Fluorinated LDHs for selective sorption of PFOS: Unveiling the roles of increased hydrophobicity and F-F interactions

Abstract

Layered double hydroxides have been proved to be effective adsorbents for the removal of per- and polyfluoroalkyl substances (PFASs) from aqueous solutions. In this study, a novel layered double hydroxides modified by perfluorooctyltriethoxysilane (LDHs-PFOTES) was synthesized at room temperature, demonstrating rapid and selective sorption of perfluorooctane sulfonic acid (PFOS). The covalent grafting of fluorosilane groups onto the LDHs surface enhanced hydrophobicity and introduced fluorine functional groups, while preserving the positive charge essential for electrostatic interactions. LDHs-PFOTES removed 97.21 % of PFOS from water within 5 min, with a sorption capacity of 1530.33 mg/g, nearly doubling the capacity of unmodified LDHs (831.79 mg/g). The thermodynamic analysis of PFOS sorption on LDHs-PFOTES revealed that the process is spontaneous and endothermic. LDHs-PFOTES maintained over 90 % PFOS removal efficiency under varying pH levels, ionic strengths, and the presence of competing compounds. Furthermore, the C-F structure on the LDHs-PFOTES enabled fluorine-fluorine (F-F) interactions with PFOS, further improving selective sorption. It was concluded that the combined effects of electrostatic interactions, hydrophobicity, F-F interactions, and ion exchange resulted in the efficient and selective removal of PFOS from water. LDHs-PFOTES demonstrated 95.37 % sorptive efficiency in removing PFOS when treating a mixture of 11 PFAS compounds. Additionally, LDHs-PFOTES effectively desorbed PFOS using a mixed solution of NaCl and methanol, maintaining approximately 95.08 % removal efficiency over three regeneration cycles. This fluorination strategy to modify LDHs offers a novel approach for selectively and efficiently removing PFASs from water.