Investigation on perfluoroalkyl acids degradation in a novel atmospheric plasma reactor for water treatment

In this thesis the results obtained in a new atmospheric plasma reactor (D-MSPD) designed for the degradation of poly- and perfluoroalkyl substances (PFAS) in water are reported and discussed, using perfluorooctanoic acid (PFOA) as model compound. PFOA is a persistent organic compound whose salt has been used, since the 1950s, as a surfactant and in the production of Teflon. The presence of this compound in surface and groundwater is a risk to human health because it is classified by the IARC as a potentially carcinogenic substance for humans (group 2B). Traditional water treatment systems are ineffective in removing perfluoroalkyl substances, and the removal techniques used are based on absorption on ion exchange resins or granular activated carbon, which however do not lead to the degradation of the contaminants, but only to their transfer from water to sorbent, generating eventually new wastes that require further treatment. For this reason, new advanced oxidation/reduction techniques are being studied to guarantee the effective removal of these substances from the water. One of these techniques is atmospheric plasma treatment, which involves the application of a non-thermal discharge in air or other gases at room temperature and atmospheric pressure to generate, in contact with the contaminated water, a plasma, a gaseous system containing free electrons, ions, radicals, and excited species. In the present work a new Double Multipin Self Pulsing Discharge (D-MSPD) plasma reactor was employed for the first time. It is based on a previous simpler design, the MSPD reactor, which was upgraded to have plasma generation not only over the water surface but also in gas bubbles within the bulk liquid in order to increase the efficiency in the degradation of pollutants with different physical chemical properties. Specifically, the presence of plasma within gas bubbles spread throughout the bulk liquid should allow for the degradation of pollutants which have no surfactant properties, such as the short chain by-products that are generated from PFOA and which are not effectively degraded in the MSPD reactor. The main reactive species produced by the effect of the discharges were determined, i.e. ozone in the gas phase, hydrogen peroxide and OH radical in the solution. Then, the degradation of PFOA was studied by treating water contaminated with known amounts of this contaminant, widely used to evaluate the reactors efficiency and to compare it with that of other reactors previously studied. In the second part of this work a catalyst based on graphene oxide (GO), specifically, reduced graphene oxide doped with boron (B-rGO), was synthesized, characterized and tested in combination with plasma for the degradation of PFOA in this new reactor. For this purpose, the results of experiments of PFOA degradation with only plasma and with plasma plus catalyst were compared. In all the experiments, solution samples were taken at different treatment times and subjected to HPLC/ESI-MS analysis to determine the amount of residual pollutant. Degradation curves were built from these data and rate constants were obtained by interpolating the experimental data with first order exponential decay functions. The effects of different gases used to generate the plasma (air or argon) and of PFOA initial concentration (10-5, 10-6, 10-7 M) on the degradation rate, on the energy efficiency and on the formation of byproducts in solution were studied. From these tests it was concluded that this recently developed reactor has promising prospects for water decontamination via fast and efficient degradation of PFAS and probably of other recalcitrant organic pollutants.