Study on barrier mechanism and performance of polymerized bentonite-amended sand cut-off wall

Polymer-amended bentonite is a kind of high-performance barrier material increasingly used in subsurface containment systems. This study investigates the barrier mechanism and performance of polymerized bentonite-amended sand backfills, including sodium carboxymethyl cellulose-modified bentonite (CMC-B) and sodium polyacrylate-modified bentonite (BPC), for use in vertical cut-off walls to control lateral migration of contaminants. Batch tests, cycled column experiments and dual-region models were conducted to assess the effectiveness of the polymerized bentonite–sand cutoff wall. Both sand–CMC-B and sand–BPC mixtures demonstrated higher lead adsorption capacities. For example, adding 5% CMC-B or BPC increased adsorption capacity by 1.2 times compared to sand–ordinary bentonite (SN). The permeability (kv) of sand–CMC-B and sand–BPC mixtures was over ten times lower than that of SN, and both met anti-seepage standards at all pressures (kv < 10−9 m/s). However, CMC-B showed poorer chemical compatibility, with permeability increasing 11-fold after lead exposure. Column tests and modeling confirmed BPC’s better preformation and retardation. The dual-region model indicated that increasing the thickness of cut-off wall from 0.8 m to 1.2 m may extend the breakthrough time by over twofold, highlighting the critical role of the thickness of cut-off wall in long-term performance.