Removal of hardness from water can be done using different methods that can be grouped into two: chemical (use of chemical softeners) and mechanical (physical) methods.

Chemical Method of Removing Water Hardness - Use of Water Softeners

Removing harness using chemical method involves the use of chemical water softeners. These are of two types: those which lead to precipitation; and those which do not precipitate.

A. Chemical Water Softeners which lead to precipitation - these are chemical substances added to the water to remove dissolved calcium, magnesium or iron(II) salt by forming precipitate or undissolved solids. The precipitate is then removed by filtration.

Examples of these chemicals are:

(i). Calcium hydroxide, Ca(OH)₂ - addition of calculated amount of Ca(OH)₂ will remove only temporary hardness from water. Ca(OH)₂ precipitates the insoluble trioxocarbonate(IV).

Example, Ca(OH)₂(s) + Ca(HCO₃)₂(aq) → 2CaCO₃(s) + 2H₂O

Note: Ca(OH)₂ is slightly soluble;  Ca(HCO₃)₂ is soluble; CaCO₃ is insoluble.

Also, notice that excess of Ca(OH)₂ could cause hardness, hence the amount added is measured.

(ii). Sodium hydroxide, NaOH - addition of caustic soda (NaOH) removes both temporary and permanent hardness by precipitating the metal ions which cause the hardness as insoluble hydroxides.

Example, 2NaOH(aq) + Ca(HCO₃)₂(aq) → 2NaHCO₃(aq) + Ca(OH)₂(s)

Note: Ca(HCO₃)₂ is soluble; Ca(OH)₂ is insoluble.

2NaOH(aq) + MgSO₄(aq) → Na₂SO₄(aq) + Mg(OH)₂(s)

Note: MgSO₄ is soluble;  is Mg(OH)₂ insoluble

(iii). Sodium trioxocarbonate(IV), Na₂CO₃ (in form of crystals (i.e. washing soda) and soda ash (anhydrous)) - addition of Na₂CO₃.10H₂O (crystals) or Na₂CO₃ (anhydrous) will remove both temporary and permanent hardness.

Example, Na₂CO₃(aq) + Ca(HCO₃)₂(aq) → 2NaHCO₃(aq) + CaCO₃(s)

Na₂CO₃(aq) + CaSO₄(aq) → Na₂SO₄(aq) + CaCO₃(s)

(iv). Borax, Na₂B₄O₇. 10H₂O - addition of borax removes both temporary and permanent hardness.

B. Chemical water softeners which do not lead to precipitation - these are chemical softeners whose process is based on the softeners ability to sequester Ca²⁺, Mg²⁺ or Fe²⁺ (i.e. without precipitating them, but keep them in solution, away from precipitating soap). These substances are Mostly of the polyphosphate family, e.g., polymetaphosphate, (NaPO₃)_n (this is sold under the trade name Calgon); and tetrasodium diphosphate, Na₄P₂O₇.

They remove both types of hardness. Mechanical Method Mechanical water softening include: A. Distillation method - this will remove all solid particles that had dissolved in the water, but it is a rather expensive method. Both temporary and permanent hardness are removed. B. Ion exchange method - this involves the use of porous solids ion exchangers.

The principle of this method is that, the ions, i.e. Ca²⁺, Mg²⁺ or Fe²⁺, which cause hardness are removed from water by exchanging them with other cations from the ion exchangers which do not cause hardness - by this, the water is free from hardness. This is a cation exchanger. Two major types of ion exchangers (also called permutit) are used in softening water.

These are: 1. The zeolites - these are naturally occurring aluminosilicate minerals, e.g., NaAl(SiO₃)₂ or NaAlSi₂O₆

2. The ion - exchange resins - these are synthetic organic polymers (e.g., polystyrene). For example, when water containing Ca²⁺ or Mg²⁺ ions is allowed to filter through thick layers of zeolite, the Na⁺ ions in the zeolite are replaced by Ca²⁺ or Mg²⁺ ions in the solution: 2NaAlSi₂O₆ + Ca²⁺ → Ca(AlSi₂O₆)₂ + 2Na⁺ . The hardness is thus removed.

A zeolite which has been used can be regenerated by allowing it to stand in contact with conc. NaCl solution. The calcium alumino silicate is reconverted to sodium aluminosilicate: Ca(AlSi₂O₆)₂ + 2Na⁺ → 2NaAlSi₂O₆ + Ca²⁺.

Hence, by this reaction, sodium zeolite is ready to serve as a water softener again - common salt (NaCl) will keep this ion exchange water softener operating for many years. Ion exchange resins are reacted with hot conc. H₂SO₄, and then neutralized with NaOH solution before being used to soften hard water. Note: mechanical water softening methods remove both temporary and permanent hardness from water.

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