Chemiluminescence in the Photooxidation of Humic Acids*

Photo‐irradiation of aqueous basic solutions of soil humic acids and synthetic melanins with UV and visible light (Λ > 320 nm) under oxygen or nitrogen atmospheres generates electronic exicted 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 paramagnetic resonance (EPR) signal with g‐value at 2.006 and ΔH∼ 3Gs. Chemiluminescence intensity and EPR signals follow multistep kinetics. An increase of the ratio of OD 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 (Λex 480 and 400 nm, respectively), and an increase of the short wavelength band with a maximum at 450 nm (Λex 260 nm). The data indicate that a complex chain of reactions initiated by reactive species leads to the degradation of the aromatic core of the polymers. Oxygen efficiently enhances the chemiluminescence intensity and the rate of photodegradation. The mechanism of photodegradative oxidation and chemiluminescence probably involves an energy transfer process and singlet oxygen formation. The possibility of its occurrence in nature and its significance are discussed.