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Biosurfactant-mediated mobility of graphene oxide nanoparticles in saturated porous media

There is currently a lack of scientific understanding regarding how bio-surfactants influence the mobility of graphene oxide (GO) through saturated porous media. In this study, the transport characteristics of GO through porous media with different heterogeneities (i.e., quartz sand and goethite-coated sand) after the addition of saponin (a representative bio-surfactant) were investigated. The results demonstrated that saponin (3-10 mg L^-1) promoted GO mobility in both types of porous media at pH 7.0. This trend was attributed to the competitive deposition between nanoparticles and bio-surfactant molecules for attachment sites, the enhanced electrostatic repulsion, the decreased strain, the presence of steric effects induced by the adsorbed saponin, and the increase in the hydrophilicity of nanoparticles. Intriguingly, saponin promoted GO mobility in goethite-coated sand (i.e., chemically heterogeneous porous media) to a greater extent than that in sand (i.e., relatively homogeneous porous media) when saponin concentrations increased, which stemmed from the differences in the extent of the deposition site competition for saponin on the two porous media and the electrostatic repulsion between GO and the porous media. Furthermore, a cation-bridging mechanism was also involved in the ability of saponin to increase GO mobility when the electrolyte solution was 0.1 mM Cu²⁺. Moreover, the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory and the colloid transport model were applicable to elucidate the mobility properties of GO with or without saponin in porous media. The findings from this work highlight the important status of bio-surfactants in the fate of colloidal carbon-based nanomaterials in subsurface systems.