Membrane behavior of clay under mixed solution conditions

Compacted clay is employed as the buffer material for landfills, and multiple ions are dissolved in the leachate restricted by the compacted clay layer. The membrane efficiency is an important indicator to assess the barrier properties of the compacted clay layer and is measured through membrane tests. However, most membrane tests are currently conducted with a single solute solution, which does not reflect the mixed solution characteristics of leachates. To assess the membrane efficiency of compacted clay under mixed solution conditions, 13 membrane tests were conducted on a bentonite-amended soil using KCl-NaCl mixed solutions, KCl-CaCl2 mixed solutions, and KCl-AlCl3 mixed solutions with different mixing ratios at a total concentration of 20 mM. Nuclear magnetic resonance (NMR) tests were conducted on the soil specimen after the membrane tests to investigate the micromechanism of the membrane behavior under mixed solution conditions. Results indicate that the membrane efficiency increased with the mixing ratio of Na+ but decreased with the mixing ratio of Ca2+ or Al3+. In the 13 membrane tests, the lowest membrane efficiency was achieved when the specimen was tested with pure AlCl3 solution. The relationship between the membrane efficiency and mixing ratio was also investigated at the microscopic scale. As the ion valence increases, the diffuse double layer thickness is smaller and the proportion of macropores is larger, as verified by NMR tests.

Three-dimensional fully coupled hydro-mechanical-chemical model for solute transport under mechanical and osmotic loading conditions

Mechanical deformation and chemico-osmotic consolidation of clay liners can change its intrinsic transport properties in all direction and can alter fluid and solute transport processes in the entire model domain. These phenomena are described inadequately by lower-dimensional models. Based on the Biot's consolidation theory, fluid and solute mass conservation equations, a three-dimensional (3D) fully-coupled hydro-mechanical-chemical (HMC) model has been proposed in this study. The impacts of mechanical consolidation and chemico-osmotic consolidation on permeability, hydrodynamic dispersion, solute sorption, membrane efficiency, and chemical osmosis are considered in the model. The model is applied to evaluate performances of a single compacted clay liner (CCL) and a damaged geomembrane-compacted clay composite liner (GMB/CCL) to contain a generic landfill contaminant. Effect of model dimensionality on solute spread for CCL is found to be marginal, but for GMB/CCL the effect is significantly large. After 50-year simulation period, solute concentration at the half-length of the GMB/CCL liner is predicted to be 40% of the source concentration during 1D simulation, which is only 6% during the 3D simulation. The results revealed approximately 74% over-estimation of liner settlement in 1D simulation than that of the 3D for GMB/CL system. Solute spread accelerates (over-estimates) vertically than horizontally since overburden load and consequent mechanical loading-induced solute convection occurs in the same direction. However, in homogeneous and isotropic soils, horizontal spread retards the overall migration of contaminants, and it highlights the importance of 3D models to study solute transports under mechanical and chemico-osmotic loading conditions in semi-permeable clays, especially, for damaged geomembrane-clay liners. The results show the utility of geomembranes to reduce soil settlement, undulation, and restriction of solute migration. Furthermore, application of geomembrane can inhibit development of elevated negative excess pore water pressure at deeper portion of a clay liner.

Landfill gas emission through compacted clay considering effects of crack pathway and intensity

Compacted clay barrier plays an important role in reducing landfill gas transport due to its low gas permeability. There is limited understanding of desiccation cracks and to what extent they can cause preferential pathways of landfill gas through compacted clay barriers. This study investigated the intensity and pathway of desiccation cracks as well as its effects on gas emission through compacted clay. The compacted clay with and without scratched compaction interface was subjected to drying to simulate desiccation cracks. The clay was then extruded from large containers into one dimensional columns to allow observation of crack propagation using an X-ray computerized tomography scanner. After that, gas emission rate was measured from each column under different gas pressures (i.e., 1, 5, 10 and 20 kPa). Furthermore, a simplified method is proposed to predict gas emission rate with consideration of intensity and characteristics of cracks. Test results demonstrated that desiccation cracks were initiated mainly at the center of each container (i.e., within 40% of container dimension). Gas emission rate can be increased at least 10 times with the presence of desiccation cracks (i.e., at gas pressure of 5 kPa). As compared to the depth and continuous pathway of cracks which significantly increased gas emission rate, the discontinuous crack pathway can reduce the gas emission rate by up to 3 times. The findings towards crack characteristics and gas emission observed in this study are crucial for safety design and long-term operation of compacted clay barriers in landfill covers.

A new method for determination of heavy metal adsorption parameters in compacted clay by batch tests

Evaluation of adsorption properties of pollutants on artificial or natural clay strata is normally considered in investigations of soil and groundwater pollution. Batch adsorption tests can be used to obtain the adsorption parameters of clay particles; however, the results from these tests are usually very different from the adsorption of actual clay strata. If the adsorption parameters obtained by batch tests are used to directly evaluate the properties of adsorption of pollutants onto compacted clay, the predicted groundwater and soil pollution will be unsafe. Although the column diffusion tests are closer to the actual situation, they may require much more time, and diffusion and adsorption occur simultaneously in tests, making it difficult to accurately determine the adsorption parameters. To solve this problem, batch adsorption and column diffusion tests were conducted using three kinds of clay materials to investigate the mechanism of the differences in adsorption properties of heavy metal on clay particles and in compacted clay. The amount of adsorption per unit particle surface area of clay particles was found to be equal to that per unit pore surface area of compacted clay. A new simplified method was proposed to determine the adsorption parameters in compacted clay. It is easy to use and provide a reference for prediction and evaluation of soil and groundwater pollution.

COD (glucose configuration) effects on the non-Darcy flow of compacted clay in a municipal solid waste landfill

Clay liners play a critical role in preventing leachate leakage and pollutant migration from landfills through their low permeability and non-Darcy behavior during seepage, and such liners exhibit a threshold-gradient characteristic. Landfill waste may produce complex, highly concentrated leachates through chemical and biological degradation. The hydraulic conductivity and threshold gradient of a clay liner is affected by high leachate concentrations. Some scholars have suggested that chemical oxygen demand (COD) can be selected as a key indicator for pollution alerts and used to assess the environmental risk posed by municipal solid waste (MSW) landfill sites. To study the influence of leachate concentration on the permeability of compacted clay, the highest concentrations of organic pollutant COD (glucose configuration) were used as the target dialysate in this study. COD is abbreviation of chemical oxygen demand. A COD solution was prepared from dissolved glucose for the experiments. The results showed that as the COD concentration increased, the hydraulic conductivity increased and the threshold gradient decreased. The permeate viscosity and the soil-water characteristic curve were measured. As the COD concentration increased, the permeate viscosity increased and the bound water content decreased. By considering the COD concentration effects on permeate viscosity and intrinsic permeability and adapting a previously established empirical relationship between the threshold gradient and apparent fluidity (K/η), this study derived an equation for calculating the hydraulic conductivity and threshold gradient with changes in the COD concentration, and good predictions were obtained.

Effects of biochar on hydraulic conductivity of compacted kaolin clay

Compacted clay is widely used as capillary barriers in landfill final cover system. Recently, biochar amended clay (BAC) has been proposed as a sustainable alternative cover material. However, the effects of biochar on saturated hydraulic conductivity (k_sat) of clay with high degree of compaction is not yet understood. The present study aims to investigate the effects of biochar on k_sat of compacted kaolin clay. Soil specimens were prepared by amending kaolin clay with biochar derived from peanut-shell at 0, 5 and 20% (w/w). The k_sat of soil specimens was measured using a flexible water permeameter. The effects of biochar on the microstructure of the compacted clay was also investigated using MIP. Adding 5% and 20% of biochar increased the k_sat of compacted kaolin clay from 1.2 × 10^-9 to 2.1 × 10^-9 and 1.3 × 10^-8 ms^-1, respectively. The increase in k_sat of clay was due to the shift in pore size distribution of compacted biochar-amended clay (BAC). MIP results revealed that adding 20% of biochar shifted the dominant pore diameter of clay from 0.01-0.1 μm (meso- and macropores) to 0.1-4 μm (macropores). Results reported in this communication revealed that biochar application increased the k_sat of compacted clay, and the increment was positively correlated to the biochar percentage.