Handbook of Solvents - George Wypych - ChemTech - Ventech!

17.1 The environmental fate and movement of organic solvents 1151
more complex, and depends on the rate of flow, depth, and turbulence of both the body of
water and the atmosphere above it. In the absence of measured values, there are a number of
estimation techniques to predict the rate of removal from water. 8
17.1.2.3 Degradation
The disappearance of a solvent from solution can also be the result of a number of abiotic
and biotic processes that transform or degrade the compound into daughter compounds that
may have different physicochemical properties from the parent solvent. Hydrolysis, a
chemical reaction where an organic solvent reacts with water, is not one reaction, but a family
of reactions that can be the most important processes that determine the fate of many organic
compounds. 9 Photodegradation is another family of chemical reactions where the
solvent in solution may react directly under solar radiation, or with dissolved constituents
that have been made reactive by solar radiation. For example, the photolysis of water yields
a hydroxyl radical:
HO 2 + hν →HO•+ H
[17.1.3]
Other oxidants such as peroxy radicals (RO2�) and ozone can react with solvents in
water. The subject of photodegradation is treated in more detail under atmospheric processes
(17.1.4).
Biodegradation is a family of biologically mediated (typically by microorganisms)
conversions or transformations of a parent compound. The ultimate end-products of
biodegradation are the conversion of organic compounds to inorganic compounds associated
with normal metabolic processes. 10 This topic will be addressed under Soil (17.1.3.3).
17.1.2.4 Adsorption
Adsorption is a physicochemical process whereby a dissolved solvent may be concentrated
at solid-liquid interfaces such as water in contact with soil or sediment. In general, the extent
of adsorption is inversely proportional to solubility; sparingly soluble solvents have a
greater tendency to adsorb or partition to the organic matter in soil or sediment (see Soil,
17.1.3.2).
17.1.3 SOIL
17.1.3.1 Volatilization
Volatilization from soil may be an important mechanism for the movement of solvents from
spills or from land disposed solvent-containing wastes. The efficacy and rate of volatilization
from soil depends on the solvent�s vapor pressure, water solubility, and the properties of
the soil such as soil-water content, airflow rate, humidity, temperature and the adsorption
and diffusion characteristics of the soil.
Organic-solvent vapors move through the unsaturated zone (the interval between the
ground surface and the water-saturated zone) in response to two different mechanisms; convection
and diffusion. The driving force for convective movement is the gradient of total
gas pressure. In the case of diffusion, the driving force is the partial-pressure gradient of
each gaseous component in the soil air. The rate of diffusion of a solvent in bulk air can be
described by Fick�s Law, viz.,
Q =−Df∇ a
[17.1.4]