A new hydraulic barrier with the gradient distribution of fixed net negative charges

Abstract

Bentonite, commonly used for geological disposal of high-level radioactive waste and landfill liners, can experience increased permeability under high groundwater salinity, compromising its effectiveness. This article presents a novel approach to enhance the anti-seepage performance of hydraulic barriers, particularly in hypersaline environments. The proposed design incorporates a gradient distribution of fixed net negative charges, based on the principles of the Donnan equilibrium and generalized Darcy's law. The Donnan equilibrium, caused by fixed net charges on clay particles, alters the salt concentration of pore water that seeps through the soil. This creates a semipermeable membrane effect on the clay, leading to a concentration difference of salts between the flowing water in the soil and the external water supply. The concentration difference is dependent on the density of fixed net negative charges on the clay surfaces. By artificially controlling the distribution of fixed net negative charges, a salinity gradient is induced in the pore water. Based on the generalized Darcy's law, the salinity gradient can counteract seepage driven by pressure gradients and effectively reduce the seepage velocity. The study discusses key factors affecting the gradient distribution of fixed net charges in the designed hydraulic barrier, including void ratio, external solution concentration, and cation exchange capacity. The new hydraulic barrier is compared with existing methods (traditional bentonite and polymer–bentonite mixtures), demonstrating the effectiveness of the proposed method in enhancing anti-seepage performance, especially in hypersaline environments.