Chlorination in drinking water treatment and naturalenvironments

Chlorine (Cl) is a member of the halogen family of elements in group 17 of the periodic table. It is an essential nutrient for all living organisms including humans, animals and plants (Winterton, 2000). It is rarely found in nature in its elemental form, and typically exists bonded to other elements in theform of chemical compounds (Deborde and Von Gunten, 2008). Its tendency to combine with other elements and compounds revolutionized the modern world in a way of producing hundreds of thousands of useful products (Evans, 2005).

Chlorine containing compounds are used in many industrial processes, such as in the production of paper, plastics, dyes, textiles, medicines, insecticides, fertilizers, solvents, paints and disinfectants (Evans, 2005). The large-scale use of Cl as disinfectant in drinking water production not only produces safe drinking water, it also produces disinfection by-products (DBPs) by reacting with natural organic matter (NOM) dissolved in the source water (Sobsey et al., 2003). Some chlorinated organic compounds (Clorg) are resistant to degradation and hence persistent (Jones and De Voogt, 1999).

 Moreover, Clorg are produced naturally in the soils. Formation and degradation of Clorg affect the Cl cycling in terrestrial environments (Montelius et al., 2015). Due to toxic nature of many Cl-containing organic compounds, (Cantor, 1997; Cemeli et al.

, 2006; IARC, 1995; IARC, 1999), it is important to explore the occurrence, formation, distribution and cycling of Cl in both society and nature. The term chlorine (Cl) will be generally used to refer to the collective forms both organic chlorine (Clorg) and inorganic chlorine (Clinorg). Moreover, Chloride (Cl?) will be the same as inorganic chlorine and compounds of organic carbon containing covalently bound chlorine will be referred to generally as organochlorine compounds (Clorg). Chlorine (Cl) is used as a disinfectant in modern supply of clean drinking water. It is presumed to be more demanded in future, because of climate changes coupled with the increased temperature and longer warm seasons. The warm seasons will yield higher amounts of waterborne pathogens (Hunter, 2003). Moreover, Cl is an essential micronutrient for plants and participates in several physiological processes such as osmotic and stomatal regulation, the water-splitting reaction in photosynthesis, and disease resistance and tolerance (Winterton, 2000). Cl not only improves the yields and quality of some crops, excess of Cl is also responsible for salinity stress and toxic to plants (Chen et al.

, 2010). The Cl is so reactive that once it is in the environment, can react with other inorganic material to form chlorides or organic materials to form chlorinated organic compounds (Clorg) (Deborde and Von Gunten, 2008). Many Clorg are very stable and highly lipophilic therefore they accumulate in the environment and affect humans through the food chain (Henschler, 1994).