Partition Coefficients

Partitioning of PCBs into other organic compound mixtures or phases found in the environment alters environmental parameters used to estimate their fate and transport. For example, dissolved phase humic substances (i.e., DPHS) can increase the apparent solubility of organic pollutants [381-390] (see Chap. 2).

The most common partition coefficient encountered in environmental work (Sect. 2.1.4) is the octanol water partition coefficient (KOW) and the solid phase carbon-water partition coefficient (KOC). A partition coefficient for dissolved organic matter-water (i. e., Kd-OM ) or dissolved organic carbon-water (i. e., Kd-OC ) occasionally appears in the literature. In the case of PCBs, Boyd and Sun [378] defined a partition coefficient for residual transformer oil and water as Kd-oil, while Sun and Boyd [379] defined a coefficient for PCB dielectric fluid-water as Kd-PCB. These authors [378,379] identified a total partition coefficient that com-

bines coefficients for several components of the soil-water system. They defined this as follows:

where Kp is the overall partition coefficient, fm is the fraction of material in medium, and Km is the partition coefficient for medium.

Any assessment of PCBs, leaching from electric utility equipments, in the environment must first consider partitioning into the various media involved in the electric equipment themselves before partitioning into other environmental solid phases. For example, most PCBs research used either pure PCB isomers or Aroclors without the fluidizers normally found in utility equipment. These fluidizers, such as mineral oil and chlorinated solvents, used in the equipment all act as partitioning media for PCB isomers. In general, lower molecular weight PCB isomers partition into higher molecular weight isomer mixtures along with partitioning into the fluidizers. Combining the work of several workers in the field [65 - 77,378,379,382] the following relationship can be defined:

Kp = fOC ■ KOC + fmo ■ Kmo + fPCB ' KPCB (55)

where Kp is the total partition coefficient, fOC is the fraction of solid particle organic carbon, KOC is the partition coefficient for organic carbon-water,fmo is the fraction of mineral oil in solid phase, Kmo is the partition coefficient for mineral oil-water, fPCB is the fraction of Aroclor in solid phase, and KPCB is the partition coefficient for Aroclor-water.

Hydrophobic pollutants such as PCBs often partition into lipid rather than into water. The KOW measures this partitioning. This coefficient provides an indication of the degree to which a pollutant accumulates into fatty tissues and any organic phase. This coefficient is especially useful for determining the release of PCBs from mineral oil transformer fluids, and Hawker and Connell [391] pro-

Log Kow = 7.2605 - 0.747 Log S (R2 = 0.8532)

Log Solubility (ug/L)

Fig. 2. PCB solubility-KOW relationship (based on data presented in Table 7)

vided a listing of the KOW for 180 PCB isomers. In general, PCB isomers partition into an oil phase rather than a water phase and residual oil in the solid phase is approximately ten times more effective for retaining PCBs than solid phase organic matter (i.e., SPOM). Partitioning into an oil phase significantly reduces the mobility of PCBs and other hydrophobic pollutants [378]. KOW can be estimated from the solubility of the PCBs themselves. The regression equation shown in Fig. 2 provides an estimate of KOW for the PCB isomers, and the coefficient is also highly correlated with the degree of chlorination of the biphenyl (Fig. 3).

Solid phase organic carbon (i.e., KOC) controls partitioning of hydrophobic contaminants such as PCB isomers [392-402]. KOC is a measure of this partitioning. KOC can be estimated from either solubility or KOW as derived in this chapter and shown in Fig. 4.

Extinct Animal Timeline

2 4 6 8 10

Number of Chlorine Atoms in Isomers

Fig. 3. The logKOW of PCBs vs the number of chlorine atoms in isomers

Koc And Kow Correlation

4 5 6 7 8

4 5 6 7 8

Fig. 4. The logKOW-KOC relationship (based on data presented in Table 7)

Continue reading here: Solubility

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