The effects of aqueous bicarbonate and calcium ions on uranium biosorption by Arthrobacter G975 strain

Uranium (VI) biosorption was investigated using carbonate-free and carbonate amended synthetic groundwater (SGW) under conditions relevant to Ca-rich carbonate-bearing subsurface environments. SGW was amended with bicarbonate concentrations ranging between 0.5 and 5 mM and a G975 Arthrobacter oxydans strain, a soil bacterium previously isolated from Hanford Site soil. Selected treatments were amended with 5 mM of calcium to evaluate its effect on U(VI) biosorption. The study confirmed that Arthrobacter sp. G975 can effectively remove soluble U(VI) ions from aqueous solution. The Langmuir, Linear, and Freundlich isotherms were fitted to the experimental data. Based on a higher correlation coefficient, the data was better fit by the Langmuir model. The presence of calcium and bicarbonate ions in SGW affects the sorption behavior of U(VI) due to the formation of highly soluble and stable uranyl-carbonate and calcium uranyl carbonate complexes that reduce adsorption of U(VI). Increasing aqueous bicarbonate concentrations exponentially reduced the U(VI) microbial adsorption compared to values obtained in carbonate-free solutions. The differences in U(VI) uptake generated by calcium additions in bicarbonate-free solutions were small enough to conclude that calcium was not a governing factor in biosorption. Results suggest that in the presence of bicarbonate and calcium, the damage to bacterial cells caused by U(VI) toxicity was reduced, yielding similar values for TOC degradation and cell density compared to U(VI)-free controls.