Photochemical Transformation of Carboxylated Multiwalled Carbon Nanotubes: Role of Reactive Oxygen Species

The study investigated the photochemical transformation of carboxylated multiwalled carbon nanotubes (COOH-MWCNTs), an important environmental process affecting their physicochemical characteristics and hence fate and transport. UVA irradiation removed carboxyl groups from COOH-MWCNT surface while creating other oxygen-containing functional groups with an overall decrease in total surface oxygen content. This was attributed to reactions with photogenerated reactive oxygen species (ROS). COOH-MWCNTs generated singlet oxygen (O-1(2)) and hydroxyl radical ((OH)-O-center dot) under UVA light, which exhibited different reactivity toward the COOH-MWCNT surface. Inhibition experiments that isolate the effects of (OH)-O-center dot and O-1(2) as well as experiments using externally generated (OH)-O-center dot and O-1(2) separately revealed that (OH)-O-center dot played an important role in the photochemical transformation of COOH-MWCNTs under UVA irradiation. The Raman spectroscopy and surface functional group analysis results suggested that (OH)-O-center dot initially reacted with the surface carboxylated carbonaceous fragments, resulting in their degradation or exfoliation. Further reaction between (OH)-O-center dot and the graphitic sidewall led to formation of defects including functional groups and vacancies. These reactions reduced the surface potential and colloidal stability of COOH-MWCNTs, and are expected to reduce their mobility in aquatic systems.