Oxidation Of Herbicides 2,4-D By Ozone And Ozone Combined With Hydrogen Peroxide

The treatment of pesticide pollutants is one of the hotspots in water environmental protection. 2,4-dichlorophenoxyacetic acid, a chlorophenoxy herbicides, is the most widely used herbicide worldwide due to its low cost and high effectiveness. In recent years, 2,4-D has become one of the most common surface and groundwater pollutants. The World Health Organization (WHO) considers chlorophenoxy herbicides, including 2,4-D, to be possible human carcinogens. The degradation of 2,4-D using O₃ and O₃/H₂O₂ advanced oxidation processes have been studied in this paper. The mathematical model of reaction kinetics and degradation mechanisms are derived and verified by the experiment. The effect of water qualities on 2,4-D degradation is investigated. The elementary study of 2,4-D degradation using O3/US combined technology is also carried out and the results are shown to be useful in the future research.Firstly, the influential factors such as temperature, pH levels, 2,4-D initial concentration, gas flow rate and hydrogen peroxide dose are investigated in order to find out felicity conditions in O₃ or O₃/H₂O₂ processes. Two treatments (O₃ and O₃/H₂O₂) are compared in the case of reaction efficiency which depends on the experimental parameters. The synergetic effects in O₃ and H₂O₂ in O₃/H₂O₂ combined technology are investigated. The results show that: 1) Temperature is the key factor of removal rate; 2) pH strongly affects the degradation process, and 3) Gas flow has very little influence on the efficiency of processes. Effectiveness of O3/H2O2 oxidation greatly depends on the H₂O₂ dosage. The degradation rate of 2,4-D increases with the increment of temperature, pH value, and hydrogen peroxide dose. The degradation efficiency is enhanced when 2,4-D initial concentration is decreased. Gas flow rate has both positive and negative influence on reaction; therefore, its overall effect is not evident. Compared with O3 process, O3/H2O2 greatly increases the removal efficiency of 2,4-D. The synergy of O3 and H₂O₂ is proved by the experiment. Hydrogen peroxide is the initiator of ozone decomposition chain reaction to generate hydroxyl radical. Hence, greater amounts of hydroxyl radicals are generated by the O₃/H₂O₂ system than ozonation alone. O₃/H₂O₂ has more advantages such as excellent degradation effectiveness, faster removal rate, more moderate conditions and lower operating cost than O3 system. O₃/H₂O₂ technology is an excellent advanced oxidation process in removal of 2,4-dichlorophenoxyacetic acid, and appeared to be more applicable for the treatment of wastewater.Secondly, the reaction kinetics of O3 and O₃/H₂O₂ processes are studied. This includes the kinetics of direct oxidation with ozone molecule and indirect oxidation with hydroxyl radical. The direct oxidation process is performed using radical scavenger to inhibit indirect oxidation. 2,4-D dissolved in water is dissociated to deprotonized and protonated species. The results show that reaction rate of dissociated species is fast than that of deprotonized species. It is confirmed through experiment that the relation of second-order rate constants is dependent on the degree of dissociation. A direct oxidation model in view of dissociation is proposed to predict 2,4-D degradation and proved to be agreeable to the simulated degradation. The overall degradation of 2,4-D follows the pseudo-first-order reaction. It has been found that the indirect oxidation of hydroxyl radical is dominant in 2,4-D degradation. Furthermore, the proposed overall kinetics model based on the observed psudo-fisrt-order kinetics is applied topractical operation, and verified by the experimental results. It is shown that the model is well constructed.In order to have a better understanding of the oxidation process, the degradation mechanism is studied for 2,4-D with ozone and ozone/hydrogen peroxide. The dominant active species is identified to be hydroxyl radical and the hydroxyl radical is determined. The results show that the radical plays the key role in 2,4-D degr...