Ultrafast Dye Removal Using Ionic Liquid–Graphene Oxide Sponge

A methylimidazolium ionic-liquid-functionalized graphene oxide (mimGO) sponge was prepared by facile amidation reaction followed by freeze-drying. Covalent functionalization of graphene oxide (GO) was confirmed by SEM, XRD, XPS, FT-IR, Raman and TGA. The mimGO sponge was employed to remove direct red 80 (DR80) dye from aqueous solutions. Results showed exceptional affinity for adsorption of azo dye direct red 80 (DR80) due to the charge-induced adsorption aided by protonated amine and cationic methylimidazolium ionic liquid pendant groups. Detailed adsorption characteristics of mimGO sponge including operational parameters, adsorption kinetics and adsorption isotherms were investigated. Adsorption on the mimGO sponge followed pseudo-second-order kinetic model, and the equilibrium capacity was described by the Langmuir adsorption model. An ultrahigh adsorption rate of 588.2 mg/(g·min) and equilibrium adsorption capacity of 501.3 mg/g for DR80 were observed for the mimGO sponge, which is much higher than that of ethylenediamine functionalized GO, unmodified GO and commercial activated carbon. Further, desorption of 99.4% of DR80 was achieved at even higher rate in aqueous solution of pH 12. The dye removal efficiency of the mimGO sponge remains at 99.2% after four adsorption–desorption cycles. Combining the ultrafast adsorption kinetics with high removal capacity and good recyclability, the mimGO sponge has great potential for effluent treatment applications.