The Breakthrough Time Analyses of Lead Ions in CCL considering Different Adsorption Isotherms

Abstract

As the 2 m thick compacted clay liner with permeability coefficient of 1 × 10⁻⁷ cm/s is required in the Chinese technical specifications about landfill, the performance of this compacted clay liner was analyzed considering three different adsorption isotherms (convex, straight, and concave). The effects of source concentration, adsorption mode, and waterhead on the breakthrough curve and breakthrough time of Pb²⁺ were discussed. The results indicate that reducing the concentration of pollution sources is beneficial to prolonging the breakthrough time. With the waterhead of 10 m, the absolute breakthrough time, respectively, increased from 2.77 to 3.7 years (concave type isotherm), from 17.63 to 26.58 years (straight type isotherm), and from 35.43 to 59.6 years (convex type isotherm), as the source concentration decreased from 1000 mg/L to 10 mg/L. The effect of adsorption isotherm type on the performance of the barrier is very obvious: with the waterhead of 10 m, the absolute breakthrough time corresponding to the convex isotherm is more than twice that of the straight adsorption isotherm, and more than 12.8 times that of the concave isotherm. The absolute breakthrough time corresponding to 0.3 m waterhead is more than 4 times that of 10 m, and reducing the waterhead can effectively increase the breakthrough time.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Compacted clay liner (CCL) is used as a common type of barrier in landfills of China. Especially in the large number of existing old landfills, CCLs are basically used as the main antifouling barriers. Compacted clay can not only be used as an important impermeable material combined with geomembrane and GCL to form a composite liner, but also be a separate liner [1]. The technical parameters of CCL are specified in many of the specifications [2–4]: the permeability coefficient of natural clay liner or modified clay liner shall not be greater than 1.0 × 10⁻⁹ m/s, and the thickness of liners at the field bottom and the four walls shall not be less than 2 m.

As the horizontal antiseepage and antifouling structure of the landfill, the performance of the liner is directly related to the effect of groundwater pollution control. In the field of barrier performance analysis and breakthrough time evaluation, the former researchers have done a lot of research work [5–15]. Acar and Haider [8] proposed a set of design methods for soil barrier in landfills based on one-dimensional pollutant convection-dispersion model and explained the analysis methods by experiments. Shackelford [9] believed that the US specification requires that 0.9 m thick compacted clay with a permeability coefficient of 1.0 × 10⁻⁹ m/s may not meet the antifouling requirements within the operational life of the antifouling facility (usually 40 to 50 years) and proposed that the design of liner thickness should meet the condition that the target pollutant within the design period should not exceed the concentration limit. Based on the one-dimensional convection-dispersion analytical solution of a semi-infinite thickness uniform medium, the transit-time design method for clay liner was proposed. Shackelford and Glade [10] discussed the pollution leakage of the soil downstream under different Rd conditions during linear adsorption by theoretical analysis. The larger the Rd is, the later the pollutant will penetrate the barrier, and its effect is affected by convection. Foose [11] deduced one-dimensional analytical solution of pollutant diffusion in composite liner and, based on this analytical solution, proposed a transit time design method for complete composite liner. Foose et al. [12] compared the permeability, mass flux, and adsorption capacity of three composite liners (GCL and two geomembrane + soil layers with different thicknesses) and used one-dimensional and three-dimensional numerical models to analyze the effects of permeability coefficient, waterhead, and diffusion coefficient on the performance of the liner. Xie et al. [13] compared and discussed the antifouling performance of four kinds of liners in China, using theoretical methods. Chen et al. [14] proposed a decoupling method for the wall thickness design of vertical separation walls. Most of these studies assume that the pollutants satisfy the linear adsorption model. However, the research results in the literatures show that the isothermal adsorption curves of heavy metal ions are usually nonlinear [15–17].

Adsorption isotherms can be divided into four categories according to the slope change of the curve closest to the origin [18–20]. The author found that the adsorption isotherm can be divided into three types: convex, straight, and concave, according to the slope change of the curve nearest the origin. And the effect of isotherm type on the contaminant transport in barrier has not been discussed.

As the contaminants migrate very slowly in CCL, the design life of CCL should be several decades. However, CCLs meeting the technical requirements of the specifications have not been used in landfill in China for more than 30 years until now. The earliest data investigated about pollutant transport in foreign landfill sites is 20 years [21], and the earliest data measured in China is 13 years [22]. So, CCL’s breakthrough time under the complicated field conditions in China has not been testified in the field project.

In this paper, the effect of isotherm type on the service performance and breakthrough time of the CCLs required in the specifications is analyzed for the first time.

2. Theoretical Analyses for Performance of CCL

2.1. Model and Main Parameters

According to the requirements of specification, we took the 2 m thick clay liner with permeability coefficient of 1×10⁻⁷cm/s as the analysis object. The common adsorbent heavy metal ion Pb²⁺ was selected as the target pollutant. Figure 1 is a schematic diagram of a horizontal clay liner. It is assumed that the source concentration keeps constant, the upstream of liner is the pollution source solution, and the downstream of the liner is aquifer. As one-dimensional migration of pollutants takes place in the liner, the function of the liner is to prevent pollutants in the leachate from entering the underlying aquifer through vertical seepage.

[figure omitted; refer to PDF]

According to the on-site investigation, the water level of landfill in China is high, which can reach more than ten meters [23, 24], while the landfill specifications require that the height of the waterhead on the liner should not exceed 30 cm. Therefore, in the analyses, the waterhead difference between the upstream and downstream barriers was considered in two cases: when the waterhead difference is high, the height of the waterhead on the liner is taken as 10 m; when the waterhead difference is low, the height of the waterhead on the liner is taken as 0.3 m.

Three source concentrations were considered in the analyses: 10 mg/L, 100 mg/L, and 1000 mg/L. According to the literature reports, the concentration range of Pb²⁺ in the leachate from abroad landfill is 0.001–5 mg/l [25, 26], while that in the leachate from the landfill of China is 0.196–57.08 mg/l [27, 28]. Therefore, according to the domestic situation, 10 mg/L is taken for low concentration. Compared with the landfill, the content of heavy metal ions in the waste liquid of slag yard and tailings pond is much higher. Chang et al. [29] surveyed several major heavy metal mining areas of Youxi lead-zinc mine, Liancheng lead-zinc mine, and Liancheng manganese mine in Fujian and found that the highest content of Mn, Zn, Pb, and Cd in the soil of the mining area, respectively, reached 92546, 27454, 23792, and 248 mg/kg. Ji et al. [30] analyzed the composition and content of heavy metals in 12 electroplating sludge samples from different electroplating enterprises. The results show that the main heavy metal species in the sludge are Cr, Cu, Ni, Pb, Zn, and Sn, and their content ranges from 40 to 14230, 22 to 16000, 14 to 20000, 5 to 120, 37 to 91000, and 32 to 19000 mg/kg. If the content of heavy metals in the solid phase is so high, the concentration of heavy metals in the corresponding pore water will also be very high. Therefore, in order to consider the effect of the source concentration value on the migration of heavy metals, the other two high concentrations in the analysis were taken as 100 mg/L and 1000 mg/L, respectively. The remaining parameters are as follows [31, 32]: the barrier porosity n was taken as 0.62, the dry density was taken as 1.04 g/cm³, the water content was taken as 0.6, the effective diffusion coefficient of Pb²⁺ was taken as 3.3 × 10⁻¹⁰m²/s, and the dispersity was taken as 0.04 m.

2.2. Scheme for Analyses

In order to study the effect of various parameters on the breakthrough time of the CCL with different adsorption models, three types of adsorption isotherms were selected: convex, straight, and concave. Langmuir sorption isotherm, linear sorption isotherm, and Freundlich sorption isotherm were used as representative models, respectively.

12 cases were set up for analysis in this paper, as shown in Table 1. Cases 1∼3, Cases 4∼6, and Cases 7∼9 aim to compare the impact of different source concentrations (1000 mg/L, 100 mg/L, and 10 mg/L) on breakthrough time. Cases 1∼3 are Straight isotherms, Cases 4∼6 are concave isotherms, and Cases 7∼9 are convex isotherms. The values of Cases 1∼3 adsorption parameters refer to the results of kaolin adsorption experiments on lead in Zeng [32]. The parameters of the Langmuir adsorption isotherm (convex) of Cases 4∼6 are Q⁰ = 7 mg/g, b = 0.00148 L/mg, and the parameters of the Freundlich model (concave) of Cases 7∼9 are K_F = 0.000132 L/g, n_f = 1.5. The maximum adsorption amount of 1000 mg/L on the adsorption isotherm corresponding to the parameters of Langmuir model and Freundlich model is equal to that of the linear model (R_d = 8) and can represent the convex and concave isotherms, respectively. Therefore, Case 1, Case 4, and Case 7, Case 2, Case 5, and Case 8, and Case 3, Case 6, and Case 9 can compare the effects of different adsorption modes, and the corresponding source concentrations are 1000 mg/L, 100 mg/L, and 10 mg/L, respectively.

Table 1

Analyses scheme.

Cases	Waterhead on liner (m)	Concentration (mg/L)	Adsorption model	Adsorption parameters	Absolute breakthrough time ta (year)	Relative breakthrough time t0.1 (year)
1	10	1000	Linear (straight)	Rd = 8(Kd = 0.00417 L/g)	17.63	35.71
2	10	100			21.28	35.71
3	10	10			26.58	35.71
4	10	1000	Langmuir (convex)	Q0 = 7 mg/g, b = 0.00148 L/mg	35.43	47.94
5	10	100			46.27	77.23
6	10	10			59.6	81.22
7	10	1000	Freundlich (concave)	KF = 0.000132 L/g, nf = 1.5	2.77	20.69
8	10	100			3.1	9.54
9	10	10			3.7	5.91
10	0.3	1000	Linear (straight)	Rd = 12.5(Kd = 0.006856 L/g)	84.16	225.42
11	0.3	1000	Langmuir (convex)	Q0 = 12 mg/g, b = 0.00133 L/mg	173.88	334.92
12	0.3	1000	Freundlich (concave)	KF = 0.000217 L/g, nf = 1.5	11.42	122.21

Case 1 and Case 10, Case 2 and Case 11, and Case 3 and Case 12 are the analysis groups for comparing the impact of waterhead on migration, respectively. Case 1 and Case 10 are straight isotherms, Case 2 and Case 11 are convex isotherms, and Case 3 and Case 12 are concave isotherms. Considering the effect of waterhead is not only the direct effect of waterhead on breakthrough time, but also the effect of velocity on adsorption [32–34]. Therefore, the adsorption parameters of different waterheads are different; refer to Zeng [32] for specific values. Case 11 and Case 12 take the adsorption isotherm, respectively, higher than Case 4 and Case 7. The Langmuir model parameters of Case 11 are taken as Q⁰ = 12 mg/g, b = 0.00133 L/mg, and the Freundlich model parameters of Case 12 are taken as K_F = 0.000217 L/g, n_f = 1.5. At the same time, the maximum adsorption amounts corresponding to 1000 mg/L on the adsorption isotherm of Case 10, Case 11, and Case 12 are equal. Therefore, similar to Case 1, Case 4 and Case 7, Cases 10∼12 can compare the effects of different adsorption modes at low waterhead. The specific adsorption model parameters used in the analyses are shown in Figure 2.

[figure omitted; refer to PDF]

Therefore, reducing the concentration of pollution source is beneficial to prolonging the breakthrough time: when the adsorption mode is convex, reducing the source concentration has the most obvious effect on prolonging the breakthrough time; when the adsorption mode is concave, reducing the source concentration has little effect on prolonging the breakthrough time. In order to improve the service performance of the barrier and prolong the breakthrough time, the concentration of the pollution source in the site should be reduced as much as possible in the project. Strongly adsorbent materials can be added to the solution of the pollution source to reduce the ion concentration in the solution. Or a thin layer of high adsorption material can be placed upstream of the barrier to reduce the concentration of pollutants entering the barrier.

4.2. Effect of Different Adsorption Models on Barrier Performance and Breakthrough Time

The comparison of the outflow concentrations corresponding to different adsorption models with source concentrations of 1000 mg/L, 100 mg/L, and 10 mg/L is shown in Figure 5(a)–5(c), respectively. When the source concentration is 100 mg/L and 10 mg/L, the outflow concentration curve corresponding to the convex type (Langmuir model) appears the latest, and the outflow concentration curve corresponding to the downward concave type (Freundlich model) appears earliest. When the source concentration is 1000 mg/L, the outflow concentration of the concave type (Freundlich model) appears first, then the linear model, and the convex type (Langmuir model) the latest. The three curves have common intersection points. After the intersection point, the outflow concentration corresponding to the convex type (Langmuir model) is the highest and reaches equilibrium first, the straight type is in the middle, and the concave type (Freundlich model) is the lowest. Figure 5(d) is a comparison of Case 10, Case 11, and Case 12, with the same rules as in Figure 5(a).

[figures omitted; refer to PDF]

The comparison between t_a and t_0.1 is shown in Figure 4. As shown in Figure 4, at the same source concentration, t_a and t_0.1 corresponding to the three adsorption models always have the same pattern: the cases with convex type have the longest breakthrough time, followed by the cases with straight type and the cases with concave type. That means that t_a and t_0.1 both correspond to the positional relationship of the isotherm: the higher the adsorption isotherm, the longer the breakthrough time, and the lower the adsorption isotherm, the shorter the breakthrough time. The t_a corresponding to the convex type isotherm is more than twice the t_a corresponding to the straight type isotherm, and more than 12.8 times that corresponding to the concave type isotherm, and the gap increases as the source concentration decreases.

The effect of adsorption isotherm on the antifouling performance of the barrier is very obvious: the cases with the convex type isotherm have the longest breakthrough time, followed by those with the straight type isotherm, and the cases with the concave type isotherm have the shortest breakthrough time. Therefore, in the project, choose the material whose isotherm has a convex shape, or modify the material to change its adsorption isotherm so that its adsorption isotherm becomes an obvious convex shape and, thus, can effectively extend the breakthrough time of the barrier.

4.3. Effect of Waterhead on Barrier Performance

Figures 6(a) and 6(b) show the pore water concentration profiles and the total concentration profiles corresponding to different waterheads, respectively, and the corresponding time is 50 years. As mentioned above, the effect of different waterheads discussed here considers the effect of flow rate on adsorption. The adsorption parameters used in low waterhead cases are larger than those in high waterhead cases. As shown in Figure 6(a), the relationship of pore water concentration is obvious, and the model with a low waterhead has to a shallow concentration front. As shown in Figure 6(b), the distribution of the total concentration profile in the soil is different from that of the pore water concentration. The pollutant migration front of the low waterhead cases (Case 10, Case 11, and Case 12) is shallow, and the corresponding total concentration front is also shallow, but as shown in Figure 2, the adsorption isotherm with a low waterhead is higher, the adsorption is larger, and the total concentration in the soil at the top of the liner is larger. Therefore, the front of the low waterhead case is shallower, but the total concentration value at the top is larger. So, there is an intersection between the total concentration profile of the low head and that of the high head.

[figures omitted; refer to PDF]

The outflow breakthrough curves corresponding to different waterheads (10 m (Cases 1, 4, 7), 0.3 m (Cases 10, 11, 12)) are shown in Figure 5(a) and 5(d). Obviously, the breakthrough curve with low waterhead flows out late. Figure 7 shows the absolute breakthrough time t_a and relative breakthrough time t_0.1 corresponding to different waterheads. As shown in Figure 7, the t_a and t_0.1 corresponding to the 0.3 m head are far greater than 10 m. It can be found that the t_a corresponding to the 0.3 m waterhead is about 4 times more than the 10 m waterhead, and t_0.1 is more than 6 times. Therefore, reducing the waterhead can effectively increase the breakthrough time. It is considered that the decrease of velocity can increase the adsorption of barrier material; that is to say, the decrease of waterhead can not only reduce the convection effect, but also increase the retardation effect of barrier. In the project, methods such as adding membranes to reduce the permeability of the barrier can be considered to reduce the seepage velocity in the clay barrier [39].

[figure omitted; refer to PDF]

5. Conclusion

The service performance of 2 m thick CCLs that meet the specifications is analyzed considering three type isotherms (convex, straight, and concave) for the first time. The effects of source concentration, isotherm type, and waterhead on the breakthrough time of Pb²⁺ are discussed. And the following conclusions can be drawn.

Reducing the concentration of the pollution source is beneficial to prolong the breakthrough time. When the source concentration decreased from 1000 mg/L to 10 mg/L, the absolute breakthrough time, respectively, increased by 0.93 years (concave type isotherm), 8.95 years (straight type isotherm), and 24.17 years (convex type isotherm). The effect of adsorption mode on the performance of the barrier is very obvious. The absolute breakthrough time corresponding to the convex type isotherm is more than twice that of the straight type isotherm, and more than 12.8 times that of the concave type isotherm. Reducing the waterhead can effectively increase the breakthrough time.

In order to improve the service performance of the barrier and prolong the breakthrough time, the following methods can be used in the project: add strong adsorption materials into the pollution source solution to reduce the concentration; or lay a thin layer of highly absorbent materials upstream of the barrier to reduce the pollutant concentration entering the barrier; select the material with convex type, or modify the material to change its adsorption mode from concave type or linear to convex type; take some measures such as adding membranes to reduce the permeability of the barrier to reduce the seepage velocity in the soil barrier.

Acknowledgments

The authors are grateful for the financial support of the National Natural Science Foundation of China (Grant no. 41702329), the Scientific Research Fund of Hunan Provincial Education Department (Grant no. 17B097), Department of Natural Resources of Hunan Province (Grant nos. 2020-15), and the Key Laboratory of Soft Soils and Geoenvironmental Engineering of the Education Ministry of China (Grant no. 2016P05).

References

[1] CJJ 113-2007, Technical Code for Liner System of Municipal Soil Waste Landfill, 2007.

[2] CJJ 17-2004, Technical Code for Municipal Soil Waste Sanitary Landfill, 2004.

[3] CJJ176-2012, Technical Code for Geotechnical Engineering of Municipal Soil Waste Sanitary Landfills, 2012.

[4] GB 50869-2013, Technical Code for Municipal Soil Waste Sanitary Landfills, 2013.

[5] S. Shu, W. Zhu, S. Wang, C. W. W. Ng, Y. Chen, A. C. F. Chiu, "Leachate breakthrough mechanism and key pollutant indicator of municipal solid waste landfill barrier systems: centrifuge and numerical modeling approach," Science of The Total Environment, vol. 612, pp. 1123-1131, DOI: 10.1016/j.scitotenv.2017.08.185, 2018.

[6] S. Shu, W. Zhu, J. Shi, "A new simplified method to calculate breakthrough time of municipal solid waste landfill liners," Journal of Cleaner Production, vol. 219, pp. 649-654, DOI: 10.1016/j.jclepro.2019.02.050, 2019.

[7] S. Shu, W. Zhu, H. Xu, "Effect of the leachate head on the key pollutant indicator in a municipal solid waste landfill barrier system," Journal of Environmental Management, vol. 239, pp. 262-270, DOI: 10.1016/j.jenvman.2019.03.065, 2019.

[8] Y. B. Acar, L. Haider, "Transport of low‐concentration contaminants in saturated earthen barriers," Journal of Geotechnical Engineering, vol. 116 no. 7, pp. 1031-1052, DOI: 10.1061/(asce)0733-9410(1990)116:7(1031), 1990.

[9] C. D. Shackelford, "Transit-time design of earthen barriers," Engineering Geology, vol. 29 no. 1, pp. 79-94, DOI: 10.1016/0013-7952(90)90083-d, 1990.

[10] C. D. Shackelford, M. J. Glade, "Analytical mass leaching model for contaminated soil and soil stabilized waste," Groundwater, vol. 35, 1997.

[11] G. J. Foose, "Transit-time design for diffusion through composite liners," Journal of Geotechnical and Geoenvironmental Engineering, vol. 128 no. 7, pp. 590-601, DOI: 10.1061/(asce)1090-0241(2002)128:7(590), 2002.

[12] G. J. Foose, C. H. Benson, T. B. Edil, "Comparison of solute transport in three composite liners," Journal of Geotechnical and Geoenvironmental Engineering, vol. 128 no. 5, pp. 391-403, DOI: 10.1061/(asce)1090-0241(2002)128:5(391), 2002.

[13] H. J. Xie, L. T. Zhan, Y. M. Chen, Z. H. Lou, "Comparison of the performance of four types of liner systems in China," China Civil Engineering Journal, vol. 44, pp. 141-149, 2011.

[14] G. N. Chen, P. J. Cleall, Y. C. Li, Z. X. Yu, H. Ke, Y. M. Chen, "Decoupled advection-dispersion method for determining wall thickness of slurry trench cutoff walls," International Journal of Geomechanics, vol. 18 no. 5,DOI: 10.1061/(asce)gm.1943-5622.0001130, 2018.

[15] C. Coles, R. Yong, "Aspects of kaolinite characterization and retention of Pb and Cd," Applied Clay Science, vol. 22 no. 1-2, pp. 39-45, DOI: 10.1016/s0169-1317(02)00110-2, 2002.

[16] E. I. Unuabonah, K. O. Adebowale, B. I. Olu-Owolabi, L. Z. Yang, "Comparison of sorption of Pb 2+ and Cd 2+ on Kaolinite clay and polyvinyl alcohol-modified Kaolinite clay," Adsorption, vol. 14 no. 6, pp. 791-803, DOI: 10.1007/s10450-008-9142-9, 2008.

[17] M.-q. Jiang, X.-y. Jin, X.-Q. Lu, Z.-l. Chen, "Adsorption of Pb(II), Cd(II), Ni(II) and Cu(II) onto natural kaolinite clay," Desalination, vol. 252 no. 1-3, pp. 33-39, DOI: 10.1016/j.desal.2009.11.005, 2010.

[18] C. H. Giles, D. Smith, A. Huitson, "A general treatment and classification of the solute adsorption isotherm. I. Theoretical," Journal of Colloid and Interface Science, vol. 47 no. 3, pp. 755-765, DOI: 10.1016/0021-9797(74)90252-5, 1974.

[19] C. Hinz, "Description of sorption data with isotherm equations," Geoderma, vol. 99 no. 3-4, pp. 225-243, DOI: 10.1016/s0016-7061(00)00071-9, 2001.

[20] G. Limousin, J.-P. Gaudet, L. Charlet, S. Szenknect, V. Barthès, M. Krimissa, "Sorption isotherms: a review on physical bases, modeling and measurement," Applied Geochemistry, vol. 22 no. 2, pp. 249-275, DOI: 10.1016/j.apgeochem.2006.09.010, 2007.

[21] R. K. Rowe, "Long-term performance of contaminant barrier systems," Géotechnique, vol. 55 no. 9, pp. 631-678, DOI: 10.1680/geot.2005.55.9.631, 2005.

[22] H.-j. Xie, Y.-m. Chen, L.-t. Zhan, "Investigation of migration of pollutant at the base of Suzhou Qizishan landfill without a liner system," Journal of Zhejiang University-SCIENCE A, vol. 10 no. 3, pp. 439-449, DOI: 10.1631/jzus.a0820299, 2009.

[23] Y. Xie, H. J. Xie, Y. M. Chen, "Comparisons of measurements of contaminant concentration in landfill bottom soils with theoretical solutions," Journal of Natural Disasters, vol. 18, pp. 64-71, 2009.

[24] T. L. T. Zhan, C. Guan, H. J. Xie, Y. M. Chen, "Vertical migration of leachate pollutants in clayey soils beneath an uncontrolled landfill at Huainan, China: a field and theoretical investigation," Science of the Total Environment, vol. 470-471, pp. 290-298, DOI: 10.1016/j.scitotenv.2013.09.081, 2014.

[25] P. Kjeldsen, M. A. Barlaz, A. P. Rooker, A. Baun, A. Ledin, T. H. Christensen, "Present and long-term composition of MSW landfill leachate: a review," Critical Reviews in Environmental Science and Technology, vol. 32 no. 4, pp. 297-336, DOI: 10.1080/10643380290813462, 2002.

[26] C. B. Öman, C. Junestedt, "Chemical characterization of landfill leachates–400 parameters and compounds," Waste Management, vol. 28, pp. 1876-1891, 2008.

[27] B. Z. Wang, L. Wang, Treatment and Disposal of Municipal Solid Waste Leachate, 2005.

[28] L. T. Zhan, R. H. Chen, Y. M. Chen, "Migration of heavy metals in soil strata below and around a simple dump of MSWs," Chinese Journal of Geotechnical Engineering, vol. 33, pp. 853-861, 2011.

[29] Q. S. Chang, X. Q. Ma, Z. Y. Wang, "Pollution characteristics of heavy metals and their pollution evaluation in the heavy metal mining areas in south China," Resources and Environment in the Yangtze Basin, vol. 16, pp. 395-399, 2007.

[30] W. J. Ji, Q. Wang, Q. F. Huang, "Heavy metal content in the electroplating sludge differences research," Environmental Engineering, vol. S1, pp. 265-267, 2010.

[31] X. Zeng, L. T. Zhan, X. L. Zhong, Y. M. Chen, "Similarity of centrifuge modeling of chloride dispersion in low-permeability clay," Journal of Zhejiang University(Engineering Ence), vol. 50 no. 2, pp. 241-249, 2016.

[32] X. Zeng, "Similitude for centrifuge modelling of heavy metal migration in clay barrier and method for evaluating breakthrough time," . Doctoral Dissertation, Zhejiang University, Hangzhou, China, 2015, in Chinese

[33] J. Basford, D. J. Goodings, A. Torrents, "“Fate and transport of lead through soil at 1 g and in the centrifuge,” physical modelling in geotechnics: ICPMG’02," pp. 379-383, .

[34] L. Pang, M. Close, D. Schneider, G. Stanton, "Effect of pore-water velocity on chemical nonequilibrium transport of Cd, Zn, and Pb in alluvial gravel columns," Journal of Contaminant Hydrology, vol. 57 no. 3-4, pp. 241-258, DOI: 10.1016/s0169-7722(01)00223-6, 2002.

[35] M. T. Van Genuchten, J. C. Parker, "Boundary conditions for displacement experiments through short laboratory soil columns," Soil Science Society of America Journal, vol. 48 no. 4, pp. 703-708, DOI: 10.2136/sssaj1984.03615995004800040002x, 1984.

[36] X. Zeng, L. T. Zhan, Y. M. Chen, "Applicability of boundary conditions for analytical modelling of advection-dispersion transport in low-permeability clay column tests," Chinese Journal of Geotechnical Engineering, vol. 39, pp. 636-644, 2017.

[37] GB/T 14848-2017, Standard for Groundwater Quality, 2017.

[38] GB 16889-2008, Standard for Pollution Control on the Landfill Site of Municipal Solid Waste, 2008.

[39] L. T. Zhan, X. Zeng, Y. C. Li, Y. M. Chen, "Analytical solution for one-dimensional diffusion of organic pollutants in a geomembrane–bentonite composite barrier and parametric analyses," Journal of Environmental Engineering, vol. 140 no. 1, pp. 57-68, DOI: 10.1061/(asce)ee.1943-7870.0000784, 2014.