Kinetics and mechanism of the degradation of two pesticides in aqueous solutions by ozonation

This study evaluated the reaction kinetics and degradation mechanism of the pesticides bromoxynil and trifluralin during conventional ozonation. The second-order rate constants for the direct molecular ozone and hydroxyl radical reactions with bromoxynil and trifluralin were determined using a rapid-scan stopped-flow spectrophotometry, competition kinetics, and an organic substrate monitoring method. High reactivity toward ozone and hydroxyl radicals was observed for bromoxynil and trifluralin. The second-order rate constants for the reaction with ozone were about 10(2) M(-1) s(-1). The values of the second-order rate constants for the reaction between hydroxyl radicals and bromoxynil and trifluralin in ultra-pure water at 20 degrees C were estimated to be around 8.4 x 10(9) and 7.5 x 10(9) M(-1) s(-1), respectively. The identification of oxidation by-products generated during ozonation 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. (C) 2009 Elsevier Ltd. All rights reserved.