Cooperative Effects of Cation type with Ionic strength and pH on Graphene Quantum Dots (GQDs) Transport in Saturated Porous Media

Graphene quantum dots (GQDs), characterized by nanostructures with diameters below 10 nm and strong hydrophilicity, exhibit advantageous migration behavior in groundwater environments. The impacts of ionic strength, cation type and pH on the transport of GQDs in porous media diverge from those observed for other nanosized particles. We investigated the cooperative effects of ionic strength (IS, 1 mM NaCl and 100 mM NaCl, 1 mM CaCl₂ and 10 mM CaCl₂), cation types (Na⁺, Ca²⁺), and pH (4, 7, 9). Through saturated column experiments and numerical modeling, we observed increased GQDs deposition with rising IS, notably enhanced by the presence of divalent cations due to bridging effects and self-aggregation. In the presence of monovalent cations, acidic environment favored the deposition of GQDs, whereas in the presence of divalent cations, an acidic environment with 1 mM CaCl₂ was the most favorable for the deposition of GQDs, and shifted to an alkaline condition at 10 mM CaCl₂, which may be due to the deprotonation process and the enhancement of self-aggregation. This research sheds light on critical factors governing the environmental behavior of ultra-small nanoparticles and thereby aids in the assessment of their ecological impact.