In Situ Remediation Of Trichloroethylene Contaminated Soil And Groundwater By Sodium Persulfate And Calcium Peroxide

With the continuous acceleration of China's economic and industrial process,the further implementation of the policy of"two for three"and"city for park",many cities carry out the relocation,transformation and shutdown of polluting enterprises which resulting in a large number of industrial pollution sites left behind in cities and their surrounding areas in recent years.Trichloroethylene?TCE?has become one of the most common pollutants in soil and groundwater due to improper disposal of production and usage process and leakage of underground tanks.In this paper,TCE was used as the target pollutant to study the factors influencing the degradation of TCE in groundwater based on CaO₂ and Na₂S₂O₈ single and double oxidation systems and the in-situ remediation of TCE polluted soil and groundwater by nCaO₂-Na₂S₂O₈ double oxidation system.The effectiveness and factors influencing the degradation of TCE by sodium citrate chelated Fe²⁺to catalyze CaO₂ or Na₂S₂O₈ single oxidation system,the ratio of CaO₂ and Na₂S₂O₈,the water quality,the influence of conditions on the degradation of TCE by the double oxidation system and the study on the in-situ remediation of TCE polluted soil and groundwater by nCaO₂-Na₂S₂O₈ double oxidation system were researched.Meanwhile,the migration and transformation of TCE during the oxidation process was clarified,which provides data support and theoretical basis for the in-situ remediation of TCE polluted sites by CaO₂-Na₂S₂O₈.The specific research results are as follows:1.The study of TCE degradation by CaO₂ or Na₂S₂O₈ single oxidation system shows that the removal rate of TCE was the highest when the ratio of TCD and Fe²⁺is 1:1 and the degradation effect of TCE was affected by too high or too low chelation ratio.The degradation of TCE by TCD chelated Fe²⁺to catalyze CaO₂ or Na₂S₂O₈ was effective and the degradation process of TCE was in accordance with pseudo first-order kinetic model.The final removal rate of TCE was 98.0%after 200 minutes when the initial concentration of TCE was 0.16mmol/l and the molar ratio of CaO₂/TCD-Fe²⁺/TCE was 18:6:1 while the final removal rate of TCE was 76.3%after 100 minutes when the molar ratio of Na₂S₂O₈/TCD-Fe²⁺/TCE was 20:10:1.Excessive or insufficient Na₂S₂O₈ and CaO₂ will affect the removal effect of TCE.2.The research results of TCE degradation by CaO₂-Na₂S₂O₈ double oxidation system show that the removal rate of TCE by CaO₂-Na₂S₂O₈ double oxidation system was significantly higher than that of single oxidation system and the removal rate of TCE reached99%after reaction 200min under the optimal condition which significantly improves the removal effect of TCE.The degradation of TCE by CaO₂-Na₂S₂O₈ catalyzed by TCD-Fe²⁺can effectively degrade TCE and the degradation process of TCE conformed to the pseudo first-order dynamic model.The rise of p H was against to the remowal of TCE and the high concentration of Cl^-,HCO₃^-,CO₃^2-,SO₄^2-and the initial concentration of TCE also affected the degradation of TCE.The molecular probe experiment confirmed the existence of HO·and SO₄^-·in the CaO₂-Na₂S₂O₈ double oxidation system,in which HO·was dominant.3.TCE contaminated soil and groundwater poisoning experiments showed that the soil and groundwater in the soil box were polluted and formed an obvious pollution source area after 30 days of natural aging.The highest concentration of TCE is 623mg/kg in the soil and9.05×10^-3?g/L in the groundwater.The target values of soil and groundwater remediation are2.8mg/kg and 70?g/L respectively according to the risk assessment and risk control value calculation.4.In-situ remediation of TCE contaminated soil and groundwater by nCaO₂-Na₂S₂O₈double oxidation system shows that the injected oxidation agent was slowly and fully contacted with TCE to degrade it under the action of gravity through peristaltic pump drip injection of nCaO₂-Na₂S₂O₈/TCD-FeSO₄ into the injection well and extraction treatment of the extraction well.The removal of TCE in groundwater can be divided into three stages:In the early stage?the first 15 days?of oxidant injection,the change of TCE concentration in groundwater is not obvious.The concentration of TCE decreased to 2.8×10³?g/L after 5 days of oxidant injection while the concentration of"rebound"TCE increases to 6.05×10³?g/L after 7 days.In the middle stage of oxidant injection,the concentration of TCE decreased continuously with oxidant injection.The concentration of TCE in groundwater dropped to370?g/L and the removal rate reached 92.4%after 51 days.In the later stage of the reaction,the concentration of TCE in groundwater changed slowly.The concentration of TCE decreased to 26?g/L and the removal rate reached 99.7%after 75 days.The concentration of TCE in the soil decreased continuously and only a few points appeared"rebound"phenomenon at the initial stage of the reaction.When the oxidant was injected for 31 days,the maximum removal rate of TCE was 78.3%.After the oxidant was injected for 75 days,the soil reached the restoration value and the maximum removal rate was 99.8%.Samples analysis of TCE concentration and intermediate products showed that trichloromethane and dichloromethane appeared in the degradation process of TCE.The evaluation of pH value and SO₄^2-concentration in water showed that the remediation technology of nCaO₂-Na₂S₂O₈double oxidation system had little impact on the secondary environment.