Our Stainfree Future? A Virtual Issue on Poly- and Perfluoroalkyl Substances

What is specific in adsorption of perfluoroalkyl acids on carbon materials?

Various activated carbon products show wide variability in adsorption performance towards perfluoroalkyl acids (PFAAs) and predictive tools are largely missing. In order to gain a better understanding on the adsorption mechanisms of PFAAs, perfluorooctanoic acid (PFOA) was compared with its fluorine-free analogon octanoic acid (OCA) as well as phenanthrene (nonionic) in terms of their response towards changes in carbon surface chemistry. For this approach, a commercial activated carbon felt (ACF) with high content of acidic surface groups was modified by amino-functionalisation as well as thermal defunctionalisation in H₂ (yielding DeCACF). While improvement by amino-functionalisation was moderate, defunctionalisation drastically enhanced adsorption of PFOA and other PFAAs. In comparison, OCA and phenanthrene were much less affected. Electrostatic interactions and charge compensation provided by positively charged surface sites (quantified by their anion exchange capacity) are obviously more crucial for PFAAs than for common organic acids (such as the tested OCA). A possible reason is their exceptionally strong acidity with pK_a < 1. Nevertheless, at the best modified ACF material (DeCACF) the sorption coefficients (K_d) for PFOA and perfluorooctylsulfonic acid (PFOS) at environmentally relevant concentrations reach the range of 10⁷ L/kg which is outstanding. DeCACF provides a surface with overall low polarity (low O-content), low density of acidic sites causing electrostatic repulsion, but nevertheless a sufficient density of charge-balancing sites for organic anions. The results of the present study contribute to an optimized selection of adsorbents for PFAA adsorption from water considering also various salt matrices and the presence of natural organic matter.

Destruction of Per- and Polyfluoroalkyl Substances (PFASs) in Aqueous Film-Forming Foam (AFFF) with UV-Sulfite Photoreductive Treatment

Ultraviolet photochemical reaction of sulfite (SO₃^2-) photosensitizer generates strongly reducing hydrated electrons (e_aq^-; NHE = -2.9 V) that have been shown to effectively degrade individual per- and polyfluoroalkyl substances (PFASs), including perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA). However, treatment of complex PFAS mixtures in aqueous film-forming foam (AFFF) remains largely unknown. Here, UV-sulfite was applied to a diluted AFFF to characterize e_aq^- reactions with 15 PFASs identified by liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF-MS) targeted analysis. Results show that reactivity varies widely among PFASs, but reaction rates observed for individual PFASs in AFFF are similar to rates observed in single-solute experiments. While some structures, including long-chain perfluoroalkyl sulfonic acids (PFSAs) and perfluoroalkyl carboxylic acids (PFCAs) were readily degraded, other structures, most notably short-chain PFSAs and fluorotelomer sulfonic acids (FTSs), were more recalcitrant. This finding is consistent with results showing incomplete fluoride ion release (up to 53% of the F content in AFFF) during reactions. Furthermore, results show that selected PFSAs, PFCAs, and FTSs can form as transient intermediates or unreactive end-products via e_aq^- reactions with precursor structures in AFFF. These results indicate that while UV-sulfite treatment can be effective for treating PFOS and PFOA to meet health advisory levels, remediation of the wider range of PFASs in AFFF will prove more challenging.

Novel remediation of per- and polyfluoroalkyl substances (PFASs) from contaminated groundwater using Cannabis Sativa L. (hemp) protein powder

Perfluoroalkyl and polyfluoroalkyl substances (PFASs) are a group of environmentally persistent, man-made chemicals used in many industrial products and everyday consumer items. Of the plant proteins trialled, those of hemp (Cannabis sativa L.) were found to be far superior for PFAS removal than the next best protein, soy. The use of hemp plant proteins as a possible pump-and-treat solution to PFAS remediation from groundwater has been successfully demonstrated with very good removals (>98%) of the main contaminants of PFOS and PFHxS in approximately 1 h of contact time, with salinity enhancing removal of short chain PFAS. Changes to the secondary structure of hemp proteins was found using FTIR spectroscopy analysis and calculated based on the integrated areas of the amide I component bands. The amount of β-turns increased from ∼9.3% (control) to 44.1% (undiluted groundwater); with a decrease in random coils (25.6-8.6%); α-helix (19.3-8.6%) and β-sheets (38.8-23.1%). These changes indicate that hemp proteins partially unfold during the reaction with PFAS with other FTIR evidence suggesting sorption at hydrophobic sites of the protein as well as with the side chains of the amino acids aspartic and glutamic acid. The absence of these side chains in soy protein, as evidenced from FTIR and amino acid analysis, being part of the reason why soy removed less (approx. half) of the Σ(PFHxS + PFOS) load when compared to hemp. The findings reported here will lead to new, environmentally friendly methods for PFAS remediation.

High Performance Nanofiltration Membrane for Effective Removal of Perfluoroalkyl Substances at High Water Recovery

We demonstrate the fabrication of a loose, negatively charged nanofiltration (NF) membrane with tailored selectivity for the removal of perfluoroalkyl substances with reduced scaling potential. A selective polyamide layer was fabricated on top of a poly(ether sulfone) support via interfacial polymerization of trimesoyl chloride and a mixture of piperazine and bipiperidine. Incorporating high molecular weight bipiperidine during the interfacial polymerization enables the formation of a loose, nanoporous selective layer structure. The fabricated NF membrane possessed a negative surface charge and had a pore diameter of ∼1.2 nm, much larger than a widely used commercial NF membrane (i.e., NF270 with pore diameter of ∼0.8 nm). We evaluated the performance of the fabricated NF membrane for the rejection of different salts (i.e., NaCl, CaCl₂, and Na₂SO₄) and perfluorooctanoic acid (PFOA). The fabricated NF membrane exhibited a high retention of PFOA (∼90%) while allowing high passage of scale-forming cations (i.e., calcium). We further performed gypsum scaling experiments to demonstrate lower scaling potential of the fabricated loose porous NF membrane compared to NF membranes having a dense selective layer under solution conditions simulating high water recovery. Our results demonstrate that properly designed NF membranes are a critical component of a high recovery NF system, which provide an efficient and sustainable solution for remediation of groundwater contaminated with perfluoroalkyl substances.