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In natural water, the Cl2 molecule as well as hypochlorite ions are not stable due to the presence of organic and inorganic matter. The half-life of hypochlorite is estimated to be less than 2 hours due to reduction and photolysis. The freely available chlorine reacts to form various chlorinated by-products, e.g. chloramines and chloromethanes. In sewage, chlorine is mainly present in the form of chloramines, as hypochlorous acid undergoes a fast decay (SIAR, 2003).

In the atmosphere, chlorine mainly undergoes photolysis: Cl2 + hν => 2Cl°.

The half-lifetime for that process has been estimated to be in the order of 1–4 hours, depending on the time of the day (Hov, 1985; Peterson, 1976). Calculations show that half-lives for chlorine photolysis can range from less than 5 minutes to several hours, depending on the time of day, with representative values for latitudes 40°N and 55°N. The resulting chlorine atoms can then react with species present in the atmosphere such as ozone and saturated and unsaturated hydrocarbons. Recombination of chlorine after dissociation can be neglected due to the fast reaction with other species and due to the low concentration of chlorine. Chlorine atoms can also be produced by the reaction of hydrogen chloride with the hydroxyl radical (Hov, 1985). The reaction of chlorine with hydrocarbons gives rise to formation of chlorinated organic compounds (SIAR, 2003). However, on a global scale, the atmospheric burden of chlorine is dominated by natural rather than anthropogenic sources, and so anthropogenic sources will make only a very small contribution to these processes.

Mobility of chlorine in soil is assumed to be of little relevance as chlorine in an aqueous solution covalently binds to soil organics within the first few millimetres or centimetres of the soil surface. The final target compartment for chlorinated substances, formed from chlorine, will be the hydrosphere. Depending on the inherent properties and the stability of chlorinated products, chlorine may reside in the atmosphere. For example, NCl3 is highly volatile and is found in similar concentrations in air and water (SIAR, 2003).

An octanol/water distribution equilibrium can not be defined for chlorine as it has strong oxidizing properties and will react with the organic phase. A potential for bioaccumulation or bioconcentration of active chlorine species can be disregarded, because of their water solubility and their high reactivity (SIAR, 2003).