• Acid-treated bauxite residue adsorbs phosphate more effectively than untreated BR. • Phosphate adsorption is enhanced under low pH conditions in acid-treated BR. • The Freundlich model better describes the phosphate adsorption process. • Phosphate adsorption by BR follows a pseudo-second-order kinetic model. • Hematite surfaces are the primary sites for phosphate adsorption, followed by boehmite. Bauxite Residue (BR), a byproduct of alumina production, is a hazardous industrial waste characterized by high alkalinity and salinity. Its accumulation has led to environmental pollution and public health risks, highlighting the urgent need for effective management strategies. BR has recently been recognized as a potentially effective adsorbent for phosphate recovery from wastewater due to its high iron oxide content. Phosphorous in wastewater is a critical yet non-renewable nutrient essential for agriculture and food production. However, excess phosphate in water bodies disrupts aquatic ecosystems and threatens human health. Utilizing BR to recover phosphate from wastewater offers a sustainable ‘waste controls waste’ approach, effectively addressing both waste management and resource recovery. Here phosphate adsorption capabilities of bauxite residue have been evaluated under different factors including acid-washing (i.e., 0, 0.25, 0.5 mol/L HCl), pH levels (e.g., pH = 4–7), temperature (e.g., room temperature and 50 °C), and initial phosphate concentration (e.g., 5 or 100 mg/L). Experimental results show that acid-washing and low-pH conditions (e.g., pH = 4) significantly improve BR’s phosphate adsorption capacity (i.e., 78 %–100 %). Increasing room temperature to 50 °C under different pH conditions improves adsorption efficiency by 15 %–50 % for samples BR-0 and BR-0.25, while changes in initial phosphate concentration show different effects among experiments. The isotherm adsorption models provide adequate fits for experimental data with high R² values, indicating the occurrence of chemical adsorption on heterogeneous surfaces. The pseudo-second-order model better describes the kinetic process of phosphate adsorption than the pseudo-first-order model. The mechanism of phosphate adsorption by acid-treated BR is related to ligand exchange of phosphate with the -FeOH site on hematite surface.
Wang et al. (Wed,) studied this question.