Chapter 12. Ion Exchange and Adsorption of Inorganic Contaminants

12.2



CHAPTER twelve



Overview

In the first part of this chapter, the fundamentals of ion exchange (IX) and adsorption processes

are explained, with the goal of demonstrating how these principles influence process design for

inorganic contaminant removal. In the second part, ion exchange and adsorption processes that

have been proven effective at bench, pilot, and full scale are described for the removal of hardness, barium, radium, nitrate, fluoride, arsenic, dissolved organic carbon (DOC), uranium, and

perchlorate. For each contaminant, the factors that influence the choice and design of a process,

for example, pH, total dissolved solids (TDS), contaminant speciation, competing ions, resins,

adsorbents, foulants, regenerants, and column flow patterns, are discussed. In the third and final

part of the chapter, ion exchange modeling using equilibrium multicomponent chromatographic

theory (EMCT) with constant separation factors (CSFs) is covered. Summary tables for cation

and anion contaminants are included at the end of the chapter to aid the reader in process selection. The basic features, advantages, and disadvantages of the ion exchange and metal oxide/

hydroxide packed bed processes covered in this chapter are summarized in Table 12-1.



Introduction and Theory of Ion Exchange

Contaminant cations such as calcium, magnesium, radium, barium, and strontium and

anions such as fluoride, nitrate, fulvates, humates, arsenate, selenate, chromate, perchlorate, and anionic complexes of uranium can be removed from water using ion exchange

Table 12-1  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 “tailored” resins available for specific target ions.

•  Beneficial selectivity reversal commonly occurs on 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.

Metal oxide/hydoxide 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 when regeneration is possible.

•  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.







ION EXCHANGE AND ADSORPTION OF INORGANIC CONTAMINANTS



12.3



with resins or adsorption onto granular hydrous metal oxides/hydroxides such as activated

alumina (AA), granular ferric oxide (GFO), and granular ferric hydroxide (GFH) or coagulated Fe(II), Fe(III), Al(III), Ti(III), Ti(IV), Zr(III), Zr(IV), and Mn(IV) surfaces. This chapter deals only with the theory and practice of IX with resins and adsorption with granular

media including GFH, GFO, and AA. 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.

IX with synthetic resins and adsorption onto hydrous metal oxides/hydroxides 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 IX resin beads or metal oxide/hydroxide 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 IX 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 the metal oxides/hydroxides may be slightly dissolved during 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 metal oxides/hydroxides, 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 the medium is exhausted. Regeneration

typically requires from 1 to 5 BV of regenerant followed by 2 to 20 BV 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 IX and adsorption design practice.

Disposal of the spent medium also may present a problem if it contains toxic or radioactive

substances such as nitrate, arsenic, perchlorate, uranium, and/or radium.

Uses of IX in Water Treatment

By far the largest application of IX to drinking water treatment is water softening, that is,

the removal of calcium, magnesium, and other polyvalent cations, including Ra2+, Ba2+,

Sr2+, Fe2+, and Mn2+, in exchange for sodium. The IX softening process is applied to residential, industrial, and municipal water treatment. In residential use, IX can be applied for

whole-house [point-of-entry (POE)] softening or for softening only the water that enters

the hot water heater. For industrial use, generally all the water is passed through the resin

to lower the hardness to zero, whereas in municipal water treatment some of the water

is bypassed around the softener because some hardness is desirable in the treated water.

Radium and barium are ions more preferred by the resin than calcium and magnesium;

thus the former are also removed effectively during IX softening. Resins beds containing

chloride-form anion exchange resins can be used to remove nitrate, arsenate, perchlorate,

chromate, selenate, natural organic matter (NOM)/color, uranium, perchlorate, and other

contaminant anions. GFO, GFH, and zirconium and titanium oxides/hydroxides are being

used to remove arsenate, whereas AA is being used to remove fluoride. These adsorption

processes are especially useful for groundwater treatment in small systems and for residential point-of-use (POU) and POE systems.

The choice between IX and metal oxide/hydroxide adsorption, for example, to remove

arsenic from water, is largely determined by (1) the background water quality including

TDS level, competing ions, alkalinity, and contaminant concentration and (2) the resin or

adsorbent medium affinity for the contaminant ion in comparison with competing ions. The

affinity sequence determines the run length, chromatographic peaking (if any), and process

costs. As mentioned previously, process selection is affected by spent regenerant and spent



12.4



CHAPTER twelve



medium disposal requirements and regenerant reuse possibilities, particularly if hazardous

contaminants are involved. Each of these requirements is dealt with in some detail in the

upcoming design sections for the specific processes.

Past and Future of IX

Natural zeolites, which are crystalline aluminosilicate minerals, were the first ion exchangers, and they were used to soften water on an industrial scale. Later, zeolites were completely replaced with synthetic resins because of the latter’s faster exchange rates, higher

capacity, greater resilience, and longer life. Aside from softening, the use of IX and inorganic adsorption for removal of specific contaminants from municipal water supplies has

been limited owing to the perceived expense involved in removing what have been seen

as minimal health risks resulting from contaminants such as fluoride and nitrate. The production of pure and ultrapure water by IX demineralization (DM) is the largest use of IX

resins on an industrial scale. The complete removal of contaminants, which occurs in DM

processes, is not necessary for drinking water treatment, however. Furthermore, treatment

costs are high compared with membrane processes (reverse osmosis and electrodialysis)

for desalting water (see Chap. 11).

Adherence to governmentally mandated maximum contaminant level (MCL) goals for

inorganic chemicals (IOCs) will result in more use of IX and adsorption systems for small

community water treatment to remove barium, radium, cadmium, chromium, mercury,

lead, fluoride, arsenic, nitrate, perchlorate, uranium, and other IOCs.

IX Materials and Reactions

An IX resin consists of a cross-linked polymer matrix to which charged functional groups are

attached by covalent bonding. The usual matrix is polystyrene cross-linked for structural stability with 3 to 8 percent divinylbenzene (abbreviated STY-DVB or simply polystyrene). The

–

common functional groups fall into four categories: strongly acidic (e.g., sulfonate, —SO3 ),

+

weakly acidic (e.g., carboxylate, —COO–), strongly basic [e.g., quaternary amine, —N(CH3)3 ],

and weakly basic [e.g., tertiary amine, —N(CH3)2] (Helfferich, 1962).

A schematic presentation illustrating resin matrix cross-linking and functionality is shown

in Fig. 12-1a. The schematic depicts a three-dimensional bead (sphere) made up of many

polystyrene polymer chains held together by divinylbenzene cross-linking. The negatively

–

+

charged IX sites (—SO3 ) in a cation exchanger and the positively charged sites [—N(CH3)3 ]

in an anion exchanger are shown in Fig. 12-1b fixed to the resin backbone, or matrix as it

is called. Mobile positively charged counterions (positive charges in Fig. 12-1a) are associated by electrostatic attraction with each negative IX site on a cation exchange resin. 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 (—SO3 ) or quaternary ammonium [—N(CH3)3 ].

Porosity, expressed by the terms microporous/gel, porous, and macroporous, is the resin

characterization referring to the degree of openness of the polymer structure. An actual resin

bead is much tighter than on the schematic, which is shown as fairly open for purposes of

illustration only. The water (40–60% by weight) present in a typical resin bead is not shown.

This resin-bound 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 wide pH range because the sulfonate group, being strongly acidic, is ionized







ION EXCHANGE AND ADSORPTION OF INORGANIC CONTAMINANTS



12.5



Figure 12-1  (a) Organic cation exchanger bead comprising polystyrene polymer crosslinked with divinylbnezene with fixed negatively charged co-ions (minus charges) balanced

by mobile positively charged counterions (plus charges). (b) Strong-acid cation exchanger

(left) in the hydrogen form and strong-base anion exchanger (right) in the chloride form.



throughout the 1 to 14 pH range. Three typical SAC exchange reactions are shown below.

In Eq. 12-1, the neutral CaCl2 salt, 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 12-2 and 12-3 are the standard

IX softening reactions in which sodium ions are exchanged for the typical hardness ions:

Ca2+, Mg2+, Fe2+, Ba2+, Sr2+, and/or Mn2+, either as noncarbonate hardness (Eq. 12-2) or as

carbonate hardness (Eq. 12-3). In all these reactions, R denotes the resin matrix, and the

overbar indicates the solid (resin) phase.





–

–

2RSO3 H+ + CaCl2 ↔ (RSO3 )2Ca2+ + 2HCl



(12-1)







–

–

2RSO3 Na+ + CaCl2 ↔ (RSO3 )2Ca2+ + 2NaCl



(12-2)







–

–

2RSO3 Na+ + Ca(HCO3)2 ↔ (RSO3 )2Ca2+ + 2NaHCO3



(12-3)



Regeneration of the spent resin is accomplished using an excess of concentrated (0.5–3 M)

HCl or NaCl, and amounts to the reversal of Eqs. 12-1 through 12-3. 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



12.6



CHAPTER twelve



can be used for carbonate hardness removal (Eq. 12-4) but fail to remove noncarbonate

hardness, as is evident in Eq. 12-5.





2RCOOH + Ca(HCO3)2 → (RCOO–)2Ca2+ + H2CO3



(12-4)







2RCOOH + CaCl2 ← (RCOO )2Ca + 2HCl



(12-5)



–



2+



If Eq. 12-5 proceeded to the right, the HCl produced would be so completely ionized

that it would protonate, that is, 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 Eq. 12-5 does not proceed to the right is to say that WAC resins will not

split neutral salts; that is, they cannot remove noncarbonate hardness. This is not the case

in Eq. 12-4, in which the basic salt Ca(HCO3)2 is split because a 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 IX for drinking water treatment

(Clifford and Weber, 1978; Guter, 1981), although anion resins have been used in water

demineralization for more than 50 years. In anion exchange reactions with SBA resins, the

+

quaternary amine functional group [—N(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 (Helfferich, 1962). This is

shown in Eqs. 12-6 and 12-7, in which nitrate is removed from water by using hydroxide- or

chloride-form SBA resins. Note that R4N+ is another way to write the quaternary exchange

+

site —N(CH3)3 .







–

R4N+ OH– + NaNO3 ↔ R4N+ NO3 + NaOH



R4N Cl + NaNO3 ↔ R4N

+



–



+



–

NO3



+ NaCl



(12-6)

(12-7)



In Eq. 12-6, the 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. Eq. 12-7 is a simple ion-exchange reaction without a pH change. Fortunately, all SBA

resins have a higher affinity for nitrate than chloride (Clifford and Weber, 1978) and Eq. (12-7)

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

can act as positively charged exchange sites for anions. In Eq. 12-8, 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 that can

act as an anion exchange site. As long as the solution in contact with the resin remains acidic

(just how acidic depends on the basicity of the R3N; sometimes pH ≤ 6 is adequate), IX can

take place, as is indicated in Eq. 12-9—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 Eq. 12-10. In all these reactions, R represents either the resin matrix or a functional group, such as —CH3 or —C2H5, and the overbars represent the resin phase.





R3N: + HCl ↔ R3NH+ Cl–



(12-8)







–

R3NH Cl– + HNO3 ↔ R3NH+ NO3 + HCl



(12-9)







R N: + NaNO3 → no reaction



+



3



(12-10)







ION EXCHANGE AND ADSORPTION OF INORGANIC CONTAMINANTS



12.7



Although WBA resins are not used commonly for drinking water treatment, such processes are possible as long as the exchange pH is in the 4 to 6 range, which can be costly

because this requires neutralization of most of the alkalinity present. Furthermore, activated alumina, when used for fluoride and/or arsenic removal, acts as if it were a WBA

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 ammonium hydroxide (NH4OH) and

lime (Ca(OH)2) can be used, and regardless of the base used, only a small stoichiometric

excess (<20 percent) normally is required for complete regeneration.

Granular Media Adsorption.  Packed beds of AA, titanium and zirconium oxides/

hydroxides, GFO and GFH, and iron oxides coated onto or incorporated into various

media, including anion resins (Cumbal and Sengupta, 2005), diatomaceous earth (NSF

International, 2005), and sand (Benjamin et al., 1996), can be used to remove contaminant anions, including fluoride, arsenic, selenium, silica, phosphate, vanadate, and natural

organic matter (fulvate) anions from water. A simple schematic of the contaminant adsorption process is shown in Fig. 12-2, where the contaminant (Adsorbate-A) attaches to an

→



+



Contaminant

ion or

molecule

A



S



Contaminants

adsorbed

onto internal

surface sites



Porous

adsorbent

with active

sites



+



→



A•S



Figure 12-2  Schematic of the contaminant adsorption process on a porous adsorbent with internal surface area containing

adsorbent sites. Adsorbate (A) + Adsorbent Site (S) react to form

Adsorbate · Site complex (A · S) on adsorbent surface.



adsorption site (Sorbent-S) to form A∙S a site with contaminant attached. Coagulated Fe(II)

and Fe(III) oxides/hydroxides (McNeill and Edwards, 1995; Scott et al., 1985; Lakshmanan

et al., 2008) also can be employed to remove these anions, but coagulation processes are

not covered in this chapter. The mechanism, which is one of exchange of contaminant

anions for surface hydroxides on the alumina or iron oxides/hydroxides, generally is called

sorption or adsorption, although ligand exchange is a more appropriate term for the highly

specific surface reactions involved (Stumm, 1992). Figure 12-3 depicts the ligand exchange

of arsenate for hydroxide on the surface of ferric oxide/hydroxide (FeOOH). Similar reactions can occur on the surfaces of activated alumina (AlOOH) and metal oxides/hydroxides

(MeOOH), including zirconium and titanium.

The typical activated aluminas used for fluoride removal in water treatment are

28 × 48 mesh (0.3–0.6 mm diameter) mixtures of amorphous and gamma aluminum

oxide (γ-Al2O3) prepared by low-temperature (300°C–600°C) dehydration of precipitated

Al(OH)3. These highly porous materials have surface areas of 50 to 300 m2/g. The granular

ferric oxides/hydroxides (GFO and GFH) used for arsenate removal are prepared by similar

methods and also have surface areas in the 50 to 300 m2/g range. Using the model of

a hydroxylated alumina surface subject to protonation and deprotonation, the following

ligand (fluoride) exchange reaction (Eq. 12-11) can be written for fluoride adsorption in

acid solution (alumina exhaustion), in which AlO represents the alumina surface and an



12.8



CHAPTER twelve



Fe .OH



Fe



O

Fe .OH

O

O

O



OH

Fe .OH

Fe .OH

Fe .OH



FeOOH

AlOOH

Zr, Ti oxides



O



O



+ O–



Fe



Arsenic (V)

H2AsO4–

Competing Ligands:

H2PO4–



Si(OH)3O–



O



O



O

Fe .OH



As OH

O



OH

As



O

O



Fe .OH



HOH + OH–



Fe .OH



Arsenic on

FeOOH

AlOOH

MeOOH



Hydroxide



Figure 12-3  Schematic of ligand exchange of hydroxide for arsenate on ferric oxide/hydroxide surface. Similar reactions may occur for other metal oxides/hydroxides (MeOOH).



overbar denotes the solid phase. FeOOH surfaces undergo similar reactions with arsenate

–

(H2AsO4 ) as the ligand (see Fig 12-2).





AlO − OH + H + + F − → AlO − F + HOH



(12-11)



The equation for fluoride desorption by hydroxide (alumina regeneration) is presented in

Eq. 12-12.





AlO − F + OH − → AlO − OH + F −



(12-12)



Metal oxide/hydroxide surfaces, including AA, GFH, GFO, and etc., are sensitive to

pH, and anions are best adsorbed below the zero point of charge (ZPC), where the metal

oxide surface has a net positive charge, and excess protons are available to fuel Eq. 12-11.

Above the ZPC (pH 8.2), 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 et al., 1988; Garg and

Clifford, 1992).

Ligand exchange, as indicated in Eqs. 12-11 and 12-12, occurs chemically at the internal and external surfaces of activated alumina, GFH, and GFO. A more useful model for

process design, however, is one that assumes that the adsorption of fluoride or arsenic onto

a metal oxide/hydroxide surface at the optimal pH of 5.5 to 6 is analogous to weak-base

–

anion exchange. For example, the uptake of F– or H2AsO4 requires the protonation of the

metal oxide/hydroxide surface, which is sometimes accomplished by preacidification of

the medium with HCl or H2SO4 and/or reducing the feed water pH into the 5.5 to 6.0 region.

The positive charge caused by excess surface protons then may be viewed as being balanced

by exchanging anions, that is, ligands such as hydroxide, fluoride, and arsenate. To reverse

the adsorption process and remove the adsorbed fluoride or arsenate, an excess of strong

base, for example, NaOH, must be applied. The following series of reactions (Eqs. 12-13

through 12-17) is presented as a model of the adsorption/regeneration cycle that is useful

for design purposes. Although less common, these reactions also may be applied to GFH

and GFO processes.







ION EXCHANGE AND ADSORPTION OF INORGANIC CONTAMINANTS



12.9



The first step in the cycle is acidification, in which neutral (water-washed) alumina

(alumina·HOH) is treated with acid, for example, HCl, and protonated (acidic) alumina is

formed as follows:





alumina ⋅ HOH + HCl → alumina ⋅ HCl + HOH



(12-13)



When HCl-acidified alumina is contacted with fluoride ions, the chloride ions are

strongly displaced, provided that the alumina surface remains acidic (pH 5.5–6). This displacement of chloride by fluoride, analogous to ion exchange, is shown as





alumina ⋅ HCl + HF → alumina ⋅ HF + HCl



(12-14)



To regenerate the fluoride-contaminated adsorbent, a dilute solution of alkali (0.25–

0.5 N NaOH) 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





alumina ⋅ HF + 2 NaOH → alumina ⋅ NaOH + NaF + HOH



(12-15)



Recent experiments have suggested that Eq. 12-15 can be carried out using fresh or

recycled NaOH from a previous regeneration. This suggestion is based on the field studies

of Clifford and colleagues (1998) in which arsenic-spent alumina was regenerated with

equally good results using fresh or once-used 1.0 M NaOH. Probably the spent regenerant,

fortified with NaOH to maintain its hydroxide concentration at 1.0 M, could have been used

many times, but the optimal 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:





alumina ⋅ NaOH + 2HCl → alumina ⋅ HCl + NaCl + HOH



(12-16)



The acidic alumina, alumina·HCl, is now ready for another fluoride (or arsenate or

selenite) ligand exchange cycle, as summarized by Eq. 12-14. 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 Eq. 12-17.





alumina ⋅ NaOH + NaF + 2HCl → alumina ⋅ HF + 2 NaCl + HOH



(12-17)



The modeling of the alumina/GFH/GFO 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 metal oxides/hydroxides. This is caused by the

need for excess acid and base to partially overcome the poor kinetics of the metal oxide/

hydroxide granules, which exhibit 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 metal oxides/hydroxides

is that they are amphoteric and react with (consume) excess acid and base to partly dissolve

in acidic and basic regenerant solutions. Resins are totally inert in this regard; that is, they

are not dissolved by regenerants.

Special-Purpose Resins.  Resins are practically without limit in their variety because polymer matrices, functional groups, 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 used in some water supply

applications because the positively charged resins tend to absorb the negatively charged bacteria



12.10



CHAPTER twelve



that metabolize the adsorbed organic material—negatively charged humate and fulvate

anions. To correct this problem, special resins have been developed that 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 et al., 1981).

The strong attraction of polyvalent humate and fulvate anions [components of natural

organic matter (NOM)] for anion resins has been used as the basis for removal these compounds and total organic carbon (TOC) from water by using special highly porous resins. Both

weak- and strong-base macroporous anion exchangers have been manufactured 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. However, it has been shown that both gel and standard macroporous resins, which are well

hydrated, highly cross-linked, 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 been solved at least partially using hydrophilic, well-hydrated, acrylic-matrix

SBA resins. More detail on the use of IX resins to remove NOM is given later in this chapter.

Special-purpose SBA resins with large, hydrophobic, widely space functional groups

have been developed for nitrate and perchlorate removal applications. These resins are

discussed in more detail later in the sections on nitrate and perchlorate removal.

Resins with chelating functional groups such as iminodiacetate (Calmon, 1979), aminophosphonate, 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 et al., 1991). Table 12-2 summarizes the

Table 12-2  Special Ion Exchangers Available Commercially

Type of resin



Functional group



Chelating



Iminodiacetic



Chelating



Aminophosphonic



Chelating



Thiouronium



Chelating



Polyamine



Chelating



Amidoxime



Boron-specific

NSS, nitrate-over-sulfate  selective (sulfate/divalent rejecting)

Perchlorate-selective



N-Methylglucamine

Triethyl and tripropyl

  quaternary amines

Tripropyl or ethylhexyl

  quaternary amines



Silver-impregnated SAC



Sulfonic



Iodine-releasing



Quaternary amine in

–

  triiodide form—R4N+I3



Typical applications

Selective removal of heavy metals

  and transition metals

Decalcification of brine and removal

  of cations of low atomic mass

Selective removal of heavy metals,

  especially mercury

Removal of trace heavy metals

  and heavy-metal complex anions

Removal of copper and iron from

  low pH water

Removal of boron from water

Nitrate removal in high sulfate

  water

Perchlorate removal from

 

water, often applied in singleuse (nonregenerated resin)

applications

Softening resins with

  bacteriostatic properties

Portable disinfection units



Note: Mention of trade names does not indicate endorsement. Similar products are available from alternative suppliers.

Source:  www.purolite.com, 2009.







ION EXCHANGE AND ADSORPTION OF INORGANIC CONTAMINANTS



12.11



features of some of the special ion exchangers available commercially from a variety of

sources (Purolite, 2009).

IX Equilibrium

Selectivity Coefficients and Separation Factors.  IX 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 is the run length. Consider, for example,

–

Eq. 12-18, the simple exchange of Cl– for NO3 on an anion exchanger with an equilibrium

constant, as expressed numerically in Eq. 12-19 and graphically in Fig. 12-4a.

Cl − + NO3 ↔ NO− + Cl −

3









K=



−

3



−



{NO }{Cl }

−

{Cl −}{NO3 }







(12-18)

(12-19)



In Eqs. 12-18 to 12-20, 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, Eq. 12-20 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; that is, it varies somewhat with different ionic strengths.

K N/Cl =







[NO− ][Cl − ] qNCCl

3

=



−

[Cl − ][ NO3 ] qClCN



(12-20)



where qN = resin-phase equivalent concentration (normality) of nitrate, eq/L



CN = aqueous-phase equivalent concentration (normality), eq/L

and the brackets represent concentration (mol/L).



Figure 12-4  (a) Favorable isotherm for nitrate-chloride exchange according to Reaction 12-18 with

constant separation factor αNO /Cl > 1.0. (b) Unfavorable isotherm for bicarbonate-chloride exchange with

3

constant separation factor αHCO /Cl < 1.0.

3



12.12



CHAPTER twelve



The binary separation factor αN/Cl, used throughout the literature on separation practice,

is a useful description of the exchange equilibria because of its simplicity and intuitive

nature:







α ij =



distribution of ion i between phases yi / xi

=

n

distributtio of ion j between phases y j / x j

α N/Cl =



yN / x N yN / x Cl (qN / q)(CCl / C )

=

=

/

yCl / x Cl x N / yCl (CNN C )(q lC/ q)



(12-21)

(12-22)



where 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 12-20 and 12-22 show that for homovalent exchange, where the exchanging ions have the same charge (i.e., monovalent/monovalent ion and divalent/divalent ion

exchange), the separation factor αij and the selectivity coefficient Kij are equal. This is

expressed for nitrate/chloride exchange as





K N/Cl = α N/Cl =



qNCCl

CN qCl



(12-23)



For exchanging ions of unequal charge, that is, heterovalent exchange, the separation

factor is not the same as the selectivity coefficient. Consider, for example, the case of

sodium IX softening, as represented by Eq. 12-24, the simplified form of Eq. 12-2.







2 Na + + Ca 2+ = Ca 2+ + 2 Na +

K Ca/Na =



(12-24)



2

qCaCNa

2

CCa qNa



(12-25)



Using a combination of Eqs. 12-21 and 12-25, we obtain





α divalent/monovalent or α Ca/Na = K Ca/Na



qyNa

Cx Na







(12-26)



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 constant resin capacity q.

The higher the solution concentration C, the lower is the divalent/monovalent separation

factor; that is, 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 below.

Selectivity Sequences for Resins.  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 12-3,