Chlorine environmental problems

In earlier editions of the Eniyclopedia there have been articles covering the properties, manufacture, capacities, etc, of polychlorinated biphenyls (PCBs), chlorinated naphthalenes, benzene hexachloride, and chlorinated derivatives of cyclopentadiene. These materials are no longer in commercial use because of their toxicity. However, they stiU impact on the chemical industry because of residual environmental problems. Their toxicity and environmental impact are discussed (see Cm.OROCARBONSANDCm.OROHYDROCARBONS, TOXIC aromatics). 

Environmental problems associated with PCBs are the result of a number of factors. Several open uses of PCBs have resulted in thein direct introduction into the environment, eg, organic diluents careless PCB disposal practices have resulted in significant releases into aquatic and marine ecosystems higher chlorinated PCBs are very stable in thein persistence in different environmental matrices and by a variety of processes (Fig. 1) PCBs are transported throughout the global ecosystem and preferentiaHy bioconcentrate in higher trophic levels of the food chain. 

Attempts have been made to apply the structure-activity concept (Hansch and Leo 1995) to environmental problems, and this has been successfully applied to the rates of hydrolysis of carbamate pesticides (Wolfe et al. 1978), and of esters of chlorinated carboxylic acids (Paris et al. 1984). This has been extended to correlating rates of biotransformation with the structure of the substrates and has been illustrated with a number of single-stage reactions. Clearly, this approach can be refined with the increased understanding of the structure and function of the relevant degradative enzymes. Some examples illustrate the application of this procedure ... 

Jendrzejewski N, Eggenkamp HGM, Coleman ML (2001) Characterization of chlorinated hydrocarbons from chlorine and carbon isotopic compositions Scope of application to environmental problems. Appl Geochem 16(9-10) 1021-1031... 

The publication of Silent Spring (authored by Rachel Carson), which outlined many environmental problems associated with chlorinated pesticides, caused a ban on the use of DDT in 1972. 

Chlorine-containing polymers such as poly(vinyl chloride) PVC undergo an autocatalytic dehydrochlorination reaction under the influence of elevated temperature and UV radiation. Since the HCl originating from the dehydro chlorination of the PVC chains is believed to sustain this autocatalytic process, stabilizers that irreversibly bond HCl can thus inhibit the degradation. Heavy metal compounds such as cadmium stearate or lead stearate are currently used for this purpose. However, alternatives are required due to environmental problems associated with the use of heavy metals. Indeed, the largest current application of LDH materials is in the polymer industry, mainly to stabilize PVC [3,229-232]. 

Destructive chlorination to chlorpicrin (Cl3CN02) has been noted on warming TNT with Ca hypochlorite (Ref 4). Small amounts of chloropicrin are formed during the above-mentioned prepn of HNS from TNT and Na hypochlorite (Ref 118). On a production scale, an environmental problem is thus created, since this compd is a toxic lachrymator... 

Around the world, the most widely used biocide for industrial cooling systems is gaseous chlorine. Typically, it is a low-cost maintenance item and generally very effective, but may present specific cooling water chemistry limitations, plus possible handling, safety, and environmental problems. (The relatively high capital costs associated with a gaseous chlorine system may also be a problem.)... 

Chlorofluorocarbon 12, a freon, was used in refrigeration systems and air conditioners and as an aerosol propellant because of its high stability and low toxicity. Its stability results in environmental problems. It does not decompose until it reaches the upper atmosphere, where ultraviolet light causes the carbon-chlorine bonds to break.The resulting chlorine atoms catalyze the destruction of ozone, resulting in the infamous ozone hole.The use of this and other stable freons has recently been phased out. 

The use of chlorinated organic compounds in agriculture and industry has caused a number of environmental problems. A good illustration is provided by the case of 1,1,1 -trichloro-2,2-bis(p-chlorophenyl)ethane, better known as dichlorodiphenyl-trichloroethane, or DDT. 

In this volume of Issues we address the sources, environmental cycles, uptake, consequences and control of many of the more important chlorinated organic micropollutants. Under this heading we have included a range of semi-volatile persistent compounds, notably polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) as well as a number of chlorinated pesticides. We have not sought to include volatile species such as CFCs which cause environmental problems of an entirely different nature. The compounds included in this volume cause no threat to the stratospheric ozone layer, but have given widespread cause for concern in relation to their environmental persistence and high toxicity, and their potential for adverse effects on humans and wildlife. 

Thermoplasticity of chemically modified wood may be increased further by chlorination, using chlorine water as a posttreatment. Chlorination depoly merizes lignin and increases its thermoplasticity [19]. However, this treatment generates organic chlorine compounds, which may cause environmental problems. 

Early production was based on the chlorohydrin process (Box 2), characterized by high yields but also encumbered by technical and environmental problems due to the use of chlorine. The introduction, in the 1940-1950s, of the oxidation of ethylene on silver catalysts with lower costs and better environmental standards, led to its phasing out, virtually completed by the end of the 1960s. 

The environmental problems associated with chlorinated aromatics continue to stimulate Interest in reactions involving photochemical replacement of chlorine by hydrogen. The efficiency of photochemical dechlorination of some... 

Historically, several processes have been developed to an industrial scale to produce phenol, including (i) sulfonation of benzene and alkali fusion of the benzene sulfonate (ii) chlorination of benzene and hydrolysis of chlorobenzene (iii) the cumene process (Section 13.2) (iv) toluene oxidation to benzoic add and subsequent oxidative decarboxylation of the latter to phenol and (v) dehydrogenation of cyclohexanol-cyclohexanone mixtures. Today, however, only the cumene process and the toluene oxidation are still run on an industrial scale, all the other processes having been given up due to economic reasons or environmental problems. 

One technical approach to solve this environmental problem is to eliminate the chlorine content of CFCs, primarily by replacing any chlorine atoms with fluorine atoms in these molecules. Hydrochlorofluorocarbons (HCFC s) do not have inqtact zero on the ozone layer and although they have been used in this period, they are already currently subject to restrictions and will be proscribed fi-om 2025. Hydrofluorocarbons (HFC s) have ODP (Ozone Depletion Potential) equal to zero and low GWP (Global Warming Potential) and they are therefore designated as substitutes to CFCs in the field of refijgeration [3]. 

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