Seawater neutralization and gypsum amelioration of bauxite refining residue to produce a plant growth medium

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Alumina production waste (bauxite refining residue) is highly alkaline, saline, and sodic, and hence cannot support plant growth for revegetation. Gypsum (CaSO4.2H2O) amendment of bauxite residue can lower alkalinity and improve the residue Ca status, but given the large gypsum requirement, efficient gypsum use is imperative. We investigated gypsum amelioration of residue sand (RS), examining changes in RS chemistry, and growth of Rhodes grass (Chloris gayana). Furthermore, we examined whether gypsum amelioration of RS should occur before or after seawater neutralization. We found that Ca from gypsum (20 t ha−1) was retained within the surface 0.2 m of RS, regardless of whether the gypsum was applied before or after seawater neutralization. This Ca was retained both as exchangeable Ca and as a precipitate (either calcite or hydrotalcite), with ca. 50% retained as exchangeable Ca in both approaches. Gypsum at 20 t ha−1, or even lower, provided sufficient Ca for maximum growth of Rhodes grass, in the surface, but higher rates would be required to allow Ca movement down the Na-dominated profile to ameliorate a larger rooting depth – this being important in environments where there are prolonged periods of water stress. The information presented will guide the efficient use of gypsum to ameliorate bauxite refining wastes.