Effective strategies toward imine-linked cationic covalent organic frameworks for rapid and selective removal of 99TcO4− from water: insights from DFT and MD calculations

Abstract

Effective removal of radioactive ⁹⁹TcO₄⁻ is crucial for both the management of spent nuclear fuel and nuclear-related environmental remediation. Recently, cationic covalent organic frameworks (cationic COFs) have received extensive attention as efficient anion-scavenging materials for capturing radioactive ⁹⁹TcO₄⁻. Herein, three kinds of imine-linked cationic COFs (named as DBBT, DBTFP and PITFP) with eclipsed (AA) and staggered (AB) structures were built and modeled to eliminate ⁹⁹TcO₄⁻ from radioactive wastewater. The adsorption performance and interaction mechanism were systematically investigated by density functional theory (DFT) calculations and molecular dynamics (MD) simulations. The anion-exchange process is driven by the electrostatic interaction between ⁹⁹TcO₄⁻ and positively charged groups of cationic COFs. The adsorption ability of DBBT is better than that of DBTFP and PITFP with a higher elimination ratio of ⁹⁹TcO₄⁻. The straight channels of DBBT-AA make both the adsorption dynamics and selectivity better than DBBT-AB. The carbonyl group in DBTFP has a slightly negative influence on the adsorption of ⁹⁹TcO₄⁻. The adverse effect of small pore size can be observed for PITFP, though more adsorption sites are an advantage. The increasing concentration of ⁹⁹TcO₄⁻ will significantly accelerate the adsorption, however, full elimination of ⁹⁹TcO₄⁻ is not observed owing to the hard desorption of inner-layer counter-anions. Our study provides an insightful understanding for the adsorption process of ⁹⁹TcO₄⁻ at the molecular level and will facilitate the development of cationic COFs with outstanding performance for ⁹⁹TcO₄⁻ elimination.