Μελέτη της φωτοκαταλυτικής αποικοδόμησης επιλεγμένων οργανοφωσφορικών παρασιτοκτόνων σε υδατικά διαλύματα

Abstract

The increasingly intensive and widespread use of pesticides has resulted in significant contamination of surface and ground water. As a result, effective purification methods for eliminating pesticides in natural waters have been in urgent demand. The already existing methods like adsorption or biological treatment appear to have many drawbacks since the former involves only phase transfer of the pollutants without degradation and the latter can’t be applied to contaminants which are toxic to the bacteria. In the recent years, research for new methods that can overcome these disadvantages has led to processes that involve light, which are called Advanced Oxidation Processes (AOP’s). Although processes based on combinations like UV-B/H2O2 and UV-B/O3 or with semiconductors like UV-A/TiO2 and UVA/ ZnO make use of different reacting systems, they are all characterized by the same chemical feature: production of OH radicals (. OH). These radicals show little selectivity of attack and are able to oxidize the pollutants due to their high oxidative capacity [Reduction potential of OH Eo =2.8 V] The organophosphorous insecticides are comprised within the 10 most widely used pesticides all over the world. They are extremely toxic acting on acetylcholinesterace activity. In this study, the photocatalytic degradation of three organophosphorous pesticides has been studied using heterogeneous and homogeneous photocatalysis. In heterogeneous photocatalysis, suspensions of TiO2 and ZnO were used as the basic photocatalytic systems while in homogenous, the photocatalytic efficiency of UV/Fe3+/H2O2 was examined. Dichlorvos, dimethoate and methyl parathion were chosen as the target compounds. The objectives were a) to examine the pesticide disappearance, b) to evaluate the kinetics of the photocatalytic oxidation, c) to compare the photocatalytic efficiency of the photocatalysts that were used, d) to examine the effect of various parameters like the mass of catalyst, the addition of an oxidant, temperature, pH etc. e) to evaluate the degree of mineralization f) to examine the toxicity during and at the end of the treatment and finally to identify the organic intermediates that are formed during photocatalytic oxidation. As far as heterogeneous photocatalysis is concerned, complete disappearance of the insecticides is achieved in the presence of TiO2 or ZnO. Titanium dioxide proved to be more efficient photocatalyst since the oxidation of the insecticides proceeded at higher reaction rates. Parameters such as pH, temperature and concentration of the catalyst play an important role affecting the reaction rate. The photooxidation of the organic compounds follows pseudo-first order kinetics according to the Langmuir- Hinshelwood model.