Study On Effect And Mechanism Of Treatment Of Organic Phosphorus Pesticide Polluted Groundwater By Prechlorination-softening Process

In the northern part of China,groundwater is an important source of water supply.However,with the extensive use of herbicides,pesticides and other organophosphorus pesticides(OPPs),pesticide pollution in underground water is becoming more and more serious.Since the removal rate of OPPs by conventional water treatment process is low,and it OPPs easy to react with oxidant in disinfection process to produce more toxic and stable organic phosphorus oxide(Oxons),how to achieve OPPs detoxification has become a research hotspot.The group has proposed the rapid elimination of malathion biotoxicity by “oxidation-alkalization hydrolysis” technique,but whether the technique can be used in real groundwater application is not clear.By combining the "oxidation-alkalization hydrolysis" technique with lime softening method which is widely used to remove hardness in groundwater,the pre-chlorination-softening process is proposed innovatively,and the biotoxicity of calcium,magnesium and other hardness ions in groundwater is eliminated.At the same time of analyzing OPPs detoxification mechanism,the kinetics and corresponding technical points of pre-chlorination and softening process were discussed.The main findings are as follows:(1)Determine the optimal reaction conditions for the pre-chlorination-softening process.The five OPPs(Chlorpyrifos?Diazinon?Dimethoate?Malathion?Parathion)were degraded by pre-chlorination-softening process.The results showed that thechlorination reaction of five OPPs was in accordance with the quasi-first-order reaction;the degradation rate of organophosphorus pesticides increased with the increase of chlorination;the oxidation rate of five OPPs was between 0.0211~0.0464 min-1 with the increase of chlorination to 2.5 mg/L;the oxidation rate of five OPPs was 0.077~0.1218min-1.Taking into account the economic benefit and treatment effect,the dosage is 2mg/L,the chlorination time is 30 min.Calcium and magnesium ions are removed by adding different amounts of lime to the groundwater.found that the optimal dosage of lime was 145 mg/L,at this time,the total hardness in the solution decreased by 60%,the calcium hardness decreased by 71%,and the alkalinity decreased by 67%.at this point,the pH value of the solution is about10.5,which provides the required alkaline conditions for the hydrolysis process.(2)The degradation path and mechanism of OPPs toxicity were deduced based on the mass spectrum information of the degradation products and the biological toxicity change of the solution.Using UPLC-ESI-MS/MS to identify its degradation products,it was found that the P=S bonds in the OPPs were oxidized to P=O bonds under the action of chlorine,thus forming its corresponding oxides.At the the OPPs and its Oxon hydrolysis kinetics also conform to the quasi-first-order reaction during subsequent softening,and the degradation rate of the Oxon is 3~10 times that of the OPPs.With the pH value gradually increased to 11,the Oxon hydrolysis rate increased continuously.It was found that hydroxylation reaction and deethylation reaction were the main ways of hydrolysis and AChE inhibition rate were decreased.By comparing the changes of toxicity of pre-oxidation-softening process and conventional treatment process,it was found that the toxicity of pre-chlorination-soft chemical process increased first and then decreased,while the toxicity of conventional process decreased first and then increased;the final solution toxicity of conventional process was 2~8 times that of pre-chlorination-softening process.(3)By adding standard real groundwater(adding trace OPPs to real groundwater),the effects of pre-oxidation-softening process and traditional groundwater treatment on hardness and OPPs were compared.and it was found that the degradation rate of five OPPs by pre-oxidation-softening process could reach more than 95%,which was better than that of conventional water treatment process,and the total hardness removal rate ofgroundwater was 62%.Although the conventional groundwater treatment process can obtain the same hardness removal rate,the relative toxicity of the treated water is 1.86 times that of the initial toxicity and 3~4 times that of the research process,which means a large increase in biological toxicity.Therefore,the pre-oxidation-softening process proposed in this study can achieve simultaneous removal of hardness and OPPs while reducing biological toxicity.