Reversibility of uranium and thorium binding on a modified bauxite refinery residue: the effects of aging temperature

The effects of aging on the pH-dependence and reversibility of uranium and thorium binding by a modified bauxite refinery residue (MBRR) were studied in laboratory uptake/leaching experiments. Natural uranium and thorium isotopes (predominantly²³⁸U^VI and ²³²Th^IV) of 0.4, and 0.2 mM were loaded for an 8-day period at the natural equilibrium pH of the MBRR (approximately 8.5) and were allowed to age for 6 months in humid sealed containers at 4, 23, and 65 °C. After aging, anthropogenic ²³²U^VI and²²⁹Th^IV were added as exchange isotopes and the pH decreased stepwise from 8 to 3 over several hours in pH dependent experiments. The relative concentrations of the isotope pairs (i.e. ²³⁸U^VI and ²³²U^VI; ²³²Th^IV and ²²⁹Th^IV) indicated that irreversibility of U^VI and Th^IV binding by the MBRR increased with aging and was dependent on the aging temperature and surface actinide loading. Linear combination fitting of the EXAFS data for the uranium loaded materials indicated that at low uranium concentrations sorption to minerals (particularly hematite) was a prime uranium removal mechanism, but at higher concentrations precipitation dominated. The data suggest that increased irreversibility and incorporation of U^VI during aging is associated with re-crystallisation of precipitates.