Rapid Removal of Perfluoroalkanesulfonates from Water by ß-Cyclodextrin Covalent Organic Frameworks.

Adsorption is an effective method for the removal of perfluoroalkanesulfonates (PFSAs) from water, and the limitation of the adsorption rate of existing adsorbents motivates efforts to develop novel adsorbents. Here, we developed four ß-cyclodextrin covalent organic frameworks (ß-CD-COFs) with a rapid removal rate and high adsorption capacity for four PFSAs in water including perfluorooctanesulfonate (PFOS), perfluorobutanesulfonate (PFBS), perfluorohexanesulfonate (PFHxS), and chlorinated polyfluorinated ether sulfonate (F53B). All ß-CD-COFs exhibited extremely fast adsorption (adsorption equilibrium <2 min) for PFSAs with high adsorption capacities (0.33-1.51 mmol/g), which were significantly better than those of traditional resins and activated carbons, probably due to the ordered pores of ß-CD-COFs and the electron-deficient cavity ß-CD. Density functional theory (DFT) calculations also showed that PFSAs could be captured in the ß-CD cavity through strong interactions with a high binding energy. The novel ß-CD-COFs were highly selective to PFSAs in simulated wastewater impacted by aqueous film-forming foams, and they could also rapidly remove them from an actual chrome plating wastewater within 2 min. Additionally, the ß-CD-COFs could be regenerated by methanol with relatively good reusability in four cycles, further highlighting their application potential as PFAS adsorbents in water or wastewater.

Contribution of Nanobubbles for PFAS Adsorption on Graphene and OH- and NH2-Functionalized Graphene: Comparing Simulations with Experimental Results.

Per- and polyfluoroalkyl substances (PFAS) are ubiquitous in aquatic environments around the world. In recent years, the enrichment of PFAS on the surface of nanobubbles on adsorbents has been proposed, but no direct evidence has been provided to support this new adsorption mechanism. In this study, we used density functional theory (DFT) and molecular dynamics (MD) to simulatively investigate the contribution of nanobubbles for PFAS adsorption on the pristine and functionalized graphene (GR). The adsorption energy of PFAS on GR-NH2 was higher than that of GR-OH, while GR showed the lowest adsorption energy. When the effect of water molecules was considered, the oleophobic property of the C-F chain made it difficult for hydrophobic interaction to be involved in the adsorption of PFAS on nonpolar GR. With the existence of nanobubbles, both GR and GR-NH2 can effectively remove PFAS, but their adsorption mechanisms were quite different. For hydrophobic GR, the nanobubbles initially attached to the surface of materials played a major role, while for hydrophilic GR-NH2, the nanobubbles dispersed in the solution were more important. Moreover, the nanobubbles had a more significant contribution to long-chain PFAS. Our degassing and aeration experiments could support the simulation results. The removal of PFOS decreased by 27.7% at maximum after degassing and increased by 21.0%-29.2% after aeration. The study could provide a theoretical basis for the environmental process and contamination control of PFAS at the solid-liquid interfaces.

Role of the air-water interface in removing perfluoroalkyl acids from drinking water by activated carbon treatment.

Contamination of drinking water by per- and polyfluoroalkyl substances (PFASs) is a worldwide problem. In this study, we for the first time revealed the role of the air-water interface in enhancing the removal of long-chain perfluoroalkyl carboxylic (PFCAs; CnF2n+1COOH, n ≥ 7) and perfluoroalkane sulfonic (PFSAs; CnF2n+1SO3H, n ≥ 6) acids, collectively termed as perfluoroalkyl acids (PFAAs), through combined aeration and adsorption on two kinds of activated carbon (AC). Aeration was shown to enhance the removal of long-chain PFAAs through adsorption at the air-water interface and subsequent adsorption of PFAA-enriched air bubbles to the AC. The removal of selected long-chain PFAAs was increased by 50-115 % with the assistance of aeration, depending on the perfluoroalkyl chain length. Aeration is more effective in enhancing long-chain PFAA removal as air-water interface adsorption increases with PFAA chain length due to higher surface activity. After removing adsorbed air bubbles by centrifugation, up to 80 % of the long-chain PFAAs were able to desorb from the sorbent, confirming the contribution of the air-water interface to the adsorption of PFAAs on AC. Aeration during AC treatment of water could enhance the removal of long-chain PFAAs, and improve the performance of AC during water treatment.