This study examines the effects of natural solar radiation on the metal-binding capacity of dissolved organic matter (DOM). Newington Bog water (35.5 mg L−1 dissolved organic carbon [DOC]) was irradiated for 20 days under UV-B lamps in the laboratory and under natural solar radiation. In the presence of irradiated DOM, IC50 (contaminant concentration required to reduce algal growth by 50%) was significantly decreased with UV-B treatment for four metals: Pb, 64%; Cu, 63%; Ni, 35% and Cd, 40%. Solar radiation also significantly decreased IC50 of Pb (58%) and Cu (49%), DOC concentration (11%), DOM fluorescence (DOMFL, 33%) and DOC-specific UV absorbance. Further experiments on Raisin River water (20.7 mg DOC L−1) exposed to 20 days of artificial UV-A and UV-B radiation produced significant decreases in IC50 for Cu (48%) with UV-A and for Pb (43%) with UV-B. DOC concentration was decreased 20% by UV-B and 24% by UV-A. DOMFL decreased 51.5% in the first 5 days of UV-A exposure, an effect that was not observed with the UV-B treatment. The UV-A treatment decreased UV absorbance more at longer wavelengths and over a broader wavelength band than did the UV-B treatment. Change in toxicity with UV irradiation was inconsistent among the metals tested in this study, indicating that some organic metal-binding ligands were more quickly removed or altered than others. The DOM remaining after irradiation appears to be qualitatively different from the unirradiated DOM. The much greater irradiance of UV-A makes its contribution to the removal and/or alteration of DOM at least as important as the influence of higher energy UV-B.
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Vol. 81 • No. 6