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 tion 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. These resins merely sorb strong acids: they cannot split salts.
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.