Ion exchange

Ion exchange is the process of exchanging ions of a solid matrix (ionite) with ions of a solution.

Ion exchange is one of the main methods of water purification from ionic pollution, water softening, desalination and deep desalination. The presence of ion-exchange materials of various properties makes it possible to solve the problems of water purification of various chemical compositions. This is one of the few technological methods that make it possible to selectively extract some components from a solution, for example, hardness salts.

Ion exchangers are solid insoluble substances containing functional groups capable of ionization in solutions and exchange of ions with electrolytes.

Ionites are divided according to the properties of functional groups into several types.

The main types of ion exchangers:

– cation exchangers;

– anion exchangers;

– selective ion exchangers.

Cation exchangers are ion exchangers with anion-exchange groups or anions fixed on the matrix, which exchange cations with the external environment.

Anion exchangers are ion exchangers with cation-exchange groups or anions fixed on the matrix, which exchange anions with the external environment.

The exchange reaction on cation exchangers (1) and anion exchangers (2) can be described by the following equations:

R ̶ H+ + Na+ ↔ R ̶ Na+ + H+ (1);

R ̶ OH– + Cl- ↔ R ̶ Cl– + OH– (2).

As a rule, these processes follow one after another, and the solution is fed to the anion exchanger after cationization. This solution is acidic. As a result of anionization, the purified solution has a neutral reaction (3):

R ̶ OH– + Cl– + H+ ↔ R ̶ Cl– + Н2О (3).

Thus, it is possible to describe the process of water desalination, which is carried out using cation exchangers in the H-form and anion exchangers in the OH-form. As a result, almost all cations and anions are extracted from water.

The ionization process proceeds until the ion exchanger is depleted – there are no functional groups capable of exchanging on the ion exchanger matrix.

To restore the exchange properties of ion exchangers, it is necessary to perform regeneration. In the case of water desalination, solutions of acids (hydrochloric, sulfuric) are used for the regeneration of cation exchangers, and a solution of sodium hydroxide is used for the regeneration of anion exchangers. The regeneration of cation exchangers (4) and anion exchangers (5) can be described by the following equations:

R ̶ Na+ + HCl ↔ R ̶ H+ + NaCl (4);

R ̶ Cl– + NaOH ↔ R ̶ OH– + NaCl (5).

As a result of regeneration, the ion exchangers go into working condition, and the regeneration solution is a mixture of salt solutions with an excess of the regenerating agent (acid and alkali).

 

In the case of water softening, ionization is carried out on a cation exchanger in the Na-form. In the process, hardness ions are removed from the water and replaced with sodium ions. The process is described by the following equation (6):

2R ̶ Na+ + Са2+ ↔ R ̶ Са ̶ R + 2Nа+ (6).

Salt solution is used for regeneration. The process proceeds according to reaction (7):

R ̶ Са ̶ R + 2NаСl ↔2R ̶ Na+ + CaCl2 (7).

The main characteristics of ion exchangers are:

– exchange capacity;

– selectivity;

– mechanical strength;

– osmotic stability;

– chemical stability;

– temperature stability;

– grading.

Ion exchange equipment.

For water treatment and wastewater treatment, devices of periodic action such as a filter with a continuous layer of ion exchanger are most often used.

In the direction of movement of the cleaned and regenerating solutions, they are divided into direct-flow (parallel-flow) and counter-current (counter-flow).

Advantages of straight-through filters:

– simplicity of design;

– ease of operation;

– the possibility of washing the ion exchanger directly in the filter.

Disadvantages of straight-through filters:

– poor water quality;

– low degree of regeneration and, as a consequence, an increase in the consumption of reagents;

– a small degree of use of volume.

Benefits of counterflow filters:

– high quality of water;

– a high degree of regeneration and a decrease in the consumption of reagents;

– high degree of use of the filter volume.

Disadvantages of counterflow filters:

– higher price due to more complex design;

– high requirements for pre-cleaning.

Application of ion exchange technologies:

  1. Water softening.
  2. Reduction of alkalinity and temporary hardness of water.
  3. Water desalination.
  4. Deep water desalination (mixed action filters).
  5. Removal of nitrates from water.
  6. Removal of organic matter.