Chemically modified covalent organic frameworks for a healthy and sustainable environment: First-principles study

Pure water is a key element for a sustainable and healthy environment of human inhabitation. Since major sources of water contamination are industrially generated heavy metal cations there is great demand for efficient methods of their treatment. Here, using density functional theory, we investigate the covalent organic framework's electronic and optical properties and their interaction with the most dangerous heavy metal pollutants, namely Hg⁺², Pb^+2, and Cd⁺². We consider biphenyl boroxine covalent organic frameworks before and after chemical modification with CN, COOH, NH₂, and NO₂ groups. In addition to the molecular geometries, such parameters as the dipole moment, chemical potential, electronegativity, chemical hardness, and binding energy are calculated. It is found that CN, COOH, and NO₂ functional groups are favorable for intermolecular bonding with harmful transition metals. The functionalization with the mentioned groups reduces the band gap of the pristine covalent organic frameworks and increases their reactivity. As a result, strong complexes with Cd⁺², Hg⁺², and Pb⁺² can form, which, as follows from our calculations, can be detected by the red shift in their optical absorption spectra.