Sorption and desorption processes of U(VI) on iron (hydr)oxide phases

In this comparative study, the surface speciation of uranium(VI) on ferrihydrite (Fh) and maghemite (Mh) were investigated by vibrational spectroscopy. The experimental setup allows the monitoring of the surface processes at the water-solid interface in real time with a time resolution in the subminute time range. The performance of the experiments under inert gas atmosphere and under ambient conditions provides further insight into the molecular events of the binary and ternary sorption systems, respectively.
The spectra clearly demonstrate a characteristic surface speciation in dependence of the solid phase. On Fh, the formation of ternary inner sphere complexes are observed as it was derived earlier [1]. In addtion, the time-resolved spectra reveal the change of the surface speciation of atmospherically derived carbonate upon U(VI) sorption [2].
From the sorption experiments on Mh, outer sphere complexation of the uranyl ion can be derived. In ambient atmosphere, contributions of carbonate ions to the U(VI) surface speciation on Mh can not be ruled out.

[1] Ulrich et al. (2006) Geochim. Cosmochim. Acta 70, 5469-5487. [2] Foerstendorf et al. (2012) J. Colloid Interface Sci. 377, 299-306.