Thermodynamics of U(VI) sorption onto Shewanella putrefaciens

We have conducted acid-base potentiometric titrations and U(Vl) sorption ex periments using the Gram negative, facultatively anaerobic bacterium Shewan ella putrefaciens. Results of reversed titration studies on live, inactive bacteria indicate that their pH-buffering properties result from the equili brium ionization of three discrete populations of functional groups. Carbox yl (pK(a) = 5.16 +/- 0.04), phosphoryl (pK(a) 7.22 +/- 0.15) and amine (pK( a) = 10.04 +/- 0.67) groups most likely represent these three resolvable fu nctionalities, based on their pK, values. Site densities for carboxyl, phos phoryl and amine groups on the bacterial surface were approximately 31.7 mu mol sites/g bacteria (0.35 +/- 0.02 sites/nm(2)), 8.95 mu mol/g (0.11 +/- 0.007 sites/nm(2)) and 38.0 mu mol/g (0.42 +/- 0.008 sites/nm2), respective ly, based on an estimated bacterial specific surface area of 55 m(2)/g. Sor ption experiments showed that U(VI) can reversibly complex with the bacteri al surface in the pH 2-8 interval, with maximum adsorption occurring at a p H of similar to 5. Sorption is not strongly sensitive to ionic strength (Na Cl) 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 b y competitive speciation constraints. Measured U(VI) sorption is accounted for by using two separate adsorption reactions forming the surface complexe s > COO-UO2+ and > PO4H-UO2(OH)(2). Using S. putrefaciens as a model organi sm for dissimilatory metal-reducing Grain negative anaerobes, our results e xtend the applicability of geochemical speciation models to include bacteri a that are capable of reductively solubilizing or precipitating a wide vari ety of environmentally and geologically important metals and metallic speci es. (C) 2001 Elsevier Science B.V. All rights reserved.