Solute Diffusion through Bentonite-Polymer Composites for Containment Applications

Abstract

Bentonite-polymer composites (BPC) have been developed for use in geoenvironmental containment projects where sodium bentonite (NaB) is likely to undergo adverse chemical interactions that degrade barrier performance. Although numerous studies have investigated hydraulic conductivity of BPCs, very little is known about diffusion rates through BPC. Thus, this experimental study measured solute diffusion through different BPC materials for calcium chloride (CaCl2) source solutions ranging in concentration (Co) from 10 to 100 mM. The five materials evaluated in the study were NaB (without polymer amendment), BPC 1%–4% and BPC 1–8% (BPC type 1 with 4% and 8% polymer, respectively), and BPC 2%–4% and BPC 2%–8% (BPC type 2 with 4% and 8% polymer). For dilute CaCl2 solutions, all four BPCs exhibited lower Da values (by half an order of magnitude) than the unamended NaB. For higher CaCl2 concentrations (Co = 100 mM) the BPC2 mixes still exhibited superior performance (lower Da) relative to NaB, whereas the BPC1 mixes exhibited similar or in some cases higher Da values than the unamended NaB. The results of the study enhance our ability to consider and predict the expected performance of containment barriers comprising either NaB or BPC for a range of ionic strengths.