Encyclopedia of Environmental Science and Engineering, Volume I and II

PCBs AND ASSOCIATED AROMATICS 883

the formation of chloride ion and the consumption of hydrox-

ide ion were approximately equal at each dose. The results

paralleled the study by Webber 1983 in which transformer

askarels were irradiated in dielectric insulating oil with Sr^90

β-particles. It was found that the dechlorination reaction did

occur but that, in the absence of potassium hydroxide and

isopropanol, the degradation required impracticably long

irradiation times.

The reaction scheme which explains the experimental

data is shown in Figure 21.

• The beta particle radiolysis of water produces
  hydroxyl radicals and solvated electrons, reac-
  tion (1).


• The solvated electrons react with PCBs to give a
  PCB radical anion, reaction (2).


• The anion stabilizes itself by eliminating chlorine
  as chloride to leave a PCB radical, reaction (3).


• In the presence of isopropanol the PCB radical
  can easily abstract hydrogen to become a PCB
  molecule with one less chlorine than at the begin-
  ning of the reaction sequence, reaction (4).
  
  
  • The isopropanol radical formed in the hydrogen
    abstraction step reacts with the hydroxyl ions
    from the potassium hydroxide in the system to
    produce acetone radical ions, reaction (5).
  
  
  • The acetone radicals become stabilized as acetone
    after interaction with PCBs to yield PCB radical
    anions, reaction (6).
  
  
  • The reaction sequence continues in this way until
    total dechlorination has occurred.
  
  
  
  

Hydroxyl radicals are extremely reactive species and

are produced in very small amounts relative to the isopro-

panol present. It is consequently more likely that reaction

will take place with isopropanol than with a small amount of

PCB or an even smaller concentration of PCB radicals. Also,

the concentration of PCB radicals is so small relative to the

macro amount of isopropanol present that coupling to pro-

duce a polychlorinated quaterphenyl is unlikely. Certainly

mixtures of PCBs are destroyed by b -particle radiolysis

most efficiently when the solution is free of oxygen and con-

tains an alkaline hydrogen donor to allow free radical chain

propagation. However, in the absence of isopropanol and

FIGURE 21 The radiolysis of PCBs with 2 MeV Sr^90 b-particles.

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