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