Research On Modified GCL Liner Based On The Improvement Of Cr(Ⅵ) Adsorption And Salt Resistance

Hexavalent chromium has high mobility and strong toxicity.It exists as a large anion group in nature,so its adsorption mechanism and migration characteristics are more complex than cationic heavy metals.In China,the ultimate disposal method for chromium residue and chromium contaminated sites is landfill.However,traditional GCL can not effectively adsorb hexavalent chromium through ion exchange,which easily leads to barrier failure,resulting in ecological environment pollution and harm to human health.Meanwhile,GCL is often faced with complex salt,acid-base and other special pollution environment in practical application.Under the action of sewage and electrolyte solution,cation exchange is easy to occur,which makes interlayer Na⁺be replaced by other cations,resulting in a substantial increase in the permeability coefficient of bentonite materials and a decline in the expansion performance,which leads to the failure of anti-seepage.In order to solve these two problems,a modified GCL liner was designed by two kinds of modification and combined use of Na bentonite.Through expansion test,adsorption test and seepage test of simulated pollutant leachate,the adsorption of composite liner structure and impermeability in salt solution environment were studied,which provided guarantee for impermeability and barrier treatment of chromium contaminated sites.The main research results are as follows:（1）Resin modified bentonite and organic modified bentonite were prepared by super absorbent resin and organic quaternary ammonium salt respectively,and their microstructure was characterized.The results show that the micro morphology of resin modified bentonite shows a more curly lamellar stacking structure,the interlayer spacing increases slightly,it carries new organic functional groups,the number of surface negative charges increases significantly,and the hydrophilicity improves.The results show that the micro morphology of organic modified bentonite tends to be smoother,the interlayer spacing increases greatly,the characteristic functional groups of modifier are intercalated successfully,the surface properties change from hydrophilicity to lipophilicity,and the surface charge properties change from negative to positive.（2）On the one hand,the free expansion of resin modified bentonite is 71.0 ml/2g,which is 91.9%higher than that of Na bentonite,and the durability index（0.1%,Ca Cl₂）is 43.5 ml/2g,which is 85.1%higher than that of Na bentonite;On the other hand,by enhancing the swelling properties of bentonite,the resin material forms a more tortuous seepage channel through water absorption and expansion,thus improving the impermeability.The permeability coefficient of resin modified soil is 1.71×10^-9m/s,86.5%higher than that of GCL.Meanwhile,the negative charges carried by hydrophilic groups also effectively enhance the number of negative charges on the surface of bentonite,and play a barrier role to the external cations for cation exchange,so as to improve its salt resistance.（3）The principle of organic quaternary ammonium salt modified bentonite is interlayer cation exchange.The organic modified bentonite overcomes the defect that Na bentonite can not effectively adsorb Cr（VI）through ion exchange.The experimental adsorption capacity of the modified bentonite is 8.3 mg/g.The optimum adsorption conditions were as follows:solid-liquid ratio 8 g/L,p H=4.0 and initial Cr（VI）concentration 5 mg/L.（4）The permeability coefficient of GCL is 1.27×10^-8 m/s,and the permeability coefficient of the modified soil based on resin modification and organic modification is 7.57×10^-9m/s,40.4%higher than GCL.The results showed that the residual concentration of Cr（VI）in the leachate was 0.97 mg/L and the adsorption rate was only 80.6%;After modified GCL,the residual concentration of Cr（VI）in leachate is0.02 mg/L,the adsorption rate is 99.6%,and the concentration of leachate is reduced by 48.5 times.When the initial concentration of Cr（VI）was increased to 20 mg/L,the effective removal rate of GCL was 71.8%,the effective removal rate of modified bentonite composite liner was 98.4%,and the leachate concentration was reduced by17.1 times.