Improved precious metal adsorption by introduction of carboxylic acid groups on methylene crosslinked calix[4]arene resin matrix

A methylene crosslinked calix[4]arene tetracarboxylic acid resin that exhibits high precious metal adsorption properties was prepared. The maximum loading capacity of this resin was compared with that of an unmodified methylene crosslinked calix[4]arene resin to verify its applicability for platinum and palladium adsorption. The introduction of multiple acetic acid groups improved the adsorption capacity for divalent palladium, while that for tetravalent platinum was slightly suppressed. The newly obtained capacity of palladium was 1.17 mmol/g, while that of platinum was 0.23 mmol/g. The stoichiometry of Pd(II) to the calix[4]arene tetracarboxylic acid monomer in the methylene crosslinked tetracarboxylic acid calix[4]arene resin was found to be 1:2, while that of Pt(IV) to the same monomer was 9:1. Linear and non-linear methods were applied to evaluate the fitting of the Langmuir and Freundlich models for the studied adsorption isotherm. The results showed that the adsorption isotherm was consistently fitted by the Langmuir model. Further, the non-linear method was found to be more suitable for obtaining the isotherm parameters.