Solubilization of Fe(III) oxide-bound trace metals by a dissimilatory Fe(III) reducing bacterium

Trace metals associate with Fe(III) oxides as adsorbed or coprecipitated sp ecies, and consequently, the biogeochemical cycles of iron and the trace me tals are closely linked. This communication investigated the solubilization of coprecipitated Co(III) and Ni(II) from goethite: (alpha -FeOOH) during dissimilatory bacterial iron reduction to provide insights on biogeochemica l factors controlling trace-element fluxes in anoxic environments. Suspensi ons of homogeneously substituted Co-FeOOH (50 mmol/L as Co0.01Fe0.99OOH; Co -57- labeled) in eight different buffer/media solutions were inoculated wit h a facultative. metal-reducing bacteria isolated from groundwater (Shewane lla putrefacians CN32), and incubated under strictly anaerobic conditions f or periods up to 32 days. Lactate (30 mmol/L) was provided as an electron d onor. Growth and non-growth promoting conditions were established by adding or withholding PO4 and/or trace metals (Co-60-labeled) from the incubation media. Anthraquinone disulfonate (AQDS; 100 mu mol/L) was added to most su spensions as an electron shuttle to enhance bacterial reduction. Solutions were buffered at circumneutral pH with either PIPES or bicarbonate buffers. Solid and liquid samples were analyzed at intermediate and final time poin ts for aqueous and sorbed/precipitated (by HCl extraction) Fe(II) and Co(II ). The bioreduced solids were analyzed by X-ray diffraction and field-emiss ion electron microscopy at experiment termination. Ni-FeOOH (Ni0.01Fe0.99OO H) was used for comparison in select experiments. Up to 45% of the metal co ntaining FeOOH was bioreduced: growth-supporting conditions did not enhance reduction. The biogenic Fe(II) strongly associated with the residual Fe(II I) oxide as an undefined sorbed phase at low fractional reduction in PIPES buffer, and as siderite (FeCO3) in bicarbonate buffer or as vivianite [Fe-3 (PO4)(2). 8H(2)O] when P was present. Cobalt(III) was reduced to Co(II) in proportion to its mole ratio in the solid. The release of bioreduced Co(II) to the aqueous phase showed complex dependency on the media and buffer com position and the fractional reduction of the Co-FeOOH. In most cases Co(II) was solubilized in preference to Fe(Il), but in select eases it was not. T hese differences were rationalized in terms of competitive adsorption react ions on the residual Fe(III) oxide surface and coprecipitation in biogenic Fe(II) solids. The bioreduced Co-FeOOH surface showed unexpectedly high sor ption selectivity for the biomobilized Co(II). The bioreductive solubilizat ion of Ni(II) from Ni-FeOOH was comparable to Co-FeOOH. Our results indicat e that Fe(III)-oxide-entrained trace metals can be mobilized during bacteri al iron reduction leading to a net increase, in most cases, in aqueous meta l concentrations. The enhancement in trace-metal aqueous concentration, e.g ., in groundwater, may proportionally exceed that of Fe(II). Copyright (C) 2001 Elsevier Science Ltd.