Geoscience Reference
In-Depth Information
where P is a photoreactive substance and P* is the excited state that reacts to form
products. The photolysis rate is related directly to the adsorption spectrum of
contaminants having the spectral distribution of sunlight. Because radiation at
wavelengths below 290 nm is adsorbed by atmospheric ozone, organo or orga-
nometallic contaminants that do not absorb radiation above this wavelength do not
undergo direct photolysis.
The rate of any type of photoreaction depends also on the rate of light
absorption by the photoactive species. The light transmission characteristics of an
aqueous system must be determined to account for the degree to which system
components other than the contaminant itself attenuate radiation in the water
column. Summing over all wavelengths absorbed by all contaminants and natural
organic compounds in the water body gives the total rate constant of light
absorption.
Indirect photolysis occurs when a species other than the organic or organo-
metallic toxic compound absorbs sunlight and initiates reactions leading to con-
taminant transformation. Natural products present in natural waters, such as humic
substances, clay colloids, and transition metals, absorb the radiation required for
subsequent reactions and act as natural sensitizers. Electrically excited molecules
are capable of greatly accelerating the process and determine a number of light-
induced transformations of toxic organic chemicals.
Singlet oxygen is a selective oxidant that reacts at appreciable rates with
electron-rich groups. Singlet oxygen is formed when an excited sensitizer transfers
its energy to ground-state (triplet) oxygen. In natural waters, the sensitizers are
generally related to dissolved humic substances. In addition, any substance dis-
solved in water, on adsorbing sunlight, crosses to a triplet state and potentially
produces singlet oxygen; such substances include hydrogen peroxide, superoxide,
alkyl peroxy radicals, and hydroxyl radicals.
In natural water, singlet oxygen originating from humic substances has been
shown, for example, to oxidize thioether pesticide contaminants such as disulfoton
(Zepp et al.
1981
). Irradiation of dilute hydrogen peroxide in the presence of
various non-sunlight-absorbing herbicides results in enhanced oxidation of these
substances (Draper and Crosby
1981
).
Coordinative interactions in natural waters change as a result of a variation in
coordinative species or coordination number, which in turn leads to a transfor-
mation of contaminant properties. Any combination of cations with molecules or
anions containing free pairs of electrons (bases) is called coordination (or complex
formation). The coordination can be electrostatic, covalent, or a mixture of both.
The metal cation is called the central atom, and the anion or molecule with which
it forms a coordinative compound is referred to as a ligand.
The actual form in which a contaminant molecule or ion is present in natural
water, as result of a change in the coordinative relationship, emphasizes a specific
chemical speciation. A chemical species is defined by IUPAC ''as the isotopic
composition, electronic or oxidation state, and/or complex or molecular structure,''
and the speciation of an element as ''the distribution of an element amongst
defined chemical species in a system'' (Templeton et al.
2000
).
