THERMODYNAMICS OF U(VI) SORPTION ONTO SHEWANELLA PUTREFACIENS

Abstract

We have conducted acid–base potentiometric titrations and U(VI) sorption experiments using the Gram negative, facultatively anaerobic bacterium Shewanella putrefaciens. Results of reversed titration studies on live, inactive bacteria indicate that their pH-buffering properties result from the equilibrium ionization of three discrete populations of functional groups. Carboxyl (pKa=5.16±0.04), phosphoryl (pKa=7.22±0.15) and amine (pKa=10.04±0.67) groups most likely represent these three resolvable functionalities, based on their pKa values. Site densities for carboxyl, phosphoryl and amine groups on the bacterial surface were approximately 31.7 μmol sites/g bacteria (0.35±0.02 sites/nm2), 8.95 μmol/g (0.11±0.007 sites/nm2) and 38.0 μmol/g (0.42±0.008 sites/nm2), respectively, based on an estimated bacterial specific surface area of 55 m2/g. Sorption experiments showed that U(VI) can reversibly complex with the bacterial surface in the pH 2–8 interval, with maximum adsorption occurring at a pH of ∼5. Sorption is not strongly sensitive to ionic strength (NaCl) in the range 0.02–0.10 M. The pH and ionic strength dependence of U(VI) sorption onto S. putrefaciens is similar to that measured for metal-oxide surfaces and Gram positive bacteria, and appears to be similarly governed by competitive speciation constraints. Measured U(VI) sorption is accounted for by using two separate adsorption reactions forming the surface complexes >COO–UO2+ and >PO4H–UO2(OH)2. Using S. putrefaciens as a model organism for dissimilatory metal-reducing Gram negative anaerobes, our results extend the applicability of geochemical speciation models to include bacteria that are capable of reductively solubilizing or precipitating a wide variety of environmentally and geologically important metals and metallic species.