Kinetics of Ozonation of 4-Chloro-2-Methyl Phenoxyacetic Acid in Aqueous Solution

In order to find an effective method to control chlorophenoxy herbicides pollution in water, ozonation of 4-chloro-2-methyl phenoxyacetic acid (MCPA) in aqueous solution was studied in a bubble reactor in laboratory. The experimental results show that ozonation is a good method to degrade MCPA pollution in aqueous solution, and all MCPA are removed in a short time. The degradation of MCPA fits well with pseudo first-order kinetics. Direct reaction of ozone molecule plays a predominant role in MCPA degradation at initial solution pH value. With the increasing of tert-butyl alcohol initial addition ([TBA]₀), degradation rate constant of MCPA decreases at first, then it reaches minimum at [TBA]₀=5 mmol·L^-1, afterwards it increases again. TBA has adverse effect by way of inhibiting the indirect reaction of hydroxyl radicals. On the other hand, it has positive effect by means of promoting mass transfer of O₃ and direct reaction of O₃ molecule. The effect of temperature on MCPA degradation was investigated in both direct reaction system by adding sufficient TBA and overall reaction systems without adding TBA. It is shown that temperature has strong effects on the MCPA degradation in both systems. The pseudo first-order rate constant in both systems increases with temperature increases. The relationship between degradation rate and temperature is processed by using Arrhenius formula. It is noteworthy that apparent activation energy of overall reaction is lower than that of direct reaction. In the overall reaction process which is involved indirect reaction of hydroxyl radical and direct reaction of O₃ molecule, the energy barrier of MCPA degradation is less and the decomposition of MCPA can be easily completed.