Removal of Sulfamethoxazole by Nano Zero-Valent Iron Loaded Yak Manure Biochar Prepared via Carbothermal Reduction

Sulfamethoxazole (SMX) is a persistent sulfonamide antibiotic increasingly detected in Qinghai-Tibet Plateau water bodies, where low temperatures and intense UV irradiation retard its natural degradation. Although nano zero-valent iron (nZVI) can reduce and adsorb SMX, bare nZVI suffers from rapid aggregation and oxidation. In this study, yak manure biochar loaded with nZVI (Fe-YBC) was prepared by a one-step carbothermal reduction method at 900 °C, using FeCl₃·6H₂O as the iron precursor at a biomass-to-iron mass ratio of 5:1. Batch adsorption experiments showed that SMX removal by Fe-YBC followed pseudo-first-order kinetics (R² = 0.984) and the Langmuir isotherm model (R² = 0.97), indicating monolayer physisorption. The maximum adsorption capacity of Fe-YBC reached 193.54 mg/g at pH 3, approximately 2.6-fold higher than that of unmodified biochar (YBC, 73.34 mg/g). Adsorption efficiency decreased with increasing pH, consistent with the shift of SMX to its anionic form above pKa2 = 5.6. Coexisting cations inhibited adsorption in the order Mg²⁺ > Ca²⁺ > K⁺ > Na⁺, attributable to competitive site occupation modulated by ionic valence and hydrated radius. After five adsorption–desorption cycles using 75% ethanol as the desorbent, Fe-YBC retained over 86% of its initial removal efficiency. These findings suggest that carbothermal-reduction-derived Fe-YBC is a cost-effective and regenerable adsorbent for SMX removal and offers a route for valorizing yak manure waste in plateau regions.