Chlorination of bromacil: Kinetics and disinfection by-products

Bromacil, a nonselective uracil compound, is applied to field crops, such as citrus and pineapple, to control annual and perennial grass weeds. In this study, the degradation kinetics and effects of influencing factors on the formation of disinfection by-products (DBPs) during bromacil chlorination were investigated. The reaction between chlorine and bromacil followed second-order kinetics, and the degradation kinetic model for bromacil chlorination was established. The rate constants of the reactions between bromacil and HOCl and CIO- were calculated as 4.33 ( +/- 1.40) x 10(1) and 3.21 ( +/- 0.47) x 10(2) M-1 s(-1) , respectively. The presence of Br- in water resulted in the formation of HOBr and accelerated the rate of bromacil degradation during chlorination. The rate constant was calculated as 6.34 ( +/- 0.09) x 10(4) M-1 s(-1) for the reaction between bromacil and HOBr at 25 degrees C. The apparent second-order rate constant (k(app)) increased with increase in temperature and pH, and the activation energy was calculated as 37.83 kJ/mol using the Arrhenius equation. Because the structure of bromacil comprises bromine atoms, brominated DBPs, including dibromochloromethane (DBCM) and bromodichloromethane (BDCM), formed abundantly during bromacil chlorination. The formation of bromoform (BF), BDCM, and DBCM increased with the reaction time, whereas the formation of chloroform (CF) increased until day 4 and then decreased. The yields of all the detected DBPs increased with the increase in pH from 5 to 8. With increasing bromide concentration, the formation of CF and BDCM gradually decreased, where as that of BF considerably increased, resulting in relatively stable bromine utilization factor in the bromide to chlorine ratio of 0.00-0.10 during bromacil chlorination.