Chemi luminescence in the photo oxidation of humic acids
Slawinski, J.; Puzyna, W.; Slawinska, D.
Photochemistry and Photobiology 28(1): 75-82
ISSN/ISBN: 0031-8655 Accession: 004935415
Photo-irradiation of aqueous basic solutions of soil humic acids and synthetic melanins with UV and visible light (.lambda. > 320 nm) under O2 or N atmospheres generates electronic excited states and radicals. These processes give rise to a long-lived chemiluminescence with emission maxima at 480-500, 570 and 615-650 nm, as well as a EPR signal with g[low field nuclear magnetic moment]-value at 2.006. Chemiluminescence intensity and EPR signals follow multistep kinetics. An increase of the ratio of OD [optical density] at 260/400 nm and 400/600 nm and a decrease of amplitude of an EPR signal after prolonged photo-irradiation were observed. Long irradiation also causes a decrease of fluorescence intensity bands with maxima at 535 nm and 495 nm (.lambda.ex 480 and 400 nm, respectively), and an increase of the short wavelength band with a maximum at 450 nm (.lambda.ex 260 nm). A complex chain of reactions initiated by reactive species apparently leads to the degradation of the aromatic core of the polymers. O2 efficiently enhances the chemiluminescence intensity and the rate of photodegradation. The mechanism of photodegradative oxidation and chemiluminescence probably involves an energy transfer process and single O2 formation. The possibility of its occurrence in nature and its significance are discussed.