Transport and Deposition of Fullerene Nanoparticles In Quartz Sands: Experiments and Modeling

A series of batch and column experiments was performed to assess the aggregation and transport of nanoscale fullerene (nC60) particles in water-saturated quartz sands as a function of electrolyte and flow conditions. As the electrolyte concentration was increased from 1 to 100 mM, changes in nC60 particle diameter were minimal in the presence of NaCl, but increased by more than seven-fold in the presence of CaCl2. The latter effect was attributed to the agglomeration of individual nC60 particles, consistent with a net attractive force between particles and suppression of the electrical double layer. At low ionic strength (3.05 mM) nC60 particles were readily transported through 40 to 50 mesh quartz sand, appearing in the column effluent after introducing less than 1.5 pore volumes of nC60 suspension, with approximately 30% and less than 10% of injected mass retained in the presence of CaCl2 or NaCl, respectively. At higher ionic strength (30.05 mM) and in 100 to 140 mesh quartz sand, greater than 95% of the introduced nC60 particles were retained in column regardless of the electrolyte species. Approximately 50% of the deposited nC60 particles were recovered from 100 to 140 quartz sand after sequential introduction deionized water adjusted to pH 10 and 12. Nanoparticle transport and deposition were simulated using a mathematical model based on clean-bed filtration theory that was modified to incorporate rate-limited attachment kinetics and limiting or maximum retention function. The numerical model successfully captured the characteristics of both the nC60 effluent concentration and column retention profiles. Results of this work demonstrate that nC60 transport and retention in water-saturated sand is strongly dependent upon electrolyte conditions, flow rate and grain size, and that release of deposited C60 nanoparticles requires substantial changes in surface charge, consistent with retention in a primary energy minimum.