Biopolymer-amended geosynthetic clay liners for landfills and contaminated sites: Gas and hydraulic performance

Gases from landfills and industrial contaminated sites adversely affect human health and the climate. Geosynthetic Clay Liners (GCLs), consisting of two geotextiles and bentonite, are widely used as a gas barrier material. However, studies on the gas permeability of biopolymer-amended bentonite as the GCLs core are limited, especially cation exchange occurs during hydration of GCL with natural soil pore water. Free swell index, gas permeability, hydraulic conductivity tests and microscopic tests were conducted to evaluate the gas/hydraulic performance of GCL. This study evaluated the impact of cation exchange on the liquid limit and free swell index of xanthan gum (XG)-amended sodium-activated calcium bentonite and its consequent impact on gas permeability of XG-amended GCL (XG-GCL). The gas permeability of XG-GCL with 1 % XG dosage hydrated with salt solution preserved low value of 6.2 × 10^-15 m² at apparent degree of saturation of 80 %, which was approximately one-eighth of the value measured in unamended GCL (4.7 × 10^-14 m²). When hydrated and permeated with salt solution, the hydraulic conductivity decreased from 2.6 × 10^-10 to 3.8 × 10^-11 m/s as XG dosage increased from 0 to 2 %. The findings demonstrated that XG dosage no less than 1.0 % could maintain low gas permeability and acceptable hydraulic conductivity of XG-GCL when exposed to salt solution. The enhanced gas/hydraulic performance were attributable to: (1) the XG hydrogels effectively filled the inter-aggregate pore within GCL core; and (2) the carboxyl groups on XG side chains interacted with Ca²⁺, forming ionic cross-linking, which effectively mitigated the adverse effects of Ca²⁺ on the diffuse double layer of montmorillonite. This study provides new insights into the role of biopolymer amended-GCL performance exposed to natural soil pore water.