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Concept of subaqueous capping of contaminated sediments with active barrier systems (ABS) using natural and modified zeolites
PII: S0043-1354(98)00432-1
Wat. Res. Vol. 33, No. 9, pp. 2083±2087, 1999 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/99/$ - see front matter
CONCEPT OF SUBAQUEOUS CAPPING OF CONTAMINATED SEDIMENTS WITH ACTIVE BARRIER SYSTEMS (ABS) USING NATURAL AND MODIFIED ZEOLITES M PATRICK H. JACOBS* and ULRICH FOÈRSTNER{*
Department of Environmental Science and Technology, Technical University of Hamburg-Harburg, 21071 Hamburg, Germany (First received May 1998; accepted in revised form September 1998) AbstractÐIn this study the concept of subaqueous capping of contaminated sediments of lakes, rivers and coastal waters with active barrier systems (ABS) in order to minimise the contaminant release into the surface water is developed. This concept is supposed to provide a low-cost alternative to conventional methods in water protection. Active barrier systems, i.e. reactive geochemical barriers on the basis of low-cost materials, shall actively inhibit contaminant release from the sediment into the surface water, without the hydraulic contact between sediment and surface water being disturbed. Theoretical considerations and ®rst experimental results regarding the retention of Pb2+ by four dierent zeolitic rocks suggest that natural zeolite as a reactive material in sediment capping meets all the economical and technical requirements posed by the active barrier concept. Natural zeolites are capable of demobilising large amounts of cationic pollutants by sorption, as shown for Pb2+, and, furthermore, they are capable of demobilising non-polar organics and anionic contaminants, when the zeolite surface is pretreated with cationic surfactants. Active barrier systems on the basis of natural zeolites thus can be applied to nearly any type of contaminated sediment. Additionally, the physical characteristics of zeolitic rocks, as grain size and density, facilitate their use in subaqueous applications. # 1999 Elsevier Science Ltd. All rights reserved Key wordsÐsediment capping, active barrier system, natural zeolite, cationic surfactants
INTRODUCTION
The concept of subaqueous capping of contaminated sediments in lakes, rivers and coastal waters is a promising approach in developing a low-cost and low-technology alternative to conventional methods in water protection since conventional osite technologies, as removing pollutants from the sediment by chemical or physical means, are too complex and thus too costly where large areas of sediments are concerned. The concept of capping sediments in-situ involves the placement of a cover over the sediment in order to seal it o and thus to minimise contaminant release into the water column. An international review of the application of sediment-capping techniques is given by Azcue et al. (1998). So far, there have been several laboratory and ®eld scale investigations into the capping of sediments with sand or gravel layers (Wang et al., 1991; Zeman, 1994). Instead of this physical armouring, the concept discussed here employs *Author to whom all correspondence should be addressed. [Tel.: +49-40-77183322; fax: +49-40-77182315; e-mail: [email protected], e-mail: [email protected]]. {Tel.: +49-40-77183208; fax: +49-40-77182315.
active barrier systems, i.e. pervious geochemical barriers capable of actively demobilising pollutants in percolating pore water by sorption or precipitation processes. Since enormous amounts of material are necessary to cover large areas of sediment, it is a crucial point in the development of the capping design to ®nd adequate low-cost sorbents. In order to minimise running costs of the barriers they must consist of materials showing physical and chemical stability in a long-term view. The requirements to potential active barrier materials, accordingly, can be summarised as follows: . . . .
Availability at low cost. Active retention of contaminants. Physical and chemical stability. Sucient hydraulic conductivity.
Concept With respect to these requirements natural zeolites are an adequate material. Zeolites are crystalline hydrated aluminosilicates of alkali and alkaline earth elements. At present, there are four types of natural zeolite that are of commercial interest due to their favourable exchange properties along with an abundant occurrence in nature. These
2083
2084
Patrick H. Jacobs and Ulrich FoÈrstner
are clinoptilolite (Na4,K4)(Al8Si40O96)24H2O, chabazite (Na2,Ca)6(Al12Si24O72)40H2O, mordenite Na8(Al8Si40096)24H2O and phillipsite (Na,K)10 (Al10Si22O64)20H2O (Mumpton, 1977). Natural zeolites occur in dierent geological settings as rock-forming minerals in many locations in the world, e.g. in the Western United States, Turkey and Italy. Due to their abundant occurrence in ¯atlying, near-surface and nearly mono-mineralic deposits they, generally, are available at low cost. Natural zeolite, generally, is mined as a brittle, solid rock and thus its grain size distribution can be controlled freely by crushing and sieving. The hydraulic properties of natural zeolite material, thus, can be ®tted to meet particular requirements. Furthermore, the availability of zeolites as a coarse grained material facilitate their application in subaqueous environments since coarse grains settle to the ground readily and, once sedimented, are relatively resistant against erosion. During the last few years natural zeolites have been tested successfully for treatment of waste water and soils (Leppert, 1990; Misaelides and Godelitsas, 1995; Shanableh and Kharabsheh, 1996; Reyes et al., 1997; Sakadevan and Bavor, 1998). Their remarkably high cation exchange capacity of up to 6 mmol(eq) gÿ1 exceeds, by far, that of smectite clays, for example, and thus permits an ecient removal of heavy-metal cations. The high cation exchange capacity results from the unique crystal structure of zeolite minerals which basically is formed by a framework of SiO4-tetrahedra, wherein all oxygen atoms are shared by two adjacent tetrahedra resulting in an overall oxygen/silicon-ratio of 2:1. The characteristic isomorphic substitution of silicon by aluminium in the tetrahedra-sites results in a net negative charge of the framework which is balanced by loosely bound, exchangeable, extraframework cations, mainly of the alkali and alkaline earth elements. In contrast to other tecto-sili-
cates, e.g. feldspar, the zeolite framework is remarkably open. It shows in®nite, three-dimensional systems of tunnels and cages where the exchangeable cations as well as water molecules are situated. The cation exchange behaviour, which controls the selectivity for particular cations in exchange processes, depends on the charge and size of the cations and the structural characteristics of the particular zeolite mineral, e.g. channel dimensions. The pre-treatment of zeolite surfaces with cationic surfactants additionally facilitates the retention of non-polar contaminants, e.g. chlorinated hydrocarbons, and of anionic contaminants, e.g. chromate and arsenate (Haggerty and Bowman, 1994; Bowman et al., 1995) (Fig. 1). Cationic surfactants, showing a strong anity to the exchange sites at the zeolite surfaces, take the place of exchangeable metal cations and thereby form a layer covering the zeolite surface. Since only the external surface, and not the internal cage and tunnel structure, of the zeolite is accessible for the large surfactant molecules, the internal surface remains an active cation exchanger, while the external surface becomes electrically neutral or even positively charged as a consequence of the surfactant loading either in a mono-layer or in a bi-layer, respectively (Chen et al., 1992; Sullivan et al., 1997). The preferential formation of mono-layers or bi-layers is controlled by the type of surfactant and its counter ion (Li and Bowman, 1997). Hence, it is possible to modify a natural zeolite in a way that it becomes a powerful sorptive agent for nearly all kinds of contaminants in aquatic systems. MATERIAL AND METHODS
Zeolite In this study we currently investigate 3 dierent natural high-grade zeolitic rocks from the Western United States
Fig. 1. Retention of anionic, cationic and non-polar contaminants by zeolites pre-treated with cationic surfactants forming a bi-layer on the zeolite surface.
Natural zeolites in active barrier systems
2085
Table 1. Physical properties of high-grade clinoptilolite and chabazite rocks Ash Meadows Clino2 Zeolite content Speci®c density (g cmÿ3) Speci®c surface area (m2 gÿ1) Cation exchange capacity (meq gÿ1) Crystal size (zeolite) (mm)
Armagosa Green Clino
90% (25%) Clinoptilolite 2.1 43.8a b 1.6±2.1 (NH+ 4 loading) ÿ
a
90% (25%) Clinoptilolite ÿ 43.8a + 2.02 (NH4 loading)b ÿ
Bowie Chabazite a
>90% Chabazitec 1.73c 521c 2.5c <1c
Data by Anaconda Research Lab./Mineral Lab.bData by Miles Industrial Mineral Research.cData by GSA Resources.
a
with a zeolite content of approximately 90% (Table 1): Ash Meadows Clino2 and Armagosa Green Clino (American Resource Corporation, Reno, NV) and Bowie Chabazite (GSA Resources, Cortaro, AZ). Furthermore, we are investigating a tuaceous rock of unknown zeolite content from Germany (Trasswerke Meurin). These natural materials we use without further processing. The grain sizes of the materials used are 0.25±0.84 mm for Bowie Chabazite, 1.0±2.0 mm for Armagosa Green Clino, 0.42± 0.84 mm for Ash Meadows Clinoptilolite and 0.2±0.63 mm for Meurin tu. The major exchangeable cations are sodium accompanied by potassium and calcium in Ash Meadows Clino2, potassium accompanied by sodium and calcium in Armagosa Green Clino and calcium accompanied by sodium and potassium in Bowie Chabazite. Sorption experiments Batch experiments are carried out to investigate the sorption of lead on the zeolite materials. Solutions of Pb(NO3)2 are prepared for initial lead concentrations from 0.5 to 25 mmol lÿ140 ml of solution is added to 1 g of zeolite in 50 ml centrifuge tubes. The samples and appropriate blanks are shaken mechanically at 208C over 24 h to reach equilibrium, and then centrifuged. The supernatant is analysed by atomic absorption spectrometry and the amount of lead sorbed on the zeolite is calculated by dierence. RESULTS AND DISCUSSION
The sorption of Pb2+ from aqueous solution on the zeolitic materials is shown in Fig. 2. The results
for the Meurin tu and the Armagosa Green Clino can be described very well with a Langmuir equation (Langmuir, 1918) although the theoretical conditions of the Langmuir equation are not ful®lled in detail. The Langmuir equation requires one single kind of sorption site, but zeolites have at least two kinds of site, at the external surface and at the internal surface, both having clearly dierent exchange properties. In contrast to the external surface, the ion exchange and the ion selection within the zeolite structure primarily depend on structural properties as the dimensions of the tunnels and cages (KaÈrger and Ruthven, 1992). The Langmuir equation can be written as S
Sm KC 1 KC
1
where S is the amount sorbed at equilibrium and C is the equilibrium concentration in the liquid phase. Sm is the maximum sorption (sorption capacity) and K is the Langmuir coecient. Due to the more complex conditions, the Langmuir equation does not provide a satisfactory ®t for the sorption of Pb2+ neither on Ash Meadows clinoptilolite nor on Bowie Chabazite. Here, ®tting a Redlich±Peterson equation (Redlich
Fig. 2. Sorption isotherms of Pb2+ on zeolite materials at pH 4.8 and 208C.
2086
Patrick H. Jacobs and Ulrich FoÈrstner
and Peterson, 1959) gives the most satisfactory results (equation 2). This equation is derived from the Langmuir equation but allows for the non-homogeneity of the ion exchanger. To lower concentrations it approaches the Henry equation. For higher concentrations it does not reach a maximum like the Langmuir equation but approaches a Freundlich-type isotherm. The Redlich±Peterson equation may be written as S
Sm KC 1 KC n
2
where n is derived from the Freundlich exponent. Sm only represents a maximum of sorption in this equation if n = 1! Both equations were ®tted to the experimental results by linear regression using the linearised forms of the equations. The linearised Langmuir equation can be written as C 1 1 C S Sm Sm K
3
and analogously the linearised Redlich±Peterson equation can be written as C 1 n 1 C S Sm Sm K
4
The parameters of the ®tted equations are summarised in Table 2 along with the coecient of determination (r2). According to the results obtained by the sorption experiments the materials dier considerably with regard to their sorption capacity for lead although the total cation exchange capacities (Table 1) are very similar. This apparently results from the selectivity of cation exchange and the dierences in the original composition of exchangeable cations. The higher original potassium content of Armagosa Green Clino compared to Ash Meadows Clinoptilolite may explain the lower lead sorption on Armagosa Green Clino although both rocks have a very similar mineralogical composition; potassium sorbed on clinoptilolite is much less readily exchanged by other cations than sodium or calcium. Similarly, chabazite shows a very low selectivity for calcium, that is the dominant cation on Bowie Chabazite, resulting in the high sorption capacity found for lead on Bowie Chabazite. This suggests that exchanging, before use, the extra-framework cations of the zeolite by cations of low selectivity,
as sodium in case of clinoptilolite, can notably improve the eciency of the zeolite material. On the basis of the sorption data calculated here and assuming an average lead-contaminated sediment with a lead concentration in the pore water of c(Pb2+) = 0.077 mg lÿ1 (Darby et al., 1986), we can roughly estimate the dimensions of an active barrier system required to eciently seal o this sediment; although parameters as pH, ionic strength and competing cations have to be neglected in this calculation, yet. For an equilibrium concentration of c(Pb2+) = 0.077 mg lÿ1 the amount of lead sorbed on Bowie Chabazite is SPb=9.1 mg gÿ1 and for Ash Meadows Clino2 it is SPb=1.4 mg gÿ1. An arbitrary cubeshaped element of sediment with a volume of 1 m3 and a water content of 50% contains an amount of 38.5 mg Pb2+ dissolved within the pore water. Considering this as the total ¯ux to the sediment± water interface, an active barrier system covering the element must contain (38.5 mg/9.1 mg gÿ1=) 4.2 g of Bowie Chabazite or (38.5 mg/1.4 mg gÿ1=) 27.5 g of Ash Meadows Clino2 in order to demobilise the total amount of Pb2+. Since the interface area of the cube-shaped element is 1 m2 and if we install a reactive barrier of 0.2 m thickness and a density r = 1.5 g cmÿ3 with a zeolite content of only 0.1% in a matrix of clean sand or gravel, we have an amount of reactive zeolite of 300 g mÿ2. This gives a safety margin approximately of a factor 70 and 11, respectively. Hence, an active barrier system covering a sediment surface of 104 m2 would consist of 3 t of zeolite at approximately US$90 to US$290 per ton (for Ash Meadows Zeolite), resulting in total costs for the natural zeolite material of approximately US$270 to US$870. To improve this ®rst calculation, still suering from some substantial simpli®cations, detailed experiments on heavy-metal sorption on zeolites and mixtures of zeolite in sand or gravel, simulating natural pore-water conditions, have to be conducted, along with computer modelling. CONCLUSION
Natural zeolites are an adequate material in subaqueous sediment capping due to their unique economical, physical and chemical properties which are: . Availability at low cost. . Retention of cationic contaminants by cation exchange.
Table 2. Parameters of the ®tted sorption equations
Bowie Chabazite Ash Meadows Clinoptilolite Armagosa Green Clinoptilolite Meurin Tu
Sorption equation
Sm (mg gÿ1)
K (l gÿ1)
n
r2
Redlich±Peterson Redlich±Peterson Langmuir Langmuir
184.4 79.8 45.6 25.9
0.733 0.235 0.084 0.021
0.87 0.85 ÿ ÿ
0.9999 0.9998 0.9987 0.9990
Natural zeolites in active barrier systems
. Retention of non-polar and anionic contaminants after appropriate modi®cation. . Favourable physical and hydraulic properties. . Physical and chemical stability under normal conditions. Nevertheless, the successful design of an active barrier system using natural or modi®ed zeolites requires further research on the contaminant retention by this material, as well as detailed knowledge about the capping-site characteristics. Especially, the particular hydraulic conditions have to be taken into account. Sites subject to strong underwater currents, tidal currents or shipping activities, for example, may require extremely coarse capping material or an additional armouring layer on top of the active barrier. Furthermore, the thickness of the active barrier, the kind of zeolite mineral used, the pre-treatment of the zeolite material and the zeolite content within the sand matrix have to be adapted to the particular contaminants and the degree of contamination. The objective of further research, accordingly, shall be to obtain all the data relevant to successfully apply the concept of active barrier to a wide variety of contaminants, sediment characteristics and hydraulic and hydrochemical conditions. AcknowledgementsÐThis study is part of the German± Australian Active-Barrier-System (ABS) Co-operation Project funded by the German Federal Ministry for Science and Education (BMBF). REFERENCES
Azcue J. M., Zeman A. and FoÈrstner U. (1998) International review of application of subaqueous capping techniques for remediation of contaminated sediments. Proceedings of the 3rd International Congress on Environmental Geotechnics, Lisbon, September 7±11. Bowman R. S., Haggerty G. M., Huddleston R. G., Neel D. and Flynn M. (1995) Sorption of nonpolar organics, inorganic cations, and inorganic anions by surfactantmodi®ed zeolites. In Surfactant-enhanced Remediation of Subsurface Contamination, eds. D. A. Sabatini, R. C. Knox and J. H. Harwell, ACS Symposium Series 594, pp. 54±64. American Chemical Society, Washington, DC. Chen Y. L., Chen S., Frank C. and Israelachivili J. (1992) Molecular mechanisms and kinetics during self-assembly
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of surfactant layers. J. Colloid Interface Sci. 153, 244± 265. Darby D. A., Adams D. D. and Nivens W. T. (1986) Early sediment changes and element mobilization in a man-made estuarine marsh. In Sediment and Water Interactions, ed. P. G. Sly, pp. 343±351. Springer, Berlin/Heidelberg/New York/Tokyo. Haggerty G. M. and Bowman R. S. (1994) Sorption of inorganic anions by organo-zeolites. Environ. Sci. Technol. 28, 452±458. KaÈrger J. and Ruthven D. M. (1992) Diusion in Zeolites. Wiley-Interscience, New York. Leppert D. (1990) Heavy metal sorption with clinoptilolite zeolite: Alternatives for treating contaminated soil and water. Mining Eng. 42, 604±609. Langmuir I. (1918) The adsorption of gases on plane surfaces of glass, mica and platinum. J. Am. Chem. Soc. 40, 1361±1403. Li Z. and Bowman R. S. (1997) Counterion eects on the sorption of cationic surfactant and chromate on natural clinoptilolite. Environ. Sci. Technol. 31, 2407±2412. Misaelides P. and Godelitsas A. (1995) Removal of heavy metals from aqueous solutions using pretreated natural zeolite material: The case of mercury(II). Toxicol. Environ. Chem. 51, 21±30. Mumpton F. A. (1977) Natural zeolites. In Mineralogy and Geology of Natural Zeolites, ed. F. A. Mumpton, Vol. 4, pp. 1±17. Mineralogical Society of America Short Course Notes. Redlich O. and Peterson D. L. (1959) A useful adsorption isotherm. J. Phys. Chem. 63, 1024. Reyes O., Sanchez E., PelloÂn A., Borja R., Colmenarejo M. F., MilaÂn Z. and Cruz M. (1997) A comparative study of sand and natural zeolite as ®ltering media in tertiary treatment of waste water from tourist areas. J. Environ. Sci. Health A 32(9±10), 2483±2496. Sakadevan K. and Bavor H. J. (1998) Phosphate adsorption characteristics of soils, slags and zeolite to be used as substrates in constructed wetland systems. Water Res. 32(2), 393±399. Shanableh A. and Kharabsheh A. (1996) Stabilization of Cd, Ni and Pb in soil using natural zeolite. J. Hazard. Mater. 45(2±3), 207±218. Sullivan E. J., Hunter D. B. and Bowman R. S. (1997) Topological and thermal properties of surfactant-modi®ed clinoptilolite studied by tapping-mode2 atomic force microscopy and high-resolution thermogravimetric analysis. Clays Clay Min. 45(1), 42±53. Wang X. Q., Thibodeaux L. J., Valsaraj K. T. and Reible D. D. (1991) Eciency of capping contaminated sediments in situ. 1. Laboratory-scale experiments on diusion±adsorption in the capping layer. Environ. Sci. Technol. 25(9), 1578±1584. Zeman A. J. (1994) Subaqueous capping of very soft contaminated sediments. Can. Geochem. J. 31(4), 570±577.
Wat. Res. Vol. 33, No. 9, pp. 2083±2087, 1999 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/99/$ - see front matter
CONCEPT OF SUBAQUEOUS CAPPING OF CONTAMINATED SEDIMENTS WITH ACTIVE BARRIER SYSTEMS (ABS) USING NATURAL AND MODIFIED ZEOLITES M PATRICK H. JACOBS* and ULRICH FOÈRSTNER{*
Department of Environmental Science and Technology, Technical University of Hamburg-Harburg, 21071 Hamburg, Germany (First received May 1998; accepted in revised form September 1998) AbstractÐIn this study the concept of subaqueous capping of contaminated sediments of lakes, rivers and coastal waters with active barrier systems (ABS) in order to minimise the contaminant release into the surface water is developed. This concept is supposed to provide a low-cost alternative to conventional methods in water protection. Active barrier systems, i.e. reactive geochemical barriers on the basis of low-cost materials, shall actively inhibit contaminant release from the sediment into the surface water, without the hydraulic contact between sediment and surface water being disturbed. Theoretical considerations and ®rst experimental results regarding the retention of Pb2+ by four dierent zeolitic rocks suggest that natural zeolite as a reactive material in sediment capping meets all the economical and technical requirements posed by the active barrier concept. Natural zeolites are capable of demobilising large amounts of cationic pollutants by sorption, as shown for Pb2+, and, furthermore, they are capable of demobilising non-polar organics and anionic contaminants, when the zeolite surface is pretreated with cationic surfactants. Active barrier systems on the basis of natural zeolites thus can be applied to nearly any type of contaminated sediment. Additionally, the physical characteristics of zeolitic rocks, as grain size and density, facilitate their use in subaqueous applications. # 1999 Elsevier Science Ltd. All rights reserved Key wordsÐsediment capping, active barrier system, natural zeolite, cationic surfactants
INTRODUCTION
The concept of subaqueous capping of contaminated sediments in lakes, rivers and coastal waters is a promising approach in developing a low-cost and low-technology alternative to conventional methods in water protection since conventional osite technologies, as removing pollutants from the sediment by chemical or physical means, are too complex and thus too costly where large areas of sediments are concerned. The concept of capping sediments in-situ involves the placement of a cover over the sediment in order to seal it o and thus to minimise contaminant release into the water column. An international review of the application of sediment-capping techniques is given by Azcue et al. (1998). So far, there have been several laboratory and ®eld scale investigations into the capping of sediments with sand or gravel layers (Wang et al., 1991; Zeman, 1994). Instead of this physical armouring, the concept discussed here employs *Author to whom all correspondence should be addressed. [Tel.: +49-40-77183322; fax: +49-40-77182315; e-mail: [email protected], e-mail: [email protected]]. {Tel.: +49-40-77183208; fax: +49-40-77182315.
active barrier systems, i.e. pervious geochemical barriers capable of actively demobilising pollutants in percolating pore water by sorption or precipitation processes. Since enormous amounts of material are necessary to cover large areas of sediment, it is a crucial point in the development of the capping design to ®nd adequate low-cost sorbents. In order to minimise running costs of the barriers they must consist of materials showing physical and chemical stability in a long-term view. The requirements to potential active barrier materials, accordingly, can be summarised as follows: . . . .
Availability at low cost. Active retention of contaminants. Physical and chemical stability. Sucient hydraulic conductivity.
Concept With respect to these requirements natural zeolites are an adequate material. Zeolites are crystalline hydrated aluminosilicates of alkali and alkaline earth elements. At present, there are four types of natural zeolite that are of commercial interest due to their favourable exchange properties along with an abundant occurrence in nature. These
2083
2084
Patrick H. Jacobs and Ulrich FoÈrstner
are clinoptilolite (Na4,K4)(Al8Si40O96)24H2O, chabazite (Na2,Ca)6(Al12Si24O72)40H2O, mordenite Na8(Al8Si40096)24H2O and phillipsite (Na,K)10 (Al10Si22O64)20H2O (Mumpton, 1977). Natural zeolites occur in dierent geological settings as rock-forming minerals in many locations in the world, e.g. in the Western United States, Turkey and Italy. Due to their abundant occurrence in ¯atlying, near-surface and nearly mono-mineralic deposits they, generally, are available at low cost. Natural zeolite, generally, is mined as a brittle, solid rock and thus its grain size distribution can be controlled freely by crushing and sieving. The hydraulic properties of natural zeolite material, thus, can be ®tted to meet particular requirements. Furthermore, the availability of zeolites as a coarse grained material facilitate their application in subaqueous environments since coarse grains settle to the ground readily and, once sedimented, are relatively resistant against erosion. During the last few years natural zeolites have been tested successfully for treatment of waste water and soils (Leppert, 1990; Misaelides and Godelitsas, 1995; Shanableh and Kharabsheh, 1996; Reyes et al., 1997; Sakadevan and Bavor, 1998). Their remarkably high cation exchange capacity of up to 6 mmol(eq) gÿ1 exceeds, by far, that of smectite clays, for example, and thus permits an ecient removal of heavy-metal cations. The high cation exchange capacity results from the unique crystal structure of zeolite minerals which basically is formed by a framework of SiO4-tetrahedra, wherein all oxygen atoms are shared by two adjacent tetrahedra resulting in an overall oxygen/silicon-ratio of 2:1. The characteristic isomorphic substitution of silicon by aluminium in the tetrahedra-sites results in a net negative charge of the framework which is balanced by loosely bound, exchangeable, extraframework cations, mainly of the alkali and alkaline earth elements. In contrast to other tecto-sili-
cates, e.g. feldspar, the zeolite framework is remarkably open. It shows in®nite, three-dimensional systems of tunnels and cages where the exchangeable cations as well as water molecules are situated. The cation exchange behaviour, which controls the selectivity for particular cations in exchange processes, depends on the charge and size of the cations and the structural characteristics of the particular zeolite mineral, e.g. channel dimensions. The pre-treatment of zeolite surfaces with cationic surfactants additionally facilitates the retention of non-polar contaminants, e.g. chlorinated hydrocarbons, and of anionic contaminants, e.g. chromate and arsenate (Haggerty and Bowman, 1994; Bowman et al., 1995) (Fig. 1). Cationic surfactants, showing a strong anity to the exchange sites at the zeolite surfaces, take the place of exchangeable metal cations and thereby form a layer covering the zeolite surface. Since only the external surface, and not the internal cage and tunnel structure, of the zeolite is accessible for the large surfactant molecules, the internal surface remains an active cation exchanger, while the external surface becomes electrically neutral or even positively charged as a consequence of the surfactant loading either in a mono-layer or in a bi-layer, respectively (Chen et al., 1992; Sullivan et al., 1997). The preferential formation of mono-layers or bi-layers is controlled by the type of surfactant and its counter ion (Li and Bowman, 1997). Hence, it is possible to modify a natural zeolite in a way that it becomes a powerful sorptive agent for nearly all kinds of contaminants in aquatic systems. MATERIAL AND METHODS
Zeolite In this study we currently investigate 3 dierent natural high-grade zeolitic rocks from the Western United States
Fig. 1. Retention of anionic, cationic and non-polar contaminants by zeolites pre-treated with cationic surfactants forming a bi-layer on the zeolite surface.
Natural zeolites in active barrier systems
2085
Table 1. Physical properties of high-grade clinoptilolite and chabazite rocks Ash Meadows Clino2 Zeolite content Speci®c density (g cmÿ3) Speci®c surface area (m2 gÿ1) Cation exchange capacity (meq gÿ1) Crystal size (zeolite) (mm)
Armagosa Green Clino
90% (25%) Clinoptilolite 2.1 43.8a b 1.6±2.1 (NH+ 4 loading) ÿ
a
90% (25%) Clinoptilolite ÿ 43.8a + 2.02 (NH4 loading)b ÿ
Bowie Chabazite a
>90% Chabazitec 1.73c 521c 2.5c <1c
Data by Anaconda Research Lab./Mineral Lab.bData by Miles Industrial Mineral Research.cData by GSA Resources.
a
with a zeolite content of approximately 90% (Table 1): Ash Meadows Clino2 and Armagosa Green Clino (American Resource Corporation, Reno, NV) and Bowie Chabazite (GSA Resources, Cortaro, AZ). Furthermore, we are investigating a tuaceous rock of unknown zeolite content from Germany (Trasswerke Meurin). These natural materials we use without further processing. The grain sizes of the materials used are 0.25±0.84 mm for Bowie Chabazite, 1.0±2.0 mm for Armagosa Green Clino, 0.42± 0.84 mm for Ash Meadows Clinoptilolite and 0.2±0.63 mm for Meurin tu. The major exchangeable cations are sodium accompanied by potassium and calcium in Ash Meadows Clino2, potassium accompanied by sodium and calcium in Armagosa Green Clino and calcium accompanied by sodium and potassium in Bowie Chabazite. Sorption experiments Batch experiments are carried out to investigate the sorption of lead on the zeolite materials. Solutions of Pb(NO3)2 are prepared for initial lead concentrations from 0.5 to 25 mmol lÿ140 ml of solution is added to 1 g of zeolite in 50 ml centrifuge tubes. The samples and appropriate blanks are shaken mechanically at 208C over 24 h to reach equilibrium, and then centrifuged. The supernatant is analysed by atomic absorption spectrometry and the amount of lead sorbed on the zeolite is calculated by dierence. RESULTS AND DISCUSSION
The sorption of Pb2+ from aqueous solution on the zeolitic materials is shown in Fig. 2. The results
for the Meurin tu and the Armagosa Green Clino can be described very well with a Langmuir equation (Langmuir, 1918) although the theoretical conditions of the Langmuir equation are not ful®lled in detail. The Langmuir equation requires one single kind of sorption site, but zeolites have at least two kinds of site, at the external surface and at the internal surface, both having clearly dierent exchange properties. In contrast to the external surface, the ion exchange and the ion selection within the zeolite structure primarily depend on structural properties as the dimensions of the tunnels and cages (KaÈrger and Ruthven, 1992). The Langmuir equation can be written as S
Sm KC 1 KC
1
where S is the amount sorbed at equilibrium and C is the equilibrium concentration in the liquid phase. Sm is the maximum sorption (sorption capacity) and K is the Langmuir coecient. Due to the more complex conditions, the Langmuir equation does not provide a satisfactory ®t for the sorption of Pb2+ neither on Ash Meadows clinoptilolite nor on Bowie Chabazite. Here, ®tting a Redlich±Peterson equation (Redlich
Fig. 2. Sorption isotherms of Pb2+ on zeolite materials at pH 4.8 and 208C.
2086
Patrick H. Jacobs and Ulrich FoÈrstner
and Peterson, 1959) gives the most satisfactory results (equation 2). This equation is derived from the Langmuir equation but allows for the non-homogeneity of the ion exchanger. To lower concentrations it approaches the Henry equation. For higher concentrations it does not reach a maximum like the Langmuir equation but approaches a Freundlich-type isotherm. The Redlich±Peterson equation may be written as S
Sm KC 1 KC n
2
where n is derived from the Freundlich exponent. Sm only represents a maximum of sorption in this equation if n = 1! Both equations were ®tted to the experimental results by linear regression using the linearised forms of the equations. The linearised Langmuir equation can be written as C 1 1 C S Sm Sm K
3
and analogously the linearised Redlich±Peterson equation can be written as C 1 n 1 C S Sm Sm K
4
The parameters of the ®tted equations are summarised in Table 2 along with the coecient of determination (r2). According to the results obtained by the sorption experiments the materials dier considerably with regard to their sorption capacity for lead although the total cation exchange capacities (Table 1) are very similar. This apparently results from the selectivity of cation exchange and the dierences in the original composition of exchangeable cations. The higher original potassium content of Armagosa Green Clino compared to Ash Meadows Clinoptilolite may explain the lower lead sorption on Armagosa Green Clino although both rocks have a very similar mineralogical composition; potassium sorbed on clinoptilolite is much less readily exchanged by other cations than sodium or calcium. Similarly, chabazite shows a very low selectivity for calcium, that is the dominant cation on Bowie Chabazite, resulting in the high sorption capacity found for lead on Bowie Chabazite. This suggests that exchanging, before use, the extra-framework cations of the zeolite by cations of low selectivity,
as sodium in case of clinoptilolite, can notably improve the eciency of the zeolite material. On the basis of the sorption data calculated here and assuming an average lead-contaminated sediment with a lead concentration in the pore water of c(Pb2+) = 0.077 mg lÿ1 (Darby et al., 1986), we can roughly estimate the dimensions of an active barrier system required to eciently seal o this sediment; although parameters as pH, ionic strength and competing cations have to be neglected in this calculation, yet. For an equilibrium concentration of c(Pb2+) = 0.077 mg lÿ1 the amount of lead sorbed on Bowie Chabazite is SPb=9.1 mg gÿ1 and for Ash Meadows Clino2 it is SPb=1.4 mg gÿ1. An arbitrary cubeshaped element of sediment with a volume of 1 m3 and a water content of 50% contains an amount of 38.5 mg Pb2+ dissolved within the pore water. Considering this as the total ¯ux to the sediment± water interface, an active barrier system covering the element must contain (38.5 mg/9.1 mg gÿ1=) 4.2 g of Bowie Chabazite or (38.5 mg/1.4 mg gÿ1=) 27.5 g of Ash Meadows Clino2 in order to demobilise the total amount of Pb2+. Since the interface area of the cube-shaped element is 1 m2 and if we install a reactive barrier of 0.2 m thickness and a density r = 1.5 g cmÿ3 with a zeolite content of only 0.1% in a matrix of clean sand or gravel, we have an amount of reactive zeolite of 300 g mÿ2. This gives a safety margin approximately of a factor 70 and 11, respectively. Hence, an active barrier system covering a sediment surface of 104 m2 would consist of 3 t of zeolite at approximately US$90 to US$290 per ton (for Ash Meadows Zeolite), resulting in total costs for the natural zeolite material of approximately US$270 to US$870. To improve this ®rst calculation, still suering from some substantial simpli®cations, detailed experiments on heavy-metal sorption on zeolites and mixtures of zeolite in sand or gravel, simulating natural pore-water conditions, have to be conducted, along with computer modelling. CONCLUSION
Natural zeolites are an adequate material in subaqueous sediment capping due to their unique economical, physical and chemical properties which are: . Availability at low cost. . Retention of cationic contaminants by cation exchange.
Table 2. Parameters of the ®tted sorption equations
Bowie Chabazite Ash Meadows Clinoptilolite Armagosa Green Clinoptilolite Meurin Tu
Sorption equation
Sm (mg gÿ1)
K (l gÿ1)
n
r2
Redlich±Peterson Redlich±Peterson Langmuir Langmuir
184.4 79.8 45.6 25.9
0.733 0.235 0.084 0.021
0.87 0.85 ÿ ÿ
0.9999 0.9998 0.9987 0.9990
Natural zeolites in active barrier systems
. Retention of non-polar and anionic contaminants after appropriate modi®cation. . Favourable physical and hydraulic properties. . Physical and chemical stability under normal conditions. Nevertheless, the successful design of an active barrier system using natural or modi®ed zeolites requires further research on the contaminant retention by this material, as well as detailed knowledge about the capping-site characteristics. Especially, the particular hydraulic conditions have to be taken into account. Sites subject to strong underwater currents, tidal currents or shipping activities, for example, may require extremely coarse capping material or an additional armouring layer on top of the active barrier. Furthermore, the thickness of the active barrier, the kind of zeolite mineral used, the pre-treatment of the zeolite material and the zeolite content within the sand matrix have to be adapted to the particular contaminants and the degree of contamination. The objective of further research, accordingly, shall be to obtain all the data relevant to successfully apply the concept of active barrier to a wide variety of contaminants, sediment characteristics and hydraulic and hydrochemical conditions. AcknowledgementsÐThis study is part of the German± Australian Active-Barrier-System (ABS) Co-operation Project funded by the German Federal Ministry for Science and Education (BMBF). REFERENCES
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