Synthesis of LaOCl nanoparticles-decorating biochar from catalytic co-pyrolysis of chestnut shells and polyethylene terephthalate with the introduction of lanthanum and its adsorption capability towards phosphate

Valorizing agricultural and industrial crop residues into functional bio-based materials offers sustainable pathways for pollution control. In this study, chestnut shells, an underutilized industrial crop residue, were co-pyrolyzed with polyethylene terephthalate (PET) under catalytic modification by lanthanum (La) to produce a high-performance biochar adsorbent (CS/PET-La-800 C). The introduction of La induced the in-situ growth of LaOCl nanoparticles (180–450 nm) and substantially enhanced the textural properties, increasing the BET surface area to 312.2 m2/g. The modified biochar achieved a maximum phosphate adsorption capacity of 8.89 mg/g, nearly one order of magnitude higher than the pristine char. Adsorption behavior followed the Langmuir isotherm (R2= 0.94) and pseudo-second-order kinetics (R2= 0.95), indicating a chemisorption-dominated process with monolayer coverage. Thermodynamic results (ΔH = 12.45 kJ/L, ΔG < 0) confirmed that adsorption was spontaneous and endothermic. The mechanism mainly involved La-O-P precipitation, electrostatic attraction, and surface complexation, aided by micro-mesoporous structure. Moreover, CS/PET-La-800 C maintained over 80 % phosphate removal efficiency after five regeneration cycles with negligible La leaching (< 0.1 mg/L across pH 3–9). These results demonstrate a practical route to convert industrial crop residues into efficient, regenerable biochar adsorbents for phosphate recovery and wastewater remediation.