Aggregation, Adsorption, and Morphological Transformation of Graphene Oxide in Aqueous Solutions Containing Different Metal Cations

The colloidal behavior of graphene oxide (GO) has been extensively studied in the presence of common environmental cations, but the aggregation, adsorption, and morphological transformation of GO under heavy metal ions have not been investigated. We observed that heavy metal cations (Cr3+, Pb2+, Cu2+, Cd2+, Ag+) destabilized GO suspension more aggressively than common cations (Ca2+, Mg2+, Na+, K+). In addition to electric double-layer (EDL) suppression, heavy metal cations can easily cross the EDL, bind to GO surface, and then change the surface potential, which is a more efficient pathway for GO aggregation. According to aggregation kinetics, the destabilizing ability of cations follows the order of Cr3+ ≫ Pb2+ > Cu2+ > Cd2+ > Ca2+ > Mg2+ ≫ Ag+ > K+ > Na+. The destabilizing capability of metal cations is consistent with their adsorption affinity with GO, which is determined by their electronegativity and hydration shell thickness. GO nanosheets can be transformed to 1D tube-like carbon material, 2D multiple overlapped GO plane, and 3D sphere-like particles during aggregation, thereby combined to form a sphere-like aggregated GO, which is for the first time observed by TEM and AFM images. Therefore, the aggregation of GO 2D nanosheets follows the Schulze-Hardy rule, which is usually used for spherical particles. An integrative process of adsorption-transformation-aggregation is proposed to better understand the nanomaterial (e.g., GO) colloidal behavior, environmental risk, self-assembly process, and application as a novel adsorbent.