Hydraulic performance of polymer-amended bentonite for containment of zinc-contaminated groundwater

Abstract

Zinc (Zn) is a prevalent contaminant, with the highest concentration and exceedance rate among heavy metals in groundwater surrounding tailing ponds in China. Bentonite is the primary material used for soil-bentonite vertical cutoff walls, which are essential for preventing the migration of contaminated groundwater. This study investigates the swell and hydraulic performances of guar gum (GG), polyacrylamide (PAM), and carboxymethyl cellulose (CMC) amended bentonite in Zn-contaminated groundwater, aiming to identify the optimal polymer amendment. Modified fluid loss (MFL) tests were conducted to compare the hydraulic conductivity (k) of unamended and amended bentonite slurries in deionized water (DW) and Zn(NO₃)₂ solutions at various concentrations. The free swell index (FSI) of the bentonites was measured through free swell tests. Additionally, the effect of polymer dosage on FSI and k was examined, and the results were compared with those of unamended and polymer-amended bentonites reported in previous studies. The results revealed that the FSI of the amended bentonites was higher, while their k was lower compared to unamended bentonite across all concentrations of Zn(NO₃)₂ solutions. Notably, CMC-amended bentonite (CB) exhibited the lowest k, which was 1 to 2 orders of magnitude lower than that of unamended bentonite. The swell and hydraulic performances of CB improved with increasing dosage under various concentrations of Zn(NO₃)₂ solutions. Furthermore, a clear linear negative correlation was observed between k and FSI for CB at lower dosages. These findings are of great practical significance for constructing cutoff walls in contaminant containment project for Zn-contaminated sites.