GEOSYNTHETIC CLAY LINERS, PART SIX: ION EXCHANGE

This paper examines the properties of geosynthetic clay liners (GCLs) and explores the chemistry of bentonite: how it is structured, how it works with GCL, and how it reacts to soil contents over time. The chemical composition of the clay (bentonite) is directly related to the hydraulic conductivity (permeability) of the GCL. High quality bentonites, which are used in GCLs, consist mainly of three-layer mineral montmorillonite of the smectite group. Bentonites used for industrial applications are divided into sodium and calcium categories, depending upon which exchangeable cation is dominant. Because of a higher-swelling capacity, the sodium-type bentonites are used in GCLs. In addition to covering the chemistry of bentonite this paper discusses the differences between calcium, sodium, and polymer-enhanced bentonites. The ion-exchange capability of bentonite is a natural process and, under typical use conditions and without covering the GCL with a geomembrane, it normally transforms original Na-bentonite into Ca-bentonite. The result is a slight change in the GCL's permeability.