Kinetic, equilibrium, and thermodynamic studies for adsorptive removal of nickel ions by thermally modified diatomite from aqueous solution

In this study, the removal of nickel ions from an aqueous solution was studied using thermally modified diatomite at different temperatures. Most of the diatomite has a circular shape, with a diameter and pore size of approximately 25 µm and 100 nm, respectively, based on the scanning electron microscopy analysis. The surface area of diatomite decreases from 48 to 13.4 m2 g−1 by increasing heating temperature from 650 to 950 °C, respectively. It is observed that the percentage of Ni(II) ion removal increases by raising the pH value until 4 and then decreases by further increasing the pH value. Based on Langmuir and Freundlich’s adsorption isotherms, the maximum adsorption capacity of diatomite is 45.96 mg g−1, and Ni(II) ion adsorption is somewhat favorable in the form of a monolayer. The adsorption energy of diatomite is found at about 6.94 kJ mol−1 from Dubinin–Radushkevich (D–R) adsorption isotherm with a combination of electrostatic interaction and ion exchange mechanisms. Finally, the mechanism of Ni(II) ion adsorption on diatomite can be explained by pseudo-second-order reaction kinetics with rate-limiting steps of film diffusion and intra-particle diffusion.