Chemical Synthesis and Reactivity

PCBs are manufactured by the direct chlorination of biphenyl in the presence of Fe or FeCl3, as shown earlier in reaction (8-11). A mixture of isomeric products is obtained, the structures of which depend on the reaction temperature and the ratio of Cl2 to biphenyl. Since the biphenyl is usually contaminated with polyphenyls such as tetraphenyl, the chloro derivatives of these products are also obtained in reactions analogous to (8-11).

PCBs are among the most persistent compounds in the environment. As noted previously, they are resistant to both hydrolysis and oxidation and, as we discuss shortly, undergo slow microbial degradation. It is not surprising that biphenyls are so stable, since the aryl halide functional group is exceedingly resistant to hydrolysis (see Section 8.3.2.3).

Highly chlorinated PCBs are photodegraded with the long-wavelength UV light (> 290 nm), which is not absorbed by the ozone layer. The initial step in the photodegradation process involves the dissociation of a chlorine atom, and the radicals that form abstract a hydrogen atom from another organic molecule, as shown in reactions (8-27) and (8-28). When the photolysis proceeds in the presence of water, phenols are obtained as products [reaction (8-29)] by the reaction of water with the triplet excited state of the PCB.

o h2o

Photochemical reactions are not a major pathway for the environmental destruction of PCBs because only the highly chlorinated derivatives absorb at wavelengths greater than 290 nm and the quantum efficiency of the reactions is low. Lightly chlorinated biphenyls are not degraded photochemically in the environment because they do not absorb light at wavelengths greater the 290 nm; they absorb at the shorter wavelengths, which do not penetrate the ozone layer.

PCBs have low water solubility 50 ppb for tetrachlorobiphenyls and 1 ppb for hexachlorobiphenyls) and are more dense than water. When discharged into lakes and rivers they sink to the bottom, where they are adsorbed on hydrophobic organic sediments. PCBs, as noted earlier, have low volatility (10~4-10~natm), with the largest amounts in the atmospheres of urban areas (0.2-20 ng/m3) and the lowest in marine air (0.02-0.34 ng/m3). The atmospheric reservoir of PCBs is less than 0.1% of the total amount in the environment. In spite of these low volatilities, atmospheric transport is the principal way PCBs are spread globally as a gas or bound to particles. The Great Lakes of North America have become a major source of atmospheric PCBs owing to the revolatilization of the PCBs present there. PCB concentrations are high in urban areas as a result of their volatilization from municipal landfills and waste disposal sites, where they were dumped prior to 1978, when their disposal was regulated.

The destructive disposal of PCBs requires extreme reaction conditions because they are such stable compounds. Since they are resistant to air oxidation at temperatures below 700°C, incineration at lower temperatures, for example, while burning trash, results in partial oxidation to chlorobenzofurans:

A polychlorinated dibenzofuran

A polychlorinated dibenzofuran

Reductive removal of the chloro groups proceeds with strong reducing agents such as LiAlH4, sodium, or titanium(III), or by catalytic hydrogenation to generate biphenyl, a compound that can be oxidatively destroyed by combustion. Complete hydrolysis can be attained by treatment with NaOH in methanol at 300-320°C and pressure of 180 kg/cm2.

High-temperature combustion (~1000°C) is used most frequently for complete destruction (> 99.9999% of the PCBs can be destroyed). Cement kilns, with their high temperatures (1500°C) and basic conditions, are ideal sites for the combustion of PCBs. In this process a slurry of crushed limestone and silicate-containing rock, containing 30-40% water, is fed in one end of a 500ft rotating tube, and this slurry slowly migrates by gravity to the other end of the tube, where oil or coal and the PCBs are burning to give the high temperature. The PCBs are pyrolyzed and oxidized to carbon dioxide while burning, and the chlorine is released as HCl. The HCl reacts with the basic compounds present in the limestone to form calcium and magnesium chloride as it passes through the cement and slurry. Cement formation is a slow process, so the PCBs have a long residence time in the kiln where the temperatures are maintained at 1370-1450°C in the presence of molecular oxygen. Almost all of the PCBs are destroyed, and the only emissions of chlorinated compounds observed are those detected when no PCBs were added to the kiln.

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