Transport Of Fullerene Nanoparticles (nC₆₀) In Saturated Porous Media And The Effect On Mobility Of Organic Contaminants

With the increasing production and use of buckminsterfullerene (C₆₀) and itsderivatives, the potential environmental implications of these engineered carbonnanomaterials have received much attention. The stable colloidal suspension of C₆₀,i.e., fullerene nanoparticles (nC₆₀) is likely the most important form of C₆₀in aqueousenvironments. The environmental effect of nC₆₀to groundwater is not only due to thepotential biological toxicity of nC₆₀to organisms, but also related to the fact that nC₆₀might significantly alter the fate and transport of common environmental organiccontaminants. Thus, understanding the subsurface transport of nC₆₀and the facilitatedtransport of organic contaminants with nC₆₀are of critical importance for the benignuse and risk management of C₆₀.The effects of several important environmental factors on the transport of nC₆₀through saturated porous media were examined in this paper. Decreasing flowvelocity from approximately10m/d to1m/d had little effect on nC₆₀transportthrough Ottawa sand （mainly pure quartz）, but significantly inhibited the transportthrough Lula soil （a sandy, low-organic-matter soil）. The difference can beattributed to both the smaller gain size and the more irregular and rougher shapes ofLula soil, and can be explained with the shadow zone effect theory. Increasing ionicstrength and switching background solution from NaCl to CaCl2enhanced thedeposition of nC₆₀in both sand and soil columns, but the effects were moresignificantly for soil. This was likely because the clay minerals （and possibly soilorganic matter） in soil responded to changes of ionic strength and ionic speciesdifferently than quartz. Anions and fulvic acid in the effluents had little effect on thetransport of nC₆₀. Compared with the modified clean-bed filtration theory （CFT）model （which incorporates the blocking effect on particle deposition/attachment, i.e.,the blocking of available deposition sites by earlier deposited nanoparticles）, thetwo-site transport model that takes into account of both the blocking-affectedattachment process and the straining effect can more accurately model the breakthrough profiles of nC₆₀.In this study, facilitated transport of2,2’,5,5’-polychloronated biphenyl （PCB） andphenanthrene by nC₆₀(a stable aqueous-phase aggregates of C₆₀) through two sandysoil columns was investigated. Experimental results indicated that low-level （from1.55to12.8mg/L） nC₆₀could significantly enhance the mobility of PCB andphenanthrene. However, none of the three model dissolved organic matters（DOM）—a humic acid, a fulvic acid and a bovine serum albumin—had noticeableeffect on the transport of PCB, when these DOMs were present at concentrationsequivalent to approximately10–11mg/L organic carbon. It is proposed in thisresearch that the contaminant-mobilizing ability of nC₆₀is a result of irreversibleadsorption of a fraction of nC₆₀-associated PCB/phenanthrene （whereasDOM-associated PCB is readily desorbable）. Additionally, slow desorption kineticsof nC₆₀-adsorbed PCB/phenanthrene is another possible mechanism. Findings inthis study indicate that nC₆₀in the subsurface environment can greatly enhance themobility of nonionic, highly hydrophobic organic contaminants, which typicallyexhibit very low mobility. Such effects should be taken into account when assessingthe potential environmental risks of engineered carbonaceous nanomaterials.