Transformation of polyfluorinated compounds in natural waters by advanced oxidation processes

Current state of knowledge of environmental occurrence, toxic effects, and advanced treatment of PFOS and PFOA

Per- and polyfluoroalkyl substances (PFAS) are anthropogenic synthetic compounds, with high chemical and thermal stability and a persistent, stable and bioaccumulative nature that renders them a potential hazard for the environment, its organisms, and humans alike. Perfluorooctane sulfonic acid (PFOS) and Perfluorooctanoic acid (PFOA) are the most well-known substances of this category and even though they are phased out from production they are still highly detectable in several environmental matrices. As a result, they have been spread globally in water sources, soil and biota exerting toxic and detrimental effects. Therefore, up and coming technologies, namely advanced oxidation processes (AOPs) and advanced reduction processes (ARPs) are being tested for their implementation in the degradation of these pollutants. Thus, the present review compiles the current knowledge on the occurrence of PFOS and PFOA in the environment, the various toxic effects they have induced in different organisms as well as the ability of AOPs and ARPs to diminish and/or eliminate them from the environment.

Advanced oxidation processes may transform unknown PFAS in groundwater into known products

Per- and polyfluoroalkyl substances (PFAS) are a group of fluorinated organic contaminants classified as persistent in the aquatic environment. Early studies using targeted analysis approaches to evaluate the degradation of PFAS by advanced oxidation processes (AOP) in real water matrices may have been misinterpreted due to the presence of undetected or unknown PFAS in these matrices. The aims of the present study were to (1) screen selected commercially available AOPs (UV, UV + H2O2, O3/H2O2) and UV photocatalysis in a pilot system using commercially used and novel photocatalysts (TiO2, boron nitride [BN]) for removing PFAS contaminants and (2) evaluate their role on the conversion of non-detected/unknown to known PFAS compounds in real groundwater used as drinking water supplies. Results indicated that, while AOPs have the potential to achieve removal of the EPA method 533 target PFAS compounds (PFDA [100%], PFNA [100%], PFOA [85-94%], PFOS [25-100%], PFHxS [3-100%], PFPeS [100%], PFBS [100%]), AOPs transformed non-detected/unknown longer-chain PFAS compounds to detectable shorter-chain ones under very high-dose AOP operating conditions, leading to an increase in ∑PFAS concentration ranging from 95% to 340%. As emerging PFAS treatment processes transition from lab-scale investigations of target PFAS to pilot testing of real water matrices, studies will need to consider impact of the presence of non-target long-chain PFAS to transform into targeted PFAS compounds. A promising approach to address the potential risks and unforeseen consequences could involve an increased reliance on adsorbable organic fluorine (AOF) analysis before and after advanced oxidation process (AOP) treatment.

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.

Hydroxyl Radical Transformations of Perfluoroalkyl Acid (PFAA) Precursors in Aqueous Film Forming Foams (AFFFs)

Historical releases of aqueous film forming foam (AFFF) are significant sources of poly- and perfluoroalkyl substances (PFASs), including perfluoroalkyl acids (PFAAs) and their precursors, to the environment. While several studies have focused on microbial biotransformation of polyfluorinated precursors to PFAAs, the role of abiotic transformations at AFFF-impacted sites is less clear. Herein, we use photochemically generated hydroxyl radical to demonstrate that environmentally relevant concentrations of hydroxyl radical (^•OH) can play a significant role in these transformations. High-resolution mass spectrometry (HRMS) was used to perform targeted analysis, suspect screening, and nontargeted analyses, which were used to identify the major products of AFFF-derived PFASs as perfluorocarboxylic acids, though several potentially semi-stable intermediates were also observed. Using competition kinetics in a UV/H₂O₂ system, hydroxyl radical rate constants (k_OH) for 24 AFFF-derived polyfluoroalkyl precursors were measured to be 0.28 to 3.4 × 10⁹ M^-1 s^-1. Differences in k_OH were observed for compounds with differing headgroups and perfluoroalkyl chain lengths. Also, differences in k_OH measured for the only relevant precursor standard available, n-[3-propyl]tridecafluorohexanesulphonamide (AmPr-FHxSA), as compared to AmPr-FHxSA present in AFFF suggest that intermolecular associations in the AFFF matrix may affect k_OH. Considering environmentally relevant [^•OH]_ss, polyfluoroalkyl precursors are expected to exhibit half-lives of ∼8 days in sunlit surface waters and possibly as short as ∼2 h during oxygenation of Fe(II)-rich subsurface systems.

Impact of biological activated carbon filtration and backwashing on the behaviour of PFASs in drinking water treatment plants

PFASs are present in surface water, tap water and even commercial drinking water and pose a risk to human health. In this study, the treatment efficiency of 14 PFASs was studied in a large drinking water treatment plant (DWTP) using Taihu Lake as the source, and it was found that the ozone/biological activated carbon (O₃-BAC) process was the most effective process for the removal of PFASs in DWTPs. For the O₃-BAC process, there were differences in the removal of PFASs by BACs (1,4,7,13 years) of different ages. The sterilization experiments revealed that for GAC, its physical adsorption capacity reached saturation after one year, while for BAC with mature biofilms, biosorption was the main mechanism for the removal of PFASs. The abundance of Alphaproteobacteria and Gammaproteobacteria in biofilms was positively correlated with the age of the BAC. The microbial community with higher abundance is beneficial to the biodegradation of organic matter and thus provides more active sites for the adsorption of PFASs. PFASs can leak in the early stage of filtration after backwashing, so it is necessary to pay close attention to the influent and effluent concentrations of PFASs during biofilm maturation after backwashing.

A review on superior advanced oxidation and photocatalytic degradation techniques for perfluorooctanoic acid (PFOA) elimination from wastewater

Perfluorooctanoic acid (PFOA) has been identified as the most toxic specie of the family of perfluorinated carboxylic acids (PFCAs). It has been widely distributed and frequently detected in environmental wastewater. The compound's unique features such as inherent stability, rigidity, and resistance to harsh chemical and thermal conditions, due to its multiple and strong C-F bonds have resulted in its resistance to conventional wastewater remediations. Photolysis and bioremediation methods have been proven to be inefficient in their elimination, hence this article presents intensive literature studies and summarized findings reported on the application of advanced oxidation processes (AOPs) and photocatalytic degradation techniques as the best alternatives for the PFOA elimination from wastewater. Techniques of persulfate, photo-Fenton, electrochemical, photoelectrochemical and photocatalytic degradation have been explored and their mechanisms for the degradation and defluorination of the PFOA have been demonstrated. The major advantage of AOPs techniques has been centralized on the generation of active radicals such as sulfate (SO₄^•-) hydroxyl (•OH). While for the photocatalytic process, photogenerated species (electron (e) and holes (h ⁺ _vb)) initiated the process. These active radicals and photogenerated species possessed potentiality to attack the PFOA molecule and caused the cleavage of the C-C and C-F bonds, resulting in its efficient degradation. Shorter-chain PFCAs have been identified as the major intermediates detected and the final stage entails its complete mineralization to carbon dioxide (CO₂) and fluoride ion (F^-). The prospects and challenges associated with the outlined techniques have been highlighted for better understanding of the subject matter for the PFOA elimination from real wastewaters.

Total Oxidizable Precursor (TOP) Assay-Best Practices, Capabilities and Limitations for PFAS Site Investigation and Remediation

The comprehensive characterization of per- and polyfluoroalkyl substances (PFASs) is necessary for the effective assessment and management of risk at contaminated sites. While current analytical methods are capable of quantitatively measuring a number of specific PFASs, they do not provide a complete picture of the thousands of PFASs that are utilized in commercial products and potentially released into the environment. These unmeasured PFASs include many PFAS precursors, which may be converted into related PFAS chemicals through oxidation. The total oxidizable precursor (TOP) assay offers a means of bridging this gap by oxidizing unknown PFAS precursors and intermediates and converting them into stable PFASs with established analytical standards. The application of the TOP assay to samples from PFAS-contaminated sites has generated several new insights, but it has also presented various technical challenges for laboratories. Despite the increased number of literature studies that include the TOP assay, there is a critical and growing gap in the application of this method beyond researchers in academia. This article outlines the benefits and challenges of using the TOP assay with aqueous samples for site assessments and suggests ways to address some of its limitations.

Spatiotemporal variations and bioaccumulation of per- and polyfluoroalkyl substances and oxidative conversion of precursors in shallow lake water

Per- and polyfluoroalkyl substances (PFAS) in water and fish from Nansi Lake, Chian and in inflowing tributaries and nearby sewage treatment plants (STPs) were determined to evaluate their distribution and bioaccumulation. The potential precursors of perfluoroalkyl acid (PFAA) present in the water were converted via hydroxyl radical oxidation. Over 3 seasons, the average concentration ranges of the 15 PFAA (∑₁₅PFAA) concentrations in Nansi Lake, inflowing tributaries, and STPs were 22.8-70.3, 19.5-43.5, and 84.1-129 ng L^-1, respectively. Perfluorooctanoic acid, perfluorooctane sulfonate (PFOS), and short-chain PFAA (perfluorocarboxlate acid <8, perfluorosulfonate acids <6) were present in high concentrations in the lake and tributaries. PFAA concentration was the lowest during the wet season and the highest during the dry season. Moreover, PFAA precursors were converted to perfluorocarboxlate acid. The concentration of C8-based precursors was higher than that of the C6-based precursors in the lake and tributaries. The concentration of PFAA in the fish liver was higher than that in fish muscles, and PFOS was the dominant chemical present in fish. Potential risk assessment based on Environment Quality Standard revealed heavy PFOS contamination in the fish. Thus, the water of Nansi Lake was heavily polluted by PFAS.

Occurrence and behavior of per- and polyfluoroalkyl substances and conversion of oxidizable precursors in the waters of coastal tourist resorts in China

Per- and polyfluorolkyl substances (PFAS) were measured in the water and fish from 20 coastal tourist resorts in China, to investigate their sources, seasonal differences, and bioconcentration. An oxidative method with hydroxyl radicals was used to extract potential perfluoroalkyl acid (PFAA) precursors in the water of resorts. The results indicated that the total concentrations of target chemicals (i.e., ΣPFAS) in the original water were 59.4-138, 32.7-77.2, and 14.6-29.9 ng L^-1 in December, April, and August, respectively. C₄-C₁₀ perfluorocarboxlate (PFCA) and perfluorooctane sulfonate (PFOS) accounted for 67%-92% of the ΣPFAS contents in all water samples. The PFAS concentrations in the muscles and liver of fish were 16.0-162 ng g^-1 ww and 186-1240 ng g^-1 ww, respectively. The dominant compounds were perfluorobutanoate acid (PFBA) and PFOS in the water, and perfluorooctanoic acid (PFOA) and PFOS in fish tissues. High bioconcentration were observed for PFCA (C ≥ 8) and perfluorosulfonate (PFSA, C ≥ 6). After oxidative conversion, the water exhibited a noticeable increase in the ΣPFAS value. Precursors that generated C₄-C₉ PFCA were more prevalent than precursors that generated other PFCA upon oxidation. The concentration of C₈-based precursor was higher than that of C₆-based precursor in wet and dry seasons. This study is the first to apply an oxidative method to investigate PFAS pollution in the water of coastal tourist resorts. The results verified that PFAA precursors exist in the water of coastal tourist resorts, and more attention should be given to the existence of PFAA precursors and the safety of water in coastal tourist resorts.

Adsorption of perfluoroalkyl acids on granular activated carbon supported chitosan: Role of nanobubbles

The safety threat posed by Perfluoroalkyl acids (PFAAs) in drinking water is a growing concern. In this study, we loaded chitosan (CS) on granular activated carbon (GAC) to adsorb PFAAs, and we explored the role of nanobubbles in the adsorption process through experiments and density functional theory (DFT) calculations. Compared with GAC, we found that the use of the composite adsorbent (CS/GAC) enhanced the removal rate of perfluorooctanoic acid by 136% with the assistance of nanobubbles. PFAAs with different chain lengths have different adsorption mechanisms owing to surface activity differences. PFAAs with longer C-F chains can be directly enriched with amino groups on the CS or air-water interface on composite adsorbents. Additionally, PFAAs can be enriched with nanobubbles in solution to form nanobubble-PFAA colloids, which are adsorbed by protonated amino groups on CS through electrostatic interactions. We found that PFAAs with shorter C-F chains are less affected by nanobubbles, and DFT calculations indicated that the adsorption of short-chain PFAAs is mainly affected by electrostatic interactions. We also proved that the electrostatic interactions between CS and PFAAs are mainly derived from the abundant protonated amino groups.

Hydroxyl-radical based advanced oxidation processes can increase perfluoroalkyl substances beyond drinking water standards: Results from a pilot study

Advanced oxidation processes (AOPs) are popular technologies employed across the U.S. for wastewater reclamation and drinking water treatment of recalcitrant chemicals. Although there is consensus about the ineffectiveness of AOPs to treat perfluoroalkyl substances (PFASs; not polyfluoro compounds by definition here), there is a lack of field data demonstrating their impact on the transformation of unknown PFAS precursors during groundwater treatment. In this study, the fate of PFASs in seven pilot-scale AOPs, including four different technologies (UV/H₂O₂, UV/Cl₂, UV/TiO₂, and O₃/H₂O₂), was assessed at four drinking water systems across New York State (NYS), USA. Seven of 18 PFASs were detected in the influent at concentrations ranging from below method detection to 64 ng/L. Across all systems, all detected PFASs showed an increase in concentration after treatment presumably due to unknown precursor transformation with specific increases for perfluorobutane sulfonate (PFBS), perfluorohexanoic acid (PFHxA), perfluoroheptanoic acid (PFHpA), perfluorohexane sulfonate (PFHxS), perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS), and perfluorononanoic acid (PFNA) averaging 405 (range: 0 - 1220) %, 1.0 (-7 - 9) %, 3.8 (0 - 9.5) %, 3.3 (-11 - 13) %, 14 (0 - 48) %, 13 (3 - 25) %, and 2 (0 - 5.2) %, respectively. The increase in PFAS concentration was dependent on UV and oxidant dose, further confirming that transformation reactions were occurring due to AOPs similar to a total oxidizable precursor assay. At one of the sites, PFOA levels exceeded the current NYS drinking water standard of 10 ng/L after, but not before treatment, highlighting the importance of considering the potential impact of AOP on treated water quality when designing treatment systems for regulatory compliance. The increase in PFAS concentration in the AOP systems positively correlated (r = 0.91) with nitrate levels in groundwater, suggesting that onsite septic discharges may be an important source of PFAS contamination in these unsewered study areas. Results from this pilot-scale demonstration reveal that hydroxyl radical-based AOPs, although ineffective in treating PFASs, can help to reveal the true extent of PFAS contamination in source waters.

A review on degradation of perfluorinated compounds based on ultraviolet advanced oxidation

Perfluorinated compounds (PFCs), as emerging persistent pollutants, can exist for a long time in the environment due to their high stability. PFCs have been detected in drinking water, wastewater, and the human body. Studies have shown that PFCs pose a threat to human health and the ecological environment, which is expected to be listed in new drinking water regulations. Traditional processes, including coagulation, biological filtration, chlorination, ozonolysis, and ultraviolet light have ineffective removal efficiency on PFCs; however, advanced oxidation processes (AOP) based on ultraviolet (UV) light have good application prospects for the removal of PFCs. This study provides an overview of the removal of PFCs by UV-based AOPs; systematically introduces the research status of various UV-based AOPs from the perspectives of degradation pathways, degradation efficiency, influencing factors, formation of by-products; and comprehensively compares these different UV-based AOPs. Finally, the limitations of existing research and future research needs are discussed. This review aims to provide an overview for a better understanding of the degradation status and prospects of UV-based AOPs for the degradation of PFCs.

Managing and treating per- and polyfluoroalkyl substances (PFAS) in membrane concentrates

Per- and polyfluoroalkyl substances (PFAS), which are present in many waters, have detrimental impacts on human health and the environment. Reverse osmosis (RO) and nanofiltration (NF) have shown excellent PFAS separation performance in water treatment; however, these membrane systems do not destroy PFAS but produce concentrated residual streams that need to be managed. Complete destruction of PFAS in RO and NF concentrate streams is ideal, but long-term sequestration strategies are also employed. Because no single technology is adequate for all situations, a range of processes are reviewed here that hold promise as components of treatment schemes for PFAS-laden membrane system concentrates. Attention is also given to relevant concentration processes because it is beneficial to reduce concentrate volume prior to PFAS destruction or sequestration. Given the costs and challenges of managing PFAS in membrane concentrates, it is critical to evaluate both established and emerging technologies in selecting processes for immediate use and continued research.

Electrochemical degradation of per- and poly-fluoroalkyl substances using boron-doped diamond electrodes

Electrochemical degradation using boron-doped diamond (BDD) electrodes has been proven to be a promising technique for the treatment of water contaminated with per- and poly-fluoroalkyl substances (PFAS). Various studies have demonstrated that the extent of PFAS degradation is influenced by the composition of samples and electrochemical conditions. This study evaluated the significance of several factors, such as the current density, initial concentration of PFAS, concentration of electrolyte, treatment time, and their interactions on the degradation of PFAS. A 2⁴ factorial design was applied to determine the effects of the investigated factors on the degradation of perfluorooctanoic acid (PFOA) and generation of fluoride in spiked water. The best-performing conditions were then applied to the degradation of PFAS in wastewater samples. The results revealed that current density and time were the most important factors for PFOA degradation. In contrast, a high initial concentration of electrolyte had no significant impact on the degradation of PFOA, whereas it decreased the generation of F^-. The experimental design model indicated that the treatment of spiked water under a current density higher than 14 mA cm^-2 for 3-4 h could degrade PFOA with an efficiency of up to 100% and generate an F^- fraction of approximately 40-50%. The observed high PFOA degradation and a low concentration of PFAS degradation products indicated that the mineralization of PFOA was effective. Under the obtained best conditions, the degradation of PFOA in wastewater samples was 44-70%. The degradation efficiency for other PFAS in these samples was 65-80% for perfluorooctane sulfonic acid (PFOS) and 42-52% for 6-2 fluorotelomer sulfonate (6-2 FTSA). The presence of high total organic carbon (TOC) and chloride contents was found to be an important factor affecting the efficiency of PFAS electrochemical degradation in wastewater samples. The current study indicates that the tested method can effectively degrade PFAS in both water and wastewater and suggests that increasing the treatment time is needed to account for the presence of other oxidizable matrices.

Effect of granular activated carbon and other porous materials on thermal decomposition of per- and polyfluoroalkyl substances: Mechanisms and implications for water purification

Thermal treatment is routinely used to reactivate the spent granular activated carbon (GAC) from water purification facilities. It is also an integral part of sewage sludge treatment and municipal solid waste management. This study presents a detailed investigation of the fate of per- and polyfluoroalkyl substances (PFAS) and one PFAS alternative (GenX) in thermal processes, focusing on the effect of GAC. We demonstrate that the thermolysis of perfluoroalkyl carboxylic acids (PFCAs), including perfluorooctanoic acid (PFOA), and GenX can occur at temperatures of 150‒200 °C. Three temperature zones were discovered for PFOA, including a stable and nonvolatile zone (≤90 °C), a phase-transfer and thermal decomposition zone (90‒400 °C), and a fast decomposition zone (≥400 °C). The thermal decomposition began with the homolysis of a C‒C bond next to the carboxyl group of PFCAs, which formed unstable perfluoroalkyl radicals. Dual decomposition pathways seem to exist. The addition of a highly porous adsorbent, such as GAC or a copolymer resin, compressed the intermediate sublimation zone of PFCAs, changed their thermal decomposition pathways, and increased the decomposition rate constant by up to 150-fold at 250 °C. The results indicate that the observed thermal decomposition acceleration was linked to the adsorption of gas-phase PFCA molecules on GAC. The presence of non-activated charcoals/biochars with a low affinity for PFOA did not accelerate its thermal decomposition, suggesting that the π electron-rich, polyaromatic surface of charcoal/GAC played an insignificant role compared to the adsorbent's porosity. Overall, the results indicate that (1) substantial decomposition of PFCAs and GenX during conventional thermal GAC/sludge/waste treatment is very likely, and (2) the presence or addition of GAC or other highly porous materials can accelerate thermal PFAS decomposition and alter decomposition pathways.

A review of the occurrence, transformation, and removal of poly- and perfluoroalkyl substances (PFAS) in wastewater treatment plants

Poly- and perfluoroalkyl substances (PFAS) comprise more than 4,000 anthropogenically manufactured compounds with widescale consumer and industrial applications. This critical review compiles the latest information on the worldwide distribution of PFAS and evaluates their fate in wastewater treatment plants (WWTPs). A large proportion (>30%) of monitoring studies in WWTPs were conducted in China, followed by Europe (30%) and North America (16%), whereas information is generally lacking for other parts of the world, including most of the developing countries. Short and long-chain perfluoroalkyl acids (PFAAs) were widely detected in both the influents (up to 1,000 ng/L) and effluents (15 to >1,500 ng/L) of WWTPs. To date, limited data is available regarding levels of PFAS precursors and ultra-short chain PFAS in WWTPs. Most WWTPs exhibited low removal efficiencies for PFAS, and many studies reported an increase in the levels of PFAAs after wastewater treatment. The analysis of the fate of various classes of PFAS at different wastewater treatment stages (aerobic and/aerobic biodegradation, photodegradation, and chemical degradation) revealed biodegradation as the primary mechanism responsible for the transformation of PFAS precursors to PFAAs in WWTPs. Remediation studies at full scale and laboratory scale suggest advanced processes such as adsorption using ion exchange resins, electrochemical degradation, and nanofiltration are more effective in removing PFAS (~95-100%) than conventional processes. However, the applicability of such treatments for real-world WWTPs faces significant challenges due to the scaling-up requirements, mass-transfer limitations, and management of treatment by-products and wastes. Combining more than one technique for effective removal of PFAS, while addressing limitations of the individual treatments, could be beneficial. Considering environmental concentrations of PFAS, cost-effectiveness, and ease of operation, nanofiltration followed by adsorption using wood-derived biochar and/or activated carbons could be a viable option if introduced to conventional treatment systems. However, the large-scale applicability of the same needs to be further verified.

Field study on the transportation characteristics of PFASs from water source to tap water

Perfluoroalkyl substances (PFASs) can occur in water sources, pass through drinking water treatment plants (DWTPs), drinking water distribution systems (DWDSs), to the consumer taps. This investigation was carried out to present the transportation behaviors of 17 PFASs, involving seven DWTPs with different water sources, raw water transportation modes, treatment processes, and DWDS structures in eastern and northern China. The results showed that the long-distance raw water transportation pipelines removed a certain extent of PFASs from raw water, probably due to the accumulation of loose deposits. The long-distance, open-channel South-to-North water diversion increased PFAS contamination risk. In the DWTPs, granular activated carbon (GAC) adsorption and ultraviolet radiation removed less than 25% of PFASs, but ozonation-biological activated carbon (O₃-BAC) was superior to GAC alone in removing PFASs. Loose deposits couldsignificantly influence PFAS accumulation and release within branch-structured DWDSs. In loop-structured DWDSs, finished water with different PFAS characteristics could mix along the pipeline, with the corresponding DWTP as the center, ultimately forming a relatively uniform distribution in the entire DWDS.

Comparison of currently available PFAS remediation technologies in water: A review

Remediation of Poly- and perfluoroalkyl substances (PFASs) in the environment has rapidly increased due to growing concerns of environmental contamination and associated adverse toxicological effects on wildlife and humans due to bioaccumulation and extreme persistence. Although, PFASs are highly recalcitrant to conventional water treatment processes, there are some effective techniques available. Those techniques involve exceedingly high costs due to high energy use, and high capital or operational costs. Thus, most remediation techniques have limitations in field applications even though the laboratory scale experiments are promising. As a result of stringent new health and environmental regulatory standards are being established, development of suitable water treatment methodology is more challenging. Most of the separation and destruction techniques have their own limitations in field applications while the biological approaches to treat PFASs are extremely limited and are not currently considered as viable. In this review, extra consideration is given to novel advanced techniques for wide array of PFAS classes including short chain PFAS removal, and compare their efficiencies, effectiveness, energy use, sustainability, cost, and simplicity in laboratory scale to field applications. Electrochemical, sonochemical, advanced oxidation processers (AOPs) and plasma together with novel hybrid techniques are considered as effective approaches for PFASs removal and have shown promising results for long chain and some short chain PFASs, as well as extremely persistent per-fluoro alkyl acids (PFAAs). Therefore, it is essential to better understand the removal mechanisms to optimise the advanced treatment processes like hybrid techniques because, the unique physicochemical characteristics of various PFASs impose difficult challenges. Careful selection of a combined effective treatment methodology in an integrated processing unit, would be a revolutionary approach for complete elimination of PFASs from the environment. Considering the site-specific water quality parameters together with community perspectives will also make it more viable in real world field applications.

Challenges of aqueous per- and polyfluoroalkyl substances (PFASs) and their foreseeable removal strategies

Per- and polyfluoroalkyl substances (PFASs) are artificial refractory organic pollutants which are widely presented in aqueous environment. Due to the unquiet strength of the highly polarized carbon-fluorine bond (C-F) and their hydrophobic/lipophobic feature as well as biological persistence properties, the remediation and treatment of PFASs is a big challenge. Preliminary studies indicate that a few kinds of technical approaches could remove or transfer PFASs, but the effectiveness is not high as expected or limited while most of the techniques are only tested at laboratory scale. A review of existing treatment technologies was thus conducted for the purpose to outlook these technologies, and more importantly, to propose the foreseeable technique. As such, a constructed wetland-microbial fuel cell (CW-MFC) technology was recommended, which is a newly emerged technology by integrating physical, chemical and enhanced biological processes plus the wetland plants function with strong eco-friendly feature for a comprehensive removal of PFASs. It is expected that the review can strengthen our understanding on PFASs' research and thus can help selecting reasonable technical means of aqueous PFASs control.

Adsorption of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) from water using leaf biomass (Vitis vinifera) in a fixed-bed column study

INTRODUCTION

Adsorption of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) onto modified activated carbons (AC-H₃PO₄) produced from leaf biomass of Vitis vinifera leaf litter in a fixed bed column experiment was investigated in this study.

METHODS

The column bed was packed with the produced activated carbons in a uniform particle size of ˃ 64 μm. Experimental parameters including the initial concentration of the solution, column bed height, the mass of adsorbent and flow rate were optimized to establish the best adsorption efficiency parameters for the system. Breakthrough and saturated time were estimated from the column fixed bed experimental data and analysed using the Adam-Bohart, Thomas model, and Yoon-Nelson models.

RESULTS

Maximum sorption capacities of produced activated carbon ACH₃PO₄ based on Thomas model were 159.61 and 208.64 mg/g for PFOA and PFOS, respectively. The results indicated the breakthrough and saturated time of the system increased concurrently with the increase in bed height and initial concentrations, while an increase in flow rate enhanced fractional bed utilization (FBU) efficiency of the column. Thomas and Yoon-Nelson model best describe the prediction of breakthrough data and sorption behaviour of PFOA and PFOS indicating suitability of AC-H₃PO₄ column design.

CONCLUSION

Findings suggest that agro based adsorbent is a good alternative to non-ago based adsorbent. The surface characteristics of the phosphoric acid modified activated carbons AC-H₃PO₄ affirmed the removal of PFOA and PFOS from the contaminated water.

Spatial distribution of perfluoroalkyl acids (PFAAs) and their precursors and conversion of precursors in seawater deeply affected by a city in China

Conversion of perfluoroalkyl acid (PFAA) precursors in the environment has been a hotspot research in recent years. This study firstly determined the spatial distribution of PFAAs and their precursors including 8:2 fluorotelomer unsaturated acid (8:2 FTUCA), perfluorooctane sulfoneamide (FOSA), and diperfluorooctane sulfonamido ethanol-based phosphate (di-SAmPAP), then investigated the conversion of the potential precursors in the seawater and sewage treatment plants (STPs) effluents. The results indicated that the target pollutants showed a typical concentration gradient from nearshore to offshore. And the obviously increased concentration of perfluorinated carboxylic acids (△[PFCAs]) after oxidation treatment can verify the existence of PFAA precursors in the seawater and STP effluents. The concentrations of PFCAs with carbon atom numbers 4-9 (PFCA_C4-C9) revealed the most increase. Moreover, the levels of △[PFCAs] and the ratios of △[PFCAs] to their concentration before oxidation (△[PFCA]/[PFCA]_{before oxidation}) indicated obvious spatial variations in the seawater and STP effluents. The higher levels of △[PFCA_C4-C12] and the lower ratios of ∑△[PFCA_C4-C12]/∑[PFAA]_{before oxidation} were observed in the STP effluents, which implied that precursors might be decomposed during the sewage treatment process. These results suggested the STP effluents might have an important effect on the PFAAs levels of seawater.

Investigating recycled water use as a diffuse source of per- and polyfluoroalkyl substances (PFASs) to groundwater in Melbourne, Australia

The purpose of this study was to investigate the contribution of per- and polyfluoroalkyl substances (PFASs) to groundwater at a location where recycled water from a wastewater treatment plant (WWTP) is used to irrigate crops. Groundwater from Werribee South, located west of Melbourne, Australia, was sampled over two campaigns in 2017 and 2018, extracted using solid phase extraction (SPE) and analysed with liquid chromatography-tandem mass spectrometry (LC-MS/MS-QQQ). PFASs were detected in 100% of the groundwater samples. The sum total of twenty PFAS compounds (∑₂₀PFASs) for all sites in the study ranged from <0.03 to 74 ng/L (n = 28) and the highest levels of which were observed in the centre of the irrigation district. Perfluorooctanesulfonic acid (PFOS) was the most detected compound overall (96%) with a mean concentration of 11 ng/L (<0.03-34 ng/L), followed by perfluorobutanesulfonic acid (PFBS; 86%, 4.4 ng/L), perfluorooctanoic acid (PFOA; 82%, 2.2 ng/L) and perfluorobutanoic acid (PFBA; 77%, 6.1 ng/L). Concentrations of PFASs found in this study are greater than background levels of PFASs detected in groundwater and are in the range of concentrations typically detected in wastewater effluent. This study presents evidence that the use of recycled water can be a source of PFAS contamination to groundwater.

Oxidative conversion of potential perfluoroalkyl acid precursors in Jiaozhou Bay and nearby rivers and sewage treatment plant effluent in China

Precursors that can be transformed into perfluoroalkyl acids (PFAAs) have not been investigated in detail. In this study, the levels of potential PFAA precursors in the Jiaozhou Bay, inflowing rivers, and STP (sewage treatment plant) effluents were investigated by converting all PFAA precursors into perfluorinated carboxylic acids (PFCAs) by chemical oxidation. The significance of controlling PFAA precursors was indicated by the ratios of PFCAs converted by the oxidative treatment of precursors to PFAAs before oxidation (∑△[PFCA_C4-C12]/∑[PFAA]_{before oxidation}). The higher levels of △[PFCA_C4-C12] (average = 18.89 ng/L) and lower ratios (∑△[PFCA_C4-C12]/∑[PFAA]_{before oxidation}, average = 0.21) were revealed in the STP effluents rather than in the water of the Jiaozhou Bay and rivers, which implied the precursors conversion during the sewage treatment process. The concentrations of △[PFCAs] and the aforementioned ratios showed apparent spatial and temporal differences. These results indicated that STPs were the important sources of precursors to other water bodies.

Application of advanced oxidation processes and toxicity assessment of transformation products

Advanced Oxidation Processes (AOPs) are the techniques employed for oxidation of various organic contaminants in polluted water with the objective of making it suitable for human consumption like household and drinking purpose. AOPs use potent chemical oxidants to bring down the contaminant level in the water. In addition to this function, these processes are also capable to kills microbes (as disinfectant) and remove odor as well as improve taste of the drinking water. The non-photochemical AOPs methods include generation of hydroxyl radical in absence of light either by ozonation or through Fenton reaction. The photochemical AOPs methods use UV light along with H₂O₂, O₃ and/or Fe⁺² to generate reactive hydroxyl radical. Non-photochemical method is the commonly used whereas, photochemical method is used when conventional O₃ and H₂O₂ cannot completely oxidize organic pollutants. However, the choice of AOPs methods is depended upon the type of contaminant to be removed. AOPs cause loss of biological activity of the pollutant present in drinking water without generation of any toxicity. Conventional ozonation and AOPs can inactivate estrogenic compounds, antiviral compounds, antibiotics, and herbicides. However, the study of different AOPs methods for the treatment of drinking water has shown that oxidation of parent compound can also lead to the generation of a degradation/transformation product having biological activity/chemical toxicity similar to or different from the parent compound. Furthermore, an increased toxicity can also occur in AOPs treated drinking water. This review discusses various methods of AOPs, their merits, its application in drinking water treatment, the related issue of the evolution of toxicity in AOPs treated drinking water, biocatalyst, and analytical methods for identification of pollutants /transformed products and provides future directions to address such an issue.

Removal of perfluoroalkyl and polyfluoroalkyl substances in potable reuse systems

Perfluoroalkyl and polyfluoroalkyl substances (PFASs) are a group of persistent contaminants that have been identified throughout the aquatic environment. In this study, ten targeted perfluoroalkyl acids (PFAAs), three targeted PFAA precursors, and non-targeted PFAA precursors were monitored in four full- and pilot-scale potable reuse plants at each stage of advanced treatment. Non-targeted PFAA precursors were quantified by applying a total oxidizable precursor assay in which PFAA precursors are oxidized by hydroxyl radicals to targeted PFAAs. Two of the potable reuse systems had membrane-based treatments with reverse osmosis and UV-advanced oxidation (RO-UV/AOP) and two used ozone, biological activated carbon filtration and granular activated carbon adsorption (O₃-BAC-GAC). The total targeted PFAAs in the four tertiary effluents, the influent sources for the potable reuse systems, ranged from 52 to 227 ng/L with non-targeted PFAA precursors accounting for 30-67% of total PFASs on a molar basis. The RO-UV/AOP treatment trains reduced PFAAs and PFAA precursors to below their method reporting limits through the barrier provided by RO. The O₃-BAC-GAC based treatment trains reduced, but did not completely remove PFAAs or PFAA precursors and the PFASs present in the product water were primarily shorter-chain PFAAs, some of which lack human health guidance values for drinking water. The relative fraction of targeted shorter-chain PFAAs increased after each treatment step indicating that there was preferential removal of the PFAA precursors and longer-chain PFAAs. This study provides new insight on the concentrations and treatment of PFAA precursors through potable reuse treatment systems.

Roles of intrinsic Mn3+ sites and lattice oxygen in mechanochemical debromination and mineralization of decabromodiphenyl ether with manganese dioxide

Commercial β-MnO₂ with a chemical formula of approximate Mn_0.77⁴⁺Mn_0.23³⁺O_1.88 was used for mechanochemical (MC) oxidative degradation of decabromodiphenyl ether (BDE209). The ball milling process initiated the degradation of BDE209 on β-MnO₂, yielding a nearly complete degradation and debromination of BDE209 within 2 h. The use of β-MnO₂ exhibited much higher MC debromination efficiency than that by using birnessite (δ-MnO₂, 40.2%), Bi₂O₃ (45.6%), CaO (65.3%), and persulfate (81.9%). It was demonstrated that the oxidative degradation of BDE209 was promoted by the redox half reactions of both Mn⁴⁺→ Mn³⁺ and Mn³⁺→ Mn²⁺, but naturally existed Mn³⁺ centers on the surface of β-MnO₂ functioned as dominant reactive species at the initial stage of the MC degradation (often before the degradation efficiency of BDE209 achieved 50%). Moreover, the surface lattice oxygen of MnO₂, rather than O₂, played a key role in the debromination and mineralization of BDE209. The Mn³⁺ sites on β-MnO₂ not only easily accepted the electron of BDE209, but also promoted the mobility of lattice oxygen from the bulk to the surface for mineralizing BDE209. These results firstly highlighted the importance of Mn³⁺ availability and oxygen mobility on the reactivity of manganese oxide for the MC oxidative degradation of organic pollutants.

Thermal- and photo-induced degradation of perfluorinated carboxylic acids: Kinetics and mechanism

Perfluorinated carboxylic acids (PFCAs) of different carbon chain lengths are chemicals of concern to human health and their removal, using conventional remediation technologies, is challenging. The present paper pursuits thermal and photo-induced degradation of PFCAs (F(CF₂)_nCOOH, n = 1-9) under various concentrations of four different acids (HNO₃, H₂SO₄, HCl, and H₃PO₄) covering a range of strong acidic to basic pH. For thermal-induced experiments, the temperature was set at 40 °C, 60 °C, and 80 °C at acid strengths of 0.04-18.4 M. Photo-induced experiments were conducted at pH 0.5, 7.0, and 13.0 under a light intensity of (150 ± 10) × 100 μW/cm². The degradation first-order rate constant (k_1, h^-1) as a function of [H⁺] was modeled by considering equilibrium of nondissociated (F(CF₂)_nCOOH, HX) and dissociated (F(CF₂)_nCOO^-, X^-) species of PFCAs (HX ⇌ X^- + H⁺, pK_a = -0.1). Species-specific rate constants, k₁^HX, reasonably described the trend of thermal and photo decay of PFCAs, where k₁^HX increased with acidity of solution and the carbon chain length of PFCAs. Mechanism of degradation of PFCAs (e.g. perfluorooctanoic acid (PFOA)) involved homolytic breakage of CC bond between alkyl and carboxyl groups, which produced radicals and subsequently decarboxylation to perfluoroheptene-1. Density functional theory (DFT) calculations supported the mechanism. The calculations indicated that a breaking of CC bond is more feasible with nondissociated HX than dissociated X^- species of PFCAs and also with increase in chain length. The potential of a combination of thermal- and photo-induced processes under acidic conditions to enhance degradation of PFOA in water is presented.