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Article

Experimental Study on Transport of Cd(II) and Cu(II) in Landfill Improved Clay Liners Building Material Containing Municipal Sludge-Activated Carbon

by
Jun Xu
1,
Haijun Lu
2,
Zhenhua Wang
1,
Qian Zhang
2,
Guanghua Cai
3 and
Meng Zang
2,*
1
Hubei Institute of Urban Geological Engineering, Wuhan 430050, China
2
School of Civil Engineering and Architecture, Wuhan Polytechnic University, Wuhan 430023, China
3
College of Civil Engineering, Nanjing Forestry University, Nanjing 210037, China
*
Author to whom correspondence should be addressed.
Buildings 2024, 14(9), 2638; https://doi.org/10.3390/buildings14092638
Submission received: 12 July 2024 / Revised: 13 August 2024 / Accepted: 23 August 2024 / Published: 25 August 2024
(This article belongs to the Special Issue Green Building Materials and Intelligent Construction Technology)
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Abstract

:
Landfills necessitate a liner barrier system to prevent the leakage of contaminants into the surrounding soil. However, the currently employed compacted clay liner (CCL) is insufficient to prevent the leakage of heavy metal ions. This study proposes a novel landfill liner system utilizing sludge-based activated carbon (SAC)-modified clay. The adsorption characteristics of SAC-modified clay liner (SAC-CCL) for Cd(II) or Cu(II) were evaluated through batch tests. The permeability coefficient and unconfined compressive strength of SAC-CCL were assessed through permeation and unconfined compression tests. The permeability coefficient of the SAC-modified clay ranged from 2.57 × 10−9 to 1.10 × 10−8 cm/s. The unconfined compressive strength of the SAC-CCL varied between 288 and 531 kPa. The migration of Cd(II) or Cu(II) within an 80 cm thick, full-scale SAC-CCL was simulated using soil column tests. The diffusion coefficient (D) was calculated by inversion using the one-dimensional solute migration equation. The diffusion coefficients (D) for Cd(II) and Cu(II) ranged from 1.9 × 10−10 to 13.5 × 10−10 m2/s. The retardant performance of SAC-CCL for Cd(II) and Cu(II) followed the order: 3% SAC-CCL > 1% SAC-CCL > CCL > 5% SAC-CCL, from strongest to weakest. Consequently, SAC-modified clay demonstrates significant potential as a landfill lining material. However, the migration behavior of heavy metal ions in SAC-CCLs under cyclic dry–wet conditions requires further investigation.

1. Introduction

Landfill leachate is a highly concentrated organic wastewater containing a significant number of organic pollutants and toxic heavy metals. The leakage of heavy metals into the soil and groundwater surrounding landfill sites poses a significant threat to the ecological environment and human health [1,2,3,4,5]. To prevent the leakage of waste leachate, a liner barrier system is required at the base of landfills, with compacted clay widely utilized due to its low permeability. The permeability coefficient of the impermeable landfill liner must not exceed 1 × 10−7 cm/s, and the compacted clay layer should have a thickness of no less than 75 cm (GB 50869-2013) [6,7,8]. However, heavy metals are highly penetrative and can still permeate the liner barrier through diffusion [9]. To enhance the liner barrier’s ability to retard heavy metals, conventional compacted clay can be modified by incorporating adsorbents with high adsorption capacity [10,11,12,13]. Several studies have utilized municipal sludge to prepare activated carbon, examining its adsorption and removal performance for heavy metals, and concluded that sludge-based activated carbon is a promising material for heavy metal adsorption [14,15,16,17,18,19]. Thus, sludge-activated carbon can be utilized as an enhancement material for compacted clay liners (CCLs). However, the retardation performance of sludge-activated carbon-modified clay liners (SAC-CCLs) against heavy metals has yet to be evaluated.
The migration of heavy metals within landfill liners is governed by convection and diffusion processes [20]. Seepage velocity is a key factor influencing convection and diffusion processes [21]. The hydraulic gradient directly determines the seepage velocity in saturated porous media. In China, the initial water content of landfill waste is 50–60%, and as degradation and compression occur, substantial amounts of leachate are generated [22,23]. Additionally, the landfill leachate drainage system becomes clogged due to microbial and physicochemical processes, leading to elevated leachate levels within the landfill [24]. The primary water level within the landfill can rise to 1–10 m after 10 years of operation [25]. Soil column tests are commonly employed to analyze the migration characteristics of the contaminants in compacted clay soils. The pressure head commonly applied in soil column tests ranges from 0.3 to 2.0 m, with compacted clay layers typically being 10–30 cm thick [26,27]. The low compaction head and limited thickness of compacted clay may not accurately represent the breakdown process of heavy metals in CCLs.
The unconfined compressive strength of the liner material is a critical factor in determining both the height and capacity of a landfill [28]. Daniel and Wu recommended that the compacted soils used in landfill liners should possess a minimum unconfined compressive strength of 200 kPa [29]. However, this compressive strength does not meet the engineering requirements of landfills in China. The natural density of municipal solid waste (MSW) in China ranges from 0.1 to 0.6 t/m3, which can increase to 0.8 to 1.0 t/m3 following mechanical compaction. Due to site limitations, large landfill heights, often exceeding 30 m, are common in China. The dissolution of montmorillonite, illite, and other minerals in clay, induced by the erosion of heavy metal ion solutions, along with the degradation of the clay structure, leads to a reduction in the unconfined compressive strength of compacted clay [30,31,32]. The CCL layer can be compromised by uneven settlement and deformation under landfill loads, while the HDPE geomembrane, covered by compacted clay, may tear, leading to the failure of the liner system’s impermeability and its ability to contain leachate [33]). Thus, the unconfined compressive strength of liner material under heavy metal ion erosion is a critical indicator for assessing the safe operation of landfills.
In this study, sludge-based activated carbon (SAC) was assessed as a sorptive additive for capturing Cd(II) and Cu(II) in CCLs through batch tests. The permeability coefficients of SAC-CCLs were determined using flexible wall permeation tests, with Cd(II) and Cu(II) solutions serving as permeation media. To investigate the migration of Cd(II) and Cu(II) within SAC-CCLs, full-scale soil column model tests were conducted on landfill liners. Diffusion coefficients were derived by fitting experimental test data to numerical calculations of one-dimensional nonlinear solute migration. To evaluate the loading capacity of the modified liner following Cd(II) and Cu(II) contamination, its unconfined compressive strength was measured after the soil column test.

2. Materials and Methods

2.1. Testing Materials

The sludge used in this study was freshly dewatered and sourced from the Wuhan Hanxi Wastewater Treatment Plant in Hubei Province. It had a moisture content of 80.3%, an organic matter content of 43.2%, and a pH of 6.96. The elemental composition of the sludge is presented in Table 1. The sludge was dried in an oven at 105 °C for 24 h. After drying, the sludge was ground in a ball mill, passed through a 1 mm sieve, sealed, and stored. The sludge particles were soaked in a 50% ZnCl2 solution at an impregnation ratio of 1:1. After 24 h of impregnation, the sludge particles were dried again in an oven at 105 °C. The activated sludge was placed in a quartz boat within a tube vacuum furnace (GSL, Hangzhou Zhuochi Instruments Co., Ltd., Hangzhou, China) and carbonized under an N2 atmosphere at 400 °C. The SAC was washed with distilled water until its pH approached neutrality [34]. The prepared SAC and its microscopic morphology are shown in Figure 1, and its physical properties are presented in Table 2.
The clay was sourced from shallow soil at a subway construction site in Wuhan, Hubei Province, China, at a depth of 2.5–3.0 m. The basic physical and chemical properties of the clay are presented in Table 3. The clay was dried at 105 °C, then crushed and passed through a 2 mm sieve. The SAC was uniformly mixed into the clay at 1%, 3%, and 5% to prepare the SAC-modified clay [35]. The geotechnical properties of the clay and the modified clay are presented in Table 4. The experimental processes are presented in Figure 2.

2.2. Batch Tests

Four grams of dried soil samples were dispersed in 25 mL of Cd(NO3)2 and Cu(NO3)2 solutions. The concentrations of Cd(II) and Cu(II) ranged from 50 to 300 mg/L. The dispersions were agitated in a water bath at 25 °C and 120 rpm for 24 h. The mixture was centrifuged at 3000 rpm for 10 min using a TDL-40B centrifuge (Shanghai Anting Electronic Instruments Co., Ltd., Shanghai, China). The concentrations of Cd(II) and Cu(II) in the supernatant were measured using an atomic absorption spectrophotometer (AA320N, Shanghai Jingke Instruments Co., Ltd., Shanghai, China) according to GB 7475-87 [36]. The adsorption of Cd(II) and Cu(II) by soil particles was calculated based on mass balance [37,38].

2.3. Permeability Tests

Compacted soil samples were prepared at maximum dry density and optimum moisture content, with a specimen diameter of 5 cm and a height of 10 cm. Permeation tests were conducted on saturated specimens using a geotechnical flexible wall permeameter (PN3230M, GEOEQUIP, Atlanta, GA, USA) under an ambient pressure of 0.55 MPa and a counter pressure of 0.50 MPa, following ASTM D 5084 [39] and ASTM D 7100 [40] standards [41]. The permeation medium was a 100 mg/L solution of Cd(NO3)2 and Cu(NO3)2. The hydraulic conductivity coefficient (K) was calculated using Equation (1).
Κ = Q × H × ρ × g 1 0 5 × A × t × P
where K is the hydraulic conductivity, Q is the permeate water volume at time t, H is the specimen height, A is the specimen area, P is the osmotic pressure, and ρ is the solution density.

2.4. Soil Column Model Tests

Soil column tests were conducted using large cylindrical columns with a diameter of 30 cm and a height of 100 cm, as shown in Figure 3. To minimize the “wall effect” during the soil column tests, a 1 cm thick layer of rubber was installed on the inner wall of each column. Soil samples with a thickness of 80 cm were compacted in the columns at maximum dry density and optimum moisture content. Initially, a constant pressure water head of 10 m of deionized water was applied to the soil surface. Once a stable saturation drainage rate was achieved, the deionized water was replaced with a constant concentration of Cd(II) and Cu(II) solutions. The constant pressure water head was adjusted to 1 m and 10 m. The concentration of Cd(II) and Cu(II) solutions was 100 mg/L. The duration of the soil column test was 360 days [42,43].
Upon completion of the soil column test, the soil sample was extracted using a steel pipe. The soil column was divided into thicknesses of 0–10 cm and 10–80 cm, and each segment was evenly cut into 5 and 14 sub-layers using stainless steel wires. The soil samples were loaded into a cylindrical steel mold with a diameter of 5 cm and a height of 10 cm, and pore water was extracted by squeezing the soil samples using a microcomputer-controlled electronic pressure tester (YAW2000, Jinan Zhongluchang Testing Machine Manufacturing Co., Ltd., Jinan, China). The pore water was centrifuged at 3000 rpm for 10 min, and the concentrations of Cd(II) and Cu(II) ions in the pore water were measured using an atomic absorption spectrophotometer.

2.5. Unconfined Compression Tests

To evaluate the effects of Cd(II) and Cu(II) solutions on the strength of compacted clay and modified clay liners, soil samples were extracted using a portable coring drill after the soil column test, with a specimen diameter of 3.5 cm and a height of 8 cm. Ten soil samples were collected from each column. A strain-controlled unconfined compression apparatus (YYW-2, Nanjing Soil Instrument Factory Co., Ltd., Nanjing, China) was used to measure the compressive strength of the soil samples according to GB/T 50123-2019 [44]. The axial strain rate was 3% per minute.

2.6. Chemical Property

After the completion of the soil column test, SAC-CCL samples were extracted from the top of the soil column with a water head height of 1 m. The samples were analyzed using a combination of SEM-EDS (Gemini SEM 300, Carl Zeiss AG, WAN, Jena, Germany) and FT-IR (Nicolet 6700, Thermo Fisher Scientific, Waltham, MA, USA) to assess the functional groups, surface microstructure, and elemental composition.

3. Results and Discussion

3.1. Adsorption Isotherms

The adsorption isotherms of the soil samples for Cd(II) and Cu(II) are shown in Figure 4. As depicted in the figure, the adsorption of Cd(II) and Cu(II) by all soil samples follows the Langmuir adsorption model, with coefficients of determination (R2) exceeding 0.97. The SAC-modified clay exhibited higher adsorption capacity for Cd(II) and Cu(II) compared to the unmodified clay liner building material, and this advantage became more pronounced with increasing SAC content. Furthermore, the adsorption of Cu(II) by the soil samples was significantly greater than that of Cd(II). Consequently, the SAC-CCL building material demonstrated a superior retarding effect on the penetration of heavy metal ions. The adsorption of these ions by soil samples primarily occurs through physical adsorption [45,46], which explains the variability in adsorption capacity due to the different specific surface areas of the adsorbents. In this study, the specific surface areas of the clay and SAC were 60.55 m2/g and 546.82 m2/g, respectively, with SAC offering more adsorption sites for heavy metal ions.
It is important to note that lower soil-to-solution ratios (e.g., 1:10) in batch tests can overestimate the retardation factor of landfill liners for heavy metal ion transport. Therefore, to obtain isotherm parameters (qm, b) that are more reflective of the soil column tests, higher soil-to-solution ratios (4:25) were employed in this study. The qm and b values for Cd(II) adsorption by clay were 346.99 mg/kg and 0.046 L/mg, respectively, while for Cu(II) adsorption, they were 426.67 mg/kg and 0.061 L/mg. For SAC-modified clay, the qm and b values for Cd(II) ranged from 348.95 to 397.70 mg/kg and 0.055 to 0.077 L/mg, respectively, and for Cu(II) ranged from 521.37 to 558.63 mg/kg and 0.077 to 0.11 L/mg, respectively.

3.2. Hydraulic Conductivity

The hydraulic conductivity of the compacted soil samples is shown in Figure 5. The hydraulic conductivity coefficients of all soil samples initially exhibited a rapid increase, followed by a gradual decrease until stabilization as the penetration time of the Cd(II) and Cu(II) solutions increased. The effective porosity of the soil sample determines its hydraulic conductivity [47]. Although the soil samples were pre-processed by vacuum saturation before the start of the permeation test, the small pore scale and high curvature of the clay resulted in a large number of disconnected pores within the sample. At the beginning of the permeation test, the solution percolated through the effective pores with a small permeation flow. As the permeation time extended, the high osmotic pressure facilitated the transport of fine soil particles, increasing pore connectivity and effective porosity, thereby leading to an increase in permeation flow. In the later stages of the permeation test, the acidic permeation solution corroded the clay particles and altered the morphology of the soil particles, causing pore blockage and reducing permeation flow [48].
The SAC particles are rich in pores, and the addition of SAC altered the close contact between clay particles, providing more effective pathways for solution percolation. This resulted in a significant increase in the hydraulic conductivity of the SAC-amended clay compared to that of compacted clay. The hydraulic conductivity of 1–5% SAC-modified clay ranged from 2.67 × 10−9 to 1.06 × 10−8 cm/s under Cd(II) penetration and from 2.57 × 10−9 to 1.10 × 10−8 cm/s under Cu(II) penetration. These values are 2–10 times higher than the hydraulic conductivities of 1.32 × 10−9 and 1.58 × 10−9 cm/s observed for the compacted clay. However, the hydraulic conductivity of the modified clay remains within the engineering requirement of 1.0 × 10−7 cm/s for sanitary landfill liners.

3.3. Transport Curve

A one-dimensional solute transport equation was employed to simulate the migration of heavy metal ions through the enhanced clay liner of the landfill [49]). Migration parameters, which are critical in assessing the liner’s ability to retard heavy metals, can be derived either from breakthrough curves of the heavy metal ions or from soil column profile concentrations [50,51]. The concentration profiles of Cu(II) and Cd(II) in the soil column after 360 days of migration are illustrated in Figure 6. The observed values were in reasonable agreement with the predictions of the one-dimensional solute transport equation. The concentrations of Cd(II) and Cu(II) in the pore water exhibited a marked decrease with increasing soil column depth, with Cd(II) showing a faster migration rate than Cu(II). According to the “Groundwater Quality Standard” (GB/T14848-2017) [52], the permissible concentrations for Cu(II) and Cd(II) in Class IV groundwater are 1.5 mg/L and 0.01 mg/L, respectively. The depths of the soil column corresponding to a Cu(II) concentration of 1.5 mg/L in pore water at a water head of 1 or 10 m were as follows: for clay, 40.3 cm or 54.8 cm; for clay containing 1% SAC, 39.3 cm or 51.2 cm; for clay containing 3% SAC, 32.6 cm or 44.8 cm; and for clay containing 5% SAC, 40.8 cm or 57.4 cm. For Cd(II), the depth of the soil column at which the pore water concentration reached 0.01 mg/L in the clay at a water head of 1 m was 67.9 cm. At a water head of 10 m, the Cd(II) concentration in the clay at a depth of 80 cm was 0.17 mg/L (Figure 6a). When the water head increased from 1 m to 10 m, the depths of the soil column corresponding to a Cd(II) concentration of 0.01 mg/L in pore water for clay containing 1% SAC and 3% SAC extended from 53.8 cm and 47.5 cm to 75.9 cm and 68.3 cm, respectively (Figure 6b,c). However, in the clay containing 5% SAC, the Cd(II) concentration in pore water at a soil column depth of 80 cm reached 0.03 mg/L or 1.13 mg/L at a water head height of 1 m or 10 m, respectively (Figure 6d). Therefore, clay containing 3% SAC exhibited the most effective retardation of Cu(II) and Cd(II), followed by clay with 1% SAC and 5% SAC. In conclusion, SAC-CCL landfill liner building material can effectively retard the migration of heavy metal ions. However, the SAC content should not exceed 3%. Excessive SAC addition increases the discrete nature of clay particles and the permeability coefficient of the liner layer, thereby accelerating the breakthrough of heavy metals through the liner’s permeability layer.
The diffusion coefficient (D) values were back-calculated by adjusting D values until the concentration profiles predicted by the FORTRAN program accurately matched the experimental data [53]. The optimal D values for all soil columns are presented in Table 5. For clay containing 1% or 3% SAC at a water head height of 1 m, the D values for Cd(II) and Cu(II) were 3.9 × 10−10 m2/s and 2.8 × 10−10 m2/s or 3.4 × 10−10 m2/s and 1.9 × 10−10 m2/s, respectively. These values represent reductions of 4.9% and 20%, or 17.1% and 45.7%, compared to the D values for unmodified clay. However, the D values for Cd(II) and Cu(II) in clay containing 5% SAC were approximately double those of unmodified clay. When the water head height was increased to 10 m, all soil columns exhibited a significant rise in D values. The D value for unmodified clay increased by a factor of 1.1. For clay containing 1%, 3%, or 5% SAC, the D values increased by factors of 0.6–1.1, 0.7–1.3, and 0.6–0.5, respectively. Previous studies have reported D values for heavy metal ions, such as Cr(VI), Cd(II), and Pb(II), ranging from 1.0 × 10−10 to 12.0 × 10−10 m2/s [54,55,56,57]. In the present study, the D values for Cd(II) and Cu(II) in the compacted soil layer (1.9 × 10−10 to 13.5 × 10−10 m2/s) are well within these expected ranges. The diffusion coefficient is influenced by several factors, including particle size, pore space, and molding density of the porous medium, as well as the fluid flow velocity [58,59]. The addition of 5% SAC to the compacted clay resulted in a significant increase in the overall particle and pore sizes of the material within the soil column. This, in turn, led to a decrease in the packing density and an increase in the actual flow velocity of the water.

3.4. Unconfined Compressive Strength

The unconfined compressive strength of compacted soil samples at varying depths is illustrated in Figure 7. The initial unconfined compressive strength of the clay was measured to be 408–584 kPa. When supplemented with 1%, 3%, or 5% SAC, the unconfined compressive strengths were reduced to 358–531 kPa, 310–452 kPa, and 288–391 kPa, respectively, demonstrating a decrease of 9.1–12.3%, 22.6–24.0%, and 29.4–33.0% compared to the original clay. This reduction in compressive strength highlights that the incorporation of SAC, while enhancing the clay’s adsorption capacity, compromises its structural integrity. However, despite this reduction, the SAC-CCL composite material remains capable of supporting landfill waste loads of up to 30 m in height, thereby significantly mitigating the risk of bending deformation and potential damage to the liner under the substantial weight of the landfill.
Furthermore, the unconfined compressive strength of the soil samples exhibited a negative correlation with the degree of contamination by Cd(II) or Cu(II), with the structural integrity of the soil being more severely compromised by Cd(II) contamination compared to Cu(II). Specifically, the compressive strength of soils contaminated with Cd(II) decreased by 3.7–6.8% relative to those contaminated with Cu(II). Additionally, within the soil column at depths of 56–80 cm (Figure 7a–c), the unconfined compressive strength at a pressure head of 10 m was observed to be higher than at 1 m. This is attributed to the reduced extent of corrosion damage inflicted by Cd(II) or Cu(II) in these lower sections of the compacted soil samples. At higher pressure heads, increased seepage velocities facilitated the deposition and consolidation of fine clay particles within the pore spaces at the lower end of the soil column, leading to a more compacted structure in these regions [60,61].

3.5. Illustration of the Mechanism

The surface of the SAC-CCL material exhibits a significant presence of fine particles and larger aggregates, which are highly conducive to both chemical and physical adsorption processes. The smaller particles, in particular, enhance the exposure of active sites, thereby increasing the material’s overall adsorption capacity, as illustrated in Figure 8a,c,e,g. The elemental analysis reveals that the C contents in SAC-CCL material range between 13 and 16%, and the O contents is between 44 and 45%, indicating an abundance of these elements within the structure, as shown in Figure 8d,h. Post-treatment analysis, following Cu and Cd adsorption, shows that the combined content of Al, Si, and Ca elements constitutes 38.5% and 42.81%, respectively. This substantial presence of silicate minerals suggests the material’s ability to serve as effective carriers for heavy metal adsorption. Moreover, the high O content indicates a strong cation exchange capacity, facilitating ion exchange processes wherein Ca ions are replaced by Cu(II) and Cd(II).
The FTIR spectrum analysis (Figure 8i) reveals a broad band at 3420 cm−1, attributable to hydroxyl groups (O-H stretching vibration), as well as monomeric alcohols and phenols. The peak at 1627 cm−1 corresponds to the C=O functional group found in carboxylic acids, anhydrides, ketones, and esters. The peak at 1425 cm−1 is associated with the stretching vibration of the carboxyl group (COOH) or the aromatic hydroxyl group (-OH) of the phenol group. Additionally, the peak observed at 1035 cm−1 can be ascribed to the symmetric C-O-C stretching present in cellulose, hemicellulose, and lignin. The bending vibration peak at 780 cm−1, corresponding to the Si-O bond, further confirms the presence of silicate minerals in the biochar-modified clay material.

4. Conclusions

To evaluate the retarding characteristics of the SAC-CCL landfill liner for heavy metal ions, specifically Cd(II) and Cu(II), we conducted a comprehensive analysis. This included assessing the adsorption capacity of SAC-modified compacted clay for these ions, determining the permeability coefficient, and systematically analyzing the migration processes of Cd(II) and Cu(II) within a full-scale SAC-CCL landfill liner. Additionally, the unconfined compressive strength of the contaminated soil was examined.
(1)
The SAC-amended clay demonstrated a significantly higher adsorption capacity for Cd(II) and Cu(II) compared to unmodified clay, consistent with the Langmuir adsorption isotherm model.
(2)
The permeability coefficient of SAC-modified clay was determined to be in the range of 2.57 × 10−9 to 1.10 × 10−8 cm/s, satisfying the engineering requirements of 1.0 × 10−7 cm/s for landfill liner anti-seepage systems. The diffusion coefficients (D) for Cd(II) and Cu(II) were found to range from 1.9 × 10−10 to 13.5 × 10−10 m2/s. The unconfined compressive strength of the SAC-CCL was measured between 288 and 531 kPa, indicating that it can support landfill waste heights exceeding 30 m.
(3)
The SAC-CCL effectively retards the migration of heavy metal ions, highlighting the potential application value of SAC-modified clay as a landfill liner material. However, further research is needed to assess the migration behavior of heavy metal ions in SAC-CCL under cyclic wet–dry conditions.

Author Contributions

Conceptualization, H.L.; Validation, Z.W.; Formal analysis, Z.W.; Investigation, G.C.; Resources, J.X.; Writing—original draft, J.X.; Writing—review & editing, Q.Z. and M.Z.; Supervision, M.Z.; Funding acquisition, H.L. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financially supported by the Natural Science Foundation of Hubei Province of China (2023AFD214).

Data Availability Statement

Data is contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Buildings 14 02638 g001
Figure 1. Prepared SAC and typical scanning electron microscope (SEM) images.
Figure 1. Prepared SAC and typical scanning electron microscope (SEM) images.
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Figure 2. A summary diagram of experimental process.
Figure 2. A summary diagram of experimental process.
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Figure 3. Schematic of the column test.
Figure 3. Schematic of the column test.
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Figure 4. Adsorption isotherms for Cd (II) or Cu (II) in the soil samples.
Figure 4. Adsorption isotherms for Cd (II) or Cu (II) in the soil samples.
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Figure 5. Hydraulic conductivity of the compacted soil samples.
Figure 5. Hydraulic conductivity of the compacted soil samples.
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Figure 6. Measured and predicted concentrations of Cd(II) or Cu(II) in the column tests.
Figure 6. Measured and predicted concentrations of Cd(II) or Cu(II) in the column tests.
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Figure 7. Unconfined compressive strength of the compacted soil samples.
Figure 7. Unconfined compressive strength of the compacted soil samples.
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Figure 8. The functional groups, surface microstructure, and elemental composition of SAC-CCL samples. (a–h) SEM-EDS images, (i) FTIR images.
Figure 8. The functional groups, surface microstructure, and elemental composition of SAC-CCL samples. (a–h) SEM-EDS images, (i) FTIR images.
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Table 1. Elemental composition of sludge.
Table 1. Elemental composition of sludge.
Element NameCOMgAlSiPSKCaFeOther
Content/(%)7.8754.11.285.8014.832.341.871.543.775.191.33
Table 2. Basic physical indices of SAC.
Table 2. Basic physical indices of SAC.
SAC Yield/(%)Total Pore Volume
/(cm3/g)		Mean Pore Diameter/(nm)BET Surface Area/(m2/g)Iodine Number/(mg/g)Carbon Content/(%)
82.610.135.75546.82477.254.14
Table 3. Basic physical and chemical indices of the clay.
Table 3. Basic physical and chemical indices of the clay.
pHSoluble Salt/(%)Organic/(%)Particles Size Distribution/(%)
>0.05/(mm)0.05–0.005/(mm)0.005–0.002/(mm)<0.002/(mm)
5.80.51.212324511
Table 4. Geotechnical mechanical indices of the soil samples.
Table 4. Geotechnical mechanical indices of the soil samples.
Soil SamplesLiquid Limit/(%)Plastic Limit
/(%)		Plasticity Index
/(%)		Maximum Dry Density
/(g/cm3)		Optimum Water Content
/(%)		Porosity RatioSpecific Gravity
Raw clay48.5126.2422.271.7219.150.362.65
Clay containing 1% SAC 43.9323.2620.671.7020.220.372.62
Clay containing 3% SAC40.1720.0420.131.6618.570.392.59
Clay containing 5% SAC36.8217.3219.501.6317.820.402.54
Table 5. D values of the landfill clay liners containing SAC.
Table 5. D values of the landfill clay liners containing SAC.
Heavy MetalsCd(II)/(m2/s)Cu(II)/(m2/s)
Soil Samples H = 1 mH = 10 mH = 1 mH = 10 m
Clay4.1 × 10−108.8 × 10−103.5 × 10−107.2 × 10−10
Clay containing 1% SAC3.9 × 10−106.2 × 10−102.8 × 10−105.9 × 10−10
Clay containing 3% SAC3.4 × 10−105.7 × 10−101.9 × 10−104.4 × 10−10
Clay containing 5% SAC8.2 × 10−1013.5 × 10−107.7 × 10−1011.3 × 10−10
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Xu, J.; Lu, H.; Wang, Z.; Zhang, Q.; Cai, G.; Zang, M. Experimental Study on Transport of Cd(II) and Cu(II) in Landfill Improved Clay Liners Building Material Containing Municipal Sludge-Activated Carbon. Buildings 2024, 14, 2638. https://doi.org/10.3390/buildings14092638

AMA Style

Xu J, Lu H, Wang Z, Zhang Q, Cai G, Zang M. Experimental Study on Transport of Cd(II) and Cu(II) in Landfill Improved Clay Liners Building Material Containing Municipal Sludge-Activated Carbon. Buildings. 2024; 14(9):2638. https://doi.org/10.3390/buildings14092638

Chicago/Turabian Style

Xu, Jun, Haijun Lu, Zhenhua Wang, Qian Zhang, Guanghua Cai, and Meng Zang. 2024. "Experimental Study on Transport of Cd(II) and Cu(II) in Landfill Improved Clay Liners Building Material Containing Municipal Sludge-Activated Carbon" Buildings 14, no. 9: 2638. https://doi.org/10.3390/buildings14092638

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