The absorption of photon energy by CDOM can lead to several types of photophysical and photochemical reactions, and we emphasize that light absorption may be the first of many steps that can ultimately lead to the chemical changes we observe in CDOM. Most CDOM photochemistry involves the excitation of humic substances, which have a large degree of double bond character (C=C and C=0) that readily absorb sunlight energy. However, beyond direct chemical reaction from absorbing photon energy, excited species may participate in a number of indirect chemical reactions.
Direct, or primary, photochemical reactions are the immediate chemical changes to CDOM such as isomerization, bond cleavage, and photolysis. Thus, we refer to the chemical rearrangements and reactions that result from absorption of a photon directly. In isomerization, the absorption of light energy leads to bond breakage and rearrangement, leading to a change in the conformation of the molecule. In direct photolysis, the excited state formed directly by the absorption of light energy undergoes bond cleavage leading to the degradation of large CDOM polymer units into smaller ones. Conceptually, this can lead to the liberation of smaller aliphatic compounds from larger aromatic compounds but, as Kieber and Mopper point out , even smaller organic compounds (a-keto acids) can also undergo photolysis Photochemical reaction with available oxygen may lead to photochemical decarboxylation and the formation of CO2 [53,54]. Iron-CDOM complexes that absorb light energy may accelerate this process, whereby the oxidation of organic matter proceeds by a ligand-metal charge transfer [54-56].
Furthermore, humic substances in CDOM can gain energy (become photosensitized) as a result of initial absorption of radiation by another molecular entity (termed the photosensitizer). The energized humic species (HS*) may be involved in charge transfer processes leading to the oxidation of organic matter and/or the formation of radicals. Most of these reactions involve the transfer of electrons to dioxygen . Radical formation is a common result of direct photolysis of CDOM and includes the formation of highly reactive oxygen species such as H202, singlet oxygen, inorganic and organic peroxy radicals (as discussed in Chapter 8), and solvated electrons [12,13].
Reactive species formed by the CDOM absorption of UVR can then undergo indirect, or secondary, photochemical reactions. In fact, the many possible reactions caused by photosensitized transient intermediates probably account for most of the photodegradation of CDOM that we observe. The many different photoprocesses involved in CDOM photochemistry make for a very complex pathway of reactions beginning with the initial absorption of light energy and ending with the final products of these multiple reactions.
Indirect, or secondary, photochemical reactions include the chemical changes brought on by photosensitizers, the molecules excited by the initial absorption of light. Sensitizers can include humic substances or other dissolved organic and inorganic species such as transition metals and nitrite and nitrate ions. Photosensitized organic matter has a short life span and tends to transfer its energy to a receptor molecule - usually dioxygen, forming singlet oxygen. Any photosensitized reaction involves the transfer of energy, hydrogen atoms, protons, or electrons [57,58], and the results of these charge transfers by intermediates are the underlying reactions that cause bond breakage or oxidation of the CDOM. Thus, indirect photochemistry has an important affect on the photochemical degradation of CDOM.
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