Researches On Material Properties Of Surface Barrier Material And Prb Medium In Risk Prevention And Control Of Cr(Ⅵ)-Contaminated Sites

With the high concentration,concealment and remediation costs,Cr（Ⅵ）-contaminated sites pose serious impacts on soil and groundwater.Polymer cement surface barrier materials were laid on the surface of contaminated sites to reduce the risk of Cr（Ⅵ）supplying infiltrated soil and groundwater with precipitation recharge,while Permeable Reactive Barrier（PRB）active materials could react with Cr（Ⅵ）in groundwater and redece its concentration,thus limiting the risk of diffusion of Cr（Ⅵ）in contaminated sites.However,the research on the performance of polymer cement surface barrier materials was limited to ideal conditions,and its propertis in outdoor application is unknown.Used as PRB reaction mediums,the modified iron-based materials not only have poor permeabilities,serious passivation and uneven distribution,but also are expensive.Therefore,the material performance changes of surface barrier materials were explored in this paper through outdoor tests,and improved its mechanical properties in order to for their potential engineering applications.Meanwhile,the superior materials with exellent Cr（Ⅵ）removal ability and high permeability were selected from natural mineral materials,and then their performance in groundwater at different depths were explored.The main results are as follows:（1）Polymer cement surface barrier material can effectively inhibit the vertical migration of Cr（Ⅵ）in soil,and also maintain soil moisture to prevente the migration of Cr（Ⅵ）particles with the wind.However,affected by temperature,sunlight and other natural factors,weatherability and anti-aging ability of materials need to be improved furthermore.Laying filament polyester fiber at the bottom of the material can improve its tensile properties and weather resistance,and the best thickness of spraying was3mm.The addition of fibers in materials can improve its tensile strength,and wood fiber was superior to others,and its optimum dosage was about 0.74%of the solid content of the material.Iron oxide red can improve the anti-aging ability of the material,and the optimum dosage was 1%.（2）Sponge iron and GZVI had better removal effects on Cr（Ⅵ）,and others had limited removal ability which was not related to the dosage.The synergistic effects were found between Sponge iron,maifanite and Pyrite,and the compound combination has the best removal rate of Cr（Ⅵ）.The higher proportion of Sponge iron,the greater saturated adsorption capacity of Cr（Ⅵ）by complex reaction columns,but the faster the permeability coefficient decreased.Considering the saturated adsorption capacity and permeability of the compound materials,Sponge iron:Maifanite=5:1 and Sponge iron:pyrite=1:1 were more suitable.（3）The removal effect of Pyrite complex system on Cr（Ⅵ）in groundwater was better than that of Maifanite system,with longer penetration time and greater adsorption capacity.Decreasing the influent concentration and extending the hydraulic retention time can promote the removal efficiency of Cr（Ⅵ）,which were more obvious for Pyrite system.In simulated deep groundwater,Cr（Ⅵ）migrated faster and took less time to reach penetration,the adsorption capacity was larger,but the adsorption rate decreased.The concentrations of TCr and Cr（Ⅵ）in the effluent of Maifanite system almost equal and the content of soluble iron was approach 0.Increasing the pressure can also reduce the concentrations difference of TCr and Cr（Ⅵ）in the pyrite system.Throughout the reaction process,the p H of the reaction column effluent first rose and then fell,but the p H was always greater than 7.5,the trend of ORP value was opposite to p H,and the system was always in the reduced state.（4）The chromium after reacting was mainly in residual state and oxidizable state,and the hydrolysis state and weak acid extraction state accounted for a relatively small amount.In deep groundwater,more Cr（Ⅵ）was reduced and precipitated,the content of chromium adsorbed in the medium increased,and the proportion of residual chromium also increased,the particles on the surface of the medium became smaller and denser in accumulation,and the pores on the surface of material were reduced.The percentage of Fe and Cr increased after pressurization,and the peak values and peak area also increased.After reaction,Cr（Ⅵ）had reduced to Cr（III）by sponge iron,and its surface was oxidized to Fe（III）and wrapped by Fe and Cr oxides.Fe-Cr reaction products migrated to the surface of Maifanite and were absorbed,formed Fe₂O₃,Cr Si O₂ and other substances,which inhibited the passivation of sponge iron.Since it is difficult for pyrite to dissolve in neutral aqueous solution,its surface changed little,and the degree of participation in Cr（Ⅵ）reduction precipitation was not high.（5）The surface barrier material and PRB material showed obvious barrier effect on Cr（Ⅵ）migration in the conceptual model.By regularly spraying the surface barrier material again,the Cr（Ⅵ）load of PRB reaction wall could be reduced,the spatial synergy between surface barrier material and PRB material could be effectively improved,and the risk of Cr（Ⅵ）diffusion and migration can be restrained significantly.