Calixarene-functionalized graphene oxide composites for adsorption of neodymium ions from the aqueous phase

Abstract

Calixarene-functionalized graphene oxide (GO) composites were synthesized via esterification and polymerization of calix[n]arenes (n = 4, 6, 8) onto graphene oxide surfaces for the first time. The GO composites were used to adsorb Nd ions from the aqueous phase due to their quite high BET surface areas. In this work, the GO composites (GO-C4, GO-C6, and GO-C8) showed high adsorption capacities (232.56, 219.78, and 311.53 mg g⁻¹, respectively) compared to GO (110.99 mg g⁻¹) during the removal process and excellent efficiency in removing a high concentration of neodymium ions (1150 mg L⁻¹) from water. The results indicate that the adsorption ability for Nd ions was in the order of GO-C8 > GO-C4 > GO-C6 > GO. The adsorption processes of GO-C4 and GO-C8 were completed within 185 min, whereas that of GO-C6 was achieved within 230 min. Furthermore, controlling the pH value of an aqueous solution in the range of 6, the GO composites exhibited the highest adsorption capacity toward Nd ions. For our process of removing Nd ions, the adsorption data of experiments mostly fit the pseudo-second-order model better than two other models. Four adsorption isotherm models were compared to study the details of the adsorption process, and the results indicated that the Freundlich model is the most suitable for describing the adsorption process of the GO composites. The adsorption mechanisms to remove Nd ions were the electrostatic attraction of Nd ions with hydroxyl functional groups and the complexation process of Nd ions with functional groups on the surface of the GO composite. Moreover, thermodynamic studies indicated that the adsorption behavior of the GO composites towards Nd ions was an endothermic and spontaneous natural process. In general, GO composites could potentially be applied to metal ion removal.