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

The singlet oxygen (O-1(2)) 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 O-1(2) formation (Phi(Delta)) 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 Phi(Delta) 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 Phi(Delta) for the spring samples. For the aquatic sampling sites Hietajarvi and Birkenes, Phi(Delta) 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 Phi(Delta) 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, O-1(2) production rates were calculated for eight representative HS.