Uptake and incorporation of Al, Cr, V, Zn and Mo in hematite: Competition, synergies and influence on structural properties.

This study investigated multiple (di-, tri- and tetra-)incorporation of selected minor and trace elements (Al3+, Cr3+, V3-5+, Zn2+, Mo6+ and As5+) into hematite. The purpose was to improve understanding of how hematite may control trace element mobility in the environment, and how physical and chemical properties of hematite are impacted by multi-element incorporation at x/Fe molar ratios of up to 10%. Simultaneous structural incorporation of Al±Cr±V±Zn into hematite was achieved, with both synergistic and antagonistic effects occurring between certain element combinations. Cr+Al had synergistic effects on their co-incorporation, while V negatively affected Al incorporation, and both V and Zn negatively affected Cr incorporation. In contrast, Mo was minimally associated with hematite, and As prevented hematite formation completely. X-ray diffraction indicated contraction and expansion of the hematite unit-cell upon substitution was related to the ionic radius of the substituting element in single-element samples, while V predominantly controlled the direction of deviation in multi-element samples. X-ray absorption near-edge structure spectroscopy indicated V was present as a mixture of V3+-V5+, with a higher average V oxidation state associated with multi-element samples. Results provide new insights into trace element geochemistry within hematite, and highlight the importance of multi-element studies to better understand natural and anthropogenic systems.

In situ neutralisation of uncarbonated bauxite residue mud by cross layer leaching with carbonated bauxite residue mud.

Unameliorated residue mud from the Bayer process generates highly alkaline leachates (pH ca. 13) after deposition in storage areas. Pre-deposition treatment of bauxite residue mud (BRM) with CO(2) gas (carbonation) lowers leachate pH to ca. 10.5. Laboratory scale leaching columns were used to investigate the potential for in situ pH reduction in existing uncarbonated BRM deposits through exposure to carbonated mud leachate. Leachates from uncarbonated and carbonated residues in single and dual-layer column configurations were analysed for pH, electrical conductivity, carbonate and bicarbonate content, and element concentrations. Air-dried solids were analysed by X-ray diffraction before and after leaching. Cross layer leaching lowers leachate pH from uncarbonated BRM. Leachate pH was significantly lower in dual layer and carbonated residue than in uncarbonated residue between one and 400 pore volumes leached. Carbonated residue porewater as well as dawsonite and calcite dissolution were identified as sources of (bi-)carbonate. Leachate concentrations of As, Cr, Cu, Ga and La were immediately reduced in dual layer treatments compared with uncarbonated residue. No element analysed exhibited a significantly higher leachate concentration in dual layer treatments than the highest observed concentration in single layer treatments. The implementation of dual layer leaching in the field therefore presents an opportunity to improve leachate quality from existing uncarbonated residue deposits and justifies further testing at field scale.