Efficient ammonia removal and toxic chlorate control by using BiVO₄/WO₃ heterojunction photoanode in a self-driven PEC-chlorine system

The efficient removal of ammonia is a difficult issue in wastewater treatment because ammonia is easily converted to nitrate instead of N₂. The oxidation of ammonia by chlorine radical (Cl^[rad]) is recognized as an effective method. However, the massive generation of toxic byproducts chlorate and nitrate pose great risk for its practical application due to the excessive oxidation capacity of hydroxyl radical. Herein, we propose a novel method to selectively generate Cl^[rad] for efficient ammonia removal using BiVO₄/WO₃ photoanode in a self-driven photoelectrocatalytic (PEC) system. Cl^[rad] was predominantly produced by regulating the valence band edge of WO₃ though modifying BiVO₄, which tuned the moderate oxidative force of hole to reduce OH^[rad] generation and thereby inhibited the formation of chlorate and nitrate. The self-driven ammonia degradation was achieved by employing BiVO₄/WO₃ and Si photovoltaic cells as composite photoanodes to improve light-absorption and electron-hole separation, thus enhancing Cl^[rad] production. These results showed that 10 mg L⁻¹ of ammonia-N was completely removed (99.3 %) in 120 min with 80.1 % of total nitrogen removal. Toxic byproducts chlorate and nitrate were inhibited by 79.3 % and 31 %, respectively, compared to WO₃. This work provides new insights to develop efficient, energy-saving and environment-friendly method for ammonia pollution treatment.