Closed-loop phosphorus recovery via engineered biochar: Synergistic co-pyrolysis of invasive water hyacinth with industrial red mud

The sustainable recovery of phosphate from aqueous systems while simultaneously valorizing industrial and biological wastes remains a critical environmental challenge. Herein, we developed an innovative red mud-modified water hyacinth biochar (RM-BC) for phosphate recovery and subsequent resource utilization. Systematic investigations revealed that the optimized RM-BC exhibited exceptional phosphate adsorption performance, achieving 99.83 % removal efficiency. RM-BC demonstrated rapid uptake kinetics (89.39 % recovery within 120 min) and maintained robust performance (>70 % efficiency) across a wide pH range (3−11). Kinetic and isotherm analyses indicated that the adsorption process followed pseudo-second-order kinetics and the Freundlich model, suggesting multilayer adsorption on heterogeneous surfaces. Thermodynamic studies confirmed the spontaneous and endothermic nature of the process, with enhanced efficiency at higher temperatures. Advanced characterization elucidated the mechanisms, including pore filling, surface precipitation, complexation, and electrostatic interactions. The phosphorus-laden biochar (RM-BC-P) showed excellent environmental compatibility, with heavy metal leaching levels below regulatory limits, qualifying it as a safe soil amendment. Furthermore, RM-BC-P exhibited controlled phosphorus release characteristics, demonstrating dual functionality as both a slow-release fertilizer and heavy metal immobilization agent in contaminated soils. This work provides fundamental insights into waste-derived adsorbent design and presents a sustainable strategy for simultaneous phosphate recovery and waste valorization.