Degradation Of Organochlorine Pesticides In Water By UV Irradiation-activated Sodium Persulfate Process

Organochlorine pesticides are one of the kinds of pesticides commonly used in the control of agricultural diseases and insect pests.They belong to persistent organic pollutants because of their persistent,biological enrichment and high toxicity.There are potential environmental risks to the water environment,especially to drinking water sources.Although the content of organochlorine pesticides in water is low,the potential harm of organochlorine pesticides still needs to be paid enough attention to.The research on degradation technology of organochlorine pesticides is one of the important topics in drinking water treatment at present.The advanced oxidation technology based on sulfate radical(SO₄^·-)can remove most of the organic matter.Because of the low efficiency of supersulfate oxidation alone,the ultraviolet（UV）irradiation of sodium persulfate（PS）is used to promote the formation of SO₄^·-.The removal level of pollutants was improved and the degradation of organochlorine pesticides in water by UV irradiation was studied.Triadimefon（TDF）and humus（PCM）were used as target pollutants to study the effects of initial concentration of pollutants,PS concentration,pH value,humic acid content and chloride concentration on the removal rate of pollutants.Comparative analysis of UV Irradiation,PS oxidation and UV/PS system alone for the degradation of organochlorine pesticides;The types of oxidants involved in the degradation of organochlorine pesticides were studied,the kinetic equation of degradation of organochlorine pesticides by UV/PS technology was established,and the degradation pathways of TDF and PCM were explored.The main conclusions of the above research are as follows:1.when PS was used alone,the removal rates of TDF and PCM were 13.52%and 16.38%,which might be due to the partial thermal activation of PS,the production of a small amount of SO₄^·-,and the degradation of trace amounts of TDF and PCM.When irradiated by UV alone,the removal rates of TDF and PCM were35.54%and 43.55%,mainly due to the photolysis of TDF and PCM under the excitation of UV energy.SO₄^·-can effectively degrade TDF and PCM.Compared with PS oxidation and UV irradiation alone,the removal rate of TDF in UV/PS system was increased by 86.22%and 64.2%,the PCM removal rate increased by 74.11%and46.94%,respectively.2.In the UV/PS reaction system,the degradation efficiency of TDF and PCM decreased with the increase of their initial concentration,and the degradation efficiency of TDF and PCM could be enhanced by increasing the dosage of PS.The existence of chloride ion and humic acid could inhibit the oxidative degradation of TDF and PCM in UV/PS system.The effect of pH on the degradation efficiency of TDF and PCM in UV/PS system was not significant.The optimum reaction conditions were as follows:TDF removal rate could reach 99.83%at TDF concentration of 200μg·L^-1,PS dosage of 250μmol·L^-1,pH 5,（25±2）℃and reaction time of 600 s.PCM removal rate could reach 92.83%at PCM concentration of 100μg·L^-1,PS dosage of 250μmol·L^-1,pH 5,（20±2）℃and reaction time of 30min.3.The effects of initial concentration of target pollutants,PS dosage,chloride ion concentration,humic acid dosage and initial pH value of solution on the apparent rate of degradation of PCM in UV/PS system showed a good linear relationship（R²>93%）.The oxidative degradation of PCM in UV/PS system accords with the quasi-first-order reaction kinetics.4.According to the experimental results of free radical capture,the species of free radicals（pH=5）in UV/PS system is SO₄^·-,and there is almost no HO·in the system.In neutral condition（pH=7）,there is a small amount of HO·in the system,SO₄^·-and HO·lead to the degradation of pollutants.However,under alkaline conditions（pH=9）,HO·is the main effective free radical for degradation of pollutants.5.The mechanism for TDF and PCM degradation with UV/PS system is a combination of direct photo-degradation by UV and radical oxidation mainly based on sulfate radical(SO₄^·-).The comparison of the removal rate of UV/PS system and UV irradiation alone showed that direct photo-degradation has little contribution to pollution degradation.radical oxidation mainly based on sulfate radical(SO₄^·-)is the main mechanism of TDF and PCM removal.