Swelling and Texture of Iron-Bearing Smectites Reduced by Bacteria

Abstract

Abstract--Microbial reduction of clay mineral structural Fe(III) decreases the swelling of nontronite gels, most importantly at intermediate oxidation states (40 to 80 cmoi Fe(II) kg -1 clay). The purpose of this study was to establish whether microbial reduction of structural Fe(III) decreased the swelling of other Fe-bearing smectites and to discern the influence that organic compounds of microbial origin (bacterial cells, cell fragments and/or exudates) may have on clay swelling and texture. Structural Fe(III) was reduced by incubating smectite suspensions with either a combination of Pseudomonas bacteria or a mixture of anaerobic bacteria. The influence of organics on clay swelling was estimated on smectites suspended in either organic or inorganic media in the absence of bacteria. The gravimetric water content of the reduced clay gels equilibrated at various applied pressures was recorded as a function of Fe oxidation state. Transmission electron microscopy (TEM) was employed to determine the influence of bacteria and type of media on the texture of reduced smectite gels. Reduction of structural Fe(III) by bacteria decreased the swelling pressure of all Fe-bearing smectites. Increased clay swelling, due to the presence of organics (organic medium, exudates or cell fragments), was correlated to the total Fe content, the extent of structural Fe reduction, as well as the initial swelling characteristics of the Fe-bearing smectites. High structural Fe(II) contents (>50 cmol Fe(II) kg -~) resulted in increased attractive forces between clay platelets that decreased clay swelling, even in organic medium suspensions. Microbial reduction resulted in increased face-face association of individual clay layers, forming larger and more distinct crystallite subunits than in nonreduced clay gels. But, perhaps more importantly, microbial reduction of structural Fe(III) resulted in an increased association between crystallite subunits and, thus, an overall larger particle size and pore size distribution, due to the interaction of bacteria ceils, cell fragments and organic exudates.

Pedogenic processes and clay transformations in bisequal soils of the Southern Taiga zone

“…In Bs horizon of Profile 3, swelling mixed layers were not present, either because of Al intercalation in the interlayer after decomposition of the organic-Al complexes, or because of charge increase upon waterlogging (e.g. Gates et al,1993Gates et al, ,1998. The horizons where gleying was more intense were the E horizons, and there, swelling was detected.…”

Effects of sediment iron mineral composition on microbially mediated changes in divalent metal speciation: Importance of ferrihydrite

“…Natural sediments were chosen to encompass the suite of physical properties and iron-bearing minerals that are commonly found in natural systems. We chose to employ natural sediments with clay mineral composition dominated by kaolinite, since microbial reduction of structural Fe III in smectite and illite clays can alter clay mineral morphology and/or surface chemical properties (e.g., Gates et al, 1998;Gates et al, 1996;Kostka et al, 1999;Dong et al, 2003). The selected sediments also contained low levels of 0.5 M HCl soluble iron, thereby allowing us to specifically test the importance of ferrihydrite content through experiments with mixtures of natural sediments and synthetic ferrihydrite.…”

A Model for the Mechanism of Fe3+ to Fe2+ Reduction in Dioctahedral Smectites

“…Redox reactions modify the chemical and physical properties of Fe-containing smectites, such as cation-exchange capacity (CEC), specific surface area, swelling behavior, and ability to fix interlayer cations (Khaled and Stucki, 1991;Stucki, 1985, 1989;Stucki et al, 1984Stucki et al, , 1996. Consequently, the texture (Gates et al, 1998;Stucki and Tessier, 1991), permeability (Shen et al, 1992), and fertility of formations in which Fe-containing smectites are present (Stucki, 1988) are affected. The potential for modification of smectite properties for industrial, engineering, and environmental applications by controlling the oxidation state of Fe is great (Chen et al, 1987;Low et al, 1983).…”