Fabrication of a permeable SnO<inf>2</inf>-Sb reactive anodic filter for high-efficiency electrochemical oxidation of antibiotics in wastewater

Abstract

Electrochemical oxidation (ECO) is an appealing technology for treating emerging organic pollutants in wastewater. However, the conventional flow-by ECO process is expensive with a low energy efficiency owing to the limitations of mass transport of contaminants to the limited surface area of the anode. In this study, a novel freestanding porous and permeable SnO2-Sb anode was fabricated by one-step sintering using micrometer-sized (NH4)2CO3 grains as the pore-forming agents. This permeable SnO2-Sb anode without Ti substrate functioned as a reactive anodic filter (RAF) in an ECO cell to treat wastewater containing ciprofloxacin (CIP). Forcing the wastewater through the porous RAF depth-wise improved the mass transport and vastly enlarged the electroactive surface area. Compared with the conventional flow-by configuration, the flow-through RAF exhibited a 12-fold increase in the mass transfer rate constant (60.7 × 10−6 m s−1) and a 5-fold increase in the CIP degradation rate constant (0.077 min−1). At a cell potential of 4.0 V, more than 92% of the CIP was degraded in a single-pass operation at a filtration flux of 54 L m−2 h−1 and a short retention time of 1.7 min through the RAF. The robustness and stability of the RAF were demonstrated by its remarkable CIP degradation efficacy of 99% during 200 h of operation. The mechanism of CIP degradation was examined using probe molecules and density functional theory calculations and found to be a combined effect of direct electron transfer and oxidation by generated radicals ([rad]OH and SO4[rad]−). The great potential of RAF in flow-through ECO was further validated by its effective removal (>92%) of various organic pollutants in actual municipal wastewater at a low energy consumption of 0.33 kWh m−3. The RAF-based ECO process thus provides an advanced environmental technology for the oxidation of toxic and recalcitrant organic pollutants in wastewater treatment.