Hydraulic conductivity, viscosity and polymer elution of polymer-modified bentonite

Polymer-modified bentonites have been widely employed as engineered barrier materials due to their favourable chemical compatibility and barrier properties. This study investigated the slurry viscosity, polymer elution, hydraulic conductivity (k), heavy metal adsorption and microstructural characteristics of bentonites modified with two anionic polymers and one non-ionic polymer exposed to Pb(NO3)2, Cd(NO3)2 and CaCl2 solutions. The results showed that hydroxypropyl methylcellulose-modified bentonite (HB) exhibited the lowest k, together with relatively high residual polymer loading and low polymer loss. Polymer elution and slurry viscosity were strongly affected by polymer charge and solution chemistry. Lower polymer elution was generally associated with higher slurry viscosity. Hydraulic conductivity was moderately negatively correlated with residual polymer loading and positively correlated with polymer loss percentage. The Langmuir model and pseudo-second-order kinetic model provided better fits to the equilibrium and kinetic adsorption data, respectively. Among the polymer-modified bentonites, HB exhibited the highest fitted adsorption capacity for Pb2+ and Cd2+. Scanning electron microscopy images showed that polymer amendment altered the particle arrangement and pore morphology of bentonite. After exposure to Cd(NO3)2 solution, HB still exhibited a relatively continuous polymer-rich phase associated with bentonite particles, whereas the anionic polymer-modified bentonites showed more aggregated and less continuous polymer–bentonite structures.