Fe-bearing Clays Research Articles

Influence of structural Fe content in clay minerals on selenite redox reactions: Kinetics and structural transformations

Redox reactions control the environmental fate of selenium, an element of concern due to its small gap between beneficial and detrimental effects on human health and due to the longevity of the radionuclide 79Se produced in nuclear reactors. Fe-bearing clay minerals are major redox-active ingredients of Earth’s critical zone and constitute an important component of the barrier in (radioactive and other) waste repositories. Here we systematically investigate selenite (Se(IV)O32−) sorption and reduction by Fe-bearing clay minerals of the smectite group, using batch experiments and X-ray absorption fine structure (XAFS) spectroscopy to elucidate reaction mechanisms and kinetics up to 720 or 3600 h. We found a linear relationship between structural Fe(II) content and selenite reduction rate. Selenite reduction depends also on redox potential and pH, with pH 7 showing a slower reduction rate than at pH 5. Selenite first sorbs to clay edge sites by forming an inner-sphere sorption complex, before being reduced to Se(0). The first redox product was amorphous (red) Se(0), which gradually transformed into trigonal (grey) Se(0), indicative of a kinetically hindered conversion of an amorphous (or less crystalline) solid phase into the thermodynamically stable and more crystalline form of the solid. Despite an Fe2+aq concentration of up to 6·10−4 M in the clay suspension, and their thermodynamic prediction for the clays at lower Eh, we did not observe formation of iron selenides. These insights into the selenite reduction mechanisms by Fe-bearing clays provides valuable information for the development of effective approaches for selenite immobilization.

Combined effects of humic substances and clay minerals on U(VI) bioreduction

In the last several decades, microbial reduction of U(VI) to U(IV) has been applied as a method to remediate uranium contamination in situ. The U(VI) bioreduction kinetics and resulting U(IV) speciation are affected by various environmental factors, such as humic substances (HS) and Fe-bearing clay minerals. Previous studies have largely focused on their individual effects on U(VI) bioreduction. However, HS and Fe(III)-bearing clay minerals often co-exist at uranium contaminated sites, their joint effects may be different from the sum of their individual ones, due to either synergistic or antagonistic interactions between the two. In this work, we investigated the combined effects of HS and Fe(III)-bearing clay minerals (Fe-rich nontronite and Fe-poor montmorillonite) on U(VI) reduction by Shewanella putrefaciens and the resulting U(IV) speciation. HS alone exhibited an inhibitory effect on U(VI) bioreduction, likely because HS retained a fraction of electrons from microbial oxidation of lactate. The resulting U(IV) was in the form of aqueous U(IV)-HS complex. Clay minerals alone also inhibited U(VI) bioreduction because a fraction of reduced U(IV) was likely re-oxidized by structural Fe(III) and thus electrons were partially diverged from U(VI) to structural Fe(III). Bioreduced U(IV) was in a solid phase. However, in the presence of both HS and clay minerals, their combined effect of inhibition on U(VI) bioreduction was not a simple addition of the two individual ones. In the co-presence of clay minerals and a low concentration of HS (10 and 20 mg C/L), their combined inhibition effect on U(VI) bioreduction was stronger than that of HS but weaker than that of clays. The stronger inhibition than that of HS alone was due to electron diversion by clay minerals. The weaker inhibition than that of clays alone was because HS and/or Fe(III)-HS/Fe(II)-HS served as electron shuttles to accelerate the rate of Fe(III) reduction. These double shuttles decreased the ability of structural Fe(III) in clays to diverge electrons from U(VI) and thus partially negated the inhibitory effect of clay minerals. In the co-presence of clay minerals and a high concentration of HS (100 mg C/L), their combined effect on U(VI) bioreduction was initially similar to that in the low concentration of HS scenario. However, over extended time, clay minerals partially negated the inhibitory effect of HS, likely because clay minerals adsorbed a fraction of HS and thus lowered the ability of residual aqueous HS to retain electrons. Biogenic U(IV) was sequestered in solid in the presence of Fe-rich nontronite and HS but partially remained soluble in the presence of Fe-poor montmorillonite and HS. This study highlights the importance of the complex interaction between HS and Fe-bearing clays in bioreduction of U(VI) and speciation of the resulting U(IV).

Synergistically enhanced activation of persulfate for efficient oxidation of organic contaminants using a microscale zero-valent aluminum/Fe-bearing clay composite

Sulfate radical-based advanced oxidation processes have received increasing attention in contaminated water treatment. As an excellent electron donor, microscale zero-valent aluminum (mZVAl) is a promising persulfate (PS) activator. However, the natural oxide layer of mZVAl impedes electron transfer from the core metal. Herein, a composite comprising mZVAl and Fe-bearing clay (mZVAl-NaNAu-2) was obtained through the simple ball milling (BM), which synergistically activates PS for the efficient degradation of various organic contaminants, such as 4-chlorophenol, benzoic acid, nitrobenzene, phenol, and ofloxacin, in water. The sulfate radicals and hydroxyl radicals were the main reactive species but Fe(IV) was minor in this reaction. The BM can cause fracture of the mZVAl powder and promote exposure of the fresh Al0 surface. Various Fe species in mZVAl-NaNAu-2 at different reaction times were evaluated by extraction experiments, Mössbauer and XPS spectra. Approximately 47.0% of structural Fe(III) was reduced to Fe(II) after BM. The dissolved Fe(II) and structural trioctahedral Fe(II)-Fe(II)-Fe(II) entities were mainly responsible for the homogeneous and heterogeneous activation of PS, and corroded mZVAl acted as an “electron pool” to regenerate the aforementioned Fe(II) species continuously. The mZVAl-NaNAu-2/PS system also presented certain resistance to a natural organic matter and some anions, such as Cl- and NO3-. This study provided a new strategy for the efficient removal of organic contaminants from water and also solved the bottleneck of mZVAl in applications due to its dense oxide film.

The lake as an iron sink - new insights on the role of iron speciation

The solubility and behavior of iron (Fe) in natural waters is tightly linked to Fe speciation, and Fe speciation likely influences how Fe distributes between the water column and sediments. In this study, the function of a lake as an Fe sink, with focus on the role of Fe speciation, was assessed for Lake Bolmen in southern Sweden. We found that a large fraction of the Fe flowing in to the lake was efficiently lost by sedimentation in the lake basin. Fe in inflowing water was a mix of organically complexed mononuclear Fe, Fe-(oxy)hydroxides and Fe-bearing clays, while surface sediments were composed of Fe-(oxy)hydroxides, Fe-bearing clays, Fe-bearing silicates and Fe sulfides. The absence of organically complexed Fe in the surface sediments indicates that the lake is mainly a sink for minerogenic fractions. Furthermore, while lakes are considered to be sinks of Fe, it has been suggested that this function may be impaired by increasing precipitation and consequently shorter water residence time. In this study there were large within- and between-year variations in precipitation and Fe concentrations. However, rather than smaller Fe losses to the sediments during wet years, within-lake losses tended to be larger due to higher loading of Fe from the catchment. Thus, forecasted increases in precipitation may result in enhanced catchment export and Fe loading to lakes, and subsequently enhanced Fe sequestration in sediments.

Enhanced reactivity of zero-valent aluminum/O2 by using Fe-bearing clays in 4-chlorophenol oxidation

Zero-valent aluminum (ZVAl) is a promising reductant because of its relatively low redox potential, which can efficiently activate molecular oxygen to generate reactive oxygen species. However, its long-term performance is limited by the intrinsic dense oxide layer and the passivation effect of the accumulative Al-(hydr)oxide on its surface during the reaction. In this study, four clay minerals with different compositions were mixed with ZVAl by ball milling to obtain four composites of ZVAl and clay (ZVAl-Clay), which were used to degrade a high concentration of 4-chlorophenol (4-CP) under ambient conditions. The oxidation efficiencies of different ZVAl-Clays were strongly relevant to Fe contained in the clay minerals. The Fe-free ZVAl-Clay presented poor oxidation performance, whereas the reaction efficiencies of those ZVAl composites with Fe-bearing clays exhibited varying degrees of improvement. In comparison with the original ZVAl, the highest oxidation rate increased by 23 times, the maximum increased OH production was approximately 8 times, and the corresponding mineralization efficiency improved by 38.7%. However, the levels of improved oxidation performance of various ZVAl-Clays were not positively correlated with their actual total Fe contents, and their degradation efficiencies might also be affected by other physical and/or chemical properties of different clays. The synergistic mechanism revealed by various characterizations was that electron transfer might occur from ZVAl to the structural Fe(III) of the clay through the basal plane or edge of clays triggered by ball milling. Thus, the partially produced Fe(II) on the clay surface promoted the Fenton-like reaction to decompose H2O2 into OH for efficient oxidation of 4-CP. In short, the ZVAl composites with Fe-bearing clays deserved further exploration as potential materials for efficient degradation of organic matters in wastewater samples.

Mutual Interactions between Reduced Fe-Bearing Clay Minerals and Humic Acids under Dark, Oxygenated Conditions: Hydroxyl Radical Generation and Humic Acid Transformation.

Hydroxyl radicals (·OH) exert a strong impact on the carbon cycle due to their nonselective and highly oxidizing nature. Reduced iron-containing clay minerals (RIC) are one of the major contributors to the formation of ·OH in dark environments, but their interactions with humic acids (HA) are poorly known. Here, we investigate the mutual interactions between RIC and HA under dark and oxygenated conditions. HA decreased the oxidation rate of structural Fe(II) in RIC but significantly promoted the ·OH yield. HA dissolved a fraction of Fe(II) from RIC to form an aqueous Fe(II)-HA complex. ·OH were generated through both heterogeneous (through oxidation of structural Fe(II)) and homogeneous pathways (through oxidation of aqueous Fe(II)-HA species). RIC-mediated ·OH production by providing H2O2 to react with Fe(II)-HA and electrons to regenerate Fe(II)-HA. This highly efficient homogeneous pathway was responsible for increased ·OH yield. Abundant ·OH significantly decreased the molecular size, bleached chromophores, and increased the oxygen-containing functional groups of HA. These molecular changes of HA resembled photochemical transformation of HA. The mutual interaction between RIC and HA in dark and redox-fluctuating environments provides a new pathway for fast turnover of recalcitrant organic matters in clay- and HA-rich ecosystems such as tropical forest soils and tidal marsh sediments.

Formation of brown carbon on Fe-bearing clay from volatile phenol under simulated atmospheric conditions

Abstract Atmospheric brown carbon (BrC) has recently attracted great concerns due to its potential contribution to climate change. It was reported that dissolved Fe(III) species can act as an efficient oxidant to catalyze the polymerization of volatile phenol substances, subsequently forming atmospheric BrC in aerosols. Since Fe in the atmosphere is commonly associated with clay mineral dusts, the transformation of phenol substances on Fe-bearing clay mineral surface is potentially important for BrC formation. In this study, taking guaiacol as the model phenol substance, its brown carbon formation process was investigated on Fe(III) modified montmorillonite under different relative humidities (RHs). The spectroscopic features and compositions of the produced BrC were systematically characterized. The results demonstrated that the interface FeOH2+ is the key species to promote the polymerization of guaiacol, and RH plays a significant role to influence the surface acidity of clay and the gas-to-particle adsorption of guaiacol. The BrC was produced with the most abundance and variety under moderate RH (~33%), while higher RH (~100%) accelerated the polymerization rate. Since aerosol moisture content might not largely influence the hydroxylation states of the surface bearing Fe(III) species, it signifies the importance of clay associated Fe(III) to catalyze the BrC formation in varied conditions Our study highlights the importance of clay mineral dust on the formation of BrC, and reveals the brown carbon formation pathways under environmentally relevant conditions.

Investigation of Siderophore-Promoted and Reductive Dissolution of Dust in Marine Microenvironments Such as Trichodesmium Colonies

Desert dust is a major source of iron (Fe) to Fe-limited ocean regions. However, Fe-limited phytoplankton often struggle to obtain dust-bound Fe (dust-Fe) due to its low solubility and short residence time in the euphotic zone. Trichodesmium, a globally important nitrogen-fixing, cyanobacterium, is uniquely adapted for utilizing dust as an Fe-source. Trichodesmium can actively collect and concentrate dust particles within the core of its colonies and enhance dust-Fe dissolution rates via two bio-dissolution mechanisms: reduction and complexation by a siderophore. Here, mimicking bio-dissolution in Trichodesmium colonies, we studied the kinetics of desert dust dissolution by a siderophore and a reductant in seawater. By concurrent measurements of dissolved Fe, silica and aluminum we recognized two major mineral pools that released Fe into seawater over an 8-day period: Fe(hydr)oxides and aluminosilicates. In the presence of the siderophore desferrioxamine-B, we observed two stages of dissolution: a short stage of fast Fe dissolution followed by a lasting stage of slow Fe dissolution that was highly correlated to aluminum and silica dissolution. In the presence of the reductant, ascorbate, Fe dissolution was not correlated to aluminum and silica dissolution and was relatively slow. Based on these observations and on dust minerology, we constructed a conceptual model for dust-Fe dissolution by a siderophore and a reductant from two major mineral pools: reductive and siderophore-promoted dissolution of Fe(hydr)oxides and slow continuous dissolution of Fe-bearing clays in the presence of a siderophore. Our findings highlight the importance of clays as an Fe source to Trichodesmium and possibly to marine phytoplankton in general and can be further used to assess the contribution of dust-Fe to the Fe requirements of natural Trichodesmium colonies. From our measured bio-dissolution rates and concentrations of dust retained within colonies from the Gulf of Aqaba, we calculated the supply of dissolved Fe from dust to Trichodesmium. Based on published Fe-quotas and growth rates we calculated Trichodesmium’s Fe requirements under Fe-limited and Fe-replete conditions. The calculated dissolved Fe supply from dust, retained by a colony, can fulfil the minimal Fe requirements of slow growing, Fe-limited colonies, but cannot support fast growth and/or higher cellular Fe quotas.

Mechanisms of Enhanced Antibacterial Activity by Reduced Chitosan-Intercalated Nontronite

Previous studies have documented the antibacterial activity of certain iron-containing clays. However, the repulsion between negatively charged bacteria and the clay surface makes this process inefficient. The objective of this study is to improve the bactericidal efficiency of clays by reversing their surface charge from negative to positive. To achieve this objective, positively charged chitosan, a nontoxic and biodegradable polymer, was intercalated into nontronite NAu-2. Chitosan-intercalated NAu-2 (C-NAu-2) was chemically reduced to obtain reduced C-NAu-2 (rC-NAu-2). Relative to reduced nontronite (rNAu-2), the antibacterial activity of rC-NAu-2 is higher and more persistent over a pH range of 6-8. The close spatial association between positively charged rC-NAu-2 and negatively charged bacteria increases the chances of cell membrane attack by extracellular ROS, the influx of soluble Fe2+ into the bacterial cell, and the yield of intracellular ROS. All these factors contribute to the enhanced antibacterial activity of rC-NAu-2. In contrast to rNAu-2 treated E. coli cells, where membrane damage and intracellular ROS/Fe accumulation are restricted to the polar regions, the close bacteria-clay association in rC-NAu-2 results in nonselective membrane damage and more uniform intracellular ROS/Fe distribution across whole bacterial cells. These results advance the antibacterial model by highlighting the importance of bacteria-clay interactions to the antibacterial activity of Fe-bearing clays.

Coupling quinoline degradation with Fe redox in clay minerals: A strategy integrating biological and physicochemical processes

Quinoline is a common hazardous pollutant especially in fossil-fuel related wastewaters. Various approaches including biological degradation and physicochemical treatments (e.g., advanced oxidation processes, AOPs) have been applied for quinolone removal, however individual efficiency of these methods was usually not desirable. Here we reported that quinoline could be extensively degraded by combining biodegradation and AOPs during Fe redox cycling in the clay minerals. Four microbial consortia containing diverse functional groups of bacteria were successfully enriched from industrial coking wastewater, which were capable of reducing Fe(III) in nontronite (NAu-2) when coupled with quinoline degradation, with rates higher than those under other anaerobic condition (e.g., nitrate or sulfate reduction). Subsequent aeration of reduced Fe(II) in NAu-2 produced •OH that non-selectively oxidized quinoline. Quinoline was largely sorbed by Fe(III)-containing clay but desorbed when Fe(III) was bio-reduced, making Fe-bearing clay an easily recyclable substance for quinoline treatment. Quinoline was transformed into diverse products through several pathways during Fe(III) bioreduction and Fe(II) oxidation. Overall, our results suggest that Fe-rich clay mineral plays an important role in biological and chemical degradation of quinoline.

Fe(II)–Fe(III) Electron Transfer in a Clay Mineral with Low Fe Content

Iron-containing clay minerals are ubiquitous in soils, sediments, and water and provide a significant source of redox-active Fe that is known to influence metal sorption, contaminant fate, and nutrient cycling. While biological reduction of clay minerals has been known for some time, it has only recently been shown that Fe(II) can abiotically reduce Fe(III) in clay minerals. Here, we used Mossbauer spectroscopy to show that Fe(II) reduces an extensive amount of Fe(III) in a low-Fe clay mineral (Wyoming montmorillonite, SWy-2, 2.3 wt % Fe). The extent of reduction ranges from 12 to 78% over a pH range of 4.0–7.5 and Fe(II) concentration from 0.4 to 2.2 mM and increases as the amount of sorbed Fe(II) increases until about half of the mineral is reduced. It is unclear how such extensive reduction occurs in an Fe-bearing clay mineral with such a low Fe concentration. With only 2.3 wt % Fe in SWy-2, Fe atoms in the clay mineral are spatially isolated and electron conduction or hopping between neighboring Fe at...

Soluble iron dust export in the high altitude Saharan Air Layer

Every summer huge amounts of desert dust particles are exported from the hyperarid subtropical Sahara to the North Atlantic the so-called Saharan Air Layer (SAL), a dry, warm and dust-laden corridor that expands from the North African coast (1–5 km.a.s.l.) to the Americas above the marine boundary layer. Because of the potential impact of the dust deposited on the ocean on marine biogeochemistry and climate, we studied the Fe solubility (in seawater) of atmospheric aerosols samples directly collected in the SAL off the North African coast, i.e. the fresh aerosols recently exported from the Sahara in the SAL. The aerosol sampling was performed at ∼2400 m.a.s.l. in Izaña observatory in Tenerife island. In the total aerosols, we found low Fe concentrations and high fractional Fe solubility (FFS ∼2%) in the North Atlantic free troposphere airflows and high Fe concentrations and low FFS (∼0.7%) within the SAL; the resulting FFS versus total dust (or total Fe) plot shows a hyperbolic trend attributed to the conservative mixing of ‘fine combustion aerosols’ and ‘lithogenic mineral dust’. We then focused on the soluble Fe in the SAL. Our results indicate that ∼70% of soluble Fe is associated with the dissolution of submicron dust particles, probably involving Fe-bearing clays. We found a FFS of submicron dust (∼6%) higher than that typically observed in submicron particles of soil dust samples (<1%). The correlation between FFS and the ammonium-sulphate/dust ratio and the low variability in the Fe/Al ratio in the dust samples, suggests that the increase in the FFS of submicron dust aerosols (with respect to soil dust particles) may be related to the presence of acid pollutants mixed with dust. Previous studies had focused on dust processing and changes of Fe solubility during the trans-Atlantic transport of dust in the SAL. We found that submicron dust exported off the coast of North Africa may have already experienced acid processing over the Sahara, i.e. before dust export to the Atlantic. Export of soluble submicron Fe dust and deposition of coarse and depleted in soluble Fe dust particles during the trans-Atlantic transport may account for the observed variability in dust, soluble Fe and FFS.

Fe-bearing Clay Mineral: A Significant Fe Pool for Abiotic Electron Transfer

Fe-bearing Clay Mineral: A Significant Fe Pool for Abiotic Electron Transfer

Insights into the Mechanism of Fe(II) Adsorption and Oxidation at Fe–Clay Mineral Surfaces from First-Principles Calculations

Interfacial reactivity of redox-active iron-bearing mineral surfaces plays a crucial role in many environmental processes including biogeochemical cycling of various elements and contaminants. Herein, we apply density functional theory (DFT) calculations to provide atomistic insights into the heterogeneous reaction between aqueous Fe(II) and the Fe-bearing clay mineral nontronite Fe2Si4O10(OH)2 by studying its adsorption mechanism and interfacial Fe(II)–Fe(III) electron transfer (ET) at edge and basal surfaces. We find that edge-bound Fe(II) adsorption complexes at different surface sites (ferrinol, silanol, and mixed) may coexist on both (010) and (110) edge facets, with complexes at ferrinol FeO(H) sites being the most energetically favorable and coupled to proton transfer. Calculation of the ET activation energy suggests that interfacial ET into dioctahedral Fe(III) sheets is probable at the clay edges and occurs predominantly but not exclusively through the complexes adsorbed at ferrinol sites and might also involve mixed sites. No clear evidence is found for complexes on basal surface that are compatible with ET through the basal sheet despite this experimentally hypothesized ET interface. This study suggests a strong pH-dependence of Fe(II) surface complexation at basal versus edge facets and highlights the importance of the protonation state of bridging ligands and proton coupled electron transfer to facilitate ET into Fe-rich clay minerals.

New redox-active material for permeable water remediation systems

Reduction of structural iron in Fe-bearing clays could be used for reductively treating contaminated waters. When used in permeable water treatment systems, clays are generally mixed with sand to satisfy required hydraulic properties. Depending on compaction and pore size, clay particles can migrate within the clay/sand mix resulting in progressive clogging and failure of the system. In this study, a novel Fe-bearing clay-material suitable for permeable water treatment systems has been developed. Particles of NAu-2 and SWy-2 smectite were attached to the surface of sand grains using a polymer binding agent, with optimum coating levels of 62 and 51mg clay/g sand, respectively. The quality and extent of the clay coating are strongly affected by the type of sand being used, very poor coatings being obtained with smooth quartz grains compared to rougher and more porous surfaces. The clay-coatings are stable under most pH and redox conditions and are reducible at room temperature with obtained Fe(II)/Total Fe ratio of 55% to 83%. The CEC of the two smectites decreases when coated on sand but the chemically reduced samples are able to reduce Cr6+ to Cr3+ at room temperature. At a compaction density of 1300kg/m3, the permeability of NAu-2 clay-coated sand remains at ~1×10−4m/s after multiple reduction and reoxidation cycles.

Stratigraphic setting, facies types, depositional environments and mechanism of formation of Ash Shumaysi ironstones, Wadi Ash Shumaysi, Jeddah district, West Central Saudi Arabia

The ironstones of Wadi Ash Shumaysi are mainly enclosed within the middle part of the fluvio-lacustrine siliciclastic succession of Ash Shumaysi Formation (Oligo-Miocene). This succession is composed mainly of conglomerates, sandstones, siltstones, muddy, sandy and glauconitic ironstones, fresh water carbonates, tuffaceous mudstone and basalts in descending order. The study aims to determining the stratigraphic setting, facies types, depositional environments, and mechanism of formation of the succession and the associated ironstones, through field and petrographic investigations. The succession comprises the following four main facies associations arranged according to their predominance from the lower to the upper parts of the succession of Ash Shumaysi Formation: I: Proximal (Conglomerate Facies) Braided Streams, II: Distal (Sandstone Facies) Braided Streams, III: Floodplain facies association, including ironstones, IV: Lacustrine delta, including ironstones. The ironstones of facies associations III and IV represent the following types: 1. Sandy ironstones (ironstone type 1 (IST 1)): which formed by the deposition of water laid detrital quartz grains and Fe-bearing clays under different water depth and currents. 2. Glauconitic muddy ironstones (ironstone type 2 (IST 2)): which formed by the deposition of Fe-bearing clays within slightly agitated starved lakes. During periods of low clastic input, the deposited Fe-bearing clays become converted into green glauconitic clays as a result of the high Fe, Mg, K, Al, and Si activities. During ultimate stages of oxidized diagenetic alterations, the green glauconitic clays become hematitized into glauconitic ironstones. 3. Oolitic muddy ironstones-oolitic ferruginous sandstone (IST 3): which is represented by lacustrine successive shallowing-upward cycles. Each of these cycles begins by friable ferruginous in situ oolitized muddy ironstone which grades upward into oolitic sandy ironstone/oolitic ferruginous sandstone. The second interval is recorded overlying the fining-upward cycle of facies F5. 4. Flaser laminated silty ironstone/ferruginous siltstone (IST 4): which consists of small-scale shallowing-upward cycles of parallel laminated hematitic siltstone and flaser laminated silty ironstone/ferruginous siltstone. In this ironstone type, the inputted depositional components are composed mainly from silt-sized detrital quartz grains embedded within Fe-bearing clays. These components are vertically and laterally distributed within the depositional environments by the impact of the currents and wave actions. The diagenetic processes play an important role in the petrographic and mineralogic evolution of ironstone type IST 1. These processes includes: (a) the diagenetic recrystallization and dehydration of the precursor amorphous Fe-bearing clays and the intimately associated Fe-oxyhydroxies and formation of ferrihydrites, goethite. and hematite; (b) corrosion and embayment of the quartz grains by the enclosing different Fe mineral phases; and (c) in situ pellitization and oolitization throughout the tangential growth of the Fe-oxhyhdroxides during their dehydration and recrystallization and formation of pseudo-ooids. The diagenetic processes in ironstone type IST 2 includes glauconitization of the remained unglauconitized clays during the depositional stages, hematitization and the related oolitization, and pelletization processes. The main diagenetic processes involved in the formation of ironstone type IST 3 are: (a) hematitization of the amorphous Fe-clays and formation of goethite and hematite cement, (b) in situ pelletization and oolitization. The most important diagenetic process in ironstone type IST 4 is the dehydration and recrystallization of the amorphous Fe-clays and formation of goethite and hematite cement.

Letter to the Editor on Fougerite

AbstractFougerite (IMA 2003-057) was accepted by the Commission on New Minerals and Mineral Names of the International Mineralogical Association (IMA) in 2004 as a new mineral to represent the green rust (GR) family. The data on which it was approved, however, are inconsistent. X-ray diffraction patterns from the Fougères soils contain no peaks that could meaningfully be attributed to a GR phase. The sequential dissolution procedure used to identify GR in the soils was not rigorously tested for selectivity. If indeed it is selective, the results indicate the presence of 40–78% Fe in minerals other than GR. Other Fe-bearing phases were not included in the interpretation of the spectroscopy data that were presented. The data are consistent with the presence of Fe-bearing clays and other silicate minerals. In light of the ambiguous and conflicting data, we recommend that the case for fougerite as a mineral be re-evaluated by the IMA.

Spectroscopic Evidence for Interfacial Fe(II)−Fe(III) Electron Transfer in a Clay Mineral

Interfacial electron transfer has been shown to occur between sorbed Fe(II) and structural Fe(III) in Fe oxides, but it is unknown whether a similar reaction occurs between sorbed Fe(II) and Fe(III)-bearing clay minerals. Here, we used the isotopic specificity of (57)Fe Mössbauer spectroscopy to demonstrate electron transfer between sorbed Fe(II) and structural Fe(III) in an Fe-bearing smectite clay mineral (NAu-2, nontronite). Mössbauer spectra of NAu-2 reacted with aqueous (56)Fe(II) (which is invisible to (57)Fe Mössbauer spectroscopy) showed direct evidence for reduction of NAu-2 by sorbed Fe(II). Mössbauer spectra using aqueous (57)Fe(II) showed that sorbed Fe(II) is oxidized upon sorption to the clay and pXRD patterns indicate that the oxidation product is lepidocrocite. Spectra collected at different temperatures indicate that reduction of structural Fe(III) by sorbed Fe(II) induces electron delocalization in the clay structure. Our results also imply that interpretation of room temperature and 77 K Mössbauer spectra may significantly underestimate the amount of Fe(II) in Fe-bearing clays. These findings provide compelling evidence for abiotic reduction of Fe-bearing clay minerals by sorbed Fe(II), and require us to reframe our conceptual model for interpreting biological reduction of clay minerals, as well as contaminant reduction by reduced clays.

Reductive Degradation of p,p′-DDT cy Fe(II) in Nontronite NAu-2

AbstractClay minerals are abundant in soils and sediments and often contain Fe. Some varieties, such as nontronites, contain as much as 40 wt.% Fe2O3 within their molecular structure. Several studies have shown that various Fe-reducing micro-organisms can use ferric iron in Fe-bearing clay minerals as their terminal electron acceptor, thereby reducing it to ferrous iron. Laboratory experiments have also demonstrated that chemically or bacterially reduced clays can promote the reductive degradation of various organics, including chlorinated pesticides and nitroaromatics. Therefore, Fe-bearing clays may play a crucial role in the natural attenuation of various redox-sensitive contaminants in soils and sediments. Although the organochlorinated pesticide p,p′-DDT is one of the most abundant and recalcitrant sources of contamination in many parts of the world, the impact of reduced Fe-bearing clays on its degradation has never been documented. The purpose of the present study was to evaluate the extent of degradation of p,p′-DDT during the bacterial reduction of Fe(III) in an Fe-rich clay. Microcosm experiments were conducted under anaerobic conditions using nontronite (sample NAu-2) spiked with p,p′-DDT and the metal-reducing bacteria Shewanella oneidensis MR-1. Similar experiments were conducted using a sand sample to better ascertain the true impact of the clay vs. the bacteria on the degradation of DDT. Samples were analyzed for DDT and degradation products after 0, 3, and 6 weeks of incubation at 30°C. Results revealed a progressive decrease in p,p′-DDT and increase in p,p′-DDD concentrations in the clay experiments compared to sand and abiotic controls, indicating that Fe-bearing clays may substantially contribute toward the reductive degradation of DDT in soils and sediments. These new findings further demonstrate the impact that clay materials can have on the natural attenuation of pollutants in natural and artificial systems and open new avenues for the passive treatment of contaminated land.

Characterization of a redox-modified clay mineral with respect to its suitability as a barrier in radioactive waste confinement

Engineered barriers for high-level nuclear waste (HLW) consist of excavated repositories in sub-surface rock formations where canisters holding the radionuclide are stored. Clay minerals, particularly the swelling 2:1 types, are used as backfill material, both in the canisters and in the bore hole, in order to prevent radionuclide transport to surrounding groundwater. One of the most important risks that can occur is the corrosion of the canister, which could be coupled with reduction of iron (Fe) in the clay structure. Such changes could greatly decrease the long-term stability of the clay and, consequently, of the barriers themselves. In order to test the potential effects of such redox interactions, an Fe-bearing clay mineral from a commercial source located in the Kutch region, India, was selected for study. This particular mineral is one of the candidate clay minerals to serve as such a barrier material, and is the one with the largest structural Fe content. Results from it should, therefore, provide maximum insight into the potential effects of redox interactions between the barrier and its surroundings. The unaltered clay was characterized by X-ray powder diffractometry (XRD), thermal gravimetric analysis (TGA/DTGA), Fourier-transform infrared (FTIR) spectroscopy, and variable-temperature Mössbauer spectroscopy. The chemically reduced and reoxidized forms of the clay were characterized by variable-temperature Mössbauer spectroscopy and chemical analysis. In the unaltered state the clay is comprised of smectite, maghemite, superparamagnetic goethite, and hematite, with a possible trace of kaolinite. In the reduced state the Fe (oxyhydr)oxides were dissolved. Upon reoxidation no six-line pattern was observed, indicating that the Fe remained only in the structure of the silicates. The final structure of the reduced–reoxidized clay contained more defects than the original clay, as revealed by greater quadrupole splitting values for structural Fe(III) in the clay. These findings indicate that upon exposure to natural redox cycles the Kutch clay could undergo permanent changes in its mineralogical composition and clay mineral structure, but further study is required to ascertain the effects that such changes would have on its long-term stability as a barrier material.