Surfactant-induced cracking and enhanced DNAPL migration in low-permeability kaolin: implications for surfactant-assisted remediation

Dense non-aqueous phase liquid (DNAPL) is a typical toxic groundwater pollutant. Surfactants are widely used to remediate DNAPL-contaminated groundwater, but the impacts of their interactions on DNAPL migration in clay layers remain unclear, which greatly increases uncertainty in site characterization and remediation. This study examined the interactions of Tween 80 (T80) and Monochlorobenzene (MCB) with kaolin to simulate aquitard conditions. Partitioning, adsorption, cracking, and pore-scale migration experiments were performed to evaluate MCB and T80 adsorption on kaolin, clay structural modifications, and preferential pathway formation. Results showed that when T80 concentrations reached 1000 mg/L or higher, interfacial aggregates formed and initially inhibited MCB dissolution. MCB adsorption capacity on kaolin increased linearly with increased T80 adsorption capacity below thresholds (46.78 and 16.16 mg/g at pH 3 and 7, respectively). Beyond thresholds, competition between dissolved and adsorbed T80 for MCB adsorption caused a decline in MCB adsorption. Characterization results showed that MCB and T80 adsorption increased kaolin hydrophobicity, induced particle agglomeration, and caused pore volume rearrangement. These changes depleted portions of free and bound water, thereby promoting crack formation and preferential pathway development in kaolin. Cracking experiments showed that maximum crack ratio and total crack length reached 2.06% and 67.1 mm at 10 mg/L T80 and pH 3. Cracking was inhibited due to interfacial aggregates formation at higher T80 concentrations. These results demonstrate that surfactants can substantially alter the retention, migration pathways, and DNAPL mobility in clay-rich media, with important implications for contaminant fate assessment, site characterization uncertainty, and groundwater remediation strategies.