Understanding Breakpoint Chlorination during in-Situ Electrochlorination: Influence of Mass Transport
Authors: Atrashkevich Aksana , Varda Dominic , Yeager Kaitlyn , Garcia-Segura Sergi 
Published: December 2024

Based on the United Nations of 2023, approximately 26% of people lack access to reliably managed drinking water services, and around 44% of water worldwide is not safely treated before releasing into the environment. Water scarcity is induced not only by impact of water stresses, but also by rapid spreading of freshwater pollution. Decentralized electrochemical water treatment technologies are becoming a sustainable and easily scalable solution for water treatment in areas experiencing water stresses, especially when powered by alternative energy sources. Electrochemical water treatment has received profound interest among researchers due to its high efficiency in oxidizing wide range of pharmaceuticals, disinfecting water from pathogens, reducing nitrate content, and removing ammonia. The electrochemical oxidation of ammonia is particularly interesting because ammonia is a widely common species in water. Dealing with high level of ammonia, either from initially contaminated water or water with elevated levels of ammonia generated during other water treatment processes presents severe challenges. Indirect electrochemical oxidation of ammonia can become a feasible solution that exploits the unavoidable oxidation of chloride ions (ubiquitous in all types of waters) to chlorine.
Despite indirect ammonia oxidation being present in many research works that utilize real water matrices, the mechanism of electrochemical breakpoint chlorination still requires further research efforts. This study aimed to examine the mechanism of breakpoint chlorination during electrochemical generation of chlorine active species and determine the main differences when comparing with homogeneous ammonia oxidation. A dimensional stable anode of Ti/IrO2, identified as one of the most selective anodes for chlorine evolution, was used. The study found that operating parameters influenced generation of intermediates and products. The difference in current density did not impact significantly on the species formation and mechanism. One of the most interesting findings revealed the significant role of operating hydraulic conditions on the electrochemical breakpoint chlorination process. Contradictory to many electrochemical water treatment reactions, electrochemical breakpoint chlorination under low mass transport conditions in batch reactor resulted in more efficient reaction performance. Two reasons were proposed that could justify the observation including (i) higher chlorine species availability on the anode surface that resulted to the elevated chlorine to ammonia ratio leading to faster ammonia oxidation, and (ii) decreased mass transport that led to formation of thicker diffusion layer on the anode surface with a low pH profile resulting in more efficient chlorine evolution reaction. The Pourbaix diagram of chlorine species formation indicates more effective chlorine evolution under lower pH conditions. The experiments with only chlorine evolution reaction without ammonia present confirmed that lower mass transport in the reactor led to more efficient chlorine evolution performance. Cyclic voltammograms confirmed more effective chlorine evolution at lower water pH. In addition, there was a significant difference between the electrochemical indirect ammonia oxidation and homogeneous breakpoint chlorination. Results showed that electrochemical treatment resulted in negligible chlorine and monochloramine formation, and simultaneous pH decrease due to immediate ammonia oxidation, which differs from homogeneous breakpoint chlorination. Heterogeneous chlorine availability on the anode surface during electrochemical reaction and corresponding local ammonia oxidation due to high chlorine to ammonia ratio suggest a more environmentally friendly approach when comparing with homogeneous oxidation. This might result to lower number of halogenated compounds in water due to electrochemical breakpoint chlorination resulted in diminutive chlorine species presence in the bulk solution until breakpoint was achieved. These findings opened a new perspective on electrochemical breakpoint chlorination. In addition, faster and more economically feasible ammonia oxidation might be achieved by lowering mass transport during chlorine evolution to maintain more acidic pH on the anode surface. The saddle point of the most efficient electrochemical breakpoint chlorination might be found through the mass transport optimization.

Figure 1