Polychlorinated dibenzodioxins , toxicity

Because of very high toxicity, ability of delayed action and high stability in the environment, polychlorinated dibenzodioxins (PCDD), dibenzofurans (PCDF) and biphenyls (PCB) are particularly hazardous for people s health if misused for committing acts of terrorism, or under the circumstances of violating the rules of toxic wastes safe storage. 

Dioxins are a particular group of chlorinated organic molecules which have been associated with pulp and paper production and are a concern because of their extreme toxicity. There are two groups of molecular types which fall into the general category referred to as dioxins. These are the polychlorinated dibenzodioxins (PCDDs) and the polychlorinated dibenzofurans (PCDFs). The structures of these molecules are shown in Figure 10.4. 

Chlorinated micropoUutants are harmful for man and environment due to their toxicity, persistence, and bioaccumulation. Persistent compounds are very stable and difficult to get metabolized and mineralized by biological and chemical processes in the environment, and as a result, they have become ubiquitous in water, sediments, and the atmosphere bioaccumulation is the result of the lipophilicity of these compounds. Polychlorinated dibenzodioxins and -furans (PCDD/F) are not produced purposely like many of other chlorinated technical products, such as chlorinated biocides DDT, lindane, and toxaphene. The production and use of persistent organic pollutants (POPs), the dirty dozen has now been banned worldwide by the Stockholm protocol. It should be mentioned that about 3000 halogenated products have now been isolated as natural products in plants, microorganisms, and animals," but the total amount of these products is much smaller compared to xenobiotics. 

Polychlorinated hydrocarbons, such as polychlorinated dibenzodioxins, dibenzofurans, and biphenyls exist as a number of different congeners. Some of these are geometric isomers. Many cause a range of toxic effects that are believed to be mediated by interaction with the aryl hydrocarbon receptor (AhR) (they are known as pleiotropic effects). However, not all the isomers cause these effects because they do not all interact with the AhR receptor. To interact with this receptor, the molecule needs to be flat (planar). It can be seen in the diagram (Fig. 5.2)... 

Phiet PH, Anh TK, Vu DQ, et al. 1989. Preliminary observations on the clinical histories, polychlorinated dibenzodioxin and dibenzofiiran tissue levels and 2,3,7,8-TCDD toxic equivalents of potentially dioxin-exposed patients living in the south of Vietnam. Chemosphere 19 937-940. 

United States because they build up in the environment and can cause harmful health effects, they are still in use in many other countries. Polychlorinated dibenzodioxins and polychlorinated dibenzofurans (dioxins) (Fu et ah, 2003) consist of 210 different compounds that have similar chemical properties (Bhandari and Xia, 2005). This class of compounds is persistent, toxic, and bioaccumulative. They are generated as by-products during incomplete combustion of chlorine containing wastes such as municipal solid waste, sewage sludge, and hospital and hazardous wastes (Bhandari and Xia, 2005). PCBs were widely used in the past and now contaminate many industrial and natural areas. 

Dioxin A family of chemicals with related properties and toxicity. There are 75 different dioxins, or polychlorinated dibenzodioxins (PCDDs) 135 different furans, or polychlorinated dibenzofurans (PCDFs) and 209 different polychlorinated biphenyls (PCBs). Each different form is called a congener, a member of a family of compounds known chemically as dibenzo-p-dioxins. Concern about them arises from their potential toxicity as contaminants in commercial products. Tests on laboratory animals indicate that it is one of the more toxic anthropogenic (man-made) compounds a term used interchangeably with 2,3,7,8-tetrachlorodibenzo-p-dioxin or TCDD Disease prevention Measures used to prevent a disease or reduce its severity Dispersoids The particles of a dispersion... 

In response to the continuing discovery of the persistence, bioaccumulative properties, and toxicity of POPs, regional, national and international policies ban the intentional production of compounds, such as polychlorinated biphenyls (PCBs), several organochlo-rine pesticides, such as mirex and dieldrin, and the brominated flame retardants poly-brominated diphenyl ethers (penta-BDE and octa-BDE, and most recently, deca-BDE). Policies and programs have also targeted the unintentional production and release of POPs such as polychlorinated dibenzodioxins and furans (PCDD/Fs). Evidence of the success of these policies has been seen in immediate reductions of air concentrations, followed by declining concentrations in water bodies, soils, biota and our food supplies... 

The acute toxicity exerted by PCBs is generally low. The most prominent effect is the pronounced induction of the MFO system in different tissues, with increased de novo synthesis and increased levels in several cytP450 isozymes. The PCBs in the environment occur as a mixture of congeners and every congener exerts specific toxicity, with some PCBs exerting exclusive effects. One such effect is the capability for planar PCBs to bind to and stimulate the cytoplasmatic aryl hydrocarbon receptor in the cell, the same receptor that is stimulated by the highly toxic polychlorinated dibenzodioxins (PCDDs) (see below). 

An industrial site contaminated with polychlorinated dibenzodioxin (PCDD) presents a set of complex problems to those charged with the assessment of site contamination and the development of a remediation plan. In the suiTe of PCDD compounds, TCDD is of particular concern. The U.S. Environmental Protection Agency (U.S. EPA) has designated TCDD as a potent animal carcinogen and very acutely toxic(l). 

I. Mechanism of toxicity. PCBs are irritating to mucous membranes. When burned, PCBs may produce the more highly toxic polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs see p 184). It is difficult to establish the specific effects of PCB intoxication because PCBs are neatly always contaminated with small amounts of these compounds. PCBs, and particularly the PCDD and PCDF contaminants, are mutagenic and teratogenic and are considered potential human carcinogens. 

In addition to the thermogravimetry experiments, batch pyrolysis experiments continuously monitoring HCl formation were performed with PVC to determine the optimum temperature for HCl formation. Under conditions of maximum HCl formation, PVC was pyrolyzed with and without oxygen in a fluidized bed reactor and the formation of polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) measured. Avoiding the formation of these highly toxic compounds would be a critical element in any waste stream processing scheme. 

One other approach that was considered was ranking chemicals by groups according to their physical and chemical properties. A number of properties were selected, for example, LD50, bioaccumulation, and persistence as the main criteria for toxicity to the environment and humans. Substances were then classified according to the range they fell within (Ministers Advisory Panel, 1995). This approach was used for the Canadian Environmental Protection Act (CEPA) assessments. For example, polychlorinated dibenzodioxins, polycyclic aromatic hydrocarbons, inorganic cadmium compounds, benzidine, trichloroethylene, and a host of others were concluded to be toxic. Others, such as chlorobenzene, toluenes, xylenes, and dibutyl pthalate, were concluded to be nontoxic. Others, such as aniline, styrene, crankcase oils, and pentachlorobenzene, do not have sufficient information for classification. 

Because PCB molecules have two benzene rings or 12 carbon atoms as well as some chlorine atoms, they wUl bum at high temperatures when ignited. The main danger from a PCB fire is not the PCB itself, but the formation of extremely toxic byproducts of combustion, such as polychlorinated dibenzodioxins, polychlorinated dibenzofurans, and hydrogen chloride gas, which often produce immediate respiratory effect. The formation of dioxins and furans... 

In an effort to develop a suitable perovskite catalyst for the oxidation of alky-laromatics to carbonyl compounds, the attention has been focused on the use of LaMOs perovskites. Thus, several ABOg-type perovskite oxides (A = La, Y, Nd, or Gd B = Fe, Mn, Cr, or Co) have been investigated as catalysts for the oxidation of 1,2-dichlorobenzene, a model compound for the highly toxic polychlorinated dibenzodioxins [39]. LaMOg (M = Cr, Co, Fe, Mn, Ni) perovskites were used as catalysts in the oxidation of alkylaromatics to benzylic ketones, under solvent-free conditions, using TBHP as oxidant [48]. LaCrOs was found to be an efficient catalyst for the oxidation of alkylarenes to phenyl ketones, under mild reaction conditions, in the presence of TBHP as oxidant. Perovskites... 

---