Graphene/δ-MnO2 composite as adsorbent for the removal of nickel ions from wastewater

A type of graphene nanosheet/δ-MnO2 (GNS/MnO2) composite was synthesized by a microwave-assisted method. The Ni (II) removal potential of GNS/MnO2 from solution was investigated. Microstructure measurements show that nanoscale δ-MnO2 particles deposit on the surfaces of GNS well. The adsorption characteristics had been examined with respect to pH effect, removal kinetic data, equilibrium isotherms and thermodynamic data in batch experiments. The equilibrium data are fitted well by the pseudo-second-order kinetics, suggesting that a chemical sorption process plays a significant role in Ni (II) ions adsorption. Adsorption isotherm can be described by the Langmuir model. The saturate adsorption capacity of Ni (II) onto GNS/MnO2 is 46.6 mg g−1 at room temperature, which is 1.5 and 15 times higher than those of pure δ-MnO2 and GNS, respectively. The positive values of both ΔH and ΔS suggest an endothermic reaction and increase in randomness at the solid–liquid interface during Ni (II) adsorption process. The negative ΔG values indicate a spontaneous adsorption process and GNS/MnO2 can be reused for 5 times with high recovery rate of 91%.