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 (UO₂ ²⁺) 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 Å 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 to hinder surface complexation, suggesting ligand-exchange mechanisms to be operative for the formation of inner-sphere surface complexes. Uranyl symmetry is maintained when coprecipitated with Al and Mg from synthetic or barren lixiviant solutions, precluding incorporation into the HTC lattice. Uranyl ions, however, are surrounded by up to 3-5 Mg/Al atoms in coprecipitated samples interfering with HTC crystal growth. Future research should explore the potential of Fe(II) or Mn(II) to reduce U(VI) to U(V), which is conducive for U incorporation into octahedral crystal lattice positions of the hydroxide sheet.