42
Pesticides, Organic Contaminants, and Pathogens in Air
The focus of this chapter is chemical volatilization. It is one of a number of possible processes (physical, chemical, biological) that contribute to
the fate of a chemical. The diverse examples showed that volatilization is
a general process for both liquids and solids. Volatilization can explain the
initial occurrence of chemicals in the atmosphere, where they eventually
will be dispersed and possibly deposited at some remote location, depending on stability. The banned insecticide DDT is an example of this. Residues
of DDT, and of its major conversion product DDE, have been found as far
away as Antarctica. Another example is the ozone-depleting soil fumigant
MeBr. After volatilizing from treated soil, MeBr has enough stability in the
environment to make its way to the stratosphere. These examples, and many
others (e.g., treated agricultural felds, pavement coatings, and spills), have
prompted the development of methods for determining volatilization fux—
fux chambers, micrometeorological methods (AG, TPS, IHF), numeric and
computer modeling, and others.
3.3 Adsorption/Absorption
Adsorption can be either physical or chemical in nature. In the bulk of the
adsorbent, other atoms fll the bonding requirements (ionic, covalent, metallic) of the constituent atoms. However, atoms on the surface of the adsorbent
are not wholly surrounded by other adsorbent atoms and therefore can attract
adsorbates. For the condensation of gases to liquids, for example, physical
adsorption depends on the van der Waals force of attraction between the
solid adsorbent and the adsorbate molecules. There is no chemical specifcity
in physical adsorption: any chemical contaminants in air tend to be adsorbed
on any solid if the temperature is suffciently low or the pressure of the contaminant suffciently high. In chemical adsorption, chemical contaminants
in air or water are held to a solid surface by chemical forces (ionic, covalent)
that are specifc for each surface and each chemical. Chemical adsorption
occurs usually at higher temperatures than those at which physical adsorption occurs; furthermore, chemical adsorption is ordinarily a slower process
than physical adsorption and, like most chemical reactions, may involve an
energy of activation.
Adsorption differs from absorption, in which a fuid (the absorbate) is dissolved by or permeates a liquid or solid (the absorbent). Absorption is made
between two phases of matter: for example, a liquid absorbs a vapor, or a
solid absorbs a liquid. When a liquid solvent absorbs a chemical vapor mixture or part of it, a mass of chemical moves into the liquid. For example,
water may absorb oxygen from the air. This mass transfer takes place at the
interface between the liquid and the chemical vapor, at a rate depending on
both the chemical and the liquid. This type of absorption depends on the
Pesticides, Organic Contaminants, and Pathogens in Air
The focus of this chapter is chemical volatilization. It is one of a number of possible processes (physical, chemical, biological) that contribute to
the fate of a chemical. The diverse examples showed that volatilization is
a general process for both liquids and solids. Volatilization can explain the
initial occurrence of chemicals in the atmosphere, where they eventually
will be dispersed and possibly deposited at some remote location, depending on stability. The banned insecticide DDT is an example of this. Residues
of DDT, and of its major conversion product DDE, have been found as far
away as Antarctica. Another example is the ozone-depleting soil fumigant
MeBr. After volatilizing from treated soil, MeBr has enough stability in the
environment to make its way to the stratosphere. These examples, and many
others (e.g., treated agricultural felds, pavement coatings, and spills), have
prompted the development of methods for determining volatilization fux—
fux chambers, micrometeorological methods (AG, TPS, IHF), numeric and
computer modeling, and others.
3.3 Adsorption/Absorption
Adsorption can be either physical or chemical in nature. In the bulk of the
adsorbent, other atoms fll the bonding requirements (ionic, covalent, metallic) of the constituent atoms. However, atoms on the surface of the adsorbent
are not wholly surrounded by other adsorbent atoms and therefore can attract
adsorbates. For the condensation of gases to liquids, for example, physical
adsorption depends on the van der Waals force of attraction between the
solid adsorbent and the adsorbate molecules. There is no chemical specifcity
in physical adsorption: any chemical contaminants in air tend to be adsorbed
on any solid if the temperature is suffciently low or the pressure of the contaminant suffciently high. In chemical adsorption, chemical contaminants
in air or water are held to a solid surface by chemical forces (ionic, covalent)
that are specifc for each surface and each chemical. Chemical adsorption
occurs usually at higher temperatures than those at which physical adsorption occurs; furthermore, chemical adsorption is ordinarily a slower process
than physical adsorption and, like most chemical reactions, may involve an
energy of activation.
Adsorption differs from absorption, in which a fuid (the absorbate) is dissolved by or permeates a liquid or solid (the absorbent). Absorption is made
between two phases of matter: for example, a liquid absorbs a vapor, or a
solid absorbs a liquid. When a liquid solvent absorbs a chemical vapor mixture or part of it, a mass of chemical moves into the liquid. For example,
water may absorb oxygen from the air. This mass transfer takes place at the
interface between the liquid and the chemical vapor, at a rate depending on
both the chemical and the liquid. This type of absorption depends on the