Encyclopedia of Environmental Science and Engineering, Volume I and II

950 PCBs AND ASSOCIATED AROMATICS

TABLE 58

PCB emulsions in water are reduced to low parts per trillion

Time (min.)/PCB Concentration (ppm)

Initial (ppm PCB) 10 min. 20 min. 30 min. 90 min.

22.5 0.028 0.007

254 13.2 8.7 4.6

3,300 25.5 2.9 0.2

TABLE 59

Polychlorinated dioxins (ppt) Polychlorinated dibenzofurans (ppt)

Component Untreated Treated Untreated Treated

2,3,7,8-TCDF N.D. N.D. 0.5 0.03

Tetra-chloro- 1.2 0.15 1.5 0.21

Penta-chloro- 0.95 0.48 1.5 0.12

Hexa-chloro- 1.1 0.17 1.5 0.15

Hepta-chloro- 1.7 0.37 2.8 0.32

Octa-chloro- 1.3 0.22 1.6 N.D.

N.D.  not detected.

No mechanistic studies have so far been done to determine

the formation of intermediate reaction products.

Once an hydroxylated product has been formed in a

dechlorination reaction, there is a possibility that, given the

right conditions for reaction, PCDFs or PCDDs might be

formed. Such reactions are known to occur under pyroly-

sis conditions and also when hydroxyl radicals are produced

during the course of a reaction such as ozone oxidation and

the interaction of solutions with high energy electrons gener-

ated by a Van der Graff machine or Sr^90 b − -particles. These

reactions have been discussed in an earlier section.

The dimerization of chlorophenate precursors to yield

dibenzofurans is a bimolecular reaction and therefore the

products formed should be highly dependent upon the chlo-

rophenate concentrations. The particular PCDD isomers

formed in the system, and their quantities, will depend upon

the relative kinetics of alternative reaction routes.

The data in Table 59 indicate that aqueous samples

which contained PCDFs and PCDDs can be dechlorinated

to below the method detection limit of 0.03 parts per tril-

lion. Work to determine the efficiency and optimal treat-

ment parameters of the method towards the elimination of

PCDFs and PCDDs has not been specifically addressed. The

sample analyses shown in the table derive from real samples

which contained PCDF and PCDD contamination together

with other contaminants. All of the contaminants were suc-

cessfully removed. During this experiment the system was

not intended to specifically reduce PCDF and PCDD com-

pounds but rather to remove other components of concern.

Since these classes of compounds are chemically more reac-

tive than chlorobenzenes it is expected that PCBs/PCDFs/

PCDDs would also react to the observed low levels found in

the current research.

Analytical chemistry at the time of the ‘Yusho’ incident

in 1968 was at a stage of development in which the deter-

mination of compounds of concern at the parts per million

level was considered to be the state-of-the-art. Vos showed

in 1970 that PCDFs could be determined in the presence of

a matrix of PCBs but it was not until about 10 years later

that analytical methodology had advanced to the point that

PCDFs and PCDDs could be determined with a standard,

quality assured protocol.

The analysis protocol used to determine the PCDF data

in Table 59 is designed to precisely and accurately quantitate

the 2,3,7,8-TCDF and 2,3,7,8-TCDD isomers which are of

greatest health concern. The remaining congeners are classi-

fied according to the number of chlorine atoms contained in

the isomeric group. The concentration of the isomeric group

is reported as the sum of the concentrations of the unsepa-

rated isomers.

The technology might be designed into a closed loop

system. For example, the excavated soil is washed, the sol-

vent mixed with supporting electrolyte and fed into the elec-

trochemical cell. Compounds of concern are destroyed and

the wash water and solvent regenerated and recirculated for

further use.

The cost-efficiency of the process can be optimized

by reducing the concentration of contaminants of concern

to an intermediate level in the closed-loop system. The

incompletely reacted mixture can be re-used to extract the

excavated soil for a second pass treatment. The solvent is

re-loaded with contaminants and treated in a second pass.

Finally, the mixture is completely reacted to a prescribed

level which will allow the clean soil to be returned to the

site. The solvent need not be treated to extremely low levels

because it is used in the closed-loop system to re-extract sub-

sequent batches of contaminated soil.

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