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

• Th binds to MBRR irreversibly from pH 3 to 8, improving with temperature and time.
• 16–52% U binding is irreversibly bound, concentration and temperature dependent.
• U binding maximal at pH range, pH 5–6, but aging increases 232U uptake at pH > 6.
• U and Th compete for similar sites affecting U binding cwf. simple 0.2 mM U aging.
• Increased U binding with aging is mostly from surface precipitate recrystallisation.

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 (predominantly 238UVI and 232ThIV) 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 232UVI and 229ThIV 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. 238UVI and 232UVI; 232ThIV and 229ThIV) indicated that irreversibility of UVI and ThIV 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 UVI during aging is associated with re-crystallisation of precipitates.

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