Synergistic Calcium‐Magnesium Modification of Rice Straw Biochar via One‐Step Pyrolysis: A pH‐Robust Adsorbent for Phosphate Recovery and Slow‐Release Fertilizer

ABSTRACT Efficient removal and reclamation of phosphorus from wastewater is essential for controlling water pollution and promoting sustainable development. In this work, a novel pH‐stabilized phosphate adsorbent (Ca‐MBC) was prepared by synergistic calcium‐magnesium modification of rice straw biochar using one‐step pyrolysis, and its feasibility as a fertilizer was further investigated. SEM and FTIR analyses revealed that Ca‐MBC possessed a large specific surface area (200.44 m 2 /g) and abundant functional groups. At an initial phosphate concentration of 7 mg/L, Ca‐MBC achieved a removal efficiency of 99.1% and a maximum adsorption capacity of 23.53 mg/g. Notably, it maintained a high removal efficiency (> 93.95%) over a wide pH range (4.0‐13.0), indicating excellent pH tolerance and practical economic feasibility. The adsorption process followed the pseudo‐second‐order kinetic model (q e = 7.23 mg/g, k 2 = 0.038/min, R 2 = 0.998), indicating chemisorption‐dominated behavior, and was well described by the Sips isotherm model. The dominant mechanisms contributing to adsorption include surface complexation, chemical precipitation, ligand exchange, and electrostatic attraction. In addition, phosphorus‐loaded Ca‐MBC‐P could enhance mung bean seed germination to 90%, demonstrating significant plant growth promotion. Overall, Ca‐MBC exhibits excellent adsorption performance in water and has potential as a carrier material for phosphorus resources.