Mechanistic insights into the adsorptive removal of PFOA and PFOS using microbubbles and nanobubbles: A review

Perfluoro- or polyfluoroalkyl substances (PFASs) have attracted research attention due to their high concentrations and severe ecotoxicity in the environment. The distinctive attributes of adsorption support its wide-scale application in the remediation of contaminants including PFASs. Previous studies mainly focused on the exploration and development of adsorbent materials, evaluating the removal of PFASs with different chain lengths, or assessing the impact of environmental factors on PFASs removal. However, the role of microbubbles (MBs) or nanobubbles (NBs), a commonly present but easily overlooked substance in natural media, in the PFASs adsorption has not been systematically summarized. Therefore, in this review, we introduce the importance of MBs/NBs in PFASs (particularly PFOA and PFOS) adsorption, by analyzing their characteristics related to adsorption. MBs/NBs can effectively remove PFASs by enriching at the interface and prolonging the retention time. The theoretical calculations of molecular dynamics and Gibbs free energy on the adsorption behavior of PFOS/PFOS with the participation of MBs/NBs were systematically analyzed. The hydrophobicity of PFOS, which is greater than that of PFOA, leads to stronger adsorption of PFOS by MBs. Environmental factors mainly affect the stability and surface properties of MBs/NBs, as well as the hydrophobicity of PFOA/PFOS, to alter the adsorption behavior of PFOA/PFOS mediated by MBs/NBs. Furthermore, the adsorption mechanisms of PFOA/PFOS on MBs/NBs were summarized, with electrostatic and hydrophobic interactions dominating this behavior. This review offers an enhanced and comprehensive insight into the adsorption of PFASs in water. Furthermore, it facilitates a more profound evaluation of the significant role played by MBs/NBs in the adsorptive elimination of PFASs from aquatic system.