Geosynthetic Clay Liner Interaction with Leachate: Correlation between Permeability, Microstructure, and Surface Chemistry

TLDR

When a geosynthetic clay liner containing sodium bentonite contacts fluids with other cations, those cations can exchange with the interlayer sodium, altering the clay’s surface chemistry, microstructure, and hydraulic conductivity. The study aimed to determine how clay surface chemistry influences microstructure and permeability after prolonged contact between a natural sodium bentonite GCL and a sodium‑activated calcium bentonite GCL with various fluids in oedometer cells. This was examined by measuring exchangeable cations, performing small‑angle X‑ray scattering, and using transmission electron microscopy. Calcium carbonate in the activated GCL redissolved in dilute permeants, releasing Ca²⁺ that fully replaced Na⁺, eliminating the low‑permeability gel phase and forming a more permeable hydrated‑solid phase; while no gel phase appeared in these samples, exposure to real leachate produced a gel phase—especially after prehydration—and hydraulic conductivity correlated with the proportions of gel phase and Na interlayer occupation.

Abstract

When a geosynthetic clay liner (GCL) containing sodium bentonite is brought into contact with fluids containing other cations, the latter may exchange with the sodium present between clay layers. This modification of clay surface chemistry may change the clay microstructure and hence its hydraulic conductivity. The influence of clay surface chemistry on microstructure and permeability, after prolonged contact between two GCLs (a natural sodium bentonite GCL and a sodium-activated calcium bentonite GCL) and different fluids in oedometer cells, was investigated using exchangeable-cation analysis, small-angle x-ray scattering, and transmission electron microscopy. Results suggest that calcium carbonate in the bentonite, formed during activation of the calcium bentonite, may redissolve during contact with a dilute permeant, releasing calcium ions that exchange with sodium in the clay. This exchange leads to obliteration of a so-called "gel" phase (beneficial in terms of low permeability) and to the development of a more permeable "hydrated-solid" phase. Sodium replacement by calcium during GCL contact with a 0.1MCaCl2 solution was found to be virtually complete, with or without GCL prehydration with dilute water. No gel phase was observed in these samples. When in contact with real leachate, however, a gel phase appeared, especially when GCL samples were prehydrated. A correlation was observed between the level of hydraulic conductivity and the relative proportions of gel phase and clay interlayer occupation by sodium.