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To simulate iron consumption in soils, iron leaching from silicate minerals due to three heterotrophic bacterial strains and a chemical treatment was studied using hybrid silica gel ( HSG) doped with two phyllosilicates, nontronite ( NAu-2) or low-iron-content montmorillonite ( SWy-2). HSG methodology, a novel way of separating bacteria cells from a colloidal mineral source, consisted in embedding colloidal mineral particles into an amorphous porous silica matrix using a classical sol-gel procedure. Pantoae agglomerans PA 1 and Rahnella aquatilis RA 1 were isolated from silicate-rich soils, that is, beech and wheat rhizospheres ( Vosges, France); Burkholderia sp. G 5 was selected from acidic and nutrient-poor podzol soils ( Vosges, France). Fe release from clay minerals and production of bacterial metabolites, that is, low molecular weight organic acids ( LMWOA) and siderophores, were monitored. Two LMWOA profiles were observed with major gluconate production (> 9000 μM) for Burkholderia sp. G 5 and moderate production of lactate, acetate, propionate, formate, oxalate, citrate, and succinate (< 300 μM) for R. aquatilis RA 1 and P. agglomerans PA 1. HSG demonstrated its usefulness in revealing clay mineral-microorganisms interactions. The effect of bacterial exsudates was clearly separated from physical contact effect. [ABSTRACT FROM AUTHOR]

Abstract: Four colloidal-size fractions of strongly anisotropic particles of nontronite (NAu-2) having different ratios of basal to edge surfaces were incubated in the presence of heterotrophic soil bacteria to evaluate how changes in mineral surface reactivity influence microbial dissolution rate of minerals. To avoid any particle aggregation, which could change the reactive surface area available for dissolution, NAu-2 particles were immobilized in a biocompatible TEOS-derived silica matrix. The resulting hybrid silica gels support bacterial growth with NAu-2 as the sole source of Fe and Mg. Upon incubation of the hybrid material with bacteria, between 0.3% and 7.5% of the total Fe included in the mineral lattice was released with a concomitant pH decrease. For a given pH value, the amount of released Fe varied between strains and was two to twelve-fold higher than under abiotic conditions. This indicates that complexing agents produced by bacteria play an important role in the dissolution process. However, in contrast with proton-promoted NAu-2 dissolution (abiotic incubations) that was negatively correlated with particle size, bacterial-enhanced dissolution was constant for all size fractions used. We conclude that bio-dissolution of nontronite particles under acidic conditions seems to be controlled by bacterial metabolism rather than by the surface reactivity of mineral. [Copyright &y& Elsevier]

Abstract: Anisotropic textural and crystallographic properties of phyllosilicate particles often influence the mineral weathering rate. The purpose of this study was to investigate how the changes in mineral surfaces (basal vs. edge) as a result of changes in crystal size control the dissolution of the mineral. Different nano-size fractions of Na-exchanged nontronites (NAu2 and NAu1) were immobilized in a silica gel and then incubated under acidic conditions using HNO3 at 28°C for 5days. For each sample, the dissolution behavior was analyzed by measuring the amount of iron released from the mineral lattice. The results showed that for a given pH, a decrease in particle size significantly increased NAu2 and NAu1 dissolution. At pH 1.5, 7.2% of the total iron of the highest size sample of NAu2 was released in solution whereas this proportion increased up to 25% for the smallest size fraction. The percentage of total iron extracted from NAu1 at the same pH (1.5) was less important: 3.5% and 6.5% for higher and smaller size fractions, respectively. The observed increase in dissolution was not directly correlated to the increase in the amount of edge faces, suggesting that all mineral surfaces contributed to mineral dissolution. In the present case this may be related to the fact that 8% and 2% of total iron of NAu2 and NAu1, respectively, are located in the tetrahedral sheet. In conclusion, the basal surface of nontronites plays an important role in the weathering process. [Copyright &y& Elsevier]