Research Of The Performance Of Reactive Soil-Bentonite Vertical Barrier Used To Contain Chlorinated Organics

The paper relies on the national key research and development plan“Key technology research“”and integrated demonstration of the remediation and safe use of pesticide contaminated sites in the Yangtze River Delta”（2018YFC1803101）,the national key research and development plan“Solid waste landfill soil pollution risk management and control and purification technology”（2018YFC1802304）,the national natural science foundation of China“Study on mechanism and performance of vertical engineering barrier of bentonite modified by polyphosphate dispersant to contain high-risk heavy metal pollutants”（41877248）,the national natural science foundation of China“Study on mechanism and performance of enhanced alkali activated mineral powder bentonite vertical engineering barrier for containing dichloromethane pollutants”（42177133）.Through experimental research and theoretical analysis,the content optimization,anti-seepage performance,adsorption/degradation performance of reactive soil-bentonite vertical barrier materials are systematically studied.Based on the results,the following conclusions can be drawn:（1）The mud standing test,mud filtration test and backfill improved filtration test were carried out,which revealed that the effects of micron zero valent iron powder dispersion of reactive soil-bentonite vertical barrier,carrier material and iron powder content on mud filtration and hydraulic conductivity.The results show that the iron powder in the bentonite slurry modified by polyanionic cellulose（PAC）has good dispersion,and the iron powder is precipitated,agglomerated and layered in the bentonite slurry modified by sodium hexametaphosphate and activated carbon;Adding 1-2%PAC can reduce the filtration of bentonite by about 42%-50%;Under the infiltration of 1.5g/L dichloromethane solution,the hydraulic conductivity of barrier material increases slightly with the increase of iron powder content（2-8%）.In addition,the hydraulic conductivity still meets the anti-seepage design requirements of vertical barrier(less than 1×10^-9m/s)（2）The flexible wall permeability test and batch adsorption and degradation test were carried out.Then anti-seepage performance and adsorption and degradation performance of reactive soil-bentonite vertical barrier material were also crarified.The results show that the hydraulic conductivity（k_w）of reactive soil-bentonite vertical barrier backfill under tap water permeability is 2.5×10^-11m/s,while the hydraulic conductivity（k_c）is 7×10^-11m/s under the penetration of 50 mg/L dichloroacetic acid solution,meeting the requirements of barrier anti-seepage design.When the content of iron powder is 2%,the removal rate of 150μg/L dichloroacetic acid solution by active backfill（73.3%）is 57%higher than that by ordinary backfill（46.7%）.（3）The field application test of reactive soil-bentonite vertical barrier material was carried out in a chlorinated organic pollution site.At the same time,the lateral displacement,settlement,hydraulic conductivity and iron powder uniformity of reactive soil-bentonite vertical barrier material were studied.The results show that the lateral displacement of reactive soil-bentonite vertical barrier and ordinary bentonite vertical barrier increases first and then decreases with the increase of depth.The lateral displacement reaches the maximum at about4.5m.which resulting in the settlement in shallow layer is large and that in deep layer is small;Under the infiltration of tap water and on-site polluted groundwater,the hydraulic conductivity of reactive soil-bentonite meets the anti-seepage design requirements(1×10^-9m/s).And the uniformity of iron powder is good at the same depth of the barrier（4）The convection adsorption diffusion migration of dichloroethane in the aquifer is simulated.Meanwhile,the effects of the embedding depth of the vertical barrier,the bottom contact and the hydraulic conductivity of the barrier on the time of dichloroethane breaking through the vertical barrier are found out.The results show that the breakdown time of the barrier increases with the increase of the embedding depth.And the minimum embedding depth increases with the increase of the hydraulic conductivity of the relative aquitard.When the hydraulic conductivity of the relative weak permeable layer is high,the breakdown time increases with the increase of the hydraulic conductivity of the bottom contact,while the hydraulic conductivity of the barrier has a great influence on the field service performance.The lower the hydraulic conductivity,the smaller the diffusion range of pollutants.