A Study For In Situ Simultaneous Remediation Of Cr(Ⅵ) Contaminated Soil And Groundwater By Integrated Water Circulation Control System Combined Thermally Active Oxalic Acid Technology

With the continuous development of urbanization,industrialization and agricultural modernization,the environmental pollution problems caused by industrial,mining and agricultural activities were becoming more and more serious.Pollutants in soil migrated to groundwater due to seepage,irrigation,rainfall leaching,etc.,resulting in groundwater pollution;groundwater level fluctuations,runoff and drainage led to further spread of pollution plumes,resulting in a wider range of groundwater contamination,so that contaminated site was always the coexistence of groundwater and soil.Commonly used remediation technologies in this field always focused on solving unilateral problem of soil or groundwater contamination,and the simultaneous remediation of contaminated soil and groundwater was less.Based on the above problems,this paper took Cr（Ⅵ）as the target pollutant,took the simultaneous remediation of contaminated soil and groundwater as the goal,combined in-situ soil washing and groundwater pump and treat technology,proposes a new water circulation control system,and coupled a thermally active OA system to achieve the in situ simultaneous removal of Cr（Ⅵ）in soil and groundwater.The specific research results were as follows:（1）The establishment of thermally active OA system.Static batch experiments were conducted to investigate the reaction efficiency and applicability in groundwater of the system.The results showed that the reduction rate of100 mg/L Cr（Ⅵ）was 100%at 120 min with a reaction temperature of 60°C,an initial OA concentration of 40 m M and an initial p H of 6.60（±0.1）,and the reduction rate was85.31%at a Cr（Ⅵ）concentration of 300 mg/L.Single-factor effect experiments showed that the rate of Cr（Ⅵ）reduction was positively correlated with the reaction temperature and initial OA concentration and negatively correlated with the initial Cr（Ⅵ）concentration,and the value of p H of the initial solution within 4.73～8.49 nearly did not affect the reduction of Cr（Ⅵ）.The system was insensitive to common ions and humic acid in groundwater,and the reduction of Cr（Ⅵ）was almost unaffected when the concentrations of Cl^-,NO₃^-,K⁺,Na⁺and Mg²⁺and humic acid were below 40 m M,HCO₃^-was below 30 m M,and SO₄^2-and Ca²⁺were below 2.5 m M.The optimal conditions were predicted through the response surface model:when the temperature was 66°C,the initial OA concentration was 35 m M,the initial Cr（Ⅵ）concentration was 83 mg/L,and the reaction time was 106 min,the Cr（Ⅵ）reduction rate was 96.43%under these conditions.（2）The construction of integrated water circulation control system for soil and groundwater.The hydrodynamic parameters for the stable operation of the system were determined through two-dimensional simulated tank experimental research.The heat transmission was explored and the solute transport was characterized.When the ratio of drenching flow rate to pumping flow rate was 1.25～1,the water level of the aquifer was relatively stable,and the water circulation control system operated stably.The study of the heat transmission under different hydrodynamic parameters showed that the heat transfer rate increased with the increase of pumping-drenching flow rate,but it showed the regulation of increasing,then decreasing and finally stabilizing,and finally forming a relatively stable temperature field;the flow rate of the water circulation control had a greater influence on the temperature field distribution,the higher the flow rate,the larger the effective temperature zone（≥50℃）and the higher the average temperature.When the hydrodynamic parameter was pumping flow rate of 300 m L/min-the drenching flow rate of 400 m L/min,the heat transfer was better.The solute transport experiment showed that under the condition of pumping flow rate of 300m L/min-drenching flow rate of 400 m L/min,the maximum vertical migration rate of the drenching fluid in the vadose zone was 0.68 cm/min,the influence radius of the pumping well was 59.25 cm,87.5%of the drenching fluid entering the aquifer would be captured by pumping well to the surface,and the residence time of the drenching fluid in the aquifer was 105 min.（3）Simulation study on the effect of simultaneous in-situ remediation of Cr（Ⅵ）contaminated soil and groundwater.The simulation study on the simultaneous in-situ remediation of Cr（Ⅵ）contaminated soil and groundwater was carried out using a two-dimensional simulation tank with the following experimental conditions:pumping flow rate of 300 m L/min-drenching flow rate of 400 m L/min,OA concentration of 40 m M,OA solution temperature of 95°C or above,and system operation time of 8 h.The results of the study showed that,with the operation of the system,when the effective temperature zone（≥50℃）was formed in the aquifer,the thermally active OA system started to play a reduction role and formed a coupling with the integrated water circulation control system to achieve the quick removal of Cr（Ⅵ）in the aquifer.At 0.5h of system operation,Cr（Ⅵ）entered the aquifer under the effect of soil washing,and at 3h,the remediation of the vadose zone was basically completed,and at 8h of system operation,the content of Cr（Ⅵ）in the soluble state in soil（vadose zone）was 0.10 mg/kg,and the concentration of Cr（Ⅵ）in extracting solution was below the groundwater environmental quality III water standard（≤0.05 mg/L）,and the residual total Cr content decreased to 1.21 mg/kg.At 3h of system operation,a 50℃temperature front entered the aquifer;as the remediation progressed,an effective temperature zone（≥50℃）was gradually formed in the aquifer,and the extent of Cr（Ⅵ）contamination plume in this zone was gradually reduced and the concentration gradually decreased;96.77%of the sampling holes in the aquifer had Cr（Ⅵ）concentrations below the groundwater environmental quality III water standard（≤0.05 mg/L）at 8 h of system operation,with30.00%of the sampling holes having total Cr concentrations below 0.05 mg/L.The extracted Cr（Ⅵ）contaminated groundwater had OA remained and reacted continuously.After stationary for 72h,the concentration of Cr（Ⅵ）was lower than the groundwater environmental quality class III water standard（≤0.05 mg/L）.The integrated water circulation control system coupled with thermally active OA in-situ reduction of Cr（Ⅵ）was effective and could achieve in-situ simultaneous remediation of Cr（Ⅵ）contaminated soil and groundwater.