Synergistic effect of graphene nanosheets and zinc oxide nanoparticles for effective adsorption of Ni (II) ions from aqueous solutions

The threat of toxic substances such as heavy metals to public health and wildlife has led to an increasing public awareness. Different techniques for neutralizing the toxic effects of heavy metals in wastewater have been used. Here, we prepared a new and efficient type of adsorbent, zinc oxide–graphene nanocomposite (ZnO–Gr), via a green method to remove Ni (II) ions from aqueous solutions. A facile microwave-assisted hydrothermal technique in the presence of an ionic liquid, 1-hexyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide [C6mim] [NTf2], was used to prepare ZnO–Gr. The synergistic effect between graphene nanosheets and ZnO nanoparticles in this new adsorbent for Ni (II) ions caused a maximum adsorption capacity of 66.7 mg g−1 at room temperature which is much higher than that of graphene nanosheets (3.8 mg g−1) and other carbonaceous nanomaterials used as an adsorbent in the literature. The maximum desorption percentage (90.32%) was achieved at pH 3.6. By thermodynamic study, we found that the adsorption of this heavy metal ion on ZnO–Gr was spontaneous (ΔG° = −6.14 kJ mol−1) and endothermic (ΔH° = 53.31 kJ mol−1) with entropy change of ΔS° = 199.45 J K−1 mol− 1.