Sorption of uranium to sulfide minerals: Insight into redox reactions from spectroscopic analysis

The transport of uranium in groundwaters is largely affected by its interaction with mineral surfaces. Redox reactions involving relatively soluble uranyl compounds at the redox front lead ultimately to precipitation of U(IV) minerals which are found to be associated with sulfides in uranium-rich sedimentary environments. This immobilization process of uranium is poorly understood. The authors have investigated this process in terms of the chemical interaction between dissolved uranyl and sulfide mineral surfaces. They exposed galena and pyrite to uranyl nitrate solutions at varying pH under rigorously controlled anoxic conditions. Subsequently, after varying equilibration times, the sulfide surfaces were analyzed by X-ray photoelectron spectroscopy (XPS). Their results indicate that (1) uranium is strongly sorbed to galena and pyrite surfaces, (2) XPS is a useful tool for determination of the oxidation state of sorbed uranium for carefully calibrated systems, and (3) reduction of U(VI) at the surface of galena is a slow process