Synergistic removal of tetracycline and cadmium in real wastewater using KHCO3-Modified biochar derived from agricultural waste: Mechanism and engineering application.

Co-contamination of tetracycline (TC) and cadmium (Cd) is common in wastewater from intensive livestock farming and aquaculture. However, quantitative understanding of the coupled removal of antibiotic-heavy metal co-contaminants by the same modified biochar remains limited, while many modification strategies rely on corrosive reagents and complex post-treatment procedures that hinder low-cost and scalable application. This study prepared KHCO3-modified biochar (KBCs: RKBC, YKBC, HKBC, CKBC) via one-step co-pyrolysis of KHCO3 with four agricultural residues (rapeseed meal-straw, corn stover, peanut shells, and Rosa roxburghii residue), for the removal of TC and Cd from wastewater. Compared to raw biochar, KBCs exhibited significantly enhanced textural properties: specific surface area increased by 35.17-305.71-fold, and total pore volume expanded by 27.71-69.98-fold. KBCs exhibited strong resistance to ionic interference, regenerability, and adaptability. At pH 6.0 and 298 K, the best-performing RKBC effectively removed TC/Cd from wastewater. Isotherm models determined maximum adsorption capacities of 496.31 ± 14.212 mg g-1 for TC and 95.67 ± 3.549 mg g-1 for Cd, representing 9.02-fold and 12.33-fold improvements over the corresponding pristine biochar. Mechanistic analyses indicate that TC retention is primarily driven by hydrogen bonding (26.50-32.60%), with electrostatic attraction (18.53-22.42%) and pore filling (6.12-20.32%) as secondary contributions, whereas Cd immobilization is dominated by pore filling (36.92-40.46%) and electrostatic attraction (20.80-27.25%), supplemented by mineral precipitation (10.51-12.81%) and π-Cd interaction (5.41-11.57%). In a pilot-scale three-stage treatment system based on RKBC, TC and Cd decreased from 136.35 to 0.36 mg L-1 to 0.89 and 0.04 mg L-1, respectively, supporting the engineering potential of KBCs in TC-Cd coexisting wastewater.