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Chapter 11.3 Clay Liners and Waste Disposal
Handbook of Clay Science Edited by F. Bergaya, B.K.G. Theng and G. Lagaly Developments in Clay Science, Vol. 1 r 2006 Published by Elsevier Ltd.
693
Chapter 11.3
CLAY LINERS AND WASTE DISPOSAL K. CZURDA Angewandte Geologie, Universita¨t Karlsruhe, D-76128 Karlsruhe, Germany
As populations grow and technologies advance, the type and quantity of waste produced keeps on growing. As a result, waste disposal became a huge environmental problem. This chapter is concerned with waste encapsulation in relation to environmental clean-up and protection. The term ‘encapsulation’ refers to the sealing of waste body by geological and engineered liner systems. In most cases, such systems partly consist of clay liners with varied modifications. Thus, encapsulation systems are as varied as the environments in which they are built, and the components of an encapsulation system are as multiple and complex as the wastes themselves. For all waste types, encapsulation is the only option if the waste has to be permanently isolated from the accessible environment (Caldwell and Reith, 1993). The requirements for an encapsulation system are basically the same, whether the waste is municipal refuse in a landfill, hospital debris in a low-level waste dump, or mixed wastes of diverse industrial production/construction activities. This leads us to the need to classify wastes because encapsulation systems consist of engineered liner components according to the magnitude of the risks which are associated with the waste. From the beginning, sophisticated engineered liners should meet two requirements: (i) to guarantee practical imperviousness so as to prevent leachates from infiltrating the environment and (ii) to possess retention or at least retardation properties preventing contaminant migration by convection and diffusion (Drescher, 1997). In many cases—but not absolutely—the surface barrier (layer) may be slightly permeable, allowing further decomposition of sanitary wastes by precipitation moisture. Because of the different functions of the surface and the base encapsulation barriers, different state-of-the-art systems were developed (Fig. 11.3.1). In practice, clay liners are designed so that most of the required properties are optimally expressed (Fig. 11.3.2).
11.3.1. WASTE CATEGORIES Different waste compositions require different sealing units. For example, it is not economical to use a multilayer system for inert construction wastes or to design an DOI: 10.1016/S1572-4352(05)01022-6
Chapter 11.3: Clay Liners and Waste Disposal
694
Surface liner Waste Supporting dam Groundsurface Ring drainage Geologic barrier
Base liner Technical barrier
Groundwater table
Drainage system
1m
Fig. 11.3.1. Multibarrier system for waste encapsulation. Geologic in situ barriers and engineered technical barriers (compacted mineral layers and geomembranes) are the main parts of the system. clay fine
silt middle coarse
gravel stones fine middle coarse 0
sand fine middle coarse
80
20
60
12
11 10
9 8 7 6
5
4
3
2
1
40
40
60
20
80
0 0.001 0.002 0.006
0.02
0.06
0.2
0.6
2
6
20
percent coarser by weight
percent finer by weight
100
100 60 100
grain diameter [mm] range of hydraulic permeability (k) [m·s-1]
Fig. 11.3.2. Practical imperviousness is the main function of liner systems. Hydraulic permeabilities are expressed as k coefficients in ms1. The range of kp107 ms1 is considered as one of the most important barrier features of mineral sealing units in the national regulations. Grain size distribution areas 10, 11, and 12 refer to this.
identical system for sanitary landfills and toxic industrial wastes (Bradshaw et al., 1992). For this reason, all regional waste repository regulations have to categorise wastes not according to the input of waste components but mainly in terms of leaching the waste and quantifying the contaminant content (Czurda, 1992). The
11.3.2. Mineral Barriers
695
European Union, most European countries, the USA, Canada, Japan, etc. follow this system, ending up with similar waste categories. Taking radioactive waste into account leads to the following classification scheme: inert construction and industrial waste, domestic waste, toxic industrial waste, incineration ashes and slags, and radioactive waste. Nuclear waste repositories have to follow special national regulations and are not further treated in this chapter. As an example for waste assignments, some threshold values according to German regulations (Gassner, 1991; TA Siedlungsabfall, 1993; Deponieverordnung, 2002) are shown in Table 11.3.1. They are close to European Union values (EU-Richtlinie, 1999).
11.3.2. MINERAL BARRIERS Clay rocks, clay mineral admixtures, and zeolite admixtures are the most important and widely used natural materials for constructing mineral barriers within engineered sealing layers. These materials are also used as constituents of in situ geological barriers, i.e., waste deposit location. The use of alternative materials such as amorphous silica, fly ashes, fly ash zeolites, clay remnants from coal flotation, etc. is not discussed.
Table 11.3.1. Assignment criteria for waste categories according to the German regulations TA-A and TA-Si. Selected examples. Category I is inert waste, category II is domestic waste, and category III is toxic industrial waste. The leachable parts are listed as examples for very common toxic waste constituents
Conductivity Uniaxial strength TOC Phenols Mercury Cadmium Lead Sulphate Soluble part Total Organic Carbon
I
II
III
p6000 mS/cm X50 kN/m2 p20 mg/L p0.2 mg/L p0.005 mg/L p0.050 mg/L p0.200 mg/L p500 mg/L p3%
p50000 mS/cm X50 kN/m2 p100 mg/L p50 mg/L p0.020 mg/L p0.100 mg/L p1000 mg/L p1400 mg/L p6%
p100000 mS/cm X50 kN/m2 p200 mg/L p100 mg/L p0.100 mg/L p0.500 mg/L p2000 mg/L p5000 mg/L p10%
696
Chapter 11.3: Clay Liners and Waste Disposal
A. Clay Rocks and Clay Minerals In using clays and zeolites and other fine-grain material for sealing purposes, two main issues are relevant: (i) leachate retention due to low-hydraulic conductivities and (ii) toxic constituent retention or retardation due to adsorption, precipitation, redox processes, and other mechanisms. Soil barriers, containing enough clay minerals with adequate properties to provide low permeability, are used extensively to prevent rapid advective migration of various leachates from waste disposal sites (Hiltmann and Stribrny, 1998). The clayey barriers vary from thin geosynthetic clay liners (GCL) of 1–3 cm thickness, to compacted clay liners (CCL) up to 300 cm in thickness, to natural undisturbed clayey barriers up to 30 m or more in thickness. The hydraulic conductivity of undisturbed clayey deposits depends on the mineralogy, environment of deposition, and stress history of the deposits. The same holds true for GCL and CCL. Clays attract water, other polar liquids, and cations. A dried-out clay will expand as it adsorbs water between its layers and particles when placed in an aqueous solution. If toxic ions are present in the solution, they can adsorb on the charged clay surface mineral by ion exchange. Thus, clays can accept or release ions depending on the concentration of the ions in solution relative to that on the surface. These ions, e.g., from the leachate, are not finally fixed but can participate in further exchange processes depending on the chemical environment. The nature of the cations initially present at the clay mineral surface (derived, for example, from the marine environment) is of decisive influence on adsorption potentials. According to the diameter of hydrated cations and their valency they are differently adsorbed by the clay surface mineral and are therefore exchangeable in different quantities. For example, Na+-bentonites are especially suitable for base liner construction. Because of their high-swelling potential and adsorption capacity, they fulfil the requirements for a high degree of imperviousness, and a high contaminant retention potential. Table 11.3.2 shows the cation exchange capacity (CEC) and specific surface areas of some clay minerals and other materials. The theoretical specific surface areas of smectites and vermiculites of 750–800 m2/g are only effective when the contaminants can fully penetrate the interlayer space. This may be the case for ion exchange with pure inorganic ions (see Chapter 12.10) and suitable organic cations (see Chapter 7.3) but does not often apply to the adsorption of neutral (nonionic) compounds.
B. Zeolites Zeolites show a high potential as contaminant adsorbents due to their highexchange 2+ capacity and selectivity for certain cations, such as NH+ , Cd2+, Sr2+, and 4 , Pb other metal ions, especially after ‘activation’ by sodium chloride. The selectivity of certain zeolites for specific chemicals is controlled by their pore size and charge
11.3.3. Waste Deposit Multibarrier Systems
697
Table 11.3.2. Cation exchange capacity (CEC) and specific surface area of some clay minerals and other materials Adsorbent
CEC (cmol(+)/kg)
Specific surface area (m2/g)
Allophane Kaolinite Illite Montmorillonite Vermiculite Fe– +Al–(hydr)oxides (pH 8.0) Humic material
50–100 3–15 10–40 70–120 130–210 3–25 150–250
500–700 10–20 50–100 10–800y 1–800y 25–40 about 800
¼ meq/100 g. y
depending on the participation of internal surfaces; lowest value represents the external specific surface area.
properties. By analogy with clay minerals, the substitution of Al3+ for Si4+ in the structure leads to a net negative charge and a high CEC for most natural zeolites. Natural zeolites occur in sediments, lava vesicules, deuteric-altered plutonic rocks, and hydrothermal systems associated with alkaline volcanic rocks. Since natural zeolites generally derive from volcanic glass, they are of widespread occurrence. Fig. 11.3.3 shows the isotherms for the adsorption of Cu2+ from deionised water and a 0.01 M CaCl2 solution by clinoptilolite (a very common natural zeolite) and Na+–clinoptilolite (Huttenloch et al., 2001). The isotherms are curvi-linear, and can be described by the Freundlich equation: cs ¼ Kf Cnw where cs denotes the amount of the solute adsorbed per unit mass of adsorbent, cw is the equilibrium solute concentration, Kf and n are empirical parameters specific of the adsorbate-adsorbent interaction.
11.3.3. WASTE DEPOSIT MULTIBARRIER SYSTEMS Waste deposits can in principle be constructed as underground storages and at the surface as slope storage, slope dump, or depression storage. Common domestic waste, incineration residues, inert construction wastes, etc. may be stored at the surface. Underground storage as a special deposition mode is not treated in this chapter. A. Base Liners Base liner systems must be able to prevent leakage of contaminants from the waste, and their infiltration into the subsoil. These systems have to prove a high potential for retaining toxic materials by adsorption, precipitation, and/or redox processes (Rowe et al., 1995). Adsorption on mineral surfaces mainly occurs by ion exchange. In many cases toxicants can be retarded during their migration through the sealing layers.
698
Chapter 11.3: Clay Liners and Waste Disposal
Fig. 11.3.3. Adsorption isotherms of Cu2+ on natural and Na+ clinoptilolite in deionised water and 0.01 M CaCl2 solution (1 g samples in 40 mL copper salt solutions, contact time 96 h, at 20 1C). The CEC of clinoptilolite is 720 mmol(+)/g and of Na+–clinoptilolite is 900 mmol(+)/g.
Figs. 11.3.4a, b shows an example of a base liner constructed according to German regulations for inert and domestic wastes. The essential components are compacted clay layers, and in case of domestic wastes, a geomembrane in addition to the mineral layers and of course the geological barrier (Fig. 11.3.5). The basal system contains a leachate collecting layer, connected to a purification plant. There are different leakage detection systems on the market. B. Surface Liners The exclusive function of the surface liner is to prevent precipitation water from infiltrating into the waste. In case of household wastes, the capping system has to have a gas drainage system. Capping layers for all types of waste are therefore constructed with a drainage layer (usually 16–32 mm gravel) in case of leaks in the system. As in the case of basal systems, CCL and geomembranes are the prevailing sealing elements (Fig. 11.3.6a, b). But there is an important difference in the clay mineral composition of the CCLs. Whereas the base CCL-clay should contain 2:1 layer clay minerals (e.g., montmorillonite, vermiculite), the surface CCL-clay should contain 1:1 phyllosilicates (e.g., kaolinite) as index minerals. The 2:1 minerals enable retardation by adsorption and a high degree of impermeability to be obtained, while the 1:1 clays of the surface sealing unit, combined with a sand/silt matrix, are practically impermeable but have a low-adsorption potential. Because of the small particle size, a sand–silt–kaolinite admixture for the mineral surface sealing can have a very low permeability (kf ¼ 1028 210212 cm=s).
11.3.4. Conclusions
699
waste
drainage
clay liner
(a)
waste drainage
geomembrane
clay liner
geologic barrier (b)
Fig. 11.3.4. (a) Base liner system for inert waste. Two CCL without geomembrane (b) Base liner system for domestic waste. Three CCL combined with a geomembrane.
11.3.4. CONCLUSIONS For hazardous industrial wastes and toxic sanitary landfills, we have to locate a site that functions primarily as a geologic barrier with kfo10–6 m/s and at least 3 m in thickness. In case of inert (non-toxic) wastes a geologic barrier is not necessary. It is essential, however, to follow the multibarrier concept and to add on top of the geologic barrier a system of engineered barriers and drainage layers. The engineered barriers comprise as a core unit the combined CCL and geomembrane double layer. A similar multibarrier system has to be constructed for the cover sealing. The difference is expressed in the type of GCL and the drainage layers. The GCL should not contain expanding 2:1 clay minerals, such as montmorillonite or vermiculite, because they tend to dry out and form desiccation cracks. Therefore non-swelling 1:1 clay
Chapter 11.3: Clay Liners and Waste Disposal
700
Base Liner System Domestic waste German regulation Category I
Industrial waste German regulation
Multimineral barrier
Category II
Drainage layer 30cm
30cm
Mineral barrier 50cm
75cm
30cm Geomembrane 150cm
Geologic barrier
Impervious Sorption layer layer
Waste ~ ~ ~ ~~ ~ ~~ ~~ ~~ Clay, ~ ~ ~ Na-bentonite ~~ ~~ ~ ~~ ~~ ~ ~ ~ optimized ~~ ~ ~~ ~~ ~~ Clay, kaolinite optimized
Fig. 11.3.5. Comparing different base liner systems. The multibarrier system consists of two clay units: an adsorbing bentonite unit and a sealing kaolinite unit.
(a)
(b)
Fig. 11.3.6. Environmental Protection Agency (EPA) recommendation for surface liner systems in the USA. Clay barriers are combined with geomembranes (a) The optional system (b) comprises additional filler systems: a biotic filter on top of the sealing unit and a gas collection layer below.
References
701
minerals (e.g., kaolinite) or tectosilicates (e.g., zeolites) should be the index minerals for the sand–silt–clay surface GCL. In case of untreated household waste, a gas drainage layer has to be provided in order to divert the methanol that develops as the waste decomposes. REFERENCES Bradshaw, A.D., Southwood, R., Warner, F., 1992. The Treatment and Handling of Wastes. Chapman & Hall, London. Caldwell, J.A., Reith, C.C., 1993. Principles and Practice of Waste Encapsulation. Lewis Publishers, Boca Raton, FL. Czurda, K.A., 1992. Deponie und Altlasten. EF-Verlag fu¨r Energie- und Umwelttechnik GmbH, Berlin. Deponieverordnung, 2002. Verordnung u¨ber Deponien und Langzeitlager, Bundesministerium fu¨r Umwelt, Naturschutz und Reaktorsicherheit, Berlin. Drescher, J., 1997. Deponiebau. Alphabet KG, Berlin. EU-Richtlinie, 1999. Richtlinie u¨ber Abfalldeponien, Rat der Europa¨ischen Union. Amtsblatt der Europa¨ischen Union, Bru¨ssel. Gassner, E., 1991. Gesamtfassung der Zweiten allgemeinen Verwaltungsvorschrift zum Abfallgesetz 1. Teil. Verlag Franz Rehm, Mu¨nchen. Hiltmann, W., Stribrny, B., 1998. Tonmineralogie und Bodenphysik. Springer-Verlag, Berlin. Huttenloch, P., Roehl, K.E., Czurda, K.A., 2001. Sorption of nonpolar aromatic contaminants by chlorosilane surface-modified natural minerals. Environmental Science and Technology 35, 4260–4264. Rowe, R.K., Quigley, R.M., Booker, J.R., 1995. Clayey Barrier Systems for Waste Disposal Facilities. Chapman & Hall, London. TA Siedlungsabfall, 1993. Technische Anleitung zur Verwertung, Behandlung und sonstigen Entsorgung von Siedlungsabfa¨llen, Ministerium fu¨r Umwelt, Rheinland-Pfalz. Bundesanzeiger, Ko¨ln.
693
Chapter 11.3
CLAY LINERS AND WASTE DISPOSAL K. CZURDA Angewandte Geologie, Universita¨t Karlsruhe, D-76128 Karlsruhe, Germany
As populations grow and technologies advance, the type and quantity of waste produced keeps on growing. As a result, waste disposal became a huge environmental problem. This chapter is concerned with waste encapsulation in relation to environmental clean-up and protection. The term ‘encapsulation’ refers to the sealing of waste body by geological and engineered liner systems. In most cases, such systems partly consist of clay liners with varied modifications. Thus, encapsulation systems are as varied as the environments in which they are built, and the components of an encapsulation system are as multiple and complex as the wastes themselves. For all waste types, encapsulation is the only option if the waste has to be permanently isolated from the accessible environment (Caldwell and Reith, 1993). The requirements for an encapsulation system are basically the same, whether the waste is municipal refuse in a landfill, hospital debris in a low-level waste dump, or mixed wastes of diverse industrial production/construction activities. This leads us to the need to classify wastes because encapsulation systems consist of engineered liner components according to the magnitude of the risks which are associated with the waste. From the beginning, sophisticated engineered liners should meet two requirements: (i) to guarantee practical imperviousness so as to prevent leachates from infiltrating the environment and (ii) to possess retention or at least retardation properties preventing contaminant migration by convection and diffusion (Drescher, 1997). In many cases—but not absolutely—the surface barrier (layer) may be slightly permeable, allowing further decomposition of sanitary wastes by precipitation moisture. Because of the different functions of the surface and the base encapsulation barriers, different state-of-the-art systems were developed (Fig. 11.3.1). In practice, clay liners are designed so that most of the required properties are optimally expressed (Fig. 11.3.2).
11.3.1. WASTE CATEGORIES Different waste compositions require different sealing units. For example, it is not economical to use a multilayer system for inert construction wastes or to design an DOI: 10.1016/S1572-4352(05)01022-6
Chapter 11.3: Clay Liners and Waste Disposal
694
Surface liner Waste Supporting dam Groundsurface Ring drainage Geologic barrier
Base liner Technical barrier
Groundwater table
Drainage system
1m
Fig. 11.3.1. Multibarrier system for waste encapsulation. Geologic in situ barriers and engineered technical barriers (compacted mineral layers and geomembranes) are the main parts of the system. clay fine
silt middle coarse
gravel stones fine middle coarse 0
sand fine middle coarse
80
20
60
12
11 10
9 8 7 6
5
4
3
2
1
40
40
60
20
80
0 0.001 0.002 0.006
0.02
0.06
0.2
0.6
2
6
20
percent coarser by weight
percent finer by weight
100
100 60 100
grain diameter [mm] range of hydraulic permeability (k) [m·s-1]
Fig. 11.3.2. Practical imperviousness is the main function of liner systems. Hydraulic permeabilities are expressed as k coefficients in ms1. The range of kp107 ms1 is considered as one of the most important barrier features of mineral sealing units in the national regulations. Grain size distribution areas 10, 11, and 12 refer to this.
identical system for sanitary landfills and toxic industrial wastes (Bradshaw et al., 1992). For this reason, all regional waste repository regulations have to categorise wastes not according to the input of waste components but mainly in terms of leaching the waste and quantifying the contaminant content (Czurda, 1992). The
11.3.2. Mineral Barriers
695
European Union, most European countries, the USA, Canada, Japan, etc. follow this system, ending up with similar waste categories. Taking radioactive waste into account leads to the following classification scheme: inert construction and industrial waste, domestic waste, toxic industrial waste, incineration ashes and slags, and radioactive waste. Nuclear waste repositories have to follow special national regulations and are not further treated in this chapter. As an example for waste assignments, some threshold values according to German regulations (Gassner, 1991; TA Siedlungsabfall, 1993; Deponieverordnung, 2002) are shown in Table 11.3.1. They are close to European Union values (EU-Richtlinie, 1999).
11.3.2. MINERAL BARRIERS Clay rocks, clay mineral admixtures, and zeolite admixtures are the most important and widely used natural materials for constructing mineral barriers within engineered sealing layers. These materials are also used as constituents of in situ geological barriers, i.e., waste deposit location. The use of alternative materials such as amorphous silica, fly ashes, fly ash zeolites, clay remnants from coal flotation, etc. is not discussed.
Table 11.3.1. Assignment criteria for waste categories according to the German regulations TA-A and TA-Si. Selected examples. Category I is inert waste, category II is domestic waste, and category III is toxic industrial waste. The leachable parts are listed as examples for very common toxic waste constituents
Conductivity Uniaxial strength TOC Phenols Mercury Cadmium Lead Sulphate Soluble part Total Organic Carbon
I
II
III
p6000 mS/cm X50 kN/m2 p20 mg/L p0.2 mg/L p0.005 mg/L p0.050 mg/L p0.200 mg/L p500 mg/L p3%
p50000 mS/cm X50 kN/m2 p100 mg/L p50 mg/L p0.020 mg/L p0.100 mg/L p1000 mg/L p1400 mg/L p6%
p100000 mS/cm X50 kN/m2 p200 mg/L p100 mg/L p0.100 mg/L p0.500 mg/L p2000 mg/L p5000 mg/L p10%
696
Chapter 11.3: Clay Liners and Waste Disposal
A. Clay Rocks and Clay Minerals In using clays and zeolites and other fine-grain material for sealing purposes, two main issues are relevant: (i) leachate retention due to low-hydraulic conductivities and (ii) toxic constituent retention or retardation due to adsorption, precipitation, redox processes, and other mechanisms. Soil barriers, containing enough clay minerals with adequate properties to provide low permeability, are used extensively to prevent rapid advective migration of various leachates from waste disposal sites (Hiltmann and Stribrny, 1998). The clayey barriers vary from thin geosynthetic clay liners (GCL) of 1–3 cm thickness, to compacted clay liners (CCL) up to 300 cm in thickness, to natural undisturbed clayey barriers up to 30 m or more in thickness. The hydraulic conductivity of undisturbed clayey deposits depends on the mineralogy, environment of deposition, and stress history of the deposits. The same holds true for GCL and CCL. Clays attract water, other polar liquids, and cations. A dried-out clay will expand as it adsorbs water between its layers and particles when placed in an aqueous solution. If toxic ions are present in the solution, they can adsorb on the charged clay surface mineral by ion exchange. Thus, clays can accept or release ions depending on the concentration of the ions in solution relative to that on the surface. These ions, e.g., from the leachate, are not finally fixed but can participate in further exchange processes depending on the chemical environment. The nature of the cations initially present at the clay mineral surface (derived, for example, from the marine environment) is of decisive influence on adsorption potentials. According to the diameter of hydrated cations and their valency they are differently adsorbed by the clay surface mineral and are therefore exchangeable in different quantities. For example, Na+-bentonites are especially suitable for base liner construction. Because of their high-swelling potential and adsorption capacity, they fulfil the requirements for a high degree of imperviousness, and a high contaminant retention potential. Table 11.3.2 shows the cation exchange capacity (CEC) and specific surface areas of some clay minerals and other materials. The theoretical specific surface areas of smectites and vermiculites of 750–800 m2/g are only effective when the contaminants can fully penetrate the interlayer space. This may be the case for ion exchange with pure inorganic ions (see Chapter 12.10) and suitable organic cations (see Chapter 7.3) but does not often apply to the adsorption of neutral (nonionic) compounds.
B. Zeolites Zeolites show a high potential as contaminant adsorbents due to their highexchange 2+ capacity and selectivity for certain cations, such as NH+ , Cd2+, Sr2+, and 4 , Pb other metal ions, especially after ‘activation’ by sodium chloride. The selectivity of certain zeolites for specific chemicals is controlled by their pore size and charge
11.3.3. Waste Deposit Multibarrier Systems
697
Table 11.3.2. Cation exchange capacity (CEC) and specific surface area of some clay minerals and other materials Adsorbent
CEC (cmol(+)/kg)
Specific surface area (m2/g)
Allophane Kaolinite Illite Montmorillonite Vermiculite Fe– +Al–(hydr)oxides (pH 8.0) Humic material
50–100 3–15 10–40 70–120 130–210 3–25 150–250
500–700 10–20 50–100 10–800y 1–800y 25–40 about 800
¼ meq/100 g. y
depending on the participation of internal surfaces; lowest value represents the external specific surface area.
properties. By analogy with clay minerals, the substitution of Al3+ for Si4+ in the structure leads to a net negative charge and a high CEC for most natural zeolites. Natural zeolites occur in sediments, lava vesicules, deuteric-altered plutonic rocks, and hydrothermal systems associated with alkaline volcanic rocks. Since natural zeolites generally derive from volcanic glass, they are of widespread occurrence. Fig. 11.3.3 shows the isotherms for the adsorption of Cu2+ from deionised water and a 0.01 M CaCl2 solution by clinoptilolite (a very common natural zeolite) and Na+–clinoptilolite (Huttenloch et al., 2001). The isotherms are curvi-linear, and can be described by the Freundlich equation: cs ¼ Kf Cnw where cs denotes the amount of the solute adsorbed per unit mass of adsorbent, cw is the equilibrium solute concentration, Kf and n are empirical parameters specific of the adsorbate-adsorbent interaction.
11.3.3. WASTE DEPOSIT MULTIBARRIER SYSTEMS Waste deposits can in principle be constructed as underground storages and at the surface as slope storage, slope dump, or depression storage. Common domestic waste, incineration residues, inert construction wastes, etc. may be stored at the surface. Underground storage as a special deposition mode is not treated in this chapter. A. Base Liners Base liner systems must be able to prevent leakage of contaminants from the waste, and their infiltration into the subsoil. These systems have to prove a high potential for retaining toxic materials by adsorption, precipitation, and/or redox processes (Rowe et al., 1995). Adsorption on mineral surfaces mainly occurs by ion exchange. In many cases toxicants can be retarded during their migration through the sealing layers.
698
Chapter 11.3: Clay Liners and Waste Disposal
Fig. 11.3.3. Adsorption isotherms of Cu2+ on natural and Na+ clinoptilolite in deionised water and 0.01 M CaCl2 solution (1 g samples in 40 mL copper salt solutions, contact time 96 h, at 20 1C). The CEC of clinoptilolite is 720 mmol(+)/g and of Na+–clinoptilolite is 900 mmol(+)/g.
Figs. 11.3.4a, b shows an example of a base liner constructed according to German regulations for inert and domestic wastes. The essential components are compacted clay layers, and in case of domestic wastes, a geomembrane in addition to the mineral layers and of course the geological barrier (Fig. 11.3.5). The basal system contains a leachate collecting layer, connected to a purification plant. There are different leakage detection systems on the market. B. Surface Liners The exclusive function of the surface liner is to prevent precipitation water from infiltrating into the waste. In case of household wastes, the capping system has to have a gas drainage system. Capping layers for all types of waste are therefore constructed with a drainage layer (usually 16–32 mm gravel) in case of leaks in the system. As in the case of basal systems, CCL and geomembranes are the prevailing sealing elements (Fig. 11.3.6a, b). But there is an important difference in the clay mineral composition of the CCLs. Whereas the base CCL-clay should contain 2:1 layer clay minerals (e.g., montmorillonite, vermiculite), the surface CCL-clay should contain 1:1 phyllosilicates (e.g., kaolinite) as index minerals. The 2:1 minerals enable retardation by adsorption and a high degree of impermeability to be obtained, while the 1:1 clays of the surface sealing unit, combined with a sand/silt matrix, are practically impermeable but have a low-adsorption potential. Because of the small particle size, a sand–silt–kaolinite admixture for the mineral surface sealing can have a very low permeability (kf ¼ 1028 210212 cm=s).
11.3.4. Conclusions
699
waste
drainage
clay liner
(a)
waste drainage
geomembrane
clay liner
geologic barrier (b)
Fig. 11.3.4. (a) Base liner system for inert waste. Two CCL without geomembrane (b) Base liner system for domestic waste. Three CCL combined with a geomembrane.
11.3.4. CONCLUSIONS For hazardous industrial wastes and toxic sanitary landfills, we have to locate a site that functions primarily as a geologic barrier with kfo10–6 m/s and at least 3 m in thickness. In case of inert (non-toxic) wastes a geologic barrier is not necessary. It is essential, however, to follow the multibarrier concept and to add on top of the geologic barrier a system of engineered barriers and drainage layers. The engineered barriers comprise as a core unit the combined CCL and geomembrane double layer. A similar multibarrier system has to be constructed for the cover sealing. The difference is expressed in the type of GCL and the drainage layers. The GCL should not contain expanding 2:1 clay minerals, such as montmorillonite or vermiculite, because they tend to dry out and form desiccation cracks. Therefore non-swelling 1:1 clay
Chapter 11.3: Clay Liners and Waste Disposal
700
Base Liner System Domestic waste German regulation Category I
Industrial waste German regulation
Multimineral barrier
Category II
Drainage layer 30cm
30cm
Mineral barrier 50cm
75cm
30cm Geomembrane 150cm
Geologic barrier
Impervious Sorption layer layer
Waste ~ ~ ~ ~~ ~ ~~ ~~ ~~ Clay, ~ ~ ~ Na-bentonite ~~ ~~ ~ ~~ ~~ ~ ~ ~ optimized ~~ ~ ~~ ~~ ~~ Clay, kaolinite optimized
Fig. 11.3.5. Comparing different base liner systems. The multibarrier system consists of two clay units: an adsorbing bentonite unit and a sealing kaolinite unit.
(a)
(b)
Fig. 11.3.6. Environmental Protection Agency (EPA) recommendation for surface liner systems in the USA. Clay barriers are combined with geomembranes (a) The optional system (b) comprises additional filler systems: a biotic filter on top of the sealing unit and a gas collection layer below.
References
701
minerals (e.g., kaolinite) or tectosilicates (e.g., zeolites) should be the index minerals for the sand–silt–clay surface GCL. In case of untreated household waste, a gas drainage layer has to be provided in order to divert the methanol that develops as the waste decomposes. REFERENCES Bradshaw, A.D., Southwood, R., Warner, F., 1992. The Treatment and Handling of Wastes. Chapman & Hall, London. Caldwell, J.A., Reith, C.C., 1993. Principles and Practice of Waste Encapsulation. Lewis Publishers, Boca Raton, FL. Czurda, K.A., 1992. Deponie und Altlasten. EF-Verlag fu¨r Energie- und Umwelttechnik GmbH, Berlin. Deponieverordnung, 2002. Verordnung u¨ber Deponien und Langzeitlager, Bundesministerium fu¨r Umwelt, Naturschutz und Reaktorsicherheit, Berlin. Drescher, J., 1997. Deponiebau. Alphabet KG, Berlin. EU-Richtlinie, 1999. Richtlinie u¨ber Abfalldeponien, Rat der Europa¨ischen Union. Amtsblatt der Europa¨ischen Union, Bru¨ssel. Gassner, E., 1991. Gesamtfassung der Zweiten allgemeinen Verwaltungsvorschrift zum Abfallgesetz 1. Teil. Verlag Franz Rehm, Mu¨nchen. Hiltmann, W., Stribrny, B., 1998. Tonmineralogie und Bodenphysik. Springer-Verlag, Berlin. Huttenloch, P., Roehl, K.E., Czurda, K.A., 2001. Sorption of nonpolar aromatic contaminants by chlorosilane surface-modified natural minerals. Environmental Science and Technology 35, 4260–4264. Rowe, R.K., Quigley, R.M., Booker, J.R., 1995. Clayey Barrier Systems for Waste Disposal Facilities. Chapman & Hall, London. TA Siedlungsabfall, 1993. Technische Anleitung zur Verwertung, Behandlung und sonstigen Entsorgung von Siedlungsabfa¨llen, Ministerium fu¨r Umwelt, Rheinland-Pfalz. Bundesanzeiger, Ko¨ln.