Quantifying Interactions between Singlet Oxygen and Aquatic Fulvic Acids

Singlet oxygen (O-1(2)) is a reactive oxygen species produced by dissolved organic matter (DOM) in sunlit waters. While the production Of O-1(2) by DOM has been studied, little is known on interactions between O-1(2) and DOM. The central objective of this work was to quantify the rate constants of reaction and quenching Of O-1(2) with Suwannee River and Pony Lake fulvic acids, the terrestrial and microbial end-member reference aquatic humic substances of the International Humic Substance Society. Fulvic acids were reacted with O-1(2) generated through visible light irradiation of Rose Bengal. Uptake Of O-1(2) by the fulvic acids was followed through changes in dissolved oxygen concentrations via membrane inlet mass spectrometry (MIMS). Results from multiple diagnostic tests for O-1(2)-processes in solution suggested that 64-70% of the observed uptake of oxygen by the fulvic acid solutions was due to reaction with O-1(2); the remaining O-1(2) uptake was likely due to non-O-1(2) processes initiated by the excited-state sensitizer. The rate constants of reaction (k(rxn)) and physical quenching (k(phys)) with O-1(2) were determined to be 2.6 x 10(5) M-C-1 s(-1) and 2.7 x 10(5) M-C1- s(-1) (k(rxn)) and 1.5 x 10(5) M-C1- s(-1) and 1.3 x 10(6) M-C1- s(-1) (k(phys)) for Suwannee River and Pony Lake fulvic acids, respectively. Resultsfrom this study demonstrated that O-1(2) reacts with microbially and terrestrially derived DOM at rate constants comparable to phenols, naphthols, or aromatic amines, on a per carbon basis.