Microstructural insights into the hydraulic performance of in situ polymerized bentonites: towards sustainable barrier design for waste containment applications

Abstract

When exposed to chemically aggressive permeants, bentonite-polymer composites often exhibit progressive increases in hydraulic conductivity ( k ) due to polymer elution, thereby compromising their long-term barrier performance in waste-containment systems. In situ polymerized bentonites (IPBs) synthesized with a crosslinker-based polymer formulation have been developed to improve chemical compatibility and mitigate polymer elution. However, the mechanisms governing their sustained low k and non-eluting behavior under aggressive chemical environments are not well understood. The present study investigated the mechanisms responsible for the long-term superior hydraulic performance of IPBs through integrated macro- and micro-scale analyses. The IPBs containing different polymer dosages were permeated with AlCl 3 solutions over a wide concentration range, with comparative assessments performed using identical concentrations of NaCl and CaCl 2 solutions. The role of the crosslinker-based polymer formulation in governing clay–polymer interactions, pore structure evolution, and hydraulic response was assessed through advanced characterization and microstructural analyses. Results showed that the IPBs consistently maintained low k values, up to three orders of magnitude below the regulatory threshold limit of 1×10 –9  m s –1 , without exhibiting polymer elution during long-term permeation. The enhanced chemical resilience and non-eluting behavior of IPBs were attributed to the formation of a stable three-dimensional crosslinked polymer network, which strengthened clay-polymer bonding and restricted polymer mobility. The polymer interacted with clay through surface adsorption, without inducing intercalation into the clay lattice. The infrared absorption spectra confirmed a dual binding mechanism involving esterification and hydrogen bonding. Morphological analyses revealed heterogeneous microstructural evolution driven by variations in polymer dosage and pore-fluid chemistry. Based on the microstructural evidence, a multiscale mechanistic framework involving pore-clogging, polymer bridging, and encapsulation was established. The findings of the study provided new mechanistic insights for designing chemically resilient, sustainable, low- k barrier materials to ensure long-term environmental protection.