Quantifying interactions between singlet oxygen and aquatic fulvic acids

Singlet oxygen( ¹O ₂)is a reactive oxygen species produced by dissolved organic matter(DOM)in sunlit waters. While the production of ¹O ₂ by DOM has been studied, little is known on interactions between ¹O ₂ and DOM. The central objective of this work was to quantify the rate constants of reaction and quenching of ¹O 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 ¹O ₂ generated through visible light irradiation of Rose Bengal. Uptake of ¹O 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 ¹O ₂ - processes in solution suggested that 64-70% of the observed uptake of oxygen by the fulvic acid solutions was due to reaction with ¹O ₂; the remaining O ₂ uptake was likely due to non - ¹O ₂ processes initiated by the excited-state sensitizer. The rate constants of reaction(k _rxn)and physical quenching(k _phys)with ¹O ₂ were determined to be 2.6 × 10 ⁵ M-C ^-1 s ^-1 and 2.7 × 10 ⁵ M-C ^1- s ^-1(k _rxn)and 1.5 × 10 ⁵ M-C ^1- s ^-1 and 1.3 × 10 ⁶ M-C ^1- s ^-1(k _phys)for Suwannee River and Pony Lake fulvic acids, respectively. Results from this study demonstrated that ¹0 ₂ reacts with microbially and terrestrially derived DOM at rate constants comparable to phenols, naphthols, or aromatic amines, on a per carbon basis.