Effects of C60 on the Photochemical Formation of Reactive Oxygen Species from Natural Organic Matter

Buckminsterfullerenes (C60) are widely used nanomaterials that are present in surface water. The combination of C60 and humic acid (HA) generates reactive oxygen species (ROS) under solar irradiation, but this process is not well understood. Thus, the present study focused on the photochemical formation of singlet oxygen (1O2), hydroxyl radical (HO)-like species, superoxide radicals (O2•–), hydrogen peroxide (H2O2), and triplet excited states (3C60*/3HA*) in solutions containing both C60 and HA. The quantum yield coefficients of excited triplet states (fTMP) and apparent quantum yields of ROS were measured and compared to the calculated values, which were based on the conservative mixing model. Although C60 proved to have only a slight impact on the 1O2 formation from HA, C60 played a key role in the inhibition of O2•–. The photochemical formation of H2O2 followed the conservative mixing model due to the reaction of C60•– with HO2/O2•–, and the biomolecular reaction rate constant has been measured as (7.4 ± 0.6) × 106 M–1 s–1. The apparent fTMP was significantly lower than the calculated value, indicating that the steric effect of HA was significant in the reaction of 3C60* with the TMP probe. In contrast, C60 did not have an effect on the photochemical formation of HO from HA, suggesting that HO is elevated from the hydrophilic surface of HA. The aforementioned results may be useful for predicting the photochemical influence of C60 on aqueous environments.