Mechanisms of Uranyl Sequestration by Hydrotalcite

Since the advent of large-scale U mining, processing, and enrichment for energy or weapons production, efficient capture and disposal of U, transuranics, and daughter radionuclides has constituted an omnipresent challenge. In this study, we investigated uranyl (UO22+) sequestration by hydrotalcite (HTC) as a coprecipitation or surface adsorption reaction scenario. The master variables of the study were pH (7.0 and 9.5) and CO2 content during the reactions (CO2-rich, CO2r vs CO2-depleted, CO2p). In addition, we compared the outcomes of U–HTC coprecipitation reactions between pristine salt precursors and barren U mine wastewater (lixiviant). Extended X-ray absorption fine structure spectra revealed that uranyl adsorbs on the HTC surface as inner-sphere complexes in CO2r and CO2p systems with U–Mg/Al interatomic distances of ∼3.20 and ∼3.35 A indicative of single-edge (1E) and double-edge (2E) sharing complexes, respectively. Partial coordination of uranyl by carbonate ligands in CO2r systems does not appear...