Deposition and release of carboxylated graphene in saturated porous media: Effect of transient solution chemistry.

Chemical perturbation of pore-water in porous media may remobilize and release deposited colloids/nanomaterials into bulk flow. This re-entrainment process is important to accurately assessing the fate and transport of colloids/nanomaterials in the subsurface. This study investigated deposition and subsequent release of carboxylated graphene nanomaterials (CG) in water-saturated sand columns by first depositing CG at 100 mM NaCl or 2 mM CaCl2 (Phase 1), followed by Phase 2 (elution with sequences of 50, 10, and 1 mM NaCl, or sequences of 0.5 and 0.1 mM CaCl2), and then Phase 3 elution using deionized water. Approximate 89.2%-98.7% of injected CG was retained in sand through Derjaguin-Landau-Verwey-Overbeek (DLVO) interactions, Ca2+ bridging, and straining in Phase 1. Sequential reduction of ionic strength in Phases 2 and 3 resulted in increased release of deposited CG mainly due to the expansion of the electrical double layer thickness and thus decreased depth of the attractive secondary minimum. With increasing pulses of flushing solution, unrecoverable CG increased because weakly associated CG via the secondary minimum was likely translated to immobile regions. Significant tailing of CG released in Phase 3 suggests that CG retained in CaCl2 was more resistant upon detachment than in NaCl. In cation exchange experiment, only 0.7% of applied CG was released, possibly ascribed to the CG remobilized by cation exchange was immediately re-entrained by the secondary minimum in 50 mM NaCl. Our findings indicate that retained nanomaterials (e.g., CG) can be remobilized and transported downward in transient solution chemistries, raising concerns about their potential migration risk to groundwater.