Model Test Study Of Limiting Migration Of Heavy Metal Of Soil-bentonite Vertical Cutoff Wall

With the financial support of National Natural Science Foundation of China under Grant No.41330641,No.50878052 and Outstanding Youth Foundation of Jiangsu Province under Grant No.BK2012022,the contaminant containment performance of sand-bentonite slurry-wall backfill was investigated by workability test,consolidation tests,hydraulic conductivity test,batch adsorption test,chemical omostic memebrane behavior test,laboratory model test and numerical analysis.The following conclusions can be obtained:(1)The workability of sand-bentonite was investigated using slump test,and the consolidation behavior and hydraulic conductivity of sand-bentonite and Nanjing Clay were investigated using consolidation tests and conductivity test.The resulting values of bentonite content is 10%for bentonite-water slurry and water content corresponding to Slump at 125 mm for sand-bentonite backfill is 40%.The permeability coefficient of sand-bentonite is not significantly affected by the consolidation pressure,which is basically in the same order of magnitude,ranging from 3.0×10-10 m/s to 1.0×10-10 m/s.The permeability coefficient of Nanjing Clay decreases with the increase of consolidation pressure,which ranges from 5.4×10-9 m/s to 4.4×10-10m/s.(2)The adsorption characteristics of sand-bentonite and Nanjing Clay for heavy metals Pb and Zn were investigated using Batch adsorption test.The adsorption capacity of heavy metals Pb and Zn increase with the increase of solute concentration and the adsorption removal rats are opposite to that for sand-bentonite and Nanjing Clay.The competitive adsorption of cations leads to the adsorption capacity of single Pb and Zn for sand-bentonite and Nanjing Clay is higher than Pb-Zn mixture solution,which ranges from 3%to 11%.(3)The chemical retardation characteristics of sand-bentonite and Nanjing Clay for heavy metals Pb and Zn were investigated using chemical soil column test.The membrane efficiency of heavy metals Pb and Zn are significant for Sand-bentonite,the membrane effect coefficient decreases with the increase of solute concentration,and shows a good linear relationship in semi-logarithm coordinate.It ranges from 0.008 to 0.03 while the concentration range of heavy metals Pb and Zn is 5 mmol/L to 60 mmol/L.The effective diffusion coefficient of heavy metals Pb and Zn for sand-bentonite and Nanjing Clay were investigated using chemical soil column test.The effective diffusion coefficient of heavy metals Pb and Zn increase with the increase of solute concentration and eventually stabilized for both sand-bentonite and Nanjing Clay.And the effective diffusion coefficient of Pb is higher than Zn.In addition,the effective diffusion coefficient of Pb-Zn mixture solution for sand-bentonite decrease 8%-15%and 13%-27%for single Pb and Zn solution.(4)The actual retardation characteristics of sand-bentonite cutoff wall for Pb-Zn mixture solution were investigated using laboratory model test.The migration rate of heavy metal Pb is faster than heavy metal Zn in Pb-Zn mixture solution.Soil environment has a significant influence on the migration of heavy metal ions.sand-bentonite cutoff wall may cause the redistribution of seepage field in aquifer-aquitard layers,and the water conditions of multi-aquifer directly affect the migration path of heavy metal ions,there may cause a leaky pollution while the head of the lower aquifer is lower than upper aquifer.(5)The actual retardation characteristics of sand-bentonite cutoff wall for Pb on different construction state were investigated using numerical analysis The construction quality has a great influence on the actual migration performance of sand-bentonite cutoff wall.The actual migration performance of well constructed wall is better than cracked wall cracks for several orders of magnitude.