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

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 was studied. 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 and assimilation 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 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.