Temporal Changes in Aqu/C₆₀ Physical–Chemical, Deposition, and Transport Characteristics in Aqueous Systems

Little is known about how temporal changes in the physical-chemical properties of C₆₀ aggregates formed in aqueous systems (termed aqu/C₆₀) can impact transport pathways contributing to ecological exposures. In this study three aqu/C₆₀ suspensions of short-term (100 days), intermediate-term (300 days), and long-term (1000 days) water exposure were first characterized for particle size distribution, water/toluene phase distribution, and surface chemistry. Then, aqu/C₆₀ deposition to a model silica surface and transport in porous media were studied by quartz crystal microbalance (QCM) and saturated sand columns. As suspension time increased, aqu/C₆₀ particle size shifted to a larger size range as determined by asymmetric flow field-flow fractionation (AF4) and the aqu/C₆₀ distribution to toluene was reduced, likely due to surface polarization as revealed by nuclear magnetic resonance (NMR) and UV-visible spectroscopy of the aqu/C₆₀ suspensions. Additionally, the deposition to silica surfaces in both QCM and column studies decreased with increased water exposure time. Although a small increase in aqu/C₆₀ aggregate size with time may partially explain the greater transport of the long-term aqu/C₆₀ because of the decreased collector efficiency for larger submicrometer particles, the polarization of the aqu/C₆₀ (thus a more hydrophilic surface) revealed by the toluene/water phase distribution and confirmed by NMR, is considered the determining factor.