Electrochemical Oxidation for Treatment of PFAS in Contaminated Water and Fractionated Foam-A Pilot-Scale Study

Suspect and non-target screening of per- and polyfluoroalkyl substances (PFAS) and other halogenated substances in electrochemically oxidized landfill leachate and groundwater

Release of persistent and potentially toxic per- and polyfluoroalkyl substances (PFAS) and other halogenated compounds into the aqueous environment is an emerging issue and advanced treatment methods are needed for their removal from contaminated water. Destructive treatment methods for PFAS exist, but there is a risk of incomplete degradation, resulting in creation of transformation products during treatment. This study assessed the potential of electrochemical oxidation (EO) for destruction of PFAS and other halogenated compounds, and their transformation products. Suspect and non-target screening were used to explore the chemical space of these samples and identify compounds present before and after the treatment, including transformation products. In total, 21 PFAS classes and 53 individual PFAS were identified using suspect and non-target screening, with confidence level (CL) 3d or higher. Two new classes of PFAS (FASHN and MeOH-FASA) were discovered for the first time. Suspect screening of PFAS revealed that hydro-substituted and ether PFAS could be formed during EO. A total of 12 chlorinated and two brominated compounds were also detected and confirmed with CL 1-3, with six compounds determined to be transformation products. Formation of ammonium oxidation byproducts was hypothesized as being responsible for most identified transformation products formed during EO.

PFAS in landfill leachate: Practical considerations for treatment and characterization

Per- and polyfluoroalkyl substances (PFAS) are widely used in consumer products and are particularly high in landfill leachate. The practice of sending leachate to wastewater treatment plants (WWTPs) is an issue for utilities that have biosolids land application limits based on PFAS concentrations. Moreover, landfills may face their own effluent limit guidelines for PFAS. The purpose of this review is to understand the most appropriate treatment technology combinations for mitigating PFAS in landfill leachate. The first objective is to understand the unique chemical characteristics of landfill leachate. The second objective is to establish the role and importance of known and emerging analytical techniques for PFAS characterization in leachate, including quantification of precursor compounds. Next, an overview of technologies that concentrate PFAS and technologies that destroy PFAS is provided, including fundamental background content and key operating parameters. Finally, practical considerations for PFAS treatment technologies are reviewed, and recommendations for PFAS treatment trains are described. Both pros and cons of treatment trains are noted. In summary, the complex matrix of leachate requires a separation treatment step first, such as foam fractionation, for example, to concentrate the PFAS into a lower-volume stream. Then, a degradation treatment step can be applied to the concentrated PFAS stream.

Analysis of wastewater treatment plant data identifies the drivers of PFAS enrichment in foams

The concept of incorporating foam fractionation in aerated bioreactors at wastewater treatment plants (WWTPs) for the removal of per- and polyfluoroalkyl substances (PFAS) has recently been proposed. The extent of PFAS enrichment in aerated bioreactors' foams, as indicated by enrichment factors (EFs), has been observed to vary widely. Laboratory evidence has shown that factors affecting PFAS enrichment in foams include conductivity, surfactant concentrations and initial PFAS concentrations. However, real wastewaters are complex heterogenous matrices with physical, chemical and biological characteristics potentially contributing to the phenomenon of PFAS partitioning into foams. In this study, we characterised mixed liquor suspensions, including conductivity, filament content, aqueous PFAS concentrations, surface tension and total suspended solids concentrations (TSS) as well as foams, including bubble size and half-life. We used statistical tools - linear mixed-effects model - to establish relationships between PFAS enrichment in aerated bioreactor foams and the examined characteristics. We found that some of the examined characteristics, specifically filament content, surface tension and TSS concentrations measured in mixed liquor suspension and foam half-life, are negatively and significantly associated with the enrichment of longer chain PFAS (with perfluorinated carbon number ≥ 6). Of these, filament content is the important determinant of PFAS enrichment, potentially leading to an increase in, for example, perfluorooctanoic acid (PFOA) EF from 3 to 100 between typical filamentous and non-filamentous suspended biomass. However, enrichment of shorter chain PFAS (with perfluorinated carbon number ≤ 5) is negligible and is not affected by the characteristics that were measured. The findings of our study may serve as valuable information for the implementation of foam fractionation at WWTPs by elucidating the drivers that contribute to the enrichment of longer chain PFAS, under conditions typically found at WWTPs.

Efficient electro-oxidation-based degradation of per- and polyfluoroalkyl (PFAS) persistent pollutants by using plasma torch synthesized pure-Magnéli phase-Ti4O7 anodes

Pure Magnéli-phase Ti4O7 were prepared by means of a Plasma Torch (PT) coating method and integrated into an advanced electro-catalytic oxidation (AEO) process in order to degrade perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) persistent pollutants present in waters. The X-ray diffraction analysis confirmed the polycrystalline nature of the pure Magnéli phase PT-Ti4O7 coatings (∼100 μm thick)). The Raman spectra of the PT-Ti4O7 coatings also exhibited the two characteristic peaks (at 138 and 183 cm-1) of the PT-Ti4O7 Magnéli phase. Scanning electron microscopy revealed the nanostructured hierarchical morphology of the PT-Ti4O7 thus conferring them high surface area. The PT-Ti4O7 anodes are shown to achieve higher degradation efficiencies towards PFOA and PFOS in comparison with the conventional boron-doped diamond anodes. By investigating several AEO parameters (including current density, treatment time, nature of the anode material), we were able to optimise the AEO process. Thus, for both PFOA and PFOS (at an initial concentration of 500 ppb in synthetic wastewaters), degradation efficiencies as high as 96.6% and 99.7% were achieved, respectively, with a current density of 20 mA/cm2, a treatment time of 120 min and PT-Ti4O7 mesh-type anodes. PFOA and PFOS can be degraded by both direct anodic electrochemical oxidation (•OH radicals) and indirect electrochemical oxidation via mediators, such as persulphate acid (H2S2O8) generated by sulphate anodic oxidation. The degradation of both compounds followed pseudo-first-order kinetics. The reaction rate constant (k) for PFOS removal was 4.63 × 10-2 min-1, whereas 2.76 × 10-2 min-1 was recorded for PFOA removal. Subsequently, we have used the above optimal AEO operating conditions to treat real wastewater effluents (containing 17 types of PFAS molecules with a total content of 8500 ppb) and achieved a degradation rate of 39.1%-87.4% for eight of the 17 PFAS compounds. The degradation rate was found to be dependent on the chemical structure and chain length of each PFOA/PFOS component.

Removal of per- and polyfluoroalkyl substances (PFAS) from municipal wastewater by foam fractionation

Municipal wastewater has a relatively low content of per- and polyfluoroalkyl substances (PFAS), compared with other point sources such as landfill leachate and industrial effluents. Nevertheless, it is considered as one of the major point pollution sources. Foam fractionation was previously shown to effectively remove PFAS from different water matrices and to reach a high PFAS enrichment. In this study, the removal of PFAS from municipal wastewater of different origins was investigated. Despite the low foaming potential, it was possible to reach an average removal of the sum of perfluorooctanesulfonic acid (PFOS), perfluorooctanoic acid (PFOA), perfluorononanoic acid (PFNA), and perfluorohexanesulfonic acid (PFHxS) of 93 %, the removal of the sum of 21 PFAS of 56 %, and the removal of the sum of PFAS expressed as PFOA equivalents (ΣPFOAeq) of 91 %, without any surfactant addition utilizing a novel approach of foam collection. The PFAS content was reduced to below the limit values for drinking water in Sweden and the anticipated future limit values for surface water in the European Union. The addition of four commercial surfactants and an extract of extracellular polymeric substances (EPS) from waste activated sludge each enhanced the foam formation. Moreover, a cationic surfactant increased the removal of short-chain PFAS. Additionally, foam fractionation of activated sludge was performed. A considerably lower ΣPFOAeq removal of only 20 % was demonstrated, which was explained by a high proportion of PFAS sorbed to sludge. Finally, the study discusses the practical implications of the application of foam fractionation at municipal wastewater treatment plants.

Integrating redox-electrodialysis and electrosorption for the removal of ultra-short- to long-chain PFAS

A major challenge in per- and polyfluoroalkyl substances (PFAS) remediation has been their structural and chemical diversity, ranging from ultra-short to long-chain compounds, which amplifies the operational complexity of water treatment and purification. Here, we present an electrochemical strategy to remove PFAS from ultra-short to long-chain PFAS within a single process. A redox-polymer electrodialysis (redox-polymer ED) system leverages a water-soluble redox polymer with inexpensive nanofiltration membranes, facilitating the treatment of varied chain lengths of PFAS without membrane fouling. Our approach combines both ion migration by electrodialysis (for PFAS with chain lengths ≤C4) and electrosorption strategies (for PFAS with chain lengths ≥C6) to eliminate approximately 90% of ultra-short-, short-chain, and long-chain PFAS. At the same time, we achieve continuous desalination of the source water down to potable water level. The redox-polymer ED exhibits remarkable PFAS removal in real source water scenarios, including from matrices with 10,000 times higher salt concentrations, as well as secondary effluents from wastewaters. Additionally, the removed PFAS is mineralized with a defluorination performance between 76-100% by electrochemical oxidation, highlighting the viability of integrating the separation step with a reactive degradation process.

Enhanced perfluorooctanoic acid (PFOA) degradation by electrochemical activation of peroxydisulfate (PDS) during electrooxidation for water treatment

Improved treatment of per- and polyfluoroalkyl substances (PFAS) in water is critically important in light of the proposed United States Environmental Protection Agency (USEPA) drinking water regulations at ng L-1 levels. The addition of peroxymonosulfate (PMS) during electrooxidation (EO) can remove and destroy PFAS, but ng L-1 levels have not been tested, and PMS itself can be toxic. The objective of this research was to test peroxydisulfate (PDS, an alternative to PMS) activation by boron-doped diamond (BDD) electrodes for perfluorooctanoic acid (PFOA) degradation. The influence of PDS concentration, temperature, and environmental water matrix effects, and PFOA concentration on PDS-EO performance were systematically examined. Batch reactor experiments revealed that 99 % of PFOA was degraded and 69 % defluorination was achieved, confirming PFOA mineralization. Scavenging experiments implied that sulfate radicals (SO4-) and hydroxyl radicals (HO) played a more important role for PFOA degradation than 1O2 or electrons (e-). Further identification of PFOA degradation and transformation products by liquid chromatography-mass spectrometry (LC-MS) analysis established plausible PFOA degradation pathways. The analysis corroborates that direct electron transfers at the electrode initiate PFOA oxidation and SO4- improves overall treatment by cleaving the CC bond between the C7F15 and COOH moieties in PFOA, leading to possible products such as C7F15 and F-. The perfluoroalkyl radicals can be oxidized by SO4- and HO, resulting in the formation of shorter chain perfluorocarboxylic acids (e.g., perfluorobutanoic acid [PFBA]), with eventual mineralization to CO2 and F-. At an environmentally relevant low initial concentration of 100 ng L-1 PFOA, 99 % degradation was achieved. The degradation of PFOA was slightly affected by the water matrix as less removal was observed in an environmental river water sample (91 %) compared to tests conducted in Milli-Q water (99 %). Overall, EO with PDS provided a destructive approach for the elimination of PFOA.

Overlooked competition and promotion effects in electrochemical oxidation of humic acid and ammonia in landfill leachate

Electrochemical oxidation (EO) can effectively reduce the degree of humification and toxicity of landfill leachate by generating highly active oxidative species in situ. However, the selective and competitive oxidation of humic acid (HA) and ammonia (NH4+) and the role of different oxidative species during the EO process in complex aqueous conditions remain unclear. In this study, a nanostructured tin-antimony electrode (Ti/Sb-SnO2 NFs) was prepared and compared with three types of commercial electrodes (Ti/Ir-RuO2, Ti4O7, Ti/Sb-SnO2) in terms of electrochemical properties and electrocatalytic oxidation of HA and NH4+. The de-humification capacity, interactive effects of HA and NH4+ on each other's oxidation by different oxidative species, as well as the related oxidation byproducts were investigated. The differences in pollutant electrooxidation among the different electrodes were found to be insignificant. The presence of HA was found to be detrimental to NH4+ degradation while reducing the N2 conversion rate. Interestingly, NH4+ initially inhibited the degradation rates of HA while promoted the degradation and reduced the accumulation of organic chlorine during the later EO process. A proposed mechanism accounts for both competitive and promotional effects for simultaneous HA and NH4+ oxidation during the EO process.

A data-driven analysis to discover research hotspots and trends of technologies for PFAS removal

The frequent detection of persistent per- and polyfluoroalkyl substances (PFAS) in organisms and environment coupled with surging evidence for potential detrimental impacts, have attracted widespread attention throughout the world. In order to reveal research hotspots and trends of technologies for PFAS removal, herein, we performed a data-driven analysis of 3975 papers and 436 patents from Web of Science Core Collection and Derwent Innovation Index databases up to 2023. The results showed that China and the USA led the way in the research of PFAS removal with outstanding contributions to publications. The progression generally transitioned from accidental discovery of decomposition, to experimentation with removal effects and mechanisms of existing methods, and finally to enhanced defluorination and mechanism-driven design approaches. The keywords co-occurrence network and technology classification together revealed the main knowledge framework, which was constructed and correlated through contaminants, substrates, materials, processes and properties. Moreover, adsorption was demonstrated to be the dominant removal process among the current studies. Subsequently, we concluded the principles, advances and drawbacks of enrichment and separation, biological methods, advanced oxidation and reduction processes. Further exploration indicated the hotspots such as alternatives and precursors for PFAS ("genx": 1.258, "f-53b": 0.337), degradable mineralization technologies ("photocatalytic degrad": 0.529, "hydrated electron": 0.374), environment-friendly remediation technologies ("phytoremedi": 0.939, "constructed wetland": 0.462) and combination with novel materials ("metal-organic framework": 1.115, "layered double hydroxid": 0.559) as well as computer science ("molecular dynamics simul": 0.559, "machine learn"). Furthermore, the future direction of technological innovation might lie in high-performance processes that minimize secondary pollution, the development of recyclable and renewable treatment agents, and collaborative control strategies for multiple pollutants. Overall, this study offers comprehensive and objective review for researchers and industry professionals in this field, enabling rapid access to knowledge guidance and insights into research frontiers.

Hydrothermal Destruction and Defluorination of Trifluoroacetic Acid (TFA)

Per- and polyfluoroalkyl substances (PFAS) have received increased attention due to their environmental prevalence and threat to public health. Trifluoroacetic acid (TFA) is an ultrashort-chain PFAS and the simplest perfluorocarboxylic acid (PFCA). While the US EPA does not currently regulate TFA, its chemical similarity to other PFCAs and its simple molecular structure make it a suitable model compound for studying the transformation of PFAS. We show that hydrothermal processing in compressed liquid water transforms TFA at relatively mild conditions (T = 150-250 °C, P < 30 MPa), initially yielding gaseous products, such as CHF3 and CO2, that naturally aspirate from the solution. Alkali amendment (e.g., NaOH) promotes the mineralization of CHF3, yielding dissolved fluoride, formate, and carbonate species as final products. Fluorine and carbon balances are closed using Raman spectroscopy and fluoride ion selective electrode measurements for experiments performed at alkaline conditions, where gas yields are negligible. Qualitative FTIR gas analysis allows for establishing the transformation pathways; however, the F-balance could not be quantitatively closed for experiments without NaOH amendment. The kinetics of TFA transformation under hydrothermal conditions are measured, showing little to no dependency on NaOH concentration, indicating that the thermal decarboxylation is a rate-limiting step. A proposed TFA transformation mechanism motivates additional work to generalize the hydrothermal reaction pathways to other PFCAs.

The role of suspended biomass in PFAS enrichment in wastewater treatment foams

Foaming in aerated bioreactors at wastewater treatment plants (WWTPs) has been identified as an operational issue for decades. However, the affinity of per- and polyfluoroalkyl substances (PFAS) for air-liquid interfaces suggests that foam harvesting has the potential to become a sustainable method for PFAS removal from sewage. Aerated bioreactors' foams are considered three-phase systems, comprising air, aqueous and solid components, the latter consisting of activated sludge biomass. To achieve a comprehensive understanding of the capability of aerated bioreactors' foams to enrich PFAS, we analysed PFAS concentrations from WWTPs in both the solid and aqueous phases of the collapsed foams (foamate) and underlying bulk mixed liquors. Our findings show that PFAS enrichment occurs not only in the aqueous phase but also in the solid phase of the foamate. This suggests that previous field studies that only analysed the aqueous phase may have underestimated the capability of the aerated bioreactors' foams to enrich PFAS. Fractions of PFOA and PFOS sorbed to the solid phase of the foamate can be as high as 60 % and 95 %, respectively. Our findings highlight the importance of implementing effective foamate management strategies that consider both the aqueous and solid phases.

Using Network Analysis and Predictive Functional Analysis to Explore the Fluorotelomer Biotransformation Potential of Soil Microbial Communities

Microbial transformation of per- and polyfluoroalkyl substances (PFAS), including fluorotelomer-derived PFAS, by native microbial communities in the environment has been widely documented. However, few studies have identified the key microorganisms and their roles during the PFAS biotransformation processes. This study was undertaken to gain more insight into the structure and function of soil microbial communities that are relevant to PFAS biotransformation. We collected 16S rRNA gene sequencing data from 8:2 fluorotelomer alcohol and 6:2 fluorotelomer sulfonate biotransformation studies conducted in soil microcosms under various redox conditions. Through co-occurrence network analysis, several genera, including Variovorax, Rhodococcus, and Cupriavidus, were found to likely play important roles in the biotransformation of fluorotelomers. Additionally, a metagenomic prediction approach (PICRUSt2) identified functional genes, including 6-oxocyclohex-1-ene-carbonyl-CoA hydrolase, cyclohexa-1,5-dienecarbonyl-CoA hydratase, and a fluoride-proton antiporter gene, that may be involved in defluorination. This study pioneers the application of these bioinformatics tools in the analysis of PFAS biotransformation-related sequencing data. Our findings serve as a foundational reference for investigating enzymatic mechanisms of microbial defluorination that may facilitate the development of efficient microbial consortia and/or pure microbial strains for PFAS biotransformation.

A review of foam fractionation for the removal of per- and polyfluoroalkyl substances (PFAS) from aqueous matrices

The detection of per- and polyfluoroalkyl substances (PFAS) in aqueous matrices is an emerging environmental concern due to their persistent, bioaccumulative and toxic properties. Foam fractionation has emerged as a viable method for removing and concentrating PFAS from aqueous matrices. The method exploits the surface-active nature of the PFAS to adsorb at the air-liquid interfaces of rising air bubbles, resulting in foam formation at the top of a foam fractionator. The removal of PFAS is then achieved through foam harvesting. Foam fractionation has gained increasing attention owing to its inherent advantages, including simplicity and low operational costs. The coupling of foam fractionation with destructive technologies could potentially serve as a comprehensive treatment train for future PFAS management in aqueous matrices. The PFAS-enriched foam, which has a smaller volume, can be directed to subsequent destructive treatment technologies. In this review, we delve into previous experiences with foam fractionation for PFAS removal from various aqueous matrices and critically analyse their key findings. Then, the recent industry advancements and commercial projects that utilise this technology are identified. Finally, future research needs are suggested based on the current challenges.

Hydrothermal Alkaline Treatment (HALT) of Foam Fractionation Concentrate Derived from PFAS-Contaminated Groundwater

While foam fractionation (FF) process has emerged as a promising technology for removal of per- and polyfluoroalkyl substances (PFASs) from contaminated groundwater, management of the resulting foam concentrates with elevated concentrations of PFASs (e.g., >1 g/L) remains a challenge. Here, we applied hydrothermal alkaline treatment (HALT) to two foam concentrates derived from FF field demonstration projects that treated aqueous film-forming foam (AFFF)-impacted groundwater. Results showed >90% degradation and defluorination within 90 min of treatment (350 °C, 1 M NaOH) of all 62 PFASs (including cations, anions, and zwitterions) identified in foam concentrates. Observed rate constants for degradation of individual perfluoroalkyl sulfonates (PFSAs, CnF2n+1-SO3-), the most recalcitrant class of PFASs, in both foam concentrates were similar to values measured previously in other aqueous matrices, indicating that elevated initial PFAS concentrations (e.g., PFHxSinit = 0.55 g/L), dissolved organic carbon (DOC; up to 4.5 g/L), and salt levels (e.g., up to 325 mg/L chloride) do not significantly affect PFAS reaction kinetics. DOC was partially mineralized by treatment, but a fraction (∼15%) was recalcitrant. Spectroscopic characterization revealed molecular features of the HALT-recalcitrant DOC fraction, and nontarget high-resolution mass spectrometry tentatively identified 129 nonfluorinated HALT-recalcitrant molecules. Analysis of process energy requirements shows that treating PFAS-contaminated foam concentrates with HALT would add minimally (<5%) to the overall energy requirements of an integrated FF-HALT treatment train.

Foam fractionation and electrochemical oxidation for the treatment of per- and polyfluoroalkyl substances (PFAS) in environmental water samples

Treatment of waters contaminated by per- and polyfluoroalkyl substances (PFAS) in large volumes remains a challenge to date. Treatment trains comprising separation and destruction technologies are promising to manage PFAS contamination. Foam fractionation (FF) and electrochemical oxidation (EO) are two cost-effective technologies for PFAS separation and destruction, respectively. This work systematically explored the performance of a treatment train of FF followed by EO (FF-EO) for treating PFAS in environmental water samples. For each treatment step, the dependence of the treatment performance on operational factors and other variables were analyzed statistically. The statistical analysis revealed PFAS enrichment and removal depend significantly on PFAS carbon chain length, solution conductivity, and PFAS concentration. Whether FF-EO treatment costs less energy than direct EO without FF mainly relies upon PFAS carbon chain length and TOC content in the sample. Both correlations were found to be linear. For all environmental water samples in this study, FF-EO is more energy-efficient than EO alone.