High-affinity phosphate capture on MgO/Mg(OH)2-decorated wattle bark biochar: adsorption mechanism and regeneration performance.

MgO modified wattle bark biochar (MgO-BC) was produced by impregnating wattle bark with MgCl2 followed by pyrolysis at 600 °C and tested for the removal of phosphate from water. Material characterization showed the successful incorporation of MgO/Mg(OH)2 phases onto the biochar matrix and the presence of surface functionalities that are relevant for phosphate binding. Response surface methodology showed that solution pH, MgO-BC dosage, contact time, and initial phosphate concentration significantly influenced phosphate removal. The optimized conditions were pH 8.0, an MgO-BC dosage of 0.059 g in 10 mL solution, a contact time of 21 h, and an initial phosphate concentration of 64.6 mg P per L. Under these conditions, MgO-BC achieved approximately 99.5% phosphate removal. Equilibrium data over an extended initial phosphate concentration range (15-500 mg P per L) were best described by the nonlinear Langmuir isotherm (q max = 11.42 ± 0.11 mg P g-1, K L = 22.43 ± 1.58 L mg-1, R 2 = 0.985), confirming high-affinity, monolayer uptake on a finite number of Mg-rich sites, whereas the nonlinear Freundlich and Temkin models provided inferior fits (R 2 = 0.729 and 0.795, respectively). These model fits, combined with spectroscopic and XRD evidence for Mg-phosphate interactions, are consistent with a mechanism dominated by high-affinity surface reactions rather than weak physisorption. XRD patterns before and after adsorption confirmed that the main reflections of MgO/Mg(OH)2 did not shift after adsorption, indicating that the immobilization of phosphate was mainly due to inner-sphere complexation and poorly crystalline surface precipitation and not to bulk phase transformation. The initial phosphate removal efficiency of MgO-BC was found to be over 80% after five adsorption-desorption cycles, which confirms good regeneration potential. Based on these results, MgO-BC could be identified as an effective and reusable phosphate removal and recovery sorbent from aqueous systems.