Encyclopedia of Environmental Science and Engineering, Volume I and II

PCBs AND ASSOCIATED AROMATICS 937

as oxidation inhibitors can be understood in terms of the

mechanism of oil oxidation as follows:

Initiator [R H] R R (Initiation)

II

R O ROO

II

ROO R H

2

+− → ⋅+ ⋅

⋅+ → ⋅

⋅+ − →

RROOH R (Chain Propagation)

III

ROO R ROOR (Termination)

R

+⋅

⋅+ ⋅ →

⋅++⋅ → −

⋅+ ⋅ → +

R R R (non-radical products)

ROO R OO ROH RCOR

IV V

The hydrocarbon in the initiation step splits into free radi-

cals (I) which, with dissolved oxygen, forms peroxide radi-

cals (II). The reactivity of a peroxide radicals depends upon

its structure. If it is sufficiently reactive, it will be able to

abstract a hydrogen atom from a nearby oil hydrocarbon

molecule and partially stabilize itself as a hydroperoxide

molecule and yield a further radical. This is the chain propa-

gation step.

The peroxide radical, (II), can be terminated by reaction

with the initial hydrocarbon radical, (I), to produce an ester.

Alternatively, two peroxide radicals can react to give an

alcohol and a ketone or two hydrocarbon radicals can react

to form non-radical products.

The hydroperoxide can decompose as follows (degenerate

branching):

ROOH RO OH

2ROOH RO H O 2 ROO

→⋅+⋅

→⋅+ + ⋅

The reaction scheme illustrates why it is that a large variety

of oxidation products is typically found in used oil. The rate

of oxidation is determined by the concentration of peroxy

radicals. When an oxidation inhibitor is added to the system,

its primary function is to scavenge peroxy radicals and in

doing so itself becomes a radical.

The important difference between the initial peroxy radi-

cal and the newly formed species is that the product radical

is more stable than the reactant radical. The degree to which

the product radical is more stable is a measure of the effec-

tiveness of the compound as an oxidation inhibitor. Such

compounds as phenol show an inhibitive effect towards oxi-

dation in white mineral oil (Ingold^109 ), but the most efficient

inhibitors are found to be sterically hindered phenols such as

di-tertiary-butyl-p-cresol, DBPC. From the above it would

be excepted that the formation of aromatic alcohols from a

chemical decontamination process in particular would give

rise to “natural” oxidation inhibitors. It is probable that

the inhibition produced in this way is less than would be

achieved using DBPC but at least the oil should not be del-

eteriously affected.

The presence of oxidation inhibitors is important for

the reasons that they are essential to the oxidation stability

of the reclaimed oil and that oxidation inhibitor retards the

free radical dechlorination reaction of reagents with PCBs.

In some cases, for example reactions with metallic sodium,

the dechlorination reaction shows a very marked “induction”

period while the alkali metal reacts with oxidation inhibitor.

The reaction products are highly colored because of their

quinoidal structure and it is these compounds which are the

cause of the yellow color normally attributed to oxidized oil.

The quinoidal product is not sufficiently polar to be adsorbed

by fuller’s earth during the reclamation process and is not

detrimental to the dielectric properties of the oil.

A characteristic of the reaction of alkali metals and

organo-metallic reagents with oil components and PCBs is

that a sludge is produced. Filtration of the fine precipitate,

followed by washing and analysis by a variety of analytical

procedures, shows (Webber et al.^61 ) that the sludge contains

predominantly non-chlorinated polyphenyls and is therefore

not a toxic hazard.

Biphenyl is a reaction product together with a large

range of high molecular weight polyphenyls. It is the pro-

duction of partially soluble compounds in the intermediate

molecular weight range which can sometimes cause prob-

lems. The reason is that long chain polyphenyls can exist as

biradicals (Brown et al.^63 ) and contribute strongly towards

a high power factor oil. The effect is particularly noticeable

when treatments are applied to mineral oils which contain

more than a trace of high molecular weight aromatics. In

these cases, if the oil is left standing in contact with organo-

metallic reagents for a period of a few hours, the aromatics

in the oil provide enough solubility for the polyphenyls to

produce a power factor of more than 50%. The problem can

usually be avoided by separating excess chemical reagent

from the decontaminated oil immediately after the dechlori-

nation reaction has taken place.

DISTILLATION AND EXTRACTION

The separation by distillation of transformer oil and askarels

has been evaluated by Battelle-Columbus Laboratories and

D.&D. Disposal Inc. Transformer oil is usually a high boiling

fraction of crude oil which is distilled under vacuum to avoid

degradation. The oil contains a large variety of low vapor

pressure compounds. PCBs also have a low vapor pressure

and the difficulty in separating a small quantity of contami-

nant from the oil lies in the overlap of the boiling ranges of

the components. Since the PCBs are typically higher boiling

than most mineral oil components, conventional distillation

requires that most of the fluid be distilled before the PCBs

can be separated as a residue. The process is both inefficient

and requires a lot of energy.

Steam stripping is an alternative to vacuum distilla-

tion. Separations can be achieved at atmospheric pressure at

temperatures which do not cause thermal degradation. The

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