Dark Fe oxides and sulfides are major discoloring impurities in mined commercial white kaolin clay. In order to evaluate the potential influence of Fe-cycle bacteria on Fe cycling during post- depositional clay-weathering alteration, Fe(III)-reducing and/or Fe(II)-oxidizing microorganisms were examined in open-pit, subsurface mine samples from kaolin lenses and smectite formations collected from sites in central Georgia. Samples of varying age were examined, including late Eocene smectite overburden, hard kaolin of Middle Eocene age, soft gray kaolin from the late Paleocene, and soft tan kaolin of late Cretaceous age. These clays contained 0.06–5.33% organic carbon, which included various potential organic electron donors for bacterial metabolism: formate (1.1–30.6 mmol/kg), acetate (0–40.5 mmol/kg), lactate (0–12.1 mmol/kg), pyruvate (0.4–78 mmol/kg), oxalate (0–141.7 mmol/kg), and citrate (0–1.4 mmol/kg). All clay samples studied had small concentrations of ‘bio-available’ Fe(III) (0.5 M HCl-extractable Fe, 0.5–2.8 mmol/kg) compared to total Fe (HF- extractable, 25–171.9 mmol/kg). The highest Fe(III)/[Fe(II)+Fe(III)] ratio and the lowest organic carbon content were in kaolin samples in which Fe(III) reduction was determined to be the dominant terminal electron accepting process by hydrogen analysis. All clay samples showed greater numbers of Fe(II)-oxidizing bacteria (22–22,000 cells/g) than Fe(III)-reducing bacteria (3–410 cells/g) as determined by MPN analysis. The Fe(III)-reducing activity in clays could be stimulated with the addition of 1 mM of the Fe(III) chelator, nitrilotriacetic acid. The addition of nitrate stimulated anaerobic Fe(II) oxidation. These results suggest that anaerobic bacteria involved in both oxidation and reduction of Fe exist in these subsurface clay formations, and might have had an influence on post-depositional weathering reactions.

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