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 (R2), 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 bestfitted models to the experimental data. Moreover, adsorption thermodynamics has also been evaluated. The negative values of Gibb’s free energy change (DG0) revealed spontaneity of the adsorption process, and the positive values of enthalpy (DH0) of adsorption indicated the 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). endothermic nature of the adsorption process. Adsorption
