Impact of Porous Media Grain Size on the Transport of Multi-walled Carbon Nanotubes

Nanoparticles possess unique physical, electrical, and chemical properties which make them attractive for use in a wide range of consumer products. Through their manufacturing, usage, and eventual disposal, nanoparticles are expected to ultimately be released to the environment after which point they may pose environmental and human health risks. One critical component of understanding and modeling those potential risks is their transport in the subsurface environment. This study investigates the mobility of one important nanoparticle (multi-walled carbon nanotubes or MWCNTs) through porous media, and makes the first measurements on the impact of mean collector grain size (d(50)) on MWCNT retention. Results from one-dimensional column experiments conducted under various physical and chemical conditions coupled with results of numerical modeling assessed the suitability of traditional transport models to predict MWCNT mobility. Findings suggest that a dual deposition model coupled with site blocking greatly improves model fits compared to traditional colloid filtration theory. Of particular note is that the MWCNTs traveled through porous media ranging in size from fine sand to silt resulting in normalized concentrations of MWCNTs in the effluent in excess of 60% of the influent concentration.