36
Pesticides, Organic Contaminants, and Pathogens in Air
mixing on either side of the interface, decreasing in width with an increase
in turbulent kinetic energy. For dilute aqueous solutions, the flm thicknesses
are assumed to be independent of chemical concentrations. In the absence of
turbulence, pure diffusion, as measured by liquid diffusion coeffcients (D l ),
is a dominant process in volatilization. Since D g > D l , the rate-limiting step for
fux from an aqueous solution will be diffusion from the liquid:
J l = − D dc/dx
l (
)
(3.11)
for an ideal solution, where J l is diffusive fux from water and dc/dx is the
liquid concentration gradient. For a nonideal solution, the relationship is
J = − (Dc /RT)(˙µ / x
˙ )
(3.12)
i
i
i
where R and T are the gas constant and temperature, respectively, and µ i is
the chemical potential for the ith species. The chemical potential µ i can be
written as
i
°
i
RT ln ˝ i
(3.13)
µ = µ +
where activity α i = X i γ i , the product of mole fraction and activity coeffcient.
This all comes together in a foggy atmosphere or cloud, in which suspended
water droplets both accumulate chemicals from the air, and also release the
accumulated chemicals to the air. Glotfelty et al. (1987) measured the concentration of a number of pesticides, polychlorinated biphenyls (PCBs) and
other semivolatile chemicals in fogwater, and found that fogwater accumulated some of the contaminants to a greater degree than predicted by Henry’s
law, apparently refecting operation of the water surface flm as a third phase
in the water-to-air partition coeffcient (see Chapter 7). During rainfall, rain
drops can absorb atmospheric components in a manner that can be predicted
by the components’ Henry’s law constants, resulting in a parameter known
as washout ratio (W), which is a function of H:
˜
(3.14)
W 1/H
As with many environmental distribution coeffcients, these processes occur
as described above for pure water and pure air systems. If the water contains
dissolved solutes such as salt, or surface-active detergent molecules, or suspended clay or other soil components, the observed air/water distribution
can be signifcantly affected.
3.2.2.2 Azeotropes
If the total vapor pressure of aqueous mixtures deviates positively or negatively from Raoult’s law, azeotropes can be formed at a particular mixture
Pesticides, Organic Contaminants, and Pathogens in Air
mixing on either side of the interface, decreasing in width with an increase
in turbulent kinetic energy. For dilute aqueous solutions, the flm thicknesses
are assumed to be independent of chemical concentrations. In the absence of
turbulence, pure diffusion, as measured by liquid diffusion coeffcients (D l ),
is a dominant process in volatilization. Since D g > D l , the rate-limiting step for
fux from an aqueous solution will be diffusion from the liquid:
J l = − D dc/dx
l (
)
(3.11)
for an ideal solution, where J l is diffusive fux from water and dc/dx is the
liquid concentration gradient. For a nonideal solution, the relationship is
J = − (Dc /RT)(˙µ / x
˙ )
(3.12)
i
i
i
where R and T are the gas constant and temperature, respectively, and µ i is
the chemical potential for the ith species. The chemical potential µ i can be
written as
i
°
i
RT ln ˝ i
(3.13)
µ = µ +
where activity α i = X i γ i , the product of mole fraction and activity coeffcient.
This all comes together in a foggy atmosphere or cloud, in which suspended
water droplets both accumulate chemicals from the air, and also release the
accumulated chemicals to the air. Glotfelty et al. (1987) measured the concentration of a number of pesticides, polychlorinated biphenyls (PCBs) and
other semivolatile chemicals in fogwater, and found that fogwater accumulated some of the contaminants to a greater degree than predicted by Henry’s
law, apparently refecting operation of the water surface flm as a third phase
in the water-to-air partition coeffcient (see Chapter 7). During rainfall, rain
drops can absorb atmospheric components in a manner that can be predicted
by the components’ Henry’s law constants, resulting in a parameter known
as washout ratio (W), which is a function of H:
˜
(3.14)
W 1/H
As with many environmental distribution coeffcients, these processes occur
as described above for pure water and pure air systems. If the water contains
dissolved solutes such as salt, or surface-active detergent molecules, or suspended clay or other soil components, the observed air/water distribution
can be signifcantly affected.
3.2.2.2 Azeotropes
If the total vapor pressure of aqueous mixtures deviates positively or negatively from Raoult’s law, azeotropes can be formed at a particular mixture