Enhanced removal of diclofenac from water using a zeolitic imidazole framework functionalized with cetyltrimethylammonium bromide (CTAB)

Abstract

Diclofenac represents one of the most common pharmaceuticals and personal care products (PPCPs) in municipal wastewater. To enhance the removal of diclofenac from water, a cationic surfactant, cetyltrimethylammonium bromide (CTAB), was introduced into a zeolitic imidazole framework (ZIF)-67. CTAB is added originally to minimize the use of ligands during the synthesis of ZIF-67; however CTAB present in ZIF-67 also increases the surface charge and thus enhances the adsorption capacity up to 10 times compared to CTAB-free ZIF-67. Factors influencing the diclofenac adsorption are investigated, including CTAB loading, temperature and pH. A loading of 0.274 mol-CTAB/mol-Co²⁺ is suggested as an optimal loading to prepare a relatively efficient CTAB-ZIF-67, while over loading of CTAB could not fully incorporate CTAB into ZIF-67. In view of the diclofenac adsorption kinetics and isotherm, the adsorption of diclofenac to CTAB-ZIF-67 was considered to involve a strong affinity between diclofenac and CTAB-ZIF-67 owing to the electrostatic attraction between the carboxylic acid of diclofenac and the quaternary amine of CTAB. Considering that the removal of diclofenac from urine is of great interest, separation of diclofenac from urine using CTAB-ZIF-67 was evaluated and also employed to investigate the effect of co-existing ions. We also determined the desorption behavior of diclofenac from CTAB-ZIF-67 to provide insight for the recyclability of CTAB-ZIF-67 for diclofenac adsorption.