3D Metallo-organic coordination assembly-based anion-enriched supramolecular material for fast and efficient removal...

Chromium(VI), especially dichromate (Cr₂O₇^2-) contamination in wastewater due to rapid industrialization with uncontrolled effluent management is still a serious concern which needs focused attention. Multiprong approaches are practiced such as chemical precipitation, reverse osmosis, ion-exchange, adsorption by granular activated carbon etc. to capture and separate this "Group A" human carcinogenic effluent from water. However, low capture capacity, non-reusability, poor selectivity, pH-limited performance are some major limitations of these techniques. Recently, metal organic frameworks (MOFs), metal organic cages (MOCs), porous organic polymers (POPs) or covalent organic frameworks (COFs), covalent organic networks (CONs) etc. emerged as new-generation materials to overcome such limitations. However, the development is still in initial stage and issues related to structural stability and integrity of many MOFs in water and in wide pH range, as well as reusability need to be addressed. At this juncture, herein we report a novel [Zn(terpyridine)₂]²⁺-templated trisimidazolium-based highly cationic three-dimensional metal-organic coordination assembly (3D MOCA), serving as a new class of efficient, fast, robust and recyclable dichromate-removal material. Not only the highly cationic assembly is enriched with a high density of Br^- anions, but its three-dimensional propagation and flexibility also exposes the exchangeable Br^- ions for facile anion-metathesis with Cr₂O₇^2-. By virtue of the benefits of these attributes, the presented supramolecular material exhibits a high capture capacity (469 mg g^-1), fast exchange kinetics (0.028 g mg^-1 min^-1), wide working pH range (pH 2-12) and reusability up to a minimum of 10 cycles without much loss of efficiency. Key mechanistic examinations highlight the evidences in favor of ion-exchange-based chemistry to be responsible for dichromate removal with the present material.