Selective Trapping of In(III) Ions under σ-π* Bond Synergistic Effects by Modulating Lewis Basicity of Capture Sites on Nanofibers

Effectively selective recovery and separation of indium from alternative resources are of great environmental and economic significance. Although adsorption plays a critical role in this process, the design of highly selective capture sites remains a great challenge. Inspired by molecular orbital theory and hard and soft acids and bases theory (HSAB), we recognize that the nonequivalent orbital hybridization of In(III) ions renders them susceptible to deformation, exhibiting softer Lewis acidity. Herein, the thioacetamide-modified nanofiber (TAANF), with the O═C─NH─C═S as the capture sites, was prepared through an electrospinning technique combined with chemical modification. As expected, the O═C─NH─C═S exhibits softer Lewis basicity via modulated Lewis basicity of C═S by C═O, which better matches the Lewis acidity of In(III) ions to improve affinity. Adsorption studies showed that TAANF exhibited excellent properties for In(III) ions, especially in terms of selectivity; the selectivity coefficients range from 20 to 276 for K(I), Ca(II), Na(I), Mg(II), Mn(II), Zn(II), and Fe(II) ions in a multicomponent system. Furthermore, the capture mechanism indicates that In(III) ions not only can accept electrons from capture sites but also donate rich <i>d</i>² orbit electrons to the π orbitals of capture sites, as demonstrated by XAFS, XPS, and DFT. This enables selective capture of In(III) ions by forming a stable six-membered ring under the synergistic effect of σ and feedback π bonds (σ-π*). Finally, this work provides a strategy to design highly selective capture sites and holds promise for recovering In(III) ions from alternative sources.