The U(vi) speciation influenced by a novel Paenibacillus isolate from Mont Terri Opalinus clay

Abstract

Bacterial cell walls have a high density of ionizable functional groups available for U(VI) binding, hence have a great potential to affect the speciation of this contaminant in the environment. The studied strain of the genus Paenibacillus is a novel isolate originating from the Mont Terri Opalinus clay formations (Switzerland) which are currently investigated as a potential host rock for future nuclear waste storage. U(VI) binding to the cell surface functional groups was studied by potentiometry combined with time-resolved laser-induced fluorescence spectroscopy (TRLFS). Four bacterial U(VI) surface complexes were identified: R–COO–UO₂⁺, R–O–PO₃–UO₂, R–O–PO₃H–UO₂⁺, and (R–O–PO₃)₂–UO₂²⁻. The corresponding complex stability constants were calculated to be 5.33 ± 0.08, 8.89 ± 0.04, 12.92 ± 0.05, and 13.62 ± 0.08, respectively. Hence UO₂²⁺ displays a moderate to strong interaction with the bacterial surface functional groups. In the acidic pH range (pH 3) UO₂²⁺ binding onto the cell envelope is governed by coordination to hydrogen phosphoryl sites. Upon increasing the pH an increasing coordination of UO₂²⁺ to carboxylic and deprotonated phosphoryl sites was found. At a pH greater than 7 uranyl hydroxides dominate the speciation. Additionally the bacteria-mediated release of inorganic phosphate in dependence on [U(VI)] at different pH values was studied to assess the influence of phosphate release on U(VI) mobilization.