Crown ether-functionalized polyimide nanofiber membranes fabricated by in-situ grafting and electrospinning for efficient selective adsorption of cesium

Abstract

A nanofiber membrane (18C6-g-18C6PI) containing crown ethers in both the main and side chains was fabricated via in-situ grafting and electrospinning through amidation between 18-crown-6 polyimide (18C6PI, Mₙ = 93 kDa) and di(aminobenzo)18-crown-6 (DAB18C6). The goal of this study is to enhance the loading amount of crown ethers so as to improve the adsorption capacity for cesium (Cs⁺) from salt lakes. Results showed that the nanofiber membrane with a fiber diameter of 310–340 nm was obtained by precisely controlling the viscosity of the spinning system (650–700 mPa·s) and the grafting reaction time (10 h). The grafting degree of crown ether was enhanced from 17.3 % (18C6PI) to 84.8 % after DAB18C6 grafting. Dynamic cyclic adsorption experiments revealed that the 18C6-g-18C6PI nanofiber membrane achieved a Cs⁺ adsorption capacity of 85.23 mg g⁻¹, which is significantly higher than that of the 18C6PI nanofiber membrane (68.21 mg g⁻¹). In addition, the 18C6-g-18C6PI nanofiber membrane showed a high selective separation for Cs⁺, with separation factors for Cs⁺/Rb⁺, Cs⁺/K⁺, Cs⁺/Na⁺, Cs⁺/Ca²⁺, and Cs⁺/Mg²⁺ of 10.3, 6.5, 29.5, 17.6, and 21.4, respectively. The 18C6-g-18C6PI nanofiber membrane maintained 96.3 % of the initial capacity after five regeneration cycles. Furthermore, density functional theory (DFT) calculations demonstrated that 18C6-g-18C6PI exhibited a stronger Cs⁺ binding energy (−436.89 kJ mol⁻¹) than 18C6PI (−360.41 kJ mol⁻¹). This enhancement was ascribed to the higher crown ether loading in 18C6-g-18C6PI, which intensified the ion-dipole interaction with Cs⁺. In summary, this work provides insights into the development of crown ether-functionalized nanofiber membranes for efficient and selective recovery of cesium.