2.4.6- Trichlorophenol General

All the chlorophenols can be separated usiag C g-grafted siUca columns. In NH2 grafted columns, the elution depends on the An electrochemical detector ia oxidation mode, more sensitive than uv detectors, is generally used to detect very low quantities, especially ia analy2iag 2,4,5-trichlorophenol. 

It would be diflBcult to estimate the quantity of TGDD which enters the environment each year. In addition to the common pesticides listed in Table I, other chlorophenols nd their derivatives are used widely. For example, large amounts of the disinfectant, hexachlorophene (2,2 -methylenebis(3,4,6-trichlorophenol)), are used in homes, hospitals, and industry, and the Dowcides 2 and B (2,4,5-trichlorophenol and its sodium salt) are industrial microbiocides. More than 50,000,000 lbs of trichloro-phenol are made in the United States each year (24), and much of it eventually must be dispersed in the environment. The dioxin content seems to be variable but is generally below 0.5 ppm (25). 

ZnO photocatalyst can also be coupled with other materials in order to improve its chemical and physical properties [183] and photocatalytic activity [184]. Nanosized ZnO was immobilized on aluminum foil for the degradation of phenol [185]. Lanthanum and ZnO were combined to degrade 2,4,6-trichlorophenol [186]. Compared with Ti02 nanomaterial, ZnO nanomaterial generally absorbs a significant amount of the solar spectrum in the visible range therefore, ZnO nanomaterials were combined with Ti02 nanomaterials used as a photocatalyst [187]. 

The environmental burden of waterways with polychlorinated dibenzo-p-dioxins (PCDD) has been at the forefront of public and regulatory concern, because of the toxicity associated with particularly the 2,3,7,8-(laterally) substituted congeners, which have a tendency to bioaccumulate throughout the trophic food chain. Contamination of aquatic sediments by dioxins includes both non-point (e.g., atmospheric deposition) and point sources (e.g., industrial effluents, combined sewage overflows), and is generally characterized by a dominance of hepta- and octa-CDD, with minor contributions of hexa- to tetra-CDD [429]. Elevated concentrations of the 2,3,7,8-TCDD isomer tend to be associated with direct discharge from sources such as 2,4,5-trichlorophenol production [54,430]. 

As pointed out above, we may assume that Kwill, in general, be significantly smaller than AfiacA. This is clearly seen for sorption of 2,4,5-trichlorophenol (pA,a = 6.94) to sediment organic matter (Fig. 9.17a). For this type of compound, sorption of the neutral species dominates Dioc up to a pH of about 2 units above the pKit of the... 

Trichlorophenols are generally poisons and may be carcinogens. They may contain 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) as a contaminant. Some trichlorophenols are used as herbicides (e.g., 2,4,5-T and silvex). Human exposure may cause chloracne, liver dysfuncdon, muscle weakness, and prophyria. 

Although it is generally believed that under anaerobic conditions dechlorination, either reductive or hydrolytic, precedes ring fission, this pathway has been clearly documented only for the methanogenic studies. The physical proof and transient appearance of each dechlorinated metabolite (e.g., 2,3,6-trichlorophenol — 2,3-dichlorophenol — 3-monochlorophenol), which is seen in methanogenic cultures, has not been observed for the sulfidogenic or denitrifying cultures. It is still unclear whether this indicates that there is indeed a different mechanistic pathway or whether the rates of metabolite transformation are so rapid that they preclude their detection. 

Presence of dioxins in pesticides in general Woolson et al. (1972) examined 129 samples of 17 different pesticides derived from chlorophenols for polychlorinated dibenzo-p-dioxins. The tetra derivatives were found primarily in 2,4,5-T samples. Twenty of 42 samples contained more than 0.5 ppm. The presence of TCDD in some pesticides is easy to understand due to the chemical reactions used for its synthesis. The herbicide 2,4,5-T is produced by condensing 2,4,5-trichlorophenol with chlo-roacetic acid in the presence of NaOH at 105°C to yield the sodium salt of 2,4,5-T. Trichlorophenol may be made by the action of alkali on 1,2,4,5-tet-rachlorobenzene produced by chlorination of the trichlorobenzenes resulting from dehydrochlorination of benzene hexachloride (BHC). Today most countries will not approve a pesticide without data on dioxin content. 

Generally a quality problem is created when heavy chlorophenols (trichlorophenols, tetrachlorophenols and pentachlorophenols) are processed. Impurities such as polychlorophenoxyphenols, polychlorodibenzodioxins, polychlorodibenzofurans and polychlorodihydroxybiphenyls, are quite often found in some industrial products. Table 3 below shows the influence of the structure of the series of dibenzodioxins on the DL 50 (refs. 2, 3) (number and position of the chlorine atoms). 

Concern about dioxin s effect on human health can be traced to a number of Industrial exposures. Its recognition as a byproduct In the production of large-scale quantities of chlorophenols such as 2,4,5- trichlorophenol (TCP) and pentachlorophenol (PCP), the finding of birth defects In animals exposed to 2,3,7,8- TCDD, and the recognition In the 1970 s of a more general environmental contamination due to emissions from combustion sources. 

Several studies have assessed the plausibility of the association between soft tissue sarcomas and exposure to phenoxy herbicides and chlorophenols. Table VI lists two case-control studies conducted in Sweden, which selected controls from the general population and concluded that exposure to the herbicides or to chlorophenols resulted in a greater than 5-fold excess in soft tissue sarcomas (38-39). A New Zealand study found no association with exposure to phenoxy herbicides (40). This study drew controls from other cancer cases in the National Cancer Registry of New Zealand. However, in the New Zealand study, a 5-fold excess of STS was noted in persons who handled animal pelts, which are sometimes preserved with trichlorophenols containing isomers of dioxin. 

The most common method for manufacturing pentachlorophenol is the progressive chlorination of phenols in the presence of Lewis acid catalysts. Another method specific for 2,4,5-trichlorophenol and pentachlorophenol is by hydrolysis of chlorobenzenes such as penta-and hexachlorobenzene. In general, the hydrolysis method always produces more impurities. The final product is a mixture often containing a couple or more of the following compounds ... 

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