Kinetics and Mechanisms of Adsorptions of Cd(II), Pb(II), and Cr(VI) Metal Cations Using Recyclable (amino)thiol...

Magnetically recyclable chelating agent was developed by immobilization of an N̂N̂S donor Schiff base ligand, (1Z,3Z)-N¹,N³-bis(2-mercaptoethyl)-N'¹,N'³ bis(3(triethoxysilyl)propyl) isophthalimidamide (L1), on iron oxide magnetic nanoparticles (Fe₃O₄) to form L1@Fe₃O₄. The immobilized chelating agent was characterized using various techniques, including scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive X-ray (EDX), Brunauer-Emmett-Teller (BET), powder X-ray diffraction (PXRD), vibrating sample magnetometry (VSM), ¹H NMR, fourier transform- infrared (FT-IR) spectroscopy, and thermal gravimetric-differential thermal analyses/differential scanning calorimetry (TGA-DTA/DSC). The efficiency of L1@Fe₃O₄ in the adsorption of Cd(II), Pb(II), and Cr(VI) metal ions from aqueous solutions was evaluated under different pH, contact times, initial concentrations, and dosage conditions. The chelating agent exhibited adsorption capacities of 14.14 mg/g and 13.87 mg/g within 15 min for Cd(II) and Pb(II) cations at pH 7 and 3, respectively. On the other hand, adsorptions of Cr(VI) anions displayed an adsorption capacity of 10.55 mg/g at a pH of 3 within 960 min. The adsorption kinetics conformed to a pseudosecond-order model, while the Freundlich isotherm model best described the equilibrium data. The thermodynamics data indicated a chemosorption adsorption process. Investigations of the metal-ligand interactions, along with the parameters of adsorption isotherms confirmed that the chelating agent preferred binding with metal ions in the order Cd(II) > Pb (II) > Cr(VI), consistent with the soft base nature of the chelating agent.