Synergistic enhancement of N, S co-modified biochar for removal of tetracycline hydrochloride from aqueous solution: Tunable micro-mesoporosity and chemisorption sites

The misuse of tetracycline hydrochloride (TCH) antibiotics has caused irreversible hazards to ecosystem. In this study, a series of porous biochar with tunable micro-mesoporosity and chemisorption sites are designed by cleverly exploiting the pyrolysis properties of active agents in various temperature regions. Benefiting from the existence of more pore structures matching the adsorbate while exposing more chemisorption sites, the maximum adsorption capacity (Qm) of NSC-1.0 for TCH is up to 1480.1 mg g−1 at 303 K. A combination of model fitting, post characterization and density functional theory (DFT) calculations confirms that the adsorption process involves the pore filling, the formation of non-covalent bonds (weak hydrogen bond, π-π stacking, electrostatic interaction) and ligand covalent bonding (Lewis acid-base interaction). The Qm is positively correlated with the volume of pore size greater than 1 nm (R2 = 0.9879), indicating that the pore filling effect is a decisive factor for adsorption performance. Post-characterization directly demonstrates that weak hydrogen bonding and π-π stacking are pivotal physical interactions for adsorption. DFT calculations show that partial N, S species and oxygen-containing functional groups can significantly reduce the adsorption energy of intrinsic graphene, especially the chemisorption sites of pyrrolic N (N5) and thiophene S (S5) containing lone-pair electrons. Furthermore, the competitiveness of NSC-1.0 is revealed in practical evaluation, including cation strength, regenerability, actual water source and fixed-bed column. This study not only contributes new insights to synergistically enhance the removal of macromolecular antibiotics by coupling matchable pore sizes and chemisorption sites, but also develops theoretical support for the fabrication of such efficient adsorbents.