TIN-BASED ION EXCHANGERS
1. Background
Although ion exchange reactions occur extensively in nature both in inanimate
systems, such as soils, and in living organisms, the chemical discovery of ion
exchange and the first systematic studies of ion exchange reactions date back to the
middle of the 19th century. Early work on clays led to the development of the first
synthetic ion exchangers which were based on aluminosilicate structures and found
extensive use as water softeners. A comprehensive review of ion exchange materials
and their properties and applications has recently been published.1
Synthetic organic ion exchange resins were developed some time later, and these
almost completely displaced inorganic materials for most practical applications.
However, organic ion exchange resins suffer from several major disadvantages.
They are thermally unstable and cannot be used at temperatures above 1500C, and
they break down when subjected to ionising radiation or strong acids and alkalis.
Consequently, there has been considerable interest in inorganic ion exchangers,
since these materials are generally stable at relatively high temperatures and are
often resistant to acids, alkalis and nuclear radiation, making them suitable for
applications such as water treatment at elevated temperatures, chemical processing
of solutions containing nuclear waste, and the column chromatographic separation of
radionuclides that are used in routine medical diagnostic procedures.2
A considerable number of inorganic ion exchangers are known and these comprise
insoluble salts and hydrolysed oxides, which are synthetically prepared, and naturally
occurring minerals, such as zeolites. Despite the large amount of published work on
inorganic ion exchange materials, their fundamental properties and the mechanism of
ion exchange are not entirely clear. Classical theory suggests that inorganic ion
exchangers remove anionic species at low pH and cationic species at higher pH
values. Hence, in acidic media, the hydrolysed proton reacts with the oxide surface to
produce an anionic exchange group:
MO2 + H+  MO2H+
In alkaline solutions, the hydroxyl ion reacts with the hydrous oxide to produce a
cationic exchange surface:
MO2 + OH-  MO2OHThe transition from cationic to anionic behaviour is said to occur at a given pH value,
known as the point of zero charge (pzc), when solution conditions prevent the
formation of active sites. Zeta potential measurements of fine particles of ion
exchangers have been used to determine the type of ion exchange behaviour that
can be expected for specific materials.3
References
1. A.A. Zagorodni, ‘Ion Exchange Materials: Properties and Applications’, Elsevier,
Amsterdam, 2006.
2. R. Paterson, ‘An Introduction to Ion Exchange’, Heyden & Son, London, 1970,
p.15.
3. R. Rautiu, D.A. White, S.A. Adeleye & L. Adkins, Hydrometallurgy, 1994, 35, 361.
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2. Evaluation of ion exchange properties
An ion exchange system essentially comprises an electrolyte solution containing
cations, anions and water, in which one or other of the ions is bound to an insoluble
microporous matrix. In the water-filled pores, the remaining ion (of opposite charge to
the fixed ion), is present in sufficient numbers to render the whole exchanger system
electrically neutral. These ‘counter ions’ are free to move through the matrix by
diffusion or under electrical field and may be replaced by other counter ions (of the
same charge) from solution, in the process of ion exchange. The material is
described as a ‘cation exchanger’ or an ‘anion exchanger’ according to the counter
ion type.
The concentration of fixed charges in the exchanger is known as the ion exchange
capacity. This property is commonly measured in milli-equivalents (of charge) per
millilitre of wet resin. Alternatively, it can be measured in terms of number of
equivalents of fixed charge per gram of dry matrix. For well-defined exchangers, the
capacity is an unambiguous quantity readily determined by batch or column
experiments. However, in exchangers containing poorly-defined functional groups, or
which are stable only over a limited pH range, true capacity values are more difficult
to obtain. In these cases, ion exchange characteristics are often best determined by
the use of pH titration curves. A further term encountered in ion exchange is
breakthrough capacity, this referring to the amount of a given ion taken up by a
column of the exchanger at the point when the ion begins to pass through the
column. This property is, however, dependent upon a number of factors including
concentration, flow rate, particle size, etc.
Many applications of ion exchangers utilise the fact that they exhibit preferences for
certain ions over others. This selectivity can depend on several factors but, in
general, an ion exchanger exhibits a preference for highly charged ions over those
with low charges, and for small ions rather than large ones. However, factors such as
ion association, complex formation and cross-linking can have major effects on
selectivity, and ion exchange systems can be engineered to optimise uptake of the
ion of choice.
In practice, there is considerable use of the concentration term selectivity
coefficient (KAB), which measures the tendency of the exchanger to select ion B over
ion A. Hence, if KAB >1, then the exchanger selects ion B. Concentrations may be
measured in molar, molal or equivalent fraction units, with selectivities termed
accordingly. Another function often used to express the position of an equilibrium set
up between the exchanger and solution phases in which A and B ions are distributed,
is the distribution coefficient. Determination of these selectivity parameters is
commonly undertaken using column chromatography and appropriate elution and
displacement methodologies.
Detailed reviews of the fundamentals of ion exchange and practical methods for
determining the above parameters have been published.1,2
References
1. R. Paterson, ‘An Introduction to Ion Exchange’, Heyden & Son, London, 1970,
p.15.
2. A.A. Zagorodni, ‘Ion Exchange Materials: Properties and Applications’, Elsevier,
Amsterdam, 2006.
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3. Inorganic tin ion exchange materials
Hydrous tin(IV) oxide ion exchangers have been studied extensively over the past 30
years or so, and have proved to be versatile amphoteric ion exchange materials. 1,2
Although SnO2 itself occurs naturally as the mineral ore Cassiterite, hydrous forms
are prepared synthetically by solution routes.
Hydrous tin(IV) oxide exists in two distinct modifications – -stannic acid (also called
‘ortho-stannic acid’), which forms as a glassy granulated gel material, and -stannic
acid (also called ‘meta-stannic acid’), which exists as a fine white powder with a
particle size typically in the m region.3 The - form is prepared by the cold hydrolysis
of tin(IV) chloride with aqueous ammonia:
SnCl4 + 4NH4OH  SnO2.2H2O + 4NH4Cl
or, alternatively, by acidification of cold aqueous solutions of sodium or potassium
hydroxystannate, using either dilute nitric or sulphuric acid:
Na2Sn(OH)6 + 2HNO3  SnO2.2H2O + 2NaNO3 + 2H2O
The industrial manufacture of -stannic acid involves the reaction of metallic tin (as a
powder or fine granules) with concentrated nitric acid. The exothermic reaction is
maintained at a temperature just above 1000C by controlled addition of the tin metal
to the acid. The white precipitate is then separated, washed with water to remove
excess acid, filtered and dried at 120 – 1500C:
Sn + 4HNO3  SnO2.2H2O + 4NO2
The chemical composition of the hydrous tin(IV) oxides is imprecise, both being
usually represented by the formula, SnO2.xH2O, where x is between 1 and 2,
depending on preparation method, drying temperature, etc. Both materials have been
characterised using x-ray powder diffraction, 119mSn Mossbauer spectroscopy,
thermal analysis, BET nitrogen adsorption surface area determinations and cation
exchange sorption measurements, and whilst the freshly prepared materials exhibit
widely differing chemical and surface reactivities in precipitate form, they appear to
be identical as regards surface area and pore structure after drying or calcination at
temperatures up to ca. 5000C.3 In summary, there are no true hydrates of SnO2 and
the difference between the - and - forms must be due to differences in degree of
agglomeration and to the amount of adsorbed water in the tin(IV) oxide particles.
Although hydrous SnO2 itself is widely used as an ion exchanger, it is well known that
its ion exchange properties, including selectivity and ion exchange capacity, can be
modified by combining it with oxides of other elements,4 particularly phosphorus,
antimony, arsenic, molybdenum, tungsten and silicon.
References
1. M.J. Fuller, ‘The Ion Exchange Properties of Tin(IV) Materials’, Ph.D. Thesis,
University of London, 1969.
2. D.A. White & R. Rautiu, Chem. Eng. J., 1997, 66, 85.
3. M.J. Fuller, M.E. Warwick & A. Walton, J. Appl. Chem. Biotechnol., 1978, 28,
396.
4. P.A. Cusack, Tin International, 1999, 72 (7), 6.
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4. Applications
(a) Removal of heavy metals1-3
Hydrous tin(IV) oxide gels show good stability towards strong acid solutions and
exhibit selectivity for higher valency cations, particularly Cr3+ and Fe3+, allowing their
separation from lower valency cations. Because of its high affinity for transition metal
ions generally, hydrous SnO2 is very useful for removal of trace transition metal
contamination from solutions of salts of alkaline earth and alkali metals. SnO2 is
amphoteric in nature and the point of zero charge occurs at a pH of 4.0. Classical
theory therefore predicts that it acts as a cation exchanger above this value and as
an anion exchanger below this value. In practice, hydrous SnO2 has been found
effective for the adsorption of Cr(VI), Mn(VII), Mo(VI) and W(VI), and anions
containing arsenic, selenium and antimony. Apart from SnO2 itself, many other
inorganic tin(IV) materials exhibit cation and anion exchange properties, including
tin(IV) phosphate, tin(IV) arsenate, tin(IV) antimonate, tin(IV) molybdate, tin(IV)
tungstate and tin(IV) bismuthate. Whilst many of these systems are used as gel
materials in column chromatographic processes, others have found application in thin
layer chromatography (TLC) and ion chromatography on impregnated papers.
(b) Separation of radiopharmaceuticals4-5
Commercially, the most important use of hydrous SnO2 and other inorganic tin ion
exchangers to date involves the column chromatographic separation of radioisotopes
that are increasing employed in medical diagnostic techniques, particularly Positron
Emission Tomography (PET). α-stannic acid appears to be the preferred form for the
majority of separations, although the β- form has also found limited application.
(c) Decontamination of nuclear waste6
Because of their demonstrated stability towards strong acids and nuclear radiation,
tin-based ion exchangers have been suggested for use in waste management. In this
connection, recent attention has focused on the potential use of tin(IV) antimonate for
decontamination of radioactive wastes generated by nuclear power plants. Effective
decontamination of radioactive cobalt, nickel, strontium and cesium from simulated
solutions has been demonstrated.
(d) Metal passivation in the petroleum industry7
Finally, it should be mentioned that hydrous metal oxide colloids are used as metal
passivating agents in the petroleum industry. Colloidal SnO2, usually in conjunction
with colloidal Sb2O5, is found to be particularly effective for removing trace levels of
vanadium and nickel, both of which can cause problems during crude oil cracking
processes.
References
1.
2.
3.
4.
5.
6.
J.D. Donaldson & M.J. Fuller, J. Inorg. Nucl. Chem., 1970, 32, 1703.
D.A. White & R.Rautiu, Chem. Eng. J., 1997, 66, 85.
S.D. Sharma & S. Misra, J. Chromatogr., 1992, 594, 379.
Anon, Focus on Tin, 1994, 6, 6.
M.R. Cackette, T.J. Ruth & J.S. Vincent, Appl. Radiat. Isot., 1993, 44, 917.
R. Koivula, ‘Inorganic Ion Exchangers for Decontamination of Radioactive Wastes
Generated by Nuclear Power Plants’, Ph.D. Thesis, University of Helsinki, 2003.
7. http://www.nyacol.com/moreapps.htm
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