Transport and Retention of Multi-Wall Carbon Nanotubes (MWNTs) in Quartz Sands.

Based on their potential to advance biomedical and material science applications, considerable effort has been expended to disperse carbon nanotubes (CNTs) in water. As the cytotoxicity of dispersed CNTs has been identified, understanding their transport behavior in the environment is of critical importance. In this study, one-dimensional column experiments were conducted to assess the transport and retention of multi-wall nanotubes (MWNTs) in water-saturated 40-50 mesh Ottawa sands, with a focus toward influences of tube lengths. In order to avoid damages to nanotube length by ultrasonication or superacids (e.g., HNO₃), a chemical modification method involving a mild acid treatment was adopted to prepare suspensions of MWNTs with three different manufacture-reported (MR) lengths (0.5-2, 10-20, and 50 μm). For each column experiment, a pulse (e.g., 5 pore volumes) of MWNT suspension was introduced into the porous medium, followed by 3 pore volumes of MWNT-free solution. Concentrations of MWNTs in the column effluent and dissected solid phase samples were used to construct breakthrough curves (BTCs) and retention profiles, respectively.  At the lowest input concentrations (5 mg/L), relative effluent concentrations of MWNTs gradually increased to plateau values before decreasing sharply to a value of zero at the conclusion of each pulse injection. When the input concentration was increased to approximately 90 mg/L, symmetric BTCs were observed and the retention of MWNTs increased with MR length. Even for an MR length of 50 μm, MWNTs were readily transported through the quartz sand, where 80% of the injected mass passed through the column. In all cases, retained concentrations of MWNTs decreased hyper-exponentially with distance from the column inlet. Analysis of these findings suggests that clean-bed filtration theory alone will not be sufficient to predict MWNT transport and retention behavior in water-saturated porous media.