Photochemistry humic substances

Since most of the riverine DOM is comprised of humic substances, considerable attention has been fiacused on its fete in seawater. Little terrestrial DOM is detectable in seawater, suggesting the existence of an efficient removal process. This is surprising given the traditional view that humic substances are relatively refractory. Marine chemists are currently investigating the redox and photochemistry of humic substances to better understand its chemical fete in the oceans. 

There are three typical classes of compounds, for which photochemistry plays a crucial role in surface and marine waters nitrate ions, humic substances, and transition metal ions. 

Boule P, Bolte M, Richard C. Transformations photoinduced in aquatic media by NOj, Fe(III) and humic substances. In Boule P, ed. Environmental Photochemistry. Berlin Springer, 1999 181-216. 

DOM, and in particular dissolved humic substances give, depending on their concentration, a yellowish or brown colour to the natural waters. To underline the fact that the light-absorbing components, and, thus, those that are photochemically active, are also responsible for the colour, the term coloured (or chromophoric) dissolved organic matter (CDOM) [24,25] has been proposed for use in environmental photochemistry literature, and will be used here. 

Cooper W. J., Zika R. G., Petasne R. G., and Fischer A. M. (1989) Sunlight-induced photochemistry of humic substances in natural waters major reactive species. In Aquatic Humic Substances Influence on Fate and Treatment of Pollutants. Advances in Chemistry Series 219 (eds. I. H. Sufifet and P. MacCarthy). American Chemical Society, Washington, DC, pp. 333-362. 

J.F. Power, D.K. Sharma, C.H. Langford, R. Bonneau, J. Joussot-Dubien (1987). Laser flash photolytic studies of a well-characterized soil humic substance. In R.G. Zika, W.J. Cooper (Eds), Photochemistry of Environmental Aquatic Systems (pp. 157-173). American Chemical Society, Washington. 

On irradiation of DOM, the one primary photochemical event is photo-ionization. This process is less sensitive to environmental factors than triplet formation. All of the remaining photochemistry depends on the subsequent fate of the three primary products of photoionization triplets, solvated electrons, and cation radicals [13]. Laser flash photolysis studies supplied early evidence for hydrated electron (e q) production on irradiation of DOM, with significant primary quantum yields [9,12,84,85]. The hydrated electron was identified by its characteristic broad absorption spectrum from 700 nm to 750 nm [86, 87]. The formation of e q is thought to result from the photo-ejection of an electron from excited-state humic substances ... 

Fisher AM, Winterle JS, Mill T (1987) Primary photochemical processes in photolysis mediated by humic substances. In Zika RG, Cooper WJ (eds) Photochemistry of environmental aquatic systems. ACS Symposium Series 237, American Chemical Society, Washington DC, p 141... 

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. 

All of the humic substance solutions and natural water samples discussed In the materials and methods section of this paper were shown to have common phototransients, T475 and T720. This Implies a global common energy sink In natural waters that contain dissolved organic matter, with quantifiable energy and electron transfer capabilities. It Is expected that continued laser studies of this system backed up by complementary field studes will enable us to better understand the human ecology of natural waters when solar flux and HS photochemistry Is taken Into perspective consideration. 

Similarly, a burgeoning literature on DOM fluorescence in aerosols is describing the abundance of atmospheric DOM with chemical properties similar to humic substances. The atmosphere also contains substantial amounts of metals in dust and photochemistry is important. Understanding the sources and reactivities of atmospheric DOM via fluorescence is important because of the many metal-ligand reactions that likely occur, some of which is clearly related to DOM photochemistry in the atmosphere. 

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. 

Both photosensitized reacticms and heterogeneous photocatalysis have been the focus of many studies and reviews for the degradation of organic and inorganic species in natural terrestrial surface water. This review will discuss their potential importance in the atmosphere for two distinct cases — photochemistry of mineral dust (which contains oxides able to initiate photocatalysis) and organic or carbonaceous aerosols (which contain aromatic compounds or humic like substances able to act as photosensitizers). Additionally, direct photochemistry of unconventional precursors, i.e., iron-dicarboxylic acid anionic complexes, will also be dealt with. 

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