Homogeneous Catalytic Oxidation for the Degradation of Chlorophenoxy Herbicides in Aqueous Media

摘要

Chlorophenoxy herbicides, such as MCPA (4-chloro-2-methylphenoxyacetic acid) and 4-rnCPA (4-chlorophenoxyacetic acid), are employed on a large scale for weed control on cereal crops, grasslands, vines, peas, potatoes and forestry applications. Information on the chronic toxicity on humans due to their extensive and prolonged use in fields and transport into surface waters that are used for drinking purposes is not yet known. These compounds resist biodegradation and have potential toxicity towards humans and animals, being considered as moderately toxic (class Ⅱ or Ⅲ) by the World Health Organization. Their widespread use in industrialized and third-world countries for agricultural and non-agricultural purposes generates continuously rinsate, waste product and contaminated soil at hundreds of thousands of application sites. They are found as pollutants in natural waters, with contamination levels up to 10μg/l in ground and surface waters. In Cyprus, the constructed waters reservoirs constitute the main source of drinking water. In these surface waters a number of herbicides and pesticides have been traced during the current years. Therefore the objective of this research was to investigate the degradation potential or complete mineralization of such chlorinated aromatic compounds in water. Advanced oxidation processes (AOPs) are potentially useful for treating pesticide wastes because they generate hydroxyl radicals (OH), a powerful non-specific oxidant. The hydroxyl radical is able to react with organics yielding dehydrogenated or hydroxylated derivatives. The organic pollutants are oxidized by hydrogen radicals and mineralized to water, carbon dioxide and inorganic ions. Fenton's reagent is a mixture of hydrogen peroxide and ferrous ions which provides a source of hydroxyl radicals. The Fenton reaction has been widely applied in the treatment of non-biodegradable wastewater in the field of AOPs. Hydrogen peroxide is environmentally friendly and it is commonly used as a source of hydroxyl radicals in Fenton's process. The Fenton's process which was applied in this study included the use of ferrous-catalyzed H_2O_2 to provide an oxidizing environment. Batch experiments were performed in duplicate, using H_2O_2 (35% w/v), in the presence of Fe(Ⅱ) as a catalyst to determine the degradation of the herbicides. The Fenton's reaction takes place in pH values ranging between 3 - 3.5. In thesernpH values, the formation of the free hydroxyl radicals is activated. In addition, the low pH values minimize the possible self-decay of hydrogen peroxide and inhibit the formation and precipitation of insoluble ferric iron hydroxide, Fe(OH)_3. Solutions of known concentrations of MCPA, 4-CPA and a mixture of MCPA and 4-CPA were used and the corresponding TOC concentrations were measured in accordance with the Standard Methods. In particular, H_2O_2 was added in various dosages to known sample volumes with the addition of various dosages of Fe(Ⅱ) in order to select the optimum quantities of the reagents for the development of the method. Sixteen different combinations of dosages of oxidant and catalyst were used for each herbicide. The optimum molar ratios obtained were 26.45 [H_2O_2]/[Fe(Ⅱ)] for MCPA , 19.83 [H_2O_2]/[Fe(Ⅱ)] for 4-CPA and 39.64 [H_2O_2]/[Fe(Ⅱ)] for the mixture of the two herbicides. According to the above results, the MCPA and 4-CPA were not completely degraded by Fenton reaction since only 62% and 70% of the TOC was converted into carbon dioxide. The TOC removal achieved for the mixture of MCPA and 4-CPA was 48%. The remaining quantity of each herbicide is considered to be partially degraded to organic by-products. These findings are now used for further examination of the oxidation process and in specific, for the study of kinetics, the mechanisms of the individual reactions that take place as well as for the determination of the main by-products generated through the development of the overall Fenton reaction. In the complet