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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 HCrO 4 − 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. The hypertoxic Cr (VI) poses a serious hazard to humans and the environment. Herein, GOPP were synthesized and applied to the adsorption process of Cr (VI), aiming to achieve effective control of Cr (VI) pollution in actual fluids. [Display omitted] • The flexible decoration of PDA and PEI could gradually enhance N content. • The protonation order of the five N-containing sites were revealed. • The interaction energies of the five configurations with HCrO 4 − were researched. • Imines and secondary amines play important roles in the adsorption of Cr(VI). • The GOPP exhibited 564.7 mg/g adsorption capacity towards Cr(VI). [ABSTRACT FROM AUTHOR]