Quantitative Photoelectrochemical Conversion of Ammonium to Dinitrogen Using a Bromide-Mediated Redox Cycle

Cite this: ACS EST Engg. 2021, 1, 9, 1287–1297
Publication Date (Web):July 26, 2021
https://doi.org/10.1021/acsestengg.1c00119
Copyright © 2021 American Chemical Society
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Abstract

Bromide ion (Br) can be oxidized to reactive bromine species (RBS; Br, Br2•–, and HOBr/OBr) which can serve as an effective alternative to chlorine disinfectant. This study investigated the generation of RBS in a photoelectrochemical (PEC) system using an electrochromic TiO2 nanotube arrays (Blue-TNTs) electrode under UV light (λ > 320 nm) and demonstrated the effect of RBS on the direct conversion of ammonium (NH4+) to dinitrogen (N2) with near 100% efficiency. The PEC system utilizing in situ generated RBS not only removed NH4+ more efficiently than photocatalytic (PC) and electrochemical (EC) systems but also prevented the generation of unwanted products (i.e., NO2 and NO3). In addition, compared with the PEC-Cl system, the PEC-Br system exhibited a superior ammonium removal efficiency (16% vs 95% for 120 min of reaction; under air-equilibrated condition). The PEC system also showed higher NH4+ removal efficiency and lower energy consumption when compared to an EC system (using a boron-doped diamond electrode). While bromate ions (BrO3) are produced as a toxic byproduct of bromide oxidation in a typical ozonation system, the Blue-TNTs PEC-Br system fully hinders the bromate formation as long as ammonium is present in the solution because RBS rapidly reacts with NH4+ with little chance of further oxidation to bromate.

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsestengg.1c00119.

  • SEM images of TNTs samples (Figure S1); XPS spectra of Ti and Pt bands of Blue-TNTs and Intact-TNTs samples (Figure S2); oxygen evolution reaction of Blue-TNTs (Figure S3); and stability test of Blue-TNTs for 100 h (Figure S4) (PDF)

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