Explain Hardness of Water
Explain Hardness of Water
The water that occurs in nature always contains salts, which exist in water as positive and negative ions. The salinity of fresh water is 1000 mg/L (1L of water contains 1000 mg of various salts or ions). The typical cations present in natural water are sodium, potassium, calcium and magnesium; the most common anions are chloride, sulfate, bicarbonate, nitrates, etc. Hardness of water refers to presence of calcium and magnesium ions. When water is heated, the reaction (Tebbutt, 1999):
takes place. Calcium carbonate, unlikely other salts, is insoluble and forms precipitate. The precipitate is called lime scale, and it causes increase in soap consumption and severe threats to hot water systems clogging the pipelines and reducing their volume (DeZuane, 2007).
The lime scale formation appears when calcium ions are present, irrespective of the anion. The temporary hardness is caused by carbonates, and the permanent is associated with presence of sulfates, chlorides, and nitrates. The temporary hardness is removed by boiling (DeZuane, 2007):
Ca(HCO3)2 → 2CaCO3↓+ 2H+
The permanent hardness cannot be removed by boiling since the salts are soluble.
The effective method of permanent hardness removal is ion exchange. Water is filtered through the packed column filled with the ion exchanger, a material capable of removing the cations (cation exchange resin) or anion (anion exchange resin). When water is filtered through the cation exchange resin, the cations are removed from water, and it is enriched with H+ ions. At the next stage, water is treated with anion exchange resin, and the anions are removed, and water is enriched with OH- ions. Hence, the method allows full desalination at certain conditions. Typically, ion exchange is applied after chemical precipitation of calcium and magnesium to cut down the expenses on the valuable resin (Tebbutt, 1999).
References
DeZuane, J. (2007). Handbook of drinking water quality. Hoboken: Wiley [Imprint.
Tebbutt, T. H. Y. (1999). Principles of water quality control. Oxford: Butterworth-Heinemann.
