Membrane behavior of clay considering the effect of fixed charges

Membrane efficiency coefficient of clay is evaluated with considering the effect of fixed charges adsorbed on clay mineral surfaces. By virtue of the concept of chemical potential, the ionic concentration of pore water is calculated. An equation is first proposed to calculate the Donnan osmotic pressure based on the activity of water (H₂O), and then a new method is developed to determine the membrane efficiency coefficient, based on the theoretical chemo-osmotic pressure difference. The proposed method is used to calculate the membrane efficiency coefficients of geosynthetic clay liners (GCLs) with different bentonite contents and porosities under different KCl concentrations. The calculated results are compared to those of van't Hoff equation, showing that if skeletal deformation is excluded, the proposed model and van't Hoff equation with average ion concentration difference yield practically the same results; if the deformation is considered, however, van't Hoff equation yields smaller membrane coefficients.

Hydraulic conductivity of novel geosynthetic clay liner to bauxite liquor from China: Modified fluid loss test evaluation

A modified sodium bentonite geosynthetic clay liner (GCL) designed for acid-and-alkaline resistance was evaluated for its potential application in the containment of bauxite residue leachate. A modified fluid loss test was employed to quickly evaluate the hydraulic conductivity (k) of the GCL using distilled water, tap water, and four bauxite liquors (BLs, leachate from bauxite residue reservoirs). The effects of swelling capacity of bentonite, prehydration, hydraulic gradient (i), ionic strength (I), and relative abundance of monovalent and multivalent cations (RMD) on the hydraulic conductivity of the GCL were analyzed. The results indicated that the BLs significantly decreased free swell index of the bentonite. As compared to increasing i, prehydration obviously enhanced hydraulic performance of the GCL. The four BLs increased k of the GCL by a factor of 4-12 relative to the tap water permeation condition, and the resultant k exceeded upper limit of 5.0 × 10^-11 m/s for GCLs. The increase in k was attributed to compression in diffuse double layer of the bentonite and dissolution in clay minerals in ion-rich and hyperalkaline BLs, manifesting that further modification on the GCL is needed. The I was found a better indicator than the RMD on correlation with chemical compatibility of the GCL.

Effect of overburden confining stress on hydraulic performance of geosynthetic clay liners (GCLs)

Geosynthetic clay liners are a rapidly evolving geosynthetic product used in most hydraulic barrier applications in the geo-environmental industry. Continuous research has led to new insights to overcome the shortcomings faced in deploying GCLs in the field. These include shrinkage due to shear failure on side slopes, the effect of temperature variation, and inadequacy of minimum timely confinement to achieve optimum hydraulic performance. This paper presents previous experimental data and an additional dataset from this research gathered to observe the effect of overburden confining stress on GCL hydraulic conductivity and how the findings can be used to predict the performance of a geosynthetic clay liner for a given field application. An inverse power relationship is identified between these two parameters along with the reduction in the order of the degree of hydraulic conductivity depending on the permeant material passing through. A relationship is determined to estimate the GCL hydraulic conductivity as a function of the overburden confining stress, given that it is pre or post hydrated and the permeant liquid passing through the product. It is proposed that the relationship can be used to predict the GCL hydraulic performance in the field and provide guidance in improving the serviceability of hydraulic barrier designs.

An analytical model for solute transport through a GCL-based two-layered liner considering biodegradation

An analytical model for solute advection and dispersion in a two-layered liner consisting of a geosynthetic clay liner (GCL) and a soil liner (SL) considering the effect of biodegradation was proposed. The analytical solution was derived by Laplace transformation and was validated over a range of parameters using the finite-layer method based software Pollute v7.0. Results show that if the half-life of the solute in GCL is larger than 1 year, the degradation in GCL can be neglected for solute transport in GCL/SL. When the half-life of GCL is less than 1 year, neglecting the effect of degradation in GCL on solute migration will result in a large difference of relative base concentration of GCL/SL (e.g., 32% for the case with half-life of 0.01 year). The 100-year solute base concentration can be reduced by a factor of 2.2 when the hydraulic conductivity of the SL was reduced by an order of magnitude. The 100-year base concentration was reduced by a factor of 155 when the half life of the contaminant in the SL was reduced by an order of magnitude. The effect of degradation is more important in approving the groundwater protection level than the hydraulic conductivity. The analytical solution can be used for experimental data fitting, verification of complicated numerical models and preliminary design of landfill liner systems.