Efficient removal of malachite green dye onto nickel oxide-based adsorbent: experimental and theoretical approaches

Waterborne hazardous organic pollutants, including malachite green (MG) dye, remain a major environmental concern due to their detrimental effects on ecosystems and humans. In the present paper, the MG dye removal from aqueous solutions by nickel oxide (NiO) is investigated; used NiO has been prepared using the precipitation method. Structural characterization and properties of the adsorbent were performed via various physicochemical methods such as X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and N₂ adsorption/desorption measurements. The results revealed that the maximum adsorption capacity and removal rate were 800 mg/g and about 99%, respectively. On the other hand, the adsorption kinetics follows the pseudo-second-order model. However, thermodynamics showed that: (i) ΔS° > 0 indicated an increase of disorder in the solid/liquid interface, (ii) ΔH° > 0 confirmed the endothermic nature of the adsorption process, and (iii) ΔG° < 0 demonstrated the spontaneous nature of MG adsorption. The calculated adsorption energies MCS/SAA E_Gas =  − 409.638 kcal and E_Aqueous =  − 416.856 kcal confirmed that the MG removal by adsorption on NiO is theoretically and experimentally favorable. In addition, MG molecules are adsorbed in a planar orientation on the NiO surface sites; the optimal interfacial contact is achieved. Finally, a mechanism was proposed to explain the adsorption of MG dye onto nickel oxide.