The role of photochemical transformations in the aggregation and deposition of carboxylated multiwall carbon nanotubes suspended in water

The photochemical transformation of carboxylated multiwall carbon nanotubes (COOH–MWCNTs) in water and the subsequent impact on their aggregation and deposition behaviors were examined. The photochemical transformation of COOH–MWCNTs was investigated under UVA (300–400 nm) irradiation, the main component of UV light in solar irradiation. Time-resolved dynamic light scattering measurement and quartz crystal microbalance with dissipation monitoring were used to study the initial aggregation and deposition kinetics. Characterization of the physicochemical properties of the COOH–MWCNTs before and after irradiation revealed a loss in surface oxygen after UV irradiation, most likely a result of decarboxylation of the nanotube surface. These changes in surface chemistry greatly reduced the colloidal stability of COOH–MWCNTs in NaCl solutions. No noticeable changes in particle surface zeta potential and stability were observed in CaCl2 solutions after irradiation. Consistent with the decreased colloidal stability in NaCl solutions, the irradiated COOH–MWCNTs had a notably higher deposition than the initial COOH–MWCNTs in NaCl solutions when aggregation did not occur. Our results suggest that the photochemical transformation plays an important role in the transport of carbon nanotubes in natural aquatic systems.