Removal of Nd(III), Sr(II), and Rb(I) Ions from Aqueous Solution by Thiacalixarene-Functionalized Graphene Oxide Composite as an Adsorbent

A thiacalixarene-functionalized graphene oxide (GO-TC4A) composite was synthesized via esterification and polymerization of thiacalix[4]arene (TC4A) onto the graphene oxide (GO) surface and used to adsorb radionuclides from aqueous solution. The GO-TC4A composite showed high adsorption capacities (337.84, 101.11, and 164.47 mg·g^-1 for Nd(III), Sr(II), and Rb(I) ions, respectively) and excellent efficiency in removing a high density of radionuclides from water, compared to GO. The adsorption capacity of GO-TC4A was found to rise with increasing solution pH, reaching a maximum at pH values of 6-7, 7-9, and 4-5 for Nd(III), Sr(II), and Rb(I) ions, respectively. The correlation coefficients (R² > 0.999) implied that the pseudo-second-order model was most suitable to represent the adsorption kinetics. Four adsorption isotherm models were discussed comparatively, and the fitting results indicate that the Freundlich model was the most suitable method to describe the adsorption process of Nd(III) ions, whereas the Langmuir model best described that of Sr(II) and Rb(I) ions. FTIR and XPS analysis showed that the adsorption of radionuclides on the GO-TC4A composite is mostly controlled by chemisorption for strong chelation. Thermodynamic studies implied that the adsorption toward Nd(III), Sr(II), and Rb(I) ions was an endothermic and spontaneous natural process.