Investigation on the adsorption properties of Cr(vi) ions on a novel graphene oxide (GO) based composite adsorbent

In this study, we prepare a polyethyleneimine (PEI) and graphene oxide (GO) composite adsorbent through an amidation reaction between the amine groups of the PEI and the carboxyl groups of the GO. Adsorption performance of the PEI–GO was tested by removing Cr(vi) ions from aqueous solution. The physico-chemical properties of the GO, pristine and Cr(vi) ions-loaded PEI–GO were investigated by FT-IR, SEM-EDX and XPS methods. To investigate the adsorption kinetics of Cr(vi) ions onto this newly developed PEI–GO, we performed a batch of experiments under different adsorption conditions: content of the PEI in the PEI–GO, solution pH, initial Cr(vi) ion concentration, adsorption temperature and contact time. The prepared PEI–GO exhibited an encouraging uptake capacity of 539.53 mg g⁻¹. The adsorption process was fast, within the first 1 h, Cr(vi) ion adsorption onto the PEI–GO was about 71%, and the adsorption equilibrium could be obtained within 14 h. Kinetics experiments indicated that the adsorption process could be described by the pseudo-second-order kinetic model. Furthermore, our adsorption equilibrium data fit the Langmuir isotherms well (R² > 0.99). The removal mechanism of Cr(vi) ions from the solution consists of two steps: (1) Cr(vi) ions bind to the 4PEI–GO composite by electrostatic interaction between the negatively charged Cr(vi) ion species and the protonated amine groups of the 4PEI–GO; and (2) a few Cr(vi) ions were reduced to Cr(iii) ions with the assistance of π electrons on the carbocyclic six-membered ring of the 4PEI–GO and then bind on the 4PEI–GO by the electrostatic attraction between Cr(iii) ions and negatively charged groups (COO⁻) of the 4PEI–GO. Based on the results obtained in this work, it can be concluded that the prepared PEI–GO can be an effective and potential adsorbent for removing Cr(vi) ions from an aqueous solution.