Biodegradation of cobalt–citrate complexes: Implications for cobalt mobility in groundwater

The bacterial consumption of chelating agents that are present in low-level radioactive and mixed wastes may help to immobilize chelated metals and radionuclides accidentally released to groundwater. We investigated the influence of the bacterial consumption of citrate complexed with cobalt on cobalt transport through packed sand columns. Experiments were conducted using each of three types of column packing material using minerals common to subsurface environments: clean quartz sand; ferric oxide (Fe(OH)3)-coated sand; hausmannite (Mn3O4)-coated sand. Separate control column experiments were conducted to examine citrateʹs influence on cobalt transport without the bacterial consumption of citrate. The bacterial community consumed all the citrate; the pore water pH decreased by up to one unit before reaching a steady-state value of 6.9–7.1, which was lower than the influent pH (7.4). These results were in contrast to open batch experiments conducted with the same culture, where the pH increased by more than one unit. The dissolved oxygen exhibited similar dynamics, reaching a steady-state value of 3–4 mg/l, well below the influent value of 7.5 mg/l. The dynamics in pore water pH and dissolved oxygen were associated with the presence of the bacterial community because these parameters remained steady in control experiments in which the bacteria were not included. Cobalt transport was most rapid for the columns packed with quartz sand followed by the Fe-coated sand and finally the Mn-coated sand. Most of the cobalt retained by the quartz sand and Fe-coated sand was easily exchanged with Mg2+ whereas most of the cobalt retained by the Mn-coated sand required an acetic acid solution for its removal. The bacterially mediated pH decrease, driven by the consumption of citrate, decreased cobalt sorption to the solid phase resulting in enhanced cobalt transport. The results of these experiments suggest that geochemical changes, driven by the bacterial consumption of citrate, enhanced cobalt transport although the complexing ligand had been removed from the system.