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Ion exchange resins are complex organic molecules having a giant hydrocarbon framework. Ion exchange resin is of two types: cation exchange resin and anion exchange resin. The ion exchange resins containing acidic groups are called cation exchange resins (e.g., R-COOH or R-So3H). The ion exchange resin containing basic groups are called anion exchange resin (e.g., R-NH3OH).
Ion Exchange Resin Method
In the Ion Exchange Resin method, resins are used, which can completely remove the cations and anions present in the hard water. The resins used are generally organic, cross-linked and insoluble polymers with some functional groups responsible for the ion-exchange properties.
Hard Water and Soft Water
Water is soft water if it produces sufficient lather with the soap. Water is hard if it forms an insoluble scum before it forms lather with soap.
The formula of soap is C17H35COONa. The ions of calcium and magnesium are responsible for the hardness of water and the formation of scum in hard water. The ions of Sodium (Na) will remain present in the water in the dissolved form.
The chemical reaction for the formation of scum will be as below:
2(C17H35COONa) = (C17H35COO)2Ca + (C17H35COO)2Mg
The hardness of natural water is generally caused by the presence of bi-carbonates[Ca(HCO3)2, Mg(HCO3)2 ], chlorides[CaCl2, MgCl2] and sulphates [CaSo4, MgSo4] of calcium and magnesium but in-fact soluble salts that form a scum with soap cause hardness.
It is important to keep in mind that bi-carbonates[Ca(HCO3)2, Mg(HCO3)2 ], chlorides[CaCl2, MgCl2] and sulphates [CaSo4, MgSo4 ] are not responsible for the hardness of water but the ions of Calcium and Magnesium (Ca2+ and Mg2+) are responsible for the hardness of the water.
Temporary hardness can also be removed by Clark’s process, which involves the addition of slaked lime Ca(OH)2.
Ca(HCO3)2 + Ca(OH)2 = 2CaCo3 + 2H2O
The hardness of water is of two types:
a) Temporary Hardness: This is due to bicarbonates of calcium and magnesium. The temporary hardness of water can be easily removed by boiling water as the bicarbonates decompose readily and the insoluble carbonates are precipitated.
Ca(HCO3)2(Heat) = CaCo3 + H2O +CO2
Mg(HCO3)2 (Heat) = MgCo3 + H2O +CO2
b) Permanent Hardness: Hardness of water due to chlorides and sulphates of Ca2+ and Mg2+ is called permanent hardness. The boiling process cannot remove this type of hardness of the water. To remove permanent hardness, some suitable chemical substances are used. These substances are named water softeners. Some of the important water softeners are:
(i) Washing Soda
(ii) Calgon/ sodium hexametaphosphate (NaPo3)6
(iii) Permutit method
(iv) Ion exchange resin
Ion Exchange Resin Method for Removing Hardness of Water
The ion exchange resin technique can remove both cations and anions that cause hardness. This process contains two chambers.
- a) Cation Exchange Resin: These are called RH+ ions. This resin contains granular insoluble organic acid having giant molecules with the COOH group. Cation exchange resins have functional groups such as – COOH, SO3H etc., which can exchange their H+ ions with the cations present in the hard water. E.g., Styrene divinylbenzene-copolymers
R-COOH + Ca2+ = (R-COO)2Ca + 2H+
R-COOH + Mg2+ = (R-COO)2Mg + 2H+
- b) Anion Exchange Resin: These are called ROH- This resin contains a giant organic molecule with basic groups derived from amines R-NH3+OH-. Anion exchange resins have functional groups such as hydroxides or hydrochlorides, which can exchange their anions with other anions in hard water. E.g., Styrene divinylbenzene or amine formaldehyde copolymers
R-NH3+OH- + Cl- = R-NH3Cl + OH-
R-NH3+OH- + So42- = (R-NH3)2So4 + OH-
Note: H+ ions obtained from cation exchange resin react with OH- ions of anion exchange resin. Hence, the neutralisation process takes place.
H+ + OH- = H2O (Netural)
Recharge of Ion Exchange Resin:
When cation and anion exchangers are exhausted, regeneration should be done. The exhausted cation exchange resin is regenerated by reaction with dilute hydrochloric acid.
(R-COO)2Ca + 2HCl = 2 R-COOH + CaCl2
The exhausted anion exchange resin is regenerated by reaction with dilute NaOH.
(R-NH3)2So4 + 2 NaOH = 2 R-NH3OH + Na2So4
Mixed–bed Deionizer
A single column containing cation and anion exchanger resins are taken in this process. When hard water is passed through a mixed –bed deionizer, it alternately comes in contact with anion exchangers and cation exchangers a number of times. So more efficient softening is done, and residual hardness of softened water is o to 1 ppm only. When the resin is exhausted, the bed is back washed. Due to its lower density, the anion exchanger forms an upper layer over the cation exchanger. The anion exchanger is regenerated by NaOH solution, and the cation exchanger is regenerated by sulphuric acid solution.
Conclusion
In this article, we discussed the ion exchange resin method, types of ion exchange resin such as cation exchange resin and anion exchange resin, removing hardness of water using ion exchange resin method, recharge of ion exchange resin and other related concepts.
