Annealing temperature effect on 3D hierarchically porous NiO/Ni for removal of trace hexavalent chromium

Abstract

Three-dimensional (3D) hierarchically porous NiO/Ni composites were fabricated by hydrothermal method combined with subsequent annealing process. First, nickel hydroxide (Ni(OH)2) nanosheets were synthesized on flexible and porous nickel foam to form Ni(OH)2/Ni. Second, 3D porous nickel oxide (NiO) nanosheets grown on nickel substrate, named NiO/Ni, were obtained by thermal decomposition of Ni(OH)2/Ni in air at 400–650 °C for 3 h. The prepared samples were applied as adsorbents for the removal of hexavalent chromium (Cr(VI)) from aqueous solutions and characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and nitrogen adsorption and desorption to analyze their structures and compositions. NiO nanosheets grow on the 3D nickel foam provide more active sites and the large connected macropores are favorable to flow of solution. Numerous mesopores and micropores emerged on NiO nanosheets after thermal calcination, leading to high surface area favorable to adsorption of Cr(VI) ions. The results show that NiO/Ni annealed at 400 °C provides the maximum specific surface area (22 m2 g−1) and best removal capacity (25.94 mg g−1) at pH 4 with excellent recyclling performance. The pseudo-first-order kinetic fitting suggests the physical adsorption of Cr(VI) on NiO/Ni surface and the possible adsorption mechanism was investigated. Moreover, the effect of contact time, initial concentration of Cr(VI), pH, and matrix anion were studied in detail.