Photocatalytic reduction of uranyl: Effects of organic ligands and UV light wavelengths

Abstract

Although previous studies demonstrate the photochemical reduction of uranyl (UO₂²⁺) in the presence of various organic compounds, the actual roles of organic molecules as ligands and electron donors during the photoreaction are poorly understood. In this study, photochemical reduction of uranyl is examined with respect to organic ligands as electron donors and complexing agents, the role of titanium oxide (TiO₂) nanoparticles as a photocatalyst, and the influence of UV light irradiation with emission peaks in the UV-A, UV-B, and UV-C ranges. Organic compounds with different binding affinities to uranyl such as acetate, ethylenediaminetetracetate (EDTA), oxalate, and hydroquinone were selected.

Uranyl solutions prepared with one organic compound in a 1:8 molar ratio were irradiated under anoxic and acidic conditions (O₂ < 1 ppm, pH 2.5). Uranyl removal by UV irradiation was better than 70 percent in the presence of oxalate and acetate, followed by hydroquinone (∼ 45 %) and EDTA (∼ 10 %). Uranyl removal was nearly constant at the UV-A, UV-B, and UV-C regions in the presence of acetate and oxalate whereas greater removal was observed in the EDTA and hydroquinone solutions exposed to UV-C and UV-A, respectively. These results reveal that uranyl reduction is mediated primarily by TiO₂ nanoparticles and is highly dependent on the electron-donor compound. Addition of acetate enhances the uranyl photoreaction in hydroquinone solution. Dissolved EDTA species act as good electron donors at limited EDTA concentrations (1:2 to 1:4 uranyl to EDTA ratios) but at higher concentrations (for example, 1:8), uranyl-EDTA complexes such as [(UO₂²⁺)HEDTA]⁻ compete for the surface sites on the TiO₂ nanoparticles, hindering the photoreduction of uranyl. X-ray photoelectron spectroscopy (XPS) of the dried TiO₂ powder shows that more than 70 percent of uranium partitioned into the solid phase is present as reduced forms with oxidation states (V) and (IV). The U4f spectra of U partitioned to the solid phase from the photoreaction with acetate reveal the predominance of U(IV) over U(V), whereas U(V) is the dominant oxidation state as a result of the photoreduction with EDTA. Our results suggest that formation of uranium-ligand complexes plays a critical role in controlling the reactivity of uranyl species and the stability of reduced uranium species in the course of the photoreaction.