Deposition and remobilization (or release) of carbon nanotube (CNTs) on natural solid surfaces are two key processes that control the fate and transport of CNTs in surface and ground water systems.  While the mechanisms for the deposition of CNTs on environmental surfaces have been investigated, the influence of solution chemistry on the remobilization of CNTs from environmental surfaces is still not well understood.  In this study, the remobilization of deposited multiwalled CNTs (MWNTs) from silica surfaces was investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D).  MWNTs were deposited on silica surfaces under favorable deposition conditions and then exposed to different solution chemistries to induce the remobilization of MWNTs.  Throughout the deposition and remobilization processes, the mass of the deposited MWNTs was monitored and obtained through Voigt-based modeling.  Our results show that deposited MWNTs were released when the electrolyte concentrations (either CaCl2 or NaCl) were decreased under neutral pH conditions.  The release of MWNTs was attributed to an increase in electrostatic repulsion between MWNTs and silica surfaces which in turn resulted in a decrease in the depth of the primary energy minimum.  The fraction of deposited MWNTs that were remobilized increased stepwise when the electrolyte concentrations of the elution solutions were sequentially decreased, which was likely due to the heterogeneity in surface charge densities of MWNTs.  The degrees of MWNT remobilization resulting from a sequential decrease in NaCl concentration were lower at pH 4.0 than at 7.1 due to the smaller electrostatic repulsion experienced between MWNTs and silica surfaces at pH 4.0.  Decreasing pH from 7.1 to 4.0 in the presence of 1.5 mM CaCl2 resulted in significant MWNT remobilization, possibly due to the elimination of calcium bridging between the carboxyl groups on MWNTs and the silanol groups on silica surfaces with the decrease in pH.

Reversibility of multiwalled carbon nanotube deposition on silica surfaces: A QCM-D study

246th National Meeting (2013)

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