Kinetic evaluation of the effects of bioavailability of organic ligands on biodegradation in the presence of common sesquioxide grain coatings

Radionuclides and metals can be mobilized by chelating agents typically present in low‐level radioactive liquid wastes that are disposed of in shallow land trenches; persistence of the organic chelating agent in the subsurface environment is a critical control on solubilization and transport of the complexed nuclide. Low‐molecular‐weight organic compounds are susceptible to uptake by bacteria, yet the biodegradation rates of nuclide‐ligand complexes in a mixture of solids (soil minerals) and liquids (ground water) are poorly known. We investigated the rate of citrate uptake in the presence of cobalt by a mixed bacterial community under experimental conditions designed to explore the impacts of citrate concentration, temperature, and chemistry of mineral surfaces. Dilute solutions with a typical sandy‐aquifer groundwater composition and equimolar amounts of cobalt and citrate ranging from 4 to 320 μM were combined with four different sand treatments: uncoated quartz sand, Fe‐coated quartz sand, Mn‐coated quartz sand, and no sand. Mineralization (net 14CO2 produced) and assimilation (14C retained on a 0.2‐μm filter) were quantified. Initial rates of citrate mineralization and of assimilation were independent of temperature in the presence of Fe‐ and Mn‐coated sands but were lower at lower temperatures (15°C versus 25°C) for the no‐sand and the uncoated sand, but the fraction of total carbon taken up (respiration plus assimilation) that the cells assimilated was greater at 15°C than at 25°C. In the presence of Fe‐coated sand, the mixed culture assimilated a greater fraction of carbon at both temperatures. The van Slyke equation, based on a model of sequential irreversible reactions, was fit to the results of the heterotrophic uptake experiments. The estimated value of the van Slyke constant indicated that mass‐transfer constraints limited the overall rate of citrate uptake, and that biokinetic limitations became more important at the lower temperature for the no‐sand and uncoated sand treatments. The sorption of citrate to the sand surface did not limit the rate of uptake. Further, complexation with cobalt did not alter the rate of citrate degradation or the rate of bacterial growth. The results suggest that transmembrane citrate transport limited the rate of uptake. The benefit that the bacteria derived from being associated with the solid sand surface was not directly related to the extent of citrate sorption, but might have resulted from a local elevation