33
Properties of Pesticides and Contaminants
decline of amphibians in the Sierras, due to the action of such compounds
as endocrine system disruptors (see Chapter 4). These examples show the
critical role in transport and fate played by volatility, as measured by vapor
pressure, in the absence of other dissipation mechanisms such as chemical
degradation, and when the surface from which evaporation is occurring is
essentially noninteractive.
3.2.2 Volatilization from Water
3.2.2.1 Solutes
The fate of organic chemicals in an aqueous environment is determined
by the complex interactions of chemical, biological, and physical processes.
Chemical processes include hydrolysis, photolysis, chemical reactions, and
the formation of metal ion complexes. Biological processes include microbial
degradation and absorption by biota. Physical processes include dispersion,
sorption by sediments, and volatilization.
For volatilization, vapor pressure is still the driving force that moves a compound into the vapor phase. But, an opposing role is played by the water’s
ability to attract or retard the chemical’s tendency to evaporate. In this case
the Henry’s constant, H, is the controlling physicochemical property. Since H
is proportional to P/S, high P (vapor pressure) can be counterbalanced by high
S (water solubility). The interplay of vapor pressure and water solubility was
shown in an earlier study that correlated fux normalized to water concentration (i.e., fux/[mg/L]) with vapor pressure/water sol. (VP/Sw) for a series of
pesticides applied to water (Woodrow et al., 1997). It was assumed that a chemical’s vapor pressure would be modifed primarily by water solubility. That
is, the lower the residue concentration in water, the lower the effective vapor
pressure (through mole fraction and activity), and thus the lower the volatilization rate. This is illustrated by the form of Raoult’s law for nonideal solutions:
o
P = x P
i ˜
(3.8)
i
i
where P i is the partial pressure of component i in the mixture, x i is its mole
fraction, γ i is its solution activity coeffcient, and P o is its saturation vapor pressure. For water-compatible compounds (i.e., exothermic mixing), the activity
coeffcient is commonly less than unity, showing the effect of solubilization
(van der Waal’s type interactions and hydrogen bonding). For non-water
compatible compounds of low solubility (i.e., endothermic mixing), such as
alkanes and chlorinated alkanes, the activity coeffcient can be much greater
than unity, leading to higher than expected values for P i (positive deviation
from the ideal).
The Woodrow et al. (1997) study led to very good correlation for about 12
cases, but only when the measured fux was normalized to actual water concentration (Table 3.2):
Properties of Pesticides and Contaminants
decline of amphibians in the Sierras, due to the action of such compounds
as endocrine system disruptors (see Chapter 4). These examples show the
critical role in transport and fate played by volatility, as measured by vapor
pressure, in the absence of other dissipation mechanisms such as chemical
degradation, and when the surface from which evaporation is occurring is
essentially noninteractive.
3.2.2 Volatilization from Water
3.2.2.1 Solutes
The fate of organic chemicals in an aqueous environment is determined
by the complex interactions of chemical, biological, and physical processes.
Chemical processes include hydrolysis, photolysis, chemical reactions, and
the formation of metal ion complexes. Biological processes include microbial
degradation and absorption by biota. Physical processes include dispersion,
sorption by sediments, and volatilization.
For volatilization, vapor pressure is still the driving force that moves a compound into the vapor phase. But, an opposing role is played by the water’s
ability to attract or retard the chemical’s tendency to evaporate. In this case
the Henry’s constant, H, is the controlling physicochemical property. Since H
is proportional to P/S, high P (vapor pressure) can be counterbalanced by high
S (water solubility). The interplay of vapor pressure and water solubility was
shown in an earlier study that correlated fux normalized to water concentration (i.e., fux/[mg/L]) with vapor pressure/water sol. (VP/Sw) for a series of
pesticides applied to water (Woodrow et al., 1997). It was assumed that a chemical’s vapor pressure would be modifed primarily by water solubility. That
is, the lower the residue concentration in water, the lower the effective vapor
pressure (through mole fraction and activity), and thus the lower the volatilization rate. This is illustrated by the form of Raoult’s law for nonideal solutions:
o
P = x P
i ˜
(3.8)
i
i
where P i is the partial pressure of component i in the mixture, x i is its mole
fraction, γ i is its solution activity coeffcient, and P o is its saturation vapor pressure. For water-compatible compounds (i.e., exothermic mixing), the activity
coeffcient is commonly less than unity, showing the effect of solubilization
(van der Waal’s type interactions and hydrogen bonding). For non-water
compatible compounds of low solubility (i.e., endothermic mixing), such as
alkanes and chlorinated alkanes, the activity coeffcient can be much greater
than unity, leading to higher than expected values for P i (positive deviation
from the ideal).
The Woodrow et al. (1997) study led to very good correlation for about 12
cases, but only when the measured fux was normalized to actual water concentration (Table 3.2):