Boosting Ammonium Oxidation in Wastewater by the BiOCl-Functionalized Anode

Abstract

Mixed metal oxide (MMO) anodes are commonly used for electrochlorination of ammonium (NH 4 + ) 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 NH 4 + 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 Cl 2 L –1 ), increased Faradaic efficiency (75.5 vs 31.0%), and improved rate constant of NH 4 + 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 Cl 2 •– ) by the BiOCl/MMO anode. Laser flash photolysis reveals significantly higher apparent second-order rate constants ((4.3–4.9) × 10 6 M –1 s –1 at pH 2.0–4.0) for the reaction between NH 4 + and Cl •, compared to the undetectable reaction between NH 4 + and Cl 2 •–, as well as the slower reaction between NH 4 + and free chlorine (10 2 M –1 s –1 at pH < 4.0) within the same pH range, emphasizing the significance of Cl • in enhancing NH 4 + 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 NH 4 + -N removal in a real landfill leachate. These findings offer valuable insights into the rational design of electrodes to improve electrocatalytic NH 4 + abatement, which holds great promise for wastewater treatment applications.

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