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

► Leaching uncarbonated bauxite residue mud with carbonated bauxite residue mud leachate lowers the pH of uncarbonated bauxite residue leachate. ► Dual layer systems maintained lower leachate pH than uncarbonated bauxite residue only systems over 400 pore volumes leached. ► Carbonated residue porewater as well as dawsonite and calcite dissolution were identified as sources of (bi-)carbonate which buffered pH. ► Leachate concentrations of As, Cr, Cu, Ga and La were immediately reduced in dual layer treatments compared with uncarbonated only. ► Deposition of carbonated bauxite residue over existing uncarbonated residue may be useful for in situ neutralisation of uncarbonated residue.