Effect of solution chemistry on multi-walled carbon nanotube deposition and mobilization in clean porous media

Abstract

There are increasing concerns over the environmental impact and health risks of carbon nanotubes (CNTs) because they may be released into soil and groundwater systems. The present work systematically investigated the transport, deposition, and mobilization behaviors of multi-walled carbon nanotubes (MWNTs) in saturated columns packed with acid-cleaned glass beads and quartz sand of two different grain sizes. Combined effects of pH (5.6 and 10) and ionic strength (IS: DI water, 1mM, and 10mM) on the fate and transport of the MWNTs in the columns were examined. MWNTs were relatively mobile in all the tested conditions with DI water as the experimental solution. Their deposition in the saturated porous media, however, was very sensitive to solution chemistry, particularly IS. Slight increase in solution IS (1 mM) caused strong deposition of MWNTs in both quartz sand (>44%) and glass beads (>39%). Mobilization experimental results indicated that most of the MWNT attachment (>73%) to the porous media was irreversible and reduction in solution IS only caused a small portion of re-entrainment (<27%) of deposited MWNT for all the tested conditions. This indicates that more MWNTs are trapped in the primary minimum, although the deposition of MWNTs in saturated porous media occurs in both primary and secondary minimum. It is suggested that, under unfavorable conditions, weak associated MWNTs in the secondary minimum may be transferred into the primary minimum due to the effect of hydrodynamic force and/or local favorable sites associated with surface heterogeneity.