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 pollut ants in soils and sediments. To better understand the factors controll ing the concentration of Fe(II) in natural deposits, samples of clayey till from sites near Havrebjerg and Sparresholm, Denmark, were incuba ted 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 stru ctural Fe(II) content, the incubation could be separated into four pha ses of which the second phase (from 4 to 10 d after inoculation) gener ated the most structural Fe(II). Microbial activity increased the stru ctural Fe(II) content from approximately 10% to 20-34% of total clay F e, a lower level than that obtained for chemically reduced subsamples (76-79%), but more similar to values typical of unweathered tills of D enmark (about 50%). In addition to microbial activity, the particle si ze (fine or coarse clay) and the Fe(II) content of the clay minerals p rior to incubation were very important factors affecting the extent of Fe reduction. The results indicate that structural Fe in day minerals of clayey till is very reactive and can undergo extensive in situ red ox 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 .