Arsenic and chromium are among the major highly toxic, carcinogenic inorganic pollutants in aquatic systems posing serious threats to human health and environmental safety. Under oxidative conditions, the most common forms of waterborne arsenic and chromium at neutral pH are arsenate and chromate, respectively. Hence, enormous research interest has been directed to the effective and efficient remediation of arsenate and chromate in water. Ionic organic frameworks are particularly attractive in toxic anion sequestration due to their unique ability to incorporate well-defined recognition groups and binding sites into chemically robust, lightweight, well-structured media. Success in small-molecule studies using guanidiniums has alluded the great potential of guanidinium-based ionic covalent organic frameworks (iCOFs) in oxoanion sequestration, yet the subject remains underexplored. Herein, we present a structure-activity relationship in the sequestration of chromate and arsenate in water using a series of guanidinium-based iCOFs. In particular, the hydrogen bonding, steric and electrostatic properties of the guanidinium-based anion binding site in iCOFs were systematically altered though a set of computationally guided structural modifications. Impacts of the implemented structural modifications in iCOFs on the efficiency and selectivity in sequestering chromate and arsenate in water were examined via batch binding experiments. The nature of the binding interactions between oxoanions and iCOFs was investigated using solid-state <sup>1</sup>H NMR and X-ray photoelectron spectroscopy. The important roles of hydrogen bonding group, steric hindrance, and guanidinium conformation and acidity in the effective and efficient uptake of chromate and arsenate were elucidated.

Structure activity relationship study on sequestration of Cr(VI) and As(V) Oxoanions in water using guanidinium based ionic covalent organic frameworks

ACS Fall 2021

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