Polythionine grafted onto magnetic SBA-15 for the removal of cadmium ions from aqueous solutions: isothermal and kinetic studies

Abstract

Magnetic SBA-15 coated with polythionine was synthesized via the grafting method and used for the adsorption of Cd²⁺ ions from aqueous solutions. The grafting method was successfully performed at a high temperature (∼600 °C) by replacing cobalt-ferrite nanoparticles with almost the same magnetic properties as Fe₃O₄ nanoparticles. Polythionine with amine and sulfur functional groups acted as a selective sorbent for Cd²⁺ ion adsorption. The sorbent was characterized using FTIR, FE-SEM, BET analysis, X-ray diffraction, TEM, VSM and zeta potential analysis. To obtain the optimum conditions for the removal process, parameters such as solution pH, adsorption time, sorbent amount and breakthrough volume were studied and optimized. The optimum conditions were as follows: pH of the solution, 8; adsorption time, 15 min; sorbent amount, 15 mg. The sorbent capacity achieved was about 35 mg g⁻¹. Three different isotherms (Langmuir, Freundlich and Temkin) were investigated, and the Langmuir isotherm with the correlation coefficient and chi square values of 0.9928 and 0.0069, respectively, was chosen. Three different kinetic studies (pseudo-first order, pseudo-second order and the Elovich model) were also performed to understand the kinetic process. The pseudo-second-order kinetic model with the correlation coefficient and chi square values of 0.9971 and 0.0061, respectively, was selected as the kinetic process for this study. The negative ΔG value confirmed the presence of an exothermic and spontaneous removal process.