The effect of biogenic Fe(II) on the stability and sorption of Co(II)EDTA22 to goethite and a subsurface sediment

Authors

John M. Zachara, Pacific Northwest National LaboratoryFollow
Steven Smith, Pacific Northwest National LaboratoryFollow
James K. Fredrickson, Pacific Northwest National LaboratoryFollow

Date of this Version

2000

Citation

Geochimica et Cosmochimica Acta, Vol. 64, No. 8, pp. 1345–1362, 2000

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 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)EDTA^2-, and Fe(II)EDTA^2- 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)EDTA^2- (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 PO₄^3- (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 Fe²⁺ increased during the experiment as surfaces became saturated; Fe_(aq)²⁺ induced the dissociation of Co(II)EDTA^2- into a mixture of Co²⁺, 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 Co²⁺ 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 Co²⁺ 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.