| In order to explore the vertical migration of Cr,Ni,Cu and Cd in “soil—phreatic stratum rock-soil—phreatic water ”system and the quantitative relationship of heavy metal in these environmental media sites,the field investigation and indoor soil column leaching experiments were carried out.Moreover,accordding to the pollution of groundwater,permeable reactive barrier was applied for the remediation of groundwater contaminated by these four kinds of heavy metals and its feasiblility and influence factors were studied.In this study,61 eyes phreatic water monitoring wells were constructed in 46 Cr,Ni,Cu and Cd contaminated sites of the Pearl River Delta for sampling,including aeration zone soil sample,phreatic water rock-soil sample and groundwater sample.The contents of Cr,Ni,Cu and Cd in the surface soil were 8.52 ~ 3800,11.9 ~ 2770,18.5 ~ 4010 and 0.078 ~ 3650 mg·kg-1,respectively.The BCR method was used to analyze the distribution of Cr,Ni,Cu and Cd in the surface soil and phreatic stratum rock-soil.The total amount of Cr,Ni,Cu and Cd in diving is the dependent variable,the surface soil or phreatic stratum rock-soil in full amount or a form of content respectively as independent variables,linear regression analysis and curve regression analysis(quadratic function,cubic function,exponential function and power function)were performed on the 61 groups of samples.Besides,three undisturbed soil columns(numbered A,B and C)with diameter of 19 cm and height of 103,81 and 79 cm,respectively,were collected for soil column leaching experiments.In addition,three reaction columns(numbered a,b,c),with different proportion of iron,activated carbon and limestone,were bulit for simulating permeable reaction wall for groundwater remediation.Every column was divided into 3 sections: first section was filled with length of 10 cm quartz sand,second section was filled with length of 60 cm iron powder and active carbon uniform mixture,the volume ratio of iron powder and activated carbon in a,b,and c reaction columns was 1:1,1:2,1:4,respectively,third section was filled with length of 30 cm limestone powder.Results showed that:(1)The best regression equations were established between the acid extractable state content of Cr,Ni,Cu and Cd in surface soil as independent variables and the content of Cr,Ni,Cu and Cd in phreatic water,with the coefficient of determination function R2 of 1.000,0.292,0.999 and 0.400,respectively.Which meaned that there was a quantitative relationship between the content of heavy metal in surface soil and phreatic water,especially Cu and Cr.(2)The best regression equations were established between the acid extractable state content of Cr,Ni,Cu and Cd in phreatic stratum rock-soil as independent variables and the content of Cr,Ni,Cu and Cd in phreatic water,with the coefficient of determination function R2 of 1.000,0.607,0.973 and 0.085,respectively.Which meaned that there was a quantitative relationship between the content of heavy metal in phreatic stratum rock-soil and phreatic water,especially Cu and Cr.(3)It was found that the total content and the acid extractable state content of Cr,Ni,Cu and Cd in the surface soil of three soil columns accounted for the proportion of the whole soil column were over 55 %,implying that at the contaminated sites which have been polluted for 5~10 years,the total content and the acid extractable state of heavy metals are still concentrated in the surface soil.(4)In the leaching experiments of soil columns,which was the rapid release of heavy metals with cumulative leaching less than 1000 mm·m2,and the the slow release of heavy metals appered when the cumulative leaching was more than 1000 mm·m2.Cr,Ni,Cu and Cd contents in the leachate was significantly related with acid extractable state content in the surface soil and influenced by the physical and chemical properties soil.(5)In the permeable reactive barrier experiments for remediation of contaminated groundwater,when hydrualic retention time was 24 h,PRB could effictively remove all these heavy metals with removal efficiency of 99.8%,which demonstrated that PRB could decontaminate the groundwater contaminated by Cr,Ni,Cu and Cd efficently.The removal efficieny for the same heavy metal was in order of column a,b and c,and the higher ratio of iron powder,the higher removal efficiency of Cr(VI),Ni(II),Cu(II)and Cd(II).With the increase of HRT,from 1h to 24 h,Cr(VI),Ni(II),Cu(II)and Cd(II)in effluent gradually decreased.In a,b and c reaction column,pH increased from 4.62±0.31 to 7.63±0.12?7.32±0.17?7.18±0.15 respectively.In the third part of all colums,limstone powder could removed total iron with an efficiency higher than 80 %,which showed that limstone powder palyed an important role in the removal of iron and avoiding of secondary pollution. |
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