Characterization of humic substances and their role in photochemical processes of environmental interest.

PEDRO M. DAVID GARA; JANINA A. ROSSO; MARCELA V. MARTIN; GABRIELA N. BOSIO; MÓNICA C. GONZALEZ; DANIEL O. MÁRTIRE

Trends in Photochemistry & Photobiology

Research Trends (P) Ltd.

Año: 2011 vol. 13 p. 51 - 70

0972-4532

Photochemical processes in surface water induced by sunlight affect the redox chemistry of these waters. The humic substances are almost half of the dissolved organic carbon in natural waters and represent the main fraction of colored dissolved organic carbon that absorb solar radiation. Upon UV-visible light absorption the excited states of humic substances interact with species present in natural waters to form reactive intermediates, such as: singlet oxygen (O2(1Δg)), hydroxyl radical (•OH), superoxide anion radicals (O2•־) and organic peroxides. In the presence of inorganic ions, additional reactive species are produced. Furthermore, experimental data indicate that the photoionization of humic substances yield solvated electrons. These reactive intermediates are responsible for the degradation of organic pollutants and the dissolved organic matter. This chapter reviews: 1) recent research concerned with the characterization of humic substances from different origins; 2) the ability of HS to generate reactive species by photosensitization, 3) the reactivity of the reactive species with the humic substances and 4) the degradation of pollutants photosensitized by humic substances. General results about the photodegradation of several contaminants are reviewed, but special attention is paid to that of two model compounds. These are clomazone and trichloroacetic acid (TCAA). The herbicide clomazone [2-[(2-chlorophenyl)methyl-4, 4-dimethyl-3-isoxazolidinone] inhibits synthesis of chlorophyll and carotenoids in sensitive plants, resulting in foliage devoid of pigmentation. Trichloroacetic acid is highly phytotoxic, and is partly responsible for conifer forest dieback due to its accumulation in the tree needles and in soils. The high water solubility of TCAA is one of the main reasons for its mobility in soil. There is abundant evidence that soil microorganisms dehalogenate TCAA. However, removal by oxidative waste water treatment methods is only effective at temperatures over 40 oC or if vacuum-UV photolysis of water is applied. For these reasons, photosensitized degradation of clomazone and trichloroacetic acid is of environmental interest.