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

ates can thus be used to estimate mass transfer coefficients for other solutes having
similar (large) H such as alkanes. Indeed, an ingenious experimental approach for
determining k 2 for oxygen is to use a volatile hydrocarbon, such as propane, as a
tracer, thus avoiding the complications of biotic oxygen consumption or generation,
which confound environmental measurements of oxygen concentration change. It is
erroneous to use k 2 to estimate the rate of volatilization of a chemical with low H,
since k 2 contains negligible air phase resistance information. A correction should
also be applied for the effect of molecular diffusivity, preferably using the dimensionless
form of diffusivity, the Schmidt number raised to a power such as –0.5 or
–0.67.
This technique of probing interfacial MTCs by measuring N for various chemicals
can be applied in other areas. When chemical is taken up by fish, it appears
that it passes through one or more water layers and one or more organic membranes
in series. By analogy with air-water transfer, we can write an organic membranewater
transfer equation simply by replacing subscript A by subscript M, giving
where
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N = k OWA(C W – C M/K OW)
1/k OW = 1/k W + 1/k MK MW
or more conveniently changing to an overall organic phase MTC, k OM,
where
N = k OMA(C WK MW – C M)
1/k OM = 1/k M + K MW/k W
A plot of 1/k OM versus K MW, the organic-water or octanol-water partition coefficient,
gives 1/k M as intercept and 1/k W as slope. This is essentially the fish bioconcentration
equation (discussed in more detail in Chapter 8) in disguise, which is
conventionally written
dC F/dt = k 1C W – k 2C F
where V F is fish volume and C O is C F/L, where L is the volume fraction lipid
(equivalent to octanol) in the fish. It follows that
dC F/dt = (k OMA/V F)(C WK MW – C F/L)
k 1 is obviously K OWk OMA/V F, and k 2 is [k OMA/(V FL)]. k 1/k 2 is then LK OW, the bioconcentration
factor. The area of the respiring gill surface is uncertain as is k OM, so
it is convenient to lump these uncertainties in one unknown k 2. This suggests plotting