Application Of Fe⁰/Na₂S₂O₈ In Situ Reaction Zone To Remediation Of Groundwater Contaminated By Landfill Leachate

An informal landfill site is located in the red bed area of Sichuan Province.Due to the lack of effective anti-seepage measures,the landfill leachate leakage caused serious organic pollution to the ground water of the site.Therefore,it is necessary to take remedial control measures to eliminate the risks to the environment and human health.The aquifer of the site has the characteristics of low permeability and low water abundance,and the ectopic extraction treatment technology has some shortcomings such as small amount of water and easy trailing rebound when applied to the site.In-situ reactive zone technology is suitable for the remediation of contaminated groundwater in the site due to its strong practicability and low difficulty in implementation.Compared with biological method and physical method,chemical oxidation method can effectively degrade macromolecular organic matter and improve the biodegradability of polluted groundwater.Sodium persulfate（Na₂S₂O₈）has strong oxidation,and the SO₄^-·produced can efficiently degrade most organic matter,and has the advantages of a wide range of application conditions,will not block the low permeability medium,wide p H range and so on.However,the decomposition rate of Na₂S₂O₈ to SO₄^-·is slow.Therefore,green and efficient zero-valent iron（Fe⁰）was selected to activate Na2S2O8 to improve the efficiency of its degradation of organic matter.Based on the above characteristics,Fe⁰/Na₂S₂O₈ in situ reaction zone technology was selected to remediation the contaminated groundwater in the landfill site.The hydrogeological condition,pollution type,pollution concentration,remediation pollution range and remediation target of the site were determined through groundwater pollution investigation.The remediation effect of Fe⁰/Na₂S₂O₈ in-situ reaction zone on contaminated groundwater was explored through laboratory experiments and pilot-scale experiments,the corresponding technological parameters were obtained,and the preliminary design of the remediation of contaminated groundwater in this site was carried out.To explore the applicability of Fe⁰/Na₂S₂O₈in-situ reaction zone technology in remediation of organically polluted groundwater in low permeability strata,and to provide certain technical support and experience accumulation for similar projects.The main research conclusions are as follows:（1）According to the investigation of groundwater pollution in the landfill site,the main aquifer of the site is the sandstone of Cangxi Formation in the lower Cretaceous series,with a permeability coefficient of 0.035-0.066 m/d and a groundwater velocity of 0.05-0.11 m/d.Groundwater at the site is polluted by some macromolecular organic compounds which are difficult to degrade,such as fulvic acid and humic acid.The COD concentration of the contaminated groundwater is23.0-161.0 mg/L,the remediation area is 9256.0 m²,and the volume of the remediation is 24916.0 m³.It is planned to reduce the COD concentration of the groundwater to less than 23.0 mg/L.（2）The removal efficiency of COD in contaminated underground by Fe⁰/Na₂S₂O₈ oxidation system and the optimal reaction conditions were investigated.The polluted groundwater with COD concentration of 161.0 mg/L was used as the experimental water to compare the effects of different ways of activating Na₂S₂O₈ on the degradation of organic matter.The zero-valent iron（Fe⁰）with better activation effect and less impact on the environment was selected as the activation way.The effects of the dosage of Na₂S₂O₈,Fe⁰,reaction time and temperature were investigated.The optimal dosage of Na₂S₂O₈ and Fe⁰ were 3.570 g/L and 0.252 g/L,respectively.After reaction at 15 for 24 h,the COD removal rate in the water sample reached 80.31%.Fe⁰/Na₂S₂O₈ oxidation system can effectively degrade humic acids and fulvic acids in polluted groundwater,and increase the BOD₅/COD value from 0.02 to 0.32.（3）The column experiment was conducted to explore the effects of groundwater flow rate and agent injection method on the COD removal rate,and to analyze the changes of p H,TFe and SO₄^2-in the effluent water.A sand column was used to simulate the Fe⁰/Na₂S₂O₈ in-situ reaction zone.When the local water flow rate changed from 0.05 m/d to 0.10 m/d,the COD removal rate decreased from 79.37%to75.68%after 14 days of reaction at 15.The removal rate of COD is 60.47%,which is smaller than that of 79.37%for continuous injection.After the reaction,the effluent p H decreased from 7.89 to 3.51-3.75,the TFe concentration increased from 6.90 mg/L to 35.2-42.1 mg/L,and the SO₄^2-concentration increased from 145.72 mg/L to1736.5-1867.19 mg/L.（4）The efficiency of Fe⁰/Na₂S₂O₈ in-situ reaction zone in remediation of COD in polluted groundwater was investigated through field pilot test,and the corresponding technological parameters were determined.The area with the most serious groundwater pollution in the site was selected for field pilot test.Three injection Wells with an interval of 1.5 m were laid out.The concentration of injection agents was Na₂S₂O₈ 35.70 g/L and Fe⁰ 2.52 g/L,the injection pressure was 0.05 MPa,and the injection flow was 3.72 L/h.Continuous injection of chemicals to repair contaminated groundwater.After 34 days of monitoring,the effective reaction zone of Fe⁰/Na₂S₂O₈in situ reaction zone was 0.0-1.5 m downstream of injection well,the COD removal rate was 70.96-75.62%,and the biochemical properties of groundwater were improved after remediation.In the experimental area,the p H of groundwater in the range of 7.85 to 5.92 showed a trend of first decline and then rise,TFe concentration was 6.90-21.68 mg/L,SO₄^2-concentration was 146.71-1347.06 mg/L,both of which showed a trend of first rise and then decline.It shows that in practical application,the underground environment of the site has a buffer effect on p H and a reduction effect on TFe and SO₄^2-.（5）The preliminary design of landfill surface seepage prevention and the preliminary design of polluted groundwater remediation were carried out.The anti-seepage reconstruction area of landfill surface is 9256.0m²,including surface cleaning,anti-seepage system construction,vegetation restoration,drainage system construction,etc.Two Fe⁰/Na₂S₂O₈ in situ reaction zones（）and（）were laid,and179 injection wells with an spacing of 1.5 m were constructed.The agent was slowly injected into the aquifer at the flow rate of 3.72 L/h and the pressure of 0.05 MPa.The concentration of the agent was dynamically adjusted according to the change of COD concentration,and a total of 35787.71 Kg Na₂S₂O₈ and 2728.99 Kg Fe⁰ were needed.After Fe⁰/Na₂S₂O₈ in situ reaction zone remediation of contaminated groundwater,the remediation target can be reached.