Microbial reduction of structural iron in clays - a renewable source of reduction capacity

The ability of microorganisms to reduce structural Fe in minerals has become recognized as an important mechanism in the oxidation of pollutants in soils and sediments. To better understand the factors controlling the concentration of Fe(II) in natural deposits, samples of clayey till from sites near Havrebjerg and Sparresholm, Denmark, were incubated for up to 160 d with a combination of Pseudomonas bacteria strains. These strains were previously shown to reduce structural Fe(III) to Fe(II) in smectites. According to Eh, pH, microbial activity, and structural Fe(II) content, the incubation could be separated into four phases of which the second phase (from 4 to 10 d after inoculation) generated the most structural Fe(II). Microbial activity increased the structural Fe(II) content from approximately 10% to 20-34% of total clay Fe, a lower level than that obtained for chemically reduced subsamples (76-79%), but more similar to values typical of unweathered tills of Denmark (about 50%). In addition to microbial activity, the particle size (fine or coarse clay) and the Fe(II) content of the clay minerals prior to incubation were very important factors affecting the extent of Fe reduction. The results indicate that structural Fe in clay minerals of clayey till is very reactive and can undergo extensive in situ redox cycling. Electron transfer to structural Fe(III) of the weathered, oxidized clay minerals may renew natural redox barriers in imperfectly drained soils and sediments and thereby control geochemical processes.