Modeling the Transport of the "New-Horizon" Reduced Graphene Oxide-Metal Oxide Nanohybrids in Water-Saturated Porous Media.

ABSTRACT

Little is known about the fate and transport of the "new-horizon" multifunctional nanohybrids in the environment. Saturated sand-packed column experiments ( n = 66) were therefore performed to investigate the transport and retention of reduced graphene oxide (RGO)-metal oxide (Fe3O4, TiO2, and ZnO) nanohybrids under environmentally relevant conditions (mono- and divalent electrolytes and natural organic matter). Classical colloid science principles (Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and colloid filtration theory (CFT)) and mathematical models based on the one-dimensional convection-dispersion equation were employed to describe and predict the mobility of RGO-Fe3O4, RGO-TiO2, and RGO-ZnO nanohybrids in porous media. Results indicate that the mobility of the three nanohybrids under varying experimental conditions is overall explainable by DLVO theory and CFT. Numerical simulations suggest that the one-site kinetic retention model (OSKRM) considering both time- and depth-dependent retention accurately approximated the breakthrough curves (BTCs) and retention profiles (RPs) of the nanohybrids concurrently; whereas, others (e.g., two-site retention model) failed to capture the BTCs and/or RPs. This is primarily because blocking BTCs and exponential/hyperexponential/uniform RPs occurred, which is within the framework of OSKRM featuring time- (for kinetic Langmuirian blocking) and depth-dependent (for exponential/hyperexponential/uniform) retention kinetics. Employing fitted parameters (maximum solid-phase retention capacity Smax = 0.0406-3.06 cm3/g; and first-order attachment rate coefficient ka = 0.133-20.6 min-1) extracted from the OSKRM and environmentally representative physical variables (flow velocity (0.00441-4.41 cm/min), porosity (0.24-0.54), and grain size (210-810 µm)) as initial input conditions, the long-distance transport scenarios (in 500 cm long sand columns) of the three nanohybrids were predicted via forward simulation. Our findings address the existing knowledge gap regarding the impact of physicochemical factors on the transport of the next-generation, multifunctional RGO-metal oxide nanohybrids in the subsurface.