Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 166

TABLE 4.42
Selected Photolysis Reaction Parameters

Photolysis Reaction Parameters
Rate Constant (Near
Surface) k p , h − 1

Compound Quantum Yield Wavelength, nm

Pesticides
Carbaryl
DDT
Parathion
Atrazine
Trifluarin

0.002-0.004
<5 × 10 −7
0.0024-0.003
0.0002
0.03

0.01
0.16
<0.001
—
0.002

Sunlight
254
>280
Sunlight
313

Monocyclic aromatics
Pentachlorophenol
2,4-Dinitrotoluene

0.23-1.2
0.016

—
0.00075

290-330
313

Polycyclic aromatic hydrocarbons
Anthracene
Benzo[a]pyrene

0.15
0.58

0.003
0.0009

360
313

Source: Schnoor, C.J.L., Environmental Modeling: Fate and Transport of Pollutants in Water, Air, and
Soil , Wiley-Interscience, New York, 1996. With permission.

Rate constants and half-lives for photolysis in sunlight can be calculated as
functions of season, latitude, time of day, depth of water body, and the ozone layer
condition. Environmental influences, such as depth of the chemical in water, sensi-
tizers, quenchers, and pH, can also affect the rate of phototransformation. 137
Water is not as good a matrix as the atmosphere for photochemical reactions
because of the attenuation of the incident light in the water column. Nevertheless,
photochemical transformations in water have been shown to be important for some
compounds. 156 Some chemical species in natural waters can absorb light energy
directly and undergo direct photochemical reactions. Examples include the direct
photolysis of nitrate, nitrite, and methyl iodide in seawater. These, in turn, can react
with compounds that are not photoreactive resulting in indirect, or sensitized pho-
tochemical reactions. If these reaction products are still reactive, as are singlet
oxygen and hydrogen peroxide, further reactions can occur.
The kinetics of photochemistry are discussed in detail in Leifer, 163 Matsumura
and Katayama, 164 and Nubbe et al. 162

4.2.3.7 Biodegradation

Biodegradation is a commonly accepted term that describes microbially mediated
alterations of a compound that are sufficient to change its identity. All biodegradable
compounds are ultimately converted to one of a very few key intermediates, such as
acetyl-coenzyme A or to one of the tricarboxylic acid cycle intermediates prior to total
oxidation. As such these compounds are used for the resynthesis of larger molecules,
only a portion of biodegradable compounds being totally oxidized to inorganic con-
stituents. Biodegradation of organic compounds appears to be the most environmentally