Kinetics of reactions between chlorine or bromine and the herbicides diuron and isoproturon

The chemical oxidation of two herbicide derivatives of the phenylurea group - diuron and isoproturon - has been carried out by means of chlorine, in the absence and in the presence of bromide ion. Apparent second-order rate constants for the reactions between chlorine and the herbicides were determined to be below 0.45Lmol(-1) s(-1). Hypobromous acid reacts faster with the investigated herbicides, especially with isoproturon (k(app) = 24.8 L mol(-1) s(-1) at pH 7). While pH exerts a negative effect on the bromination rate, the maximum chlorination rate was found to be at circumneutral pH. In a second stage, the oxidation of each compound was conducted in different natural waters, in order to simulate the processes which take place in water purification plants. Again, chlorine was used as an oxidant, and bromide ion was added in some experiments with the aim of producing the more reactive HOBr oxidant. The herbicide oxidation rate was inversely proportional to the organic matter content of the natural water. However, the formation of trihalomethanes (THMs) was directly proportional to the organic matter content and constitutes a limitation for the application of chlorine during drinking water treatment. Finally, the evolution of herbicide concentration was modeled and predicted by applying a kinetics approach based on the rate constants for the reactions between the herbicides and the active oxidants.