Humic Substances photochemical reactions

Zepp RG, GL Baugham, PA Scholtzhauer (1981a) Comparison of photochemical behaviour of various humic substances in water. I. Sunlight induced reactions of aquatic pollutants photosensitized by humic substances. Chemo sphere 10 109-117. 

Labile and refractory DOM undergo abiotic photochemical reactions in the photic zone, especially in the sea surfece microlayer where physical processes concentrate DOM into thin films. Some of these reactions appear to be important in the formation of refractory DOM and others in its degradation. For example, DOM exuded by diatoms during plankton blooms has been observed to be transformed into humic substances within days of release into surfece seawater. Laboratory experiments conducted in seawater have demonstrated that photolysis of labile LMW DOM promotes the chemical reactions involved in humification and produces chemical structures foimd in marine humic substances. 

From the preceding discussions, it should be clear that photochemistry within all phases of the atmosphere is a major driver of chemical transformations in relatively short time scales. With increasing knowledge of the ever-widening array of chromophoric compounds emitted and produced in the atmosphere, there is definitely room for much more fundamental research into primary and secondary photochemical reactions of relevance. In particular, the role of humic-like substances in aerosol, cloud and ice phases needs to be studied. 

A pseudo first-order reaction with a half-life of 2 h was deduced for the photooxidation kinetics of anthracene in aqueous media under simulated solar radiation conditions [11]. Bertilsson and Anneli report that photochemical degradation of anthracene and phenanthrene in water proceeds rapidly (half-lives 1 and 20.4 h, respectively) while that of naphthalene (half-life > 100 h) does not [12], In the presence of humic substances, the anthracene degradation rate was unaffected while that of phenanthrene was slowed down considerably. Differences in the spectral absorbance of anthracene and... 

Some researchers pointed out that extracted humic materials might cause problems for the extraction and photolysis of probe (pollutant) molecules. First, the extraction efficiency may decrease, and second, humics are known to quench photochemical reaction. For example, Zepp R..G. et al. (1975) suggested that phenolic humic substances present on most inland waters could inhibit free radical chain reactions. Thus, quantum yields for direct photolysis of chlorinated compounds in the aquatic environment are not likely to exceed unity. 

Figure 6.14 Rate of photoisomerization, in sunlight of a r-l,3-pentadiene in the presence of DOM. The fraction of the diene in the trans form at equilibrium, too, expressed in relation to that at time, t. [Reproduced from R. G. Zepp, G. L. Baughman, and P. F. Schlotzhauer, Comparison of photochemical behavior of various humic substances in water 1. Sunlight induced reactions of aquatic pollutants photosensitized by humic substances , Chemo-sphere 10, 109. Copyright 1981, with permission from Elsevier.]...

The field of aquatic photochemistry encompasses a wide diversity of areas within environmental science. Natural waters receiving solar radiation are active photochemical reactors. Within these reactors, primary and secondary processes are occurring. Heterogeneous reactions are associated with both living and nonliving particulate matter. Naturally occurring humic substances are relatively efficient initiators of photochemical reactions. Many xenobiotic chemicals in natural waters undergo either direct or indirect photochemical transformations. 

Zepp, R.G., G.L. Baughman, and PF. Schlotzhauer. Comparison of Photochemical Behavior of Various Humic Substances in Water. I. Srmlight Induced Reactions of Aquatic Pollutants Photosensitized by Humic Srrbstances, Chemosphere, 10 109-117 (1981). 

Oxidized quinones are relatively poor fluorophores however, the reduced forms, hyd-roquinones, are highly fluorescent, a property that has been utilized in developing photochemical-reaction fluorescence detectors for liquid chromatography (Poulson and Bitks, 1989). Klapper et al. (2002) used the differences in fluorescence properties between oxidized and reduced samples of humic substances isolated from a range of environments to infer the presence of quinone-like moieties. These authors proposed that quinone moieties contribute significantly to the fluorescence of humic substances and that fluorescence analyses could be used to assess the redox status of humic substances. 

Sakkas VA, Lambropoulou DA, AlbanisTA. Photochemical degradation study of irgarol 1051 in natural waters influence of humic and fulvic substances on the reaction. J Photochem Photobiol A Chem 2002 147 135-41. 

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