Insight into the inhibitory mechanism of soluble ionic liquids on the transport of TiO2 nanoparticles in saturated porous media: Roles of alkyl chain lengths and counteranion types.

Abstract Column experiments were carried out to investigate the transport of TiO2 nanoparticles (nTiO2) in water-saturated porous media in the presence of various imidazolium-based ionic liquids (ILs) with different alkyl chain lengths and counteranions. The results indicated that the effects of ILs on nTiO2 transport were considerably dependent upon IL species. In general, the transport-inhibition effects increased with the increasing length of branched alkyl chain on the ILs (i.e., [C6mim]Cl > [C4mim]Cl > [C2mim]Cl). The trend was dominated by the hydrophobicity effects of ILs. Meanwhile, the inhibitory effects of ILs were strongly related to the counteranions and followed the order of [C4mim]Cl > [C4mim][TOS] > [C4mim][PF6], mainly due to different electrostatic repulsion force between nanoparticles and porous media in the presence of various ILs. Furthermore, the inhibitory role of [C4mim][TOS] in nTiO2 transport under acidic conditions (i.e., pH 6.5) was greater than that under alkaline conditions (i.e., pH 8.0). The dominant mechanism was that the differences in the extent of electrostatic repulsion between sand grains and nTiO2 with or without ILs at pH 6.5 were larger than that at pH 8.0. Moreover, two-site kinetic retention model and DLVO theory provided good descriptions for the transport behaviors of nTiO2 with different ILs.