Abstract--The effect of Fe oxidation state on the surface charge (CEC) and solubility of smectites were studied using the <2-t~m, Na+-saturated fraction of an Upton, Wyoming; a Czechoslovakian; and a New Zealand montmorillonite; and a Garfield, Washington, nontronite. The reduction of structural Fe 3+ in the octahedral sheet of each clay produced a net increase in the negative surface charge of the clay. The observed cation-exchange capacities deviated from the linear relationship predicted by charge-deficit calculations, assuming changes only in the Fe2+/Fe a+ ratio, and reversibly followed Fe reduction according to a 2nd-degree polynomial function. The deviations suggest reversible changes in mineral structiite and composition during Fe reduction.These clays were susceptible to partial dissolution in citrate bicarbonate (CB) and citrate-bicarbonatedithionite (CBD) solutions. Small amounts of Fe and Si dissolved as a result of Fe reduction in CBD, but affected < 1% of the total clay mass except for the Czechoslovakian clay in which 2% of the clay dissolved. Although slightly more Fe dissolved than Si, no change in surface charge was noted. Almost no dissolution of these elements was detected in CB solution. In contrast, significant Al was detected in the CB solution, suggesting a heterogeneous dissolution mechanism. The CEC, however, was unchanged by the CB treatment. These results may be explained by the adsorption of hydrogen ions into the vacated AP + sites in the mineral structure. Dissolution seems to have been independent of the effects of Fe oxidation state on surface charge.

Abstract--Using Garfield, Washington, nontronite as the model mineral system, methods and apparatus were developed to prepare reduced suspensions in citrate-bicarbonate-dithionite (CBD) solution. These techniques were effective in removing excess, undesired solutes from reduced suspensions while maintaining a high Fe 2 § content. They also enabled the preparation of dried, reduced films preferentially oriented with respect to the crystallographic c-axis. Supernatant solutions were collected and analyzed for Fe, A1, and Si, from which the extent of dissolution of the clay as a result of CBD treatment was assessed. Results indicated that very little Fe and Si were released to solution, but as much as about 8% of the total A1 was solubilized. The highest levels of A1 in solution Were observed in CB treatments without dithionite.

A quantitative study of microbial clay mineral reduction coupled to the oxidation of organic carbon was carried out using an Fe(III)-reducing bacterium (Shewanella putrefaciens strain MR-1). Total CO2 production, organic acid depletion, and Fe(III) reduction were measured in the same cultures with formate or lactate as the carbon source and clay as the sole electron acceptor. Mean ratios of 1.6:1 and 4.9:1 were observed for structural Fe(III) reduction coupled to formate oxidation and lactate oxidation, respectively. When organic ligands were added under similar culture conditions, the extent of clay reduction was enhanced up to 2-fold in the order of nitrilotriacetic acid (NTA) > oxalate > citrate > malate. Further, dissolution of the clay mineral structure was inferred as dissolved Fe(II) comprised up to 50% of the total clay-bound Fe reduced in cultures to which organic ligand was added. Here we provide the first direct measurements which show that (1) bacteria may couple the respiration of Fe(III) bound in smectite clay minerals to carbon cycling, (2) organic ligands increase the bioavailability of Fe(III) bound in clay minerals, and (3) bacterial Fe(III) reduction in the presence of organic ligands may lead to clay mineral dissolution. These discoveries have important implications for the biogeochemistry of soils where Fe(III)-bearing clay minerals are abundant.

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Abstraet--Shewanella putrefaciens is a species of metal-reducing bacteria with a versatile respiratory metabolism. This study reports that S. putrefaciens strain MR-1 rapidly reduces Fe(III) within smectite clay minerals. Up to 15% of the structural Fe within ferruginous smectite (sample SWa-1, Source Clays Repository of the Clay Minerals Society) was reduced by MR-1 in 4 h, and a range of 25% to 41% of structural Fe was reduced after 6 to 12 d during culture. Conditions for which smectite reduction was optimal, that is, pH 5 to 6, at 25 to 37 ~ are consistent with an enzymatic process and not with simple chemical reduction. Smectite reduction required viable cells, and was coupled to energy generation and carbon metabolism for MR-1 cultures with smectite added as the sole electron acceptor. Iron(III) reduction catalyzed by MR-1 was inhibited under aerobic conditions, and under anaerobic conditions it was inhibited by the addition of nitrate as an alternate electron acceptor or by the metabolic inhibitors tetrachlorosalicylanilide (TCS) or quinacrine hydrochloride. Genetic mutants of MR-1 deficient in anaerobic respiration reduced significantly less structural Fe than wild-type cells. In a minimal medium with formate or lactate as the electron donor, more than three times the amount of smectite was reduced over no-carbon controls. These data point to at least one mechanism that may be responsible for the microbial reduction of clay minerals within soils, namely, anaerobic respiration, and indicate that pure cultures of MR-1 provide an effective model system for soil scientists and mineralogists interested in clay reduction. Given the ubiquitous distribution and versatile metabolism of MR-I, these studies may have further implications for bioremediation and water quality in soils and sediments.

The reduction of structural Fe in smectite may be mediated either abiotically by reaction with chemical reducing agents or biotically by reaction with various bacterial species. The effects of abiotic reduction on clay surface chemistry are much better known than the effects of biotic reduction, and differences between them are still in need of investigation. The purpose of the present study was to compare the effects of dithionite (abiotic) and bacteria (biotic) reduction of structural Fe in nontronite on the clay structure as observed by variabletemperature Mössbauer spectroscopy. Biotic reduction was accomplished by incubating Na-saturated Garfield nontronite (sample API 33a) with Shewanella oneidensis strain MR-1 (Fe II /total Fe achieved was ~17 %). Partial abiotic reduction (Fe II /total Fe ~23 %) was achieved using pH-buffered sodium dithionite. The nontronite was also reduced abiotically to Fe II /total Fe ~96 %. Parallel samples were reoxidized by bubbling O 2 gas through the reduced suspensions at room temperature prior to Mössbauer analysis at 77 and 4 K. At 77 K, the reduction treatments all gave spectra composed of doublets for structural Fe II and Fe III in the nontronite. The spectra for reoxidized samples were largely restored to that of the unaltered sample, except for the sample reduced to 96 %. At 4 K, the spectrum for the 96 % reduced sample was highly complex and clearly reflected magnetic order in the sample. When partially reduced, the spectrum also exhibited magnetic order, but the features were completely different depending on whether reduced biotically or abiotically. The biotically reduced sample appeared to contain distinctly separate domains of Fe II and Fe III within the structure, whereas partial abiotic reduction produced a spectrum representative of Fe II -Fe III pairs as the dominant domain type. The 4 K spectra of the partially reduced, fully reoxidized samples were virtually the same as at 77 K, whereas reoxidation of the 96 % reduced sample produced a spectrum consisting of a magnetically ordered sextet with a minor contribution from a Fe II doublet, indicating significant structural alterations compared to the unaltered sample.

A photochemical method for measuring Fe3+ and total Fe in minerals is described. The method determines Fe2+ concentration by measuring the Fe(phen)32+ (phen = 1,10‐phenanthroline) complex formed during HF‐H2SO4 digestion of the mineral. To measure Fe2+ accurately in the presence of Fe3+ from the mineral, the sample digestion and analysis are performed under red light to prevent photochemical reduction of the ferric‐phen species. Total iron is measured by converting any Fe3+ in the digestate to Fe(phen)32+ by photochemical reduction using a fluorescent lamp. This procedure avoids the problems associated with adding chemical reducing agents to iron‐phen solutions. The calibration curves were linear up to 8‐µg Fe/ml with a lower detection limit of 0.011 µg/ml. The absorptivities of the calibration curves were 0.1852 ± 0.0017 and 0.1960 ± 0.0018 ml/cm‐µg for Fe2+ and total Fe, respectively. Ferric iron in the mineral samples was calculated by difference.

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