Enhanced chromium removal from contaminated groundwater using modified bentonite-sand liners: Experimental and simulation insights

Chromium and its compounds, as industrial solid wastes featuring high storage capacity but low utilization rate and high environmental risk. Therefore, a liner composed of cetyltrimethylammonium bromide modified bentonite (CMB), polymer modified bentonite (PMB), and sand was developed to remove chromium pollution in groundwater of chromium slag landfills. Microscopic characterizations and molecular dynamic simulations were employed to elucidate the modification and adsorption/impermeability mechanisms of CMB and PMB. The cetyltrimethylammonium (CTMA⁺) chains were intercalated into the interlayer space of NaB, with the adsorption mechanism attributed to electrostatic interactions between the head groups of CTMA⁺ and CrO₄^2-. SEM-EDS revealed that PMB exhibited an enhanced three-dimensional network structure woven by entangled polymer chains and the orthogonally aligned stacks of bentonite plates prevented NaB from undergoing cation exchange, a key mechanism for impermeability enhancement. Five long-term column tests were conducted to determine the transport parameters of chromium in the modified liner. The hydraulic conductivity of the single-liner CPBS and double-liner D-PC was 2.18 × 10^-10 m/s and 2.50 × 10^-12 m/s, respectively, meeting the typical requirement of hydraulic barriers for contaminant containment. Analytic calculations were employed to evaluate the service life of CPBS and D-PC liners under varying hydraulic gradients in chromium-contaminated groundwater. Notably, the D-PC exhibits minimal sensitivity to changes in hydraulic gradient, with a required thickness of approximately 0.09 m for a 50-year service life, indicating broad prospects in control of environmental risks associated with groundwater contamination at polluted sites.