Sustainable Phosphate Recovery Using Novel Ca–Mg Bimetallic Modified Biogas Residue-Based Biochar

Elevated phosphorus levels in aquatic ecosystems have been identified as a critical driver of eutrophication processes, necessitating resource-recovery remediation strategies. Adsorption techniques show particular promise for nutrient recovery due to their selective binding capacities and operational feasibility. In this study, the Mg- and Ca-modified biogas residue-based biochar (Ca-Mg/BC) was successfully prepared via a “bimetallic loading-pyrolysis” modification strategy. The optimum temperature for the calcination of the material and the salt solution impregnation concentrations were determined experimentally through optimization of the synthesis conditions. Structural and chemical analyses of Ca–Mg/BC demonstrated that the material contains MgO and CaO. The specific surface area of Ca–Mg/BC was 8.49 times higher than that of the unmodified biochar (BC). The optimized Ca–Mg/BC achieved 95% phosphate removal rate (157.13 mg/g adsorption capacity). FTIR and XPS characterization results indicated the importance of Ca/Mg loading in phosphate capture. MgO and CaO were mainly loaded on the surface of the material and adsorbed phosphate through a chemical reaction. Crucially, the phosphate-laden biochar exhibited potential as a nutrient-enriched soil amendment, opening the material loop from wastewater treatment to agricultural applications. This sustainable strategy simultaneously addresses water pollution control and sustainable development, providing environmentally benign solutions compatible with industrial effluent treatment and sustainable agriculture practices.