Feasibility Study on Using Calcium Lignosulfonate-Modified Loess for Landfill Leachate Filtration and Seepage Control

Abstract

Prolonged exposure to landfill leachate can weaken the impermeability of liner systems, leading to leachate leakage and the contamination of surrounding soil and water. To improve loess impermeability to enable its use as a liner material, this study uses synthetic landfill leachate to investigate its effects on loess permeability via a series of laboratory tests. This study focused on the influence of varying dosages of calcium lignosulfonate (CLS) on loess permeability, along with its capacity to adsorb and immobilize heavy metal ions. Microscale characterization techniques, including Zeta potential analysis, X-ray fluorescence spectroscopy (XRF), and scanning electron microscopy (SEM), were employed to investigate the impermeability mechanisms of CLS-modified loess and its adsorption behavior toward heavy metals. The results indicate that the permeability coefficient of loess decreases significantly with increasing compaction, while higher leachate concentrations lead to a notable increase in permeability. At a compaction degree of 0.90, the permeability coefficient was reduced to 8 × 10⁻⁸ cm/s. In contrast, under conditions of maximum leachate concentration, the permeability coefficient rose markedly to 1.5 × 10⁻⁴ cm/s. Additionally, increasing the dosage of the compacted loess stabilizer (CLS) effectively reduced the permeability coefficient of the modified loess to 7.1 × 10⁻⁵ cm/s, indicating improved impermeability and enhanced resistance to contaminant migration. With the prolonged infiltration time of landfill leachate, the removal efficiency of Pb²⁺ gradually decreases and stabilizes, while the Pb²⁺ removal efficiency of the modified loess increased by approximately 40%. CLS-modified loess, through multiple mechanisms, reduces the fluid flow pathways and enhances its adsorption capacity for Pb²⁺, thereby improving the soil’s protection against heavy metal contamination. While these results demonstrate the potential of CLS-modified loess as a sustainable landfill liner material, the findings are based on controlled laboratory conditions with Pb²⁺ as the sole target contaminant. Future work should evaluate long-term performance under field conditions, including seasonal wetting–drying and freeze–thaw cycles, and investigate multi-metal systems to validate the broader applicability of this modification technique.

1. Introduction

Leachate, with its chemical complexity and high pollutant concentrations, gradually compromises liner system integrity, accelerating landfill aging and shortening its operational lifespan [1,2,3,4]. Leachate leakage can contaminate nearby soil and groundwater, especially when it contains heavy metals like lead from discarded batteries and electronic waste, posing severe environmental and health risks. As a vital and non-renewable resource, groundwater pollution threatens regional water security and sustainability. Furthermore, leachate infiltration alters the hydraulic properties of surrounding soil, increasing environmental instability and triggering geohazards that endanger public safety and infrastructure.

The performance and lifespan of a landfill are heavily influenced by the properties of its liner materials [5,6,7,8]. Engineered landfill liners are commonly constructed using compacted clay, geosynthetic clay liners (GCLs), bentonite–sand mixtures, or regionally available soils such as lateritic clays, often amended to improve performance [2,4,7,8]. High-quality compacted clays typically exhibit permeability coefficients on the order of 10⁻⁷–10⁻⁹ cm/s and provide strong cation retention through ion exchange and surface complexation [2]. GCLs, which combine bentonite with geotextiles, offer similarly low hydraulic conductivity but can be sensitive to chemical attack and desiccation [9]. Lateritic soils have been used successfully where available, but their performance varies with mineralogy and leachate chemistry [8]. Huang et al. [5] evaluated the impact of landfill leachate on groundwater contamination and found a positive correlation between landfill scale and pollution levels. Zhao et al. [6] developed a coupled model incorporating unsaturated soil characteristics and geological material permeability to study how preferential flow paths influence seepage evolution. Zhang et al. [7] examined the impermeability performance of composite materials made of modified loess and attapulgite clay for landfill liner applications. Daramola et al. [8] analyzed the mineralogical and sorption characteristics of lateritic soils from southwestern Nigeria to evaluate their potential as landfill liners, focusing on competitive heavy metal sorption and the influence of parent rock on leachate attenuation. Moghaddam et al. [10] evaluated leachate-contaminated clays (LCC) for landfill engineering through static and dynamic tests, applying machine learning and optimization algorithms to assess shear modulus and damping behavior under different loading and leachate conditions. Marques et al. [4] conducted a systematic review of alternative materials for enhancing the retention of toxic metals in soil/clay liners, identifying commonly studied amendments, application methods, and research gaps—especially regarding field-scale implementation and material sustainability. Loess in northwestern (NW) China offers potential as a liner material if its naturally higher permeability and lower contaminant retention can be improved [11]. In our study, CLS-modified loess compacted to a degree of 0.90 achieved a permeability coefficient of 8 × 10⁻⁸ cm/s, comparable to compacted clay liners, and a Pb²⁺ removal efficiency increase of ~40% relative to unmodified loess. These results place CLS-modified loess within the performance envelope of established liner materials while providing a cost-effective and locally sourced alternative for clay-deficient regions. Although significant progress has been made in understanding how landfill leachate affects soil permeability, research on the suitability of loess from northwestern China as a liner material remains scarce. To enhance the impermeability and heavy metal adsorption of loess for landfill liners—and prevent leachate leakage—the material must exhibit both low permeability and strong adsorption capacity.

In recent years, calcium lignosulfonate (CLS) has gained attention as an environmentally friendly material, owing to its strong adsorption and binding abilities attributed to abundant oxygen-containing functional groups [11,12,13,14]. Li et al. [14] examined the engineering properties of loess stabilized with CLS, demonstrating its effectiveness in improving soil strength and reducing energy dissipation through chemical reactions with clay minerals. Its performance was also compared with traditional stabilizers such as NaOH and Na₂O·nSiO₂. Ji et al. [15] studied the modification effects of CLS on natural foundation soils. Wang et al. [16] explored how CLS affects the physical and mechanical properties of weak expansive soils, showing that CLS significantly improves compressive strength. Liu et al. [17] reported that CLS improves the particle structure of loess, thereby enhancing its engineering performance. CLS has been reported to improve the shear strength, compressibility, and overall engineering performance of soils, as supported by targeted experimental and field studies in the literature [18,19,20,21,22]. However, most existing studies have focused on modifying uncontaminated soils, with limited attention paid to how landfill leachate exposure affects the permeability of loess and its capacity to adsorb and immobilize heavy metals [23,24]. Investigating the impermeability and heavy metal immobilization performance of CLS-modified loess under contaminated conditions is of great theoretical and practical significance.

Despite considerable research on landfill liners and chemical soil stabilization, the long-term hydraulic and contaminant retention behavior of CLS-modified loess under leachate infiltration remains poorly understood. Existing studies often focus on uncontaminated soils or mechanical enhancement, overlooking how chemical interactions with aggressive leachates influence permeability and pollutant immobilization. This study addresses this gap by integrating macro-scale permeability and Pb²⁺ removal tests with micro-scale electrochemical and mineralogical analyses to elucidate the mechanisms by which CLS improves loess performance. Using synthetic landfill leachate, we assess the effects of varying CLS dosages on permeability and heavy metal retention, supported by Zeta potential, XRF, and SEM characterization. The results identify optimal modification conditions, link performance gains to specific physicochemical interactions, and propose a comprehensive mechanism for improved impermeability and contaminant immobilization. These findings position CLS-modified loess as a viable, locally sourced alternative to conventional liners, offering both theoretical insight and practical guidance for landfill barrier design in clay-deficient regions.

2. Materials and Methods

2.1. Loess

The loess samples used in this study were collected from a slope near Shiziyuan Village in Xi’an, Shaanxi Province, at depths of 3.0–4.5 m. The initial physical and chemical properties of the samples were determined in accordance with the Standard for Geotechnical Testing Methods (GB/T 50123–2019) [25]. As shown in

Table 1. Physicochemical properties of the loess (reproduced from He et al. [26]).

Physical Index	Data
Fines (%)	91.18
Sand (%)	8.82
Gravel (%)	0
Specific gravity, Gs	2.72
Void ratio, e	0.88
Dry density, ρdmax/(g/cm3)	1.78
Initial water content, wn/%	16.4
The Atterberg limit
Liquid limit, wL/%	33.42
Plastic limit, wP/%	20.43
Soil classification	CL

, the samples are classified as Late Pleistocene collapsible loess (Q₃), characterized by high collapsibility and strong moisture sensitivity, which significantly affects their mechanical properties. The chemical composition of the loess was analyzed using inductively coupled plasma mass spectrometry (ICP-MS-7900, Agilent Technologies Inc., Santa Clara, CA, USA), as presented in

Table 2. Chemical element composition of the loess specimen (reproduced from He et al. [26]).

Chemical Element	Content (%)
Silicon (Si)	73.66
Aluminum (Al)	15.5
Iron (Fe)	7.93
Potassium (K)	1.09
Magnesium (Mg)	0.95
Sodium (Na)	0.54
Calcium (Ca)	0.33

. The main chemical components in the loess are SiO₂, followed by Al₂O₃, CaO, and Fe₂O₃. Figure 1 presents the particle size distribution curve, illustrating the soil’s gradation and physical characteristics. During sample preparation, the loess was air-dried, ground, and sieved to remove impurities and achieve uniform particle size distribution. The processed samples were sealed to prevent moisture fluctuations during storage. After pretreatment, the soil was classified as low-plasticity clay, indicating low plasticity and cohesive behavior, which makes it suitable for mechanical testing.

2.2. Landfill Leachate Preparation

In this study, synthetic landfill leachate was prepared using deionized water. Its chemical composition was formulated based on typical concentrations of common pollutants in municipal solid waste landfills. Following field investigations and the procedure outlined by Wang et al. [9], the synthetic leachate used in this study was prepared in accordance with established laboratory protocols [27,28,29]. To ensure accuracy, deionized water was used as the solvent, and analytical-grade reagents (NaHCO₃, KCl, MgCl₂, CaCl₂, Pb(NO₃)₂, and NaCl) were obtained from Sinopharm Chemical Reagent Co., Ltd. The solubility of each reagent at 20 °C is listed in Table 2: NaHCO₃ (96 g/L), KCl (340 g/L), MgCl₂ (167 g/L), CaCl₂ (740 g/L), Pb(NO₃)₂ (52 g/L), and NaCl (360 g/L). This solubility information guided the formulation of the synthetic leachate. Accordingly, the target ion concentrations in the leachate were set as follows: Na⁺ (3000 mg/L), K⁺ (750 mg/L), Mg²⁺ (500 mg/L), Ca²⁺ (5 mg/L), Pb²⁺ (500 mg/L), and Cl⁻ (5500 mg/L). The synthetic leachate was formulated to reproduce the ionic strength and dominant cation–anion chemistry of typical municipal landfill leachates [30,31]. It contained Na⁺ 3000 mg/L, K⁺ 750 mg/L, Mg²⁺ 500 mg/L, Ca²⁺ 5 mg/L, Cl⁻ 5500 mg/L, and Pb²⁺ 500 mg/L in deionised water, with concentrations selected from reported ranges [27,28,29,30,31] to ensure representative double-layer compression, ion exchange, and mineral dissolution effects. Pb²⁺ was chosen as the model heavy metal because it is a high-toxicity, regulation-critical contaminant in landfill leachates, forms stable complexes with calcium lignosulfonate functional groups, and can be accurately quantified by atomic absorption spectroscopy. Although Fe, Al, and Cu are also common, they tend to hydrolyze or precipitate at near-neutral pH, potentially masking permeability mechanisms; these will be investigated in future multi-metal studies. To study the effect of leachate concentration on loess, the solution was diluted to four levels: 10%, 30%, 50%, and 100%. Deionized water was used as a control to assess the impact of landfill leachate on the mechanical properties of loess.

2.3. Calcium Lignosulfonate

The CLS used in this study was obtained from Sinopharm Chemical Reagent Shaanxi Co., Ltd., with a purity above 95% and a particle size of 100–200 mesh, appearing as a brownish powder. CLS, a lignin-derived anionic polyelectrolyte, dissolves in water through a sequence of ion dissociation, molecular dispersion, and surface-active interactions that underpin its soil-modifying capabilities. Upon hydration, Ca²⁺ ions are partially liberated from sulfonate groups into solution, increasing ionic strength and enabling subsequent cation exchange with loess minerals. The lignosulfonate macromolecules, enriched in sulfonic acid (–SO₃H), hydroxyl (–OH), and carboxyl (–COOH) groups, undergo extensive hydration, resulting in chain expansion and colloidal dispersion through hydrogen bonding with water molecules [31,32,33]. Weakly acidic functional groups partially deprotonate at near-neutral pH, generating negatively charged sites that enhance electrostatic attraction toward multivalent cations such as Pb²⁺. In solution, lignosulfonate anions readily form coordination complexes with dissolved metal ions, a process that governs both pollutant immobilization and microstructural modification when CLS interacts with soil. Over extended timescales, slow hydrolytic or oxidative changes may alter molecular weight distribution, reflected in a characteristic shift from brown to dark brown in solution. These dissolution and complexation processes collectively impart CLS with its dual capacity to improve particle bonding, reduce pore connectivity, and enhance heavy metal sequestration in treated loess [11,13,16,31,32,33,34,35,36]. According to experimental requirements, a 1% aqueous CLS solution was prepared, with a pH of ~6.5, indicating near-neutrality and suitability for most soil remediation applications. As the CLS concentration increases, the solution color changes from brown to dark brown. This color change may result from chemical reactions during CLS dissolution, indicating possible interactions with other components in the solution. In addition, the viscosity of the solution increases with higher CLS concentrations. This may be related to the polymeric nature of CLS and its enhanced surface activity, which could affect subsequent experiments, especially those involving permeability and adsorption. These physicochemical properties suggest that CLS not only improves soil structure but also interacts with other components during remediation via surface activity and chemical reactions, thereby enhancing its impermeability and pollutant adsorption capacity. Therefore, both CLS dosage and its dissolution behavior significantly affect experimental outcomes, especially in optimizing permeability and heavy metal adsorption [36].

2.4. Sample Preparation

Building on the work of Liu et al. [17], this study incorporated calcium lignosulfonate into dry loess at mass ratios ranging from 0% to 4%. To mitigate the influence of loess density and moisture content on permeability, the maximum dry density of remolded loess was employed as a reference, establishing a compaction range between 0.75 and 0.90. During the sample preparation process, air-dried loess, which had passed through a 2 mm sieve, was mixed with CLS and subsequently dried at 60 °C for 24 h to ensure uniformity and complete drying of the sample. Following this, deionized water was utilized to adjust the moisture content of the mixed soil sample to its natural water content (16.5%), thereby preserving the soil’s natural moisture state. According to the predetermined compaction, various masses of the mixed soil were accurately weighed and compacted to create samples for permeability testing. The dimensions of the samples were set at 39.1 mm × 80 mm (diameter × height) to ensure uniformity and repeatability of the experimental results. Through this series of meticulous sample preparation steps, the mixing ratio of loess and calcium lignosulfonate, along with the physical properties of the samples, were effectively controlled, providing a reliable foundation for subsequent permeability performance tests.

2.5. Permeability Test and Experimental Design

Permeability behavior was evaluated using a constant-head method in a GDS triaxial permeability apparatus, with specimens saturated to a B-value of 0.96 and consolidated under an effective confining pressure of 200 kPa. The permeability coefficient k (cm/s) was computed from the steady-state seepage rate, specimen dimensions, and applied hydraulic gradient, in accordance with GB/T 50123–2019 [25]. This coefficient provides a quantitative measure of the soil’s resistance to fluid flow and underpins the assessment of barrier performance. To investigate the effects of landfill leachate on the seepage characteristics of loess and the mechanism of loess modification using calcium lignosulfonate, an infiltration testing apparatus manufactured by Geotechnical Design and Simulation (GDS) was employed for the experiments [37,38]. Initially, the samples were subjected to curing for specific durations. Following the curing process, the samples were placed in the GDS permeability apparatus and saturated with deionized water until the saturation degree (B-value) of the samples reached 0.96, indicating that the saturation process was complete. Subsequently, the samples underwent consolidation treatment under an effective confining pressure of 200 kPa. After consolidation, permeability tests were conducted using landfill leachate of varying concentrations, with effluent samples collected every 0.5 days. The concentration of Pb²⁺ was determined using atomic absorption spectrophotometry. The removal efficiency was calculated based on the measured Pb²⁺ concentrations using the following formula:

R = (C_n−C_t)/C₀

(1)

where C_n is the initial concentration of Pb²⁺ (mg/L), Cₜ is the concentration of Pb²⁺ in the effluent (mg/L), and R is the removal efficiency at time t (d).

The experimental design for the permeability tests systematically investigates the effects of various compaction degrees, landfill leachate concentrations, and calcium lignosulfonate (CLS) dosages on the permeability and heavy metal adsorption capacity of loess samples (see

Table 3. Experimental design of the permeability test of loess.

Test	Degree of Compaction	Concentration of Landfill Leachate (%)	Concentration of CLS (%)	Curing Time (d)	Seepage Time	Quantity of Specimens
Exp-01	0.75, 0.80, 0.85, 0.90	/	/	1	0–4	4
Exp-02	0.75	10, 30, 50, 100	/	1	4	4
Exp-03	0.75	100	0.5, 1, 1.5, 2.0, 3.0, 4.0	1–14	4	6

Note: “/” indicates that the corresponding test was not used in the treatment.

). In Exp-01, loess samples were compacted at degrees ranging from 0.75 to 0.90, with permeability measured over a 0–4-day period to assess the relationship between compaction and permeability. Exp-02 examined the impact of different landfill leachate concentrations (0%, 10%, 30%, 50%, and 100%) on loess permeability, with samples undergoing a 4-day infiltration period. In Exp-03, the effectiveness of CLS was evaluated by adding varying concentrations (0.5% to 4%) to loess samples under 100% landfill leachate conditions, with curing times ranging from 1 to 14 days. These experiments were designed to provide insights into how compaction, leachate concentration, and CLS modification influence the permeability and pollutant removal capacity of loess, generating valuable data for the development of more efficient landfill liner materials. An error bar has already been added to figures where necessary. Statistical analysis indicates that the coefficient of variance for the permeability and heavy metal adsorption is <15%, which is within the requirement being the coefficient of variance for usual, accessible experimental measurements.

2.6. Sample Characterization

This study employed various characterization methods to analyze loess samples, including X-ray fluorescence (XRF), Zeta potential analysis, and scanning electron microscopy (SEM) [39,40,41]. These methods facilitated the examination of changes in oxide composition, Zeta potential, and microstructural morphology of the samples before and after modification and infiltration treatment [42,43,44,45,46,47,48]. The results provided a comprehensive evaluation of the effects of calcium lignosulfonate on loess, offering detailed insights into structural and surface properties that elucidate the mechanisms underlying changes in permeability. XRF (Panalytical Axios Series, Panalytical, The Netherlands) was used for quantitative analysis of the mineral composition of the samples, particularly the content of oxides. XRF testing primarily works by using X-rays to excite elements within the sample, causing them to emit fluorescence, which is then analyzed to determine the sample’s elemental composition. Testing details include adjusting excitation conditions and test duration to ensure high-precision composition analysis. Zeta potential testing (Zetasizer Nano ZS90, Malvern Instruments, UK) measures the charge distribution on the surface of particles to understand inter-particle interactions. Testing details include dispersing the sample in an appropriate liquid medium, measuring the Zeta potential, and analyzing changes in surface charge density. SEM (Zeiss Sigma 300 Series, Zeiss, Germany) is used to observe the microstructure of samples, particularly particle arrangement and pore structure. Testing details include coating the sample surface with a conductive film, followed by scanning the surface with a high-voltage electron beam and capturing secondary electron images for surface morphology analysis.

3. Results

3.1. Effect of Degree of Compaction on Permeability of Loess Samples

Figure 2 illustrates the effect of compaction degree on the permeability coefficient of loess samples. As compaction increases, the pore structure of remolded loess becomes increasingly compacted, leading to a continuous decrease in the permeability coefficient. Specifically, at lower compaction levels, increasing compaction significantly reduces the permeability coefficient. However, at higher compaction levels, while further compaction continues to reduce permeability, the rate of decrease becomes progressively smaller. This behavior is governed by the progressive densification of the soil skeleton, in which particles are rearranged into a more closely packed configuration under mechanical loading. As compaction increases, the interconnected void space between particles becomes less continuous, forcing seepage to occur through narrower and more tortuous flow paths, which reduces the hydraulic conductivity [44]. At low compaction, the relatively loose particle framework contains abundant, well-aligned voids that facilitate direct flow paths, resulting in higher permeability. When compaction approaches the upper range tested, further densification yields diminishing returns because the particle arrangement is already highly constrained and void connectivity is minimal. These results emphasize that the control of interparticle void geometry through compaction is a primary factor in improving the impermeability of loess, with practical implications for landfill liner design where targeted compaction levels can be used to achieve regulatory hydraulic conductivity limits.

3.2. Effect of Concentration of Landfill Leachate on Permeability Coefficient of Loess Samples

Figure 3 presents the effect of landfill leachate concentration on the permeability coefficient of loess samples at a compaction degree of 0.75. This response is attributed to the combined physicochemical effects of leachate components on the particle framework. High concentrations of multivalent cations (e.g., Ca²⁺, Mg²⁺, Pb²⁺) compress the electrical double layer surrounding soil particles, reducing electrostatic repulsion and promoting aggregation into flocculated clusters. This clustering disturbs the uniform packing density achieved by compaction, creating preferential flow channels and increasing void connectivity. Furthermore, the slightly acidic nature of the leachate promotes the dissolution of carbonate and other cementing minerals within the soil matrix, weakening interparticle bonds and allowing the framework to relax under seepage forces. As a result, the hydraulic resistance of the compacted loess decreases, and its permeability increases. These findings highlight that chemical aggressiveness of leachate can partially offset the benefits of mechanical compaction, underscoring the importance of combined mechanical–chemical considerations in the long-term performance assessment of liner materials.

This effect correlates with changes in the electrochemical environment of loess particles, as reflected in the measured zeta potential. The zeta potential of natural loess shifted from –16.3 mV in the control to –9.8 mV after exposure to full-strength leachate, indicating a substantial reduction in surface charge magnitude. This compression of the electrical double layer reduces interparticle repulsion and allows particles to rearrange into aggregated domains, which disrupts the uniform packing established by compaction and facilitates the formation of preferential flow channels. The “cementing materials” in this context refer to fine-scale mineral phases—principally carbonates such as calcite, clay mineral edge contacts, and iron/aluminum oxide coatings—that bond particles together at points of contact. The slightly acidic and chemically aggressive nature of the leachate promotes the partial dissolution of these mineral bonds, weakening the particle framework and further increasing void connectivity. The observed reduction in permeability resistance is thus attributed to two concurrent processes: (i) electrochemical destabilization of particle contacts through the compression of the double layer by multivalent cations present in the leachate, and (ii) chemical degradation of interparticle cementing phases. While the adsorption of Pb²⁺ onto particle surfaces was confirmed by effluent analysis, comprehensive assessment of other cation species in the post-contact solution was beyond the present scope. Such analysis, along with expanded ion-specific adsorption studies, will be incorporated in future work to further resolve the mechanistic pathways.

Figure 4 illustrates the trend of Pb²⁺ removal efficiency over time in the loess seepage test at 100% landfill leachate concentration. Initially, during the first 1.5 days of the experiment, the concentration of Pb²⁺ in the effluent was nearly zero, with removal efficiency approaching 100%. However, as the infiltration time increased, the Pb²⁺ concentration in the effluent gradually increased, and the removal efficiency declined. This phenomenon can be explained from several perspectives. In the early stages, carbonate minerals in the loess underwent a displacement reaction with Pb²⁺, releasing a large amount of Ca²⁺ and effectively reducing the Pb²⁺ concentration in the effluent. Additionally, the infiltration of Pb²⁺ disrupted the electric double layer structure on the surface of loess particles, causing an imbalance in charge and initiating ion exchange reactions between Pb²⁺ and low-valence cations in the diffuse double layer. This ion exchange resulted in a low Pb²⁺ concentration in the effluent, leading to high removal efficiency. However, as the experiment continued, the Pb²⁺ displacement capacity weakened. As the displacement reaction progressed, Pb²⁺ became increasingly fixed in the loess, and the soil’s adsorption capacity for Pb²⁺ became saturated, causing the Pb²⁺ concentration in the effluent to rise. Simultaneously, loess adsorbs Pb²⁺ both physically and chemically, and the increase in Pb²⁺ concentration in the later stages is closely linked to the balance between adsorption capacity and concentration. As the adsorption capacity gradually saturated, further Pb²⁺ removal was limited. Therefore, although the Pb²⁺ removal efficiency was high at the beginning of the seepage test, it decreased over time, primarily due to the weakening of the displacement reaction and the saturation of the adsorption capacity, leading to a diminished removal effect.

3.3. Effect of Dosage of CLS on Permeability Coefficient of Loess Samples

When the compaction degree is 0.75 and the landfill leachate concentration is 100%, the effect of different dosages of CLS on the permeability coefficient and Pb²⁺ removal efficiency of loess is shown in Figure 5 and Figure 6. The addition of CLS significantly reduced the permeability coefficient of loess under landfill leachate infiltration and enhanced its impermeability. Specifically, CLS has strong adhesive properties, and when added to loess, it enhances the cementation between soil particles, reducing the degradation of loess structure caused by leachate infiltration. On the other hand, the hydrophobic groups in CLS have a significant water-repellent effect, which forms an effective water barrier on the surface of soil particles, further improving the impermeability of loess. Additionally, during the modification of loess with CLS, Ca²⁺ ions preferentially exchange with low-valence cations in the soil, thinning the diffuse double layer of loess, and causing the attractive forces between soil particles to exceed the repulsive forces. This change promotes the aggregation of soil particles, facilitating the formation of pore channels. However, due to the low solubility of CLS in water, excessive dosages may lead to the formation of precipitates, hindering the formation of pore structures. Therefore, as the dosage of CLS increases, the permeability coefficient of loess may gradually decrease.

Regarding Pb²⁺ removal efficiency, an increase in CLS dosage results in improved Pb²⁺ removal. The maximum removal efficiency is achieved when the CLS dosage reaches 4%. This effect occurs because CLS adheres to the surface of loess particles and fills soil pores. When Pb²⁺-containing landfill leachate infiltrates the soil, CLS on the surface of loess preferentially reacts with Pb²⁺, forming complexes through its functional groups. This process delays the interaction between Pb²⁺ and minerals, such as carbonates and soluble salts, in the loess, effectively preventing the rapid dissolution and migration of Pb²⁺. However, as the seepage time increases, the number of available adsorption sites for CLS decreases, leading to a decline in Pb²⁺ removal efficiency. Concurrently, the dissolution of carbonates and soluble salts in loess occurs, and the diffuse double layer effect gradually becomes more pronounced, potentially further affecting Pb²⁺ removal. Ultimately, while the Pb²⁺ removal efficiency is highest when the CLS dosage is at its maximum, it gradually decreases over time, reflecting the saturation of adsorption sites and the weakening of ion exchange capacity.

The variation in permeability due to the effect of curing time for the seepage test is illustrated in Figure 7. As shown in Figure 7, under the conditions of a compaction degree of 0.75, 4% CLS addition, and 100% landfill leachate concentration, extending the curing time of CLS-modified loess samples effectively reduced their permeability coefficient. This reduction is attributed to the dissolution of CLS, which, when mixed with loess particles, forms cementing materials that bond tightly with the soil. These cementing materials adjust the porosity between soil particles, enhancing the stability of the soil structure. As CLS dissolves, Ca²⁺ ions are gradually released and undergo ion exchange with low-valence cations on the surface of loess particles. This ion exchange alters the charge distribution on the surface of the particles, affecting the soil’s double layer structure. As the curing time increases, the ion exchange and charge distribution adjustments gradually stabilize, eventually reaching dynamic equilibrium. This process significantly enhances the soil’s physicochemical properties, particularly its permeability and the strength of particle bonding. Therefore, extending the curing time promotes the bonding between CLS and loess particles, further optimizing the soil structure, reducing permeability, and improving soil stability. This phenomenon demonstrates that CLS modification not only improves loess impermeability but also enhances its mechanical properties, offering considerable value for engineering applications.

3.4. XRF, Zeta Potential and SEM Test Results

XRF elemental analysis was used to quantify the bulk elemental composition of loess before and after leachate infiltration. Figure 8 shows the change in oxide content in both unmodified and CLS-modified loess samples before and after landfill leachate seepage. In standard XRF reporting, the measured elemental contents are expressed as their corresponding oxides for consistency and comparability. For example, Pb detected in the post-infiltration samples is reported as PbO in oxide-equivalent form, which reflects the relative mass percentage of Pb present, rather than the identification of discrete Pb oxide minerals. Likewise, Na is reported as Na₂O based on its elemental concentration. To improve transparency, these results are now presented in Table S1 in the Supplementary Information, showing the numerical oxide-equivalent percentages before and after infiltration for both unmodified and CLS-modified loess. The observed increase in PbO in CLS-modified loess after infiltration reflects the retention of Pb²⁺ from the leachate via sorption and possible precipitation reactions involving functional groups in CLS and carbonate phases in the soil matrix. The measured reductions in CaO and minor decreases in other oxides in unmodified loess may be attributed to limited dissolution or mobilization of carbonate and associated phases during infiltration [48,49]. While SiO₂ dissolution in a weakly acidic solution is thermodynamically possible, its rate is extremely low under the conditions of this study, and no significant loss was confirmed by the XRF data [50]. Overall, the changes in oxide-equivalent composition indicate Pb²⁺ retention in CLS-modified loess and minor mobilization of certain native mineral constituents during infiltration. The interpretation is thus limited to observed compositional changes and supported by direct analytical evidence, without inferring unverified dissolution of silicate minerals or the involvement of contaminants not present in the model solution.

As shown in Figure 8 and Table S1, the oxide composition of both unmodified and CLS-modified loess changes significantly after landfill leachate infiltration, reflecting complex geochemical interactions. In unmodified loess, the contents of CaO and SiO₂ decreased markedly. This reduction is primarily due to the dissolution of carbonate minerals (e.g., calcite) and silicate minerals (e.g., quartz) under the weakly acidic conditions of landfill leachate, which contains organic acids and dissolved CO₂ [51,52,53]. The dissolution of Ca-bearing minerals reduces the CaO content, while acid-induced weathering of quartz and aluminosilicates contributes to the loss of SiO₂ [52]. These mineral losses weaken particle cementation, enlarge pore spaces, and increase permeability, as also observed in previous studies [54,55,56]. The results indicate that in unmodified loess, the contents of CaO and SiO₂ significantly decreased, whereas in CLS-modified loess, the content of PbO significantly increased, suggesting that CLS significantly enhances the heavy metal adsorption capacity of loess. Additionally, the changes in other oxides, such as Fe₂O₃ and Al₂O₃, were more complex and may be related to interactions with various chemical reactions in the leachate. X-ray fluorescence (XRF) analysis of the oxide composition in loess samples after landfill leachate infiltration (Figure 8) revealed significant changes in the oxide content due to leachate exposure. As shown, the content of CaO and SiO₂ in unmodified loess decreased significantly after leachate infiltration. This reduction is primarily attributed to the reaction between carbonates (e.g., calcite) and quartz minerals in loess with the acidic components in the leachate, leading to mineral dissolution. Specifically, the decrease in CaO content reflects the dissolution of carbonates in the acidic environment, while the reduction in SiO₂ suggests that quartz minerals may decompose under the influence of the acidic leachate. These findings indicate that the acidic components in landfill leachate, such as organic acids and heavy metal ions, have an erosive effect on loess minerals, resulting in mineral decomposition and loss, which negatively impacts the mechanical properties and permeability of the soil.

In contrast, CLS-modified loess exhibited distinct changes in oxide composition after leachate infiltration, particularly a significant increase in PbO content. This suggests that the modified loess has an enhanced capacity for heavy metal adsorption. As an organic modifier, CLS forms complexes with heavy metal ions, such as Pb²⁺, thereby improving the loess’s ability to adsorb these ions. This finding aligns with the observed increase in Pb²⁺ removal efficiency in macro-scale experiments, confirming the effectiveness of CLS-modified loess in remediating heavy metal pollution. The concentration changes in other oxides, such as Fe₂O₃, Al₂O₃, and K₂O, were more complex, likely resulting from the combined effects of various chemical reactions. These changes highlight the complexity of leachate components and their multifaceted impact on soil structure and chemical properties. Therefore, when considering the use of modified loess as a liner material for landfills or polluted areas, it is crucial to assess the chemical behavior of different leachate pollutants and their long-term effects on soil minerals. In summary, XRF analysis demonstrates the complex chemical interactions between landfill leachate and loess, highlighting the positive effects of CLS modification on loess oxide composition and heavy metal adsorption capacity. CLS enhances loess’s ability to adsorb heavy metals, significantly improving its stability and impermeability in polluted environments. This modification effectively reduces the risk of leachate contamination and its environmental impact.

Figure 9 shows the changes in Zeta potential before and after landfill leachate infiltration for natural and CLS-modified loess, while Figure 10 illustrates the evolution of the electric double layer in loess under leachate infiltration. The initial Zeta potential of unmodified loess was −16.3 mV, which increased to −9.8 mV after leachate infiltration. This change indicates that polyvalent cations (e.g., Ca²⁺, Mg²⁺) from the leachate entered the electric double layer of loess particles, partially neutralizing the surface negative charge and lowering the absolute surface potential (Figure 10). The weakening of electrostatic repulsion and the enhancement of other attractive forces promote particle aggregation, causing small pores to merge into larger seepage channels, which increases the permeability coefficient and significantly affects loess permeability. This phenomenon is critical in engineering applications, as it may accelerate pollutant migration, particularly in soils with inadequate impermeability, leading to environmental issues. In contrast, the Zeta potential of CLS-modified loess under leachate infiltration decreased from −15.8 mV to −12.1 mV, showing a significantly smaller reduction compared to unmodified loess. This indicates that CLS effectively reduces particle aggregation by adsorbing onto the particle surface, stabilizing the charge distribution, enhancing particle repulsion, and preventing aggregation. This suppressive effect not only protects the loess structure from leachate-induced damage but also reduces the permeability coefficient, resulting in a more stable soil structure. These improvements have significant implications for practical applications, especially in landfill liner materials, where enhanced impermeability helps prevent pollutant leakage and protect groundwater resources.

Figure 11 presents the microstructural images of CLS-modified loess before and after leachate infiltration. The upper image shows that the particles of unmodified loess are loosely arranged with larger porosity and weaker inter-particle bonding, indicating that unmodified loess has larger pores and lower compaction, which may result in a higher permeability coefficient (Figure 5). This structure is susceptible to change under external erosion or leachate infiltration. In contrast, the CLS-modified loess exhibits a denser particle arrangement, significantly improved particle bonding, and reduced porosity. This indicates that the addition of CLS enhances particle bonding, suppresses aggregation through increased chemical bonding between particles, and stabilizes the soil’s microstructure. These structural optimizations correlate with the observed macroscopic improvements in permeability reduction and heavy metal adsorption capacity (Figure 5 and Figure 6).

4. Discussion

This study employed X-ray fluorescence (XRF) analysis to investigate changes in the oxide composition of loess samples following landfill leachate infiltration (Figure 8). The results revealed significant alterations in oxide content due to leachate exposure. In unmodified loess, the concentrations of CaO and SiO₂ decreased markedly after infiltration. This reduction is attributed to chemical reactions between carbonate minerals (e.g., calcite) and quartz in the loess and the acidic constituents of the leachate, resulting in mineral dissolution and elemental loss. These findings are consistent with previous studies by Eyo et al. [46], Lu et al. [47], and Sun et al. [51]. Specifically, the decline in CaO reflects the dissolution of carbonates under acidic conditions, while the decrease in SiO₂ suggests that quartz minerals may undergo structural degradation due to acid-induced erosion. These reactions demonstrate that organic acids and heavy metal ions in landfill leachate chemically interact with the primary mineral constituents of loess, promoting mineral breakdown and loss, which in turn deteriorates the soil’s mechanical properties and permeability.

In contrast to unmodified loess, CLS-modified loess exhibited notable differences in oxide composition following leachate infiltration (see Figure 12). Most significantly, the PbO content increased substantially, indicating an enhanced capacity for heavy metal adsorption. As an organic modifier, CLS can form stable complexes with heavy metal ions such as Pb²⁺, thereby improving the loess’s adsorption performance. This result is consistent with the observed increase in Pb²⁺ removal efficiency in macro-scale experiments, further confirming the effectiveness of CLS-modified loess in mitigating heavy metal contamination. The variations in other oxides—such as Fe₂O₃, Al₂O₃, and K₂O—were more complex, likely due to a series of simultaneous chemical reactions involving leachate constituents. Prior studies [51,52,53,54,55,56] have demonstrated that iron and aluminum oxides can dissolve and reprecipitate under acidic conditions, with their behavior strongly influenced by interactions with other chemical species in the leachate. These findings align with the results of this study, highlighting the multifaceted impact of leachate components on the mineralogical and chemical properties of loess.

Traditionally, compacted clay has been widely used as a liner material in engineering applications due to its low permeability and excellent durability. However, in regions such as northwestern China, where clay resources are scarce, loess has emerged as a promising alternative. The results of this study show that when the compaction degree of loess reaches 0.90, its permeability coefficient meets the requirements specified in the Technical Specifications for Sanitary Landfill of Municipal Solid Waste (GB 50869-2013) [57], indicating that loess can achieve satisfactory impermeability under high-density compaction. Under lower compaction conditions (e.g., 0.75), the addition of CLS effectively reduced the permeability coefficient and enhanced Pb²⁺ adsorption capacity; however, it did not fully satisfy the regulatory requirements. Therefore, to meet the performance standards for landfill liner materials, it is recommended to increase the compaction degree to 0.90 and incorporate 4% CLS.

Moreover, the findings reveal that a 14-day curing period further optimized the loess microstructure, enhancing material compactness and uniformity. This microstructural improvement led to a more pronounced reduction in permeability and a greater capacity for heavy metal adsorption. Such enhancements not only improve the impermeability of loess but also extend its functional lifespan as a landfill liner. Accordingly, the integrated approach of high compaction combined with CLS modification provides an effective and economically viable solution for landfill barrier systems in clay-deficient regions. In conclusion, the interaction between landfill leachate and loess—particularly the resulting changes in mineral composition and adsorption behavior—reflects the complex chemical dynamics of leachate pollutants. Future studies should focus on evaluating the long-term performance of CLS-modified loess under varying compaction levels and pollutant concentrations. In addition, exploring alternative soil modifiers could further enhance the physical and chemical properties of loess, thereby improving its impermeability and pollutant retention capabilities. Such research will be essential to ensure the broad applicability and effectiveness of modified loess materials in practical engineering contexts.

While laboratory results demonstrate that CLS-modified loess can achieve permeability and contaminant retention comparable to conventional liners, several practical challenges must be addressed before large-scale application. Field variability in loess composition and structure may affect modification efficiency, necessitating site-specific mix design. The complex chemistry of real landfill leachates, often containing multiple contaminants, could alter CLS–soil interactions and long-term stability. In addition, achieving uniform compaction and homogeneous CLS distribution under field conditions requires careful quality control. Future work should therefore prioritize pilot-scale installations and extended in situ monitoring to validate performance under operational stresses, seasonal climate fluctuations, and variable leachate compositions.

5. Conclusions

This study investigated the permeability behavior, Pb²⁺ removal efficiency, and microstructural evolution of both natural and CLS-modified loess under artificial landfill leachate infiltration. The physicochemical interactions among CLS, loess particles, and heavy metal ions were systematically investigated using Zeta potential analysis, XRF, and SEM. Based on the experimental findings and mechanistic insights, the following conclusions can be drawn:

(1)

This study demonstrates that the permeability coefficient of remolded loess decreases with increasing compaction. Beyond a threshold compaction degree of 0.85, the rate of reduction diminishes, indicating that the pore network approaches a densely packed and structurally stable configuration. While initial compaction markedly enhances impermeability, further densification yields only marginal gains. Under landfill leachate infiltration, multivalent cations such as Ca²⁺ and Mg²⁺ penetrate the electrical double layer of loess particles, reducing the absolute Zeta potential. The resulting decrease in electrostatic repulsion promotes particle flocculation, enlarges pore pathways, and increases the permeability coefficient.

(2)

The moisture and chemical constituents of landfill leachate act corrosively on the loess matrix, dissolving binding minerals, weakening structural integrity, and increasing permeability. This effect intensifies with leachate concentration, leading to pronounced structural degradation. At full-strength leachate, Pb²⁺ removal efficiency declines steadily during curing, indicating that prolonged exposure diminishes adsorption capacity and accelerates mineral destabilization.

(3)

The incorporation of CLS lowers the Zeta potential and facilitates particle flocculation, yet its strong Pb²⁺ binding affinity and pore-filling capacity dominate the overall response. An optimal dosage of 4% was identified. During curing, CLS forms stable cementitious linkages between particles, reduces porosity, and reinforces the soil fabric. Under seepage conditions, CLS-modified loess maintains structural integrity and demonstrates superior barrier performance, effectively immobilizing heavy metals while preserving low permeability.

(4)

Future research should focus on evaluating the long-term performance and environmental durability of CLS-modified loess under diverse field conditions, including seasonal wetting–drying and freeze–thaw cycles. Additionally, further investigation is required to assess the feasibility, scalability, and cost-effectiveness of applying this modification technique in real-world landfill barrier systems, ensuring its practical viability in engineering applications.