Study On The Barrier Mechanism And Application Of Fly Ash-Bentonite Barrier Wall To Cadmium And Lead In Groundwater

Heavy metal pollution in groundwater is a concerned environmental issue.Remediation and risk management technology of heavy metal pollution are common methods for prevention and control of groundwater pollution.Compared with the limitations of high cost and low efficiency of groundwater pollution remediation technology,pollution risk management technology is widely used due to its advantages of high processing efficiency and fast processing speed.Barrier technology,which is one of the technologies of risk pollution control,has broad application prospects in the control of heavy metal pollution in groundwater.Therefore,the research on the barrier mechanism and application of barrier wall to heavy metals in groundwater is of great significance for the prevention and control of heavy metal pollution in groundwater and the promotion of barrier wall.Relying on the "Research on the barrier performance and mechanism of grouting wall to groundwater pollutants in the site",which is a subject of State Key Laboratory of Geological Prevention and Geological Environment Protection,based on the fly ash-bentonite barrier wall studied by the research group,the paper selected cadmium and lead as representative heavy metals,and carried out research on the barrier mechanism of cadmium and lead by fly ash-bentonite barrier wall and research on the application of it.The main research contents and conclusions are as follows:?1?The adsorption characteristics of fly ash-bentonite barrier materials on cadmium and lead were studied.The adsorption kinetics,isothermal adsorption model and adsorption mechanism of cadmium and lead by barrier materials were studied by adsorption experiments and SEM and XRD characterization techniques.The results show that the adsorption of Cd²⁺by the barrier material accords with quasi-first-order kinetics,and the adsorption of Pb²⁺by the barrier material follows quasi-secondary kinetics.The isothermal adsorption of Cd²⁺and Pb²⁺is consistent with the Freundlich model.The Cd²⁺and the hydrolyzate of aluminum-silicon mineral in the barrier material generate CdSiO₃ through ion exchange,and Cd²⁺forms a complex with the kaolin mineral on the surface of the barrier material.Pb²⁺will precipitate PbCO₃ with carbonate in the barrier material.The Pb²⁺and the hydrolyzate of quartz form a stable complex at the point of permanent charge on the surface of the inner layer of the clay mineral.?2?The migration characteristics of cadmium and lead in fly ash-bentonite barrier wall were studied.The effective diffusion coefficient of ions in the barrier wall,the percolation velocity of different solutions,the breakthrough curve of cadmium and lead pollutants,and the morphological transformation characteristics of cadmium and lead in the barrier wall were studied through the diffusion test and penetration test.The results show that the effective diffusion coefficient of ions in the barrier wall is 4.47×10^-10 m²/s.The seepage velocity of the polluted liquid in the barrier wall gradually decreases as the penetration time increases.The stable permeation rate is on the order of 10^-9 m/s.The relationship between the steady seepage velocity of different solutions in the barrier wall is as follows:Pb²⁺<Cd²⁺<NaCl solution<distilled water;and the relationship of penetration time in the barrier wall between Cd²⁺and Pb²⁺is as follows:Pb²⁺>Cd²⁺.The Cd²⁺and Pb²⁺mainly existed in the weak acid extraction state in superficial of the barrier wall during the penetration process.The content of the reduced state,the oxidation state and the residual state of Cd²⁺and Pb²⁺increased gradually with the increase of the migration depth.?3?Feasibility analysis of centrifugal test for migration characteristics of cadmium and lead in barrier walls was also analyzed.The results of the centrifugal test of N g?where N=25,50,g is gravitational acceleration?indicate that the centrifugation test time is 1/N² of the column test time,and the percolation velocity of the centrifugal test is N times of the column test percolation velocity.In the centrifugal test,the content of Cd²⁺and Pb²⁺on the surface of the barrier wall is different.The deeper the thickness,the similarity ratio of Cd²⁺and Pb²⁺is close to 1.It is basically feasible to apply the centrifugal test method to study the migration characteristics of cadmium/lead in the barrier wall.?4?The engineering characteristics of fly ash-bentonite barrier wall were studied.The basic engineering characteristics of the barrier wall were studied by particle analysis,compression test,slump test,etc.The service life of the barrier wall was calculated.The results show that the barrier wall has a porosity of 0.17,a dry density of 1.75 g/cm³,and a slump of 11.3 cm.The maximum compressive strength of the barrier wall is 149.92 kPa after 28 days of optimal curing age.The service life of the barrier wall,calculated according to different standard values of breakdown concentration,is quite different under the same seepage conditions.Both the retardation factor and the thickness of the barrier wall can significantly extend the service life.The service life increases linearly with thickness and retardation factor.?5?The application of fly ash-bentonite blocking wall in contaminated sites was studied.A funnel-type gate vertical barrier wall was installed combined with the hydrogeology,engineering geological conditions of the contaminated site and the status of cadmium pollution in the groundwater.Modflow was used to simulate the migration of cadmium in groundwater before and after the barrier wall was installed.The results show that the maximum migration distance is 628 m and 859 m after the cadmium migration in the groundwater for 1000 d and 4566 d before the barrier wall is installed.The influence range is 0.022 km² and 0.089 km² respectively.After the barrier wall is installed,the migration distance of cadmium in the groundwater is within the effective control range of the barrier wall and the maximum migration distance in the groundwater is 200 m and 310 m for 1000 d and 4566 d.The barrier wall provides an effective barrier to cadmium in groundwater.