Chapter 8
Multicomponent Ion-Exchange Equilibria in Chabazite Zeolite
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S. M . Robinson, W. D. Arnold, and C. H . Byers Oak Ridge National Laboratory, Oak Ridge, TN 37831-6044
Efficient design of ion-exchange columns, using Ionsiv IE-96 chabazite zeolite, for the decontamination of process wastewater that contains ppb levels of Sr and Cs requires a detailed study of binary and multicomponent ion-exchange equilibria. Experimental isotherms were acquired for Ca-Na, Mg-Na, Sr-Na, Cs-Na, Sr-Cs-Na, Ca-Mg-Ng, Sr-Ca-Mg-Na, Cs-Ca-Mg-Na, and Sr-Cs-Ca-Mg-Na comparing batch and column experimental approaches. Binary isotherms obtained by the batch technique were most successfully fitted with a modification of the DubininPolyani equilibrium model. Prediction of the multicomponent equilibria from binary data will require more sophisticated modeling. 90
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Fixed-bed ion exchange in which resins serve as the granular medium has been established for many years as an important technique for water purification and for the recovery of ionic components from mixtures. Inorganic media, such as porous, crystaline, aluminosilicate zeolites, have had limited application as ion exchangers in the past. However, zeolite molecular sieves have several characteristics which are unique compared to ion-exchange resins. They are porous crystalline aluminosilicates with a framework which consists of an assemblage of Si0 and A10 tetrahedra. These tetrahedra are joined together in various regular arrangements through shared oxygen atoms to form an open crystal lattice containing pores of precisely uniform molecular dimensions with no distribution of pore size. The guest molecules must diffuse into the micropores before they can exchange with cations which are attached to the aluminum atoms in the framework of the zeolite. Therefore, zeolites exhibit both molecular sieve and ion exchange properties. They also tend to be cheaper than many organic resins and are resistant to thermal and radiation degradation. As 4
4
0097-6156/91/0468-0133$06.00/0 © 1991 American Chemical Society Tedder and Pohland; Emerging Technologies in Hazardous Waste Management II ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
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EMERGING TECHNOLOGIES IN HAZARDOUS WASTE MANAGEMENT II
the field of applications for ion exchange broadens, a class of applications has developed where economic considerations, a high thermal and/or radiation flux, or the molecular sieve properties of zeolites make them more attractive than ionexchange resins. Their molecular sieve properties give zeolites the advantage of selectivity sometimes not obtainable with adsorbers and resins. Because of their unique properties, growing interest has focused on molecular sieve zeolites for use in ion-exchange separations in water softening, pollution abatement energy production, agriculture, animal husbandry, aquaculture, metals processing, and biomedical areas (i). Some of these considerations have led Oak Ridge National Laboratory (ORNL) to use chabazite zeolites for decontamination of process wastewater containing ppb levels of and Cs. A typical characterization of the ORNL process waste stream is shown in Tables I and II. Treatability studies (2, 3) indicate that chabazite zeolites are highly selective for cesium and strontium while admitting high loadings of these metals. Thus they are suited to the removal of trace amounts of Cs and ^ r from wastewater that contains relatively high concentrations of calcium and magnesium. These studies also indicate that the efficiency of the zeolite system depends strongly on the column design and operating conditions and that through optimization of the design of full-scale columns, one could halve the generation rate of loaded zeolite requiring disposal. Models of multicomponent liquid ion-exchange systems were virtually nonexistent before the 1980s. Although a considerable effort has been made in this area, multicomponent models have not been developed to the point where they can be used in general industrial applications without using laboratory- or pilot-scale data to predict the equilibrium and mass transfer relationships (4, 5). A knowledge of multicomponent equilibrium is essential for modeling ionexchange separation processes. While some studies have indicated significant progress in the field (6, 7), much remains to be elucidated, especially in the area of multicomponent systems or ion exchange in inorganic species. Applications of zeolites for treatment of contaminated groundwater began to emerge over the last decade. The decontamination of radioactive waste solutions using zeolites and other inorganic ion exchangers have been studied since the 1950s. Unfortunately very little fundamental studies were done, and it has been difficult to make use of much of the literature because of the lack of standard procedures and theoretical bases (8). Predictive modeling for most multicomponent systems is complicated by competitive interactions among species. In the case of microporous materials such as zeolites, these interactions are further complicated by mutual interference in intraparticle mass transport as well as competition for available ion-exchange sites (9). Since mathematical models have not been available for column design of such systems, users have been restricted to designing columns based on pilot-plant tests and/or crude models. Neither of these approaches have been very successful for efficient column design. The objective of this paper is to present experimental binary and multicomponent equilibrium data for synthetic mixtures that simulate ORNL's process wastewater. Binary equilibrium models are compared with experimental 137
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Tedder and Pohland; Emerging Technologies in Hazardous Waste Management II ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
8. ROBINSON ET A L
135
Ion-Exchange Equilibria in Chabazite Zeolite
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Table I. Radiochemical Composition of O R N L Process Wastewater (26, 27) Radionuclide
Concentration (Bq/L)
Gross alpha Gross beta "Co "Sr Cs Ru
5 6000 25 4000 400 10
137
106
Concentration (N) _
14
2.0 χ 10 1.7 χ HT" 9.1 χ 10 2.3 χ 10
13
15
Data are from refs. 26 and 27.
NOTE:
Table Π. Chemical Composition of O R N L Process Wastewater (26, 27) Component
Concentration (Mq/L)
Concentration (N)
40 8 5 2 2 0.1 0.1 0.1 0.1 93 23 10 11 7 1
2.0 χ lO^ 6.6 χ ΙΟ" 2.2 χ ΙΟ" 5.1 χ 10" 2.1 χ 10" 2.3 χ 10" 1.1 χ 10" 3.6 χ 10" 3.1 χ 10 1.5 χ 104.8 χ ΙΟ" 2.8 χ 101.8 χ ΙΟ" 2.3 χ 10" 5.3 χ ΙΟ""
2+
Ca Mg * Na K Si Sr* Al Fe Zn HCOjso CI" NO, C0 " F 2
+
+
3+
3 +
2+
2+
2
4
2
3
NOTE:
Data are from refs. 26 and 27.
Tedder and Pohland; Emerging Technologies in Hazardous Waste Management II ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
3
04
04
05
04
06
05
06
46
03
04
04 04
04
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EMERGING TECHNOLOGIES IN HAZARDOUS WASTE MANAGEMENT II
data to assess the most appropriate model for the ion-exchange systems relevant to the problem at hand. The applicability of simple multicomponent relationships, based strictly on binary data, to the prediction of multicomponent data is examined. As noted above, zeolites have several characteristics which are unique compared to ion-exchange resins. These properties and their effects on ion exchange are noted throughout the text.
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Theory Binary ion exchange in zeolites may be represented by the following chemical reaction equation (10): (1)
+ - gjfié
s
g B{z) ' • gXJ A
* 8A*
where g and g are the charges of the exchanging cations A and B, and the subscripts ζ and s refer to the zeolite and solution phases, respectively. The equivalent fractions of the exchanging cations in the solution and zeolite are defined by A
B
(2) A
s
• 8A J(2A A C
C
+
C
*B J
and ^
_ z
(3)
equivalents of exchanging cation A total equivalents of cations in the zeolite
where c and c are the molalities of the ions A and B, respectively in the equilibrium solution. The complexity and diversity of the mechanisms of single- and multicomponent ion-exchange equilibrium behavior have led to the development of a large number of equations, both theoretical and empirical in nature. Useful reviews of this area by Soldatov and Bichkova (11), Myers and Byington (12), and Shallcross et al. (13) are available. The mostfrequentlyused models are the following: A
B
the binary Langmuir model, ίA 1+bc
ac
4
()
the binary Freundlich model, (5)
Ac"
Tedder and Pohland; Emerging Technologies in Hazardous Waste Management II ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
8. ROBINSON ET AL.
Ion-Exchange Equilibria in Chabazite Zeolite
137
and the Dubinin-Polyani model, 2
2
( 6 )
q = * exp{-J« 7*[ln(-^)] } c
In these equations, q and c are the equilibrium concentrations in the solid and liquid phases, respectively, q is the saturation concentration in the solid, and b, n, and k are coefficients fitted to the experimental data. These models may be extended in a logical manner to describe multicomponent equilibrium. For instance, the multicomponent Langmuir model is
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8
=
1l
V< l
