The key role of hydrophobic forces during the electrocoagulation of perfluorooctanoic acid substitute (HFPO-TA) revealed by the interfacial thermodynamics

Abstract

Per- and polyfluoroalkyl substances (PFASs) have been widely found in water bodies and may generate negative impacts on the ecosystem. Eectrocoagulation (EC) with a zinc anode is an effective technology to remove PFASs but the dominating interaction forces between zinc flocs and PFASs are still not well elucidated. In this study, we used hexafluoropropylene oxide trimer acid (HFPO-TA), a new perfluorooctanoic acid substitute, as a model to investigate the influence of EC operational parameters on its removal efficiency, including the current density, the pH value, and the electrolyte type. 99 % removal of 40 μM HFPO-TA were achieved in 3-h at optimal conditions. The extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory was innovatively applied to reveal the interfacial free energies between zinc flocs and HFPO-TA and other PFASs. It was found that the hydrophobic force can significantly promote the adsorption of HFPO-TA on the surface of zinc flocs and a strong correlation may exist between the calculated total interfacial free energies and the pseudo-first-order kinetic constants of removing PFASs. FT-IR spectra indicated the specific interactions occurring between carboxylate head groups and zinc flocs. These findings advance the mechanistic understanding of PFASs removal by metal flocs and could support the development of electrocoagulation for removing PFASs.