Study On The In-situ Remediation Of 2,4-dichlorophenol Contaminated Groundwater With Nano Calcium Peroxide Chemical Reaction Zone

In the past few decades,the rapidly increasing population has a huge contradiction with limited farming capacity.In order to increase grain yield,pesticides and herbicides are widely used.Chlorophenols,as important intermediates in pesticide production,are often detected in groundwater and soil,posing a serious threat to human health and the ecological environment.The advanced oxidation technology based on Fenton reaction is widely used in actual site repair due to its good effect on pollutant removal and simple operation.However,the practice has showed that the instability of hydrogen peroxide in subsurface severely affect the remediation efficiency.Large amount of injected hydrogen peroxide directly decompose into water and oxygen,without the participation of fenton reaction to generate hydroxyl radicals,resulting in lower utilization of hydrogen peroxide.Besides,the release oxygen gas clogs pores around injection wells and promotes contaminant volatilization which leads to the spreading of contaminated zone.In view of the above problems,this study intends to use calcium peroxide as slow-released agent of hydrogen peroxide,and then,the slow-released agent is synthesized into nano particles,which can be injected into subsurface to form reactive zone.This approach can improve the utilization of the hydrogen peroxide and the remediation performance of chlorophenol-contaminated groundwater.In this study,the preparation method of nano-CaO₂（nCaO₂）was firstly optimized,The nCaO₂ particles had high content of active ingredients and good dispersibility.Secondly,the degradation performances,mechanisms,pathways and influencing factors of 2,4-dichlorophenol（2,4-DCP）by modified Fenton system based on nCaO₂in the system containing dissolved ferrous iron,solid ferrous iron and iron-free conditions were researched.Thirdly,the migration characteristics of nCaO₂ in aquifer media were analyzed by one-dimensional simulated column dynamic experiments.Finally,a two-dimensional simulated tank dynamic experiment was designed to evaluate the effectiveness of the nCaO₂ chemical reaction zone in repairing 2,4-DCP contaminated groundwater.The results were as follows:（1）The optimum nCaO₂ can be prepared by using CTMAB as a dispersant and drying by nitrogen.The effective content of nCaO₂ reached over 85%and the size of single particle was 10-50 nm.The preparation method avoids the disadvantages of complicated operation and high cost of the traditional preparation method,which is more suitable for the actual site.（2）According to the background concentration of Fe²⁺in groundwater,the system containing dissolved Fe²⁺can be divided into three situations:high concentration of iron,low concentration of iron and no iron.The effects,rules and mechanisms of activated nCaO₂ degradation of 2,4-DCP were investigated respectively.In systems containing iron ions,increasing Fe²⁺dosage can promote the degradation of 2,4-DCP,but excessive addition of Fe²⁺or nCaO₂ inhibited the degradation reaction.SO₄^2-,Mg²⁺,CO₃^2-,and HCO₃^-had little effect on the reaction.Mn²⁺promoted degradation,which is structurally similar to Fe²⁺.In the condition of high concentration of Fe²⁺,Fe²⁺was more conducive to nCaO₂ degradation of 2,4-DCP than Fe³⁺.Fe³⁺catalyzed nCaO₂ degradation of 2,4-DCP was better in low-concentration iron system.In high concentration iron system,nCaO₂ can degrade 2,4-DCP with·OH as the main free radical,which was mainly oxidized to decompose 2,4-DCP into monochlorophenol,phenol,small molecular acid and other intermediate products.However,under low concentration of Fe²⁺,·OH and O2^-?worked together,oxidation and reduction reactions existed simultaneously.Dechlorination reaction is mainly carried out in the degradation process.The degradation of 2,4-DCP by nCaO₂ in iron-free buffer system mainly depended on heterogeneous catalytic reaction.2,4-DCP was decomposed under alkaline condition when the pH of iron-free buffer-free system was above 12.Under the condition of solid phase iron catalysis,the oxiding ability of nCaO₂catalyzed by iron oxides is:chromite>magnetite>hematite.（3）Through the one-dimensional simulation column migration experiment,it was found that although nCaO₂ had certain migration ability in aquifer media,most of the particles remained in the saturated porous medium due to the blocking effect of the medium.Increasing the size of medium and groundwater flow rate was beneficial to nCaO₂ migration;although the increase of the injection concentration can increase the flux concentration of nCaO₂ to a certain extent,it also increased the interception ratio of the aquifer to nCaO₂.（4）The two-dimensional simulated tank experiment was used to simulate the injection of nCaO₂ into the two wells to repair the groundwater contaminated by2,4-DCP.The results showed that a continuous and obvious chemical reaction zone was formed in the simulated tank.The location and degradation rate of the main degradation zone were affected by pH.When the pH was less than 10,the degradation reaction should occur.Therefore,the position of the reaction zone was divided into three stages with the change of pH:From 2nd to 5th day of the reaction,it was located in the upstream and downstream of the nCaO₂ distribution area,the average degradation rates in the tank were above 25%.Then,it moved to the upstream of the distribution area.On the 28th day,the distribution area and its upstream became the main reaction area,and the average degradation rate in the tank was about 38.8%.The above results indicate that the nCaO₂ chemical reaction zone can effectively repair 2,4-DCP contaminated groundwater for a long time.