ROLE OF ADSORBENTS IN ION–EXCHANGE ADSORPTION

Role of adsorbents in Ion – exchange adsorption

Definition

Ion – exchange adsorption is defined as, “the process of releasing”, the one kind of ion and adsorbing another same kind of ion by an adsorbent”.

Classification of ion-exchangers

1. Cation exchange resin or cation exchanger or Decationizer resin.
2. Anion exchange resin or anion exchanger or Deanionizer resin.

1. Cation exchanger or Decationizer resin :
These resins are insoluble in water. When water containing salts like MgCl2, CaCl2 is passed through the cation exchanger, it exchanges cations with other cations present in the hardwater. The resins are made from styrene divi.,benzene,copolymer containing, acidic functional groups like sulphonic(-SO3H) acid. It is represented as RH+.

Definition :

It is defined as cross – linked polymer containing sulphonic group as integral part of resin and an equivalent amount of H+ ions.

Process :

When the solution, containing certain Ca2+, Mg+ is allowed to pass through it H+ ions are exchanged for cations present in solution.

2RH+ + Ca2+ --> R2 Ca2+ + 2H+
2RH+ + Mg2+ --> 2RMg2+ + 2H+
This process consists of desorption of H+ ions and adsorption of Ca2+, Mg2+ ions by the resin.

2. Anion exchanger or Deanionizer resin

When water is passed through anion exchange column, the resin is capable of exchanging their anions with other anions. The resin consists of quaternary ammonia hydroxide groups (-NR3OH) or amino group etc., on the benzene rings of styrene – divinyl benzene copolymer. This may be represented as R’OH_.

Definition :

Anion exchange is defined as “Cross-linked polymer containing amino group as integral part of resin and an equivalent amount of OH_ ions.

Process :

When the solution of another anion is allowed to pass through it, OH_ ions are exchanged by anion present in the solution.
R+ OH_ + Cl_ --> R+Cl_ + OH_

It also involves desorption of OH_ ions and adsorption of Cl_ ion by the resin.

Regeneration

When the cation exchange resin is exhausted it can be regenerated by passing a solution of dil. HCl.
R2 Ca 2+ 2H+ --> 2RH+ + Ca2+

When the anion exchange column is exhausted it can be regenerated by passing a solution of dil.NaOH.
R+ Cl_ + OH_ --> R+ OH_ + Cl_

Applications of Ion – exchange Adsorption

a. In this process not only hardness causing salts are removed, but also salts of trace metal from the water. E.g. Zeolite process and demineralization process etc.
b. Complex metal ions such as copper and nicked also can be removed from water used in industries.
c. Ion exchange technique can be applied for concentration of uranium in pitchblende ore.