McKay, Donald. "Front matter" Multimedia Environmental Models ...

and that sorbed is
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j A = C A /C T = 1/(1 + K BAv B)
j B = (1–C A/C T) = K BAv B/(1 + K BAv B)
The key quantity is thus K BAv B or the product of the dimensionless partition coefficient
and the volume fraction of the dispersed sorbing phase. When this product is
1.0, half the solute is in each state. When it is smaller than 1.0, most is dissolved,
and when it exceeds 1.0, more is sorbed. When the phase B is solid, it is usual to
express the concentration C B in units of moles or grams per unit mass of B in which
case K BA has units of volume/mass or reciprocal density. For example, it is common
to use mg/L for C A, mg/kg for C B, and L/kg for K P; then, with M in mg and V A in
L, then it can be shown that
M = C A(V A + m BK P) = C TV A
where m B is the mass of sorbing phase (kg) from which
C A = C T/(1 + K Pm B/V A) = C T/(1 + K PX B)
where X B is the concentration of sorbent in kg/L. The units of the partition coefficient
K BA or K P and concentration of sorbent v B or X B do not matter as long as their
product is dimensionless and consistent, i.e., the amounts of sorbing phase, continuous
phase, and chemical are the same in the definition of both the partition coefficient
and the sorbent concentration.
Care must be taken when interpreting sorbed concentrations to ascertain if they
represent the amount of chemical per unit volume or mass of sorbent, or the per
unit volume of the environmental phase such as water.
The analogous fugacity equations are simply
j A = V AZ A/(V AZ A + V BZ B)
j B = V BZ B/(V AZ A + V BZ B)
In some cases, it is preferable to calculate a Z value for a bulk phase consisting
of other phases in equilibrium. Examples are air plus aerosols; water plus suspended
solids; and soils consisting of solids, air, and water. If the total volume is V T, the
effective bulk Z value is Z T, and equilibrium applies, then the total amount of
chemical must be
Thus,
V TZ Tf = Sv iZ if