Ion exchange reactions

Introduction

This page shows the general concept of ion exchange equilibrium, selectivity coefficients, and typical reactions of the different types of ion exchange resins.

Equilibrium

An ion exchange resin in the ionic form A is in contact with a solution containing an ion B, an equilibrium reaction is observed. Here an example of cation exchange:

R-A⁺ + B⁺ R-B⁺ + A⁺

In the ion exchange process, ions A will migrate into the solution and be replaced by ions B from the solution until equilibrium is reached. This equilibrium is described by the mass action law as follows:

where [..] denotes concentrations, and γ activity coefficients, and where the bar indicates the resin phase. K is the selectivity coefficient of B⁺ over A⁺ for the ion exchange reaction:

As a first approximation, the activity coefficients are considered constant, so that the simplified selectivity coefficient is:

For the most current reaction between a strongly acidic resin in the H⁺ form and sodium ions (Na⁺) in water, equation (1) becomes:

R-H⁺ + Na⁺ R-Na⁺ + H⁺

And the corresponding selectivity coefficient is

The value of coefficient K(^Na/_H) is about 1.7 for a standard, gel type sulphonic strongly acidic exchange resin (see the table of selectivity coefficients). This means that if a resin with a known composition of Na and H ions is placed in pure water, sodium ions will migrate into the water until sodium concentration reaches the equilibrium determined by equation (6).

For divalent ions, e.g. (softening reaction):

2 R-Na⁺ + Ca⁺⁺ R₂-Ca⁺⁺ + 2 Na⁺

The selectivity coefficient is:

Regeneration

Regeneration consists of reversing the equilibrium in increasing the concentration of the ion displaced in the equilibrium reaction. For instance, a softener is regenerated by using a high concentration of [Na⁺] ions on the right side of reaction (7). This causes the reaction to be shifted to the left.

Column operation

In normal practice of ion exchange, the resin is not left at equilibrium with the solution. Instead, the solution is withdrawn at the outlet of the column so the equilibrium is permanently shifted to the right. Look again at equation (1):

R-A⁺ + B⁺ R-B⁺ + A⁺

The treated solution — enriched in ions A⁺ and depleted of ions B⁺ — is carried away and replaced by untreated solution in which the ions A⁺ have a low concentration while [B⁺] is high. This allows the resin to remove more B⁺ ions from the solution, because the equilibrium is displaced to the right.

Let us now look at the specific behaviour of common ion exchange resins.

Strongly acidic resins

When the resin is initially in the hydrogen form H⁺ (R-SO₃^–H⁺, abbreviated here as R-H), it can remove all cations from solution:

R-H + Na⁺Cl^– R-Na + H⁺Cl^–

Second example with calcium bicarbonate:

2 R-H + Ca⁺⁺(HCO₃^–)₂ R₂-Ca + 2 H₂CO₃

Here the reaction is a neutralisation, as the bicarbonate anion is alkaline. It is not reversible, as you cannot regenerate a SAC resin wiith carbonic acid.

Reactions (9) and (10) are common as a first demineralisation step. See decationisation.

When the resin is initialy in the sodium form Na⁺, SAC resins remove di-valent cations from water, but not other monovalent catinons, because the selectivity diifference with K⁺ or NH₄⁺, for instance, is too small.

2 R-Na + Ca⁺⁺(HCO₃^–)₂ R₂-Ca + 2 Na⁺HCO₃^–

This softening reaction is reversible, but you would not regenerate with sodium bicarbonate, because of the risk of calcium carbonate precipitation. Instead, sodium chloride is used.

Weakly acidic resins

These react differently, as they are only weakly ionised in regenerated (H⁺) form. With neutral salts, no reaction is observed:

R-COOH + Na⁺Cl^– nothing

Here an explanation: assuming that the reaction would take place, it would create HCl as a product. But as the resin is a weaker acid as the hydrochloric acid product, the resin would immediately return to its acidic form.

For the weak acid resin to react, it must be dissociated. Therefore, the H⁺ ion in the acid must be taken away by an alkali. For instance, the following reaction is immediate and irreversible:

R-COOH + Na⁺OH^– R-COO^–Na⁺ + H⁺OH^–

This is a neutralisation reaction in which the product is water (hence the irreversibility).

In the following reaction:

2 R-COOH + Ca⁺⁺(HCO₃^–)₂ (R-COO^–)₂Ca⁺⁺ + 2 [H⁺HCO₃^–]

the bicarbonate ion "rips off" the H from the carboxylic group so that it becomes ionised. Weak acid cation resins are used in water treatment to remove bicarbonate hardness from water. This is the de-alkalisation process. See the complex formed in the resin structure page. With sodium bicarbonate instead of calcium or magnesium bicarbonate, the reaction also occurs, but the WAC resin has much less affinity for monovalent ions, so that the operating capacity is low:

R-COOH + Na⁺HCO₃^– R-COO^–Na⁺ + H⁺HCO₃^–

Once converted to the Na⁺ form, divalent ions can be exchanged even in the absence of alkaline anions:

2 R-COO^–Na⁺ + Ca⁺Cl^–₂ (R-COO^–)₂Ca⁺⁺ + 2 Na⁺Cl^–

Softening with a WAC resin is very efficient due to the strong selectivity of these resins for hardness ions. The residual hardness in the treated water is very low, so that the process described by reaction (16) is used to soften water in presence of a high sodium concentration background. In practice, the WAC resin is converted to the Na⁺ form using soda ash (Na₂CO₃) or caustic soda (NaOH) before it is used for softening.

Conclusions: in normal use (H-cycle, regeneration with HCl) the WAC resin removes only hardness, and only when alkalinity is present; for softening (Na-cycle) the WAC resin must first be converted to the Na form with an alkali.

Strongly basic resins

When the resin is initially in the hydroxide form OH^– (R-CH₂-N(CH₃)₃⁺OH^–), abbreviated here as R-OH), it can remove all anions from solution:

R-OH + Na⁺Cl^– R-Cl + Na⁺OH^–

Regeneration of SBA resins is done by reversal of reaction (17) with a relatively concentrated solution of caustic soda.

After cation exchange [reaction (9)], the reaction is:

R-OH + H⁺Cl^– R-Cl + H⁺OH^–

This is a neutralisation reaction of a strong acid in which the product is water. Therefore, it is irreversible.

The OH^– form SBA resin also exchanges weak acids, such as carbonic acid:

R-OH + H⁺HCO₃^– R-HCO₃ + H⁺OH^–

A second reaction occurs as long as there are OH^– ions on the resin:

R-HCO₃ + H⁺HCO₃^– R₂-CO₃ + H⁺OH^–

The carbonate ions sit on two functional groups of the resin. Note that in practice, OH^– form resins are not used to remove bicarbonate or carbonate from neutral water when it contains hardness, for fear of precipitating calcium hydroxide or calcium carbonate.

The resin also reacts with very weak acids, such as silica (SiO₂) or boric acid (H₃BO₃). In the following reaction, silica is represented as H₂SiO₃:

R-OH + H⁺HSiO₃^– R-HSiO₃ + H⁺OH^–

When the SBA resin is initially in the chloride form Cl^–, it can exchange any anions for which it has a higher selectivity (see the selectivity table):

R-Cl + NO₃^– R-NO₃ + Cl^–

Reaction (21) represents the nitrate removal process, for which special SBA resins with different functional groups are used with an increased nitrate selectivity.

Weakly basic resins

The most common functional group is R-CH₂-N(CH₃)₂. These resins are not ionised in their regenerated free base form. Therefore they do not react with neutral salts:

R-CH₂-N(CH₃)₂ + Na⁺Cl^– nothing

For WBA resins to react, they must first be ionised. Only a strong acid can protonise the amine, which is transformed into a quaternary ammonium. In many cases, the reaction takes place after decationisation of the water or the solution to be treated (9):

R-CH₂-N(CH₃)₂ + H⁺Cl^– R-CH₂-NH⁺(CH₃)₂⁺Cl^–

The uptake of the whole hydrochloric acid compound converts the WBA resin to its hydrochloric (.HCl) form. Similar reactions take place wiith sulphate and nitrate after cation exchange.

The alkalinity of the WBA resin is not strong enough, however, to protonise the active group with weak acids, so that these resins cannot remove silica or carbon dioxide from water:

R-CH₂-N(CH₃)₂ + H⁺HCO₃^– nothing

Weakly basic resins are used for the removal of strong acids only, but they have a higher capacity and are easier to regenerate than SBA resins.

Chelating resins

These resins, with several different functional groups (see examples of selective resin types), are able to form complexes with metals. See also an example of the complex in the resin structure page.

Other information

• Selectivity (general).
• Selectivity tables for cation and anion exchange resins.
• Water treatment processes for practical use of the above reactions.
• Ion exchange applications (water treatment and many other areas).
• Various resin types with examples.
• Ion exchange resin structure in general.
• Mendeleev table with information about the removal of several ions with resins.