Ion exchange is a water treatment process that removes ions from water through an exchange of ions between a resin and water. There are cation exchange resins that remove positively charged ions and anion exchange resins that remove negatively charged ions. Ion exchange is commonly used for water softening by exchanging calcium and magnesium ions for sodium or potassium ions held on the resin. The resin becomes exhausted over time and must be regenerated by flushing it with a brine solution to restore the ions on the resin.

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ION EXCHANGE PROCESSES
Ion exchange is a water treatment process commonly used for water softening or demineralization,
but it also is used to remove other substances from the water in processes such as dealkalization,
deionization, and disinfection.
Ion exchange describes a specific chemical process in which unwanted dissolved ions are
exchanged for other ions with a similar charge.Ions are atoms or molecules containing a total
number of electrons that are not equal to the total number of protons.There are two different groups
of ions, cations, which are positively charged, and anions, which are negatively charged.
When contaminants dissolve in water they typically form ions. When contaminants are dissolved
in water, the water is typically crystal clear. If the water is cloudy or otherwise discolored it is
likely that some, or all, of the contaminants are in a solid form. Solid particles are not intended to
be removed by IE and solids will clog the treatment media.
Ion Exchange Resins
The ion exchange resins can be divided into two major categories .If the contaminant has a positive
charge, it would be called a cation ,and would be removed by use of an IE media called a cation
exchange resin.If the contaminant has a negative charge, it would be called an anion, and the
appropriate treatment media would be called an anion exchange resin. One is strong acid
exchange resins and another one is weak acid exchange resins. In anion exchange resins also there
is strong base exchange resins and weak base exchange resins.
A strong acid exchange resin is a one whose functional group is derived from a strong acid. Their
degree of ionization is analogous to that of a strong acid. That means it will be ionizing very fast
which permits the hydrogen to be dissociated and ready for exchange over a wide pH range. For
weak acid exchange resin the functional group is derived from a weak acid like carboxylic or
phenolic acid and these resins are able to work only in pH range. So the difference is this is derived
from a strong acid and it undergoes ionization very fast and it can function in a wide pH range
whereas in a weak acid based resin the functional groups are generated or developed from weak
acid like phenolic acid or carboxylic acid and the major disadvantage is such resins can be
functional only in a small pH range below pH 7. The major disadvantage of this strong acid
exchange resin is because it is having high affinity compared to other cations so the regeneration
efficiency will be very very less, the maximum regeneration you can get is 30 to 50 percentage but
it can be used for a wide pH range. The advantage of the weak acid exchange resin is that the
regeneration capacity is very very high, the reason is the affinity because it is functioning as a
weak acid so this affinity will be not so high so we can easily replace this one that is why we are
getting high regeneration efficiency.

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General Characteristics Of Ion Exchange Resin
a) Ion Exchange Capacity
The ion exchange capacity is a measure of the total quantity of the ions which theoretically
can be exchanged. That means it is nothing but the number of ionizable group that is present
in the ion exchange resin per unit mass or per unit volume of resin.
b) Operating capacity
Operating capacity is a measure of the actual useful capacity of the resin for exchanging
ions from a solution flowing through a fixed bed of resin particles under specified
conditions. The operating capacity can be represented by the term X. It is equal to number
of equivalents of the ions of interest applied to the column per resin volume minus number
of equivalents of the ions of interest passing through the column per resin volume.
X = number of equivalents of the ions - number of equivalents of the ions of
interest passing through the column of interest applied to the column
c) Selectivity coefficient and affinity differences among different ionic species
The order affinity is
SO4
2-
> NO3
-
> Cl-
> HCO3
-
> OH-
> F-
for anions
Pb+
> Ca2+
> Mg2+
> Na+
> H+
for cations.
d) Characteristics of the ion exchange resins
Ion exchange resin type and form of resin (hydrogen/ sodium/ hydroxide/ chloride forms):
– Strong/weak anion/cation exchange resins; selective ion exchange resin; metal ion
selective chelating resins
– Regeneration chemical used determines the form of resin
– Hydrochloric or sulfuric acid forms hydrogen form of cation exchange resin
– Sodium chloride forms sodium form of cation exchange resin or chloride form of anion
exchange resin
– Sodium hydroxide or potassium hydroxide forms hydroxide form of anion exchange
resins

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Modes Of Operation:
i. Batch process:
Resin is stirred with water for process completion, and spent resin is removed, regenerated
and reused.
ii. Fixed Bed
A fixed bed through which the wastewater from the batch mode is allowed to flow
and the treated water is obtained.
iii. Fluidized Bed
The ion exchange resin will be present in a reactor and because of the wastewater flow
velocity it will be in a fluidized form so the waste components will be having good contact
with the ion exchange resin and ion exchange will be taking place.
iv. Continuous process:
water is passed through a packed resin column, and, when exhausted, the resin is
regenerated & reused. This is the most commonly used operational mode. In continuous
process if you want to operate an ion exchange resin we can classify the operation into four
categories. One is the service time, second one is the backwash time, then regeneration and
fourth one is rinse. Service is the time when the pollutants are getting removed from the
system and backwash means we are using this backwash to remove any turbid particles
whatever is getting attached to the fixed bed column, in regeneration we are removing the
exchanged ions with the initial ions. Usually some loose ions will be present in the column
and if you want to remove those loose ions we will be using rinse.

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Principles of Ion Exchange to Soften Water:
Calcium and magnesium ions are atoms having a positive electrical charge, as do sodium and
potassium ions. Ions of the same charge can be exchanged. In the ion exchange process, a granular
substance (usually a resin) that is coated with sodium or potassium ions comes into contact with
water containing calcium and magnesium ions.
Two positively charged sodium or potassium ions are exchanged (released into the water) for every
calcium or magnesium ion that is held by the resin. This “exchange or trade” happens because
sodium or potassium are loosely held by the resin. In this way, calcium and magnesium ions
responsible for hardness are removed from the water, held by the resin, and replaced by sodium or
potassium ions in the water.
This process makes water “soft.” Eventually, a point is reached when very few sodium or
potassium ions remain on the resin, thus no more calcium or magnesium ions can be removed from
the incoming water. The resin at this point is said to be “exhausted” or “spent,” and must be
“recharged” or “regenerated.
Ion Exchange or De-Ionization or De-Mineralization
Process:
Ion-exchange resins are insoluble, cross-linked, long chain organic polymers with a
microporous structure, and the “functional groups” attached to the chains are responsible for the
ion-exchanging properties.
Resins containing acidic functional groups (− C00H, − SO3H etc.) are capable of exchanging their
H+
ions with other cations, which comes in their contact; whereas those containing basic functional

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groups (− NH2 = NH as hydrochloride salt) are capable of exchanging their anions with other
anions, which comes in their contact. The ion-exchange resins may be classified as:
(i) Cation exchange resins (RH+) are mainly styrene-divinyl benzene copolymers, which on
sulphonation or carboxylation, become capable to exchange their hydrogen ions with the cations
in the water.
(ii) Anion exchange resins (R’OH–) are styrene-divinyl benzene or amine formaldehyde
copolymers, which contain amino or’ quaternary ammonium or quaternary phosphonium or
tertiary sulphonium groups as an integral part of the resin matrix. These, after treatment with dil.
NaOH solution, become capable to exchange their OH–
anions with anions in water.
Process of Ion Exchange to Soften Water:
Figure.2. Process of Ion Exchange to Soften Water.

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Figure.3. Ion Exchanger to Soften Water.
The hard water is passed first through cation exchange column, which removes all the cations like
Ca2+, Mg2+, etc. from it, and equivalent amount of H+
ions are released from this column to water.
Thus;
2RH+ + Ca2+ ——–.> R2Ca2+ + 2H+
2RH+ + Mg2+ ——.> R2Mg2+ + 2H+
After cation exchange column, the hard water is passed through anion exchange column, which
removes all the anions like SO4
2−, Cl−, etc. present in the water and equivalent amount of OH−
ions are released from this column to water. Thus:
R’OH– + Cl– —..——.> R’Cl– + OH–
2R’OH– + SO4
2- —..–> R’2SO4
2- + 2OH–
2R’OH– + CO3
2- —-.-> R’2CO3
2- + 2OH–
H+
and OH–
ions (released from cation exchange and anion exchange columns respectively) get
combined to produce water molecule.
H+ + OH- –> H2O

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Thus, the water coming out from the exchanger is free from cations as well as anions. Ion-free
water, is known as deionized or demineralised water.
Regeneration:
When capacities of cation and anion exchangers to exchange H+
and OH–
ions respectively are
lost, they are then said to be exhausted. The exhausted cation exchange column is regenerated by
passing a solution of dil. HCl or dil H2SO4. The regeneration can be represented as :
R2Ca2+ + 2H+ —-> 2RH+ + Ca2+ (washing)
The column is washed with deionised water and washing (which contains Ca2+, Mg2+, etc. and
Cl– or SO4
2- ions) is passed to sink or drain The exhausted anion exchange column is regenerated
by passing a solution of dil. NaOH. The regeneration can be represented as :
R’2S04
2- + 2OH– —> 2R’OH– + SO4
2-(washing)
The column is washed with deionised water and washing (which contains Na+ and SO4
2- or
Cl– ions) is passed to silk or drain. The regenerated ion exchange resins are then used again.
Ion Exchange Cycle
Although ion exchange has a wide range of applications, water softening with gel resins continues
to be a major one. Usually a fixed bed is used, which is operated in a cycle of 4 steps:
(1) loading,
(2) displacement,
(3) regeneration, and
(4) washing.
As the feed water solution flows down the bed, solute ions (mainly Ca2+
and Mg2+
) are removed
from water to the resin during the loading step, while an equivalent amount of Na is transferred
from the resin to the water.
A concentration profile (wave front) similar to that in adsorption can develop as the ion exchange
takes place in the bed. As the front moves down the through the bed, the resin behind or upstream
of the front is in equilibrium with the feed concentration. Ahead or downstream of the front, the
water is essentially free of the solute ion. Breakthrough occurs when the front reaches the end of
the bed.

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At the end of the loading step, the bed voids are filled with feed solution, which must be displaced
from the bed. This is best done with a regeneration solution, which is usually a concentrated salt
solution that flows upwards through the bed. Thus displacement and regeneration steps are
combined.
Following displacement, mass transfer of Ca2+
from the resin beds to the regenerating solution
takes place while an equivalent amount of Na+
is transferred from the solution to the resin.
During displacement and regeneration, 2 concentration wave fronts move through the bed. The
first is the displacement front, and the second, the regeneration front.
The cycle is completed by displacing, with water, the salt solution in the bed void.
Design
 Design of the process involves
– Resin selection and dimensioning of the resin bed and column
– Regeneration chemical selection and working out the regeneration process
– Hydraulic design of the run and regeneration processes
 Analysis of the feed water
– Run a complete cation-anion analysis of the water
(cations: Na+
, Ca2+
, Mg2+
, K+
; Anions: HCO3
-,
SO4
2-
, Cl-
. NO3
-
)
– Obtain information on TDS, dissolved CO2 and SiO2 and pH
 Estimation of the volume of the selected resin required
– Operating ion exchange capacity of the selected resin
– Quantity of feed water to be processed per cycle
– Ionic strength of the feed water to be processed
 Decide on the number of resin columns
– Reliability of operation and enabling the use of the resin bed to exhaustion point can
be the bases

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 Decide on the resin bed and column dimensions
– Assume feed water application rate (12-24 m3
/m2
.hr) and work out diameter and depth of the
resin bed
– Decide on the freeboard to be provided (typically ~ 50% of the resin bed depth)
– Make provisions for the under-drain system and for the backwash water collection and
drainage system
 Provide feed water (and regenerant) inlet, product water outlet
(backwash water and rinse water inlet) and backwash water
collection/energy dissipation structures
 Provide air vents, access manholes and sight glasses
 Estimate the regenerating chemical requirements
– Degree of attaining theoretical ion exchange capacity depends on the amount (and
concentration) of the regenerant employed
– Supplier of the resin, in this regard, may provide the performance curves.
 Find backwash velocity and estimate the regeneration wastewater
generation rates
– Calculations for backwash velocity are similar to those for a rapid gravity filter
– Water required for the backwashing (backwash velocity and backwash duration are
needed)
– Find out rinse water requirements (liters/unit volume of resin)
 Can be determined in laboratory or may also be available from the resin
manufacturers
– Find regeneration chemical solution volumes
 Regeneration wastewater management
– Segregation of the four streams of wastewater and management
– Internal recycling of the rapid rinse water and backwash water

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– Recycle the later stage regeneration chemical solution in the initial regeneration stage (in
the next regeneration cycle)
 Design the resin column and its inlets/outlets for chemical conservation
and minimization of regeneration wastewater
Problems associated with the ion exchange process
1. Need for extensive pre-treatment of the feed water
• High influent TSS can clog or plug the ion-exchange beds.
• Residual chlorine in water can be damaging to the resin.
• Residual organics can cause resin binding .
• pH and temperature of the feed water can influence the process.
2. Fouling of ion exchange resin and limited life of the resin
• Pre-filtration, dechlorination and use of scavenger exchange resin can be the pre-treatments.
• Regenerants used should be capable of removing the fouling (inorganic and organic) materials
from the resin during regeneration .
Ion exchange process applications
1. Removal of undesirable anions and cations from water/ wastewater
2. Softening (removal of hardness)
3. De-alkalisation (removal of bicarbonate)
4. Decationisation (removal of all cations)
5. Demineralisation (removal of all ions) - mixed bed for polishing (effective and versatile
means of conditioning boiler feed water)
6. Nitrate removal
7. Selective removal of various other contaminants
• Many of these require special resins, like chelating resins making stable metal complexes
• Removal of boron (boric acid), perchlorate and heavy metals (from wastewater), like,
Cd, Cr, Fe, Hg, Ni, Pb and Zn
• Some contaminants (As, F, Li) are difficult to remove with ion exchange, due to a poor
selectivity of the resins

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By adequate selection of ion exchangers most of the inorganic wastewater problems can be
handled.
Advantages Of Ion Exchange Process
1. It is a very effective and efficient method of water softening.
2. No perforation of substances into the soft water.
3. Most of the heavy metals can be reused.
4. The wastewater that is produced by ion exchange machines is also used for water treatment.
Disadvantages Of Ion Exchange Process
1. The level of acidity in the water can be increased because of entry of sodium ions into the
softened water. It may make the water not to be very safe for use.
2. The machines used to soften the water are known as Iron exchangers. Their greatest
impediment is the fact that they must be cleaned because of their high level of saturation.
3. The iron exchangers also require high operational costs.