Protocol for the Derivation of Environmental and Human ... - CCME

Appendix C
Rearranging [2] and substituting in [4]:
and
Substituting [5] into [3]:
which, after rearrangement yields
Y × M s = (C s × M s ) + (Cw × ? m × M s )
Y = C s + (C w × ? m )
C s = Y - (C w × ? m ) [5]
K d = Y - (C w × ? m )
C w
Dissociating Organic Contaminants
Y = C w (K d + ? m ) [6]
Equilibrium partitioning isotherms effectively describe the behaviour of non-dissociating organic
contaminants in soils. This description may be extended to dissociating organic contaminants provided
sorption of both the dissociated and non-dissociated forms is understood and easily treated.
These conditions are met for some weak organic acids, such as chlorophenols, because only the nondissociated
form is appreciably sorbed. Like many other anions, the phenate generated by the
dissociation of the parent chlorophenol is mobile in soils. Because of this difference, chlorophenol
partitioning can be predicted from the concentration of the non-ionized form, which is a function of pH.
The pH-dependent distribution coefficient can then be calculated (Schellenberg et al. 1984) as the
product of the partitioning coefficient for the chlorophenol and the proportion of the non-ionized form:
K d = K oc × F oc × Q [7]
where
K oc = organic carbon-normalized coefficient for non-ionized chlorophenol
F oc = fraction of organic carbon in soil
Q = proportion of chlorophenol in non-ionized form
It is important to note that experimental data that nominate K oc have been referenced to the
concentration of non-dissociated chlorophenol.
Q is derived from the equilibrium acidity expression for the chlorophenol:
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