Plasma technology is a promising tool to degrade organic contaminants in water treatment. Compared to conventional advanced oxidation processes (AOPs), plasma operation does not require the input of chemicals and utilizes only electricity to generate reactive species (e.g., hydroxyl radical, hydrated electron). To reduce the requirement of electricity per organic contaminant degradation, plasma application in water treatment may require concentrating organic contaminants in brine solutions generated by ion exchange or reverse osmosis. These brine solutions often contain high concentrations of halides (e.g., 0.1-2 M chloride) that may largely impact the performance of plasma by changing the solution conductivity, altering radical speciation, and forming hypohalous acids. In this study, we evaluated the impact of halides on the degradation of selected organic contaminants during treatment by a dielectric barrier discharged plasma. We first compared the degradation of organic probes (i.e., benzoate, para-hydroxybenzoate) together in the solution during plasma treatment. Compared to the ionic strength control (i.e., 0.5-2 M perchlorate), halides (i.e., 0.5-2 M chloride and 1-20 mM bromide) selectively increased the degradation of para-hydroxybenzoate (i.e., by 4- to 10-fold) relative to benzoate (i.e., no change). The increased degradation of para-hydroxybenzoate was attributed to the formation of hypochlorous acid (HOCl) or hypobromous acid (HOBr), which was detected in solutions after plasma treatment. We further investigated the formation pathways of HOCl and HOBr during plasma treatment of high salinity solutions. By using a radical quencher (i.e., isopropanol), we found that the degradation of para-hydroxybenzoate was decreased to a level comparable to that in ionic strength control, indicating that the formation of HOCl and HOBr was via a radical-mediated pathway. Our study potentially demonstrated that in addition to their known impact in conventional AOPs, halides also alter organic compound degradation during plasma treatment.

Effects of halides on organic compound transformation during plasma treatment

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