Surface Charge and Phosphorus Retention in Metal-Activated Biochars from Different Pyrolysis Temperatures

Biochar is a promising material for phosphorus (P) removal from water, but its surface chemistry can limit adsorption efficiency. In this study, biochars produced at 440 °C and 880 °C from the same feedstock were functionalized post-pyrolysis using aqueous solutions of AlCl3, CaCl2, and FeCl3 at two concentrations (0.5 M and 2.0 M). The aim of this work was to assess how both pyrolysis temperature and post-pyrolysis activation with different metals affect the surface charge of biochar and its capacity to retain P from aqueous solution. The treated materials were characterized for pH, point of zero charge (pHpzc), and phosphorus retention from solution. Results showed that Al- and Fe-activation significantly reduced the biochar pH and shifted the pHpzc to more acidic values, enhancing electrostatic attraction toward phosphate species. Phosphorus adsorption was most effective for biochar obtained at 440 °C and treated with AlCl3 and FeCl3, achieving up to 10.2 mg P g−1. These findings highlight the importance of surface charge modulation in tuning biochar performance for phosphate removal from aqueous solution. Based on the obtained results, electrostatic attraction was the main mechanism by which activated biochar adsorbed P from aqueous solution.