Photoconversion of Cyanide to Dinitrogen Using Durable Electrode of TaON Overlayer-Deposited WO3 Film and Visible Light

Abstract

Chemical treatments of toxic cyanide (CN–) typically involve its conversion to cyanate (OCN–), which is less toxic. An ideal treatment should be its conversion to N2 and CO2. This study proposed and demonstrated an engineered photoelectrochemical (PEC) system that converts CN– to N2. WO3 has been often used as a visible light active photoanode in the PEC system, but it is stable only in the acidic condition. Since cyanide is generally treated under the alkaline condition, it is necessary to develop a PEC system that is stable in the alkaline condition. To overcome this drawback, the TaON overlayer was electrodeposited on a WO3 film, which enhanced not only the stability of the WO3 electrode in the neutral/alkaline condition but also the interfacial charge transfer efficiency. The WO3/TaON electrode under visible light effectively generated reactive chlorine species (RCS) from Cl– oxidation along with concurrent production of H2O2 via O2 reduction on a graphite cathode. CN– was oxidized to OCN–, which subsequently reacted with RCS to produce NH3 and further to N2 as the final product. In particular, in situ generated H2O2 works synergically with RCS to enhance the CN– oxidation and the total nitrogen (TN) removal efficiency. This study demonstrated a new method that transforms CN– to N2 and CO2 as final products without using any toxic chemical reagents, which is environmentally friendly.