Release kinetics of multiwalled carbon nanotubes deposited on silica surfaces: quartz crystal microbalance with dissipation (QCM-D) measurements and modeling.

Understanding the kinetics of the release of carbon nanotubes (CNTs) from naturally occurring surfaces is crucial for the prediction of the environmental fate and transport of CNTs. In this study, the release kinetics of multiwalled CNTs (MWNTs) from silica surfaces was investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D). MWNTs were first deposited on silica surfaces under favorable deposition conditions (1.50 mM CaCl2 and pH 7.1) and the deposited MWNTs were then rinsed at different electrolyte solutions to induce the release of MWNTs from the primary energy minimum. The kinetics of MWNT release was shown to be first order with respect to the deposited MWNTs when complete release took place. A model that accounts for the releasable and unreleasable components of MWNTs was used to fit the experimental data in order to derive the release rate coefficients. When the CaCl2 concentration in the eluent was decreased, a larger fraction of deposited MWNTs was released and the release rate coefficient of the releasable MWNTs also increased. The rise in the surface charges of both MWNTs and silica surfaces with the drop in CaCl2 concentration likely resulted in the decrease in the height of the energy barrier, thus facilitating the release of the nanotubes. Moreover, when the initial surface concentrations of deposited MWNTs were over 1000 ng/cm(2), the release rate coefficient was lower than expected. The reduced release kinetics was likely due to the formation of large surface-bound MWNT clusters which had considerably lower diffusion coefficients than dispersed MWNTs or MWNT aggregates.