Improving biochar's phosphate adsorption using active metal substances in Byer red mud

Modifying biochar with solid wastes enhances phosphate adsorption capacity and facilitates solid waste utilization. However, there is limited research on utilizing bulk solid wastes to improve the phosphates adsorption performance of biochar. In this study, a bulk industrial solid waste, Bayer red mud, was used as a modifier to improve the phosphate adsorption capacity of biochar. Several factors including mixing method and blending ratio of biomass and red mud, acid concentration, and pyrolysis temperature were investigated to determine their influence on the phosphate adsorption capacity of red mud modified biochar (RMB). The adsorption mechanisms were examined by analyzing adsorption kinetics, isotherms, thermodynamics, approximate site energy distribution, and biochar's physical and chemical properties. The results showed that acid impregnation with optimal blending ratio and pyrolysis temperature significantly increased the phosphate adsorption capacity of RMB up to 137.69 mg/g. The adsorption of phosphates by RMB followed the Pseudo 2nd order and Langmuir isotherm models, suggesting monolayer chemisorption on a homogeneous surface, as supported by the analysis of approximate site energy distribution. The characterizations of RMB suggested that the dominant mechanism for phosphate adsorption by RMB was precipitation, rather than ligand exchange or electrostatic attraction. Acid impregnation improves the extraction of Fe, Al, and Ca from Byer red mud, enhancing the dispersion of their oxides in biochar. Moreover, the P-rich RMB improved the growth of grass seedlings, suggesting its potential as a P fertilizer.