Transport and Retention of Fullerene Aggregates in Heterogeneous Two Dimensional (2-D) Aquifer Cells Containing Natural Aquifer Sands
Abstract
Understanding the processes governing nanomaterial fate in subsurface environments is important due to their widespread use in commercial and manufacturing settings. To date, the majority of studies on nanoparticle transport in porous media have been conducted under idealized conditions in columns packed with uniform, clean sands. To evaluate the transport of nanoparticles in more representative subsurface systems, column and aquifer cell experiments were completed to investigate the transport of fullerene aggregates (nC60) in unwashed Ottawa sands and in the presence of low permeability lenses. To obtain independent measurements of attachment parameters, nC60 breakthrough and retention profiles were measured in 1-D columns (2.5 dia x 10.8 cm length) with three different size fractions (40-50 mesh, d50=335 μm; 80-100 mesh, d50=165 μm; and 100-140 mesh, d50=125 μm) of unwashed Ottawa sand. A three pore volume pulse of nC60 (ca. 2.5 mg/L) suspension containing 6 mM NaCl resulted in 71, 5 and 3 percent breakthrough in the 40-50, 80-100 and 100-140 mesh unwashed Ottawa sand, respectively. Solid phase retention profiles for all three size fractions exhibited hyper-exponential decline with distance from the column inlet, consistent with a physical straining process. The aquifer cells (63 x 36 x 1.4 cm) were packed with unwashed 40-50 mesh Ottawa sand as the background medium, in which rectangular lenses (2.4 cm x 9.6 cm) of lower permeability (80-100 and 100-140 mesh) were emplaced. Following completion of nonreactive tracer tests, 200-400 mL pulses of nC60 (ca. 2.5 mg/L) suspension were injected through side-ports up-gradient of the lenses. Only trace amounts of nC60 were detected in aqueous samples collected from down-gradient sampling ports. Post-experiment dissection of the aquifer cell demonstrated that solid phase concentrations of nC60 were largest surrounding the injection ports and decreased monotonically with no preferential accumulation observed at the textural interface between the low permeability lens and background medium. Retained nC60 increased proportionally with the amount of mass injected and maximum solid phase concentrations exceeded those measured in the columns by an order-of-magnitude. For both the column and aquifer cell studies, nC60 breakthrough and deposition could not be adequately described by either classical or modified filtration theory. The enhanced retention observed near the injection point in both systems was attributed to physical straining in pore throats due to the presence of fines on the unwashed sands. Additionally, variations in the velocity field due to the lower permeability lenses influenced retention in the aquifer cells, resulting in location dependent attachment parameters. The results of this study indicate that subsurface heterogeneities, including the presence of lower permeability lenses and fine materials, may strongly influence nanomaterial transport behavior in subsurface environments.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.H53B1421W
- Keywords:
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- 1800 HYDROLOGY;
- 1831 HYDROLOGY / Groundwater quality;
- 1832 HYDROLOGY / Groundwater transport