Mechanistic insights into tetracycline adsorption by alkaline-modified biochar derived from anaerobically digested sludge

Tetracycline (TC) is highly adsorbed by NaOH-modified biochar (MBC), derived from anaerobically digested sludge (ADS). NaOH modification of pristine ADS-BC (PBC, prepared at 300 °C for 1 h) increased the surface area, pore volume, aromaticity, and hydrophobicity. Adsorption kinetics and isotherm analyses of MBC showed that chemisorption was the predominant mechanism governing its adsorption capacity. The surface structure and functionality results indicated that NaOH modification markedly enhanced graphite-like structures and aromatic compounds, thereby facilitating π-π electron donor-acceptor and π-π stacking interactions with TC. The presence of oxygen, silicon-, and calcium-based functional groups also indicates strong potential for hydrogen bonding and surface complexation, respectively. Interestingly, the maximum adsorption capacity of MBC was found to be 392.07 mg/g, comparable to that of commercial activated carbon (342.22 mg/g). In the reusability test using NaOH as the solvent, MBC maintained more than 0.9 of its initially determined adsorption capacity over five cycles. The fixed-bed experiment with effluent from a wastewater treatment facility further confirmed its practical applicability. Future studies focusing on optimized preparation conditions and techno-economic analyses are needed to reduce the environmental burden of ADS through MBC production, while establishing MBC as a promising candidate adsorbent for the removal of contaminants from aquatic solutions.