Transport and retention of fullerene (nC60) nanoparticles in unsaturated porous media: Effects of solution chemistry and solid phase coating

The retention and release of aqueous aggregates of fullerene nanoparticles (nC60) were studied under dynamic unsaturated conditions. Porous media containing nC60 were taken through multiple drainage/imbibition (drying/wetting) cycles to explore the effects of solution conditions and solid surface modification on transport and ultimate fate in unsaturated porous media. In experiments conducted with NaCl as the background electrolyte, the retention of nC60 during drainage was found to be negligibly small over a wide range of ionic strengths (I = 0.2 to I = 6 mM), significantly lower than the retention of titanium dioxide nanoparticles studied previously under similar conditions. In contrast, experiments conducted with CaCl2 as the background electrolyte found that retention of nC60 during drainage was significant at higher ionic strengths, particularly at the highest ionic strength studied (I = 6 mM). Experiments examining the influence of dissolved natural organic matter on nC60 retention in unsaturated media found no measurable impact on the transport. The effects of solid surface modification were examined by creating coatings that modified surface hydrophobicity and charge. Experiments found that a hydrophobic coating had no measurable impact on nC60 retention, when compared with retention by unmodified media. In contrast, a porous medium with surfaces that were both hydrophobic and positively-charged retained 5–10 times more nC60 during drainage than an unmodified porous medium. This result suggests that electrostatic interactions play a more important role than hydrophobic interactions in the transport and fate of nC60 in the unsaturated zone. For all conditions where retention was observed, experiments found very little release or retained nC60 after subsequent flushing with water, suggesting that once retained, the environmental mobility of nC60 may be extremely limited.

► nC60 retention in unsaturated porous media is minimal in the presence of NaCl. ► nC60 retention is more significant with CaCl2, esp. at higher ionic strengths. ► Positively charged solid surfaces increase nC60 unsaturated retention. ► Hydrophobic surfaces do not appear to increase nC60 unsaturated retention. ► Once retained, nC60 is not readily released.