Dual Heteroatom Synergy in Rice Husk–Derived Biochar: A Nitrogen/Sulfur Co-Grafting Strategy for Superior Adsorptive Removal of Tetracycline Hydrochloride

Agricultural byproducts, particularly rice husks, represent an underutilized carbon resource for developing low-cost and robust adsorbents. However, their adsorption performance under complex ionic environments relevant to real wastewater remains insufficiently understood. In this study, rice husks, an abundant agricultural residue in Vietnam, were converted into nitrogen- and sulfur-cografted biochar (N,S-BC) through a facile pyrolysis-doping for tetracycline hydrochloride (TCH) removal. The N,S-BC was comprehensively evaluated by SEM, TEM, BET, XRD, Raman, and FTIR techniques to elucidate its surface morphology, pore structure, and chemical functionalities. The optimal adsorption conditions were determined as pH 4, material dosage 0.15 g/L, initial TCH concentration 40 mg/L, and temperature 30 °C. Under these conditions, N,S-BC achieved a remarkable maximum uptake ability of 618.31 mg/g. Thermodynamic results discovered that the uptake was spontaneous (ΔG° < 0), exothermic (ΔH° = -78.17 kJ/mol), and accompanied by decreased system randomness (ΔS° = -169.87 J/mol K). The adsorption remained highly stable in the presence of monovalent cations (Na⁺, K⁺, and NH₄⁺), maintaining 98-99% removal efficiency, whereas divalent Ca²⁺ and NO₃^- caused pronounced inhibition, reducing efficiency to 27.45 and 60.50%, respectively. Furthermore, N,S-BC retained over 75% of its initial adsorption capacity after five regeneration cycles, indicating good structural stability. The high performance originates from the synergistic N,S codoping, which enhances surface polarity, active site density, and π-π/hydrogen-bond interactions with TCH, offering a sustainable route for valorizing rice husk waste into efficient adsorbents.