Permanent hardness of water can be removed by treating hard water with synthetic resins which involve the exchange of cations (e.g., Na+, Ca+2, Mg+2 etc) and anions (e.g., Cl-, SO42-, HCO3- etc) present in water by H+ and OH- ions respectively.
Synthetic resins are of two types:
1) Cation exchange resins:
Cation exchange resins are large organic molecules that contain the –SO3H group. The resin is firstly changed to RNa (from RSO3H) by treating it with NaCl. This resin then exchanges sodium ions with calcium and magnesium ions, thereby making the water soft.
2RNa + M2+(aq)→ R2M(s) + 2Na+(aq)
There are cation exchange resins in H+ form. The resins exchange hydrogen ions for sodium, calcium, magnesium ions.
2) Anion exchange resins:
Anion exchange resins exchange hydroxyl ions for anions like chloride, bicarbonate, and sulphate present in water.
During the hardness treatment process, water is first passed through the cation exchange process. The water obtained after this process is free from mineral cations and is acidic in nature.
This acidic water is further passed through the anion exchange process where hydroxyl ions neutralize the H+ ions and de-ionize the water obtained.