Transport and retention of fullerene-based nanoparticles in water-saturated porous media
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Commercial production and use of carbon-based nanomaterials will inevitably lead to the release of nanoscale compounds into to the environment. While fullerene nanoparticles, such as nC60 and multi-wall nanotubes (MWNTs), exhibit toxicity to certain microbes and human cell lines, their transport and deposition in subsurface environments are largely unknown. In this study, nanoparticle transport experiments were conducted in one-dimensional columns packed with water-saturated glass beads, quartz sands, or natural soil. Results demonstrated that nC60 transport was strongly influenced by electrolyte species and concentration, as well as mean grain size and flow rate. The attachment of nC60 was largely irreversible, with introduction of pH 12 water required to detach substantial quantities of retained nC60. Measured nC60 breakthrough curves and retention profiles in quartz sands suggest that the retention of nC60 was primarily due to attachment in a first energy minimum and that clean-bed filtration theory alone was not sufficient to describe the experimental data. In the presence of stabilizing agents, including surfactant, fulvic and humic acids, significant enhancement of nC60 transport in quartz sands was observed. In two natural soils, Appling and Webster soil, complete retention of nC60 was observed, even after introducing up to 65 pore volumes of nC60 suspension. However, nC60 readily transported through Appling soil in the presence of surfactant. For MWNTs with a manufacture-reported (MR) length of 50 Î¼m readily transported through sand columns and the retention of MWNTs at higher input concentrations increased with the MR length. The data also suggested MWNTs exhibited higher mobility in quartz sands than nC60 or single-wall nanotubes under similar chemical conditions. These findings advance our current understanding of fundamental processes governing nanoparticle transport and retention in porous media and provide reliable experimental data for the development of nanoparticle transport models.