Adsorption of Zn2+ ions using regenerated spent CoMo/γ-Al2O3 hydrodesulfurization catalyst

Abstract

Recycling of spent hydrodesulfurization (HDS) catalysts is viewed as an alternative approach to achieving sustainability objectives in environmental protection, particularly in waste management and water purification applications. Against this backdrop, the present research work focuses on the reusability of a spent CoMo/γ-Al₂O₃ HDS catalyst subjected to a set of operations that involve n-hexane solvent extraction of soluble carbon, an oxalic acid wash for foulant removal, and calcination at 500 ºC for 4 hours. The recovered catalyst is characterized via X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and atomic force microscopy (AFM). The surface area is estimated using BET technique and found it to be 189.13 m²/g. The obtained information reveals successful restoration, which is confirmed by the indicated crystal lattice parameters that correspond to a nanocrystallite size of roughly 2.03 nm. Batch experiments are conducted to test the ability of the restored catalyst in zinc ions (Zn²⁺) adsorption from aqueous solution under different operating conditions. Results indicate that regeneration improves performance and reaches around 99% for the restored catalyst compared with 59% for the fresh sample. As the adsorbent dose and initial Zn²⁺ ion concentration is increased, the removal efficiency is increased; however, it is decreased with increasing temperature. The optimum adsorption capacity reaches 557.50, which is obtained at dose of 0.01 g, concentration of 120 ppm, pH of 6, time of 30 min, and temperature of 298 K. This is attributed to the exothermic nature of the process. Based on the system's thermodynamics, the obtained ΔGº values are negative; this indicates that the process is spontaneous. Whereas negative ΔHº and ΔSº values indicate that the process is exothermic and associated with a decrease in randomness at the solid-liquid boundary.