Oxygen-Vacancies-Mediated BrO3- Electroreduction: Positively Charged Microenvironment Enables Directional Diffusion and Spontaneous Adsorption

Abstract Ozonation can disinfect the drinking water without producing chlorinated byproducts, but bromate (BrO₃⁻) emerges as a carcinogenic by-product. Although the electroreduction could theoretically convert BrO₃⁻ into non-toxic bromide (Br⁻), the detoxification process was hindered by the electrostatic repulsion of BrO₃⁻ from the negatively charged cathode. In this work, a commercial Fe foam was employed for electrochemically reducing 1.0 mg/L BrO₃⁻ via direct electron transfer mechanism (k1, 0.7796 min⁻ ¹), during which microcrystalline magnetite (Fe₃O₄) was in-situ generated with abundant oxygen vacancies (Ovs). The Ovs could shape neighboring Fe²⁺ atoms into positively charged microenvironment for accelerating the directional diffusion of BrO₃⁻ toward themselves. Compared to negatively charged surface, the positive microenvironment could enhance the BrO₃⁻ diffusion with coefficient change from 0.0059 m²/s to 0.0387 m²/s. Furthermore, Ovs activated the neighboring Fe²⁺ atom into a highly active site for BrO₃⁻ adsorption with an adsorption energy (Eads) of 4.21 eV, in comparison to the energy-demanding adsorption on intact Fe₃O₄ lattice (Eads, -2.17 eV). Afterwards, BrO₃⁻ underwent complete detoxification through sequential deoxygenation, with Ovs assisting throughout the whole process from BrO₃⁻ to Br⁻. By the in-situ defect engineering, the research pointed out a high-efficient approach to create positively charged microenvironment for enhancing oxyanion electroreduction.