Decomposition of pesticides in water with ionizing radiation

Within the scope of this thesis the degradation of the herbicides 2,4-dichlorophenoxyacetic acid (2,4-D), 2,4,5-trichloro-phenoxyacetic acid (2,4,5-T) and 4-chloro-2-methylphenoxyacetic acid (MCPA) has been studied by high energy radiation and ozone. The effect of high energy radiation on 2,4-D, 2,4,5-T and MCPA was investigated by pulse radiolysis and γ-radiolysis. Pulse radiolysis experiments showed that OH-radicals react with the pesticides by addition to the aromatic ring yielding hydroxycyclohexadienyl radicals, k (OH + S) = 6,6 to 8,6 x 10-9 M-1 s-1. The rate constants for the reactions of H-atoms with the substrates, k (H + S) = 1,4 to 2,1 x 10-9 M-1 s-1, as well as those of the solvated electrons, k (e-aq+ S) = 1,1 to 6,5 x 10-9 M-1 s-1, have been determined in argon saturated solutions. The absorption spectra of H- and OH-adducts were measured in the range of 300 to 500 mn. No reaction of the OH-adducts of 2,4-D and 2,4,5-T with oxygen was observable. The products formed during γ-radiolysis were analyzed by HPLC (2,4-dichlorophenol, 2,4,5-trichlorophenol and 4-chloro-2-methylphenol) and ionchromatography (formic acid, acetic acid, glyoxylic acid, oxalic acid and chlorine). They are presented in dependence of dose for N2O, air, constantly air- and oxygen saturated basic (pH 9) solutions. The dehalogenation process is enforced by higher oxygen concentrations. For total decomposition of 500 μM solutions doses from 3,5 kGy up to 4 kGy are required. To investigate the influence of inorganic water components on the degradation process experiments were carried out in Viennese Tap Water. At higher doses the decomposition is about 20 % less than in triply distilled water. In order to get a better insight into the kind of the radiolytic products, whether they are still of organic nature or already mineralized, DOC measurements were performed. Saturation with oxygen during irradiation is necessary to achieve a reduction of about 40 % of the DOC from a starting concentration of 500 μM and a dose of 5 kGy. To get information on the potential toxic effects of the organic radiolytic products on aquatic biota a bacterial bioluminescent test was carried out. Total detoxification could be attained for 5 x 10-5 M solutions by carrying out the irradiation procedure under air saturation and by doses of 500 Gy. The decomposition of MCPA, 2,4-D and 2,4,5-T by ozone (20 mg O3/l O2, 50 l O2/h) was studied in triply distilled water (pH 3 and 9) and in Viennese Tap Water (pH 7). The decay of all three pesticides is completed within 10 minutes. For all of them the decomposition at pH 9 is faster than at pH 3 due to radical chain reactions. (author)