Construction of malononitrile-functionalized conjugated microporous polymers as adsorbents for effective adsorption of Rhodamine B and density functional theory perspective

Abstract

The escalating challenges of water pollution, driven by rapid industrialization and population growth, necessitate innovative solutions for removing hazardous contaminants such as synthetic dyes. Rhodamine B (RhB), a prevalent pollutant in wastewater, poses significant environmental and human health risks due to its toxicity and persistence. Conjugated microporous polymers (CMPs) have emerged as promising adsorbents for dye removal owing to their high surface area, tunable porosity, and versatile functionalization. In this study, we synthesized and characterized three CMPs [PTB-FO CMP, PTB-ANQ 2CN CMP, and PTB-ANQ 4CN CMP] differing in cyano group content and systematically evaluated their RhB adsorption performance under varying conditions and contact times. Among these, PTB-ANQ 4CN CMP exhibited the highest adsorption capacity across a broad pH range, demonstrating superior stability and enhanced dye-polymer interactions facilitated by cyano functionalities. Adsorption kinetics followed a pseudo-second-order model, indicative of a chemisorption-dominated process, while equilibrium adsorption data were best described by the Langmuir isotherm, suggesting monolayer adsorption. Density Functional Theory (DFT) calculations revealed that the calculated ΔN values for electron transfer from the dye to PTB-ANQ 4CN CMP, PTB-ANQ 2CN CMP, and PTB-FO CMP are 0.31, 0.26, and 0.05, respectively. These results indicate that PTB-ANQ 4CN CMP and PTB-ANQ 2CN CMP possess markedly stronger electronic interactions with the dye molecules compared to PTB-FO CMP. Based on the density functional theory (DFT) calculations, the calculated ΔN values for electron transfer from the dye to PTB-ANQ 4CN CMP, PTB-ANQ 2CN CMP, and PTB-FO CMP are 0.31, 0.26, and 0.05, respectively. Notably, PTB-ANQ 4CN CMP and PTB-ANQ 2CN CMP exhibit significantly stronger interactions with dye than PTB-FO CMP. These findings highlight the potential of cyano-functionalized CMPs as effective and sustainable materials for wastewater treatment, offering a viable strategy for mitigating synthetic dye pollution.