Influence of cation valence on sol-gel transition of xanthan gum and gas permeability of compacted biopolymer-amended soils

Biopolymers are increasingly being employed to reduce engineered barrier permeability and to control the migration of contaminants in soils or groundwater. This study evaluated the effects of sol-gel conversion in pore fluid on the gas permeability of compacted biopolymer-amended clayey soil. The sol-gel conversion was implemented using divalent calcium (Ca 2+ ) and trivalent chromium (Cr 3+ ) ions as bridging cations and xanthan gum (XG) as the source material of gel matrix. The chemical features and morphology of XG gels were characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, zeta potential, and particle size measurements. The strength and viscosity of XG gels were assessed. The results showed that the strength of XG gels initially increased and then decreased as concentration rose from 10 to 200 mM. With Ca 2+ and Cr 3+ concentrations of 50 mM and 10 mM, respectively, the viscosity of the 0.5 wt.% XG gel increased to 35 Pa s and 87 Pa s, which are approximately 2 and 5 times larger than that of the 0.5 wt.% XG solution (18 Pa s). Additionally, the gas permeability tests were conducted on the compacted XG-amended soils immersed in Ca 2+ /Cr 3+ solutions and cured under standard condition. The gas permeability of the XG-amended soil specimen immersed in the 10 mM Cr 3+ solution was measured to be 1.0 × 10 -15 m 2 , which is approximately one-quarter of that of the specimen (4.5 × 10 -15 m 2 ) immersed in deionized water and one-half of that of the specimen immersed in the 10 mM Ca 2+ solution (2.0 × 10 -15 m 2 ), highlighting the role of sol-gel conversion in improving gas barrier performance.