Nanobubble-Driven Interfacial Interactions of Carbon-Based Adsorbents with Legacy PFAS: Impact of Concentration, pH, and Coexisting Ions

Nanobubbles (NBs) have been utilized to enhance the removal of perfluoroalkyl and polyfluoroalkyl substances (PFAS) from water. However, the effectiveness of NBs under various operational conditions, including varying PFAS concentrations, pH levels, and the presence of coexisting ions, has remained insufficiently explored. A deeper understanding of these interfacial interactions is crucial to optimizing and maximizing the efficiency of NBs in PFAS removal. This study aims to bridge these knowledge gaps by systematically exploring the role of NBs in augmenting the adsorption of legacy PFAS on various carbon-based adsorbents, including granular activated carbon (GAC), carbon nanotubes (CNTs), and graphene. Our experimental results demonstrated a significant improvement in PFAS adsorption on carbon-based adsorbents in the presence of NBs, with the enhancement effect particularly pronounced at a high initial PFAS concentration (50 mg L^-1). Acidic conditions notably facilitated PFAS removal in the NB-assisted carbon adsorption system, aligning well with the predictions from the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Furthermore, ionic strength was found to play a critical role, with lower levels stabilizing NBs and promoting interactions with adsorbent surfaces, while higher levels reduced the effectiveness of NBs. Additionally, multivalent cations (particularly Fe³⁺) showed a substantially greater enhancement in PFAS removal efficiency compared to Na⁺ and Ca²⁺. This study deepens the understanding of NB-assisted PFAS removal using carbon-based adsorbents and provides practical insights into optimizing treatment processes.