High nitrogen content bimolecular co-functionalized graphene nanoflakes for hypertoxic Cr(VI) removal: Insights into adsorption behavior and mechanisms

The proven high carcinogenicity to humans and high destructive force to the environment determine the extreme urgency of eliminating hypertoxic Cr(VI) in water bodies. Herein, a route of room temperature synthesis and secondary grafting was proposed to fabricate graphene oxide-based nanoadsorbent co-functionalized with polydopamine and branched polyethyleneimine (GOPP) to remove Cr(VI). The flexible decoration of polydopamine and polyethyleneimine on GO flakes could gradually enhance the amount of N-containing functional groups and realize selective removal of Cr(VI) with the maximum experimental adsorption capacity of 564.7 mg/g, displaying a significantly high separation factor against alkali metal, alkaline earth metal, and other transition metal ions. Various combination mechanisms, such as electrostatic attraction, reduction, complexation, and hydrogen bonding, were demonstrated to be involved in the adsorption process of Cr(VI) by XPS, ESP, and DFT calculations. And the interaction energies of the five protonated configurations of primary amine, tertiary amine, secondary amine, imine, and secondary amine on the ring with HCrO4− were: −22.66, −12.08, −24.92, −24.26, −27.64 kcal/mol. In the actual industrial wastewater study, a Cr(VI) removal rate of 85.8% was realized. This work provided a viable idea for the elimination of Cr(VI) and was expected to be applied in the field of wastewater treatment.