In this talk, we present results from resonantly enhanced second harmonic generation (SHG) technique to probe, with molecular-level sensitivity, in situ, in real time, and under relevant aqueous conditions, how U(VI) interactswith mica/water interfaces. Using these techniques, we previously determined the binding constants, adsorption free energies, charge states and speciation, interfacial potentials, and molecular insights into the mechanics of binding for uranium adsorption at the fused silica/water interface. For instance, resonantly enhanced SHG was used to measure an in situ electronic spectrum of adsorbed uranyl species at both pH 7 and pH 4 in the presence of aqueous carbonate. By monitoring the SHG response, adsorption isotherms revealed that the magnitude of the adsorption free energy for U(VI) at the silica/water interface decreased from �42.7(3) kJ�mol-1 at pH 7 to �32.5(3) kJ�mol-1 at pH 4. Using the non-resonant Chi(3) technique, surface charge densities of adsorbed uranyl species were determined to be 0.0069(7) C�m-2 at pH 7 and 0.0031(5) C�m-2 at pH 4. When these values are referenced to the initial charge density of the interface, results suggest a bidentate mode of binding at pH 7 and a monodentate mode of binding at pH 4. Free energy vs. interfacial potential analysis revealed that neutral and monovalent cationic uranyl species are surface active at both aqueous pH values. Using these same methodologies, this work is now applied to the muscovite (mica) mineral/water interface. Preliminary Chi(3) results measuring surface charge density are comparable to literature results. The measured surface charge density of mica at the interface is -0.03(4) Cm-2 and -0.056(7) Cm-2 at pH 6 and pH 9. These results and uranyl adsorption studies at the mica/water interface will be discussed.

Clay mineral/water interfaces interacting with uranyl studied by second harmonic generation

244th National Meeting (2012)

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