Diffusion through sodium and polymer enhanced bentonites exposed to dilute and aggressive solutions

Abstract

Chemical incompatibility between sodium bentonite (NaB) and aggressive waste solutions has led to the development of enhanced bentonites for geoenvironmental applications. Enhanced bentonites, such as bentonite-polyacrylic-acid composite (BPC), have been shown to maintain low enough values of hydraulic conductivity (e.g., <10−10 m/s) for diffusion to be the dominant transport mechanism, even upon exposure to aggressive solutions. However, quantification of diffusion properties of enhanced bentonites has been limited. In this study, apparent diffusion coefficients (Da) for chloride were measured for NaB and polymer-enhanced NaBs. Diffusion tests were performed using dilute (5 mmol/L) to aggressive (100 mmol/L) calcium chloride (CaCl2) solutions. In addition, scanning electron microscopy was performed to support interpretation of diffusion results. For CaCl2 solutions <100 mmol/L, Da values for BPC were lower (by approximately half an order of magnitude) than Da values for NaB. However, differences in Da due to polymer enhancement diminished as CaCl2 concentration increased. Predicted steady-state flux through a BPC geosynthetic clay liner (GCL) was dominated by diffusion and up to two orders of magnitude lower than flux through an NaB GCL. These results provide insight regarding diffusion in polymer-enhanced bentonites and expected performance of containment barriers with enhanced-bentonite GCLs.