Inhibition of Uranium(Ⅵ) Sorption on Titanium Dioxide by Surface Iron(Ⅲ) Species in Ferric Oxide/Titanium Dioxide Systems

摘要

Uranium (U(Ⅵ)) sorption in systems containing titanium dioxide (TiO_2) and various Fe (Ⅲ)-oxide phases was investigated in the acidic pH range (pH 2.5-6). Studies were conducted with physical mixtures of TiO_2 and ferrihydrite, TiO_2 with coprecipitated ferrihydrite, and with systems where Fe(Ⅲ) was mostly present as crystalline Fe(Ⅲ) oxides. The presence of ferrihydrite resulted in decreased U(Ⅵ) sorption relative to the pure TiO_2 systems, particularly below pH 4, an unexpected result given that the presence of another sorbent would be expected to increase U(Ⅵ) uptake. In mixtures of TiO_2 and crystalline Fe(Ⅲ) oxide phases, U(Ⅵ) sorption was higher than for the analogous mixtures of TiO_2 with ferrihydrite, and was similar to U(Ⅵ) sorption on TiO_2 alone. X-ray absorption spectroscopy of the TiO_2 system with freshly precipitated Fe(Ⅲ) oxides indicated the presence of an Fe(Ⅲ) surface phase that inhibits U(Ⅵ) sorption-a reaction whereby Fe(Ⅲ) precipitates as lepidocrocite and/or ferrihydrite effectively blocking surface sorption sites on the underlying TiO_2. Competition between dissolved Fe(Ⅲ) and U(Ⅵ) for sorption sites may also contribute to the observed decrease in U(Ⅵ) sorption. The present study demonstrates the complexity of sorption in mixed systems, where the oxide phases do not necessarily behave in an additive manner, and has implications for U(VI) mobility in natural and impacted environments where Fe(Ⅲ) (oxyhydr) oxides are usually assumed to increase the retention of U(Ⅵ).