Plasma Treatment and Ozonation of Binary Mixtures: The Case of Maleic and Fumaric Acids

With respect to ozonation, plasma treatment involves direct contact between the discharge and the contaminated water therefore benefiting, in addition to ozone, also of short-lived reactive species. This paper focuses on mechanistic aspects of water treatment based on plasma activation (in-situ discharge) and ozonation (ex-situ discharge), using maleic acid and fumaric acid as model substrates and dielectric barrier discharges (DBDs) for producing plasma and ozone. Both types of experiments were carried out at different pH values and degradation profiles of residual concentration vs treatment time were compared in experiments in which each acid was treated individually and in mixture with the other. It was found that, under all conditions examined, plasma treatment was more efficient than ozonation for both acids, and that fumaric acid was always more reactive than maleic acid. Peculiar S-shaped degradation curves were obtained for the decay of maleic acid when treated in mixture with fumaric acid under acidic and neutral pH conditions in ozonation and in plasma experiments. This effect was not observed when maleic acid was treated in mixture with phenol instead of fumaric acid. The experimental data are nicely fitted with a simple kinetic model which assumes that a single reactive species, in steady state concentration, is responsible for the attack initiating the pollutants degradation. Based on the complete set of results obtained the conclusion is reached that, in the DBD reactor used, under acidic and neutral pH conditions ozone plays a major role in the degradation of maleic and fumaric acids also in direct plasma treatment.