Fate of typical organic halogen compounds during electrodialysis process and improvement of their recoveries

Abstract

According to an earlier study, electrodialysis (ED) can efficiently separate most organic halogenated micropollutants from inorganic halogens (i.e., halides and oxyhalides) in water, with only a few micropollutants being poorly-recovered. To optimize the ED process and to gain in-depth knowledge of the mechanisms responsible for their low recoveries, this study designed some experiments to examine the fates and transports of three hard-to-separate organic halogen compounds. Results show that the dominant mechanism responsible for the loss of trichlorophenol (TCP) is migration and retention in the ion exchange membrane (IEM), the dominant mechanism for the loss of trichloroacetic acid (TCAA) is ion exchange with the IEM, and the dominant mechanism for the loss of trichloroacetonitrile (TCAN) is volatilization and chemical decomposition during the ED process. Optimizing specific operating conditions (e.g., sealing, voltage, and membrane type, etc.) can improve recovery of these micropollutants to 70.4–95.8% while maintaining 90.4–99.5% removal of chloride. Even though TCAN underwent hydrolysis, we recovered 82.0% chlorine from TCAN and its hydrolytic products. These results may help not only better understand the fates and transports of organic micropollutants during the ED process, but also help facilitate application of electrodialysis technique for measuring total organic halogen.