Insights into microplastic migration through polymer-modified geosynthetic clay liners under dry-wet cycles

To elucidate the hydraulic conductivity and polyethylene (PE) migration behavior of polymer-modified geosynthetic clay liners (GCLs) under dry-wet cycling, this study investigated xanthan gum (XG)-modified GCLs (XG GCLs) with XG loadings ranging from 3% to 12%. Flexible-wall permeameters were used to perform six dry-wet cycles with simulated landfill leachate (SLL). Multiscale analyses were conducted, including quantitative crack image processing, residual polymer determination, and SEM-EDS microstructural characterization. The results showed that successive dry-wet cycles gradually expanded surface cracks, reduced residual XG loadings, and caused a stepwise increase in hydraulic conductivity. The combined evidence indicated that the coupled effects of flow path enlargement and polymer elution led to the deterioration of XG GCL's barrier performance. PE was not detected in the effluents of uncycled XG GCLs during the stable period; however, under the present experimental conditions, PE became consistently detectable in all XG GCL effluents after three dry-wet cycles, with PE concentrations increasing progressively as the number of cycles increased. SEM-EDS observations revealed that PE was mainly distributed within the XG hydrogel regions lining the walls of preferential flow paths. Under the combined influence of crack expansion and accelerated polymer elution induced by dry-wet cycling, PE was entrained by the XG hydrogel and transported through the XG GCL. The findings reveal the risks of barrier performance deterioration and PE breakthrough under dry-wet cycling, underscoring the need for risk-informed design, long-term monitoring, and maintenance when using XG-modified GCLs in landfill barrier systems.