Hydraulic Conductivity of Sand/Biopolymer-Amended Bentonite Backfills in Vertical Cutoff Walls Permeated with Lead Solutions

This study investigated the hydraulic conductivity and microscopic properties of two backfills, conventional sand/bentonite (SB) and sand/xanthan gum (XG)-amended bentonite/sand (XG-SB), for use in vertical cutoff walls for controlling lateral migration of lead-contaminated groundwater. A series of laboratory experiments were conducted on the two backfills to assess slump height, specific gravity, liquid limit, and hydraulic conductivity using distilled water, tap water, and lead nitrate [Pb(NO3)2] solutions at different concentrations. The results showed that the moisture content corresponding to the target slump height was higher for XG-amended backfill as compared with unamended backfill. XG amendment slightly decreased specific gravity (Gs), but substantially increased liquid limit (wL) of the backfill. An increase in concentration of Pb(NO3)2 solution increased Gs, but decreased wL for both unamended and XG-amended backfills. The hydraulic conductivity (k) of both backfills increased with increasing concentrations of Pb(NO3)2 solutions. XG-amended backfill hydraulic conductivity (k) was found to be less than 10−9 m/s regardless of type of permeating liquid, whereas unamended backfill k was found to be higher than 10−9 m/s when permeated with Pb(NO3)2 solutions. Scanning electron microscopy (SEM) coupled with energy-dispersive spectrometry (EDS) analyses indicated that XG hydrogels filled the intergranular pores of XG-amended backfill and formed a thin coating over the bentonite particles and bentonite-coated sand granules. The X-ray diffraction (XRD) results showed no intercalation of XG into montmorillonite platelets. The zeta potential of XG-amended bentonite was higher negative relative to unamended bentonite. The microscopic properties elucidated the mechanisms for superior hydraulic performance of XG-amended backfill. A comprehensive comparison of k for conventional backfill and polymer-amended backfills based on this study and previously published studies revealed that the bentonite content, bentonite type, effective confining stress, and cation concentration of a permeating liquid can significantly affect the k of the backfills.