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261
exchangers; they have acidic functional groups bound to the resin matrix.
Active Group
Structure
Cation-exchange resins
Sulfonic acid
SO3H
Carboxylic acid
CH
2
CH
C
OH
O
Phosphonic acid
OP(OH)
2
Anion-exchange resins
Quaternary ammonium salt
CH2N
+
(CH
3)3
Cl
-
Seconddary amine
CH
2
NHR
Tertiary amine
CH
2
NR
2
Carriers of exchangeable anions are called anion exchangers; they have
basic functional groups bound to the resin matrix (Table 23). Chelating
ionexchange resins contain chemically bound chelating functional groups,
which sorb metal ions by chelation.
Tabel 23 – Types of Ion-Exchange Resins
Polystyrene (PS) cross-linked with divinyl benzene (DVB) is the
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matrix on which most of the commercial ion-exchange resins are based; the
ionic groups are introduced by post-functionalization of the cross-linked
polymer matrix. By varying the DVB content, the degree of cross-linking
can be adjusted in a simple and reproducible manner. The nominal DVB
content, which refers to mole% of DVB in the polymerization mixture, is
used to indicate the degree of cross-linking. General-purpose ionexchangers contain between 8 and 12 mole% DVB, the most common
being 8 mole%. For special purposes, resins with as little as 0.25% DVB
and as much as 25% DVB have been prepared. Resins with low DVB
content swell strongly and are soft and gelatinous, while those with very
high DVB content swell very little and are tough and mechanically more
stable.
The styrene-DVB copolymer beads are prepared by suspension
(pearl) polymerization technique. The monomers are mixed and a
polymerization catalyst such as benzoyl peroxide is added. The mixture is
then dispersed into small droplets in a thoroughly agitated aqueous solution
that is kept at a temperature required for polymerization (usually 85°C100°C). A suspension stabilizer (gelatin, polyvinyl alcohol, sodium oleate,
magnesium silicate, etc.) in the aqueous phase prevents agglomeration of
the droplets. The size of the droplets depends chiefly on the stabilizer, the
viscosity of the solution, and the agitation, and it can be varied within wide
limits. As polymerization takes place, the droplets are transformed into
polymer beads. For most purposes, a bead size of 0.1-0.5 mm is preferred,
but beads from 1 mm to 2 mm in diameter can be prepared without much
difficulty.
The above method gives beads of gel-type polymer matrix. Porous
beads can be made by incorporating a component (for example, styrene
homopolymer) that is soluble in the monomer mixture. After
polymerization this component is removed from the matrix with, for
example, toluene, thus leaving pores within the structure in the final
product.
Highly porous, so-called macroreticular beads can be prepared by
a variation of the conventional pearl polymerization technique. An organic
solvent is added in which the mixture of monomers is soluble, but the
polymer, when it is formed, is insoluble. Thus, as polymerization
progresses, the solvent is squeezed out by the growing polymer regions. In
this way, one can obtain spherical beads with large pores (several hundred
angstrom units), which guarantee access to the interior of the beads even
when nonpolar solvents are used.
263
Styrene-DVB copolymer beads are sulfonated to produce the most
widely used strong-acid type cation-exchange resins. To make them, the
copolymer precursor beads are dispersed in about 10 times their weight of
concentrated sulfuric acid and heated slowly to 150°C. The sulfonic acid
group is normally introduced into the para position. Though the reaction is
very simple in principle, it involves delicate operations and close control of
parameters in order to achieve beads of suitable structure and durability. A
fully mono-sulfonated, polystyrene has a theoretical ion content of 5.1
equivalents/kg (dry) but many commercial resins have about 4.4-5.2 eq/kg.
Amberlite IR-120, Dowex-50, Nalcite HCR, Permutit Q, Duolite C-20 and
C-25, and Lewatit S-100 are resins of this type.
Weak-acid, cation exchange resins are prepared by
copolymerization of an organic acid or acid anhydride and a cross-linking
agent. As a rule, acrylic or methacrylic acid is used in combination with
divinyl benzene, ethylene dimethacrylate, or similar compounds with at
least two vinyl groups. The pearl polymerization technique described above
can be used if esters instead of the water-soluble acids are polymerized.
The esters are hydrolyzed after polymerization. The final products have
ionic contents of 9-10 eq/kg (dry). Resins of this type are Amberlite IRC50, Duolite CS-101, Permutit H-70, and Wofatit CP-300.
Strong-based anion-exchange resins, also very common, are made
(Figure 5.62) by chloromethylating the styrene-DVB copolymer, then
aminating the product with a tertiary alkyl amine. The quaternization of
chloromethylated resins with tertiary alkyl amines takes place smoothly and
quantitatively. The two most common strong-base resins are made with
trimethylamine and contain the quaternary amine groups –N(CH3)3 and –
N(CH3)2CH2OH. The first type of resin is classed as a Type I strongbased
anion exchanger and the second type as a Type II exchanger. Dowex-1,
Amberlite IRA-400, Permutit S-1, Nalcite SBR, Duolite A-42, and De
Acidite FF are Type 1 resins. Dowex-2, Amberlite IRA-410, Permutit S-2,
Nalcite SA-R, and Duolite A-40 are Type II resins. The resins Amberlite
IRA-401 and 411 differ from the standard types 400 and 410 only by
having a lower DVB content. The ion contents of strong-based resins are 3–
4 eq/kg (dry).
Type I resins have better thermal and oxidative stability and
maintain the integrity of the quaternary groups over a long period of time.
Type II resins, when used in the hydroxide form, are limited to a maximum
temperature of approximately 408C and should not be used under oxidizing
conditions. Type II resins are often used because of lower operations costs.
264
They regenerate somewhat more easily and have higher operating
capacities than the Type I products. The resins have a useful industrial life
of 3-5 years.
Weak-base resins are made by treating a chloromethylated
intermediate with primary or secondary alkyl amines (Figure 57). The
treatment with secondary amines leads to monofunctional weak-base resins
having tertiary amino groups. By treatment with primary alkyl amines,
however, polyfunctional weak-base resins having secondary and tertiary
amino groups are obtained, the latter being formed by the reaction of the
primary amine with two chloromethyl groups:
Additional cross-linking results if the chloromethyl groups belong
to different chains (Occasionally, polyamines such as
tetramethylenepentamine are used; they can react in a similar way with two
or more chloromethyl groups.) The resins Amberlite IR-45, Dowex-3,
Nalcite WBR, and Duolite A-14 are polyfunctional weak-base anion
exchangers. De-Acidite G is a monofunctional resin with tertiary amino
groups.
Figure 57 - Reaction scheme for the preparation of weak-base
and strong-base resins starting with polystyrene
Deionization by ion-exchange is usually confined to relatively
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dilute solutions, i.e., concentrations less than 0.03N (1500 ppm CaCO3).
Since ion-exchange resins have finite capacities, economics are most
favorable for dilute solutions, although concentrations up to 0.1N (5000
ppm CaCO3) can be treated quite satisfactorily.
Cation exchange resins having strongly acidic sulfonic acid groups
are the major cation exchangers used for deionization purposes. When
dilute solutions are passed through beds of such resins, either in acid form
or sodium salt form, cation exchange takes place according to the following
reactions (shown for MgCl2):
The above reactions are reversible, and the exhausted resin can be
regenerated with moderate concentrations of strong acids or NaCl. The
relative affinities for the resin of various cations have an important
influence on the efficiency of exchange. In general, the order of ease of
replacement of common cations is Li+>Na+>K+>Mg2+>Ca2+>Al3+>Fe3+.
Strongly basic anion-exchange resins can act as acid neutralizers or
can split salts in the same manner as the sulfonic acid cation exchangers.
The following are typical reactions:
The resins show marked affinity relationships depending on ion
size and valene. The order of affinity for common anions is: SO4>Cl
–
>HCO3>F–>>HSiO3. Reaction an be reversed by regenerating with
moderate concentrations of sodium hydroxide solutions. However, since
the reactions are not as easily reversed, regeneration levels of 150-200% of
the stoichiometric requirements are frequently employed.
In addition to the commonly employed sulfonic acid cation
exchanger, resins based on the carboxylic acid groups are sometimes
employed under special conditions. These resins are effective for exchange
reactions in neutral and alkaline solution. For example, the effluent from a
saltsplitting reaction, Equation, can be treated with a carboxylic exchanger
according to the reaction:
266
The reaction is reversed very effectively by the hydrogen ion, and
regeneration can be accomplished readily at efficiencies approaching
100%.
The role of weak-base anion-exchange resins in deionization is
often confined to acid neutralization, as shown by the following equation:
The reaction can be reversed by addition of some alkaline reagent
such as NaOH, Na2CO3, NH3 and NaHCO3. The regeneration is
accomplished readily at efficiencies approaching 100%. The ease with
which the hydroxyl ion can be replaced by other anions is SO4>Br–>F
–
>CH3COO–>>HCO3.
The weak-acid and weak-base resins do not totally dimineralize the
water like their strong counterparts. However, the water they produce,
containing 100-200 ppm of dissolved solids, is adequate for many
purposes. The regeneration of the weak resins is also accomplished more
readily than the strong resins.
A relatively recent development is a thermally regenerable
ampholytic resin containing both weak acid and a weak-base functionality
within the one bead (Sirotherm by ICI), which absorbs significant
quantities of salt at ambient temperatures and releases salt on heating to 7090°C. Thus, when the resin is fully exchanged or exhausted, it can be rinsed
in hot water and reused. The adsorption step involves the transfer of
protons form carboxylic acid groups to amino groups to form the cation and
anion exchange sites:
The equilibrium is temperature sensitive, with both types of groups
showing weaker electrolyte behavior on heating. The large increase in the
ionization of water that occurs on heating, about 30-fold from 25°C to
85°C, releases additional protons and hydroxyl ions, which suppress the
ionization of the weak electrolyte resins. The hot water can thus be looked
on as providing the acidic and basic regenerants that usually have to be
added separately.
Operating data indicate that Sirotherm TR-20 reins can produce
267
waters of salinities as low as 50-100 ppm dissolved dalts, the economic
upper range of salinities in feed water being restricted to 2000-3000 ppm.
The resins are expected to find application in the demineralization of mildly
brackish surface and underground waters for industrial and municipal use
and as a roughing stage in the production of high-quality boiler feed water.
Some polystyrene resins (cross-linked with DVB) are specially
modified to have chelating functional groups bound to the matrix so as to
make them selective towards certain ions. Such resins with iminodiacetic
acid groups are marketed under the trade names Dowex A-1 (Dow
Chemical) and Chelex 100 (Bio-Rad Laboratories). The complex (1)
formation constants with metal ions of the chelating resin are so large that
the resin absorbs metal ions equivalent to the iminodiacetic acid groups
(used in sodium salt form), i.e., the efficiency of metal ion adsorption is
near 100%. A particular metal ion can be removed by controlling the pH of
aqueous solution. For example, at pH 2, mercury and copper ions are
preferentially adsorbed, while zinc, cobalt, and cadmium ions are little
adsorbed.
(1) (2)
Polystyrene resins that have aminophosphonate chelating groups
are highly selective towards calcium ion. Such a resin, e.g., Duolite ES467
(Rohm and Haas), when added to a strong brine solution (25%)
contaminated with 10 mg/liter of calcium ion will selectively remove the
calcium ion, forming calcium amino-phosphonate complex (2), until o nly
0.02 mg/liter remains. This process is particularly useful for purifying the
brine used in the chlor-alkali cell described earlier.
Many other specific resins have been prepared. To give only a few
examples, resins with hydroxamic acid groups (3) are specific for Fe3+ ions
and those with mercapto groups (4) prefer Hg
2+
ions. Resins containing
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chlorophyll and haemin derivatives or similar compounds form extremely
strong chelates with ions such as Fe3+. In fact, the chelated counterions are
held so strongly that they can hardly be displaced. Another undesired
consequence of such strong association is that the mobility of the
counterion in the resin is greatly reduced. Hence for any application one
should choose the resin carefully, seeking a reasonable compromise
between selectivity, ease of regeneration, and rate of ion exchange.
(3) (4)
Prior to the development of polystyrene resins, phenolformaldehyde (P-F) condensates were used as matrices, but they have now
been replaced. A few weak-base types still exist (e.g., Duolite ES562 of
Rohm and Haas), which are made by adding an amine during
polycondensation. These P-F condensates are used for enz ym e fixation.
Applications
Applications of ion-exchange resins are extremely varied, ranging
from water-softening to purification of chemicals and therapeutic
applications. An extremely useful industrial development of the ionexchange technique is the production of demineralized water rivaling that
of distilled water in purity.
Most ion-exchange reactions for industrial applications are done
with columns of resin in which the ions in solution (say B) are depleted,
proceeding from top to the bottom of the column, by exchange with ions
(say A) in the resin. With the progress of ion exchange the resin bed shows
an exhausted portion at the top, an ion-exchange zone in the middle and a
regenerated portion at the bottom (Figure 58).
As the ion-exchange zone moves down through the resin column,
the B ions eventually reach the outlet, at which time breakthrough occurs.
In most ion-exchange installations, the vertical column is the most
commonly employed unit. The system may consist of a single column with
one type of resin (single column system), two or more columns containing
a variety of cation and/or anion exchange resins (multiple-bed system), or a
single column containing a mixed bed of two (or more) resins (monobed or
269
mixed-bed system).
Figure 58 - Ion-exchange column in service: (+, resin containing A cations;
, resin containing B cations).
Much of the appeal of the ion-exchange process stems from the
simplicity of the single-column system. Water softening by ion-exchange is
the most widely used example of this system (Figure 59). The hardness
ions, calcium and magnesium, are exchanged for sodium as the hard water
flows down through a column of cation exchange resin used in the sodium
form. Conversion of the exhausted resin back to the sodium form is
accomplished in a regeneration step by contacting the column with an
excess of sodium chloride solution. Water softeners range in size from
small household units (e.g., 20 cm in diameter and 60 cm deep) to large
industrial units (e.g., 360 cm in diameter and 150 cm deep).
270
Figure 59 - Water softening by ion-exchange in single-column system.
Two-bed deionization of water is widely practiced. Such systems
generally use a strong-acid cation exchange followed by either a weak-base
or a strong-base anion exchanger. Three beds or more are used either to
achieve operating economy or a greater degree of deionization. For
example, a three-bed system of strong acid cation exchanger/weak-base
anion exchanger/strong-base anion exchanger offers economy in
regeneration. In service, the weak-base resin removes the mineral acids
coming from the cation exchanger and the strong-base resin removes
principally carbonic and silicic acids.
For installations requiring very high effluent water quality,
monobed or mixed-bed deionization has been used widely, yielding a
demineralized water in one operation. The standard mixed-bed is a special
case of a single column. It usually consists of an intimate mixture of a
strong-acid cation exchange resin and a strong-base anion exchange resin in
the hydrogen and hydroxide forms, respectively. When water is passed
through a fixed bed of such a mixture, the ions in solution are alternately
exposed to numerous contacts with cation and anion exchange sites, the
effect being similar to that obtained with a very large number of multiple
beds, with the result that almost complete deionization occurs. The process
follows the reaction: