Transport and deposition of functionalized multi-walled carbon nanotubes in porous media

The aim of this study was to gain more profound knowledge on the transport and deposition of functionalized multi-walled carbon nanotubes (MWCNTs) in porous media. The use of 14C-labeled MWCNTs allowed investigations into very low concentrations and the determination of retention profiles. Transmission electron micrographs revealed that the MWCNTs exhibited average outer diameters of 10–50 nm and average lengths of up to several µm. The functionalization of the MWCNTs with nitric acid induced oxygen containing functional groups and reduced the amount of metal catalysts on the nanotubes. Since nanoparticles do not behave like solutes but rather like colloids, the applicability of the available experimental setups and procedures was evaluated for carbon nanotubes. The nanoparticles could not be injected using a sample loop or an irrigation head. Therefore, the MWCNTs were applied to the columns directly by a pump or a pipette, respectively. The effect of the input concentration (Co) and sand grain size on the transport and retention of MWCNTs was investigated in water-saturated sand columns at conditions unfavorable for attachment (repulsive electrostatic forces). These experiments were performed at very low Co (0.005–1 mg L-1), low ionic strength (1 mM KCl), and high flow rate (0.64 cm min-1). The breakthrough curves (BTCs) for MWCNTs typically did not reach a plateau, but exhibited an asymmetric shape that slowly increased during breakthrough. The retention profiles (RPs) exhibited a hyper-exponential shape with greater retention near the column inlet. The collected BTCs and RPs were simulated using a numerical model within the HYDRUS-1D code that accounted for both time- and depthdependent blocking functions on the retention coefficient. For a given Co, the depthdependent retention coefficient and the maximum solid phase concentration of MWCNTs were both found to increase with decreasing grain size. These trends reflect greater MWCNTs retention rates and a greater number of retention locations in the finer textured sand. The normalized concentration of MWCNTs in the effluent increased and the RPs became less hyper-exponential with higher Co due to enhanced blocking/filling of retention locations. This concentration dependency of MWCNT transport increased with smaller grain size because of the effect of pore structure and the shape of MWCNTs on their retention. In particular, MWCNTs have a high aspect ratio, and it was hypothesized that MWCNTs may create a porous network with an enhanced ability to retain further MWCNTs, especially in smaller grain-sized sand and at higher Co. Results demonstrate that model simulations should accurately account for observed behavior of both BTCs and RPs to make reliable predictions on MWCNT transport.