Geosynthetic clay liners (GCL) consisting of bentonites are used globally to prevent the permeation of hazardous waste landfill leachates (HWL) into the soil and subsequently into the groundwater. However, the swelling of bentonite in hazardous waste leachate decreases with respect to pollutant loading, resulting in a significant reduction in GCL’s effectiveness as a barrier. This research proposes a polymerization method for sodium bentonite (NaB) to enhance its barrier properties. The polymerization process was developed specifically to improve the effectiveness of GCL in adverse conditions. The HWL collected from a hazardous waste dumping site was analyzed for its physical properties and heavy metals to prepare the synthetic hazardous waste leachate (SHWL), which was used as a permeation liquid for the tests. The NaB was effectively intercalated and grafted with sodium polyacrylate (PAM) in two ways (Dry and Wet), which resulted in the formation of layered structures in the polymer-modified bentonite (PMB). The swell index of Dry and Wet PMBs in SHWL was 1.63 and 1.9 times higher, respectively, compared to NaB. Consequently, both Dry and Wet PMBs exhibited excellent barrier characteristics with hydraulic conductivity to SHWL, 4.87 × 10–10 m/s and 3.88 × 10–10 m/s, respectively, being three orders of magnitude lower than that of NaB (3.08 × 10–07 m/s). In addition, the polymer elution rate of Wet PMB was only 0.15% compared to that of Dry PMB.