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 U-238 and Th-232 isotopes were contacted with MBRR in an 8 day loading period (equilibrium pH approximate to 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 U-232 and Th-229. Relative concentrations of the thorium isotope pair (Th-232 and Th-229) indicate that Th is very strongly bound to MBRR, and although at pH 3, some de-sorption is observed Th-232 (approximate to 3%) and Th-229 (approximate to 2.5%), released thorium is partially re-adsorbed during an overnight equilibration. During the initial equilibration, approximately 50% of the U-238 was adsorbed, and a U adsorption maximum occurs between pH 5 and pH 6, where <0.5% of the U remains in solution. However, at a pH between 5 and 3, some 60% of the bound U releases, hence the pH range of maximum U retention on the MBRR is relatively narrow. When equilibrated overnight, the MBRR releases additional U, suggesting a kinetically controlled de-sorption linked to mineral dissolution. Plots of U isotope exchange between U-232 and U-238 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. (C) 2011 Elsevier Inc. All rights reserved.