Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 164

TABLE 4.41
Rate Constants of Oxidation by Singlet Oxygen in Water at 25

° C

Compound log k Ox M − 1 s − 1

Unsubstituted aliphatic carbons 3.5

Cyclic olefins 5.3

Substituted olefins 6.0

Dialkyde sulfide 6.8

Diene 7.0

Imidazole 7.6

Furan 8.2

Trialyleneamine 8.9

The concentration of singlet oxygen was estimated as 10-12 M.

Source: Jorgensen, S.E. and Gromiec, M.J., Mathematical SubModels in Water Quality
Systems , Elsevier, Amsterdam, 1989, p. 183. With permission.

absorption by intermediary compounds. 162 Direct decomposition predominates in sys-
tems with little extraneous dissolved organic matter or particulates.
Indirect photolysis was discovered when researchers noticed that some com-
pounds degraded faster in natural water than in distilled water. The scattering of
light by reflection from particulate matter, and its absorption by non-target molecules
causes light disappearance. Absorbed energy can be converted to heat or can cause
photolysis. Light disappearance is a function of wavelength and water quality (e.g.,
color, suspended solids, and dissolved organic carbon). Thus, in more turbid or
highly colored systems, sensitized photolysis, as compared to direct photolysis, could
be a very significant decomposition mechanism for certain contaminants. 45 Photol-
ysis rate is a function of the quantity and wavelength distribution of the incident
light, the light absorption characteristics of the compound to be photolyzed, and the
efficiency at which the absorbed light produces a chemical reaction. 78
Only light that is absorbed by a molecule can produce chemical changes. Photo-
chemical energy reaches the Earth from the sun in the form of photons with wavelengths
covering the spectra from infrared to the far ultraviolet, including the visible. The
principal energy sources of photons for environmental degradation reactions are those
in the ultraviolet (UV) region. 156 This first law of photochemistry, the Grotthus-Draper
law, has been recognized since the early 19th century. 143 The photolysis rate expression
is given in Section 4.2.4.
At low latitudes, under a cloudless sky and the summer sun, the irradiance at
sea level is of the order of 1000 W m −2 . An overwhelming fraction of this energy

input is converted to heat. However, even a small fraction (about 0.04% on an
average) entering the water is captured by photosynthetic processes is enough to be
the driving force shaping the entire aquatic biological domain. Considering such a
massive input of energy into the complex aquatic environment, the possibility of
occurrence of biological light-induced chemical reactions and their importance in
determining the chemical composition of natural waters can hardly be overlooked.

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