Enhanced Chemical Repair And Mechanism Of Degrades 1,2-dichloroethane In Groundwater

In recent years,with the closure and relocation of heavily polluting industrial enterprises,a large number of industrial enterprises legacy sites with serious soil and groundwater pollution have been generated.1,2-Dichloroethane（1,2-DCA）is one of the typical pollutants in chemically polluted sites.As an important chemical raw material,1,2-DCA has carcinogenic,teratogenic and mutagenic"three causes"effects.At the same time,it has the characteristics of easy migration and refractory degradation,which will cause harm to the ecological environment.The main remediation methods for 1,2-DCA-contaminated groundwater include ex-situsoilremediation,in situ remediation,and monitoring of natural attenuation.ex-situsoilremediation is time-consuming and expensive,and it is difficult to completely degrade 1,2-DCA.In-situ remediation includes thermal desorption,steam leaching,and in-situ chemical oxidation techniques.Among them,advanced oxidation technology can efficiently treat pollutants in groundwater to an acceptable level in a short period of time.In this study,based on nZVI-activated PS,the activation energy,salinity,chloride ion extraction rate,and kinetics of EDTA-enhanced nZVI-activated PS were used to explore the mechanism and influencing factors of the system.Fe-MCM-41 and nZVI-MCM-41 were loaded with iron-based materials,and the removal ability of the materials to 1,2-DCA was evaluated by adsorption kinetics,adsorption capacity,activated sodium persulfate,etc.A series of modern characterization methods SEM,FTIR,BET reveal the micro and macro morphological structure of materials.The research results are as follows:1)Ethylenediaminetetraacetic acid（EDTA）was used to strengthen Nano-zero-valent iron（nZVI）to activate sodium persulfate（PS）for degradation of1,2-dichloroethane（1,2-DCA）in groundwater.By analyzing the specific degradation rate（Sq）,mineralization,degradation kinetics of 1,2-DCA and mass concentration change rules of Fe²⁺and Fe³⁺,clarifying the enhancement effect of EDTA and its mechanism.The effects of EDTA dosage,pH,anion and cation on the strengthening effect of EDTA were investigated.The results showed that after EDTA strengthened nZVI/PS,Sq and the degree of mineralization increased by 21%and 190%,respectively.Reaction rate increased by about 10.06%～22.95%and activation energy decreased by 6.98～8.91 kJ·mol^-1.EDTA provided a weak acid environment for the reaction,which promoted the formation of Fe²⁺.EDTA formed a chelate with Fe²⁺reducing the probability of the reaction between Fe²⁺and free radicals,which improved the sustained effectiveness of nZVI/PS.Sq first increased and then decreased with the growing use of EDTA.Sq reached the maximum value of 214.80mg·g^-1when the dosage of EDTA was 1.8 mmol.The strengthening effect of EDTA was better in acidic environment.Except NH₄⁺had a promoting effect,Cl^-,K⁺and HCO₃^-all inhibited the strengthening effect of EDTA,indicating that EDTA might be difficult to achieve an ideal strengthening effect in remediation of actual groundwater due to the existence of multiple ions.The research results provide basic parameters and technical supports for degradation of 1,2-DCA in groundwater.2)Fe-MCM-41 and nZVI-MCM-41 were synthesized by hydrothermal synthesis.Fe-MCM-41 and nZVI-MCM-41 adsorb and activate PS to degrade1,2-DCA in groundwater.We studied the effects of reaction system,material dosage and water quality conditions on the removal rate of 1,2-DCA in detail.The first-order kinetics,pseudo-second-order kinetics and intraparticle diffusion models were analyzed to explore the adsorption mechanism.By comparing Fe（NO₃）₃and nZVI,the degradation rates of the three materials were analyzed,and their degradation mechanisms were discussed.The results show that the introduction of metal elements in Fe-MCM-41 and nZVI-MCM-41 can well solve the neutral structure of MCM-41,few defects,low ion exchange capacity,low acid content,low acid strength,disadvantages of low reactivity.On the one hand,Fe-MCM-41 and nZVI-MCM-41can increase the number of in-frame active sites and enhance their activity.On the other hand,they can prevent the accumulation of iron in a short time,so that they can be well dispersed and improve their repair ability.Both Fe-MCM-41 and nZVI-MCM-41 fit the quasi-second-order kinetic equation.The intraparticle diffusion model showed that 1,2-DCA was adsorbed on the outer surface of the material at the initial stage of the reaction,and gradually migrated to the pores,and then reached the pore surface,and finally the adsorption reached a dynamic equilibrium.Metal elements can improve the hydrophobicity of MCM-41,accelerate the formation of complexes between 1,2-DCA and metal-loaded MCM-41,and enhance the degradation of 1,2-DCA by activated PS.The experimental results provide basic parameters and technical support for the degradation of 1,2-DCA in groundwater.3)The degradation effects of MCM-41,Fe-MCM-41 and nZVI-MCM-41 on1,2-DCA in the actual groundwater environment were simulated by using quartz columns filled with quartz sand.The effects of MCM-41,Fe-MCM-41 and nZVI-MCM-41 on the adsorption and activation of sodium persulfate in the simulated column to degrade 1,2-DCA were investigated respectively.The results show that the C_t/C₀ratio of the adsorption group is maintained at about 0.8 after reaching the adsorption equilibrium,which indicates that the quartz sand in the water column will also have a certain adsorption effect on 1,2-DCA.Adding MCM-41,Fe-MCM-41 and nZVI-MCM-41 to the water column can effectively adsorb 1,2-DCA,and the breakthrough point time is slightly prolonged with the addition of materials,and the C_t/C₀reaches 0.77,0.67,0.63 at 250min respectively.The activated sodium persulfate group found that the degradation effect of unactivated PS on 1,2-DCA was not optimistic.After adding the material,MCM-41,Fe-MCM-41 and nZVI-MCM-41 had the dual effects of adsorption and activation of sodium persulfate to degrade 1,2-DCA,and the breakthrough time reached 1,2-DCA respectively.15,25,and 45 min were significantly prolonged compared with the adsorption group,indicating that MCM-41,Fe-MCM-41 and nZVI-MCM-41 could effectively activate PS and prolong the penetration time.After 250 min,the C_t/C₀of the three materials reached 0.70,0.63,and 0.59,respectively,which was close to the adsorption group.The reactive sites of MCM-41,Fe-MCM-41 and nZVI-MCM-41 were occupied and could not effectively activate PS,and the removal effect of 1,2-DCA only depended on the adsorption of the material itself.It shows that in the actual groundwater,the initial stage of adding materials mainly depends on the activation of PS to degrade 1,2-DCA,and after the number of reactive sites is insufficient,the removal of 1,2-DCA mainly depends on the material itself.adsorption.