The pH-dependence and reversibility of uranium and thorium binding on a modified bauxite refinery residue using isotopic exchange techniques

The pH-dependence and reversibility of uranium and thorium binding onto a modified bauxite refinery residue (MBRR) were studied in laboratory uptake/leaching experiments. Natural ²³⁸U and ²³²Th isotopes were contacted with MBRR in an 8 day loading period (equilibrium pH ≈ 8.5) then leached in pH-dependent experiments, where the pH was decreased from 8 to 3 over several hours following addition of exchange isotopes ²³²U and ²²⁹Th. Relative concentrations of the thorium isotope pair (²³²Th and ²²⁹Th) indicate that Th is very strongly bound to MBRR, and although at pH 3, some de-sorption is observed ²³²Th (≈3%) and ²²⁹Th (≈2.5%), released thorium is partially re-adsorbed during an overnight equilibration. During the initial equilibration, approximately 50% of the ²³⁸U was adsorbed, and a U adsorption maximum occurs between pH 5 and pH 6, where 232U and ²³⁸U are linear, and suggest that U adsorption is mostly reversible. Data for adsorption in mixed systems of U and Th suggest that Th and U compete for similar binding sites.