Solid phases responsible for MnII CrIII CoII Ni CuII and Zn immobilization by a modified bauxite refinery residue (red mud) at pH 7.5

Although modified bauxite refinery residues (MBRR) immobilize trace metals, the retention mechanisms in this complex multiphase material have not been examined in detail. In this study, X-ray absorption spectroscopy was used to investigate the host minerals, and retention mechanisms, responsible for binding Mn^II, Cr^III, Co^II, Ni, Cu and Zn in MBRR. High metal loadings (∼10 mM at pH 3) in aqueous phases simulated highly contaminated mine drainage environments, whereas a final pH of 7.5 represents the typical MBRR pH buffering providing surface loadings of 1.7–2.6 wt%. Under these conditions, Cr^III precipitated as a solid phase similar to γ-CrOOH and/or amorphous Cr(OH)₃, regardless of host mineral, whereas Mn^II immobilization was associated with (partial) oxidation in all cases. Cobalt and Ni precipitated primarily as hydrotalcite-like solids when in contact with the MBRR, and sodalite, gibbsite and hydrotalcite tended to dominate this process. Cu uptake by MBRR resulted from the precipitation of Cu(OH)₂ although sorption processes may have made an additional contribution, whereas Zn precipitated as a solid phase similar to that of hydrozincite. The nature of the trace metal precipitate formed was often influenced by the host mineral constituent of MBRR, indicating that specific minerals in the MBRR can be primarily responsible for its metal immobilization properties, with gibbsite, hematite and hydrotalcite featuring prominently.