Comparison of Cd2+ adsorption onto amphoteric, amphoteric-cationic and amphoteric-anionic modified magnetic bentonites

Abstract

Organic-magnetic bentonites (OMBts), i.e., amphoteric modified MBt (BS-MBt), amphoteric-cationic modified MBt (BS-CT-MBt) and amphoteric-anionic modified MBt (BS-SDS-MBt), obtained by modifying magnetic bentonite (MBt) with amphoteric surfactant (BS), cationic surfactant (CT) and anionic surfactant (SDS) were investigated with the aim to remove cadmium (Cd²⁺). The modifier contents, surface charge and Cd²⁺ adsorption performances of OMBts were compared, and the influences of pH, temperature and ionic strength on Cd²⁺ removal were evaluated. Results showed that modifier contents of OMBts increased in the order: BS-CT-MBt > BS-MBt > BS-SDS-MBt. Although CEC of adsorbents increased in the order: MBt > BS-MBt > BS-SDS-MBt > BS-CT-MBt. The BS-MBt exhibited the highest Cd²⁺ adsorption capacity (233.19 mmol kg^-1) than other adsorbents. The adsorption isotherms could be well described by Langmuir model. The Cd²⁺ adsorption capacities on MBt and OMBts increased with an increase in pH, temperature and with a decrease of ionic strength. According to characterizations (FT-IR and XPS) and experiments, Cd²⁺ adsorption on MBt and OMBts most possibly involved electrostatic interaction, ion exchange, and surface complexation. Furthermore, the adsorption of Cd²⁺ on BS-MBt was also attributed to the chelation. The amidocyanogen group of BS-CT-MBt inhibited adsorption of Cd²⁺ due to electrostatic repulsion, while Cd²⁺ was adsorbed on BS-SDS-MBt through electrostatic attraction induced by the sulfo group.

Keywords: Adsorption; Bentonite; Cadmium ions; Magnetic; Surfactants.