Utilizing RSM-CCD for optimizing the removal of Cr(VI) and tetracycline over a new recyclable Fe3O4/SiO2/ZIF-67 composite: Isotherms, kinetics, and mechanism

Abstract

Pollution of water resources by a range of organic and inorganic non-degradable contaminants is becoming increasingly problematic. This study presents a novel Fe₃O₄/SiO₂/ZIF-67 (FSZ) nanocomposite for removing tetracycline (TC) and hexavalent chromium (Cr⁶⁺). The synthesis began with the preparation of Fe₃O₄, SiO₂, and ZIF-67 components. Following this, various loadings of the binary SiO₂/ZIF-67 were prepared. Finally, the ternary FSZ adsorbents were synthesized by impregnating 1, 3, 5, 7, and 9 wt% of Fe₃O₄ nanoparticles (NPs) onto the optimized binary SiO₂/ZIF-67 NPs. Characterization techniques such as SEM, TEM, BET, EDS, XRD, and XPS were performed. The removal efficiencies were optimized using the RSM-CCD, considering factors like composite dosage, reaction time, pH levels, and contaminants concentration. Optimal composite dosages of 0.69 g/L for TC and 1.68 g/L for Cr⁶⁺, pH values of 6.05 for TC and 6.06 for Cr⁶⁺, reaction times of 90 min for TC and 57 min for Cr⁶⁺, and contaminant concentrations of 25.47 mg/L for TC and 2683.2 ppb for Cr⁶⁺ were achieved. Removal rates of 98.53 % for TC and 97.45 % for Cr⁶⁺ were achieved under optimal conditions. Moreover, various kinetic and isothermal adsorption models were applied. The FSZ composite maintained high removal efficiencies for Cr⁶⁺ (81.14 %) and TC (79.82 %) after four cycles. Its performance was assessed in various water types, including deionized water, river water, and municipal wastewater. Additionally, the FSZ composite effectively removed other contaminants, achieving high efficiencies for cadmium (95.18 %), lithium (83.42 %), arsenic (88.29 %), sulfamethoxazole (87.38 %), doxycycline (96.15 %), and cefixime (81.97 %). Finally, the mechanisms responsible for the removal of TC and Cr⁶⁺ were evaluated. The sustainable synthesis of FSZ provides a valuable foundation for future studies focused on Cr⁶⁺ and TC removal, offering a potential solution to pressing environmental challenges.