Highly efficient electrochemical oxidation of ammonium in strong acid via low-frequency sonication: The role of ammonium desolvation

The use of electrochemical advanced oxidation process (EAOP) for treating NH 4 + -rich wastewater via reactive halogen species offers a promising alternative to biological methods but faces challenges in strong acid due to poor NH 4 + oxidation kinetics. Herein, we identify the strong solvation of NH 4 + as the key limiting factor, where surrounding H 2 O molecules form a protective shield that hinders oxidative species. To address this, low-frequency sonication (LFS, e.g., 40 kHz) was introduced, disrupting the solvation environment through cavitation and enhancing NH 4 + exposure to oxidative species. At pH 2, electrolysis alone exhibited only 8.0–28.8 % NH 4 + oxidation efficiency within 2.5 h, while this value significantly improved (60.73–100 %) in the LFS-Cl/EAOP systems and reached 100 % in the LFS-Br/EAOP systems. The LFS-Cl/EAOP systems showed strong dependence on anode materials, with boron-doped diamond (BDD) and PbO 2 outperforming mixed metal oxides (MMO) and Pt; in contrast, the LFS-Br/EAOP systems achieved high NH 4 + removal regardless of anode type. It is further revealed that bromate formation was suppressed by rapid HBrO consumption and cathodic reduction. Moreover, the feasibility of complete NH 4 + elimination and acid solution regeneration for NH 3 adsorption via LFS-Br/EAOP was demonstrated, highlighting its potential for practical application.