Swelling and texture of iron-bearing smectites reduced by bacteria

Microbial reduction of clay mineral structural Fe(III) decreases the swelli ng of nontronite gels, most importantly at intermediate oxidation states (4 0 to 80 cmol Fe(II) kg(-1) clay). The purpose of this study was to establis h whether microbial reduction of structural Fe(III) decreased the swelling of other Fe-bearing smectites and to discern the influence that organic com pounds 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 ba cteria or a mixture of anaerobic bacteria. The influence of organics on cla y swelling was estimated on smectites suspended in either organic or inorga nic 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) w as employed to determine the influence of bacteria and type of media on the texture of reduced smectite gels. Reduction of structural Fe(III) by bacte ria 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 str uctural Fe reduction, as well as the initial swelling characteristics of th e Fe-bearing smectites. High structural Fe(II) contents (>50 cmol Fe(II) kg (-1)) resulted in increased attractive forces between clay platelets that d ecreased clay swelling, even in organic medium suspensions. Microbial reduc tion resulted in increased face-face association of individual clay layers, forming larger and more distinct crystallite subunits than in nonreduced c lay gels. But, perhaps more importantly, microbial reduction of structural Fe(III) resulted in an increased association between crystallite subunits a nd, thus, an overall larger particle size and pore size distribution, due t o the interaction of bacteria cells, cell fragments and organic exudates.