The effect of biogenic Fe(II) on the stability and sorption of Co(II)EDTA 2− to goethite and a subsurface sediment

Abstract

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)EDTA 2−, a representative metal-ligand complex of intermediate stability (log K Co(II)EDTA = 17.97). The goethite was synthetic (ca. 55 m 2/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 Co 2+, Fe 2+, Co(II)EDTA 2−, and Fe(II)EDTA 2− on the two sorbents in 0.001 mol/L Ca(ClO 4) 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)EDTA 2− (10 −5 mol/L, 60Co and 14EDTA labeled), inoculated with BrY (1–6 × 10 8 organisms/mL), and the headspace filled with a N 2/H 2 gas mix. The experiments were conducted under non-growth conditions. The medium did not contain PO 4 3− (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)EDTA 2−/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 Fe 2+ increased during the experiment as surfaces became saturated; Fe (aq) 2+ induced the dissociation of Co(II)EDTA 2− into a mixture of Co 2+, Co(II)EDTA 2−, and Fe(II)EDTA 2− (log K Fe(II)EDTA = 15.98). The extent of dissociation of Co(II)EDTA 2− was greater in the subsurface sediment because it sorbed Fe(II) less strongly than did goethite. The post dissociation sorption behavior of Co 2+ was dependent on pH and the intrinsic sorptivity of the solid phases. Dissociation generally lead to an increase in the sorption (e.g., K d) of Co 2+ relative to EDTA 4− (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 EDTA 4−, may become immobilized in groundwater zones where dissimilatory bacterial iron reduction is operative.