Properties of Organochlorine Chemicals
Before examining organochlorines in detall, it is useful to set out the reasons that organic compounds containing chlorine (organochlorines) are found so frequently on priority lists, describe some of the organochlorine chemicals of particular concern, and identify critical chemical characteristics of these compounds.
The element chlorine has several physical chemical properties which prove to be both useful commercially and of concern environmentally. First, when present in organic compounds it tends to reduce reactivity. This stability is manifested as environmental persistence, which increases with increasing chlorination. Thus, dichlorobenzene is not viewed as very persistent, but hexachlorobenzene is.
The second property of importance is the relatively large size and mass of the chlorine atom. This results in a larger molecule which has a reduced vapour pressure, increased boiling point and a reduced solubility in water. Addition of chlorine thus increases the tendency to partition into lipids and therefore bioconcentrate. From an environmental risk assessment perspective, it is very important to realize that addition of chlorine usually produces an incremental effect on properties that is generally understood and, to an extent, predictable. Accordingly, the assessment of the effects of chlorine dioxide substitution for chlorine is, in large measure, an assessment of the extent to which this substitution changes the nature and quantities of the organochlorine chemicals which are produced and which survive processing and waste water treatment to enter the receiving environment. This requires consideration of the fundamental chemistries of the chlorine dioxide and chlorine oxidation processes and the nature of the organochlorine compounds produced.