The unamended bentonite (i.e., Calcium-based bentonite, CB) used in the experiments is a commercial-grade sodium bentonite sourced from Zhenjiang, Jiangsu Province. The selection of CB tested here is mainly to its wide distribution and easy availability in China as compared to the Sodium-based bentonite (NB). Although NB possesses superior engineering performance in slurry wall construction, it has limited stock in China and is relatively expensive. The fundamental physical properties are presented in Table 1, and the particle size distribution curve illustrated in Figure 1. The grain size fractions <75 μm and <2 μm accounted for 100% and 32.5%, respectively. The liquid limit (w _L), plastic limit (w _P), and plasticity index (IP) were determined to be 272.3%, 34.2%, and 233.5%, respectively. The index properties of polymers provided by the manufacturer are summarized in Table 2. The ion types of polymers include anionic and nonionic types. The average molecular weight of all polymers is equal to or greater than 800,000, and the specific gravity of the polymers is less than 1.32, which is lower than that of soil minerals.

Calcium chloride (CaCl₂) solution was used as the simulated contaminated groundwater. The CaCl₂ concentrations were set at 5, 20, 50, and 100 mM, corresponding to Ca²⁺ mass concentrations ranging from 0 to 2,400 mg/L. This range aligns with the groundwater pollutant levels reported in previous literature (Du et al., 2021; Malusis and McKeehan, 2013). Deionized water (DW) served as the control group. The index properties of DW and CaCl₂ solutions are summarized in Table 3. Triplicate samples were tested for determining these five property indexes presented in this table, and thus, a total of 75 trials were made in these tests.

A preliminary experiment was conducted to rapidly identify suitable polymers for modifying bentonite. Five polymer-amended bentonites were prepared by mixing dried bentonite with 2% polymer (by dry weight of CB). The mixtures were poured into hermetically sealed polyethylene bottles and vigorously shaken with an overhead stirrer for 1 h at a speed of 60 r/min and a controlled temperature of 20 °C ± 2 °C. To assess improvements in the constructability of polymer-amended bentonite, slurries were made by blending the polymer-amended bentonite with liquid (tap water or CaCl₂ solution) using a high-speed stirrer for 5 min. The slurries were then stored in plastic bottles for 24 h to hydrate. The engineering properties, including density, marsh funnel viscosity, filtrate volume, and pH, of the bentonite slurries were measured in accordance with API 13B-1 (API, 2009). Target values for the slurry were based on guidelines from USEPA (1984) and USACE (2010). The target values for slurry workability can be found in Table 7 of the following Section 3.2.2.

After thoroughly evaluating the swell index, engineering properties in saline conditions, and hydraulic performance of the bentonite filter cake, xanthan gum was selected as the amendment for further study of polymer-amended bentonite. The preparation process for the Xanthan gum-amended bentonite (XB) mixture followed the method previously described. The polymer dosage varied from 0% to 4% by weight of dry bentonite, as determined by Equation 1. The bentonite dosage (CB), representing the ratio of dry bentonite mass to total slurry mass, ranged from 6% to 10%, calculated via Equation 2. The mass of slurry can be calculated as the following Equation 3. Table 4 outlines the experimental design used to analyze the slurry properties of the amended bentonite. C P = m P m s × 100 % (1) C B = m s m slurry × 100 % (2) m slurry = m s + m P + m w (3) where C _P is Polymer dosage (%), C _B is bentonite dosage (%), m _p is Polymer mass (g), m _s is dry bentonite mass (g), m _w is the mixing water, and m _slurry is the total slurry mass (g).

Before performing the modified filtrate loss test, the slurries were stirred for an extra 2 min. The tests used the API standard filtration apparatus with a 76.2-mm inner diameter, following ASTM D5891 (Chung and Daniel, 2008; Liu et al., 2014; Shen and Wei, 2018). Figure 2 shows the schematic diagram of the modified filtrate loss test. The filtrate volume was recorded within 5 min, while the entire experiment lasted 30 min. As shown in Table 3, the concentrations of CaCl₂ were established at 5, 20, 50, and 100 mM, with tap water (i.e., 0 mM) serving as a control for all samples. Experiments were performed under an applied air pressure of 100 kPa. Following the completion of the test, the thickness of the filter cakes (L) was measured on two occasions utilising a vernier calliper, and the mean values were documented. Subsequently, the filter cakes were meticulously separated from the filter paper to evaluate their water content. This study calculated and reported the average void ratio. The hydraulic conductivity (k) of the bentonite filter cake was measured using the following equation (Chung and Daniel, 2008; Liu et al., 2014; Shen and Wei, 2018): k c = β γ w V t 2 2 P 0 A 2 t = β γ w 2 A 2 φ (4) where γ _w is the unit weight of the chemical solution (kN/m³), V _t is the filtrate volume at time t (mL), and P ₀ denotes the total applied pressure, which corresponds to the air pressure used in this study (kPa); A represents the cross-sectional area of the bentonite filter cake (cm²); φ is the slope of the P ₀ V _t/t–V _t relation curve (kPas/cm⁶); and β is determined using the following Equation 5 (Chung and Daniel, 2008; Liu et al., 2014; Shen and Wei, 2018): β = B C ρ L 1 + e a v e 1 − B C ρ s − e a v e B C ρ L (5) where BC indicates the bentonite dosage of the slurry (%); ρ _L and ρ _s represent the densities of the solution and bentonite (g/cm³), respectively; and e _ave is the average void ratio of the bentonite filter cake.

At the end of the MFL test, a thin layer of a jelly-like bentonite-chemical mixture atop the bentonite filter cake was carefully and quickly removed with filter papers. The filter cake was then placed in an aluminum container for water content analysis. After oven drying, the bentonite was ground into a fine powder to determine its specific gravity according to ASTM D854. The average void ratio was calculated based on the water content and specific gravity, assuming full saturation. The total pressure in Equation 4 was taken as the air pressure, as advised by Chung and Daniel (2008).

Scanning electronic micrograph (SEM) analyses were performed on CB and XB filter cake samples to examine the microstructure of Xanthan gum-modified bentonite before and after exposure to Pb(NO₃)₂ solutions, using a Hitachi SU3500 Scanning Electron Microscope. The bentonite filter cakes were frozen for 5 min utilizing liquid nitrogen and subsequently transferred to a vacuum freeze-drying apparatus for a duration of 24 h at a temperature of −79 °C. Following the freeze-drying process, the primary dry filter cakes were sectioned into small blocks with an approximate surface area of 0.25 cm². These samples were then coated with a thin layer of gold and subjected to SEM analyses.

Figure 3 shows the variation in swell index (SI) of polymer-amended bentonites in CaCl₂ solutions. In deionized water (0 mM CaCl₂), all polymer amendments significantly increased SI values compared to CB. Specifically, the HPMC-amended bentonite demonstrated the most significant improvement, with SI increasing from 15.9 mL/2 g (unamended) to 23.2 mL/2 g. Under CaCl₂ solutions, the SI of all polymer-amended bentonites decreased as CaCl₂ concentration increased. At 20 mM CaCl₂, the CMS-amended bentonite exhibited SI values similar to those of CB. By 50 mM CaCl₂, all amended bentonites displayed SI values (around 7–10 mL/2 g) that matched CB, reflecting the typical swell behavior of calcium bentonite.

These findings align with previous research indicating that high-valence cations like Ca²⁺ decrease swell capacity by shrinking the diffuse double layer, as observed in untreated calcium bentonites. When ionic strength reaches 100 mM CaCl₂, both amended and unamended bentonites exhibit similar SI values of 5–6 mL/2 g, suggesting that polymer amendments are less effective under high ionic strength. Adding high-swell polymers enhances swelling in deionized water; however, increasing the CaCl₂ concentration reduces swelling for both types. The positive effect of polymers diminishes at high salt levels, with SI values ranging from 5 to 6 mL/2 g at 100 mM. This finding aligns with that of Scalia et al. (2014) and Li et al. (2025), suggesting that polymer addition does not significantly enhance seepage resistance in aggressive environments. The decreased effectiveness of polymers at high ionic strength reflects their limited ability to stabilize the structure amid strong electrostatic interactions with divalent cations.

Figure 4 illustrates the changes in Marsh viscosity of polymer-amended bentonite slurries under CaCl₂ solution environment. In deionized water, all polymer amendments increased the Marsh viscosity relative to unamended bentonite; HPMC-amended bentonite exhibited the highest viscosity (53 s). The viscosity values of other amended bentonites ranged between 40 and 50 s, satisfying the workability requirements for bentonite slurry. With CaCl₂ concentration increasing, Marsh viscosity decreased in both unamended and amended bentonites. At 20 mM CaCl₂, the viscosity of unamended bentonite dropped to 38 s (below the required 40–50 s threshold), whereas amended bentonites retained compliance with workability standards. At 50 mM CaCl₂ (Ca²⁺ concentration: 2000 mg/L), PAC- and XG-amended bentonite-maintained viscosities of 43.5 s and 44.6 s, respectively, still meeting workability requirements. Conversely, other amended bentonites fell below 40 s and failed compliance. These results demonstrate that polymer amendment enhances the salt resistance of bentonite slurry workability, PAC and XG amendments exhibit enhanced stability in saline environments.

Figure 5 shows how fluid loss varies in polymer-amended bentonite slurries exposed to CaCl₂ solutions. Both unamended and polymer-amended slurries show a steady increase in fluid loss as CaCl₂ concentration rises. However, at the same concentrations, polymer-amended slurries consistently have lower fluid loss than unamended bentonite. At 20 mM CaCl₂, unamended bentonite exhibited excessive fluid loss (58 mL), well above the engineering workability threshold (<25 mL), while all polymer-amended slurries kept fluid loss below 25 mL, meeting operational standards. At 50 mM CaCl₂, only PAC- and XG-amended slurries remained within limits, with fluid losses of 16.8 mL and 14.6 mL, respectively. By 100 mM CaCl₂, all slurries (amended and unamended) showed significant fluid loss (>50 mL), with XG-amended bentonite having the lowest value (still over 50 mL). Mud-water separation occurred within 3–5 min after mixing, consistent with the sedimentation behavior reported by Yang et al. (2020). This indicates that all bentonite slurries lose workability at this concentration, showing the limits of polymer amendments in reducing fluid loss under high ionic strength conditions.

Figure 6 presents the hydraulic conductivity of filter cakes from polymer-amended bentonite slurries exposed to CaCl₂ solutions. Both amended and unamended bentonite slurries are influenced by Ca²⁺ to different extents, as evidenced by their inability to form stable filter cakes above certain CaCl₂ concentrations, along with a significant increase in fluid loss and visible mud-water separation after slurry standing. The hydraulic conductivity of filter cakes from both unamended and amended bentonites rises with increasing CaCl₂ concentration. However, polymer-amended bentonites exhibit one order of magnitude lower hydraulic conductivity than unamended bentonite. At the same concentrations, PAC- and XG-amended bentonites have the lowest conductivity values among all amendments, consistent with their superior fluid loss performance observed in previous tests. Unamended bentonite fails to form a filter cake at CaCl₂ concentrations above 5 mM, whereas polymer-amended counterparts maintain structural integrity until concentrations exceed 50 mM. This indicates that polymer amendment greatly enhances resistance to saline erosion and prevents structural collapse.

Through a comprehensive evaluation of swell index, workability (i.e., fluid loss), and hydraulic performance of bentonite slurry, the optimal polymer was selected for subsequent investigation; the test results are summarized in Table 5. Herein, c _max is defined as the threshold concentration of CaCl₂ solutions at which polymer-amended bentonite maintains workability criteria (Marsh viscosity ≥40 s; fluid loss ≤25 mL). A higher c _max value signifies superior chemical compatibility of the amended bentonite. For instance, PAC-amended bentonite achieved acceptable Marsh viscosity (42.2 s) at 50 mM CaCl₂ but failed at 100 mM (37.5 s), resulting in c _max = 50 mM. Although polymer amendment increased the swell index in deionized water, it considerably decreased swelling in CaCl₂ solutions. This demonstrates the limited effectiveness of polymers in enhancing salt-affected swell performance. As a result, swell behavior was excluded from polymer screening criteria, with a focus on workability and impermeability metrics.

A comparative analysis of the c _max values shows that PAC-, HPMC-, and XG-amended bentonites have better workability retention in saline environments. In 50 mM CaCl₂, these three polymer-enhanced bentonites keep Marsh viscosity at or above 40 s and fluid loss at or below 25 mL, fully meeting construction standards; their fluid loss volumes were 16.8 mL, 23.8 mL, and 14.6 mL, respectively, with filter cake hydraulic conductivities (k _c) of 1.9 × 10⁻¹⁰ m/s, 4.8 × 10⁻¹⁰ m/s, and 1.4 × 10⁻¹⁰ m/s. Other tested polymers, however, failed to meet these thresholds under the same salinity conditions.

The findings demonstrate that PAC-, HPMC-, and XG-amended bentonites effectively contribute to fluid loss control and permeability reduction, as evidenced by the minimal fluid loss and hydraulic conductivity observed under identical ionic conditions. Among these polymer-modified bentonites, the XG-enhanced variant displays the lowest fluid loss and hydraulic conductivity. Consequently, xanthan gum (XG) has been identified as the most suitable additive for subsequent research into the anti-seepage and contaminant containment capabilities of polymer-enhanced bentonite barrier systems. Henceforth, XG-enhanced bentonites shall be referred to as XB.

The variation in specific gravity, swell index, and liquid limit of XB with varying polymer dosage is summarized in Table 6. The specific gravity (G _s) of XB exhibits an inverse relationship with polymer dosage. Within the 0%–4% dosage range, G _s decreased from 2.72 to 2.66. This pattern is consistent with the literature, which documents lower G _s in polymer-amended sodium bentonites. Guler et al. (2018) reported G _s of polymer-amended soils as similar to or lower than untreated soil; Scalia and Benson (2017) also recorded a G _s of 2.67 for BPN polymer-amended bentonite compared to 2.71 for unamended soil (G _s = 2.71). The observed G _s reduction is attributed to the intrinsic low density of superabsorbent polymers (1.24 for XG), analogous to sodium carboxymethyl cellulose, propylene carbonate, and polyacrylate (Bohnhoff and Shackelford, 2014). These polymers expand interlayer spacing and generate a more porous fabric, which is consistent with their significantly lower density (1.26–1.24 g/cm³) compared to bentonite (G _s = 2.72). Consequently, increased polymer dosage correlates with progressively declining in amended bentonites.

The SI of XG shows a positive correlation with polymer dosage. The unamended bentonite (CB) recorded an SI of 15.9 mL/2 g, while XB with 4% dosage achieved an SI of 24.2 mL/2 g. These values fall within the typical SI range (20–30 mL/2 g) of sodium bentonites. The liquid Limit (w _L) variations for both amended bentonites show that w _L increases proportionally with polymer dosage. CB exhibited a w _L of 267%, while XB, with a 4% dosage, reached a w _L of 403%, indicating exponential growth compared to CB. The improved swelling behavior and liquid limits were attributed to the numerous hydrophilic functional groups (such as carboxylate, COO⁻, and hydroxyl, -OH) present in Xanthan gum. These groups provide the material with excellent water-absorbing capacity, which enhances the swell index of polymer-amended bentonites.

Table 7 delineates the workability indices of unamended bentonite slurries at various dosages. The results indicate that slurries with bentonite dosages exceeding 8% satisfy the construction standards. To evaluate the efficacy of polymer amendments on bentonite, a slurry with suboptimal workability (specifically a 6% dosage) was selected for experimental analysis.

Figure 7 shows the workability indices of XB slurries with xanthan gum (XG) content from 0% to 4%. As the XG dosage increases, the filtration loss, pH, and slurry density decrease, while the Marsh funnel viscosity increases. At a critical XG dosage of 2%, the slurry has a filtration loss of 12.5 mL, Marsh viscosity of 42.5 s, pH of 9.03, and density of 1.032 g/cm³, meeting all the specified requirements for construction workability. The results indicate that XB slurry significantly improves: (1) trench wall stabilization to prevent collapse in isolation walls; (2) pumping efficiency and cuttings suspension capacity; (3) the anti-flocculation performance of bentonite; and (4) formation pressure equilibrium to reduce fluid loss. Although pH was not independently controlled in this study, the measured pH differences between CaCl₂ and Pb(NO₃)₂ solutions provide insight into their distinct impacts on ionic transport behavior. CaCl₂ solutions exhibited near-neutral pH values, whereas Pb(NO₃)₂ solutions were moderately acidic. The lower pH in Pb(NO₃)₂ solutions increases H⁺ concentration, which suppresses the ionization of functional groups on xanthan gum molecular chains and weakens polymer–cation complexation. As a result, a higher proportion of free Pb²⁺ ions participates in cation exchange within bentonite interlayers, leading to stronger compression of the diffuse double layer, enhanced particle flocculation, and increased permeability. In contrast, the near-neutral pH environment of CaCl₂ solutions favors polymer hydration and pore-sealing effects, thereby reducing ionic transport and maintaining lower hydraulic conductivity. These results indicate that pH indirectly influences ion solubility and transport by regulating polymer conformation and cation exchange processes in polymer-amended bentonite systems.

Figure 8 illustrates the curve of filter cake hydraulic conductivity versus xanthan gum (XG) dosage. The results indicate that the hydraulic conductivity of bentonite filter cakes steadily decreases as the polymer content increases. The filter cake from unamended bentonite slurry (6% dosage) has a hydraulic conductivity of 1.5 × 10⁻⁹ m/s; in contrast, adding 4% XG reduces this value to 3.7 × 10⁻¹⁰ m/s for the amended slurry. This represents a nearly order-of-magnitude reduction compared to the unamended system, confirming that XG amendment effectively reduces permeability in bentonite filter cakes. However, more research is needed to understand its permeability behavior when exposed to contaminated leachate and the microscopic mechanisms involved (Lee et al., 2016).

To examine the effect of cation species on the chemical compatibility of bentonite, this section compares the filtrate test results when exposed to lead nitrate (Pb(NO₃)₂) and calcium chloride (CaCl₂). Equal contaminant concentrations and test pressure (P₀ = 100 kPa) were used to evaluate the impacts of Pb²⁺ and Ca²⁺ ions on filtrate loss volume, filter cake thickness, and permeability properties.