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

Uranium (U­(VI)) sorption in systems containing titanium dioxide (TiO₂) and various Fe­(III)-oxide phases was investigated in the acidic pH range (pH 2.5–6). Studies were conducted with physical mixtures of TiO₂ and ferrihydrite, TiO₂ 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 TiO₂ 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 TiO₂ and crystalline Fe­(III) oxide phases, U­(VI) sorption was higher than for the analogous mixtures of TiO₂ with ferrihydrite, and was similar to U­(VI) sorption on TiO₂ alone. X-ray absorption spectroscopy of the TiO₂ 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 TiO₂. 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).