Surface charge characteristics and sorption properties of bauxite-processing residue sand

Abstract

Bauxite-processing residue sand (BRS) is the primary growth medium used to rehabilitate Alcoa’s residue storage areas (RSAs) in south-west Western Australia. This material is typically coarse-textured, highly saline, highly alkaline, extremely sodic, and deficient in plant nutrients. To develop appropriate fertiliser strategies for optimising rehabilitation performance, a fundamental understanding of the surface charge and nutrient retention properties of BRS is essential. The contribution of permanent (σp) and variable (σv) charge to the overall magnitude and sign of the surface charge, and ammonium (NH4) and phosphorus (P) sorption, as a function of pH were studied. Samples of BRS were obtained from Alcoa’s Kwinana (KW), Pinjarra (PJ), and Wagerup (WG) Refineries. Each sample exhibited predominantly variable charge (σv ≈ 8–12 cmol/kg at pH 12), and negligible permanent negative charge (σp ≈ 0.2 cmol/kg). The point of zero net charge (PZNC) was observed at pH 6.96, 6.89, and 5.98 for the KW, PJ, and WG samples, respectively. These values are consistent with those reported for soils dominated by Fe and Al oxides and hydroxides but containing negligible organic matter. Solution and adsorbed NH4 decreased with increasing pH (pH 7–11) for BRS. It was suggested that ammonia volatilisation was a major loss pathway for NH4 applied to BRS. Phosphorus sorption decreased with increasing pH for each BRS. It was suggested that the presence of competing anions (i.e. carbonate) and increasing negative surface charge density were the major causes for this behaviour. The results from this study have major implications for the selection of suitable types of fertilisers (particularly nitrogen) for rehabilitating alkaline BRS.