The role of bare and modified nano nickel oxide as efficient adsorbents for the removal of Cd²⁺, Cu²⁺, and Ni²⁺ from aqueous solution

Abstract

The purpose of this research work is to investigate sorption characteristics of bare NiO, and modified NiO nanoparticles (NPs) with humic acid (H) for the removal of Cd²⁺, Cu²⁺, and Ni²⁺ ions from aqueous solutions. Bare NiO NPs and modified with humic acid (NiO–H) were prepared as new adsorbents. The bare and modified NPs were fully characterized by FT-IR, XRD, and SEM-EDX measurements. The influences of parameters including pH, dosage of adsorbent, temperature, and contact time were investigated in order to find the optimum adsorption conditions for isotherm studies and obtain adsorption capacity of Cd²⁺, Cu²⁺, and Ni²⁺ on NiO and NiO–H NPs. The experimental data were fitted to Langmuir and Freundlich isotherms, and the data were analyzed on the basis of Lagergren pseudo-first-order, pseudo-second-order and Webere–Morris models. On the basis of the values of the correlation coefficient (R ²), the kinetics of the metal ions adsorption on NiO and NiO–H were best-described by pseudo-second-order model. Depending on the system, the Langmuir or Freundlich isotherms have been found to provide the best correlation. Freundlich on NiO–Cd²⁺, NiO–H–Cd²⁺, and NiO–H–Cu²⁺ and Langmuir models on NiO–Ni²⁺, NiO–H–Ni²⁺, and NiO–Cu²⁺ were the best-fitted models to the experimental data. Moreover, adsorption thermodynamics has also been evaluated. The negative values of Gibb’s free energy change (∆G ⁰) revealed spontaneity of the adsorption process, and the positive values of enthalpy (∆H ⁰) of adsorption indicated the endothermic nature of the adsorption process. Adsorption studies showed that affinity of both NiO and NiO–H was the highest average for Cd²⁺ (45.4 and 55.0 mg g⁻¹) and was much lower for Cu²⁺ (22.6 and 60.9 mg g⁻¹) and Ni²⁺ (12.4 and 10.5 mg g⁻¹).