Inorganic Ion Exchangers, Chemia, Chemia Nieorganiczna
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Inorganic Ion Exchangers
Further Reading
Helfferich F (1962)
Ion Exchange
, 2nd edn. New
York: McGraw-Hill.
Hwang S-T and Kammermeyer K (1975)
Membranes in
Separations
. New York: Wiley.
Marinsky JA and Marcus Y (eds) (1973)
Ion Exchange
and
Alberti G, Casciola M, Costantino U and Vivani R (1996)
Layered and pillared metal(IV) phosphates and phos-
phonates.
Advanced Materials
8(4): 291.
Amphlett CB (1964)
Inorganic Ion Exchangers
. Amster-
dam: Elsevier.
Clear
Solvent
Extraction
. New York: Marcel
Dekker.
Osborn GH (1961)
Synthetic Ion-Exchangers
:
Recent De-
velopment in Theory and Application
. London: Chap-
man
eld A (ed.) (1982)
Inorganic Ion Exchange Mater-
ials
. Boca Raton, FL: CRC Press.
Fritz JS, Gjerde DT and Pohlandt C (1982)
Ion Chromato-
graphy
. Heidelberg: Hu
R
Hall.
Weiss J (1994)
Ion Chromatography
, 2nd edn. Weinheim:
Wiley.
&
thig.
Greig JA (ed.) (1996)
Ion Exchange Developments and
Applications
. Cambridge: Royal Society of Chemistry.
Inorganic Ion Exchangers
2. charge-compensating groups, electrostatically as-
sociated with, and not covalently bonded to,
a charged moiety
Type 1 sites, illustrated in
Figure 1A
, are respon-
sible for the ion exchange properties of materials
such as hydrous oxides and single-layer clays.
All oxidic materials have these sites to some degree,
at the surfaces of particles or crystals or at defect
sites within the structure. Ion exchange reactions
involving these types of sites may be regarded as
chemical reactions, which may display amphoteric
nature.
Type 2 sites, illustrated in Figure 1B, are respon-
sible for most of the ion exchange capacity of zeolites,
double-layer clays and zirconium phosphates. These
sites arise in structures possessing, for instance,
charged layers or charged porous frameworks. The
exchangeable ions are present to retain overall elec-
troneutrality. When materials such as zeolites are
concerned, a mixture of Type 1 and Type 2 sites is
available, although Type 2 sites will usually greatly
outnumber Type 1 sites, and the latter are often
ignored. Exchange interactions involving Type 2 sites
are physical in nature, as chemical bonds are neither
made nor broken.
E. N. Coker
, BP Amoco Chemicals,
Sunbury-on-Thames, Middlesex, UK
^
Copyright
2000 Academic Press
Summary
In the
rst part of this chapter, the origins of
ion exchange in inorganic materials are discussed
in relation to the structure of the exchanger.
Thereafter, the various types of inorganic ion
exchangers are introduced and categorized according
to their ion exchange properties. Descriptions of
particular materials follow, with special emphasis
on some structure-speci
R
c
ion exchange properties. The materials which are
discussed include zeolites and zeolite-like materials,
clays and other layered materials, zirconium
phosphates, heteropolyoxometalates and hydrous
oxides.
c and composition-speci
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R
Types of Ion Exchange Sites in
Inorganic Materials and their
Origin
For the purposes of
this chapter,
ion exchange
interactions will be de
ned as those involving the
interchange of positively or negatively charged
species (atomic or molecular) at an ion exchange
site.
There are two types of chemical species which
constitute the vast majority of ion exchange sites in
inorganic materials:
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Types of Inorganic Ion Exchange
Material
An important distinction between ion exchange ma-
terials is whether they exhibit capacity for cations,
anions, or both. Cation exchangers, and in particular
zeolites, clays and zirconium phosphates, are the
most common and best understood of the ion ex-
changers. Anion exchangers are also important but
1. structure-terminating, covalently bonded groups
such as
}
OH
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Inorganic Ion Exchangers
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Most crystalline inorganic ion exchangers are por-
ous. This porosity may arise through the presence of
void space between the layers in clay materials and
layered double hydroxides, or through the intrinsic
microporosity present in zeolitic materials. Many of
the layered materials have the versatility to (revers-
ibly) change their interlayer spacing and hence the
size of the voids, which allows the ion exchange
properties to be adjusted. The more rigid zeolite
structures give rise to exchange reactions which may
show extremely high selectivity to certain cations, or
perform ion sieving.
Zeolites
Zeolites are microporous crystalline aluminosilicate
minerals which occur naturally and may be syn-
thesized easily in the laboratory. An introduction
to the structures and properties of zeolites is given
in the article by Dyer. Zeolites are used on a large
scale as ion exchangers in many
elds; most notable
are their use as ‘builders’ or water softeners for laun-
dry detergents, and their use in the decontamination
of various types of waste streams. Typical applica-
tions of zeolites as ion exchangers are given in
Table 1
. Additionally, the ion exchange capability of
zeolites can be used as a tool to modify their catalytic
and sorptive properties. Some attention will be paid
to structural parameters which in
R
Figure 1
The two major types of ion exchange site. (A) Type 1,
structure-terminating and defect groups; (B) Type 2, charge-com-
pensating groups. M is an oxide-forming metal with oxidation
state 4; T is an oxide-forming metal with oxidation state 3. The
regions enclosed in dotted lines are those giving rise to ion
exchange where Z
#
(or Z
uence the ion ex-
change properties of zeolites in the following para-
graphs.
Besides the conditions under which an ion ex-
change reaction is performed, a number of factors
may in
S
) is exchangeable. Shaded areas
represent a continuation of the oxidic network.
}
O
uence the ion exchange properties of zeolites,
including:
S
the exchange of anions is often not fully reversible,
thus the exchangers cannot be easily regenerated and
the reactions are more dif
the structure of the zeolite, particularly the dia-
meters of the windows allowing access to the pores
and cavities
cult to treat thermo-
dynamically. Multiply charged anions, in particular,
may be held tenaciously by the exchanger. Examples
of anion exchangers are certain clays such as hydroxy
double salts (e.g. [CuNi(OH)
3
]Cl) and layered
double hydroxides (e.g. hydrotalcite, Mg
6
Al
2
(OH)
16
(CO
3
)
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the location of the ion exchange sites; different
cation environments lead to different ion ex-
change properties. The number of charge-balanc-
ing cations required for an electroneutral material
is often less than the number of available ion ex-
change sites, thus partial occupancy of sites is com-
mon. Some of the possible cation positions in
zeolites A and X (two of the most widely used
synthetic zeolite ion exchangers) are indicated in
Figure 2
4H
2
O). Amphoteric ion exchangers possess
predominantly Type 1 exchange sites, e.g. hydrous
oxides.
While ion exchange properties may be exhibited by
both amorphous and crystalline solids, studies of the
ion exchange properties of amorphous solids are of-
ten hampered by dif
)
culties in preparing mater-
ials reproducibly and the dif
the composition of the zeolite framework;
varying the Si : Al ratio or changing the frame-
work substituent elements may change, for
example, the density of exchange sites, the electric
R
R
culties in character-
izing them fully. With crystalline materials, however,
reproducible preparations can be easily veri
R
ed and
R
well-de
ned structural data aids in the interpretation
of the results of ion exchange experiments.
eld strength or the hydrophobicity of the sample
as a whole
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Table 1
Principal applications of zeolites as ion exchangers
Application
Typeof zeolitefrequentlyused
Ionexchangeprocess
Detergent building
A (synthetic)
Removal of Ca
2#
and Mg
2#
from solution
MAP (synthetic)
X (synthetic)
Wastewater treatment
Clinoptilolite (natural)
Uptake of NH
4
and heavy metals from waste
Chabazite (natural)
streams
Mordenite (natural)
Phillipsite (natural)
Nuclear waste treatment
Clinoptilolite (natural)
Uptake of
137
Cs
#
,
90
Sr
2#
and other radionuclides
Chabazite (natural)
Phillipsite (natural)
Mordenite (natural)
Mordenite (synthetic)
Ionsiv IE-96 (synthetic)
Ionsiv A-51 (synthetic)
Animal food supplement
Various (natural)
Regulation of NH
4
and NH
3
levels in stomach
Animal food supplement
Various (natural)
Scavenging of radionuclides following contamina-
tion of livestock
Fertilizer
Various NH
4
forms (natural), often those
used to remove NH
4
Slow release of NH
4
(and other cations)
from wastewater
nM
n
#
cations
are present to counterbalance the
x
units of
negative charge on the framework due to the presence
of
x
AlO
2
groups. In many cases, ion exchange
reactions in zeolites may reach completion, that is,
all of the charge-balancing cations (
M
) initially
present are capable of being replaced by the ingoing
cation.
The
empirical
structural
formula
for
an
occupy the interstitial space. The
x
/
aluminosilicate zeolite may be given as
M
(n)
x
n
[(AlO
2
)
x
(SiO
2
)
y
]
w
H
2
O
)
/
where the framework is constructed from the
entities within the square brackets and the water
molecules
and
charge-balancing
cations
(
M
)
Figure 2
A representation of some of the possible positions of exchangeable cations in the structures of zeolites A (A) and X (B).
Note: the two structures are not shown on the same scale. Reproduced with permission from Stucky GD and Dwyer FG (eds) (1983)
IntrazeoliteChemistry. ACS Symposium Series, vol. 218, p. 288. Washington, DC: American Chemical Society.
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Incomplete ion exchange reactions
In some cases,
some of the cations are constrained within the struc-
ture and are nonexchangeable. Such cations are intro-
duced into small cavities in the structure during
growth of the zeolite crystal. This situation is
common with feldspars and feldspathoids, which are
similar in composition to zeolites, but possess more
limited porosity. Even in instances when all charge-
balancing cations in the zeolite are physically ex-
changeable, the total theoretical exchange capacity
might not be obtained practically.
There are several reasons for incomplete ion ex-
change; the three most important of these are given
below and illustrated schematically in
Figure 3
.
1. The most obvious cause of partial or nonexistent
exchange is ion-sieving, where the cation to be
exchanged into the zeolite is too large, or has
a hydration sphere which is too large and robust
for it to have unrestricted access to the pores of the
zeolite. Univalent cations will typically reach
100% exchange, except in limiting cases such as
large cations combined with small-pore zeolites.
Ion-sieving is more commonly observed with
multiply charged cations, which tend to have lar-
ger hydration spheres on account of their higher
charge densities. Zeolites which possess more than
one ion exchange site (see Figure 2) may display
ion-sieving properties depending on the thermo-
dynamics of the exchange reactions occurring at
the various sites. The sites which offer the
greatest thermodynamic advantage are exchanged
R
rst, while the less favourable sites may not ex-
change at all.
2. Volumetric exclusion may occur if bulky (organic)
cations are exchanged into zeolites of high charge
density. Here, the volume occupied by the cations
may reach that available in the pores of the crystal
before complete exchange has occurred.
3. A third reason for limited exchange to be observed
is when multivalent cations are exchanged into
zeolites of low charge density. As the density of
ion exchange sites decreases, the mean separation
between adjacent sites increases, until a point is
reached where multivalent cations are unable to
satisfy two or more cation exchange sites because
of the distance between them.
Table 2
illustrates
this point by listing the maximum exchange limits
observed for several multivalent cations in samples
of zeolites ZSM-5 and EU-1 possessing a range of
Si
Figure 3
The principal reasons for limitations to ion exchange
reactions found in zeolites. (A) Ion-sieving; (B) volume exclusion;
(C) low charge density (with multivalent cations). The lightly
shaded regions represent an extract of the zeolite framework. For
clarity, only ingoingcations are shown.
exchange is kinetically limited but still capable of
reaching 100% of the theoretical capacity is the sof-
tening of water.
Zeolites are used in vast quantities in the detergent
industry as a water-softening additive for laundry
detergents
up to 30% by weight of most modern
washing powders is zeolite. The zeolite is added prin-
cipally to remove calcium and magnesium and thus
prevent their precipitation with surfactant molecules.
Zeolite A is most commonly used, due to its high ion
exchange capacity, which is a consequence of the
framework possessing the maximum possible number
of aluminium atoms (Si : Al
}
/
Al ratios.
It is easy to visualize the limiting factors of ion
exchange under equilibrium conditions; however,
practical ion exchange may have also kinetic limita-
tions. A particular example of when the desired ion
"
1 : 1). Recently, zeolite
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Table 2
Ion exchange limits (mole fraction) for various multivalent cations and temperatures in samples of zeolites ZSM-5 and EU-1
with varying numbers of aluminium atoms in the framework. In all cases, the ingoing cation replaces sodium
Zeolitetype Alper
u.c.
a
Ca
2#
(253C)
Sr
2#
(253C)
Ba
2#
(253C)
La
3#
(253C)
Ca
2#
(653C)
Sr
2#
(653C)
Ba
2#
(653C)
La
3#
(653C)
ZSM-5
1.1
0.28
0.31
0.36
0.50
0.51
0.52
ZSM-5
2.0
0.31
0.36
0.56
0.54
0.64
0.76
ZSM-5
2.4
0.36
0.48
0.67
0.39
0.50
0.67
0.77
0.48
ZSM-5
4.2
0.37
0.42
0.90
0.62
0.85
0.93
EU-1
1.2
0.54
0.56
0.56
EU-1
2.1
0.62
0.67
0.67
0.85
0.89
0.89
EU-1
3.8
0.86
0.93
0.93
0.96
0.97
0.97
a
Number of aluminium atoms in framework per unit cell.
order to provide acceptable kinetics of Ca
2
#
exchange.
MAP
(Maximum Aluminium P),
also with
Si : Al
1 : 1, has been introduced into some deter-
gents. Although the Mg
2
#
ion (radius 0.07 nm) is
considerably smaller than the Ca
2
#
ion (radius
0.1 nm), its exchange into the zeolite is far less
facile than that of Ca
2
#
, due to its large, tight
hydration sphere (the radii of the hydrated Ca
2
#
and Mg
2
#
cations are estimated to be 0.42 and
0.44 nm, respectively).
Figure 4
shows the kinetics of
exchange of Ca
2
#
and Mg
2
#
into Na-A zeolite.
The major restriction to the hydrated Mg
2
#
cation
is the 0.42 nm window in zeolite A through which
it must pass to gain access to the exchange sites
within the structure. In order for the ion exchanger
to be effective as a water softener for detergents,
it must reduce water hardness within a fewminutes of
beginning the wash cycle. While zeolites A and MAP
perform well at removing calcium from hard water
quickly, their performance towards magnesium is
generally poor. Despite the kinetic limitations, Ca
2
#
and Mg
2
#
are fully exchangeable into zeolite A, al-
though selectivity is greater for Ca
2
#
(
Figure 5
). De-
tergent-grade zeolites possess small crystallite sizes in
"
Materials closely related to zeolites
Semicrystalline zeolites
Some interest has been
shown in the ion exchange properties of zeolite pre-
cursors, which are obtained by quenching a zeolite
synthesis mixture before it has fully crystallized. In
these semicrystalline materials, some larger windows
and pores are present than in the crystalline counter-
part because the structure has not fully formed. This
leads to ion exchange selectivities which are dif-
ferent from the crystalline material. Also, their ion
exchange capacities are lower than the corresponding
crystalline zeolites. The materials typically show
weak zeolite X-ray diffraction patterns, and are
Figure 4
Kinetics of exchange of Ca
2#
and Mg
2#
for 2Na
#
in
zeolite A. Circles, Ca
2#
exchange; triangles, Mg
2#
exchange.
Data were determined at 25
Figure 5
Isotherms for Ca
2#
/
2Na
#
exchange
in zeolite A. Circles, Ca
2#
exchange; triangles, Mg
2#
exchange.
Data were determined at 25
2Na
#
and Mg
2#
/
3
C, pH 10 and at a solution concentra-
3
C, pH 10 and at a solution concentra-
tion of 0.05 mol equiv. L
1
.
tion of 0.05 mol equiv. L
1
.
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