Enhanced Transport of TiO2-Reduced Graphene Oxide Nanocomposites in Saturated Porous Media: the Impact of Loaded TiO2 Shape and Solution Conditions

Laboratory sand column experiments were conducted to model the transport behavior of TiO2 nanoparticle-reduced graphene oxide nanocomposite (TiO2 NP/rGO) and TiO2 nanowire-reduced graphene oxide nanocomposite (TiO2 NW/rGO) using different electrolyte solutions and pH values. The breakthrough curve of TiO2/rGO nanocomposite shows that the mobility is highly sensitive to ionic strength and pH. Experimental results found that the zeta potential of TiO2 NW/rGO is more negative due to more hydroxide ions in solution from the TiO2 NWs. The mobility of TiO2 NW/rGO is slightly greater than that of TiO2 NP/rGO at lower ionic strength (1-50 mM NaCl and 1-5 mM CaCl2), whereas at 10 mM CaCl2, TiO2 NW/rGO had weak transport because of physical straining. The ratio of the hydrodynamic diameter (4214 nm) to sand diameter was as high as 0.83. Mobility increased for both TiO2 NP/rGO and TiO2 NW/rGO with respect to ionic strength because of electrostatic repulsions. When the pH was 9 with a 10 mM NaCl background solution, the stronger energy barrier between the nanocomposite and sand contributed to the enhanced transport behavior. However, with a solution at pH 3-6, the ripening effect controlled the transport of TiO2 NW/rGO. The normalized concentrations rapidly climbed to a maximum (0.05 and 0.14) and then decreased gradually after 2 pore volumes. In general, these behaviors may well predict the fate of carbon-based nanoparticles with tailwater or wastewater flowing into soil environments.