Study On Controlling Cadmium Pollution Of Groundwater By Fly Ash Bentonite Blocking Wall

Landfill leachate,tailings leachate and industrial site wastewater leakage have caused serious groundwater contamination in local sites.Groundwater pollution is concealment,having difficult to restitute,which leads to the limitation of the existing repair technology with unsatisfactory repair effect and high repair cost.Therefore,it is necessary to study a cost-effective method to control the diffusion of pollutants in groundwater.Based on this,the groundwater blocking wall technology was researched and proposed to build a low-permeability vertical barrier downstream of the groundwater pollution source,intercepting the migration of pollutants and preventing their further diffusion,thus achieving the purpose of risk control.The paper selects fly ash which is solid waste and bentonite with strong adsorption as the intercepting material.The best ratio of the intercepting wall is selected through penetration test and adsorption experiment.At the same time,the microscopic characterization and stability performance of the intercepting wall were studied by scanning electron microscopy,energy spectrum analysis and erosion test.In the groundwater medium contaminated by cadmium,the adsorption properties of the intercepting wall were studied by adsorption kinetic equations and adsorption thermodynamic equations.Penetration tests and software simulations were conducted to study the regularity and penetration time of contaminants through the barrier wall.The specific findings are as follows:（1）The effective mass ratio of fly ash to bentonite in the intercepting wall material was five to one,of which the cement quality accounts was 5%.With this ratio,the permeability coefficient of the intercepting wall was 1.11×10^-8 m/s.（2）SEM shows that fly ash acted as the main skeleton in the curing system,and that bentonite was filled in the voids and surfaces of fly ash spheres.Fly ash and bentonite both produced hydration products and cementation with the cement.At the same time,EDS elemental analysis found that the presence of Cd²⁺was detected on the surface of fly ash spheres,hydration products and cements,indicating that the barrier material could adsorb Cd²⁺effectively.（3）The system of the interceptor wall adsorption met the pseudo-first-order kinetic model and the Freundlich equation.The total adsorption rate of the system was affected by both liquid film diffusion and particle diffusion.The process of adsorption of Cd²⁺,which was mainly physical adsorption,was an endothermic process in the intercepting wall.With the increase of temperature,the system would shift from physical adsorption to ion exchange adsorption.（4）The analysis of the migration law of Cd²⁺in the interdiction wall showed that the permeability coefficient of the wall would not change more than one order of magnitude when the Cd²⁺passes through the wall.The permeability coefficient decreased with higher initial concentration of contaminants.It will take about 40years to obtain a Cd²⁺penetrating wall with an initial concentration of 500 mg/L by formula calculation and software simulation.Analysing the migration process,Cd²⁺played a leading role in the diffusion of molecules through the barrier wall,meanwhile it adsorbed with the intercepting material physically,resulting in slower transmission of pollutants in the wall.（5）The corrosion resistance of the barrier wall was studied by immersion test with acid,alkali,cadmium sulfate and phenol.The alkali corrosion resistance of the wall was stronger than that of acid corrosion resistance.The wall was also resistant to cadmium sulfate corrosion,except the erosion resistance would be weakened when the concentration was more than 5 g/L.In the environment of organic pollution,the anti-corrosion performance of the intercepting wall was strong and the interception effect was unaffected.