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

Laboratory experiments were conducted with suspensions of goethite (alpha-F eOOH) and a subsurface sediment to assess the influence of bacterial iron r eduction on the fate of Co(II)EDTA(2-), a representative metal-ligand compl ex of intermediate stability (log K-Co(II)EDTA = 17.97). The goethite was s ynthetic (ca. 55 m(2)/g) and the sediment was a Pleistocene age, Fe(III) ox ide-containing material from the Atlantic coastal plain (Milford). Shewanel la alga strain BrY, a dissimilatory iron reducing bacterium (DIRB), was use d 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 t he two sorbents in 0.001 mol/L Ca(ClO4)(2) to aid in experiment interpretat ion. 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, Co-60 and (14)EDTA labeled), inoc ulated with BrY (1-6 x 10(8) organisms/mL), and the headspace filled with a N-2/H-2 gas mix. The experiments were conducted under non-growth condition s. The medium did not contain PO43- (with one exception), trace elements, o r vitamins. The tubes were incubated under anoxic conditions at 25 degrees 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+) Co-60, a nd (14)EDTA. 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 DIRE generate d Fe(II) from both goethite and the Milford sediment that was strongly sorb ed by mineral surfaces. Aqueous Fe2+ increased during the experiment as sur faces 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 goe thite. The post dissociation sorption behavior of Co2+ was dependent on pH and the intrinsic sorptivity of the solid phases. Dissociation generally le ad 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+) a nd reduced its sorption relative to unreduced material. It is concluded tha t cationic radionuclides such as Co-60 or Pu-239/240, Which may be mobilize d from disposed wastes by complexation with EDTA(4-), may become immobilize d in groundwater zones where dissimilatory bacterial iron reduction is oper ative. Copyright (C) 2000 Elsevier Science Ltd.