sensitive, and the authors proposed that the color change of the sediments at the redox boundary from tan to green in a marine sediment core of the Peru Basin coincides with a change in the relative Fe(II) content in layer silicates from 11 to 37%. In these studies, the authors speculated that Fe(III) reduction to Fe(II) is a chemical process, although it is equally likely that this reduction may be microbially catalyzed (Vorhies and Gaines 2009). Upon contact with atmospheric oxygen and other oxidants such as nitrate in pore water, this latent fraction of reactive Fe(II) can be readily oxidized and may represent the major barrier to the movement of oxidation fronts in pelagic subsurface sediments (Konig et al. 1999).
In summary, many studies have observed microbial reduction of Fe(III) in clay minerals (smectite, illite/vermiculite) in natural soils and sediments, but because clay minerals are intimately associated with iron oxides with complex textural relationships, it is often not straightforward to separately determine their respective effects on physical and chemical properties of soils and sediments. This represents an area of future research, possibly using more visual-based imaging techniques at high spatial resolutions.
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APPLICATION OF REDUCED CLAY MINERALS TO REMEDIATION OF HEAVY METALS AND ORGANIC COMPOUNDS |
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In parallel to the studies on the interaction between phyllosilicates and chromium, investigations of clay minerals with other heavy metals, such as U and Tc, were also undertaken. Cui and Eriksen (1996) studied reduction of pertechnetate in solution by heterogeneous electron transfer from Fe(II)-containing geological materials and concluded that Fe(II)-containing chlorite was reactive toward Tc(VII) reduction, although its reduction efficiency was not as high as magnetite. Similarly, biogenic Fe(II) produced from sediment materials that contained abundant iron oxides has been shown to be highly reactive toward Tc(VII) reduction to insoluble TcO2·nH2O (Fredrickson et al. 2004), whereas that from sediments that contained plentiful layer silicates (illite, vermiculite, smectite) was the least reactive.
Jaisi et al. (2008a) investigated the reactivity of Fe(II) in a reduced pure clay mineral, nontronite NAu-2, toward Tc(VII) reduction (Fig. 7). The authors concluded that when the extent of Fe(III) reduction is higher than ~30%, a certain amount of Fe(II) is released from the structure and partitioned into four species: aqueous Fe2+, surface complexation Fe(II), ion exchangeable Fe(II), and structural Fe(II). The released Fe(II) has the highest affinity for the surface complexation site, but this site has limited capacity (141 meq/kg NAu-2). The ion exchangeable site has much higher capacity (697 meq/kg). Experiments performed to understand the reactivity of these various Fe(II) species showed that the structural Fe(II) and surface-complexed Fe(II) exhibited greater reactivity toward Tc(VII) reduction than the ion-exchangeable Fe(II). The authors concluded that the surface-complexed and structural Fe(II) are the desirable species if the rate of Tc(VII) is to be maximized.
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More recently, Jaisi et al. (2009) investigated the kinetics of Tc(VII) reduction by Fe(II) in pure clay mineral nontronite and observed that the rate of Tc(VII) reduction was highest at neutral pH when Fe(II) concentration was low (<1 mmol/g). The effect of pH, however, was insignificant when Fe(II) concentration was high (>1 mmol/g). The reduction of Tc(VII) by Fe(II) in biologically reduced nontronite was also studied in the presence of common subsurface oxidants including iron and manganese oxides, nitrate, and oxygen, to evaluate the effect of these oxidants on enhancement or inhibition of Tc(VII) reduction or reoxidation of Tc(IV). Goethite and hematite enhanced reduction of Tc(VII), apparently as a result of re-distribution of reactive Fe(II) from nontronite to goethite/hematite surfaces. Addition of manganese oxides inhibited Tc(VII) reduction, and K+-birnessite reoxidized previously reduced Tc(IV). Nitrate neither enhanced reduction of Tc(VII) nor promoted reoxidation of Tc(IV), whereas oxygen was capable of reoxidizing Tc(IV). When the same oxidants were added to aged Tc reduction products (mainly remaining NAu-2 and TcO2·nH2O), the extent of Tc(IV) reoxidation decreased significantly relative to fresh Tc(IV) products. Increasing NAu-2 concentration also resulted in decreased extent of Tc(IV) reoxidation. The results suggest that NAu-2 aggregation effectively retained Tc(IV) in the NAu-2 matrix and decreased its vulnerability to reoxidation.
In addition to their reactivity toward inorganic heavy metal immobilization, smectites that were either biologically or chemically reduced have also been used to degrade organic contaminants including pesticides, chlorinated aliphatics, and nitroaromatics (Hofstetter et al. 2003, 2006; Neumann et al. 2008; Stucki 2006). Stucki (2006) recently reviewed this subject and we will not repeat it here. It is important for readers to recognize that the degradation reactions between redox-active clay minerals and organic compounds only constitute a small subset of a much broader clay mineral-organic matter interactions as recently reviewed by Lagaly et al. (2006).
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FUTURE PERSPECTIVES |
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TopAbstractIntroductionMicroorganisms used in...Clay minerals used in...Rate and extent of...Relative reducibility of Fe(iii)...Mechanisms of microbial...Mineral transformations as a...Field demonstration of microbial...Application of reduced clay...Future perspectivesAcknowledgmentsReferences cited |
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Future studies should be focused on quantifications of the extents and rates of clay mineral-microbe interactions, especially in natural systems. For example, what are the overall impacts of such interactions on soil fertility and contamination migration and fate in the field? How does chemistry of water bodies (surface water, groundwater, and seawater) change in response to such interactions? What is the relative importance of iron oxides vs. iron-rich clay minerals in sustaining microbial ecosystems in surface and subsurface systems? How are the clay mineral-microbe interactions linked to the global cycles of carbon, nitrogen, and phosphorous? All of these questions require an integrated approach, combining clay mineralogy and crystal chemistry, molecular microbiology and microbial ecology, aqueous geochemistry, contaminant hydrology, and quantitative modeling. In addressing these questions, advanced techniques are likely to play increasingly important roles. In situ microscopy, spectroscopy, and synchrotron-based X-ray techniques may provide key information necessary to understand clay mineral-microbe interactions.
To answer these questions, it is important that we establish interdisciplinary research teams, where clay mineralogists, geochemists, microbiologists, hydrologists, and mathematical modelers, all work together to share their expertise and train the next generations of scientists by offering workshops, short courses, and other training opportunities. We hope that this review paper promotes future collaborations among people with diverse backgrounds and inspires young students to become interested in this subject area.
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ACKNOWLEDGMENTS |
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TopAbstractIntroductionMicroorganisms used in...Clay minerals used in...Rate and extent of...Relative reducibility of Fe(iii)...Mechanisms of microbial...Mineral transformations as a...Field demonstration of microbial...Application of reduced clay...Future perspectivesAcknowledgmentsReferences cited |
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Footnotes |
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MANUSCRIPT RECEIVED March 18, 2009; MANUSCRIPT ACCEPTED July 17, 2009
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REFERENCES CITED |
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