Microbial Reduction of Structural Iron(III) in Smectites

Abstract

Octahedral Fe(III) in the crystal structures of three different smectites was reduced to Fe(II) by actively growing microorganisms indigenous to the clay. The smectites were SWa‐1 ferruginous smectite from Grant County, Washington; API 33a, Garfield Nontronite; and API 25, Upton montmorillonite. Bacterial growth was supported by incubating clay suspensions at room temperature in a nutrient broth solution consisting of peptone and beef extract. Some samples were first sterilized (by autoclaving), then seeded with bacteria that had been isolated previously from the SWa‐1 sample. The effect of O2 on microbial reduction of Fe(III) was also tested. Results revealed that, in all three clays, about 0.30 mmol Fe(III)/g clay was reduced to Fe(II) by bacteria in a 28‐day period. The specific organism responsible for Fe reduction has yet to be classified, but it was more efficient in samples that had not been purged of O2, and it appears to be indigenous to the SWa‐1 clay.

Dissimilatory Fe(III) and Mn(IV) Reduction

“…Fe(III)-reducing microorganisms can readily reduce the Fe(III) oxide coatings on the exterior of clay particles (193,195). However, there is only weak evidence that microorganisms may be capable of reducing some of the structural Fe(III) in clay minerals (297,338).…”

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

“…Therefore, oxido-reduction reactions of layer silicates often play an important role in environmental processes including weathering, microbial activity, and diagenetic transformations (Egashira and Ohtsubo, 1983;Stucki et al, 1987Stucki et al, , 1996. 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.…”

Reduction of Structural Fe(III) in Smectite by a Pure Culture of Shewanella Putrefaciens Strain MR-1

“…Though clay reduction within soils and sediments is thought to occur primarily as a result of the activity of indigenous microorganisms (Stucki et al 1987;Gates et al 1993), most research has focused on chemical mechanisms of Fe reduction within clays. Chemical reduction studies have employed potent inorganic t Current Address: Skidaway Institute of Oceanography, 10 Ocean Science Circle, Savannah, GA 31411. joel @ skio.peachnet.edu reductants such as dithionite or hydrazine, which are not likely to play a significant role in clay reduction for natural environments (Stucki 1988).…”