Photogeneration of singlet oxygen by humic substances: comparison of humic substances of aquatic and terrestrial origin

Abstract

The singlet oxygen (¹O₂) luminescence of 27 isolated humic substances (HS), natural organic matter, ultrafiltrates, and the synthetic fulvic acid HS1500 has been investigated by time-resolved spectroscopy in buffered D₂O. The samples include both reverse osmosis isolates from lakes in Scandinavia, Canada, and Germany, and IHSS fulvic and humic acids of aquatic and terrestrial origin. The quantum yields of ¹O₂ formation (Φ_Δ) obtained on laser excitation at 480 nm ranged between 0.06 (HS1500) and 2.7% (fulvic acid from soil, IHSS). In our study, a general trend towards higher Φ_Δ in terrestrial HS was observed. The comparison of reverse osmosis isolates from surface waters collected during fall 1999 and spring 2000 from five Scandinavian sites yielded, in all cases, higher Φ_Δ for the spring samples. For the aquatic sampling sites Hietajärvi and Birkenes, Φ_Δ even exceeded values of 0.6%, which were found to be typical for terrestrial or soil water material. Investigation of the excitation wavelength dependence of Φ_Δ in the spectral range 355–550 nm yielded different spectral shapes for aquatic HS and “non-aquatic” HS, respectively. On the basis of these excitation spectra, ¹O₂ production rates were calculated for eight representative HS.