3D Metallo-Supramolecular Polymer for Fast and Efficient Removal of Anionic Dyes from Water

Toxic dye contaminants in wastewater are of severe concern to human and aquatic ecosystems. Consequently, proper treatment and control of these effluents are of utmost urgency. Despite the advancement of various classes of materials, such as metal–organic frameworks, covalent organic frameworks, porous organic polymers, and poly(ionic liquids), challenges such as low capture capacity, high capturing time, non-reusability, poor selectivity, and pH-limited performance still remain to be addressed. Therefore, developing a potential class of materials to address such challenges is desirable. Herein, a [Zn(terpyridine)₂]²⁺-templated tris-imidazolium based highly cationic three-dimensional (3D) metallo-supramolecular polymer (MSP) ({Zn_1.5L1}_n) was synthesized and utilized for the selective adsorption of anionic dyes [methyl orange (MO), acid orange-7 (AO-7), alizarin red S (AR), Congo red (CR), and eriochrome black T (EBT)]. The role of geometry, cationic framework, and counter anions present at the imidazolium unit and the metal center (Cl^–) were investigated by comparing the capture capacity with two other MSPs {Zn_1.5L2}_n and {Zn_1.5L3}_n, which were designed by altering the ligand backbone in a controlled manner. {Zn_1.5L1}_n exhibited comparatively high efficiency out of the three MSPs and the capture capacity for MO, AO-7, AR, CR, and EBT were 966, 1116, 1002, 908, and 800 mg/g, respectively. The pseudo-second-order model and the Langmuir model suit all adsorption kinetics data and isotherms well. Further, for {Zn_1.5L1}_n, the adsorption in the presence of different competing anions at different pH (2–12) and a glass column setup experiment was carried out. The reusability of the polymer up to 10 cycles for MO and AO-7 was also demonstrated. Finally, the adsorption mechanism was established by treating the material with different cationic and neutral dyes. The selective capture toward anionic dyes revealed that ion exchange and electrostatic interactions dominated the adsorption process.