Insights into the uranium(VI) speciation with Pseudomonas fluorescens on a molecular level

Microorganisms have great potential to bind and thus transport actinides in the environment. Thus microbes indigenous to designated nuclear waste disposal sites have to be investigated regarding their interaction mechanisms with soluble actinyl ions when assessing the safety of a planned repository. This paper presents results on the pH-dependent sorption of U(VI) onto Pseudomonas fluorescens isolated from the granitic rock aquifers at Äspö Hard Rock Laboratory, Sweden. To characterize the U(VI) interaction on a molecular level, potentiometric titration in combination with time-resolved laser-induced fluorescence spectroscopy (TRLFS) were applied. This paper as a result is one of the very few sources which provide stability constants of U(VI) complexed by cell surface functional groups. In addition the bacteria-mediated liberation of inorganic phosphate in dependence on [U(VI)] at different pHs was studied to judge the influence of phosphate release on U(VI) mobilization. The results demonstrate that in the acidic pH range U(VI) is bound by the cells mainly via protonated phosphoryl and carboxylic sites. The complexation by carboxylic groups can be observed over a wide pH range up to around pH 7. At neutral pH fully deprotonated phosphoryl groups are mainly responsible for U(VI) binding. U(VI) can be bound by P. fluorescens with relatively high thermodynamic stability.