11
Chlorine [Cl, 17]

11.1 INTRODUCTION
Chlorine is a common element of the halogen group 17 in the periodic table of
elements. Its total content in the upper Earth’s crust is estimated at 640  mg/kg.
In igneous rocks, its concentrations vary between 80 and 200 mg/kg, being the highest in granites. It is likely to accumulate in shales (200 mg/kg) and in some limestones (150 mg/kg). Its lowest mean contents are in sandstones, 10 mg/kg. High Cl
amounts are in coal, within the range 200–1000 mg/kg, but contents up to about
9000 mg/kg are also reported (Finkelman 1999).
Under standard condition, Cl is a yellowish-green gas, of a specific irritating odor.
Its chemical properties are similar to Br and I. It is chemically very reactive and can
have several valence states. Most commonly its oxidation state is −1. It is a strong
oxidizing agent. Chlorine is a lithophilic and a very mobile element; it may be a
minor constituent of several minerals, mainly amphiboles, apatite, and micaceous
and clay minerals. During weathering processes, Cl is released from its compounds
and is transferred to water, mainly in oceans and seas.
Chlorine forms some minerals such as cerargyrite, AgCl; carnallite, KMg(H 2 O) 6 Cl 2 ;
and sylvite, KCl. However, halite—commonly known as rock salt—NaCl, is the
most abundant in both terrestrial and sea environments, and is the most important in
human activities.
World total salt production in 2010 was about 280 Mt, of which China and the
United States produced 22% and 15%, respectively (USGS 2010). Salt is obtained
either from solid sedimentary deposits or from seawater and brines.
Major uses of salt (in %) are chemicals, 42%; road ice control, 38%; distributors
(groceries, etc.), 8%; agricultural and food processing, 4% each; general industrial
and primary water treatment, 2% each; and other uses less than 1%. These proportions of salt use may differ from countries; in some countries the use of NaCl as road
salt exceeds 50% of its total use.
Chlorine is broadly used as ClO 2 in bleaching materials, for paper pulp and
cellulose bleaching. Chloride dioxide is used in sterilization in hospitals. It is also
used in supplied drinking water and in food preservations. Chlorine is a component of some pesticides and various chlorinated organic compounds. Chlorine
dioxide load of various environmental compartments may infiltrate the food chain
and is of a health concern (Coenen 2004).
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