Study on microstructure and permeability of modified cut-off wall materials under chemical environmental effects

Pb²⁺ in groundwater poses a serious threat to ecosystems and human health. Sand-bentonite cut-off walls are widely used to control the groundwater pollution. However, their long-term impermeability may deteriorate due to chemical interactions. In this study, natural calcium bentonite (CaB) was modified by sodium activation and sodium polyacrylate treatment to obtain sodium bentonite (NaB) and sodium polyacrylate-modified bentonite composites (PBC), respectively. These bentonites were mixed with sand to assess the effects of modification on impermeability and chemical compatibility under Pb²⁺ contamination. Compression consolidation tests, rigid-wall hydraulic conductivity tests, swelling tests, and microstructural analyses were conducted. The results show that adding NaB and PBC significantly improves the hydraulic performance of cut-off wall materials, maintaining hydraulic conductivities below 1 × 10^-9 m/s. Additionally, the minimum hydraulic conductivity (k_c, obtained from consolidation tests) values of sand-NaB and sand-PBC can reach 2.42 × 10^-11 m/s and 2.46 × 10^-11 m/s, respectively. PBC exhibits superior chemical compatibility under Pb²⁺ permeation. At a Pb²⁺ concentration of 10 mM, the sand-PBC material containing 10% bentonite exhibits a hydraulic conductivity (k_s, obtained from rigid-wall hydraulic conductivity test) of 3.24 × 10^-11 m/s, which is only 49.1 and 54.6 times lower than that of the sand-NaB and sand-CaB materials, respectively. Microstructural analysis indicates that Pb²⁺ suppresses the swelling of NaB and increases its hydraulic conductivity, while the polymer network in PBC effectively blocks seepage channels and preserves structural integrity. The findings offer valuable insights into the development of durable cut-off wall materials for sites contaminated with heavy metals.