Immobilization of cobalt by sulfate-reducing bacteria in subsurface sediments

Abstract

We investigated the impact of sulfate-reduction on immobilization of metals in subsurface aquifers. Co2+ was used as a model for heavy metals. Factors limiting sulfate-reduction dependent Co2+ immobilization were tested on pure cultures of sulfate-reducing bacteria, and in sediment columns from a landfill leachate contaminated aquifer. In the presence of 1 mM Co2+, the growth of pure cultures of sulfate-reducing bacteria was not impacted. Cultures of Desulfovibrio desulfuricans, Desulfotomaculum gibsoniae, and Desulfomicrobium hypogeia removed greater than 99.99% of the soluble Co2+ when CoCl2 was used with no chelators. The above cultures and Desulfoarcula baarsi removed 98-99.94% of the soluble Co(II) when the metal was complexed with the model ligand nitrilotriacetate (Co-NTA). Factors controlling the rate of sulfate-reduction based Co2+ precipitation were investigated in sediment-cobalt mixtures. Several electron donors were tested and all but toluene accelerated soluble Co2+ loss. Ethanol and formate showed the greatest stimulation. All complex nitrogen sources tested slowed and decreased the extent of Co2+ removal from solution relative to formate-amended sediment incubations. A range of pH values were tested (6.35-7.81), with the more alkaline incubations exhibiting the largest precipitation of Co2+. The immobilization of Co2+ in sediments was also investigated with cores to monitor the flow of Co2+ through undisturbed sediments. An increase in the amount of Co2+ immobilized as CoS was observed as sulfate reduction activity was stimulated in flow through columns. Both pure culture and sediment incubation data indicate that stimulation of sulfate reduction is a viable strategy in the immobilization of contaminating metals in subsurface systems.