Clark, M.W., Payne, T.E., Collins, R.N., Harrison, J.J., Johnston, M. and Comarmond, M.J.

Seawater neutralised modified bauxite refinery residues (MBRR), such as Bauxsol™, are used in environmental remediation, especially in the treatment of acidic, trace-metal-rich fluids, however, there is very little knowledge on how metals are retained by MBRR. The binding for Cu, Cr, Co, Mn and Zn to Bauxsol™, determined by X-ray absorption spectroscopy (XAS); and reversibility of U(VI) and Th, studied by isotopic exchange, are presented in this paper. XAS data indicate that metal uptake primarily occurred through precipitation reactions, and that sodalite, gibbsite and hematite are the three key minerals involved in the immobilisation within Bauxsol™. Cu uptake was a function of both sorption (dimeric surface complexes) and surface precipitation (Cu(OH)2) reactions on hematite. Cr(III) immobilisation occurred on hematite and sodalite as multinuclear sorption complexes and/or precipitation as g-CrOOH on mineral surfaces. Zn and Co are immobilised primarily through surface precipitation of hydrotalcite- and amorphous hydroxide-like solids on sodalite and/or gibbsite surfaces. Mn(II) was observed in association with hematite and sodalite, where partial oxidation to Mn(IV) plays a role in Mn immobilisation. Furthermore, U(VI) and Th isotopic exchange data shows that Th is almost entirely bound by MBRR surfaces, whereas U(VI) has pH-dependent sorption with a much larger degree of reversibility. Initial sorption data indicates a competition between U and Th for binding sites, and that about 20% of U is irreversibly bound. However, aging of the bound materials results in increased irreversibility, where intra-particle diffusion to deep mineral lattice defects is a primary mechanism of incorporation.