Multivalent iron in waste steel rust serves as efficient electrocatalyst for flow-through removal of tetracyclines in wastewater

Tetracycline antibiotics (TCs) pose escalating environmental threats due to their persistence and resistance gene dissemination. Herein, this research pioneered a waste-derived structured electrode by immobilizing polycrystalline waste steel rust (WSR) on honeycomb activated carbon (HAC@WSR) as a third-electrode for flow-through TCs removal. Under the optimal conditions of cell voltage 10 V, initial solution pH 7.5 ± 0.2, and flow rate 5 mL min −1 , the system achieved a removal rate ( k obs ) of 0.055 min −1 , approximately double that of commercial Fe powder, FeO, or Fe 3 O 4 with k obs of 0.025–0.032 min −1 . Integrated analyses reveal dual enhancement mechanisms: (1) Electric-field-promoted adsorption via WSR-mediated chemisorption (HAC@WSR exhibits an equilibrium adsorption capacity of 118.46 mg g −1 for tetracycline hydrochloride (TCH), 2.02 times higher than static adsorption), where adsorbed TCs self-optimize electrode activity; (2) Multivalent iron-triggered generation of diverse radicals (•OH, •O 2 - , H*) at the electrode surface, accelerating ring-opening mineralisation with negligible aquatic toxicity in final products. Under continuous operation, the three-dimensional electrochemical system (3DES) achieved about 95 % removal for TCs, with effluent TCs concentrations remained below 0.1 mg L −1 . Compared with the two-dimensional electrochemical system (2DES) without HAC@WSR, the 3DES exhibited a 40.6 % enhancement in TCH removal efficiency. Under 360 min of continuous flow operation, the 3DES maintained a removal efficiency of > 90 % for TCs with negligible iron leaching, while the 2DES had a breakdown. Additionally, the 3DES significantly improving wastewater’s biodegradability (BOD 5 /COD increased from 0.15 to 0.44), enabling practical integration as a biological pretreatment. This work establishes a "waste-to-clean-water" paradigm aligned with circular economy principles, simultaneously addressing metal waste recycling and antibiotic contamination.