Enhanced Degradation Of Imidacloprid By Advanced Oxidation Technology Based On Metal Activated PMS

Imidacloprid（IMI）is one of the most widely used herbicides in agriculture.As the first-generation neonicotinoid insecticide,imidacloprid has a relatively large residual amount in the environment.Although imidacloprid is a low-toxic pesticide,studies have shown that while it has an effect on target organisms,it also has an adverse effect on non-target organisms.In addition,due to its large content in the environment,it will migrate through various environmental media,and finally enter the human body along the food chain,causing certain harm to human health.Therefore,it is very important to take certain measures to reduce the exposure of IMI to the environment and human body.The advanced oxidation technology based on potassium peroxymonosulfate（PMS）removes refractory organic matter by generating strong oxidizing free radicals(SO₄^·-、·OH).It is a low-energy,green and environmentally friendly technology,which is obtained in industry widely used.In this paper,with imidacloprid as the target pollutant,two systems of ZnCo₂O₄/PMS and MoS₂/Fe²⁺/PMS were constructed to oxidatively degrade imidacloprid,and the influencing factors and activation mechanism of the two systems were studied respectively.Firstly,the ZnCo₂O₄bimetal oxide catalyst was prepared by the gel method,and the catalyst was characterized by scanning electron microscopy（SEM）,X-ray diffraction（XRD）,Fourier infrared spectroscopy（FTIR）and X-ray photoelectron spectroscopy（XPS）.Peroxymonosulfate（PMS）was activated by ZnCo₂O₄to generate SO₄^·-for the oxidative degradation of IMI,the effects of the PMS concentration,catalyst concentration,IMI concentration,p H and co-existing anions(Cl^-、CO₃^2-、NO₃^2-)on the degradation of IMI were investigated.The active free radicals,degradation products and the performance of the catalyst for repeated use were also studied.The results show that when the IMI concentration is 20 mg·L^-1,the dosage of ZnCo₂O₄catalyst is 0.4 g·L^-1,and the dosage of PMS is 15 mmol·L^-1,the degradation rate of IMI can reach more than 95%within 60 min.AS the initial concentration of IMI and the p H of the system increases,the degradation rate of IMI decreases.The degradation rate of IMI is positively correlated with the dosage of PMS in a range of 5～30 mmol·L^-1.when the amount of the catalyst is below 0.4 g·L^-1,the degradation rate increases with the increase of the catalyst amount.Cl^-and CO₃^2-have an inhibitory effect on the degradation rate of IMI,while NO₃^2-has no obvious effect.The main free radical in the system is SO₄^·-.The products in the system were identified by GC-MS,five intermediate products were found based on the mass spectrum,and the degradation path of IMI was speculated.The degradation rate of the catalyst can maintain more than 80%for 3 times repeated.Fixing ZnCo₂O₄through sodium alginate is easy for reusing and effectively reduce the dissolution of metal ions.Secondly,the hydrothermal method was used to prepare MoS₂,and the effect of promoting Fe²⁺to activate PMS to remove IMI was studied.The MoS₂before and after the reaction was characterized by SEM,XRD and XPS,results showed that MoS₂maintained good stability before and after the reaction.The batch experiment results show that when the IMI concentration is 20mg·L^-1,the dosage of MoS₂is 0.4 g·L^-1,the dosage of PMS and Fe²⁺respectively are 5mmol·L^-1and 0.1 mmol·L^-1,the system can completely degrade IMI within 30 minutes.The effects of MoS₂dosage,Fe²⁺dosage,PMS dosage,p H and four common ions in water(Cl^-,HCO₃^2-,NO₃^2-,NH₄⁺)on the removal of IMI in MoS₂/Fe²⁺/PMS system were studied.The dosage of MoS₂can reach the highest degradation rate at 0.02 g,and then increasing its dosage has little improve on the degradation rate;with the increase of Fe²⁺dosage and p H,the degradation rate of IMI decreases;the increase of PMS dosage is beneficial to improve the degradation rate of IMI;Cl^-,HCO₃^2-,NO₃^2-and NH₄⁺all have different degrees of inhibition on the degradation rate of IMI in the system.Among them,Cl^-and NH₄⁺have strong inhibitory effects,and HCO₃^2-and NO₃^2-have weaker inhibitory effects.The changes in the concentration of Fe²⁺,Fe³⁺and PMS during the reaction of different systems were measured,and the results again verified that MoS₂can strengthen the effect of Fe²⁺/PMS system in degrading IMI.Through free radical quenching experiment and EPR analysis to identify the main free radical species in the system,it was found that SO₄^·-,·OH,O₂^·-,¹O₂four active free radicals were produced in the system,of which SO₄^·-is the dominant free radical.