Cation and anion resins (ion exchange)

Ion exchange

The process of ion exchange has been used since ancient times. With the help of this process, Aristotle Thales brought salt water into contact with soil and produced fresh water.

In 1845, Thomson showed that whenever water containing ammonium sulfate passed through soil, calcium sulfate would be produced in the effluent.

(Calcium soil) NH4(2SO4 + CaZ) → NH4(2Z + CaSO4)

This discovery marked the beginning of the development of ion exchange science, and in the second half of the 19th century it was shown that compounds of alkaline earth metals, silica, and aluminum, called alum inosilicates,

(Al2O3 / mNa2O / xSiO2 / yH2O)

They are capable of ion exchange. The minerals with the highest ion exchange capacity were diatomaceous earth and a type of green sand called zeolite.

In 1930, the first ion exchangers were made from the reaction of anthracite and sulfuric acid, which became known as sulfonated coal. The maximum capacity of these ion exchangers is about 0.7eq kg-1. Then, polymer ion exchange resins were made from a combination of phenol or its derivatives with formaldehyde, which had a capacity of about 1.2eq kg-1. In the continuation of these studies, various resins were made, and finally in 1945, with the help of copolymerization of styrene and divinylbenzene, spherical weak and strong cationic and anionic resins were prepared, the capacity of these resins sometimes reaching several times that of previous types.

Ion exchange phenomenon

Ion exchange resins are water-insoluble solids that can be used to adsorb cations and anions by ion exchange. The ion exchange phenomenon is a combination of surface adsorption and diffusion processes, and the reaction rate is determined by the mass transfer operation that transports ions from the fluid to the resin surface or from the resin surface to the fluid.

Ion exchange is a chemical equilibrium and follows the principles governing equilibria. Ion exchange also occurs in electrolytes, but due to the homogeneity of the ions, in which the ion exchangers are both liquids, the effective factor is chemical kinetics, while in ion exchange, it is due to the presence of electrostatic forces, and the exchange power depends on these forces, the amount of which is shown based on stoichiometric calculations, and in such a way that the materials before and after ion exchange will be electrically neutral.

Some cationic resins have an active carboxyl group (─COOH) that only ion-exchanges with cations of weak acids. For example, if all the alkalinity of bicarbonate water is due to calcium and magnesium salts, then the ion exchange is shown as follows.

Ca(HCO3) + H2R → CaR + 2CO2 +2H2O

Mg(HCO3)2 + H2R → MgR + 2CO2 + 2H2O

CaSO4 + H2R → Ion exchange does not occur

MgCl2 + H2R → Ion exchange does not occur

The water produced has much lower hardness. The advantages of this type of resin are:

a) High capacity

b) Regeneration with less and more dilute acid

c) Less water required for rinsing

Cation and anion resins are used to produce ion-free water. These resins are produced in weak and strong types. Strong cation resins have the ability to remove all cations.

H2R + CaCl2 ↔ CaR + 2HCl

H2R + 2NH2NO3 ↔ (NH4)2R + 2HNO3

H2R + K2R + H2SO4 ↔ K2R + H2SO4

H2R + Na2SO4 ↔ Na2R + H2SO4

Strong anions present in water also exchange ions with anion exchange resins. The anion exchange process is shown below.

R(OH)2 + H2SO4 ↔ SO4R + 2H2O

R(OH)2 + 2HCl ↔ Cl2R + 2H2O

R(OH)2 + 2HNO3 ↔ (NO3)2R + 2H2O

Cationic resins

Cationic resins were prepared by reacting phenol or its derivatives with formaldehyde and then sulfonating them with sulfuric acid to form a mass that was crushed, sieved, and used. These resins were also prepared from the sulfonation of polystyrene.

Whenever a certain amount of divinylbenzene is added to the polymerization of styrene, a polymer with a network structure is produced, which, upon sulfonation, produces a strong cationic resin.

By changing the amount of divinylbenzene, the degree of cross-linking and, consequently, the porosity and stability of the resin are controlled. Increasing cross-linking reduces porosity and, consequently, increases the temperature resistance of the resin.

To produce spherical beads, the monomer must be suspended in water with the help of a mixer and copolymerization is carried out under these conditions. Strong cationic resins can exchange ions with all cations present in water.

When the sulfonic acid group is replaced by a carboxylic acid group (─COOH), weak cationic resins are produced, but a simpler method for preparing these resins is to combine methacrylic acid with divinylbenzene. Typically, the capacity of weak cationic resins is greater than that of strong types, sometimes reaching twice the capacity.

Anionic resins

Anionic resins have basic groups instead of acid groups. Basic groups are created from ammonia or an amine, and for better conditions, other compounds such as chloromethyl groups can be used and then the reaction is completed with ammonia or amine.

a) By reacting with ammonia, an anionic resin produces a primary amine.

b) By combining with a primary amine, a secondary amine resin is produced.

c) By reacting with a secondary amine, a tertiary amine resin is produced.

d) By reacting with a tertiary amine, an ammonium anion resin is produced.

Resins with primary, secondary, and tertiary amine basic groups are weak and have the ability to exchange ions with anions of salts or strong acids, but they do not exchange ions with anions of salts or weak acids.

SO42- + Weak anion resins ↔  Ion exchange takes place

 NO3– + Weak anion resins ↔  Ion exchange takes place

Cl– + Weak anion resins ↔  Ion exchange takes place

HSiO3– + Weak anion resins ↔  Ion exchange takes place

HCO3– + Weak anion resins ↔  Ion exchange takes place

Ammonium resins are strong and have the ability to exchange ions with all anions, even carbonic acid-based and silicic acid-based.

HSiO3– + Strong anion resins ↔  Ion exchange takes place

HCO3– + Strong anion resins ↔  Ion exchange takes place

Weak anion resins are typically regenerated with sodium carbonate or caustic soda, but they can be regenerated with most alkalis.

RCl2 + Na2CO3 +H2O → R(OH)2 + 2NaCl + CO2

The regeneration of strong anion resins is done with sodium hydroxide.

RCl2 + 2NaOH → R(OH)2 + 2NaCl

 Mixed bed columns ion-exchange resins

The removal of cations and anions in water is carried out simultaneously by a mixture of cationic and anionic ion exchange resins. In this process, the mixture of resins is placed in a column and the water passes through the resin bed, and the outlet water will have a very low T.D.S.

The regeneration of cationic and anionic resins is carried out in several stages. In the first stage, the resins are mechanically separated from each other, then the resins are regenerated, and in the final stage, the two cationic and anionic resins are mixed and used.

Advantages of the ion exchange method

Compared to other methods, the ion exchange method has the following advantages.

a) Ease of installation and operation.

b) High useful life of the resins (under proper operating conditions, the life of the resins is more than 15 years).

c) Ability to be implemented in different capacities.

d) No polluted wastewater for the environment.

Limitations of the ion exchange method

Along with the advantages of the ion exchange method, there are also some limitations as follows:

a) It is not cost-effective in the range of T.D.S. > 700 ppm.

b) This method is common for industrial units and is not usually used for drinking water.

c) The chemicals consumed are more than all methods and the produced water is more expensive.

d) The temperature of the ion exchange process should not be lower than 10 ◦C.

e) The maximum temperature of the ion exchange process is limited based on the type of resin and the coating of the ion exchange column.

c) Non-ionized substances are not capable of ion exchange.

f) The permissible limit of iron, manganese and heavy metals in total should be less than 0.1 ppm.

h) The water passing through the ion exchange resins should be free of suspended salts, colloidal substances, fat and organic matter.

h) The permissible limit of free chlorine (ClO–) in the passing water is 0.05 ppm.

d) The permissible turbidity limit is 5 A.P.H.A. Units.