Sustainable degradation of tetracycline via biochar-based Fe/Co-alginate beads in a fixed-bed peroxymonosulfate reactor

The widespread occurrence of tetracycline antibiotics (TCs) in aquatic environments poses serious ecological and human health risks. In this study, novel biochar-iron/cobalt cross-linked alginate beads (BFC-beads) were synthesized from waste coffee grounds and sodium alginate via a polymerization process. These BFC-beads were applied to activate peroxymonosulfate (PMS) for the continuous degradation of tetracycline hydrochloride (TCH) in a fixed-bed reactor. Under optimal conditions (flow rate: 4.4 ×10^-5 L/s; 2 g BFC-beads; 0.6 g/L PMS), the system achieved a TCH removal efficiency exceeding 93 %. The BFC-beads exhibited excellent operational stability across a broad pH range (3-11) and maintained performance in the presence of various anions. Reactive species analysis via quenching experiments and electron paramagnetic resonance (EPR) identified singlet oxygen (¹O₂), sulfate radicals (SO₄^-•), hydroxyl radicals (•OH), and superoxide (O₂^•-) as key contributors to TCH degradation. Electrochemical measurements indicated a synergistic Co-Fe bimetallic effect at the catalytic interface, enhancing redox cycling and reducing charge-transfer resistance. Liquid chromatography-mass spectrometry (LC-MS) revealed a stepwise degradation pathway involving demethylation, hydroxylation, and ring-opening reactions, leading to progressive mineralization. Furthermore, toxicity assessments using the Toxicity Estimation Software Tool (T.E.S.T.) demonstrated a marked reduction in the biotoxicity of degradation intermediates. Overall, the BFC-bead-packed fixed-bed reactor offers a promising and sustainable approach for continuous treatment of antibiotic-contaminated wastewater.