| Due to environmental persistence and health risk of pesticides, the treatment of micro-pollution of drinking water sources by them become one of the research hotspots in the field of water environmental protection in recent years. Among use the various treatment methods in micro-pollution water, advanced oxidation processes (AOPs) have proven to be effective technologies for organic contaminant degradation.However, toxicity assessment and formation of byproducts during AOPs scarcely been addressed. This work focused on assessing the feasibility and efficacy of two AOPs processes for the detoxification of water matrices contaminated with pesticides. In order to do so, the suitability of AOPs was investigated in ultrapure water, river water, and return flow water. Bromoxynil and trifluralin, two of frequent pesticides found in the world were used as target compounds.The main study results were summarized as follows:(1) The degradation of two pesticides, bromoxynil and trifluralin, was studied by using ozone as oxidant. The main objectives of this study were to evaluate the kinetics of the pesticide degradation and to assess the detoxification efficiency of the process. The second-order rate constants for direct and indirect reactions of ozone with bromoxynil and trifluralin were determined by using a rapid-scan stopped-flow spectrophotometry, competition kinetics, and an organic substrate monitoring method. High reactivity was obtained for bromoxynil and trifluralin in their reaction with ozone in the test environment. The rate constants for the reaction with ozone were in order of 102M-1s-1. A hydroxyl radical pathway was found to be the primary mechanism for their reaction in water. To asses the acute toxicity the Microtox bioassay was used before and after treatment. The toxicity tests indicated a decrease in toxicity as result of ozonation of bromoxynil. However, an increase in toxicity was observed after ozonation of trifluralin.(2) Ozone and the combination of ozone plus hydrogen peroxide processes were used to oxidize bromoxynil and trifluralin in ultrapure and natural waters. The rate constants for the reaction with ozone in ultrapure water were determined to be 5.2×102M-1s-1 for bromoxynil and 1.1×102M-1s-1 for trifluralin. For the reactions with hydroxyl radicals, the rate constants were 2.0×1010M-1s-1and 7.1×109M-s-1 for bromoxynil and trifluralin, respectively. In experiments performed with natural water, removals lower than 50% were achieved for both pesticides during conventional ozonation. An enhancement of the rate of degradation was observed using O3/H2O2 process. Toxicity analysis showed a decrease in the toxic effects of the samples on the luminescent bacteria in the initial stages of ozonation, followed by an increase of the toxic effects at the end of the treatment for both pesticides. During conventional ozonation in natural waters, ozone residuals showed biphasic behaviour. Depending on the conditions, the rate constant for ozone decomposition in river water were estimated to be between 3.2×102M-1s-1～5.8×102M-1s-1, and 1.2×102M-1s-1～4.2×102M-1s-1 for bromoxynil and trifluralin, respectively. A kinetic model of pesticide oxidation based on the ratio between hydroxyl radicals and ozone exposure (Rct parameter) was developed. Accurate predictions for all the tested were noted.(3) The photodegradation of bromoxynil and trifluralin in ultrapure water and natural water by ultraviolet (UV) irradiation and combination UV plus hydrogen peroxide process was investigated. The results showed a clear effect of the pH on the pesticide degradation. It was also found that under direct photolysis with monochromatic light at 254nm, the photochemical rates followed first order kinetics, with fluence-based rate constant ranging from 9.15×10-5 to 6.37×10-4m2J-1 and for bromoxynil and 7.63×10-4 to 1.47×10-3m2J-1 for trifluralin under different conditions. It was noted that the UV/H2O2 advanced oxidation process enhanced the oxidation rate in comparison to direct photolysis. In presence of 8.8×10-4 M H2O2, removal of 90% with a UV dose of 333 mJ·cm-2 and 188 mJ·cm-2 were observed for bromoxynil and trifluralin in natural water. The Microtox 81.9 screening test protocol was used before and after treatment to assess the toxicity. The outcomes showed a decrease in the toxic effect of the samples after treatment for both pesticides.(4) The formation of bromoxynil intermediate products generated during conventional ozonation, ozone combined with hydrogen peroxide, and UV irradiation coupled with H2O2 processes was investigated by using high-performance liquid chromatography and mass spectrometry. The results indicated that the main intermediate products formed during conventional ozonation and O3/H2O2 in ultrapure water were determined to be 3-bromo-4,5-hydroxybenzonitrile,3-bromo-4-hydroxybenzonitrile, and 4-hydroxybenzonitrile. The predominant species under O3 and O3/H2O2 in natural water was 4-hydroxybenzonitrile, achieving concentration equivalent to 52% and 49% of initial bromoxynil concentration, respectively. A similar speciation of the intermediate products was detected under UV/H2O2, but at low concentrations. The most abundant oxidation product formed during UV/H2O2 treatment was 3-bromo-4,5-hydroxybenzonitrile. Hydroxylation and debromination were found to be the primary pathways for the bromoxynil degradation under O3/H2O2 and UV/H2O2 treatments.(5) The identification of trifluralin intermediate products generated during AOPs was also addressed. The results showed that trifluralin by action of conventional ozonation and O3/H2O2 process in ultrapure water decomposed to several products, the most predominant species being a,a,a-trifluoro-2,6-dinitro-N-dipropyl-p-toluidine,2,6-dinitro-4-trifluoromethyaniline> trifluoromethyaniline. In natural water, the predominant species dinitro-4-trifluoromethyaniline and 4-trifluoromethyaniline, concentration equivalent to 2% and 3% the initial trifluralin concentration, respectively. A similar speciation of the intermediate products was detected under UV/H2O2, but at low concentrations. Hydroxylation and dealkylation were found to be the primary pathways for the trifluralin degradation under O3/H2O2, whereas dealkylation was the main mechanism during UV/H2O2 treatment. |
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