Transport of graphene oxide in saturated porous media: effect of cation composition in mixed Na-Ca electrolyte systems

The influence of cation composition in mixed Na-Ca electrolyte systems on the transport of graphene oxide particles in saturated porous media was studied. Column experiments were conducted to elucidate the transport behavior of GO by varied molar ratios of Ca2+/Na+ but of constant ionic strength (IS). The results show that retention of GO in sand column is strongly dependent on IS in the presence of Ca2+, featuring serious deposition rates (Rd) at the higher IS of 10 mM. The maximum Rd was 48.22% at 1 mM and 98.53% at 10 mM. However, there was no obvious difference in GO retention in solutions that only contained Na+ when the IS increased from 1 to 10 mM, and the Rd was 35.17% and 38.21% respectively. The molar ratio of Ca2+/Na+ in solution was much more influential in altering the particle retention behavior at the higher IS of 10 mM, compared with little influence at 1 mM. It was supposed that compression of diffuse double layers mainly controlled GO deposition under lower IS, while charge neutrality and metal (Ca2+) bridging played a significant role at the higher IS. A numerical advection-dispersion-retention model considering the combined processes of Langmuirian dynamics blocking and depth-dependent straining was successfully developed to simulate the transport process of GO through the sand column. Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction energy calculations were also performed to better understand the mechanisms of GO mobility. Coupling analysis of breakthrough experiments, DLVO theory and numerical modeling in this work provides insight into the mechanisms of GO transport in saturated porous media and is useful for reliable prediction of nanoparticle penetration through the vadose zone.