Purification of water

    The simplest method for the removal of all solid solutes from water is by distillation, but because of the high boiling point and enthalpy of vaporization (Table 6.1), this method is expensive. If the impurities are ionic, ion exchange is an effective (and relatively cheap) means of purification. The treatment involves the passage of water down a column of an organic resin containing acidic groups (e.g. ^_SO₃H) and then down a similar column containing basic groups (e.g. ^_NR₃OH):

    After treatment, deionized water is produced. The resins are reactivated by treatment with dilute H₂SO₄ and Na₂CO₃ solutions respectively. Reverse osmosis at high pressures is also an important process in water purification, with cellulose acetate as the usual membrane; the latter prevents the passage of dissolved solutes or insoluble impurities. The purification of drinking water is a complicated industrial process. Water may be abundant on the Earth, but impurities such as microorganisms, particulate materials and chemicals usually make it unfit for human consumption.   Coagulation and separation methods are used to remove

many particles. Aluminium and iron(III) salts are widely used in the coagulation stages, and the treatment relies upon the formation of polymeric species in solution. Pre-polymerized coagulants are now available and include polyaluminium silicate sulfate (PASS) and polyferric sulfate (PFS). About two-thirds of all Al₂(SO₄)₃ manufactured goes into water treatment processes, with the paper manufacturing industry consuming about a half of this amount.