Computer simulation of the sorption of heavy metal ions by a sulphur-containing modified zeolite

Abstract

This paper considers complex-chelating and complex-coordinating mechanisms of sorption of heavy metal ions, which have an affinity for sulphur atoms, by sulphur-containing sorbents. The advantages and disadvantages of sulphur-containing sorbents obtained by modification of natural materials are shown. The STATGRAPHICS Plus environment was used to analyse the applicability of classical linearized Freundlich and Langmuir equations for describing the adsorption isotherms of zinc, copper and nickel ions from aqueous solutions by the Kholinsky zeolite modified with a sulphur-containing polymer. It is shown that the classical Freundlich equation accurately describes the sorption isotherms of Zn 2+ and Ni 2+ by the sorbent under study at the aqueous solution temperatures of 20-40 °C and 20 °C, respectively. The use of the Freundlich equation to describe the sorption isotherms of Cu 2+ is impractical due to its low accuracy. For this purpose, regression models for approximating experimental data or a linearized Langmuir equation are more suitable. It is shown that the Langmuir equation describes 99.35, 98.41 and 92.12% of the experimental values of Cu 2+ sorption at the aqueous solution temperatures of 20, 40 and 60 °C, respectively. It was established that an increase in the temperature of the model solution leads to an increase in the equilibrium constant and equilibrium adsorption capacity, at the same time as decreasing the accuracy of isotherm description. The sorption isotherms of Zn 2+ and Ni 2+ do not obey the Langmuir equation, which can be explained by the Pearson acid-base concept (HSAB). According to this concept, Zn 2+ and Ni 2+ ions are acids of intermediate hardness, while Cu 2+ ions refer to ‘soft’ acids capable of forming the strongest compounds with ‘soft’ bases (polymer sulphur atoms). Zn 2+ and Ni 2+ ions bind not only to sulphur atoms, but also to oxygen atoms (‘hard’ base) of the zeolite matrix, thus moving along the sorbent surface in search of sites more convenient for coordination. Therefore, for Zn 2+ and Ni 2+ ions, one of the basic principles of the Langmuir theory is violated, i.e. localization of the sorbate on the sorbent surface. Apparently, it is for this reason that the Langmuir equation is not applicable for describing the Zn 2+ and Ni 2+ sorption.