Sequential oxidation-reduction process for active chlorine distribution regulation on ammonia oxidation in the flow-through electrode system

Chlorine-mediated flow-through electrode systems (Cl-FES) are considered a promising method for water treatment. However, the transformation and distribution of various reactive species in the Cl-FES remain unclear. In this study, the active chlorine (AC) distribution and electro-oxidation performance of oxidation-reduction (Ox-red) mode (anode-cathode) were systematically investigated. The pH distribution and linear sweep voltammetry curve revealed that the acid-to-alkaline transition zone in the Ox-red mode is conducive to suppressing the side reaction of oxygen evolution. The Ox-red mode achieved a 2.16-fold enhancement in AC generation rate compared to the reduction-oxidation (Red-ox) mode (cathode-anode). Meanwhile, SHapley Additive exPlanation (SHAP) analysis confirmed that flow rate was the predominant electrolytic parameter in the AC generation, with an optimal output of 92.31 mg/L achieved at 1.5 mL/min. The AC distribution at different flow rates indicated that an increased flow rate promotes the formation of hypochlorous acid (HClO) in the vicinity of the anode and inhibits the formation of chlorates. Furthermore, the ammonia electro-oxidation results demonstrated that the Ox-red mode exhibited a 2.38-fold improvement in ammonia removal rate and a high nitrogen selectivity of 94.51 %. Additionally, the chloramine byproducts generated in the interelectrode region decreased by 98.33 % after passing the cathode region. The overall findings offer insights into reactive species and byproducts regulation for effective and environmental-friendly electro-oxidation applications in flow-through electrode systems.