Effectiveness of two advanced oxidation processes and ozonation for the degradation of two pesticides in different water matrices

The negative impact caused by agrochemical contaminants on the environment has turned attention to the use of economically-feasible treatment technologies for their removal. This work focused on assessing the feasibility and efficacy of advanced oxidation processes (AOPs) for the detoxification of water matrices contaminated with pesticides. Two of the most frequent pesticides (bromoxynil and trifluralin) found in Alberta surface waters were used as target compounds. The suitability of AOPs was investigated in ultrapure water, river water, return flow water, and membrane concentrates. The results indicated high reactivity for bromoxynil and trifluralin in their reaction with ozone (O3) in ultrapure water. A hydroxyl radical pathway was found to be the primary mechanism for their reaction. The results also showed that bromoxynil and trifluralin could not be significantly degraded during conventional ozonation in natural water, achieving removals of less than 50%. For the ozone-based AOPs, a kinetic model of pesticide oxidation based on the ratio between hydroxyl radicals and ozone exposure was developed. Accurate predictions for all the natural waters tested were established. It was found that the level of total organic carbon was an important water quality parameter affecting the quantum yield during the photodegradation experiments. Results from an acute toxicity bioassay indicated a decrease in toxicity as result of ozonation of these pesticides in natural waters during the first minutes of reaction, however, during the treatment process an increase in the toxicity was detected for both pesticides. The toxicity outcomes from the ultraviolet light (UV)-based AOPs indicated a decrease in the toxic effect of the samples after treatment for both pesticides. Treatability studies on membrane concentrates showed high pesticide removals by using O3 + H2O2 for all concentrate matrices. However, the toxic effects obtained during the O3 + H2O2 process were higher than those obtained during UV irradiation coupled with H2O2. The identification of oxidation byproducts generated during these two AOPs was also addressed. It was found that hydroxylation and debromination were the primary pathways for the bromoxynil degradation, whereas hydroxylation and dealkylation were found to be the major mechanisms for trifluralin oxidation.