Ion exchange is a reversible chemical reaction wherein an ion (an
atom or molecule that has lost or gained an electron and thus acquired
an electrical charge) from solution is exchanged for a similarly
charged ion attached to an immobile solid particle. These solid
ion exchange particles are either naturally occurring inorganic
zeolites or synthetically produced organic resins. The synthetic
organic resins are the predominant type used today because their
characteristics can be tailored to specific applications.
An organic ion exchange resin is composed of high-molecular-weight
polyelectrolytes that can exchange their mobile ions for ions of
similar charge from the surrounding medium. Each resin has a distinct
number of mobile ion sites that set the maximum quantity of exchanges
per unit of resin.
Most plating process water is used to cleanse the surface of the
parts after each process bath. To maintain quality standards, the
level of dissolved solids in the rinse water must be regulated.
Fresh water added to the rinse tank accomplishes this purpose, and
the overflow water is treated to remove pollutants and then discharged.
As the metal salts, acids, and bases used in metal finishing are
primarily inorganic compounds, they are ionized in water and could
be removed by contact with ion exchange resins. In a water deionization
process, the resins exchange hydrogen ions (H+) for the positively
charged ions (such as nickel. copper, and sodium). and hydroxyl
ions (OH-) for negatively charged sulfates, chromates. and chlorides.
Because the quantity of H+ and OH ions is balanced, the result of
the ion exchange treatment is relatively pure, neutral water.
Ion exchange reactions are stoichiometric and reversible, and in
that way they are similar to other solution phase reactions. For
NiSO4 +Ca(OH)2 = Ni(OH)2 + CaSO4
In this reaction, the nickel ions of the nickel sulfate (NiSO4)
are exchanged for the calcium ions of the calcium hydroxide [Ca(OH)2
molecule. Similarly, a resin with hydrogen ions available for exchange
will exchange those ions for nickel ions from solution. The reaction
can be written as follows:
2(R-SO3H)+ NiSO4 = (R-SO3)2Ni+ H2SO4 (2)
R indicates the organic portion of the resin and SO3 is the immobile
portion of the ion active group. Two resin sites are needed for
nickel ions with a plus 2 valence (Ni+2). Trivalent ferric ions
would require three resin sites.
As shown, the ion exchange reaction is reversible. The degree the
reaction proceeds to the right will depend on the resins preference.
or selectivity, for nickel ions compared with its preference for
hydrogen ions. The selectivity of a resin for a given ion is measured
by the selectivity coefficient. K. which in its simplest form for
is expressed as: K = (concentration of B+ in resin/concentration
of A+ in resin) X (concentration of A+ in solution/concentration
of B+ in solution).
The selectivity coefficient expresses the relative distribution
of the ions when a resin in the A+ form is placed in a solution
containing B+ ions. Table 1 gives the selectivity's of strong acid
and strong base ion exchange resins for various ionic compounds.
It should be pointed out that the selectivity coefficient is not
constant but varies with changes in solution conditions. It does
provide a means of determining what to expect when various ions
are involved. As indicated in Table 1, strong acid resins have a
preference for nickel over hydrogen. Despite this preference, the
resin can be converted back to the hydrogen form by contact with
a concentrated solution of sulfuric acid (H2SO4):
(R--SO4)2Ni + H2SO4 -> 2(R-SO3H) + NiSO4
This step is known as regeneration. In general terms, the higher
the preference a resin exhibits for a particular ion, the greater
the exchange efficiency in terms of resin capacity for removal of
that ion from solution. Greater preference for a particular ion,
however, will result in increased consumption of chemicals for regeneration.
Resins currently available exhibit a range of selectivity's and
thus have broad application. As an example. for a strong acid resin.
the relative preference for divalent calcium ions (Ca+2) over divalent
copper ions (Cu+2) is approximately 1.5 to 1. For a heavy-metal-selective
resin. the preference is reversed and favors copper by a ratio of
2.300 to 1.
Selectivity of ion Exchange Resins.
in Order of Decreasing Preference
Strong acid cation Strong base anion
Ion exchange resins are classified as cation exchangers, which have
positively charged mobile ions available for exchange, and anion
exchangers, whose exchangeable ions are negatively charged. Both
anion and cation resins are produced from the same basic organic
polymers. They differ in the ionizable group attached to the hydrocarbon
network. It is this functional group that determines the chemical
behavior of the resin. Resins can be broadly classified as strong
or weak acid cation exchangers or strong or weak base anion exchangers.
Strong Acid Cation Resins. Strong acid resins are so named
because their chemical behavior is similar to that of a strong acid.
The resins are highly ionized in both the acid (R-SO3H) and salt
(R-SO3Na) form. They can convert a metal salt to the corresponding
acid by the reaction:
2(R-SO3H)+ NiCl2 --> (R-SO4),Ni+ 2HCI (5)
The hydrogen and sodium forms of strong acid resins are highly dissociated
and the exchangeable Na+ and H+ are readily available for exchange
over the entire pH range. Consequently, the exchange capacity of
strong acid resins is independent of solution pH. These resins would
be used in the hydrogen form for complete deionization; they are
used in the sodium form for water softening (calcium and magnesium
removal). After exhaustion, the resin is converted back to the hydrogen
form (regenerated) by contact with a strong acid solution, or the
resin can be convened to the sodium form with a sodium chloride
solution. For Equation 5. hydrochloric acid (HCl) regeneration would
result in a concentrated nickel chloride (NiCl,) solution.
Weak Acid Cation Basins. In a weak acid resin. the ionizable
group is a carboxylic acid (COOH) as opposed to the sulfonic acid
group (SO3H) used in strong acid resins. These resins behave similarly
to weak organic acids that are weakly dissociated.
Weak acid resins exhibit a much higher affinity for hydrogen ions
than do strong acid resins. This characteristic allows for regeneration
to the hydrogen form with significantly less acid than is required
for strong acid resins. Almost complete regeneration can be accomplished
with stoichiometric amounts of acid. The degree of dissociation
of a weak acid resin is strongly influenced by the solution pH.
Consequently, resin capacity depends in part on solution pH. Figure
1 shows that a typical weak acid resin has limited capacity below
a pH of 6.0. making it unsuitable for deionizing acidic metal finishing
Strong Base Anion Resins. Like strong acid resins. strong
base resins are highly ionized and can be used over the entire pH
range. These resins are used in the hydroxide (OH) form for water
deionization. They will react with anions in solution and can convert
an acid solution to pure water:
R--NH3OH+ HCl -> R-NH3Cl + HOH (6)
Regeneration with concentrated sodium hydroxide (NaOH) converts
the exhausted resin to the hydroxide form.
Weak Base Anion Resins. Weak base resins are like weak acid
resins. in that the degree of ionization is strongly influenced
by pH. Consequently, weak base resins exhibit minimum exchange capacity
above a pH of 7.0 (Figure 1). These resins merely sorb strong acids:
they cannot split salts.
Exchange Capacity of Weak Acid Cation and Weak Base Anion Resins
as a Function of solution pH
In an ion exchange wastewater deionization unit. the wastewater
would pass first through a bed of strong acid resin. Replacement
of the metal cations (Ni+2. Cu+2) With hydrogen ions would lower
the solution pH. The anions (SO4-2. Cl-) can then be removed with
a weak base resin because the entering wastewater will normally
be acidic and weak base resins sorb acids. Weak base resins are
preferred over strong base resins because they require less regenerant
chemical. A reaction between the resin in the free base form and
HCl would proceed as follows:
R-NH2 + HCl -> R-NH3Cl (7)
The weak base resin does not have a hydroxide ion form as does the
strong base resin. Consequently. regeneration needs only to neutralize
the absorbed acid: it need not provide hydroxide ions. Less expensive
weakly basic reagents such as ammonia (NH3) or sodium carbonate
can be employed.
Heavy-Metal-Selective Chelating Resins. Chelating resins behave
similarly to weak acid cation resins but exhibit a high degree of
selectivity for heavy metal cations. Chelating resins are analogous
to chelating compounds found in metal finishing wastewater; that
is, they tend to form stable complexes with the heavy metals. In
fact. the functional group used in these resins is an EDTAa compound.
The resin structure in the sodium form is expressed as R-EDTA-Na.
The high degree of selectivity for heavy metals permits separation
of these ionic compounds from solutions containing high background
levels of calcium, magnesium, and sodium ions. A chelating resin
exhibits greater selectivity for heavy metals in its sodium form
than in its hydrogen form. Regeneration properties are similar to
those of a weak acid resin; the chelating resin can be converted
to the hydrogen form with slightly greater than stoichiometric doses
of acid because of the fortunate tendency of the heavy metal complex
to become less stable under low pH conditions. Potential applications
of the chelating resin include polishing to lower the heavy metal
concentration in the effluent from a hydroxide treatment process
or directly removing toxic heavy metal cations from wastewaters
containing a high concentration of nontoxic, multivalent cations.
Table 2 shows the preference of a commercially available chelating
resin for heavy metal cations over calcium ions. (The chelating
resins exhibit a similar magnitude of selectivity for heavy metals
over sodium or magnesium ions.) The selectivity coefficient defines
the relative preference the resin exhibits for different ions. The
preference for copper (shown in Table 2) is 2300 times that for
calcium. Therefore, when a solution is treated that contains equal
molar concentrations of copper and calcium ions, at equilibrium.
the molar concentration of copper ions on the resin will be 2300
times the concentration of calcium ions. Or, when solution is treated
that contains a calcium ion molarconcentration 2300 times that of
the copper ion concentration, at equilibrium. the resin would hold
an equal concentration of copper and calcium.
a Ethylenediaminetetraacetic acid.
Chelating Cation Resin Selectivities
for Metal ions
Ni+2 .......................... 57
Co+2....... .................... 6.7
a Selectivity coefficient for the metal over calcium ions at a pH of 4.
Their high cost is the disadvantage of using the heavy-metal-selective
chelating resins. Table 3 compares the cost of these with the costs
of the other commercially available resins.
Cost of Commercially Available Resins
Strong acid cation.......... 70-120
Weak acid cation............ 150-200
Strong base anion........... 180-250
Weak base anion............. 180-200
Chelating cation............ 330-600
Batch and Column Exchange Systems
Ion exchange processing can be accomplished by either a batch method
or a column method. In the first method, the resin and solution
are mixed in a batch tank, the exchange is allowed to come to equilibrium,
then the resin is separated from solution. The degree to which the
exchange takes place is limited by the preference the resin exhibits
for the ion in solution. Consequently, the use of the resins exchange
capacity will be limited unless the selectivity for the ion in solution
is far greater than for the exchangeable ion attached to the resin.
Because batch regeneration of the resin is chemically inefficient,
batch processing by ion exchange has limited potential for application.
Passing a solution through a column containing a bed of exchange
resin is analogous to treating the solution in an infinite series
of batch tanks. Consider a series of tanks each containing 1 equivalent
(eq) of resin in the X ion form (see Figure 2). A volume of solution
containing 1 eq of Y ions is charged into the first tank. Assuming
the resin to have an equal preference for ions X and Y. when equilibrium
is reached the solution phase will contain 0.5 eq of X and Y. Similarly.
the resin phase will contain 0.5 eq of X and Y. This separation
is the equivalent of that achieved in a batch process.
If the solution were removed from Tank 1 and added to Tank 2, which
also contained 1 eq of resin in the X ion form, the solution and
resin phase would both contain 0.25 eq of Y ion and 0.75 eq of X
ion. Repeating the procedure in a third and fourth tank would reduce
the solution content of Y ions to 0.125 and 0.0625 eq. respectively.
Despite an unfavorable resin preference. using a sufficient number
of stages could reduce the concentration of Y ions in solution to
any level desired.
This analysis simplifies the column technique, but it does provide
insights into the process dynamics. Separations are possible despite
poor selectivity for the ion being removed.
Ion Exchange Process Equipment and Operation
Most industrial applications of ion exchange use fixed-bed column
systems, the basic component of which is the resin column (Figure
3). The column design must:
- Contain and support
the ion exchange resin
- Uniformly distribute
the service and regeneration flow through the resin bed
- Provide space
to fluidize the resin during backwash
- Include the piping,
valves, and instruments needed to regulate flow of feed, regenerant.
and backwash solutions
Regeneration Procedure. After the feed solution is processed to
the extent that the resin becomes exhausted and cannot accomplish
any further ion exchange, the resin must be regenerated. In normal
column operation, for a cation system being converted first to the
hydrogen then to the sodium form, regeneration employs the following
1. The column is backwashed to remove suspended solids collected
by the bed during the service cycle and to eliminate channels that
may have formed during this cycle. The back- wash flow fluidizes
the bed. releases trapped particles. and reorients the resin particles
according to size.
Concentration Profile in a Series of ion Exchange Batch Tanks
During backwash the larger, denser panicles will accumulate at the
base and the particle size will decrease moving up the column. This
distribution yields a good hydraulic flow pattern and resistance
to fouling by suspended solids.
2. The resin bed is brought in con- tact with the regenerant solution.
In the case of the cation resin. acid elutes the collected ions
and converts the bed to the hydrogen form. A slow water rinse then
removes any residual acid.
3. The bed is brought in contact with a sodium hydroxide solution
to convert the resin to the sodium form. Again, a slow water rinse
is used to remove residual caustic. The slow rinse pushes the last
of the regenerant through the column.
4. The resin bed is subjected to a fast rinse that removes the last
traces of the regenerant solution and ensures good flow characteristics.
5. The column is returned to service.
Figure 3. Typical ion Exchange Resin Column
For resins that experience significant swelling or shrinkage during
regeneration, a second backwash should be performed after regeneration
to eliminate channeling or resin compression. Regeneration of a
fixed-bed column usually requires between 1 and 2 h. Frequency depends
on the volume of resin in the exchange columns and the quantity
of heavy metals and other ionized compounds in the wastewater.
Resin capacity is usually expressed in terms of equivalents per
liter (eq/L) of resin. An equivalent is the molecular weight in
grams of the compound divided by its electrical charge. or valence.
For example. a resin with an exchange capacity of 1 eq/L could remove
37.5 g of divalent zinc (Zn+2, molecular weight of 65) from solution.
Much of the experience with ion exchange has been in the field of
water softening: therefore, capacities will frequently be expressed
in terms of kilograins of calcium carbonate per cubic foot of resin.
This unit can be converted to equivalents per liter by multiplying
by 0.0458. Typical capacities for commercially available cation
and anion resins are shown in Figure 4. The capacities are strongly
influenced by the quantity of acid or base used to regenerate the
resin. Weak acid and weak base systems are more efficiently regenerated;
their capacity increases almost linearly with regenerant dose.
Cocurrent and Countercurrent Regeneration. Columns are designed
to use either cocurrent or countercurrent regeneration. In cocurrent
units, both feed and regenerant solutions make contact with the
resin in a downflow mode. These units are the less expensive of
the two in terms of initial equipment cost. On the other hand, cocurrent
flow uses regenerant chemicals less efficiently than countercurrent
flow: it has higher leakage concentrations (the concentration of
the feed solution ion being removed in the column effluent), and
cannot achieve as high a product concentration in the regenerant.
Efficient use of regenerant chemicals is primarily a concern with
strong acid or strong base resins. The weakly ionized resins require
only slightly greater than stoichiometric chemical doses for complete
regeneration regardless of whether cocurrent or countercurrent flow
Resin Exchange Capacities
Regenerant Reuse. With strong acid or strong base resin systems.
improved chemical efficiency can be achieved by reusing a part of
the spent regenerants. In strongly ionized resin systems, the degreeof
column regeneration is the major factor in determining the chemical
efficiency of the regeneration process. (See Figure 5.) To realize
42 percent of the resin's theoretical exchange capacity requires
1.4 times the stoichiometric amount of reagent [2 lb HCl/ft3 (32
g HCI/L)]. To increase the exchange capacity available to 60 percent
of theoretical increases consumption to 2.45 times the stoichiometric
dose [5 Ib HCl/ft3 (80 g HCI/L)].
The need for acid doses considerably higher than stoichiometric
means that there is a significant concentration of acid in the spent
regenerant. Further. as the acid dose is increased incrementally,
the concentration of acid in the spent regenerant increases. By
discarding only the first part of the spent regenerant and saving
and reusing the rest.
greater exchange capacity can be realized with equal levels of regenerant
consumption. For example, if a regenerant dose of 5 Ib HCl/ft3 (80
g HCI/L) were used in the resin system in Figure 5, the first 50
percent of spent regenerant would contain only 29 percent of the
original acid concentration. The rest of the acid regenerant would
contain 78 percent of the original acid concentration. If this second
part of the regenerant is reused in the next regeneration cycle
before the resin bed makes contact with 5 Ib/ft3 (80 g/L) of fresh
HCI, the exchange capacity would increase to 67 percent of theoretical
capacity. The available capacity would then increase from 60 to
67 percent at equal chemical doses. Figure 5 shows the improved
reagent utilization achieved by this manner of reuse over a range
of regenerant doses.
Effect of Reusing Acid Regenerant on Chemical Efficiency
Regenerant reuse has disadvantages in that it is higher in initial
cost for chemical storage and feed systems and regeneration procedure
is more complicated. Still. where the chemical savings have provided
justification, systems have been designed to reuse parts of the
spent regenerant as many as five times before discarding them.
Summary Report: Control and Treatment Technology for the Metal Finishing
Ion Exchange USEPA EPA 625/-81-007 June 1981 pp 4-10 (updated by