A Study On Heavy Metals Transport In Landfill Liners And Performance Of Evapotranspiration Covers

The cover system and landfill liner are two important barrier systems which affect the leachate generation and the migration of contaminants. Compacted clay liners and compacted clay covers are used in landfill for common. Many kinds of contaminants can be adsorpted by Clay. However, the adsorption capacity of clay is relatively low. Granuar activated carbon, acid-activated bentonite and loess are added to clay to improve its adsorption capacity. The adsorption characteristics of heavy metals on clay and modified clay are studied by a series of batch tests, and the critical soil solids concentrations and the corresponding adsorption parameters are determined. The hydrodynamic dispersion coefficients and diffusion coefficients of contaminants transport through clay and modified clay are estimated according to column model tests and the horizontal diffusion tests. Three evapotranspiration covers are built with large-scale laboratory tank. Rainfall simulator is used to simulate the actual rainfall on the base of weather data. Real-time data of temperature and water content at different depths of the cover are collected with temperature and moisture sensors buried in soil. Parameters such as plants, precipitation intensity and initial conditions are analysed. A series of meaningful conclusions are obtained through the tests and numerical analyses mentioned above.1. The adsorption of heavy metal Cd(Ⅱ) on loess-modified clay occurs within a fairly short time of30min. The unit equilibrium adsorption amount decreases as soil solids concentration increases; Cd(Ⅱ) removal increases as the initial pH of the solution increases; the amount of unit equilibrium adsorption increases with increasing temperature. The pseudo-second order kinetics fit the test data of Cd(Ⅱ) adsorption best with the highest correlation coefficients. The Langmuir model was found to show the best-fit to the test data. Based on the analysis with the Langmuir isotherm model, the maximum Cd(Ⅱ) adsorption capacities of loess-modified clay were estimated to be5.173mg/g,5.735mg/g,6.193mg/g and7.077mg/g as temperatures ranging from298K to313K,323K and333K, respectively. Thermodynamic studies have shown that the adsorption process is spontaneous and endothermic. The activation energy of adsorption is about14.95kJ/mol, which shows the adsorption process belongs to activated chemical reaction.2. Granuar activated carbon, acid-activated bentonite and loess are added to clay to improve the adsorption capacity of clay liner. Batch test data show that the adsorption capacities of Cd(Ⅱ) on modified clay have been significantly improved, indicating the method of using absorbent to modifiy clay liner in landfill is effective. The unit equilibrium adsorption amount decreases as soil solids concentration increases. The critical soil solids concentrations of clay and modified clay are found to be320g/L,280g/L,280g/L and280g/L. The values of Langmuir adsorption parameter qm of clay and modified clay decrease as soil solids concentrations increase.3. The hydrodynamic dispersion coefficients and diffusion coefficients of Cd(Ⅱ) through clay and modified clay are back-calculated with VS2DI on the base of test data. The dispersion coefficients of Cd(Ⅱ) through clay, clay containing6%Granuar activated carbon, clay containing6%acid-activated bentonite and clay containing6%loess are6.0×10-10m2/s,5.4×10-10m2/s,5.1×10-10m2/s and4.6×10-10m2/s from column tests. The diffusion coefficients of Cd(Ⅱ) are4.8×10-10m2/s,4.3×10-10m2/s,4.1×10-10m2/s and3.7×10-10m2/s from horizontal diffusion tests.4. It is shown that the horizontal diffusion device used in this paper is effective to analyze the diffusion of contaminant through clay soil. The concentration of source solution reduces gradually with time, because some ions diffuse into the pore solution, the others are adsorbed by soil. The diffusion coefficient of Cr(Ⅵ) in Dalian red clay varies from1.50×10-11m2/s to2.08×10-11m2/s. After1000d diffusion, the concentration of source solution drops down to1.27mg/L from62.5mg/L, and the diffusion distance is only3.5cm. Numerical simulation is made with VS2DI. The calculation results show that when diffusion coefficient is constant, the diffusion effect becomes more obvious with lower density, lower temperature, higher pH value, and much more time.5. Laboratory model test results show that:these three proposed evapotranspiration covers can store rainfall during strong precipitation and release pore water of soil in clear day effectively. The shallow biobarrier evapotranspiration cover shows the best performance. Laboratory test data fit well with numerical calcualtion values. Numerical simulation results show that:Plants play an important role in water balance of evapotranspiration cover system. After24-day simulation cycle, water contents in evapotranspiration covers are higher with higher initial water contents of soil. Evapotranspiration covers perform better when the ratio of permeability coefficient between sand and silty clay is larger. It is useful to increase the initial porosity of soil for improving the performance of evapotranspiration cover, and increasing the porosity of silty clay has more obvious effect than sand. The shallow biobarrier evapotranspiration cover can effectively control water infiltration and evapotranspiration under the precipitation and evapotranspiration boundary conditions during flood season of Dalian in1976.