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Title: Transport and retention of multi-walled carbon nanotubes in saturated porous media: Effects of input concentration and grain size

Author
item KASEL, DANIELA - Agrosphere Institute
item Bradford, Scott
item SIMUNEK, JIRI - University Of California
item HEGGEN, MARC - Agrosphere Institute
item VEREECKEN, HARRY - Agrosphere Institute
item KLUMPP, ERWIN - Agrosphere Institute

Submitted to: Water Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/13/2012
Publication Date: 2/1/2013
Citation: Kasel, D., Bradford, S.A., Simunek, J., Heggen, M., Vereecken, H., Klumpp, E. 2013. Transport and retention of multi-walled carbon nanotubes in saturated porous media: Effects of input concentration and grain size. Water Research. 47(2):933-944.

Interpretive Summary: Knowledge of the subsurface transport behavior of multi-walled carbon nanotubes (MWCNT) is needed to assess potential health risks to ecosystems and humans. The objective of this study was to investigate the influence of sand grain size and input concentration on the transport and retention of MWCNT in saturated sand. Findings from this study demonstrated that risk assessment simulations of MWCNT transport and fate need to accurately account for observed behavior of breakthrough curves and retention profiles with input concentration and grain size. This information will be of interest to scientists and engineers concerned with predicting the fate of MWCNT in soil and groundwater environments.

Technical Abstract: Water-saturated column experiments were conducted to investigate the effect of input concentration (Co) and sand grain size on the transport and retention of low concentrations (1, 0.01, and 0.005 mg L/1) of functionalized 14C-labeled multi-walled carbon nanotubes (MWCNT) under repulsive electrostatic conditions that were unfavorable for attachment. The breakthrough curves (BTCs) for MWCNT typically did not reach a plateau, but had an asymmetric shape that slowly increased during breakthrough. The retention profiles (RPs) were not exponential with distance, but rather exhibited a hyper-exponential shape with greater retention near the column inlet. The collected BTCs and RPs were simulated using a numerical model that accounted for both time- and depth-dependent blocking functions on the retention coefficient. For a given Co, the depth-dependent retention coefficient and the maximum solid phase concentration of MWCNT were both found to increase with decreasing grain size. These trends reflect greater MWCNT retention rates and a greater number of retention locations in the finer textured sand. The fraction of the injected MWCNT mass that was recovered in the effluent increased and the RPs became less hyper-exponential in shape 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 MWCNT shape on MWCNT retention. In particular, MWCNT have a high aspect ratio and we hypothesize that solid phase MWCNT may create a porous network with enhanced ability to retain particles in smaller grain sized sand, especially at higher Co. Results demonstrate that model simulations of MWCNT transport and fate need to accurately account for observed behavior of both BTCs and RPs.