The impact of structural Fe(III) reduction by bacteria on the surface chemistry of smectite clay minerals

Abstract

Although clay mineral reduction is thought to occur primarily as a result of the activity of indigenous microorganisms in soil, most research has focused on chemical mechanisms of Fe reduction within clay minerals. Here we show that bacteria isolated from soils and sediments catalyze the rapid reduction of structural Fe(III) in the smectite clay minerals. The extent of Fe(III) reduction is large, from 46% to >90%. Furthermore, the effects of structural Fe(III) reduction by bacteria on the surface chemistry of smectites are dramatic. Swelling pressure, as measured by water content, was shown to decrease by 40% to 44% in smectites reduced by bacteria as compared to unaltered or reoxidized smectites. Particle surface area decreased by 26% to 46% in response to bacterial reduction, and the surface charge density as measured by the ratio of cation exchange capacity to specific surface area increased over the same scale. Measurements of swelling pressure in smectite saturated with the organic cation trimethylphenylammonium (TMPA) indicated that the hydrophilic character of the clay mineral surface was enhanced upon reduction. The valence state of Fe in the octahedral layer of smectite, as revealed through reflectance spectra, correlated to the amount of Fe(III) reduced in bacterial cultures, providing information on the mechanism of intervalence electron transfer in bacterially reduced clay minerals. The extent of reduction and surface chemical effects catalyzed by bacteria in this study are similar in magnitude to those observed previously for potent inorganic reductants. Given that clay minerals dominate the solid phase of porous media and that Fe(III)-reducing bacteria are abundant in soils and aquatic sediments, these data suggest that bacterial clay mineral reduction may play an important role in soil biogeochemistry, affecting processes such as nutrient cycles and the fate of organic contaminants.