URANIUM(VI) ADSORPTION TO FERRIHYDRITE - APPLICATION OF A SURFACE COMPLEXATION MODEL

A study of U(VI) adsorption by ferrihydrite was conducted over a wide range of U(VI) concentrations, pH, and at two partial pressures of car bon dioxide. A two-site (strong- and weak-affinity sites, =Fe3OH and = FewOH, respectively) surface complexation model was able to describe t he experimental data well over a wide range of conditions, with only o ne species formed with each site type: an inner-sphere, mononuclear, b identate complex of the type (=FeO2)UO2. The existence of such a surfa ce species was supported by results of uranium EXAFS spectroscopy perf ormed on two samples with U(VI) adsorption density in the upper range observed in this study (10 and 18% occupancy of total surface sites). Adsorption data in the alkaline pH range suggested the existence of a second surface species, modeled as a ternary surface complex with UO2C O30 binding to a bidentate surface site. Previous surface complexation models for U(VI) adsorption have proposed surface species that are id entical to the predominant aqueous species, e.g., multinuclear hydroly sis complexes or several U(VI)-carbonate complexes. The results demons trate that the speciation of adsorbed U(VI) may be constrained by the coordination environment at the surface, giving rise to surface specia tion for U(VI) that is significantly less complex than aqueous speciat ion.