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

Abstract

Multilayer heterojunction SbSn/CoTi/Ir anodes, which consist of Ir_(0.7)Ta_(0.3)O_2 bottom layers coated onto a titanium base, Co-TiO_2 interlayers, and overcoated discrete Sb-SnO_2 islands, were prepared by spray pyrolysis. The Ir_(0.7)Ta_(0.3)O_2 bottom layer serves as an Ohmic contact to facilitate electron transfer from semiconductor layers to the Ti base. The Co-TiO_2 interlayer and overcoated Sb-SnO_2 islands enhance the evolution of reactive chlorine. The surficial Sb-SnO_2 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, Cl_2·–. 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 NH_4^+ can be removed after 2 h of electrolysis with minimal energy consumption (370 kWh/kg COD and 383 kWh/kg NH_4^+). 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.

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