Contaminant Fate and Transport - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 283

alcohols and organic acids are metabolized and degraded rapidly by microbes, perhaps
because these compounds also occur naturally and microbes have evolved to degrade
them more efficiently (Hemond and Fechner-Levy 2000). Halogenated synthetic or
anthropogenic compounds, however, are not easily degraded by microbes (USEPA 2006),
and some contaminants that escape biotic degradation bioaccumulate in the bodies of
organisms.
The rate of biodegradation by microorganisms generally slows if the organic contami-
nants possess the following (Hemond and Fechner-Levy 2000):

• High molecular weight
• Low water solubility
• Presence of benzene or aromatic rings
• A large amount of branching within the molecular structure
• Presence of halogen atoms in the structure (chlorine, fluorine, bromine, or iodine)

8.3.2.2 Abiotic Degradation

Abiotic degradation refers to degradation processes accomplished without microorgan-
isms. Common abiotic degradation processes include

• Photolysis
• Hydrolysis
• Reduction-oxidation
• Radioactive decay

Photolysis (sometimes referred to as photodegradation or photochemical degradation)
occurs in the presence of sunlight. Common examples of this process include the fading of
colored and dyed objects and the transformation of plastic object textures from pliable to
brittle. Photolysis is most common in the atmosphere, surface water, and at Earth's surface.
The degree of degradation caused by sunlight depends on the wavelength spectrum of the
light, intensity of light exposure, and duration (Hemond and Fechner-Levy 2000). If the
energy per photon is sufficient to break a specific chemical bond, photolysis can be initiated.
Once begun, increased light intensity will result in a faster rate of degradation. Ultraviolet
light is especially effective at degrading many organic contaminants (USEPA 1996a).
Degradation by photolysis is often observed in organic compounds with double bonds
between their carbon atoms. Many VOCs, PAHs, and semivolatile organic compounds
(SVOCs) characterized by a benzene ring fit this pattern. Under favorable conditions, the
degradation of many organic compounds by photolysis may occur in a short period of
time—from a few hours to a few days (Lyman et al. 1990).
The process of hydrolysis occurs when a water molecule breaks. Contaminant degrada-
tion by hydrolysis also destroys a molecule of contaminant. Two types of chemical com-
pounds are susceptible to degradation by hydrolysis (Schwartzenbach et al. 1993):
• Alkyl halides, straight-chained or branch-chained hydrocarbons, where one or
more hydrogen atoms have been replaced by a chlorine, fluorine, bromine, or
iodine atom. Using an “X” to represent a halogen atom and an “R” to represent the
hydrocarbon group, the basic hydrolysis reaction is shown in Equation 8.4:

+ −

H O R X + − → − R OH H X + + (8.4)

2