PCBs AND ASSOCIATED AROMATICS 939
A 40,000 gallon per day (gpd) Ultrox system consisting of
a reactor and ozone generator, costs about $125,000 with
O&M costs per day of about $175. For a 150,000 gpd system
the equipment cost is about $300,000. O&M costs for the
large system are about $330 per day so that the cost to pro-
cess 1000 gal. of waste water, including monitoring labor, is
about $4.35 for the smaller unit and about $2.21 for the larger
system.
Virontec has applied the UV/O 3 technology in its
RADINOX® process. A full-scale treatment system was
applied to a PCP-contaminated wood preserving site in
Oregon during 1988. Data from bench scale tests showed
that PCP concentrations were reduced from 10 mg/L to
0.5 mg/L in a 30 gal./min. continuous flow reactor.
C.H. Langford and his co-workers^115 have studied the UV
photodecomposition of several chloroaromatic compounds
using a semiconductor catalyst. It was suggested that a photo-
generated reactive species is desorbed from the TiO 2 surface
by band-gap excitation. The species was not identified but it
was pointed out that hydroxyl radicals have been considered
as the reactive species in several studies and that the superox-
ide ion (O 2 ) shows nucleophilic and oxidative characteristics
in a variety of systems (Matthews^116 ). In particular, Sugimoto
et al.^117 showed that polychlorinated aromatics were degraded
by a superoxide ion initiated pathway.
R. Niessen et al.^118 have studied the phototransformation
of phenols by excitation of nitrate ions. They pointed out that
nitrates are probably one of the main sources of hydroxyl
radicals in natural waters exposed to daylight. The gamma-
ray radiolysis of halogenated benzenes was investigated by
Choi and Willard” using a high activity Co^60 source. It was
found that many similarities existed between UV photoly-
sis reactions and those initiated by gamma rays. The work
was applied in 1971 to an environmentally persistent pesti-
cide, DDT, and in the following year Arai et al.^58 applied the
method to solutions of PCBs.
A high energy gamma-ray passing through matter typi-
cally leaves a track of ionized particles and electrons. The
solvated electrons have a very high affinity for atoms such
as chlorine and abstract them from the PCB molecule as
chloride ion. The reaction leaves the original PCB in a highly
reactive “free radical” state with one chlorine less than it had
originally. A free radical chain reaction can then occur in
which there is a cascade of progressively more dechlori-
nated free radical products. Eventually, a stable configura-
tion is reached and biphenyl and polyphenyl products are
observed.
Van de Graff generated electrons have been used to
study the effects of high energy electron irradiation (3
MeV) on persistent toxic trace components in sludges and
wastewater by Merrill et al.^119 at MIT. It was observed that
in pure, deoxygenated solutions dechlorination did occur
and that the reaction products became more water soluble
than the starting compounds. Aromatic carbons and typi-
cal insulating oil compounds are resistant to breakdown
by high energy radiations. A project was therefore under-
taken for the Canadian Electrical Association (Webber^60 )
to investigate the possibility of decontaminating electricalinsulating oils using beta-particles (2 MeV electrons) from
a radioactive Sr^90 source.
Irradiations were conducted at Simon Fraser University
using different askarels in transformer oil both with and
without isopropanol, potassium hydroxide and air. The PCB
mixtures were found to be dechlorinated by beta-particle radi-
olysis most efficiently in isopropanol/KOH solutions. The
solution acts as an alkaline hydrogen donor and allows free
radical chain propagation to occur in the absence of oxygen
as a free radical scavenger. The reaction also occurs when
commercial askarels are dissolved in transformer oils but
very large radiation doses, and hence reaction times, are then
required for complete dechlorination. The efficiency of the
Sr^90 induced beta-particle reaction measured in terms of its
G(Cl − ) value, was found to be only about 7 10 −3 i.e., about
10 5 times smaller than the efficiency of the Co^60 gamma-
induced reaction in Willard and Choi’s work^57 but about 400
times more efficient than the reaction induced by high energy,
accelerated electrons. The work was stopped because of the
lack of applicable reaction kinetics and because the method
has the potential to produce the hydroxylated precursors of
PCDFs and PCDDs. The reaction mechanism was discussed
in an earlier section on the formation of compounds of con-
cern and shown in Figure 18.BACTERIOLOGICALThe distribution and fate of PCBs in the environment depends
on their dispersion and partition as well as their susceptibil-
ity to degradation by various routes.
A number of papers have been written on the sub-
ject 120 – 132 and this area of investigation may become
increasingly important for the in-situ decontamination of
soils and the cleanup of spills. Biological treatments for
industrial waste systems have been in use for many years
but it is only recently that new microbial strains have been
developed which show promise for the treatment of envi-
ronmentally persistent compounds.
In a study on the persistence of Aroclor 1254 in vari-
ous California soils it was found by Iwata et al.^132 that some
considerable changes had taken place after 1 yr. of incuba-
tion at 30°C and 40% moisture content. It was pointed out,
however, that the loss of some of the components may have
been due to evaporation. Bailey et al.^133 used C^14 -labelled
compounds to obtain material balances in their PCB deg-
radation study in river water. They found that monochloro-
biphenyl degraded rapidly (t 1/2 2 days) to chlorobenzoic
acid which, in turn, degraded to carbon dioxide and water.
By way of contrast, the 2,2’,4,4’-tetrachlorobiphenyl stud-
ied under similar conditions showed no measurable deg-
radation during a period of 50 days. The degradability of
PCBs in lake sediments has been found to decrease with
an increasing degree of chlorination of the biphenyl. Half-
lives for the different congeners range from a few days for
dichlorobiphenyl to 200 days for pentachloro-compounds.
Biodegradability is also affected by the position of chlorineC016_003_r03.indd 939C016_003_r03.indd 939 11/18/2005 1:12:44 PM11/18/2005 1:12:44 PM