Physical chemistry of microbially-reduced smectites

Abstract

The ability of bacteria to reduce smectite structural Fe and the effect microbial reduction has on clay swelling, flow behavior, and the crystal structure was studied to better understand the reduction process. Structural Fe reduction by combinations of Pseudomonas bacteria is a diffusion driven process in which extracellular reductants may be involved in the transfer of electrons from the solution phase to crystalline Fe. The absolute magnitude of structural Fe(II) depended on the total structural Fe content of the clay mineral. Reduction of structural Fe by various bacteria cultures decreased the swelling pressure of the Fe-bearing clays studied, as measured by the gravimetric water content (m$\sb{\rm w}$/m$\sb{\rm c}$). The depression of m$\sb{\rm w}$/m$\sb{\rm c}$ by microbial reduction of structural Fe(II) was greater at Fe(II) contents less than 0.4 mmol g$\sp{-1}$ than in chemically-reduced smectites. The presence of structural Fe(II) apparently overcomes the opposing effect of organics on the ability of clay gels to retain water. The flow behavior of microbially-reduced smectite suspensions depended on smectite, medium used, incubation time, and Fe(II) content. The yield stress ($\sigma\sb{\rm e}$) for the Fe-bearing smectite suspensions increased with increasing Fe(II) content above 0.2 mmol Fe(II), indicating increased inter-particle forces of attraction. A general direct relationship existed between $\sigma\sb{\rm e}$ and the plastic viscosity ($\eta\sb{\rm pl}$) for the Fe-bearing smectite suspensions. Multi nuclear ($\sp{29}$Si, $\sp{27}$Al, and $\sp{23}$Na) magic-angle sample spinning nuclear magnetic resonance (MAS NMR) of dithionite- and microbially-reduced montmorillonite revealed that reduction of structural Fe caused reversible structural distortions in the tetrahedral sheet, lifted the paramagnetic effect of Fe(III) on quadrupolar nuclei, and resulted in an increased association of charge-balancing cations with the clay surface. The structural changes that occur upon reduction of structural Fe are consistent with numerous studies detailing the effects of reduction of structural Fe(III) on physical and chemical properties of smectites.