Effectiveness of acidic industrial wastes for reclaiming fine bauxite refining residue (Red mud)

Land disposal of bauxite refining residues produced by the aluminum industry has resulted in areas devoid of plants because of the high salinity and alkalinity of the residues. In addition, the fine fraction (red mud) of these residues is prone to wind and water erosion, which can pollute the surrounding soils and surface waters. A glasshouse pot-leaching study was performed to evaluate the effectiveness of waste gypsum (CaSO₄) and copperas (FeSO₄) as ameliorants for the red mud fraction. Red mud was amended with the wastes at rates of 0, 2, 5, and 8% (w/w), and leached with 1200 ml (126 min rainfall) of deionized water before conducting a seedling emergence test using Agropyron elongatum. Leachate and soil analyses indicated that gypsum and copperas amendments were able to reduce the pH, electrical conductivity (EC) and Na content of red mud significantly. Copperas was more effective in reducing soil Na contents and maintaining a lower soil EC. On the other hand, gypsum-amended red mud maintained a lower pH and a higher Ca content because of the low solubility of gypsum. Both amendments also resulted in significant reductions in soluble Al content and exchangeable sodium percentage (ESP). The reduced pH, ESP and Al content were correlated to the improved seedling emergence of Agropyron at application rates of >2%. The results demonstrate that both gypsum and copperas are effective ameliorants for red mud. However, gypsum appears to provide a more persistent pH buffering capacity and lower ESP for red mud.