Date of this Version
Geochimica et Cosmochimica Acta, Vol. 64, No. 8, pp. 1345–1362, 2000
Laboratory experiments were conducted with suspensions of goethite (α-FeOOH) and a subsurface sediment to assess the influence of bacterial iron reduction on the fate of Co(II)EDTA2-, a representative metal-ligand complex of intermediate stability (log KCo(II)EDTA = 17.97). The goethite was synthetic (ca. 55 m2/g) and the sediment was a Pleistocene age, Fe(III) oxide-containing material from the Atlantic coastal plain (Milford). Shewanella alga strain BrY, a dissimilatory iron reducing bacterium (DIRB), was used to promote Fe(III) oxide reduction. Sorption isotherms and pH adsorption edges were measured for Co2+, Fe2+, Co(II)EDTA2-, and Fe(II)EDTA2- on the two sorbents in 0.001 mol/L Ca(ClO4)2 to aid in experiment interpretation. Anoxic suspensions of the sorbents in PIPES buffer at pH 6.5–7.0 were spiked with Co(II)EDTA2- (10-5 mol/L, 60Co and 14EDTA labeled), inoculated with BrY (1–6 X 108 organisms/mL), and the headspace filled with a N2/H2 gas mix. The experiments were conducted under non-growth conditions. The medium did not contain PO43- (with one exception), trace elements, or vitamins. The tubes were incubated under anoxic conditions at 25°C for time periods in excess of 100 d. Replicate tubes were sacrificed and analyzed at desired time periods for pH, Fe(II)TOT, Fe(aq)2+ , 60Co, and 14EDTA. Abiotic analogue experiments were conducted where Fe(aq)2+ was added in increasing concentration to Co(II)EDTA2-/mineral suspensions to simulate the influence of bacterial Fe(II) evolution. The DIRB generated Fe(II) from both goethite and the Milford sediment that was strongly sorbed by mineral surfaces. Aqueous Fe2+ increased during the experiment as surfaces became saturated; Fe(aq)2+ induced the dissociation of Co(II)EDTA2- into a mixture of Co2+, Co(II)EDTA2-, and Fe(II)EDTA2- (log KFe(II)EDTA = 15.98). The extent of dissociation of Co(II)EDTA2- was greater in the subsurface sediment because it sorbed Fe(II) less strongly than did goethite. The post dissociation sorption behavior of Co2+ was dependent on pH and the intrinsic sorptivity of the solid phases. Dissociation generally lead to an increase in the sorption (e.g., Kd) of Co2+ relative to EDTA4- (form unspecified). Sorbed biogenic Fe(II) competed with free Co(aq) 2+ and reduced its sorption relative to unreduced material. It is concluded that cationic radionuclides such as 60Co or 239/240Pu, which may be mobilized from disposed wastes by complexation with EDTA4-, may become immobilized in groundwater zones where dissimilatory bacterial iron reduction is operative.