Comparative Study of the Degradation of Bromoxynil and Bromoxynil heptanoate by UV photolysis and by Chemical Oxidation (H2O2/UV; O3; Cl2)
In the aquatic environment hydrolysis and phototransformation processes are the main abiotic degradation routes of chemicals. The aim of this study is to investigate the degradation pathway of 2 pesticides (bromoxynil heptanoate and its hydrolysis product bromoxynil) by UV photolysis and by chemical oxidation. The chemical oxidation processes studied are ozonation chlorination and radical oxidation with OH radicals. These radicals are produced by photolysis of hydrogen peroxide. Bromoxynil heptanoate (-4 cyano -2,6 dibromophenyl heptanoate) is an ester which is quite unstable in aqueous medium. Preliminary experiments, carried out in diluted solution (C[inf]o=10-[sup]6 mol l-1) and in phosphate buffer (μ=10-2 mol l-1), showed that 20 to 25% of bromoxynil heptanoate is hydroIyzed after 24 hours at pH 7. The hydrolysis decreases strongly with pH; it is only 3 % at pH 4 after one day. In order to limit the hydrolysis reaction, the following experiments were carried out at pH 7 for bromoxynil (C[inf]o=5 10-[sup]5 mol l-1) and at pH 4 for bromoxynil heptanoate (Co=10-[sup]6 mol l-1). The pH was adjusted with a phosphate buffer. Absorption spectra of bromoxynil heptanoate and bromoxynil do not show any absorption band up to 320 nm (bromoxynil heptanoate l max=218, 290 nm; bromoxynil I max=222, 280 nm; fig. 1). Therefore, the removal of these two pesticides by direct photolysis with the sun light will be very small. The measurement of the quantum yield (number of molecules undergoing photodegradation transformation per number of photons absorbed by these molecules) in the UV region in both monochromatic and polychromatic lights (HPK, Philips; I[inf]0=1.48 10-[sup]8 Einstein sec-1, indicates that these 2 compounds are easily eliminated by a UV irradiation treatment (10-2 < F < 10-1 at I=254 nm and 290 nm). The irradiation of these two pesticides at 254 nm (low pressure vapor mercury lamp, l[inf]0=5.8 10-[inf]8 Einstein sec-1) in the presence of hydrogen peroxide does not seem to significantly improve the efficiency of the treatment by UV irradiation alone. For example, the half life time of bromoxynil is 11 minutes by UV compared to 8 minutes when hydrogen peroxide is present. This small difference of reactivity of H2O2/UV system is due to the small quantity of H202 photolyzed during this irradiation time; at t=8 min only 1 % of H202 is decomposed. The oxidation of bromoxynil heptanoate and bromoxynil by ozonation was performed in a semi-batch reactor (Q=5 mg O3 min-1). Results show that these two compounds are very reactive with ozone. 50 % of elimination was obtained after 5 and 2 minutes of ozonation for bromoxynil heptanoate and bromoxynil respectively. In both cases the transfer of ozone was the limiting factor. Finally chlorination of bromoxynil at a rate of 35.5 mg Cl2-l-1 indicates that this pesticide is also oxidized by chlorine but the degradation rate is quite slow; a total degradation is observed after 90 min of reaction. Comparison of the different treatments studied shows that ozonation is certainly the most effective process. Only a few minutes with reasonable ozone doses are necessary to eliminate these pesticides from aqueous medium. The other treatment (UV, H2O2/UV, Cl2) can also be a good alternative for the removal of these compounds from water. For the different oxidation treatments, the HPLC analysis of the bromoxynil reaction mixture shows the formation of by-products. Only UV and H2O2/UV byproducts were identified in this work; but comparison of the retention times from the HPLC analysis indicates that certain by-products from ozonation are similar to the ones observed by UV and H202/UV oxidation. On the other hand, the products formed during chlorination, probably chlorinated products, have different retention times. The reaction intermediates werc identified by GC-MS analysis after 10 min of irradiation (with and without H2O2). This time corresponds to the maximum concentration of one of the photoproduct. Two kinds of compounds wers identified from the GC-MS analysis: hydroxylated compounds (-3,4 dihydroxy -5 bromobenzonitrile; -3,4,5 trihydroxybenzonitrile) and hydrogenated compounds (-3,4 dihydroxybenzonitrile; -4 hydroxybenzonitrile). In both cases the bromine atoms were substituted either by an H atom or by an OH group. A reaction mechanism of photodegradation of bromoxynil heptanoate is proposed in this study (Fig. 6).