Removal, mechanistic and kinetic studies of Cr(VI), Cd(II), and Pb(II) cations using Fe₃O₄ functionalized Schiff base...

The Schiff base chelating ligands; (E)-2-(3,3-dimethoxy-2-oxa-7,10-diaza-3-silaundec-10-en-11-yl)phenol (L1), (E)-N-(2-((pyridine-2ylmethylene)amino)ethyl)-3-(trimethoxysilyl)propan-1-amine (L2) and (E)-N-(2-((thiophen-2-ylmethylene)amino)ethyl)-3-(trimethoxysilyl)propan-1-amine (L3) were immobilized on Fe₃O₄ magnetic nanoparticles (MNPs) and utilized in the extraction of Cr(VI), Cd(II) and Pb(II) metal cations from aqueous solutions. The compounds synthesized, denoted as L1@ Fe₃O₄, L2@Fe₃O₄, and L3@Fe₃O₄, were characterized using FT-IR spectroscopy, TEM-SEM, VSM, and BET/BHJ techniques for analysis of functional groups, surface morphology, magnetic properties, and degree of porosity of the adsorbents, respectively. BET/BHJ technique confirmed the mesoporous nature of the compounds as their pore diameters ranged between 15 and 17 nm. The initial optimization conditions of pH, adsorbent dosage, initial metal concentration, and contact time on adsorption were studied using L1@ Fe₃O₄. The optimum efficiencies recorded were 68% and 46% for Cr(VI) and Cd(II), respectively, obtained at pH 3, and a metal concentration of 20 ppm while an efficiency of 99% was recorded for Pb(II) cations at pH 7 and a metal concentration of 100 ppm. Compounds L2@Fe₃O₄ and L3@ Fe₃O₄ were also used in the extraction of metal cations from aqueous solution and gave efficiencies of 22%, 56%, and 78% for L2@ Fe₃O₄ and 19%, 90%, and 59% using L3@ Fe₃O₄ for Cr(VI), Cd(II), and Pb(II), respectively. The maximum adsorption capacities of L1@ Fe₃O₄ for Cr(VI), Cd(II), and Pb(II) cations were obtained from the Langmuir isotherm as 32.84, 41.77, and 450.45 mg/g, respectively. The experimental data was analyzed using pseudo-first-order, pseudo-second-order, intraparticle diffusion, and Elovich kinetic models. Both linear and non-linear forms of kinetic isotherms; Langmuir, Freundlich, Redlich-Peterson, and Temkin were utilized to investigate the nature of adsorption on L1@Fe₃O₄. The mechanistic studies deduced that the Langmuir isotherm and pseudo-second-order kinetic model better described the adsorption process both yielding high correlation coefficient values (R² > 0.98).