Enhanced uranium sorption on aluminum oxide pretreated with arsenate. Part II: Spectroscopic studies.

In a companion study, we demonstrated that pretreatment of gamma-alumina surface with arsenate enhances uranyl uptake under acidic conditions, where uranyl otherwise sorbs poorly. Here, we examine the local structure and long-range order of the sorption products by using X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD). Arsenate was chosen for the pretreatment because of its high affinity for binding with uranyl and alumina, and because it is an analog for environmentally abundant and commercially accessible phosphate. It also facilitates characterization of sorption products using As K-edge XAS, which complements U LIII-edge XAS. Fitting results suggest the formation of U-As precipitates with structures similar to UO2HAsO4 x 4H2O (trögerite) and likely U polymeric species at high U concentrations. The ratios among surface-sorbed uranyl, U-As precipitates, and uranyl polymeric species are dependent on the [As]initial/[U]initial ratio and absolute initial U concentration. XRD suggests the precipitates are likely to be highly disordered and poorly crystalline. Current findings evaluate the mechanism by which the pretreatment results in enhanced U uptake and stability and provides a conceptual basis for designing other pretreatment technologies for uranium remediation.