Boosting Ammonium Oxidation in Wastewater by the BiOCl-Functionalized Anode

Mixed metal oxide (MMO) anodes are commonly used for electrochlorination of ammonium (NH₄⁺) in wastewater treatment, but they suffer from low efficiency due to inadequate chlorine generation at low Cl^- concentrations and sluggish reaction kinetics between free chlorine and NH₄⁺ under acidic pH conditions. To address this challenge, we develop a straightforward wet chemistry approach to synthesize BiOCl-functionalized MMO electrodes using the MMO as an efficient Ohmic contact for electron transfer. Our study demonstrates that the BiOCl@MMO anode outperforms the pristine MMO anode, exhibiting higher free chlorine generation (24.6-60.0 mg Cl₂ L^-1), increased Faradaic efficiency (75.5 vs 31.0%), and improved rate constant of NH₄⁺ oxidation (2.41 vs 0.76 mg L^-1 min^-1) at 50 mM Cl^- concentration. Characterization techniques including electron paramagnetic resonance and in situ transient absorption spectra confirm the production of chlorine radicals (Cl^• and Cl₂^•-) by the BiOCl/MMO anode. Laser flash photolysis reveals significantly higher apparent second-order rate constants ((4.3-4.9) × 10⁶ M^-1 s^-1 at pH 2.0-4.0) for the reaction between NH₄⁺ and Cl^•, compared to the undetectable reaction between NH₄⁺ and Cl₂^•-, as well as the slower reaction between NH₄⁺ and free chlorine (10² M^-1 s^-1 at pH < 4.0) within the same pH range, emphasizing the significance of Cl^• in enhancing NH₄⁺ oxidation. Mechanistic studies provide compelling evidence of the capacity of BiOCl for Cl^- adsorption, facilitating chlorine evolution and Cl^• generation. Importantly, the BiOCl@MMO anode exhibits excellent long-term stability and high catalytic activity for NH₄⁺-N removal in a real landfill leachate. These findings offer valuable insights into the rational design of electrodes to improve electrocatalytic NH₄⁺ abatement, which holds great promise for wastewater treatment applications.