Removal of Tecnazene From Water Using UV Light and Hydrogen Peroxide (1998)

This thesis describes the efficiency of UV irradiation treatment for removal of tecnazene (1,2,4,5-tetrachloro-3-nitrobenzene) from water in the presence and absence of hydrogen peroxide. It also includes the role of hydrogen peroxide in the treatment, and the identification of decomposition products of tecnazene. Tecnazene is a fungicide used on potatoes for the purpose of sprout suppression and dry rot control. It is used mainly in the UK, and applied at the time of store loading at a rate of about 0.1g a.i./kg potatoes. In general, toxicity, persistence in the environment and fate are of great concern when using agrochemicals. In the case of tecnazene these aspects are not well understood in spite of the fact the chemical has been used for a long time. However, questions about tecnazene have been raised from the safety point of view since tecnazene has been found to be highly toxic to several aquatic species including fish (Whale et a1.,1988). In addition to that tecnazene and its metabolites have been identified in sediments and in fish down stream of a potato washing plant (Whale et al., 1988). In this respect, proper treatment and disposal of tecnazene has been of interest. Therefore, the removal of tecnazene from water was studied in this thesis. The method employed for the treatment was UV irradiation treatment combined with hydrogen peroxide (advanced oxidation method) since it is one of the strongest oxidation methods. Hydrogen peroxide was studied as a photooxidant. UV irradiation treatment of tecnazene in pure water leads to a rapid decomposition, and almost complete decomposition of tecnazene was observed within half an hour. Hydrogen peroxide enhanced the rate of photolysis although hydrogen peroxide alone hardly oxidised any tecnazene. This is because of the formation of hydroxyl radicals, which are known as strong oxidants, as a consequence of photolysis of hydrogen peroxide. The decomposition rate increased with the increase in the concentration of hydrogen peroxide. Investigation of transformation pathways is an important part of a study for the removal of pesticides, since there is a possibility that more toxic products might be produced as by-products. Therefore, it is necessary to identify as many decomposition products as possible, and investigate the transformation pathways. However, identification of pesticide decomposition products is difficult due to their low concentration versus the concentration of their parent compounds. In this respect, a cosolvent is usually employed for the study of decomposition products due to its capability of dissolving hydrophobic organic chemicals which covers the majority of pesticides. Although acetonitrile was recommended as a cosolvent by researchers (Leifer, 1988; Bunce, 1978), it was found to be decomposed by UV light and affected the decomposition rate of tecnazene in the case where hydrogen peroxide was present, while the affect of the presence of acetonitrile on the formation of decomposition products was obscure on this occasion. In terms of identification of decomposition products, trichloronitrobenzene and tetrachlorophenol were identified by GC-MS with the sample irradiated by UV light for 80min in 10% (v/v) acetonitrile-water mixtures. The identification of anionic products was also carried out by ion exchange chromatography, and chloride, nitrite and nitrate were identified as inorganic anions, and lactate, formate, acetate, succinate and oxalate were identified as organic anions. These results may indicate several pathways of tecnazene decomposition, such as reductive dechlorination pathway, replacement of the ring nitro group by hydroxyl and ring. opening reaction of tecnazene. In terms of the role of hydrogen peroxide on the formation of decomposition products, faster oxidation reactions were observed in the case of UV irradiation in the presence of hydrogen peroxide. For instance, the nitrite was oxidised quickly to nitrate permitting control over the form of N. Moreover. organic anions detected as decomposition products were oxidised quicker than in the absence of hydrogen peroxide. These are advantages of UV irradiation in the presence of a hydrogen peroxide treatment These supporting results do suggest that LTV irradiation treatment of tecnazene brought about reductive dechlorination and the ring opening reaction following the formation of organic anions, and in the case of UV irradiation in the presence of hydrogen peroxide one further advantage was observed which was a faster oxidation reaction toward tecnazene and its decomposition products. These should not cause undue concern. However, further work, such as the study of the influence of the presence of suspended sediment and other substances (humate etc.), temperature and pH on the photodecomposition rate may be required before such an approach could be recommended with confidence for commercial situations, unless it was confined to a final polishing treatment as for drinking water.