Batch/Columnar Operation

Ion exchange processing can be accomplished in either a batch or column mode. The batch mode involves mixing the resin and solution in a batch tank, allowing the exchange to come to equilibrium, and then separating the resin from solution. The degree to which the exchange takes place is limited by the preference the resin exhibits for the ion in solution. Consequently, the utilization of the resin's exchange capacity is limited unless the selectivity for the ion in solution is far greater than for the exchangeable ion attached to the resin. Because batchwise regeneration of the resin is chemically inefficient, batch processing by ion exchange has, in the author's opinion, limited potential application in treating water for purposes outlined in Chapter 1 and will not be discussed further.

Basic Columnar Operation

The primary function of most fixed-bed (columnar) ion exchange operations is the removal of an ionic species from solution in exchange for another ionic species for the purpose of concentrating a desirable material into a small volume, or removing an undesirable constituent. The latter may be accomplished by exchange with an unobjectionable ionic species, for example the softening of water using a cation exchanger in the sodium cycle. In most cases, the removal of all ions or particular ionic species must be as complete as possible, and the ion exchange resin must have the capacity to treat a large volume of liquid per unit volume of exchanger.

In Chapter 5, we will see that ion exchange resins in fixed-bed operations are primarily designed for removing soluble ionic species and, therefore, cannot be expected to adsorb macro- or colloidal-ionic species except at the surface of the particle. Because of their ionic nature and particle size, ion exchange beds are very effective filters; however, they are not normally used for this purpose. If they are used and the filter load is heavy, one cannot expect them to function efficiently as ion exchangers. Readers are directed to Wachinski's Ion Exchange Treatment for Drinking Water (2004) for his thinking on the use of low-pressure membranes for pretreatment of ion exchange systems; see also Wachinski's Membrane Processes for Water Reuse (2013) for an in-depth treatment of membrane processes role in pretreatment.

Figure 3.1 shows a typical column operation used to soften a hard water containing calcium, magnesium, and sodium salts. Water with the indicated constituents is passed through an ion exchange column containing an ion exchanger or resin that has been regenerated such that the exchange sites are in the sodium form. The chemical constituents in the treated water are shown leaving the ion exchange column. Figure 3.2 shows typical hardware.

Figure 3.1   Typical columnar softening.

Figure 3.2   Typical ion exchange unit.

Synthetic Cation Exchangers

Cation exchangers are available with numerous fixed ionic groups exhibiting a range of different properties and acid strengths. The most common are strong-acid resins (SARs) with sulfonic groups (–SO₃–) and weak-acid resins (WARs) with carboxylic acid groups and phosphonic acid groups.

In general, strong-acid cation exchangers substitute one ion for another depending on the resin's selectivity, and as such operate at all pH values. Their capacity is less than stoichiometric, however, and they must be regenerated more frequently than WARs, which exhibit much higher capacities and regenerate almost stoichiometrically. WARs operate only over a limited pH range.

Table 3.1   Classification of the Major Ion Exchange Resins

Type	Active Group	Typical Configuration
Cation Exchange Resins
Strong acid	Sulfonic acid
Weak acid	Carboxylic acid
Anion Exchange Resins
Strong base	Quaternary ammonium
Weak base	Secondary amine
Weak base	Tertiary amine (aromatic matrix)

Strong-Acid Cation Exchange Resins

All major strongly acidic cation exchange resins involved in water and wastewater treatment applications have a chemical matrix consisting of styrene and DVB. The functional groups are sulfonic acid radicals. These resins differ mainly in DVB content and gelular or macroreticular structure.

SARs are so named because their chemical behavior resembles that of a strong acid. A strong-acid cation resin can convert a neutral salt into its corresponding acid if operated in the acid form. This ability is known as salt-splitting and distinguishes SARs from WARs, which cannot salt-split. The equation for salt-splitting is

$$\text{NaCl}+\left(\text{resin}-{\text{H}}^{+}\right)\to \text{HCl}+\left(\text{resin}-{\text{Na}}^{+}\right)$$

Strong-acid exchangers can be operated in any form but are typically operated in the hydrogen cycle, where the resin is regenerated with a strong acid such as HCl or H₂SO₄, or in the sodium cycle where the resin is regenerated with NaCl.

Author's note: Probably the most important thing you will get from this book is that a resin (in this case a strong-acid resin) can be regenerated or put into any cycle that you wish. Ion exchangers can be operated in the K⁺ cycle, the Ca⁺⁺ cycle, and the Mg⁺⁺ cycle—any cycle that will allow the ion exchanger to work for you.

A strong-acid cation exchanger in the hydrogen cycle will remove nearly all major raw water cations and is the first step in demineralizing water. The equation for removing calcium sulfate from a raw water with an ion exchanger in the hydrogen cycle is

$${\text{CaSO}}_4+2\left(\text{R}-{\text{H}}^{+}\right)\rightleftharpoons 2\left({\text{R}}^{-}\right)-{\text{Ca}}^{++}+{\text{H}}_2{\text{SO}}_4$$

Figure 3.4 shows the same water treated by the sodium cycle exchanger in Figure 3.1, treated by a strong-acid cation resin in the hydrogen cycle. The resin in the bed is regenerated by either HCl or H₂SO₄. Regeneration will be further discussed later in this chapter. “In general, a strong acid can be regenerated using any monovalent, divalent, or trivalent acid or salt.”

The strong-acid exchangers require excess strong-acid regenerant (typical regeneration efficiency varies from 25% to 45% in concurrent regeneration), and they permit little leakage. In addition, they have rapid exchange rates, are stable, and may last 20 years or more with little loss of capacity. They exhibit less than 7% swelling going from Na⁺ to H⁺ form and are useful for softening and demineralization (removal of all cations with little leakage).

Generally, in most domestic water softening, one would use a strongly acidic cation exchange resin with an 8% DVB cross-linking. However, if a condition called oxidative decross-linking occurs, it would be best to use a resin with greater resistance to oxidative attack. Amberlite IR-122 resin, for example, with its higher DVB content is typical. In very severe decross-linking problems, a macroreticular type is recommended, where up to 20% DVB can be used as the cross-linker.

Figure 3.4   Hydrogen cycle strong acid cation exchange.

Weak-Acid Cation Exchange Resins

Carboxylic acid is the functional group associated with weak-acid cation exchangers (see Table 3.1). As such, weak-acid cation resins are not highly dissociated and do not exchange their H⁺ as readily as strong resins. Because they exhibit a higher affinity for hydrogen ions than SARs, WARs show higher regeneration efficiencies. They are usually regenerated with strong acids such as HCl or H₂SO₄. WARs do not require the same concentration driving force that is required to convert SARs to the hydrogen form. The carboxylic functional groups have a high affinity for H⁺ and will utilize up to 90% of the acid (HCl or H₂SO₄) regenerant, even with low acid concentrations. This is in contrast to SAR regeneration, where a large excess of regenerant (of which 60%–75% goes unutilized) is required to create the concentration driving force.

On account of their high affinity for the hydrogen ion, WARs can be used only at pHs above 4 or 5, as shown in Figure 3.5.

WAR exchangers differ from SARs in that WARs require the presence of some alkaline species to react with the more tightly held hydrogen ions of the resin, for example,

$$\text{Ca}{\left({\text{HCO}}_3\right)}_2+2\left(\text{R}-{\text{H}}^{+}\right)\rightleftharpoons \left(2{\text{R}}^{-}\right)-{\text{Ca}}^{++}+2\left({\text{H}}_2{\text{CO}}_3\right)$$

Figure 3.5   Capacity of weak-acid cation resin as a function of solution pH.

The exchange is, in effect, a neutralization with the alkalinity (HCO₃⁻), neutralizing the H⁺ of the resin. WAR will split alkaline salts but not nonalkaline salts (e.g., NaHCO₃ but not NaCl or Na₂SO₄).

Weak-acid cation resins are used to remove the cations associated with high alkalinity, that is, ${\text{CO}}_3^{=}$

, OH⁻, and ${\text{HCO}}_3^{-}$

, and low in dissolved CO₂ and sodium. WARs are used primarily for achieving simultaneous softening and dealkalization. They are sometimes used in conjunction with a strong-acid polishing resin, which allows for economic operation in terms of regenerant requirements, but also produces a treated water of quality comparable to the use of just an SAR.

Synthetic Anion Exchangers

Anion exchangers were developed almost exclusively as synthetic resins; organic exchangers were among the earliest ion exchange resins produced. The first patents issued for anion exchangers were for resins having weak-base amino groups. Later, resins with strong-base quaternary ammonium groups were prepared.

Anion exchangers are available with numerous fixed ionic groups. The most common are strong-base resins (SBRs) with quaternary ammonium groups (CH₂N(CH₃)₃Cl) and weak-base resins (WBRs) with tertiary amines in an aromatic or aliphatic matrix.

In general, strong-base anion exchangers operate at all pH values, but their capacity is less than stoichiometric, and they must be regenerated more frequently than WBRs, which exhibit much higher capacities and regenerate almost stoichiometrically. WBRs operate only over a limited pH range.

Strong-Base Anion Exchange Resins

SBRs are available. In Type I resins, the functional groups consist of three methyl groups:

$$-{\text{N}}^{+}--{\text{N}}^{+}{\left({\text{CH}}_3\right)}_3-$$

In Type II resins, an ethanol group replaces one of the methyl groups. Type I resins exhibit greater stability. Type II resins exhibit slightly greater regeneration efficiency and capacity.

Strong-base exchangers are so named because they have the ability to split strong or weak salts. This ability distinguishes them from WBRs, which cannot salt-split.

The reactions with sulfate and chloride and a strong-base anion exchanger in the hydroxyl form are

$$\begin{array}{c}{\text{SO}}_4^{=}\\ 2{\text{Cl}}^{-}\end{array}\}+2\text{R}-{\left(\text{OH}\right)}^{-}\rightleftharpoons 2\text{R}-\left(\begin{array}{c}{\text{SO}}_4^{=}\\ 2{\text{Cl}}^{-}\end{array}\right)+2{\left(\text{OH}\right)}^{-}$$

Figure 3.6 shows the same water treated by the hydrogen cycle exchanger as the one shown in Figure 3.4. The resin in this case is regenerated by sodium hydroxide.

In general, when placed in the hydroxide form, strong-base exchangers require an excess of regenerant (with typical efficiencies varying between 18% and 33%). Typically, high-quality sodium hydroxide is used as the regenerant, but again the regenerant used depends on the desired form in which the resin is to be used. A problem with SBRs is that they tend to irreversibly sorb humic acid substances, losing their capacity. Activated carbon or a weak-base organic trap is typically used to prevent fouling.

Figure 3.6   Strong-base ion exchange reactions.

Type I exchangers are typically used for maximum silica removal. They are more difficult to regenerate and swell more (from Cl⁻ to OH⁻ form) than Type II exchangers. The principal use of Type I is to make the highest quality water. When they are loaded with silica, they must be regenerated with warm NaOH (see resin manufacturer's literature.)

Weak-Base Anion Exchangers

The functional groups associated with weak-base anion exchangers are secondary and tertiary amines and are often based on phenol-formaldehyde or epoxy matrices instead of polystyrene-DVB. They do not remove anions above a pH of 6 (see Figure 3.7). They regenerate with a nearly stoichiometric amount of base (with the regeneration efficiency possibly exceeding 90%) and are resistant to organic fouling. In addition, they swell about 12% going from the OH⁻ to salt form, they do not remove CO₂ or silica, and they have capacities about twice as great as that of strong-base exchangers. They are useful, following strong-acid exchangers to save cost of regenerant chemicals, as organic “traps” to protect strong-base exchangers, and to remove color.

Weak-base anion exchange resins behave much like their weak-acid counterparts and will not remove CO₂, that is, weakly ionized acids such as carbonic and silicic acids. WBRs remove free mineral acidity (FMA) such as HCl or H₂SO₄, that is, anions associated with the hydrogen ion and that are strong acid formers such as sulfate, chloride, or nitrate. For this reason, WBRs are often called “acid adsorbers.” Any ions associated with cations other than hydrogen are not removed.

WBRs do not have a hydroxide ion form as do SBRs. Consequently, regeneration need not provide hydroxide ions, only neutralize the absorbed acid. Less expensive weakly basic reagents such as ammonia or sodium carbonate may be used, along with sodium hydroxide.

Once again, the regeneration efficiencies of these resins are much greater than those for SBRs. Weak-base exchangers are used in conjunction with SBRs in demineralizing systems to reduce regenerant costs and to attract organics that might otherwise foul the SBRs. Where silica removal is not critical, WBRs may be used by themselves in conjunction with an air stripper to remove CO₂ (see Chapter 5).

Figure 3.7   Capacity of weak-base anion resin as a function of solution pH.

The major advantage of weak-base anion exchange resins is that they can be regenerated with stoichiometric amounts of regenerant and are, therefore, much more efficient. They also have a much higher capacity for the removal of chlorides, sulfates, and nitrates. There are three typical weakly basic anion exchange resins: styrene-DVB, acrylic-DVB, and epoxy. Again, gel or macroreticular varieties of resins based on these raw materials are available and used. For treatment of waters that do not present organic fouling problems, the gel type of weakly basic resin is used.

For waters containing organic contaminants (humic and fulvic acids), macroreticular weakly basic anion resins are preferred. Since their development in the late 1960s, acrylic resins are beginning to be used more and are found to be more effective from an economic standpoint. Figure 3.8 shows a typical weak-base ion exchange application.

Order of Selectivity (Relative Affinity)

In general, the higher the valence of an ion, the higher its ability to be removed by a resin. For example, for a resin in the hydrogen form, if a divalent cation came along, it would be taken preferentially to the monovalent cation (H⁺) on the resin. If a trivalent cation came along, it would preferentially displace the divalent cation (in nearly all circumstances).

Now imagine the ion exchange material is in the hydrogen form. If a sodium ion is introduced, the sodium ion would displace the hydrogen ion, that is, the sodium ion would stay on the resin and the hydrogen ion would be carried out. If we then introduce a calcium ion, the calcium ion would displace two sodium ions and the calcium ion would stay on the resin. If we then introduce aluminum ions, two aluminum ions would displace six calcium ions. Table 3.2 shows the order of selectivity of a typical strong-acid resin. Table 3.3 shows the order of selectivity of a typical strong-base resin.

In the design of ion exchange systems, one must be careful about the cycle in which one chooses to operate the resin, because this will determine whether one gets the job done.

“Changing the degree of cross-linking does not change the order of selectivity but does change relative affinities.”

Figure 3.8   (a) Weak-acid and (b) weak-base ion exchange reactions.

As shown in Table 3.4, the higher the number, the easier for that ion to displace ions lower than it. For example, to remove Ag from a wastewater stream, use Dowex cation resin with 16% cross-linking. If there is any sodium in water, with a selectivity of 2.37, Ag will easily displace it and the Ag is monovalent. If you are trying to remove a specific ion, you want the highest spread you can get, and if you can get different levels of cross-linking to do that, even better. A minimum selectivity delta of two is required for most exchanges.

This order varies with different resins (depending on the manufacturer), but usually the order is shifted by only a few elements. Trivalent replacing divalent and divalent replacing monovalent are always the same.

Table 3.4 also shows the relative affinity of selected cation resins as a function of valence and cross-linking with DVB at 4%, 8%, and 16%. Lithium is assigned a value of 1.00. Table 3.5 presents the relative selectivity of selected anions for Dowex strong base anion resin. Chloride is assigned a value of 1.00. Contact the resin manufacturer you have chosen for the relative exchange order of a particular resin.

Use the following rules as a guide to understanding the relative selectivity patterns of exchange.

1. At low aqueous concentrations and ordinary temperatures, the extent of exchange or the exchange potential increases with increasing valence of the exchanging ion Na⁺ < Ca⁺⁺ < Al⁺⁺⁺
2. At low aqueous concentrations, ordinary temperatures, and constant valence, the exchange potential increases with increasing atomic number. Li < Na < K; Mg < Ca < Sr < Ba; F < C1 < Br < 1
3. At high concentrations, the differences in exchange potentials of ions of different valence diminish and, in some cases, the ion of lower valence has the higher exchange potential. Na⁺ vs. Ca⁺⁺
4. The exchange potentials of various ions may be approximated from their activity coefficient—the higher the activity coefficient the greater the potential.
5. The exchange potential of the hydrogen ion or the hydroxyl ion depends on the strength of the acid or base formed between the functional group, and the hydrogen or hydroxyl ion. The stronger the acid or the base, the lower the exchange potential.

Selectivity Determination

Select the resin (see Chapter 5). Put the resin in the lithium cycle. Run sodium ions through and monitor how much sodium stays on the resin and how much runs through. Do this until sodium-in is equal to sodium-out. Analyze the resin to determine how much sodium is on the resin. Repeat the same for every ion. Compare the percentage of material retained as a percentage of lithium (e.g., retains twice as much lithium, i.e., 2.0).

Backwash

Backwash is carried out with a new installation and after each “run.” Water is introduced through the bottom of the ion exchange column at a rate sufficient to expand the bed by 50%–75%. The primary purpose of the backwash step is to remove any silt, dirt, iron, or other insoluble matter that has accumulated during the exchange operation. Also, any clumps of resin formed because of tight packing are dispersed, and the resin beads are reoriented. Most of the problems associated with ion exchange operation occur because of insufficient backwash.

Low-pressure membrane pretreatment to an ion exchange accomplishes many things, the most significant is the significant reduction of all particulate matter including silt, clays, insoluble iron, and microorganisms. Dissolved iron and manganese can be oxidized and removed to levels less than 0.05 mg/L. In all cases, the economics of adding low-pressure membrane pretreatment should be considered. With proper pretreatment, the purpose of backwash is now to reorient the resin and remove any resin particles, which is less because the cause of resin degradation is particulate matter abrading the softer resin. Resin life is also increased.

Figure 3.13 provides hydraulic expansion data for Amberlite^® IR-120 Plus. It shows bed expansion in percent as a function of temperature and flow rate in gpm/ft². Typical flow rates are upflow at 5–7 gpm/ft² of exchanger cross-sectional area.

Regeneration

Although the columnar ion exchange process consists of the basic steps of backwash, regeneration, rinsing, and service, the actual process is regeneration with the appropriate backwash and rinsing steps, and the service cycle. A more in-depth discussion of regeneration is in order.

Regeneration requires 5%–15% of the treated water. Typical recovery of an ion exchange process varies from a low of 75% to 90%. The reader can calculate the recovery of a process using the principles described in this chapter. The regeneration process consists of three steps:

Figure 3.13   Hydraulic backwash data. (From Rohm and Haas, Amberlite® IR-120 Plus Specification Sheet, January 1982. With permission.)

1. Backwashing: Wash out any dirt in bed.
2. Regeneration: Actual chemical addition.
3. Rinsing: Rinse out regeneration chemicals.

The regeneration process uses only 5% of the water treated, and thus all of the ions removed will be concentrated in about 5% of the water (20 times as concentrated).

Recall that the ion exchange resin looks very much like a whiffle ball with exchange sites on the inside and the outside. If insoluble compounds, such as calcium sulfate, are formed upon regeneration, it can block or plug the internal exchange sites.

Regeneration or brining follows backwash and is the displacement of the ions held on the resin sites. These ions were removed from the process feed water during the service cycle. If the column contains an SAR regenerated with an acid for example, hydrogen ions (H⁺) are exchanged onto the resin in place of those released. This resin is now in the hydrogen form. If the column is an SBR and is regenerated with sodium hydroxide, hydroxyl ions (OH⁻) are exchanged on the resin. It is then in the hydroxyl form. The regeneration step can be accomplished in either a downflow or upflow mode. When softening water, or using the column in the sodium cycle, the regeneration step is often referred to as “brining.” In most cases, concentrated sodium chloride (NaCl) is passed through the bed to drive the calcium and magnesium (softening), or other metals (industrial uses) off of the resin. Again, brining can be accomplished in either an upflow or downflow mode. Table 3.8 provides typical regeneration data for regenerating a Rohm and Haas strong-acid ion exchange resin. All resin manufacturers provide regeneration data for their resins used in standard applications. If the reader chooses to use a resin for a nonstandard application, the regeneration requirements can be determined using software and/or the bench-scale procedures in Chapter 4.

Resin specifications show greater resin capacity when HCl is used versus H₂SO₄. As an example, you might get 22 Kgr/ft³ using HCl and only 18 Kgr/ft³ using H₂SO₄. Any calcium exchanged onto the resin—is eluted during regeneration as CaSO₄, which has a very limited solubility of only about 2200 mg/L. Thus, if there is more than 110 mg/L Ca in the raw water, there will probably be a solubility problem with H₂SO₄ regeneration. CaCl₂ is almost infinitely soluble, so there is no problem with HCl.

When using a sodium cycle system to soften water containing iron, resin capacity decreases over time. This is because iron is being exchanged onto internal sites and becoming oxidized to the ferric form (insoluble), thus blocking exchange sites. This effect can be minimized by more frequent regeneration, so that the iron has less time to oxidize and block the resin. Even with frequent regeneration there will still be some loss of capacity. The resin manufacturer will recommend that once or twice per year you regenerate with sodium chloride mixed with sodium bisulfite. The sodium bisulfite is such a strong reducing agent that it will reduce the insoluble ferric iron to soluble ferrous form so that you can clean up the bed. Usually 1 or 2 oz. of sodium bisulfite per ft³ of resin is sufficient. Loss of resin capacity attributed to iron fouling is all but eliminated with low-pressure membrane pretreatment that reduces iron and manganese to typically less than 0.05 mg/L.

Rinse

Rinsing follows regeneration. The purpose is to remove excess regenerant prior to putting the unit into service. This is usually conducted in a downflow mode. Softening operations involve a displacement rinse, a fast rinse, and a final rinse. After brining, the resin bed is full of brine. During the displacement rinse, the brine (concentrated NaCl) is slowly forced through the resin bed. The objective is to obtain optimum contact. The displacement rinse is always done at the same rate and direction as brining. Raw water is used for all rinsing, except in certain deionization operations. A high flow rate is used to remove any residual brine from the resin bed. The fast rinse is usually in the same direction as the displacement rinse (see Table 3.8).

Service

Service is the operational mode of the system. This step follows rinsing. Process water, for example plating rinse water, is passed through the ion exchanger to remove cations or anions. The end of the service run is detected by a sharp increase in the cation or anion level of the effluent. In most industrial applications, an acceptable or predetermined level of cations or anions in the effluent is set. When this level is reached, “breakthrough” has occurred. The unit is then taken off-line and is backwashed, regenerated, and rinsed before being put back into service (see Table 3.5 and Figure 3.14).

Pretreatment of process water prior to ion exchange is crucial to prevent fouling or damage to the resin. In the first edition of this book, the author recommended cartridge filtration and granular activated carbon pretreatment. “At a minimum, the water must be filtered to remove suspended solids. Cartridge filters work best because they do not require backwashing (another side stream requiring disposal). Organic substances in the process water can foul most resins and should be removed by pretreatment with activated carbon. Fouling occurs when organics in the water affix themselves to the resin by occupying the exchange sites. This phenomenon decreases the capacity of the resin. Strong oxidizing agents (e.g., chlorine) degrade most resins. These agents must be removed or destroyed by a pretreatment step.”

Figure 3.14   Pressure drop versus flow rate. (From Rohm and Haas, Amberlite® IR-20 Plus Specification Sheet, January 1982. With permission.)

The author wrote in Ion Exchange Treatment for Drinking Water (2004), “Today's thinking is to not use ion exchange resins at $50 per cubic foot and higher as filtration media, but to properly pretreat the water using, for example, hollow fiber microfiltration to remove all particulate and microorganisms, so that the exchanger does its job by exchanging dissolved ionic species, to provide particle free water to the ion exchanger and only use the exchanger to remove dissolved ionic species. Clarifiers, multimedia filters, and cartridge are often replaced with hollow fiber microfiltration systems capable of treating source water of any quality to turbidity levels less than 0.1 ntu, nonmeasurable suspended solids, iron less than 0.05 mg/L, and manganese less than 0.02 mg/L. Microfiltration removes practically all inorganic foulants, and all microorganisms are removed, reducing resin fouling and allowing for infrequent backwashing and regeneration. Low pressure membranes for particulate removal followed by granular activated carbon for organics reduction and oxidizing agent reduction or a scavenging SAR—when appropriate—should be considered. Nanofiltration or reverse osmosis can be used to further concentrate regenerant brines and in some cases to recover or reuse brines.”