Intrinsic nanoconfinement design in halloysite nanotubes enhances the affinity of zirconium phosphate for efficient removal of heavy metal ions

Confining the active adsorption materials within the nanostructure space can significantly improve adsorption performance. However, the fundamental mechanisms by which intrinsic nanoconfinement in clay minerals modulates the microstructure of active materials and regulates interfacial interactions remain unclear. In this work, a comprehensive analysis was conducted to compare the adsorption performance of two structures: a nonconfined structure (ZrP/HNTs), where zirconium phosphate (ZrP) nanocrystals are grown on the outer surface of halloysite nanotubes (HNTs), and a nanoconfined structure (ZrP@HNTs), where ZrP nanocrystals are confined within HNTs' lumen. The resulting ZrP@HNTs adsorption capacity for Pb (II) was 3 times that of ZrP/HNTs (163.7 vs. 54.6 mg/g), and the adsorption rate increased by 2.8 times. Additionally, the confined structure endowed ZrP@HNTs with remarkable stability, only 6.7% capacity decrease after five cycles. Furthermore, finite element simulations revealed that the nanoconfined structure of ZrP@HNTs effectively enriches Pb(II), increasing its concentration and facilitating adsorption. According to the above result, low and high Pb(II) concentrations were used to simulate the adsorption environments of ZrP/HNTs and ZrP@HNTs, respectively, with atomic force microscopy (AFM) providing key quantitative insights. AFM force spectroscopy further confirmed that the nanoconfined structure enhanced the adhesion energy between Pb(II) and phosphate groups on the ZrP surface from 4.34 mJ·m −2 to 10.81 mJ·m −2 , directly contributing to improved adsorption stability. These findings offer valuable insights into the role of nanoconfinement in enhancing adsorption efficiency, providing a basis for the development of advanced heavy metal removal technologies.