Processi d'ossidazione avanzata indotti da plasma non-termico per la purificazione dell'acqua

Every day numerous sources contribute to contaminate water resources with different types of pollutants among which organic compounds represent a major fraction. Conventional remediation processes are not able to remove completely all these contaminants, especially when the compounds are persistent and resistant to oxidation or when they are present in small concentrations, with the consequent increase of probability of find them even in drinking water. The most innovative techniques recently applied to water purification are based on advanced oxidation processes that use highly reactive species and oxidants such as O3, and OH radicals. Examples are ozonation, the Fenton process and all techniques that combine UV radiation, H2O2, O3, with also the possible presence of a catalyst. A more recent approach, still under study and development, applies electrical discharges in the water or in the air above the water. Electrical discharges are the manifestation of the passage of current in a gas and are generated by applying a high voltage between an active electrode and a grounded electrode. The electrical discharge generates a plasma, a highly reactive environment which contains electrons, ions, excited molecules and radicals and as a whole is electrically neutral. When an electric non-thermalizing discharge is applied in a gas at atmospheric pressure and ambient temperature a non-thermal plasma is produced, which is a system thermodynamically not at equilibrium, with high energy electrons in a gas which remains at room temperature. If the gas is air, the generated plasma is rich in active oxygen species such as O3, •OH, O, O•-, O2•-, etc. It is therefore a strongly oxidizing environment that can be employed for the oxidation of any organic pollutant dissolved in the water. Electrical discharges can be applied directly in the water or above the water. From the point of view of energy discharges in the water are more costly. This is one reason that led the research group in which this thesis was carried out to develop a plasma reactor based on discharge above the water. Non-thermal plasma processing is a strongly interdisciplinary field covering engineering, physics and chemistry. It is evident from the literature that our present understanding of the chemical processes occurring in the plasma, and especially the mechanisms of pollutants degradation in water, is still rather incomplete. The aim of my thesis was to study the degradation products and mechanisms of non-thermal plasma induced advanced oxidation of some organic pollutants under well characterized experimental conditions. Phenol was chosen as a model pollutant, to study the role of the main oxidizing species of non-thermal plasma in air, the OH radical and ozone. The research was then extended to a few important emerging organic contaminants. The experiments were carried out with two prototype reactors built in the Department of Chemical Sciences in collaboration with the Department of Electrical Engineering of University of Padova. In this reactors plasma is generated by dielectric barrier discharges in air above the aqueous solution to be treated. In both reactors, the liquid phase is stationary while the air flows above the solution. They differ in size and in the number of wires that constitute the active electrode. In the first part of my project I tested and compared the performance of the two reactors and found that the larger one has an efficiency similar to that of the smaller but allows to treat larger quantities of solution (200 mL instead of 70 mL). This is a good result in view of possible scaling up of the process. Moreover larger samples of treated solution were available for the analyses. I first performed experiments at different pH and with different salts which showed that the oxidation of phenol is much favored in a basic environment. There appears to be no correlation between the reaction rate and the nature and concentration of the salts used (carbonate and phosphate). By means of ionic chromatography I identified and quantified some reaction intermediates. These results integrated with data obtained previously by our group by HPLC and ionic chromatography, TOC analysis and with data on CO2 quantified by FT-IR spectroscopy, allowed me to determine the carbon balance. From these analyses it appeares that the rate of formation of CO2 is much less than that of disappearance of phenol and that an important fraction of the organic carbon present in solution at the end of the treatment of phenol is yet to be identified. I measured the release of CO2 as a function of the treatment time not only for phenol but also for numerous of its reaction intermediates treated individually under the same experimental conditions (formic acid, acetic acid, oxalic acid, glyoxylic acid, malonic acid and glyoxal). Formic acid, the most advanced intermediate of phenol oxidation to CO2, is the only one among those tested which is completely mineralized to CO2 under the conditions investigated. The mechanism of phenol mineralization proceeds through several consecutive reaction steps and possibly involves also some competitive reactions. The slow determining step or steps in the oxidation are before formic acid degradation. I studied also the effect of the pollutant initial concentration on the process efficiency and found that efficiency increases as the pollutant concentration is reduced. Notably at the low pollutant concentrations present in the environment, the DBD discharge induced oxidation leads to complete mineralization of phenol. To determine the role of the main reactive species, O3 and OH, I conducted a series of experiments of treatment of aqueous solutions containing phenol with ozone produced by ex situ discharge, i.e. ozonation, and in situ discharge, i.e. discharge applied directly above the aqueous solution. These experiments have shown that at the same concentration of ozone, the treatment of phenol with in situ discharge is more efficient. So I studied the phenol oxidation with DBD in the presence of tert-butanol, a compound that reacts very slowly with ozone and is used as an efficient OH radicals scavenger. I verified that in the presence of a large excess of tert-butanol the oxidation of phenol is significantly slowed down. The tert-butanol, treated individually, has proved very useful as a mechanistic probe because it reacts only with the OH radical and not with ozone. Therefore it allowed me to estimate the concentration of OH radicals present in solution under the different types of treatment and pH conditions investigated. Finally, the use of argon instead of air allowed me to generate a plasma without ozone and to study the oxidation process of phenol and of tert-butanol, treated individually and together, by the OH radical. The results of all these experiments allowed me to conclude that the greater efficiency of non-thermal plasma applied directly above the solution to be treated with respect to ozonation, is to be attributed to OH radicals produced directly from the in situ discharge. My attention finally turned to so-called emerging organic contaminants. Among them there are several drugs, whose presence in surface waters is documented since a couple of decades in many European countries. They are continuously released into waters from various sources especially from livestock and patients treated pharmacologically and, thanks to the increased sensitivity of today's detection techniques, they have been also detected in trace amounts in drinking water. I chose three drugs detected in Italian waters, especially in the Po Basin: carbamazepine, an antiepileptic, hydrochlorothiazide, a diuretic, and atenolol, a beta-blocker. I determined the rate of degradation in mQ water and in phosphate buffer at pH 7 and I found that the reactivity of the three compounds follows the order: carbamazepine > hydrochlorothiazide > atenolol. I determined the mineralization yields and I found that they considerably increase with decreasing concentration. This is a very encouraging result considering that the low concentrations used in my experiments are still ten thousand times greater than those found in natural waters. Finally, through HPLC analysis coupled with mass spectrometry (LC-ESI) I have identified numerous oxidation intermediates of these drugs and found that in turn they undergo oxidative degradation in the course of the treatment. This thesis provides evidence that plasma treatment of aqueous solutions contaminated by organic compounds is a promising approach to develop novel techniques for water purification that can help the traditional remediation techniques, especially in the complete removal of persistent pollutants in low concentrations