Describe the process of water softening and purification.

Explanation

(i) An idealised image of water softening process involves replacement of cations such as Mg, Ca and Fe in water with sodium ions by a cation exchange zeolite. The ion exchange zeolites or resins are used to replace the Mg and Ca ions found in hard water with sodium ions.
(ii) They can be recharged by washing it with a solution containing a high concentration of sodium ions.
(iii) The calcium and magnesium ions migrate from the zeolite or resin being replaced by sodium ions from the solution until a new equilibrium is reached. That is, the salt is used to recharge an ion exchange medium, which itself is used to soften the water.
(iv) A couple of other methods, namely chelating method and reverse osmosis are also used to soften hard water. Chelating method employs a polydentate ligand such as EDTA, while reserve osmosis uses high pressure to force the water through a semi-p.0.. permeable membrane.
(v) In the case of water purification application, ion exchange zeolites or resins are used to remove toxic (e.g., copper) and heavy metal (e.g., cadmium or lead) ions from solution, replacing them with harmless sodium or potassium ions.