Polychlorinated like PCBs

PCDFs are similar in many respects to PCDDs but have been less well studied, and will be mentioned only briefly here. Their chemical structure is shown in Figure 7.1. Like PCDDs, they can be formed by the interaction of chlorophenols, and are found in commercial preparations of chlorinated phenols and in products derived from phenols (e.g., 2,4,5-T and related phenoxyalkanoic herbicides). They are also present in commercial polychlorinated biphenyl (PCB) mixtures, and can be formed... 

In addition to the chemicals included on the other lists, the CDC also included heavy metals such as arsenic, lead, and mercury volatile solvents such as benzene, chloroform, and bromoform decomposition products such as dioxins and furans polychlorinated biphenyls (PCBs) flammable industrial gases and liquids such as gasoline and propane explosives and oxidizers and all persistent and nonpersistent pesticides. Agents included in this volume are limited to those that are most likely to pose an acute toxicity hazard. 

For halogenated aromatic hydrocarbons like polychlorinated biphenyls (PCBs), polychlorinated dibenzofurans (PCDFs), and polychlorinated dibenzo-p-dioxins (PCDDs) the binding to the aryl hydrocarbon (Ah) receptor regulates their toxicity [89]. The Ah receptor controls the induction of one of the cytochrome P450 enzymes in the liver. Toxic responses such as thymic atrophy, iveight loss, immu-notoxicity and acute lethality are associated ivith the relative affinity of PCBs, PCDFs and PCDDs for the Ah receptor [89]. The quantitative structure-activity relationship (QSAR) models predicting the affinity of the halogenated aromatic hydrocarbons ivith the Ah receptor describe the electron acceptor capability as well as the hydrophobicity and polarizability of the chemicals [89[. 

Meanwhile, many other chemicals have enabled our society to accomplish great technical advances. For example, we have learned to recover fossil hydrocarbons from the earth and use these for heating, for transportation fuels, and for synthetic starting materials. Likewise, synthetic compounds like tetraethyllead, chlorinated solvents, freons, methyl /-butyl ether (MTBE), polychlorinated biphenyls (PCBs), and many others (see Chapter 2) have enabled us to develop products and perform industrial processes with greater efficiencies and safety. However, it has become quite apparent that even such contained applications always result in a certain level of discharge of these compounds to the environment. 

Gevao et al. (2000) recently reported the presence of polychlorinated naphthalenes in a lake sediment core taken in northwest England (see Table below for profile of pentachloronaphthalenes or PCNs). Like PCBs, these compounds were used by the electric industry as dielectric fluids in transformers and capacitors. 

For many years now, we have seen the concentrations of banned compounds like the polychlorinated biphenyls (PCBs) decrease (or at least remain constant) in Lake... 

It would lie far beyond the aim of this chapter to introduce the state-of-the art concepts that have been developed to quantify the influence of colloids on transport and reaction of chemicals in an aquifer. Instead, a few effects will be discussed on a purely qualitative level. In general, the presence of colloidal particles, like dissolved organic matter (DOM), enhances the transport of chemicals in groundwater. Figure 25.8 gives a conceptual view of the relevant interaction mechanisms of colloids in saturated porous media. A simple model consists of just three phases, the dissolved (aqueous) phase, the colloid (carrier) phase, and the solid matrix (stationary) phase. The distribution of a chemical between the phases can be, as first step, described by an equilibrium relation as introduced in Section 23.2 to discuss the effect of colloids on the fate of polychlorinated biphenyls (PCBs) in Lake Superior (see Table 23.5). 

Urban air concentrations were multiplied by relative potencies (REPs) (listed in Table 2) to calculate the contribution of PCNs to dioxin toxic equivalents (TEQ) and to compare the contributions of the dioxin-like PCBs using REPs from Giesy et al. (1997) [143]. On average, PCNs contributed 64% of total PCN and dioxin-like PCB (PCN+PCB) TEQ in downtown air and 48% in north Toronto. The PCN contribution to PCN+PCB TEQ in Chicago air was similar (68%) when recalculated using the same REPs [97,126]. Although PCNs are as important as PCBs in air on a TEQ basis, polychlorinated dioxins and furans remain the dominant contributors of TEQ in downtown Toronto air [126]. 

Although this public health statement will focus on CDDs, it is important to note that CDDs are found in the environment together with other structurally related chlorinated chemicals, such as chlorinated dibenzofurans (CDFs) and polychlorinated biphenyls (PCBs). Therefore, people are generally exposed to mixtures of CDDs and other classes of toxicologically and structurally similar compounds. 2,3,7,8-TCDD is one of the most toxic and extensively studied of the CDDs and serves as a prototype for the toxicologically relevant or dioxin-like CDDs. Based on results from animal studies, scientists have learned that they can express the toxicity of dioxin-like CDDs as a fraction of the toxicity attributed to 2,3,7,8-TCDD. For example, the toxicity of dioxin-like CDDs can be half or one tenth or any fraction of that of 2,3,7,8-TCDD. Scientists call that fraction a Toxic Equivalent Factor (TEF). More information on TEFs can be found in Section 2.5. 

EPA. 1997c. Addition of dioxin and dioxin-like compounds modification of polychlorinated biphenyls (PCBs) listing toxic chemical release reporting community right-to-know. U. S. Environmental Protection Agency. Federal Register. 62 FR 24887. 

In addition to transformation by corrodable metals (such as Fe° and Zn°), bimetallic combinations of a catalytic metal with a corrodable metal (such as Pd/Fe or Ni/Fe) have also been shown to transform a variety of contaminants. In most cases, rates of transformation by bimetallic combinations have been significantly faster than those observed for iron metal alone [26,96,135-139]. Not only have faster transformation rates been observed with bimetallic combinations, but, in some cases, transformation of highly recalcitrant compounds, such as polychlorinated biphenyls (PCBs), chlorinated phenols, and DDT has been achieved [24,140,141]. The mechanism responsible for the enhanced reactivity with bimetallic combinations is still unclear however, it has been suggested that electrochemical effects, catalytic hydrogenation, or intercalation of H2 may be responsible. A likely limitation to the full-scale application of bimetallic combinations to groundwater remediation is deactivation of the catalytic surface either by poisoning (e.g., by sulfide) or by formation of thick oxide films [136,142,143]. 

Giesy, J.P., Kannan, K. (1998). Dioxin-like and non-dioxin-like toxic effects of polychlorinated biphenyls (PCBs) implications for risk assessment. Crit. Rev. Toxicol. 28 511-69. 

Loganathan, B.G., Kumar, S., Iseki, N., Masunaga, N. (2001). Polychlorinated dibenzo-p-dioxin/furan and dioxin-like PCB concentrations in sediments and mussel tissues from Kentucky Lake, USA. Organohalogen Compounds 51 158-61. 

Using traditional methods of whole-water analysis, concentrations of these HCs are usually underestimated. Indeed, by these methods HCs may not even be detected, although they may occur on sediment at concentrations likely to have toxic effects on biota. The conventional approach for determining the concentration of HCs on suspended sediment is to analyze a whole-water sample and a filtered water sample and to assume that the difference between the two represents the fraction sorbed to suspended sediment. The major problem with this approach is that the amount of suspended sediment and associated contaminant in the whole-water sample may not be sufficient to produce a detection by whole-water analysis methods. This is particularly true if the suspended sediment concentration in the sample is small, as is generally the case for springs relative to surface water. For example, if a sample contains 50 mg/L of suspended sediment, and the sediment contains 300 pg/kg of polychlorinated biphenyls (PCBs) (a concentration likely to adversely affect biota health (Environment Canada, 1998)), the concentration of PCBs in the whole-water sample will be 0.015 pg/L. This concentration is well below most laboratory method detection limits—for example, the USGS National... 

Polychlorinated biphenyls (PCBs) are a major environmental problem. These oily substances have many uses, but they resist breakdown by bacterial action when spilled in the environment and, being fat-soluble, can accumulate to dangerous concentrations in the fatty tissues of fish and animals. One little-appreciated complication in controlling the problem is that there are 209 different PCBs, all now in the environment. They are generally similar, but their solubilities in fats differ considerably. The best measure of this is Kay, the equilibrium constant for the partition of a PCB between the fat-like solvent octanol and water. 

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