Multilayer Heterojunction Anodes for Saline Wastewater Treatment: Design Strategies and Reactive Species Generation Mechanisms

• Published in: Environmental science & technology Vol. 50; no. 16; pp. 8780 - 8787
• Main Authors: Yang, Yang, Shin, Jieun, Jasper, Justin T, Hoffmann, Michael R
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 16.08.2016
• Subjects: active sites; ammonium compounds; anodes; chemical oxygen demand; Chlorine; electrochemistry; Electrodes; Electrolysis; Electron transfer; energy; Energy consumption; Experiments; Free radicals; humans; islands; Oxidation-Reduction; pyrolysis; reaction kinetics; reaction mechanisms; Saline water; semiconductors; simulation models; titanium; Titanium - chemistry; Waste Water - chemistry; wastewater; wastewater treatment; Water treatment
• ISSN: 0013-936X; 1520-5851; 1520-5851
• DOI: 10.1021/acs.est.6b00688

Multilayer heterojunction SbSn/CoTi/Ir anodes, which consist of Ir0.7Ta0.3O2 bottom layers coated onto a titanium base, Co-TiO2 interlayers, and overcoated discrete Sb-SnO2 islands, were prepared by spray pyrolysis. The Ir0.7Ta0.3O2 bottom layer serves as an Ohmic contact to facilitate electron transfer from semiconductor layers to the Ti base. The Co-TiO2 interlayer and overcoated Sb-SnO2 islands enhance the evolution of reactive chlorine. The surficial Sb-SnO2 islands also serve as the reactive sites for free radical generation. Experiments coupled with computational kinetic simulations show that while ·OH and Cl· are initially produced on the SbSn/CoTi/Ir anode surface, the dominant radical formed in solution is the dichlorine radical anion, Cl2·–. The steady-state concentration of reactive radicals is 10 orders of magnitude lower than that of reactive chlorine. The SbSn/CoTi/Ir anode was applied to electrochemically treat human wastewater. These test results show that COD and NH4 + can be removed after 2 h of electrolysis with minimal energy consumption (370 kWh/kg COD and 383 kWh/kg NH4 +). Although free radical species contribute to COD removal, anodes designed to enhance reactive chlorine production are more effective than those designed to enhance free radical production.