16593 09 tủ tài liệu bách khoa

CHAPTER 9

ION EXCHANGE AND
INORGANIC ADSORPTION
Dennis A. Clifford, Ph.D., P.E., DEE
Professor and Chairman
Department of Civil and Environmental Engineering
University of Houston
Houston, Texas

INTRODUCTION AND THEORY
OF ION EXCHANGE
Contaminant cations such as calcium, magnesium, barium, strontium, and radium,
and anions such as fluoride, nitrate, fulvates, humates, arsenate, selenate, chromate,
and anionic complexes of uranium can be removed from water by using ion
exchange with resins or by adsorption onto hydrous metal oxides such as activated
alumina (AAl) granules or coagulated Fe(II), Fe(III), Al(III), and Mn(IV) surfaces.
This chapter deals only with the theory and practice of ion exchange with resins and
adsorption with activated alumina (AAl). The reader interested in cation and anion
adsorption onto hydrous metal oxides in general is referred to Schindler’s and
Stumm’s publications on the solid-water interface (Schindler, 1981; Stumm, 1992) as
a starting point.
Ion exchange with synthetic resins and adsorption onto activated alumina are
water treatment processes in which a presaturant ion on the solid phase, the adsorbent, is exchanged for an unwanted ion in the water. In order to accomplish the
exchange reaction, a packed bed of ion-exchange resin beads or alumina granules is
used. Source water is continually passed through the bed in a downflow or upflow
mode until the adsorbent is exhausted, as evidenced by the appearance (breakthrough) of the unwanted contaminant at an unacceptable concentration in the
effluent.
The most useful ion-exchange reactions are reversible. In the simplest cases, the
exhausted bed is regenerated using an excess of the presaturant ion. Ideally, no permanent structural change takes place during the exhaustion/regeneration cycle.
(Resins do swell and shrink, however, and alumina is partially dissolved during

9.1


9.2

CHAPTER NINE

regeneration.) When the reactions are reversible, the medium can be reused many
times before it must be replaced because of irreversible fouling or, in the case of alumina, excessive attrition. In a typical water supply application, from 300 to as many
as 300,000 bed volumes (BV) of contaminated water may be treated before exhaustion. Regeneration typically requires from 1 to 5 bed volumes of regenerant, followed by 2 to 20 bed volumes of rinse water. These wastewaters generally amount to
less than 2 percent of the product water; nevertheless, their ultimate disposal is a
major consideration in modern design practice. Disposal of the spent media may
also present a problem if it contains a toxic or radioactive substance such as arsenic
or radium.

Uses of Ion Exchange in Water Treatment
By far the largest application of ion exchange to drinking water treatment is in the
area of softening, that is, the removal of calcium, magnesium, and other polyvalent
cations in exchange for sodium. The ion-exchange softening process is applicable to
both individual home use and municipal treatment. It can be applied for wholehouse (point-of-entry or POE) softening or for softening only the water that enters
the hot water heater. Radium and barium are ions more preferred by the resin than
calcium and magnesium; thus the former are also effectively removed during ionexchange softening. Resins beds containing chloride-form anion exchange resins can
be used for nitrate, arsenate, chromate, selenate, dissolved organic carbon (DOC),
and uranium removal, and more applications of these processes will be seen in the
future. Activated alumina is being used to remove fluoride and arsenate from drinking water, particularly high total dissolved solids (TDS) waters, at point-of-use
(POU), (POE), and municipal scales.
The choice between ion exchange or alumina adsorption (to remove arsenic from
water, for example) is largely determined by (a) the background water quality—
including TDS level, competing ions, alkalinity, and contaminant concentration—
and (b) the resin or alumina affinity for the contaminant ion in comparison with the

competing ions. The affinity sequence determines the run length, chromatographic
peaking (if any), and process costs. As previously mentioned, process selection will
be affected by spent regenerant and spent medium disposal requirements, and
regenerant reuse possibilities, particularly if hazardous materials are involved. Each
of these requirements is dealt with in some detail in the upcoming design sections
for the specific processes summarized in Table 9.1.

Past and Future of Ion Exchange
Natural zeolites (i.e., crystalline aluminosilicates) were the first ion exchangers used
to soften water on a commercial scale. Later, zeolites were completely replaced by
synthetic resins because of the latters’ faster exchange rates, longer life, and higher
capacity. Aside from softening, the use of ion exchange for removal of specific contaminants from municipal water supplies has been limited. This is primarily because
of the expense involved in removing what is perceived as only a minimal health risk
resulting from contaminants such as fluoride, nitrate, or chromate.The production of
pure and ultrapure water by ion-exchange demineralization (IXDM) is the largest
use of ion exchange resins on a commercial scale. The complete removal of contaminants, which occurs in demineralization (DM) processes, is not necessary for drinking water treatment, however. Furthermore, costs associated with these treatments


ION EXCHANGE AND INORGANIC ADSORPTION

9.3

TABLE 9.1 Advantages and Disadvantages of Packed-Bed Inorganic Contaminant
Removal Processes
Ion exchange
Advantages
● Operates on demand.
● Relatively insensitive to flow variations, short contact time required.
● Relatively insensitive to trace-level contaminant concentration.
● Essentially zero level of effluent contaminant possible.

● Large variety of specific resins available.
● Beneficial selectivity reversal commonly occurs upon regeneration.
● In some applications, spent regenerant may be reused without contaminant removal.
Disadvantages
● Potential for chromatographic effluent peaking when using single beds.
● Variable effluent quality with respect to background ions when using single beds.
● Usually not feasible at high levels of sulfate or total dissolved solids.
● Large volume/mass of regenerant must be used and disposed of.
Activated alumina adsorption
Advantages
● Operates on demand.
● Relatively insensitive to total dissolved solids and sulfate levels.
● Low effluent contaminant level possible.
● Highly selective for fluoride and arsenic.
Disadvantages
● Both acid and base are required for regeneration.
● Relatively sensitive to trace-level contaminant concentration.
● Media tend to dissolve, producing fine particles.
● Slow adsorption kinetics and relatively long contact time required.
● Significant volume/mass of spent regenerant to neutralize and dispose of.

are high compared with those of the alternative membrane processes (i.e., reverse
osmosis and electrodialysis) for desalting water (see Chapter 11).
Adherence to governmentally mandated maximum contaminant levels (MCLs)
for inorganic contaminants (IOCs) will mean more use of ion exchange and alumina
for small-community water treatment operations to remove barium, arsenic, nitrate,
fluoride, uranium, and other IOCs. An AWWA survey (1985) indicates that 400 communities exceeded the 10 mg/L nitrate-N MCL, 400 exceeded the 4.0 mg/L fluoride
MCL (USEPA, 1985), and 200 exceeded the 2.0 mg/L secondary limit on barium.
Regarding radiological contaminants, an estimated 1,500 communities exceed the
proposed 20 µg/L MCL for uranium (USEPA, 1991), and many others may exceed

the MCL goal for radon (Rn) contamination when it is established. In most of these
cases, new contaminant-free sources cannot readily be developed, and a treatment
system will eventually be installed.

ION EXCHANGE MATERIALS AND REACTIONS
An ion exchange resin consists of a crosslinked polymer matrix to which charged
functional groups are attached by covalent bonding. The usual matrix is polystyrene


9.4

CHAPTER NINE

crosslinked for structural stability with 3 to 8 percent divinylbenzene. The common
functional groups fall into four categories: strongly acidic (e.g., sulfonate, ᎏSO3−);
weakly acidic (e.g., carboxylate, ᎏCOO−); strongly basic (e.g., quaternary amine,
ᎏN+(CH3)3); and weakly basic (e.g., tertiary amine—N(CH3)2).
A schematic presentation of the resin matrix, crosslinking, and functionality is
shown in Figure 9.1.The figure is a schematic three-dimensional bead (sphere) made
up of many polystyrene polymer chains held together by divinylbenzene crosslinking. The negatively charged ion exchange sites (ᎏSO3−) or (ᎏCOO−) are fixed
to the resin backbone or matrix, as it is called. Mobile positively charged counterions (positive charges in Figure 9.1) are associated by electrostatic attraction with
each negative ion exchange site. The resin exchange capacity is measured as the
number of fixed charge sites per unit volume or weight of resin. Functionality is the
term used to identify the chemical composition of the fixed-charge site, for example
sulfonate (ᎏSO3−) or carboxylate (ᎏCOO−). Porosity (e.g., microporous, gel, or
macroporous) is the resin characterization referring to the degree of openness of the
polymer structure. An actual resin bead is much tighter than implied by the
schematic, which is shown as fairly open for purposes of illustration only. The water

(a)

(b)

FIGURE 9.1 (a) Organic cation-exchanger bead comprising polystyrene polymer
cross-linked with divinylbenzene with fixed coions (minus charges) of negative
charge balanced by mobile positively charged counterions (plus charges). (b) Strongacid cation exchanger (left) in the hydrogen form and strong-base anion exchanger
(right) in the chloride form.


ION EXCHANGE AND INORGANIC ADSORPTION

9.5

(40 to 60 percent by weight) present in a typical resin bead is not shown. This resinbound water is an extremely important characteristic of ion exchangers because it
strongly influences both the exchange kinetics and thermodynamics.
Strong- and Weak-Acid Cation Exchangers
Strong acid cation (SAC) exchangers operate over a very wide pH range because the
sulfonate group, being strongly acidic, is ionized throughout the pH range (1 to 14).
Three typical SAC exchange reactions follow. In Equation 9.1, the neutral salt CaCl2,
representing noncarbonate hardness, is said to be split by the resin, and hydrogen
ions are exchanged for calcium, even though the equilibrium liquid phase is acidic
because of HCl production. Equations 9.2 and 9.3 are the standard ion exchange
softening reactions in which sodium ions are exchanged for the hardness ions Ca2+,
Mg2+, Fe2+, Ba2+, Sr2+, and/or Mn2+, either as noncarbonate hardness (Equation 9.2) or
carbonate hardness (Equation 9.3). In all these reactions, R denotes the resin matrix,
and the overbar indicates the solid (resin) phase.
2R
ෆS
ෆO
ෆ−ෆ3ෆH

ෆ+ෆ + CaCl2 ⇔ ෆ(R
ෆS
ෆO
ෆ3ෆ−ෆ)ෆ2ෆC
ෆaෆ2ෆ+ෆ + 2HCl

(9.1)

2ෆ
RS
ෆO
ෆ3ෆ−ෆN
ෆaෆ+ෆ + CaCl2 ⇔ ෆ(R
ෆS
ෆO
ෆ3ෆ−ෆ)ෆ2ෆC
ෆaෆ2ෆ+ෆ + 2NaCl

(9.2)

2R
ෆS
ෆO
ෆෆෆN
ෆaෆෆ + Ca(HCO3)2 ⇔ ෆ(R
ෆS
ෆO
ෆෆෆ)ෆෆC
ෆaෆෆෆ + 2NaHCO3
−

3

+

−
3 2

2+

(9.3)

Regeneration of the spent resin is accomplished using an excess of concentrated (0.5
to 3.0 M) HCl or NaCl, and amounts to the reversal of Equations 9.1 through 9.3.
Weak acid cation (WAC) resins can exchange ions only in the neutral to alkaline
pH range because the functional group, typically carboxylate (pKa = 4.8), is not ionized at low pH.Thus,WAC resins can be used for carbonate hardness removal (Equation 9.4) but fail to remove noncarbonate hardness, as is evident in Equation 9.5.
2ෆ
RC
ෆO
ෆO
ෆH
ෆ + Ca(HCO3)2 ⇒ (ෆR
ෆC
ෆO
ෆO
ෆ−ෆ)ෆ2ෆC
ෆaෆ2ෆ+ෆ + H2CO3
2R
ෆC
ෆO
ෆO

ෆH
ෆ + CaCl2 ⇐ (ෆR
ෆC
ෆO
ෆO
ෆ−ෆ)ෆ2ෆC
ෆaෆ2ෆ+ෆ + 2HCl

(9.4)
(9.5)

If Equation 9.5 were to continue to the right, the HCl produced would be so completely ionized that it would protonate (i.e., add a hydrogen ion to the resin’s weakly
acidic carboxylate functional group, and prevent exchange of H+ ions for Ca2+ ions).
Another way of expressing the fact that Equation 9.5 does not proceed to the right
is to say that WAC resins will not split neutral salts (i.e., they cannot remove noncarbonate hardness). This is not the case in Equation 9.4, in which the basic salt,
Ca(HCO3)2, is split because a very weak acid, H2CO3 (pK1 = 6.3), is produced.
In summary, SAC resins split basic and neutral salts (remove carbonate and noncarbonate hardness), whereas WAC resins split only basic salts (remove only carbonate hardness). Nevertheless, WAC resins have some distinct advantages for
softening, namely TDS reduction, no increase in sodium, and very efficient regeneration resulting from the carboxylate’s high affinity for the regenerant H+ ion.
Strong- and Weak-Base Anion Exchangers
The use of strong-base anion (SBA) exchange resins for nitrate removal is a fairly
recent application of ion exchange for drinking water treatment (Clifford and W. J.
Weber, 1978; Guter, 1981), although they have been used in water demineralization


9.6

CHAPTER NINE

for decades. In anion exchange reactions with SBA resins, the quaternary amine
functional group (ᎏN+[CH3]3) is so strongly basic that it is ionized, and therefore

useful as an ion exchanger over the pH range of 0 to 13. This is shown in Equations
9.6 and 9.7, in which nitrate is removed from water by using hydroxide or chlorideform SBA resins. (Note that R4N+ is another way to write the quaternary exchange
site, ᎏN+(CH3)3)
R
R4ෆN
ෆෆ4N
ෆ+ෆO
ෆH
ෆ−ෆ + NaNO3 ⇔ ෆ
ෆ+ෆN
ෆO
ෆ3ෆ−ෆ + NaOH

(9.6)

R
R4ෆN
ෆ4ෆN
ෆ+ෆC
ෆlෆ−ෆ + NaNO3 ⇔ ෆ
ෆ+ෆN
ෆO
ෆ3ෆ−ෆ + NaCl

(9.7)

In Equation 9.6 the caustic (NaOH) produced is completely ionized, but the quaternary ammonium functional group has such a small affinity for OH− ions that the
reaction proceeds to the right. Equation 9.7 is a simple ion exchange reaction without a pH change. Fortunately, all SBA resins have a much higher affinity for nitrate
than chloride (Clifford and W. J. Weber, 1978), and Equation 9.7 proceeds to the
right at near-neutral pH values.

Weak-base anion (WBA) exchange resins are useful only in the acidic pH region
where the primary, secondary, or tertiary amine functional groups (Lewis bases) are
protonated and thus can act as positively charged exchange sites for anions. In Equation 9.8 chloride is, in effect, being adsorbed by the WBA resin as hydrochloric acid,
and the TDS level of the solution is being reduced. In this case, a positively charged
Lewis acid-base adduct (R3NH+) is formed, which can act as an anion exchange site.
As long as the solution in contact with the resin remains acidic (just how acidic
depends on basicity of the R3N:, sometimes pH ≤ 6 is adequate), ion exchange can
take place as is indicated in Equation 9.9—the exchange of chloride for nitrate by a
WBA resin in acidic solution. If the solution is neutral or basic, no adsorption or
exchange can take place, as indicated by Equation 9.10. In all these reactions, R represents either the resin matrix or a functional group such as ᎏCH3 or ᎏC2H5, and
overbars represent the resin phase.
R
ෆෆ3N
ෆ:ෆ + HCl ⇔ R
ෆ3ෆN
ෆH
ෆ+ෆC
ෆlෆ−ෆ

(9.8)

R3ෆN
R3ෆN
ෆ
ෆH
ෆ+ෆC
ෆlෆ−ෆ + HNO3 ⇔ ෆ
ෆH
ෆ+ෆN
ෆO

ෆ3ෆ−ෆ + HCl

(9.9)

R
ෆෆ3N
ෆ:ෆ + NaNO3 ⇒ no reaction

(9.10)

Although no common uses of WBA resins are known for drinking water treatment,
useful ones are possible (Clifford and W. J. Weber, 1978). Furthermore, when activated alumina is used for fluoride and arsenic removal, it acts as if it were a weakbase anion exchanger, and the same general rules regarding pH behavior can be
applied. Another advantage of weak-base resins in water supply applications is the
ease with which they can be regenerated with bases. Even weak bases such as lime
(Ca[OH]2) can be used, and regardless of the base used, only a small stoichiometric
excess (less than 20 percent) is normally required for complete regeneration.

Activated Alumina Adsorption
Packed beds of activated alumina can be used to remove fluoride, arsenic, selenium,
silica, and humic materials from water. Coagulated Fe(II) and Fe(III) oxides
(McNeill and Edwards, 1995; Scott, Green et al., 1995) and iron oxides coated onto
sands (Benjamin, Sletten et al., 1996) can also be employed to remove these anions,


ION EXCHANGE AND INORGANIC ADSORPTION

9.7

but these processes are not covered in this chapter. The mechanism, which is one of
exchange of contaminant anions for surface hydroxides on the alumina, is generally

called adsorption, although ligand exchange is a more appropriate term for the
highly specific surface reactions involved (Stumm, 1992).
The typical activated aluminas used in water treatment are 28- × 48-mesh (0.3- to
0.6-mm-diameter) mixtures of amorphous and gamma aluminum oxide (γ-Al2O3)
prepared by low-temperature (300 to 600°C) dehydration of precipitated Al(OH)3.
These highly porous materials have surface areas of 50 to 300 m2/g. Using the model
of an hydroxylated alumina surface subject to protonation and deprotonation, the
following ligand exchange reaction (Equation 9.11) can be written for fluoride
adsorption in acid solution (alumina exhaustion) in which ϵAl represents the alumina surface and an overbar denotes the solid phase.
ϵෆ
Aෆlෆ
−ෆ
Oෆ
H + H+ + F− ⇒ ϵ
ෆ
ෆA
ෆlෆ−
ෆF
ෆ−ෆ + HOH

(9.11)

The equation for fluoride desorption by hydroxide (alumina regeneration) is presented in Equation 9.12.
ϵ
ෆA
ෆlෆ−
ෆF
ෆ + OH− ⇒ ϵ
ෆA
ෆlෆ−

ෆO
ෆH
ෆ + F−

(9.12)

Another common application for alumina is arsenic removal, and reactions similar
to Equations 9.11 and 9.12 apply for exhaustion and regeneration when H2AsO4− is
substituted for F−.
Activated alumina processes are sensitive to pH, and anions are best adsorbed
below pH 8.2, a typical zero point of charge (ZPC), below which the alumina surface
has a net positive charge, and excess protons are available to fuel Equation 9.11.
Above the ZPC, alumina is predominantly a cation exchanger, but its use for cation
exchange is relatively rare in water treatment. An exception is encountered in the
removal of radium by plain and treated activated alumina (Clifford, Vijjeswarapu et
al., 1988; Garg and Clifford, 1992).
Ligand exchange as indicated in Equations 9.11 and 9.12 occurs chemically at the
internal and external surfaces of activated alumina. A more useful model for process
design, however, is one that assumes that the adsorption of fluoride or arsenic onto
alumina at the optimum pH of 5.5 to 6.0 is analogous to weak-base anion exchange.
For example, the uptake of F− or H2AsO4−, requires the protonation of the alumina
surface, and that is accomplished by preacidification with HCl or H2SO4, and reducing the feed water pH into the 5.5 to 6.0 region. The positive charge caused by excess
surface protons may then be viewed as being balanced by exchanging anions (i.e.,
ligands such as hydroxide, fluoride, and arsenate). To reverse the adsorption process
and remove the adsorbed fluoride or arsenate, an excess of strong base (e.g., NaOH)
must be applied.The following series of reactions (9.13–9.17) is presented as a model
of the adsorption/regeneration cycle that is useful for design purposes.
The first step in the cycle is acidification, in which neutral (water-washed) alumina (Alumina⋅HOH) is treated with acid (e.g., HCl), and protonated (acidic) alumina is formed as follows:
A
ෆlෆu

ෆm
ෆiෆn
ෆaෆ⋅ෆH
ෆO
ෆH
ෆ + HCl ⇒ A
ෆlෆu
ෆm
ෆiෆn
ෆaෆ⋅ෆH
ෆC
ෆlෆ + HOH

(9.13)

When HCl-acidified alumina is contacted with fluoride ions, they strongly displace
the chloride ions providing that the alumina surface remains acidic (pH 5.5 to 6.0).
This displacement of chloride by fluoride, analogous to ion exchange, is shown as
A
ෆlෆu
ෆm
ෆiෆn
ෆaෆ⋅ෆH
ෆC
ෆlෆ + HF ⇒ A
ෆlෆu
ෆm
ෆiෆn
ෆaෆ⋅ෆH
ෆF

ෆ + HCl

(9.14)


9.8

CHAPTER NINE

To regenerate the fluoride-contaminated adsorbent, a dilute solution of 0.25 to 0.5 N
NaOH alkali is used. Because alumina is both a cation and an anion exchanger, Na+
is exchanged for H+, which immediately combines with OH− to form HOH in the
alkaline regenerant solution. The regeneration reaction of fluoride-spent alumina is
A
ෆෆlu
ෆm
ෆiෆn
ෆaෆ⋅ෆH
ෆF
ෆ + 2NaOH ⇒ A
ෆlෆu
ෆm
ෆiෆn
ෆaෆ⋅ෆN
ෆaෆO
ෆH
ෆ + NaF + HOH

(9.15)

Recent experiments have suggested that Equation 9.15 can be carried out using
fresh or recycled NaOH from a previous regeneration. This suggestion is based on
the field studies of Clifford and Ghurye (1998) in which arsenic-spent alumina was
regenerated with equally good results using fresh or once-used 1.0 M NaOH. The
spent regenerant, fortified with NaOH to maintain its hydroxide concentration at
1.0 M, probably could have been used many times, but the optimum number of
spent-regenerant reuse cycles was not determined in the field study.
To restore the fluoride removal capacity, the basic alumina is acidified by contacting it with an excess of dilute acid, typically 0.5 N HCl or H2SO4:
A
ෆlෆu
ෆm
ෆiෆn
ෆaෆ⋅ෆN
ෆaෆO
ෆH
ෆ + 2HCl ⇒ A
ෆlෆu
ෆm
ෆiෆn
ෆaෆ⋅ෆH
ෆC
ෆlෆ + NaCl + HOH

(9.16)

The acidic alumina, alumina⋅HCl, is now ready for another fluoride (or arsenate or
selenite) ligand-exchange cycle as summarized by Equation 9.14. Alternatively, the
feed water may be acidified prior to contact with the basic alumina, thereby combining acidification and adsorption into one step as summarized by Equation 9.17:
A
ෆlෆu

ෆm
ෆiෆn
ෆaෆ⋅ෆN
ෆaෆO
ෆH
ෆ + NaF + 2HCl ⇒ A
ෆlෆu
ෆm
ෆiෆn
ෆaෆ⋅ෆH
ෆF
ෆ + 2NaCl + HOH

(9.17)

The modeling of the alumina adsorption-regeneration cycle as being analogous to
weak-base anion exchange fails in regard to regeneration efficiency, which is excellent for weak-base resins but quite poor on alumina. This is caused by the need for
excess acid and base to partially overcome the poor kinetics of the semicrystalline
alumina, which exhibits very low solid-phase diffusion coefficients compared with
resins that are well-hydrated, flexible gels offering little resistance to the movement
of hydrated ions. A further reason for poor regeneration efficiency on alumina is
that alumina is amphoteric and reacts with (consumes) excess acid and base to produce soluble forms (Al(H2O)6 3+, Al(H2O)2(OH)−4) of aluminum. Resins are totally
inert in this regard (i.e., they are not dissolved by regenerants).
Special-Purpose Resins
Resins are practically without limit in their variety because polymer matrices, functional groups, and capacity and porosity are controllable during manufacture. Thus,
numerous special-purpose resins have been made for water-treatment applications.
For example, bacterial growth can be a major problem with anion resins in some
water supply applications because the positively charged resins tend to “adsorb” the
negatively-charged bacteria that metabolize the adsorbed organic material—negatively charged humate and fulvate anions.To correct this problem special resins have
been invented, which contain bacteriostatic long-chain quaternary amine functional

groups (“quats”) on the resin surface. These immobilized quats kill bacteria on contact with the resin surface (Janauer, Gerba et al., 1981).
The strong attraction of polyvalent humate and fulvate anions (natural organic
matter, [NOM]) for anion resins has been used as the basis for removal of these total
organic carbon (TOC) compounds from water by using special highly porous resins.
Both weak- and strong-base macroporous anion exchangers have been manufactured


ION EXCHANGE AND INORGANIC ADSORPTION

9.9

to remove these large anions from water. The extremely porous resins originally
thought to be necessary for adsorption of the large organic anions tended to be structurally weak and break down easily. More recently, however, it has been shown that
both gel and standard macroporous resins, which are highly crosslinked and physically
very strong, can be used to remove NOM (Fu and Symons, 1990). Regeneration of
resins used to remove NOM is often a problem because of the strong attraction of the
aromatic portion of the anions for the aromatic resin matrix. This problem has at least
been partially solved using acrylic-matrix SBA resins. More details on the use of ion
exchange resins to remove NOM appears later in this chapter.
Resins with chelating functional groups such as imino-diacetate (Calmon, 1979),
amino-phosphonate, and ethyleneamine (Matejka and Zirkova, 1997) have been
manufactured that have extremely high affinities for hardness ions and troublesome
metals such as Cu2+, Zn2+, Cr3+, Pb2+, and Ni2+. These resins are used in special applications such as trace-metal removal and metals-recovery operations (Brooks,
Brooks et al., 1991). The simplified structures of these resins are shown in Figure 9.2.
Table 9.2 summarizes the features of some of the special ion exchangers available
commercially from a variety of sources (Purolite, 1995).

FIGURE 9.2 Structure of highly selective cation exchangers
for metals removal.

ION EXCHANGE EQUILIBRIUM
Selectivity Coefficients and Separation Factors
Ion exchange resins do not prefer all ions equally. This variability in preference is
often expressed semiquantitatively as a position in a selectivity sequence or, quantitatively, as a separation factor, αij, or a selectivity coefficient, Kij, for binary exchange. The selectivity, in turn, determines the run length to breakthrough for the
contaminant ion; the higher the selectivity, the longer the run length. Consider, for
example, Equation 9.18, the simple exchange of Cl− for NO3− on an anion exchanger


9.10

CHAPTER NINE

TABLE 9.2 Special Ion Exchangers—Commercially Available
Type of resin

Functional group

Typical application

Chelating

Thio-uronium

Selective removal of metals,
especially mercury.

Chelating

Imino-diacetic

Selective removal of polyvalent
ions, especially transition metals.

Chelating

Amino-phosphonic

Decalcification of brine and
removal of metals from
wastewaters

Silver impregnated, SAC

Sulfonic

Softening resin with bacteriostatic
properties

NSS, Nitrate-over-sulfate
selective (sulfate rejecting),
hydrophobic, SBA

Triethyl and tripropyl
quaternary amines

Nitrate removal in high sulfate
waters

Iodine releasing

Quaternary amine
SBA in triiodide
form, ᎏR4N+I3−

Disinfection by iodine release
into product water

Source: Purolite, 1995.

whose equilibrium constant is expressed numerically in Equation 9.19 and graphically in Figure 9.3a:
Cl− + NO3− ⇒ N
ෆ
ෆO
ෆ3− + Cl−
−
3

(9.18)

−

{N
ෆO
ෆ } {Cl }
K = ᎏᎏ
{C
ෆl−} {NO3−}

(9.19)

In Equations 9.18 to 9.20, overbars denote the resin phase, and the matrix designation R has been removed for simplicity; K is the thermodynamic equilibrium constant, and braces denote ionic activity. Concentrations are used in practice because
they are measured more easily than activities. In this case, Equation 9.20 based on
concentration, the selectivity coefficient KN/Cl describes the exchange. Note that KN/Cl
includes activity coefficient terms that are functions of ionic strength and, thus, is not
a true constant (i.e., it varies somewhat with different ionic strengths).
[N
ෆO
ෆ3−] [Cl−] qN CCl
=ᎏ
KN/Cl = ᎏᎏ
[C
ෆl−] [NO3−] qCl CN

(9.20)

where [ ] = concentration, mol/L
qN = resin phase equivalent concentration (normality) of nitrate, eq/L
CN = aqueous phase equivalent concentration (normality), eq/L
The binary separation factor αN/Cl, used throughout the literature on separation
practice, is a most useful description of the exchange equilibria because of its simplicity and intuitive nature:
distribution of ion i between phases yi / xi
αij = ᎏᎏᎏᎏ = ᎏ
distribution of ion j between phases yj / xj
(yN/xN) yN xCl (qN/q) (CCl/C)
αN/Cl = ᎏ = ᎏ = ᎏᎏ
(yCl/xCl) xN yCl (CN/C) (qCl/q)

(9.21)
(9.22)

ION EXCHANGE AND INORGANIC ADSORPTION

where

9.11

yi = equivalent fraction of ion i in resin, qi/q
yN = equivalent fraction of nitrate in resin, qN/q
xi = equivalent fraction of ion i in water, CN/C
xN = equivalent fraction of nitrate in water, CN/C
qN = concentration of nitrate on resin, eq/L
q = total exchange capacity of resin, eq/L
CN = nitrate concentration in water, eq/L
C = total ionic concentration of water, eq/L

Equations 9.20 and 9.22 show that for homovalent exchange (i.e., monovalent/
monovalent and divalent/divalent exchange), the separation factor αij and the selectivity coefficient Kij are equal. This is expressed for nitrate/chloride exchange as
qN CCl
KN/Cl = αN/Cl = ᎏ
CN qCl

(9.23)

For exchanging ions of unequal valence (i.e., heterovalent exchange), the separation
factor is not equivalent to the selectivity coefficient. Consider, for example, the case
of sodium ion-exchange softening as represented by Equation 9.24, the simplified
form of Equation 9.2:
2N
ෆaෆ+ + Ca2+ = C

ෆaෆ2+ + 2 Na+
qCa CNa2
KCa/Na = ᎏ2
CCa qNa

(9.24)
(9.25)

Using a combination of Equations 9.21 and 9.25,
αdivalent/monovalent

or

q yNa
αCa/Na = KCa/Na ᎏ
C xNa

(9.26)

The implication from these equations is that the intuitive separation factor for
divalent/monovalent exchange depends inversely on solution concentration C and
directly on the distribution ratio yNa/xNa between the resin and the water, with q constant. The higher the solution concentration C, the lower the divalent/monovalent
separation factor [i.e., selectivity tends to reverse in favor of the monovalent ion as
ionic strength—I (which is a function of C)—increases]. This reversal of selectivity is
discussed in detail in the following paragraphs

Selectivity Sequences
A selectivity sequence describes the order in which ions are preferred by a particular
resin or by a solid porous oxide surface such as AlOOH (activated alumina granules or hydrated aluminum oxide precipitate), FeOOH (hydrous iron oxide), or
MnOOH (hydrous manganese oxide). Although special-purpose resins (such as

chelating resins) can have unique selectivity sequences, the commercially available
cation and anion resins exhibit similar selectivity sequences. These are presented in
Table 9.3, where the most-preferred ions (i.e., those with the highest separation factors) are listed at the top of the table and the least-preferred ions are at the bottom.
For example, the αCa/Na value of 1.9 means that at equal concentration in the aqueous
phase, calcium is preferred by the resin 1.9/1.0 over sodium (on the basis of equivalents, not moles). Weak acid cation resins with carboxylic functional groups exhibit
the same selectivity sequence as SAC resins except that hydrogen is the most pre-


9.12

CHAPTER NINE

TABLE 9.3 Relative Affinities of Ions for Resins*
Strong acid cation resins†

Strong base anion resins‡

Cation, i

αi/Na+

Anion, i

Ra2+
Ba2+
Pb2+
Sr2+
Cu2+
Ca2+
Zn2+

Fe2+
Mg2+
K+
Mn2+
NH4+
Na+
H+

13.0
5.8
5.0
4.8
2.6
1.9
1.8
1.7
1.67
1.67
1.6
1.3
1.0
0.67

UO2(CO3)34−
ClO4−¶
CrO42−
SeO42−
SO42−
HAsO42−
HSO4−

NO3−
Br−
SeO32−
HSO3−
NO2−
Cl−
BrO3−
HCO3−
CH3COO−
F−

αi/Cl−§
3200
150
100
17
9.1
4.5
4.1
3.2
2.3
1.3
1.2
1.1
1.0
0.9
0.27
0.14
0.07

* Above values are approximate separation factors for
0.005–0.010 N solution (TDS = 250–500 mg/L as CaCO3).
†
SAC resin is polystyrene divinylbenzene matrix with sulfonate functional groups.
‡
SBA resin is polystyrene divinylbenzene matrix with
−N+(CH3)3 functional groups (i.e., a Type 1 resin).
§
Separation factors are approximate and are based on various literature sources and on experiments performed at the
University of Houston.
¶
ClO4−/Cl− separation factor is for polystyrene SBA resins;
on polyacrylic SBA resins, the ClO4−/Cl− separation factor is
approximately 5.0.

ferred cation, and the magnitude of the separation factors differ from those in Table
9.3. Similarly, WBA resins and SBA resins exhibit the same selectivity sequence,
except that hydroxide is most preferred by WBA resins, and the WBA separation
factors differ in magnitude but have the same trend as those in Table 9.3.
Some general rules govern selectivity sequences. In dilute solution (e.g., in the
TDS range of natural waters) the resin prefers the ion with the highest charge and
lowest degree of hydration.
Selectivity is affected by the nature of the ion. Hydrophobic ions (e.g., nitrate and
chromate) prefer hydrophobic resins (i.e., highly crosslinked macroporous resins
without polar matrices and/or functional groups), whereas hydrophilic ions (e.g.,
bicarbonate and acetate) prefer moderately crosslinked (gel) resins with polar
matrices and/or functional groups. Divalent ions, (e.g., sulfate and calcium) prefer
resins with closely spaced exchange sites, where their need for two charges can be
satisfied (Clifford and W. J. Weber, 1983; Sengupta and Clifford, 1986; Subramonian
and Clifford, 1988; Horng and Clifford, 1997).

Activated alumina operated in the acidic to neutral pH range for anion adsorption has a selectivity sequence that differs markedly from anion exchange resins.
Fortunately, some of the ions such as fluoride, which is least preferred by resins (and
therefore not amenable to removal by resins) are highly preferred by the alumina.
Based on research by the author, his coworkers, and other investigators (Trussell and


ION EXCHANGE AND INORGANIC ADSORPTION

9.13

Trussell et al., 1980; Singh and Clifford, 1981; Rosenblum and Clifford, 1984; Schmitt
and Pietrzyk, 1985), activated alumina operated in the pH range of 5.5 to 8.5 prefers
anions in the following order:
OH− > H2AsO4− , Si(OH)3O− > F− > HSeO3− > SO24 − > CrO24 −
>> HCO3− > Cl− > NO3− > Br− > I−

(9.27)

Humic- and fulvic-acid anions are more preferred than sulfate, but because of their
widely differing molecular weights and structures, and the different pore-size distributions of commercial aluminas, no exact position in the selectivity sequence can be
assigned. Reliable separation factors for ions in the above selectivity sequence (such
as fluoride, arsenate, silicate, and biselenite) are not available in the literature, but this
is not particularly detrimental to the design effort because alumina has an extreme
preference for these ions. For example, when fluoride or arsenate is removed from
water, the presence of the usual competing ions—bicarbonate and chloride—is
nearly irrelevant in establishing run length to contaminant ion breakthrough (Singh
and Clifford, 1981; Rosenblum and Clifford, 1984). Sulfate does, however, offer some
small but measurable competition for adsorption sites. The problem with the
extremely preferred ions is that they are difficult to remove from the alumina during
regeneration, which necessitates the use of hazardous, chemically strong (e.g., NaOH

and H2SO4), and potentially destructive (of the medium) regenerants.

Isotherm Plots
The values of αij and Kij can be determined from a constant-temperature, equilibrium plot of resin-phase concentration versus aqueous-phase concentration (i.e., the
ion-exchange isotherm). Favorable and unfavorable isotherms are depicted in Figure 9.3a and b, where each curve depicts a constant separation factor, αNO3/Cl for Figure 9.3a and αHCO3/Cl for Figure 9.3b.
A “favorable” isotherm (convex to x-axis) means that species i (NO3− in Figure
9.3a), which is plotted on each axis, is preferred to species j (Cl− in Figure 9.3a), the
hidden or exchanging species. An “unfavorable” isotherm (concave to the x-axis)
indicates that species i (HCO3− in Figure 9.3b) is less preferred than j (Cl− in Figure

(a)

(b)

FIGURE 9.3 (a) Favorable isotherm for nitrate-chloride exchange according to reaction (9.18)
with constant separation factor αNO3/Cl > 1.0. (b) Unfavorable isotherm for bicarbonate-chloride
exchange with constant separation factor αHCO3 /Cl < 1.0.


9.14

CHAPTER NINE

9.3b). During column exhaustion processes, favorable isotherms result in sharp
breakthroughs when i is in the feed and j is on the resin, whereas unfavorable
isotherms lead to gradual breakthroughs under these conditions. (This is discussed
in detail later, under the heading “Column Processes and Calculations” where Figure 9.8 is explained.) In viewing these binary isotherms, note that
xi + xj = 1.0
Ci + Cj = C

(9.28)
(9.29)

yi + yj = 1.0

(9.30)

qi + qj = q

(9.31)

Therefore, the concentration or equivalent fraction of either ion can be directly
obtained from the plot, which in Figure 9.3a and b is a “unit” isotherm because equivalent fractions (xi , yj ) rather than concentrations have been plotted in the range 0.0 to
1.0. Figure 9.3a represents the favorable isotherm for nitrate-chloride exchange, and
Figure 9.3b the unfavorable isotherm for bicarbonate-chloride exchange.
For nonconstant separation factors (e.g., the divalent/monovalent [Ca2+/Na+]
exchange case described by Equations 9.24 and 9.26, a separate isotherm exists for
every total solution concentration C. As the solution concentration or TDS level
decreases, the resin exhibits a greater preference for the divalent ion, as evidenced by
a progressively higher and more convex isotherm.The phenomenon can be explained
by solution theory: As the solution concentration increases, the aqueous phase
becomes more ordered.This results in polyvalent ion activity coefficients that are significantly less than 1.0 (i.e., the tendency for polyvalent ions to escape from the water
into the resin is greatly diminished, leading to a reduction in the height and convexity
of the isotherm). This phenomenon of diminishing preference for higher-valent ions
with increasing ionic strength I of the solution has been labeled electroselectivity and
can eventually lead to selectivity reversal, whereupon the isotherm becomes concave
(Helfferich, 1962). This trend is shown in Figure 9.4, where the sulfate-chloride
isotherm is favorable in 0.06 N solution and unfavorable in 0.6 N solution.
The exact ionic strength at which electroselectivity reversal occurs is dependent
on the ionic makeup of the solution, and highly dependent on the resin structure

(Boari, Liberti et al., 1974) and its inherent
affinity for polyvalent ions. Electroselectivity
reversal is very beneficial to the sodium ion
exchange softening process in that it causes
the divalent hardness ions to be highly preferred in dilute solution (I ≤ 0.020 M) during
resin exhaustion and highly nonpreferred
(i.e., easily rejected) during regeneration with
relatively concentrated (0.25 to 2.0 M) salt
solution.

FIGURE 9.4 Electroselectivity of a typical type 1 strong-base anion-exchange
resin used for divalent-monovalent (SO2−
4 /
Cl−) anion exchange.

EXAMPLE 9.1 The following solved example
problem briefly describes the experimental
technique necessary to obtain isotherm data
and illustrates the calculations required to
construct a nitrate-chloride isotherm for a
strong-base anion exchange resin. By using
the isotherm data or the plot, the individual
and average separation factors αij can be calculated. Only minor changes are necessary to


9.15

ION EXCHANGE AND INORGANIC ADSORPTION

apply the technique to weak-base resins or to cation resins. For example, acids (HCl

and HNO3) rather than sodium salts would be used for equilibration of weak-base
resins.
To obtain the data for this example, weighted amounts of air-dried chloride-form
resin of known exchange capacity were placed in capped bottles containing 100 mL
of 0.005 N (5.0 meq/L) NaNO3 and equilibrated by tumbling for 16 hours. Following
equilibration, the resins were settled, and the nitrate and chloride concentrations of
the supernatant water were determined for each bottle. The nitrate/chloride equilibrium data are in Table 9.4. The total capacity q of the resin is 3.63 meq/g. Note that
the units of resin capacity used here are meq/g rather than eq/L, because for precise
laboratory work a mass rather than volume of resin must be used.
SOLUTION

1. Verify that, within the expected limits of experimental error, the total concentration C of the aqueous phase at equilibrium is 0.005 N. Large deviations from this
value usually indicate that concentrated salts were absorbed in the resin and
leached out during the equilibration procedure. This problem can be avoided by
extensive prewashing of the resin with the same normality of salt, in this case
0.005 N NaCl, as is used for equilibration.
2. Calculate the equivalent fractions, xN and xCl, of nitrate and chloride in the water
at equilibrium.
3. Using the known total capacity of the resin, qCl, calculate the milliequivalents
(meq) of chloride remaining on the resin at equilibrium by subtracting the meq of
chloride found in the water.
4. Calculate the meq of nitrate on the resin, qN, by assuming that all the nitrate
removed from solution is taken up by the resin.
5. Calculate the equivalent fractions, yN and yCl, of nitrate and chloride in the resin
phase at equilibrium.
6. Calculate the separation factor, αij, which is equal to the selectivity coefficient,
Kij, for homovalent exchange.
7. Repeat steps (1) through (6) for all equilibrium data points, and plot the
isotherm.
Solution (with the Equilibrium Data Point for 0.2 g Resin as an Example):

1. C = CN + CCl = 1.17 + 3.78 = 4.95 meq/L (This is well within the expected ±5 percent limits of experimental error; (5.00 − 4.95)/5.00 = 1.0 percent error)
TABLE 9.4 Example Data for Plot of Nitrate/Chloride Isotherm
g resin
ᎏ
100 mL

CN meq/L

CCl meq/L

C meq/L

xN

xCl

yN

yCl

αij

0.020
0.040
0.100
0.200
0.400
1.20

4.24

3.56
2.18
1.17
0.53
0.185

0.722
1.32
2.77
3.78
4.36
4.49

4.96
4.88
4.98
4.95
4.89
4.68

0.854
0.730
0.440
0.236
0.108
0.040

0.146
0.270
0.560

0.764
0.892
0.960

0.980
0.920
0.760
0.523
0.300
0.110

0.020
0.091
0.240
0.477
0.700
0.890

8.6
4.25
4.12
3.55
3.59
2.99

The first three columns represent experimental data. The remaining italicized columns were obtained by
calculation as described in the example.


9.16

2.

CHAPTER NINE

CN 1.17 meq/L
xN = ᎏ = ᎏᎏ = 0.236
C
4.95 meq/L
CCl 3.78 meq/L
xCl = ᎏ = ᎏᎏ = 0.764
C
4.95 meq/L

Checking: xN + xCl = 0.236 + 0.764 = 1.00
3. Calculate chloride remaining on the resin at equilibrium, qCl:
qCl = qCl, initial − chloride lost to water per gram of resin
qCl, initial = q = 3.63 meq/g

΂

΃

0.100 L
qCl = 3.63 meq/g − 3.78 meq/L ᎏ = 1.74 meq/g
0.200 g
4. Calculate nitrate on resin at equilibrium
qN = qN, initial + nitrate lost from water per gram of resin

΂

΃

0.100 L
qN = 0 + [(5.00 − 1.17) meq/L] ᎏ = 1.91 meq/g
0.200 g
Checking: qN + qCl = 1.74 + 1.91 = 3.65 meq/g (within 5 percent of 3.63)
5. Calculate the resin-phase equivalent fractions, yN and yCl, at equilibrium.
1.91 meq/g
yN = ᎏᎏ = 0.523
3.65 meq/g
1.74 meq/g
yCl = ᎏᎏ = 0.477
3.65 meq/g
6. Calculate the separation factor, αij.
yN xCl 0.523 × 0.764
αij = ᎏ = ᎏᎏ = 3.55
xN yCl 0.236 × 0.477
Note: Each data point will have an associated αij value. These αij values can be
averaged, but it is preferable to plot the isotherm data, construct the best-fit
curve, and use the curve at xN = 0.5 to obtain an average αij value. The bad data
points will be evident in the plot and can be ignored. Due to mathematical sensitivity, resin inhomogeneity, and imprecise experimental data, the calculated αij
values are not constant, as can be seen in Table 9.4. The αij values at the ends of
the isotherm are particularly nonrepresentative.
7. Plot the isotherm of yN versus xN. The nitrate versus chloride isotherm plot should
appear similar to that in Figure 9.3a.

ION EXCHANGE AND ADSORPTION KINETICS
Pure Ion Exchange Rates
As is usual with interphase mass transfer involving solid particles, resin kinetics is

governed by liquid- and solid-phase resistances to mass transfer. The liquid-phase
resistance, modeled as the stagnant thin film, can be minimized by providing turbu-


ION EXCHANGE AND INORGANIC ADSORPTION

9.17

lence around the particle such as that resulting from fluid velocity in packed beds or
mechanical mixing in batch operations. The speed of “pure” ion-exchange reactions
[i.e., reactions not involving (a) WAC resins in the RCOOH form or (b) free-base
forms of weak-base resins] can be attributed to the inherently low mass-transfer
resistance of the resin phase that is caused by its well-hydrated gelular nature. Resin
beads typically contain 40 to 60 percent water within their boundaries, and this water
can be considered as a continuous extension of the aqueous phase within the flexible polymer network. This pseudo-continuous aqueous phase in conjunction with
the flexibility of the resin phase can result in rapid kinetics for “pure ion-exchange”
reactions (i.e., ion exchange of typical inorganic ions using fully hydrated strong
resins). Reactions involving the acid or base forms of weak resins, reactions involving large ions, and reactions of chelating resins are not considered “pure ion
exchange”; these reactions are generally not rapid.
Alumina and SBA Resins Compared
Unlike adsorption onto granular activated carbon (GAC) or activated alumina,
requiring on the order of hours to days to reach equilibrium, pure ion exchange
using resins is a rapid process at near-ambient temperature. For example, the halftime to equilibrium for adsorption of arsenate onto granular 28- × 48-mesh (0.29- to
59-mm-dia) activated alumina was found to be approximately 2 days (Rosenblum
and Clifford, 1984), while the half-time to equilibrium during the exchange of arsenate for chloride on a strong-base resin was only 5 min (Horng, 1983; Horng and
Clifford, 1997). Similarly, the exchange of sodium for calcium on a SAC resin is
essentially complete within 5 min (Kunin, 1972).
Rates Involving Tight Resin Forms
In contrast, ion exchange with WAC and WBA resins can be very slow because of
the tight, nonswollen nature of the acid form ෆ(R

ෆC
ෆO
ෆO
ෆH
ෆ)ෆ of WAC resins or free-base
forms (e.g., R
ෆ3ෆN
ෆ:ෆ) of WBA resins. In reactions involving these tight forms, the average solid-phase diffusion coefficients change drastically during the course of the
exchange, which is often described using the progressive-shell, shrinking-core model
(Helfferich, 1965; Helfferich, 1966) depicted in Figure 9.5. In these reactions, which
are effectively neutralization reactions, either the shell or the core can be the
swollen (more hydrated) portion, and a rather sharp line of demarcation exists
between the tight and swollen zones. Consider, for example, the practical case of
softening with WAC resins in the H+ form (Equation 9.4). As the reaction proceeds,
the hydrated, calcium-form shell comprising ෆ(R
ෆC
ෆO
ෆO
ෆ−ෆ)ෆ2ෆC
ෆaෆ2ෆ+ෆ expands inward and
replaces the shrinking, poorly hydrated core of R
ෆC
ෆO
ෆO
ෆH
ෆ. The entire process is
reversed upon regeneration with acid, and the tight shell
of R
ෆC
ෆO

ෆO
ෆH
ෆ thickens as it proceeds inward and replaces
the porous, disappearing core of (ෆR
ෆC
ෆO
ෆO
ෆ−ෆ)ෆ2ෆC
ෆaෆ2ෆ+ෆ.
In some cases, “pure ion exchange” with weak resins
is possible, however, and proceeds as rapidly as pure ion
exchange with strong resins. For example, the “pure”
exchange of sodium for calcium on a WAC resin (Equation 9.32) does not involve conversion of the resin
RCOOH in contrast with Equation 9.4 and would take
FIGURE 9.5 Progressiveplace in a matter of minutes as with SAC resins (Equashell model of ion exchange
with weak resins.
tion 9.3).


9.18

CHAPTER NINE

CaCl2

R2Ca
Exhausted
Ion exchange zone

2 RNa + Ca2+ = R2Ca + Na+

RNa
Fresh resin

NaCl

FIGURE 9.6 Resin concentration profile for binary ion
exchange of sodium for calcium.

2ෆ
RC
ෆO
ෆO
ෆ−ෆN
ෆaෆ+ෆ + Ca(HCO3)2 = (ෆR
ෆC
ෆO
ෆO
ෆ−ෆ)ෆ2ෆC
ෆaෆ2ෆ+ෆ + 2NaHCO3

(9.32)

Although weak resins involving ෆ
Rෆ
Cෆ
Oෆ
Oෆ
H and ෆ
Rෆ3ෆ

N; may require several hours to attain
equilibrium in a typical batch exchange, they may still be used effectively in column
processes where the contact time between the water and the resin is only 1 to 5 min.
There are two reasons for the column advantage: (1) an overwhelming amount of
unspent resin is present relative to the amount of water in the column; and (2) the resin
is typically exposed to the feed water for periods in excess of 24 h before it is exhausted.
Prior to exhaustion, the overwhelming ratio of resin exchange sites present in the column to exchanging ions present in the column water nearly guarantees that an ion will
be removed by the resin before the water carrying the ion exits the column.The actual
contaminant removal takes place in the “adsorption” or “ion-exchange” or “mass
transfer” zone (see Figure 9.6) which characterized the breakthrough curve of interest.
In summary, ion exchange of small inorganic ions using strong resins is fundamentally a fast, interphase transfer process because strong resins are well-hydrated gels
exhibiting large solid-phase diffusion coefficients and little resistance to mass transfer. This is not the case with weak resins in the acid (RCOOH) or free-base (R3N:)
forms, nor is it true for alumina, because these media offer considerably more solidphase diffusion resistance. Irrespective of fast- or slow-batch kinetics, all these media
can be effectively used in column processes for contaminant removal from water,
because columns exhibit enormous contaminant-removal capacity and are exhausted
over a period of many hours to many days. Leakage of contaminants, will, however,
be much more significant with media that exhibit relatively slow mass transfer rates.

COLUMN PROCESSES AND CALCULATIONS
Binary Ion Exchange
Ion-exchange and adsorption column operations do not result in a fixed percentage of
removal of contaminant with time, which would result, for example, in a steady-state
coagulation process.These column processes exhibit a variable degree of contaminant


ION EXCHANGE AND INORGANIC ADSORPTION

Na+

9.19

Ca2+

Ceffluent
meq/L

Time or bed volumes
FIGURE 9.7 Effluent concentration histories
(breakthrough curves) for the softening reaction
in Figure 9.6.

removal and gradual or sharp contaminant breakthroughs similar to (but generally
much more complicated than) the breakthrough of turbidity through a granular filter.
First, we consider the hypothetical case of pure binary ion exchange before proceeding to the practical drinking water treatment case of multicomponent ion exchange.
If pure calcium chloride solution is softened by continuously passing it through a
bed of resin in the sodium form, ion exchange (Equation 9.2) immediately occurs in
the uppermost differential segment of the bed (at its inlet). Here all the resin is converted to the calcium form in the moving ion-exchange zone, where mass transfer
between the liquid and solid phases occurs.These processes are depicted in Figure 9.6.
The resin phase experiences a calcium wave front that progresses through the column until it reaches the outlet.At this point, no more sodium-form resin exists to take
up calcium, and calcium “breaks through” into the effluent, as shown in Figure 9.7. In
this pure binary ion-exchange case, the effluent calcium concentration can never
exceed that of the influent; this is, however, generally not true for multicomponent
ion exchange, as we will show later. The sharpness of the calcium breakthrough curve
depends on both equilibrium (i.e., selectivity) and kinetic (i.e., mass transfer) considerations. Imperfect (i.e., noninstantaneous) interphase mass transfer of sodium and
calcium, coupled with flow channeling and axial dispersion, always act to reduce the
sharpness of the breakthrough curve and result in a broadening of the ion-exchange
zone.This is equivalent to saying that nonequilibrium (noninstantaneous) mass transfer produces a diffuse calcium wave and a somewhat gradual calcium breakthrough.
A breakthrough curve can be gradual even if mass transfer is instantaneous, and
flow channeling and axial dispersion are absent, because the first consideration in
determination of the shape is the resin’s affinity (an equilibrium consideration) for

the exchanging ions. Mass transfer is the second consideration. If the exchange
isotherm is favorable, as is the case here (i.e., calcium is preferred to sodium), then a
perfectly sharp (square-wave) theoretical breakthrough curve results. If the ionexchange isotherm is unfavorable, as is the case for the reverse reaction of sodium
chloride fed to a calcium-form resin, then a gradual breakthrough curve results even
for instantaneous (equilibrium) mass transfer. These two basic types of breakthrough curves, sketched in Figure 9.8, result from the solution of mass balance
equations assuming instantaneous equilibrium and constant adsorbent capacity.
Multicomponent Ion Exchange
The breakthrough curves encountered in water supply applications are much more
complicated than those in Figures 9.7 and 9.8. The greater complexity is caused by
the multicomponent nature of the exchange reactions when treating natural water.
Some ideal resin concentration profiles and breakthrough curves for hardness


9.20

CHAPTER NINE

Ceffluent

Favorable
(self-sharpening)
Unfavorable
(broadening)
Time or bed volumes

FIGURE 9.8 Theoretical breakthrough curves for equilibrium ion exchange with no mass transfer limitations.An
unfavorable isotherm (Figure 9.3b) results in a broadening wave front (breakthrough), while a favorable isotherm
(Figure 9.3a) results in a self-sharpening wave front.

removal by ion-exchange softening and for nitrate removal by chloride-form anion

exchange are sketched in Fig. 9.9a and b. The important determinants of the shapes
of these breakthrough curves are (1) the feed water composition, (2) the resin capacity, and (3) the resin’s affinity for each of the ions as quantified by the separation factor, αij, or the selectivity coefficient, Kij. The order of elution of ions from the resin,
however, is determined solely by the selectivity sequence, which is the ordering of
the components from i = 1 − n, where 1 is the most-preferred and n is the leastpreferred species. Finally, before continuing with our discussion of multicomponent
ion-exchange column behavior, we must remind ourselves of Equation 9.26, which
shows that the αij values for di- and higher-valent ions, and thus the order of elution
of ions, will be determined by the total ionic concentration C of the feed water.
In carrying out the cation-or anion-exchange reactions, ions in addition to the target ion (e.g., calcium or nitrate) are removed by the resin. All the ions are concentrated, in order of preference, in bands or zones in the resin column, as shown in the
resin concentration profiles of Figures 9.9a and 9.9b. As these resin boundaries
(wave fronts) move through the column, the breakthrough curves shown in the figures result. These are based on theory (Helfferich and Klein, 1970) but have been
verified in the actual breakthrough curves published by Clifford (1982 and 1995),
Snoeyink et al. (1987), and Guter (1995).
Some useful rules can be applied to effluent histories in multicomponent ionexchange (and adsorption) systems (Helfferich and Klein, 1970; Clifford, 1982; Clifford, 1991):
1. Ions higher in the selectivity sequence than the presaturant ion tend to have long
runs and sharp breakthroughs (like all those except HCO3− in Figure 9.9b); those
less preferred than the presaturant ion will always have early, gradual breakthroughs, as typified by HCO3−.
2. The most-preferred species (radium in the case of softening, and sulfate in the
case of nitrate removal) are last to exit the column, and their effluent concentrations never exceed their influent concentrations.
3. The species exit the column in reverse preferential order, with the less preferred
ions (smaller separation factors with respect to the most-preferred species) leaving first.
4. The less-preferred species will be concentrated in the column and will at some
time exit the column in concentrations exceeding their influent concentrations
(chromatographic peaking). This is a potentially dangerous situation, depending
on the toxicity of the ion in question. Good examples of chromatographic peak-


ION EXCHANGE AND INORGANIC ADSORPTION

9.21

(a)

FIGURE 9.9 (a) Ideal resin concentration profile (above) and
breakthrough curves (below) for typical softening and radium
removal. Note that the column was run far beyond hardness
breakthrough and slightly beyond radium breakthrough. The
most preferred ion is Ra2+, followed by Ba2+ > Ca2+ > Mg2+ > Na+.

ing (i.e., effluent concentration greater than influent concentration) are visible in
Figure 9.9a and b. A magnesium peak is shown in Figure 9.9a, and bicarbonate
and nitrate peaks in Figure 9.9b.
5. When all the breakthrough fronts have exited the column, the entire resin bed is
in equilibrium with the feed water. When this happens, the column is exhausted,
and the effluent and influent ion concentrations are equal.
6. The effluent concentration of the presaturant ion (Na+ in Figure 9.9a, and Cl− in
Figure 9.9b) decreases in steps as each new ion breaks through, because the total
ionic concentration of the water (C, meq/L) must remain constant during simple
ion exchange.
One way to eliminate the troublesome chromatographic peaking of toxic ions such as
nitrate and arsenate is by inverting the selectivity sequence so that the toxic contam-


9.22

CHAPTER NINE

(b)

FIGURE 9.9 (Continued) (b) Ideal resin concentration profile (above) and breakthrough curves (below) for nitrate
removal by chloride-form anion exchange with a strong-base

resin. Note that the column was run far beyond nitrate breakthrough and somewhat beyond sulfate breakthrough. The most
preferred ion is SO42−, followed by NO3− > Cl− > HCO3−.

inant is the ion most preferred by the resin. This requires the preparation of specialpurpose resins.This has been done in the case of nitrate removal and will be discussed
later under that heading. Potential peaking problems still remain with other inorganic
contaminants, notably arsenic [As(V)] and selenium [Se(IV)] (Clifford, 1991). An
alternative means of eliminating or minimizing peaking is to operate several columns
in parallel, as will be discussed in the section on “Multicolumn Processes.”

Breakthrough Detection and Run Termination
Clearly an ion-exchange column run must be stopped before a toxic contaminant is
“dumped” during chromatographic peaking. Even without peaking, violation of the


ION EXCHANGE AND INORGANIC ADSORPTION

9.23

MCL will occur at breakthrough, when the contaminant feed concentration exceeds
the MCL. Effective detection and prevention of a high effluent concentration of
contaminant depend on the frequency of sampling and analysis. Generally, continuous on-line analysis of the contaminant (e.g., nitrate or arsenate) is too sophisticated
for small communities, where most of the inorganic contaminant problems exist
(AWWA, 1985). On-line conductivity detection, the standard means of effluent
quality determination in ion-exchange demineralization processes, is not easily
applied to the detection of contaminant breakthrough in single-contaminant processes such as radium, barium, nitrate, or arsenate removal. This is because of the
high and continuously varying conductivity of the effluents from cation or anion
beds operated on typical water supplies. Nevertheless conductivity should not be
ruled out completely, because even though the changes may be small, as the various
ions exit the column a precise measurement may be possible in selected applications.
On-line pH measurement is a proven, reliable technique that can sometimes be

applied as a surrogate for contaminant breakthrough. For example, pH change can
be used to signal the exhaustion of a weak-acid resin (ᎏRCOOH) used for carbonate hardness removal. When exhausted, the WAC resin ceases to produce acidic
carbon dioxide, and the pH quickly rises to that of the feed water. This pH increase
is, however, far ahead of the barium or radium breakthrough. The pH can sometimes
be used as an indicator of nitrate breakthrough, as discussed in the section on nitrate
removal.
The usual method of terminating an ion-exchange column run is to establish the
relevant breakthrough curve by sampling and analysis and then use these data to
terminate future runs based on the metered volume of throughput with an appropriate safety factor. If a breakthrough detector such as a pH or conductivity probe is
applied, the sample line to the instrument can be located ahead (e.g., 6 to 12 in.) of
the bed outlet to provide advance warning of breakthrough.

Typical Service Cycle for a Single Column
Ion-exchange and adsorption columns operate on similar service cycles consisting
of six steps: (1) exhaustion, (2) backwash, (3) regeneration, (4) slow rinse, (5) fast
rinse, and (6) return to service. (Backwash may not be required after every exhaustion.) A simple single-column process schematic is shown in Figure 9.10, which
includes an optional bypass for a portion of the feed water. Bypass blending will be
a common procedure for drinking water treatment applications because ionexchange resins can usually produce a contaminant-free effluent that is purer than
that required by law. Therefore, to minimize treatment costs, part of the contaminated feed water, typically 10 to 50 percent, will be bypassed around the process
and blended with the effluent to produce a product water approaching some fraction (e.g., 70 percent) of the MCL acceptable to the regulatory agency. An alternative means of providing efficient column utilization when significant contaminant
leakage is allowed is to operate several columns in parallel as discussed in the section on “Multicolumn Processes.”

Partial Regeneration and Regenerant Reuse
Yet another means of optimizing column utilization and minimizing process costs is
to use the technique of partial regeneration. This involves the use of only a fraction
(e.g., 25 to 50 percent) of the regenerant required for “complete” (e.g., 90 to 100 per-


9.24

CHAPTER NINE

Backwash out
Feedwater, QF

Regenerant
Typical service cycle
Ionexchange
bed

Effluent
bypass, QB
Blended
product
water, QP

E

Exhaustion (at service rate)
Backwash
Regeneration (co- or countercurrent)
Slow rinse (displacement rinse)
Fast rinse (at service rate)
Repeat cycle

Spent regenerant
Backwash

FIGURE 9.10 Schematic and service cycle of a single-column ion-exchange
process.

cent) removal of the contaminant from the exhausted resin. The result is often, but
not always, a large leakage of contaminant on the next exhaustion run, caused by the
relatively high level of contaminant remaining on the resin. Such large leakage can
often be tolerated without exceeding the MCL. Partial regeneration is particularly
useful in nitrate removal, as will be discussed in detail later. Generally, either bypass
blending or partial regeneration will be used; simultaneous use of both processes is
possible, but creates significant process control problems.
Reuse of spent regenerant is another means of reducing costs and minimizing
waste disposal requirements. In order for a spent regenerant to be reused, the target
contaminant ion must either be removed from the regenerant before reusing it, or
the resin must have a strong preference in favor of the regenerant ion as compared
to the contaminant ion, which accumulates in the recycle brine. The recent literature
suggests that spent brine reuse is possible in more applications than were previously
thought possible. Removing nitrate from the recycle brine by means of biological
denitrification was the approach used by the author and his colleagues (Clifford and
Liu, 1993b; Liu and Clifford, 1996) for their nitrate ion-exchange process with brinereuse. In their pilot-scale experiments, a denitrified 0.5 M Cl− brine was reused 38
times without disposal. Clifford, Ghurye, et al. (1998) also determined that spent
arsenic-contaminated brine could be reused more than 20 times by simply maintaining the Cl− concentration at 1.0 M without removing the arsenic. Kim and Symons
(1991) showed that DOC anions could be removed from drinking water by strongbase-anion exchange with regenerant reuse. No deterioration of DOC removal was
noted during 9 exhaustion-regeneration cycles with spent brine (a mixture of NaCl
and NaOH) reuse when the Cl− and OH− levels were maintained at 2.0 and 0.5 M,
respectively. Further information on these processes is provided in the “Applications” section of this chapter.
Reusing the entire spent-regenerant solution is not necessary. In the case where
there is a long tail on the contaminant elution curve, the first few bed volumes of
regenerant are discarded, and only the least-contaminated portions are reused. In
this case a two-step roughing-polishing regeneration can be utilized. The roughing
regeneration is completed with the partially contaminated spent regenerant, and the

ION EXCHANGE AND INORGANIC ADSORPTION

9.25

polishing step is carried out with fresh regenerant. The spent regenerant from the
polishing step is then used for the next roughing regeneration.
Regenerant reuse techniques are relatively new to the ion-exchange field and are
yet to be proved in full-scale long-term use for water supply applications. Although
possessing the advantages of conserving regenerants and reducing the volume of
waste discharges regenerant reuse can also result in some significant disadvantages,
including (1) increased process complexity; (2) increased contaminant leakage; (3)
progressive loss of capacity caused by incomplete regeneration and fouling; (4) the
need to store and handle spent regenerants; and (5) buildup (concentration) of trace
contaminants as the number of regenerant reuse cycles increases.

Multicolumn Processes
Ion-exchange or adsorption columns can be connected (1) in series to improve product
purity and regenerant usage, or (2) in parallel to increase throughput, minimize peaking, and smooth out product water quality variations. If designed properly, multiplecolumn systems can be operated in parallel, series, or parallel-series modes.
Columns in Series. A series roughing-polishing sequence is shown in Figure 9.11.
In such a process, a completely exhausted roughing column is regenerated when the
partially exhausted polishing column effluent exceeds the MCL. This unregenerated
polishing column becomes the new roughing column, and the old roughing column,
now freshly regenerated, becomes the new polishing column. Often three columns
are used. While two are in service, the third is being regenerated. A multicolumn system consisting of three or more columns operated in this manner is referred to as a
“merry-go-round system,” which should not be confused with “carousel system”
(AST, 1995), which is usually operated as columns in parallel.
Columns in Parallel. In addition to bypass blending (see Figure 9.10), an alternative means of allowing a predetermined amount of contaminant leakage in the product water is to employ multiple columns
in parallel operated at different stages of
exhaustion. For example, with three
columns operated in parallel, the first one

could be run beyond MCL breakthrough
while the second and third columns
Column
Column
Column
would not have achieved breakthrough.
1
2
3
Thus, even after contaminant breakthrough in the first column, the average
concentration of the three blended efflluents would be below the MCL. MultipleRoughing
Polishing
Column in
parallel-column operation will give a
column
column
standby or
more consistent product water quality
regeneration
and can also prevent, or at least smooth
out, chromatographic peaks from serious
FIGURE 9.11 Two-column roughing-polishing
system operated in a merry-go-round fashion.
overruns. During normal operation of
After exhaustion of column 1, it will be taken out
multiple-parallel-column systems, some
of service, and the flow sequence will be column 2
columns are being exhausted while
and then column 3. Following exhaustion of
others are being rinsed, regenerated,

column 2 and regeneration of column 1, the
or are in standby mode. A recently deroughing-polishing sequence will be column 3
then column 1.
scribed carousel-ion-exchange process