Aggregation of reduced graphene oxide and its nanohybrids with magnetite and elemental silver under environmentally relevant conditions

The aggregation and long-term (25 d) sedimentation behaviors of reduced graphene oxide (RGO) and its three successively self-assembled nanohybrids with magnetite (Fe₃O₄) and zerovalent silver (Ag⁰) nanoparticles have been investigated. The aggregation behaviors of the nanomaterials in NaCl and CaCl₂ were found to be in good agreement with the Derjaguin-Landau-Verwey-Overbeek (DLVO)-type interactions and the Schulze-Hardy rule. The colloidal stability decreased with the increasing ratios of the edge-based functional groups (COO^- and C=O) to the total oxygen-containing functional groups decorated on the basal planes (C-O) and edges of RGO, as quantified by X-ray photoelectron spectroscopy analysis. In the presence of natural organic matter (NOM), the aggregation of RGO and its nanohybrids was greatly inhibited as a result of the enhanced electrosteric repulsions arising from the adsorbed NOM macromolecules. The long-term sedimentation kinetics results showed that the RGO nanohybrids were less stable in synthetic groundwater containing higher electrolyte concentrations, which was likely because of the greater charge screening or neutralization effect imparted by higher monovalent and divalent electrolyte concentrations. Our findings have important implications for evaluating the environmental impact and toxicity of the emerging class of multifunctional nanohybrids whose environmental behaviors are currently largely unknown.