David L. Sedlak

Perfluoroalkyl sulfonamides (FASAs) have been detected in aqueous film-forming foams (AFFF), municipal wastewater, and consumer products, among others. Although the compounds can be quantified directly by high-performance liquid chromatography/tandem mass spectrometry (HPLC/MS–MS), methods that are frequently used to measure poly- and perfluoroalkyl substances (PFASs) for which standards are not available, like the total oxidizable precursor (TOP) assay, 19F-NMR and nontarget analysis often employ solid-phase extraction followed by elution in a polar organic solvent, such as methanol. Volatilization of FASAs during open blowdown of polar organic solvent extracts from soil, sediment, or aqueous samples leads to underestimation of PFAS concentrations. To assess the importance of this loss mechanism, we measured recovery of three FASAs under conditions relevant to environmental analysis. Recoveries of FASAs and other neutral PFAS, including FASAAs and FTCAs, from methanol during the nitrogen blowdown step decreased as analyte concentrations decreased, with recoveries of less than 1% at 2 μg/L (i.e., a concentration corresponding to about 4 ng/L in water samples prior to solid-phase extraction). The addition of 10 mM NaOH prior to the blowdown step improved recoveries to greater than 80% for all FASAs. NaOH addition prior to blowdown also resulted in an approximately 40% increase in the concentrations of precursors detected in diluted AFFF as measured with the total oxidizable precursor (TOP) assay. The increased concentrations could not be explained by analytes for which analytical standards were available. These findings highlight that volatilization during solvent evaporation can be a significant source of analytical bias for a broader class of neutral PFAS, with FASAs serving as an important example.

The desire to protect aquatic ecosystems and drinking water supplies from the adverse effects of nutrients, trace organic contaminants, and other constituents in municipal wastewater has led to a need for additional treatment, particularly in watersheds with insufficient dilution. Constructed wetlands have emerged as viable alternatives to advanced wastewater treatment processes for effluent polishing due to their low cost and ancillary benefits. Starting in the late 1980s, experiences gained from several decades of operating wetlands in small communities increased confidence that these nature-based treatment systems could be effective and reliable. As a result, investments were made in larger constructed wetlands, with surface areas greater than a million square meters (1 Mm²) and flows often exceeding 1 m³ s^-1, on effluent-dominated rivers and as part of potable water reuse projects. More recently, new wetland designs have improved the constructed wetland system performance by taking advantage of sunlight-mediated processes in the water column and microbial processes on subsurface porous media. Research that provides additional insight into contaminant removal mechanisms, demonstrates long-term viability, and further improves treatment performance could expand the application of constructed wetlands to other difficult-to-solve water quality challenges, including the treatment of municipal water reuse concentrate and the mitigation of nonpoint source pollution.

Aqueous film-forming foams (AFFFs) are a major source of per- and polyfluoroalkyl substances (PFAS) contamination in groundwater and soil. Although PFAS sorption is known to depend on the properties of solid matrices, the roles of organic matter and iron (hydr)oxides remain poorly understood, especially for polyfluorinated compounds. To investigate the interplay between particulate organic matter and iron (hydr)oxides in PFAS-contaminated subsurface environments, we studied the partitioning of neutral, cationic, and anionic PFAS in AFFF to well-characterized solids using the mixed-mode solid-phase extraction (SPE) technique combined with the total oxidizable precursor (TOP) assay. Our results indicated that organic matter consistently enhanced PFAS sorption. In contrast, the effect of goethite, a representative iron-containing mineral, on PFAS sorption varied with PFAS charge. Using three different AFFFs, we found that anionic PFAS exhibited strong sorption regardless of organic matter content, while cationic and zwitterionic PFAS sorbed poorly at circumneutral pH. These trends were more pronounced for iron-coated sand, which showed higher affinity for anionic PFAS. Treatment with ascorbate, a mild reductant, released PFAS associated with iron (hydr)oxides. These findings highlight the importance of iron (hydr)oxides in the retention of anionic PFAS and could be leveraged to manage PFAS mobility through subsurface iron amendments or in situ iron (hydr)oxide formation/dissolution.

Most dissolved organic contaminants readily react with sulfate radicals (SO4•–) formed when persulfate is activated by thermolysis or base addition during in situ chemical oxidation (ISCO). However, under conditions encountered in the subsurface, hydrophobic contaminants associate with particles. To determine the potential of solids to protect contaminants from oxidation, we measured the stoichiometric efficiency (i.e., moles of contaminant transformed per mole of SO4•–) for a homologous series of chlorinated benzenes using solid-to-water ratios approaching subsurface conditions. Sorption to inorganic surfaces (i.e., sand and clays) reduced the stoichiometric efficiency by 3 orders of magnitude relative to contaminants in solution. At low initial oxidant concentrations (i.e., 10 mM), adsorbed contaminants were oxidized after desorbing to reestablish equilibrium. At higher oxidant concentrations (i.e., 500 mM), contaminant loss was attributable to SO4•– that reacted at the particle surface. Absorption by particulate organic matter (i.e., Pahokee peat) offered greater protection. For the most hydrophobic compounds (i.e., tetra-, penta-, and hexachlorobenzene), 1.5% organic matter by mass reduced the stoichiometric efficiency by an additional order of magnitude. The effect of sorption on the efficacy of persulfate ISCO can be predicted using contaminant hydrophobicity (i.e., the octanol–water partition coefficient, Kow), persulfate dosage, and particulate organic matter content.

Recognition of the risks associated with exposure to low levels of hexavalent chromium (Cr­[VI]) coupled with the high costs and complexity of conventional treatment methods necessitates the development of inexpensive, reliable technologies for removing Cr­[VI] from drinking water sources. In response to this need, we created a filter medium consisting of molybdenum disulfide (MoS2) nanoparticles attached to cotton wool. Under conditions relevant to drinking water, Cr­[VI] was reduced to less toxic Cr­[III] and sequestered on the filter medium. Under air-saturated solutions, competition by dissolved oxygen for reductive sites lowered the capacity of the medium for Cr­[VI] removal and resulted in the release of appreciable amounts of molybdenum. Removing oxygen from water prior to treatment increased the capacity of the MoS2-cotton by over an order of magnitude and decreased the mass of molybdenum released by over 80%. After Cr­[VI] breakthrough, the activity of the MoS2-cotton could be restored by storing the column for 24 h a dilute hydrochloric acid solution. The MoS2-cotton filter media is attractive relative to existing Cr­[VI]-removal technologies due to its low cost, lack of a need for pH adjustment and its ability to convert Cr­[VI] into Cr­[III]. In the absence of oxygen, the medium exhibits a treatment capacity that is similar or better than commercially available strong base anion exchange resins.

Perfluoroalkyl acid (PFAA) precursors are a diverse subclass of per- and polyfluoroalkyl substances (PFASs) that can be transformed into PFAAs of public health concern. Unlike strongly acidic PFAAs, precursors can be anionic, cationic, neutral, or zwitterionic. Precursor charge affects the environmental fate, but existing quantification techniques struggle to ascertain the abundance of compounds within each charge group. To fill this gap, we developed and validated a solid-phase extraction procedure that separates precursors by charge and quantifies the sum of the precursors in each fraction with the total oxidizable precursor (TOP) assay. Method performance was demonstrated by spiking known concentrations of ten precursors into aqueous film-forming foam (AFFF)-impacted groundwater, municipal wastewater, and soil samples. Precursor fractionation and recovery were greater in groundwater and soil samples than in wastewater. Use of the method provided results that were consistent with expectations based on precursor transport properties. In surficial soils near an AFFF source zone, anionic precursors with five or fewer perfluorinated carbons accounted for about 95% of PFASs, but less than half of PFASs in the underlying groundwater. In municipal wastewater influent, the sum of precursors exceeded the sum of PFAAs and was approximately equally distributed among all charge fractions.

To oxidize trace concentrations of organic contaminants under conditions relevant to surface- and groundwater, air-diffusion cathodes were coupled to stainless-steel cathodes that convert atmospheric O₂ into hydrogen peroxide (H₂O₂), which then was activated to produce hydroxyl radicals (^·OH). By separating H₂O₂ generation from its activation and employing a flow-through electrode consisting of stainless-steel fibers, the two processes could be operated efficiently in a manner that overcame mass-transfer limitations for O₂, H₂O₂, and trace organic contaminants. The flexibility resulting from separate control of the two processes made it possible to avoid both the accumulation of excess H₂O₂ and the energy losses that take place after H₂O₂ has been depleted. The decrease in treatment efficacy occurring in the presence of natural organic matter was substantially lower than that typically observed in homogeneous advanced oxidation processes. Experiments conducted with ionized and neutral compounds indicated that electrostatic repulsion prevented negatively charged ^·OH scavengers from interfering with the oxidation of neutral contaminants. Energy consumption by the dual-cathode system was lower than values reported for other technologies intended for small-scale drinking water treatment systems. The coordinated operation of these two cathodes has the potential to provide a practical, inexpensive way for point-of-use drinking water treatment.

Per- and polyfluoroalkyl substances (PFASs) from aqueous film forming foams (AFFFs) can hinder bioremediation of co-contaminants such as trichloroethene (TCE) and benzene, toluene, ethylbenzene, and xylene (BTEX). Anaerobic dechlorination can require bioaugmentation of Dehalococcoides, and for BTEX, oxygen is often sparged to stimulate in situ aerobic biodegradation. We tested PFAS inhibition to TCE and BTEX bioremediation by exposing an anaerobic TCE-dechlorinating coculture, an aerobic BTEX-degrading enrichment culture, and an anaerobic toluene-degrading enrichment culture to n-dimethyl perfluorohexane sulfonamido amine (AmPr-FHxSA), perfluorohexane sulfonamide (FHxSA), perfluorohexanesulfonic acid (PFHxS), or nonfluorinated surfactant sodium dodecyl sulfate (SDS). The anaerobic TCE-dechlorinating coculture was resistant to individual PFAS exposures but was inhibited by >1000× diluted AFFF. FHxSA and AmPr-FHxSA inhibited the aerobic BTEX-degrading enrichment. The anaerobic toluene-degrading enrichment was not inhibited by AFFF or individual PFASs. Increases in amino acids in the anaerobic TCE-dechlorinating coculture compared to the control indicated stress response, whereas the BTEX culture exhibited lower concentrations of all amino acids upon exposure to most surfactants (both fluorinated and nonfluorinated) compared to the control. These data suggest the main mechanisms of microbial toxicity are related to interactions with cell membrane synthesis as well as protein stress signaling.