Synergistic Photogeneration of Reactive Oxygen Species by Dissolved Organic Matter and C60 in Aqueous Phase

We investigated the photogeneration of reactive oxygen species (ROS) by C-60 under UV irradiation, when humic acid (HA) or fulvic acid (FA) is present. When C-60 and dissolved organic matter (DOM) were present as a mixture, singlet oxygen (O-1(2)) generation concentrations were 1.2-1.5 times higher than the sum of O-1(2) concentrations that were produced when C60 and DOM were present in water by themselves. When C-60 and HA were present as a mixture, superoxide radicals (O-2(center dot-)) were 2.2-2.6 times more than when C-60 and HA were present in water by themselves. A synergistic ROS photogeneration mechanism involved in energy and electron transfer between DOM and C-60 was proposed. Enhanced O-1(2) generation in the mixtures was partly due to (DOM)-D-3* energy transfer to O-2. However, it was mostly due to (DOM)-D-3* energy transfer to C-60 producing C-3(60)*. C-3(60)* has a prolonged lifetime (>4 mu s) in the mixture and provides sufficient time for energy transfer to O-2, which produces O-1(2). The enhanced O-2 generation for HA/C-60 mixture was because C-3(60)* mediated electron transfer from photoionized HA to O-2. This study demonstrates the importance of considering DOM when investigating ROS production by C-60.