Humic acid induced weak attachment of fullerene nC₆₀ nanoparticles and subsequent detachment upon reduction of solution ionic strength in saturated porous media

Sand column experiments were performed under saturated conditions to investigate impact of humic acid (HA) on attachment of nC₆₀ nanoparticles (NPs) in NaCl and CaCl₂ at ionic strengths (ISs) from 1 mM to 100 mM and subsequent detachment via reducing solution IS. The attachment increased with increasing IS due to reduced repulsive Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction energy and accordingly increased retention in primary energy wells. More attachments occurred in CaCl₂ compared to NaCl because Ca²⁺ exhibited greater charge screen ability and served as a bridging agent between the NPs and sand surfaces. The presence of HA significantly reduced nC₆₀ NPs attachment on sand surfaces (especially on nanoscale physical heterogeneities) in 10 mM NaCl and 1 mM CaCl₂ because of enhanced electrostatic and steric repulsions. Interestingly, although the HA did not cause reduction of attachment in 100 mM NaCl and 10 mM CaCl₂ compared to the case in absence of HA, the HA caused weak attachment of nC₆₀ on sand surfaces and then much more significant detachment by decreasing IS. The HA did not alter both attachment and detachment in 100 mM CaCl₂, because the Ca²⁺ at the high concentration caused formation of very stable complex of HA and NPs, and strong interaction of the complex with the sand surfaces via cation bridge. Our study highlighted that the HA can not only enhance the transport of NPs by inhibiting attachment as revealed in the literature, but also by the continuous capture and release of the NPs from surfaces in subsurface environments.