Synthesis of calix[4]arene bearing pyridinium units supported silica gel for sorption of arsenate and dichromate anions

In the present study two new silica gel-immobilized calix[4]arene polymers 4a and 4b have been prepared via modification of calix[4]arene, which contains pyridinium units 3a or 3b with silica-based polymer (ECHES) and investigated their sorption ability towards arsenate and dichromate anions at different pH. The prepared polymers were characterized by Fourier transform infrared spectroscopy, elemental analysis and thermal gravimetric analysis. Finally, the sorption properties of polymers (4a and 4b) towards dichromate and arsenate anions were reported. It was found that these calix[4]arene-based polymers exhibit a high level of sorption abilities towards dichromate and arsenate anions. It was observed that the percentage of arsenate removal reached 62–57% for polymer 4a and 59–50% for 4b when the pH of the aqueous solution was kept in the range of 3.5–4.5. The sorption of arsenate and dichromate anions by calix[4]arenes-based polymers indicates that the partially protonated pyridyl groups play a major role for the formation of hydrogen bonds and electrostatic interactions. In order to fully comprehend the selectivity of 4a, we also examined the retention of dichromate anions in the presence of Cl−, NO3− and SO42− anions at pH 1.5.