Activated carbon versus metal-organic frameworks: A review of their PFAS adsorption performance

A critical review of biochar for the remediation of PFAS-contaminated soil and water

Per- and polyfluoroalkyl substances (PFAS) present significant environmental and health hazards due to their inherent persistence, ubiquitous presence in the environment, and propensity for bioaccumulation. Consequently, the development of efficacious remediation strategies for soil and water contaminated with PFAS is imperative. Biochar, with its unique properties, has emerged as a cost-effective adsorbent for PFAS. Despite this, a comprehensive review of the factors influencing PFAS adsorption and immobilization by biochar is lacking. This narrative review examines recent findings indicating that the application of biochar can effectively immobilize PFAS, thereby mitigating their environmental transport and subsequent ecological impact. In addition, this paper reviewed the sorption mechanisms of biochar and the factors affecting its sorption efficiency. The high effectiveness of biochars in PFAS remediation has been attributed to their high porosity in the right pore size range (>1.5 nm) that can accommodate the relatively large PFAS molecules (>1.02-2.20 nm), leading to physical entrapment. Effective sorption requires attraction or bonding to the biochar framework. Binding is stronger for long-chain PFAS than for short-chain PFAS, as attractive forces between long hydrophobic CF2-tails more easily overcome the repulsion of the often-anionic head groups by net negatively charged biochars. This review summarizes case studies and field applications highlighting the effectiveness of biochar across various matrices, showcasing its strong binding with PFAS. We suggest that research should focus on improving the adsorption performance of biochar for short-chain PFAS compounds. Establishing the significance of biochar surface electrical charge in the adsorption process of PFAS is necessary, as well as quantifying the respective contributions of electrostatic forces and hydrophobic van der Waals forces to the adsorption of both short- and long-chain PFAS. There is an urgent need for validation of the effectiveness of the biochar effect in actual environmental conditions through prolonged outdoor testing.

Perfluoroalkyl and polyfluoroalkyl substances (PFAS) and nutrients from two constructed wetlands in a city of southeastern China

Perfluoroalkyl and polyfluoroalkyl substances (PFAS) are a large class of toxic contaminants. Nutrients are closely related to the ecological health of aquatic systems. Both have received widespread global attention. This study investigated the concentrations, compositions, and spatial distributions of PFAS and nutrients in surface water from two constructed wetlands and the nearby drinking water treatment plants (DWTPs). We explored the natural environmental factors and human activities that affect the composition and distribution of pollutants in wetlands and assessed the ability of the DWTPs to remove contaminants. Concentrations of ∑32PFAS varied from 153 to 405 ng/L. Hexafluoropropylene oxide trimer acid (HFPO-TA) was the predominant substance accounting for 45 % of ∑32PFAS concentrations. It might originate from the emissions of indirect sources of PFAS related manufacturers. The detection rate of 6:2 fluorotelomer carboxylic acid (6:2 FTCA) was 100 % with concentrations ranging from 0.915 to 19.7 ng/L 6:2 FTCA might come from the biotransformation of indirect sources in the air. Concentrations of total nitrogen (TN) and total phosphorus (TP) were from 1.47 to 3.54 mg/L, and non-detect (ND) to 0.323 mg/L, respectively. Constructed wetlands could effectively remove PFAS under nutrient stress, however, the removal of PFAS depends on the characteristics of specific compounds and their sources. The removal rates for PFAS and nutrients could be promoted through artificial dredging. But wetland bioremediation could have two opposing effects. On the one hand, plants can take up pollutants from water via roots, leading to pollutant removal and purification. On the other hand, plants may also absorb precursor intermediates from the air through leaves and release them into the water, leading to increased pollutant concentrations. Thirty-two emerging PFAS were identified by high resolution mass spectrum. The drinking water treatment process removed PFAS and nutrients below the drinking water quality standards of China, however, 9 non-target PFAS compounds were still found in tap water. These results provide case support and a theoretical basis for the pollution control and sustainable development of typical ecological wetlands used as drinking water sources.

Efficient PFAS removal from contaminated soils through combined washing and adsorption in soil effluents

This study investigates soil washing as a viable strategy to remove poly- and perfluoroalkyl substances (PFAS) from contaminated soils using various washing agents including water, methanol, ethanol, and cyclodextrin ((2-Hydroxypropyl)-β-cyclodextrin HPCD)). Water was less effective (removing only 30 % of PFAS), especially for long-chain hydrophobic PFAS. Methanol (50 % v/v) or HPCD (10 mg g-1 soil) achieved > 95 % PFAS removal regardless of PFAS type, soil size fraction (0-400 µm or 400-800 µm), or experimental setups (batch or column, at liquid/solid (L/S) = 1). Column optimization studies revealed improved efficiency at L/S = 10 with diluted washing solutions, where HPCD exhibited rapid PFAS mobilization even at lower concentrations (1 mg mL-1). We then applied a first-order decay model to effectively predict PFAS breakthrough curves and mobilization within soil columns. Subsequent treatment of wash effluents by activated carbon and biochar effectively reduced PFAS concentrations below detection limits. The performance of both soil washing and subsequent adsorption was found to depend strongly on the specific characteristics of PFAS compounds. These findings highlight the significant potential of methanol and HPCD in soil washing and the effectiveness of integrated soil washing and adsorption for optimizing PFAS removal.

Vesicles exhibit high-performance removal of per-and polyfluoroalkyl substances (PFAS) depending on their hydrophobic groups

The removal of per- and polyfluoroalkyl substances (PFAS) from drinking water is urgently needed. Here, we demonstrated high performance of vesicles on PFAS adsorption. Vesicles used in this study were enclosed amphiphile bilayers keeping their hydrophobic groups inside and their hydrophilic groups outside in water. The distribution coefficient Kd of perfluorooctane sulfonic acid (PFOS) for vesicles was 5.3 × 105 L/kg, which is higher than that for granulated activated carbon (GAC), and Kd of perfluorooctanoic acid (PFOA) for vesicles was 103-104 L/kg. The removal efficiencies of PFOA and PFOS adsorption on DMPC vesicles were 97.1 ± 0.1% and 99.4 ± 0.2%, respectively. The adsorption behaviors of PFOA and PFOS on vesicles were investigated by changing the number of cis-double bonds in the hydrophobic chains of the vesicle constituents. Moreover, vesicles formed by membranes in the different phases were also tested. The results revealed that, when vesicles are formed of a membrane in the liquid-crystalline (liquid-like) phase, the adsorption amounts of both PFOA and PFOS increased as the cis-double bond in the hydrocarbon chains decreased, which is considered due to molecular shape similarity. When vesicles are formed of a membrane in the gel (solid-like) phase, they do not adsorb PFAS as much as in the liquid-crystalline phase, even though the hydrocarbon chains do not have any cis-double bond. Our findings demonstrate that vesicles can be utilized as PFAS adsorbents by optimizing the structure of vesicle constituents and their thermodynamical phase. Indeed, the vesicles (DMPC) were demonstrated that they can adsorb PFOA and PFOS, and be coagulated by a coagulant even in environmental water. The coagulation will enable the removal of PFOA and PFOS from the water after adsorption.

Per- and polyfluoroalkyl substances removal from landfill leachate by a planting unit via interactions between foamed glass and Typha domingensis

Sustainable removal of per- and polyfluoroalkyl substances (PFAS) from landfill leachate remains a pressing global challenge. To develop an effective PFAS removal technology that utilizes nature-based solutions, we considered a planting unit comprised of a microbial carrier (foamed glass) and Typha domingensis. This study evaluated the possibility of removing PFAS from landfill leachate using a planting unit through a pot experiment. The planting unit effectively removed various short- and long-chain PFAS from the landfill leachate, including perfluorocarboxylic acids (PFCAs [C4-C10]), perfluorosulfonic acids (PFSAs [C4, C6, and C8]), fluorotelomer carboxylic acids (FTCAs [5:3 and 7:3]), and 6:2 fluorotelomer sulfonic acid (FTS), with initial concentrations of 43-9100 ng L-1, achieving a removal efficiency of 53-83% in 21 d. Mass balance analysis indicated that the contribution of accumulation on foamed glass and plant adsorption and uptake played no major role in the removal of PFCAs (C4-C9), PFSAs (C4), and FTCAs (5:3 and 7:3), and that other removal processes played a key role. Although not the most effective removal process, the contribution of accumulation on foamed glass tended to be more notable in the removal of longer-chain PFCAs. In addition, plant adsorption and uptake showed that longer-chain PFCAs were more likely to remain in roots, whereas shorter-chain PFCAs were more likely to be transferred to aboveground plant part. On the other hand, 6:2 FTS removal occurred primarily due to accumulation on foamed glass. These results suggest that differences in the physicochemical properties of PFAS affect removal mechanisms. This study provides valuable insights into development of environmentally friendly technologies capable of removing a variety of short- and long-chain PFAS.

Comparison of perfluoroalkyl substance adsorption performance by inorganic and organic silicon modified activated carbon

Owing to the persistence and increasingly stringent regulations of perfluoroalkyl substances (PFAS), it is necessary to improve their adsorption capacities using activated carbon (AC). However, their adsorption capacities are suppressed by dissolved organic matter (DOM). In this study, two ACs modified with organic silicon (C-OS) and inorganic silicon (C-IS) were synthesized and used for the adsorption of PFAS in raw water (RW). The results showed that the PFAS adsorption capacity of C-IS was much less influenced by DOM than that of the original AC (C-virgin). In RW, perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) adsorption capacities on C-IS were 15.08 and 3.65 times higher than those on C-virgin, respectively. DOM had less influence on the PFOA and PFOS adsorption kinetics of C-IS than C-OS and C-virgin. Under multi-PFAS condition, C-IS also exhibited slower desorption of short-chain PFAS and breakthrough in batch and column tests, respectively. Characterization of the ACs before and after adsorption and independent gradient modelling indicated that hydrogen bond interactions between the O-Si of C-IS and the -COOH or -CSO3H groups of PFAS contributed to PFAS adsorption. Density functional theory calculations demonstrated that the adsorption energy of C-IS was much lower than that of C-OS and C-virgin. The arrangement of PFAS molecules on C-OS was chaotic owing to the hydrophobic siloxane chain, whereas the arrangement of PFAS on C-IS was orderly in multi-layer or semi-micelle status and more favorable to PFAS adsorption. This study provides a new strategy for avoiding adverse effects of DOM on PFAS adsorption.

Zirconium-Based Metal-Organic Frameworks with Free Hydroxy Groups for Enhanced Perfluorooctanoic Acid Uptake in Water

Perfluorooctanoic acid (PFOA) is a highly recalcitrant organic pollutant, and its bioaccumulation severely endangers human health. While various methods are developed for PFOA removal, the targeted design of adsorbents with high efficiency and reusability remains largely unexplored. Here the rational design and synthesis of two novel zirconium-based metal‒organic frameworks (MOFs) bearing free ortho-hydroxy sites, namely noninterpenetrated PCN-1001 and twofold interpenetrated PCN-1002, are presented. Single crystal analysis of the pure ligand reveals that intramolecular hydrogen bonding plays a pivotal role in directing the formation of MOFs with free hydroxy groups. Furthermore, the transformation from PCN-1001 to PCN-1002 is realized. Compared to PCN-1001, PCN-1002 displays higher chemical stability due to interpenetration, thereby demonstrating an exceptional PFOA adsorption capacity of up to 632 mg g-1 (1.53 mmol g-1), which is comparable to the reported record values. Moreover, PCN-1002 shows rapid kinetics, high selectivity, and long-life cycles in PFOA removal tests. Solid-state nuclear magnetic resonance results and density functional theory calculations reveal that multiple hydrogen bonds between the free ortho-hydroxy sites and PFOA, along with Lewis acid-base interaction, work collaboratively to enhance PFOA adsorption.

Elucidating the degradation mechanisms of perfluorooctanoic acid and perfluorooctane sulfonate in various environmental matrices: a review of green degradation pathways

Given environmental persistence, potential for bioaccumulation, and toxicity of Perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), the scientific community has increasingly focused on researching their toxicology and degradation methods. This paper presents a survey of recent research advances in the toxicological effects and degradation methods of PFOA and PFOS. Their adverse effects on the liver, nervous system, male reproductive system, genetics, and development are detailed. Additionally, the degradation techniques of PFOA and PFOS, including photochemical, photocatalytic, and electrochemical methods, are analyzed and compared, highlighted the potential of these technologies for environmental remediation. The biotransformation pathways and mechanisms of PFOA and PFOS involving microorganisms, plants, and enzymes are also presented. As the primary green degradation pathway for PFOA and PFOS, Biodegradation uses specific microorganisms, plants or enzymes to remove PFOA and PFOS from the environment through redox reactions, enzyme catalysis and other pathways. Currently, there has been a paucity of research conducted on the biodegradation of PFOA and PFOS. However, this degradation technology is promising owing to its specificity, cost-effectiveness, and ease of implementation. Furthermore, novel materials/methods for PFOA and PFOS degradation are presented in this paper. These novel materials/methods effectively improve the degradation efficiency of PFOA and PFOS and provide new ideas and tools for the degradation of PFOA and PFOS. This information can assist researchers in identifying flaws and gaps in the field, which can facilitate the formulation of innovative research ideas.

Separable Metal-Organic Framework-Based Materials for the Adsorption of Emerging Contaminants

Thousands of chemicals have been released into the environment in recent decades. The presence of emerging contaminants (ECs) in water has emerged as a pressing concern. Adsorption is a viable solution for the removal of ECs. Metal-organic frameworks (MOFs) have shown great potential as efficient adsorbents, but their dispersed powder form limits their practical applications. Recently, researchers have developed various separable MOF-based adsorbents to improve their recyclability. The purpose of this review is to summarize the latest developments in the construction of separable MOF-based adsorbents and their applications in adsorbing ECs. The construction strategies for separable MOFs are classified into four categories: magnetic MOFs, MOF-fiber composites, MOF gels, and binder-assisted shaping. Typical emerging contaminants include pesticides, pharmaceuticals and personal care products, and endocrine-disrupting compounds. The adsorption performance of different materials is evaluated based on the results of static and dynamic adsorption experiments. Additionally, the regeneration methods of MOF-based adsorbents are discussed in detail to facilitate effective recycling and reuse. Finally, challenges and potential future research opportunities are proposed, including reducing performance losses during the shaping process, developing assessment systems based on dynamic purification and real polluted water, optimizing regeneration methods, designing multifunctional MOFs, and low-cost, large-scale synthesis of MOFs.

In situ synthesis of porous metal-organic frameworks NH2-UiO-66 on tea stem biochar and application in odours adsorption

Multiple odour nuisance in livestock farming is a notorious problem that has a significant impact on the living environment of surrounding communities. Adsorbents based on metal-organic framework (MOF) materials show great promise for controlling odour pollution, as they offer a high specific surface area, a controllable structure and an abundance of active sites. However, the MOF formation process is prone to problems such as pore clogging or collapse and reduced porosity, which limits its further application. In this study, a series of odour adsorbents were prepared by in situ growth of NH2-UiO-66 on tea stem biochar (TSBC) using a hydrothermal method and named UiO (Zr)-TSBCx. The physical and chemical properties and composition of UiO (Zr)-TSBCx have been systematically characterized using SEM, TEM, XRD, FT-IR, N2 adsorption-desorption and XPS. The release of odours from the pig farm effluent was monitored using in-situ continuous Proton-Transfer-Reaction Mass Spectrometry (PTR-MS), and the obtained primary compositions were tested for further adsorption. In dynamic adsorption experiments focused on butyric acid, UiO (Zr)-TSBC2 showed a high adsorption capacity of 3.99 × 105 μg/g and exceptional structural stability. UiO (Zr)-TSBC2 showed variable adsorption efficiencies for different odorous gases, with the best performance for the removal of ammonia, toluene and butyric acid. It also demonstrated the ability to rapidly mitigate instantaneous high concentrations of hydrogen sulfide (H2S), methanethiol and toluene resulting from agitation. Additionally, based on the relationship between the adsorption amount and the structural characteristics of the adsorbent as well as the nature of the odours, a possible adsorption mechanism of UiO (Zr)-TSBC2 for a variety of odours released from pig farm effluent was proposed. This work demonstrates a novel approach to promote deodorization applications in livestock and poultry farming environments by the in-situ growth of NH2-UiO-66 on biochar prepared from tea stem.

Roles of varying carbon chains and functional groups of legacy and emerging per-/polyfluoroalkyl substances in adsorption on metal-organic framework: Insights into mechanism and adsorption prediction

Metal-organic frameworks (MOFs) are promising adsorbents for legacy per-/polyfluoroalkyl substances (PFASs), but they are being replaced by emerging PFASs. The effects of varying carbon chains and functional groups of emerging PFASs on their adsorption behavior on MOFs require attention. This study systematically revealed the structure-adsorption relationships and interaction mechanisms of legacy and emerging PFASs on a typical MOF MIL-101(Cr). It also presented an approach reflecting the average electronegativity of PFAS moieties for adsorption prediction. We demonstrated that short-chain or sulfonate PFASs showed higher adsorption capacities (μmol/g) on MIL-101(Cr) than their long-chain or carboxylate counterparts, respectively. Compared with linear PFASs, their branched isomers were found to exhibit a higher adsorption potential on MIL-101(Cr). In addition, the introduction of ether bond into PFAS molecule (e.g., hexafluoropropylene oxide dimeric acid, GenX) increased the adsorption capacity, while the replacement of CF2 moieties in PFAS molecule with CH2 moieties (e.g., 6:2 fluorotelomer sulfonate, 6:2 FTS) caused a decrease in adsorption. Divalent ions (such as Ca2+ and SO42-) and solution pH have a greater effect on the adsorption of PFASs containing ether bonds or more CF2 moieties. PFAS adsorption on MIL-101(Cr) was governed by electrostatic interaction, complexation, hydrogen bonding, π-CF interaction, and π-anion interaction as well as steric effects, which were associated with the molecular electronegativity and chain length of each PFAS. The average electronegativity of individual moieties (named Me) for each PFAS was estimated and found to show a significantly positive correlation with the corresponding adsorption capacity on MIL-101(Cr). The removal rates of major PFASs in contaminated groundwater by MIL-101(Cr) were also correlated with the corresponding Me values. These findings will assist with the adsorption prediction for a wide range of PFASs and contribute to tailoring efficient MOF materials.

Innovations in metal-organic frameworks (MOFs): Pioneering adsorption approaches for persistent organic pollutant (POP) removal

Adsorption is a promising way to remove persistent organic pollutants (POPs), a major environmental issue. With their high porosity and vast surface areas, MOFs are suited for POP removal due to their excellent adsorption capabilities. This review addresses the intricate principles of MOF-mediated adsorption and helps to future attempts to mitigate organic water pollution. This review examines the complicated concepts of MOF-mediated adsorption, including MOF synthesis methodologies, adsorption mechanisms, and material tunability and adaptability. MOFs' ability to adsorb POPs via electrostatic forces, acid-base interactions, hydrogen bonds, and pi-pi interactions is elaborated. This review demonstrates its versatility in eliminating many types of contaminants. Functionalizing, adding metal nanoparticles, or changing MOFs after they are created can improve their performance and remove contaminants. This paper also discusses MOF-based pollutant removal issues and future prospects, including adsorption capacity, selectivity, scale-up for practical application, stability, and recovery. These obstacles can be overcome by rationally designing MOFs, developing composite materials, and improving material production and characterization. Overall, MOF technology research and innovation hold considerable promise for environmental pollution solutions and sustainable remediation. Desorption and regeneration in MOFs are also included in the review, along with methods for improving pollutant removal efficiency and sustainability. Case studies of effective MOF regeneration and scaling up for practical deployment are discussed, along with future ideas for addressing these hurdles.

Magnetic surfactant-modified clay for enhanced adsorption of mixtures of per- and polyfluoroalkyl substances (PFAS) in snowmelt: Improving practical applicability and efficiency

The extensive use of per- and polyfluoroalkyl substances (PFAS) in many industrial and consumer contexts, along with their persistent nature and possible health hazards, has led to their recognition as a prevalent environmental issue. While various PFAS removal methods exist, adsorption remains a promising, cost-effective approach. This study evaluated the PFAS adsorption performance of a surfactant-modified clay by comparing it with commercial clay-based adsorbents. Furthermore, the impact of environmental factors, including pH, ionic strength, and natural organic matter, on PFAS adsorption by the modified clay (MC) was evaluated. After proving that the MC was regenerable and reusable, magnetic modified clay (MMC) was synthesized, characterized, and tested for removing a wide range of PFAS in pure water and snowmelt. The MMC was found to have similar adsorption performance as the MC and was able to remove > 90% of the PFAS spiked to the snowmelt. The superior and much better performance of the MMC than powdered activated carbon points to its potential use in removing PFAS from real water matrices at an industrial scale.

Regeneration of exhausted adsorbents after PFAS adsorption: A critical review

The adsorption process efficiently removes per- and polyfluoroalkyl substances (PFAS) from water, but managing exhausted adsorbents presents notable environmental and economic challenges. Conventional disposal methods, such as incineration, may reintroduce PFAS into the environment. Therefore, advanced regeneration techniques are imperative to prevent leaching during disposal and enhance sustainability and cost-effectiveness. This review critically evaluates thermal and chemical regeneration approaches for PFAS-laden adsorbents, elucidating their operational mechanisms, the influence of water quality parameters, and their inherent advantages and limitations. Thermal regeneration achieves notable desorption efficiencies, reaching up to 99% for activated carbon. However, it requires significant energy input and risks compromising the adsorbent's structural integrity, resulting in considerable mass loss (10-20%). In contrast, chemical regeneration presents a diverse efficiency landscape across different regenerants, including water, acidic/basic, salt, solvent, and multi-component solutions. Multi-component solutions demonstrate superior efficiency (>90%) compared to solvent-based solutions (12.50%), which, in turn, outperform salt (2.34%), acidic/basic (1.17%), and water (0.40%) regenerants. This hierarchical effectiveness underscores the nuanced nature of chemical regeneration, significantly influenced by factors such as regenerant composition, the molecular structure of PFAS, and the presence of organic co-contaminants. Exploring the conditional efficacy of thermal and chemical regeneration methods underscores the imperative of strategic selection based on specific types of PFAS and material properties. By emphasizing the limitations and potential of particular regeneration schemes and advocating for future research directions, such as exploring persulfate activation treatments, this review aims to catalyze the development of more effective regeneration processes. The ultimate goal is to ensure water quality and public health protection through environmentally sound solutions for PFAS remediation efforts.

Effect of ligand interactions within modified granular activated carbon (GAC) on mixed perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) adsorption

A new adsorbent based on commercial granular activated carbon (GAC) and loaded with Cu(II) (GAC-Cu) was prepared to enhance the adsorption capacity of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS). The surface area (SA) and pore volume of GAC-Cu decreased by ∼15% compared to those of pristine GAC. The scanning electron microscopy-energy dispersive spectrometry (SEM-EDS) and leaching test results indicated that, compared with GAC, the Cu atomic ratio and Cu amount in GAC-Cu increased by 2.91 and 2.43 times, respectively. The point of zero charge (PZC) measured using a salt addition method obtained a pH of 6.0 (GAC) and 5.0 (GAC-Cu). According to the isotherm models obtaining highest coefficient of determination (R2), GAC-Cu exhibited a 20.4% and 35.2% increase for PFOA and PFOS in maximum uptake (qm), respectively, compared to those of GAC. In addition, the adsorption affinity (b) for GAC-Cu increased by 1045% and 175% for PFOA and PFOS, respectively. The pH effect on the adsorption capacity of GAC-Cu was investigated. The uptake of PFOA and PFOS decreased with an increase in pH for both GAC and GAC-Cu. GAC-Cu exhibited higher uptake than GAC at pH 6 and 7, but no enhanced uptake was observed at pH 4.0, 5.0, and 8.5. Therefore, ligand interaction was effective at weak acid or neutral pH.

Activation of sawdust biochar with water and wastewater treatment residuals for sorption of perfluorooctanesulfonic acid in water

Recent research has found biochar to be a cost-effective adsorbent for removal of perfluoroalkyl substances in water. To promote cleaner production and sustainable waste management, this study explored the potential to produce activated biochars by co-pyrolyzing sawdust with iron-rich biosolids and polyaluminum sludge. The maximum capacity to adsorb perfluorooctanesulfonic acid (PFOS) reached 27.2 mg g-1 with biosolids-activated biochar and 19.2 mg g-1 with aluminum sludge-activated biochar, compared to 6.2 mg g-1 with sawdust biochar. The increased adsorption capacities were attributed to electrostatic interactions between the anionic PFOS and metal functionalities on the biochar surface. In contrast, hydrophobic interaction was the dominant adsorption mechanism of sawdust biochar. The presence of dissolved organic matter at 5-50 mg L-1 was found to inhibit adsorption of PFOS in water, while pH as low as 3.0 and sodium chloride concentrations up to 100 mM enhanced removal of PFOS by all the three adsorbents. In batch adsorption tests at environmentally relevant PFOS dosages and adsorbent dosage of 0.25 g L-1, the biosolids-sawdust biochar and Al sludge-sawdust biochar removed 71.4% and 66.9% of PFOS from drinking water and 77.9% and 87.9% of PFOS from filtrate of sludge digestate, respectively. The biosolids-sawdust biochar additionally removed Fe, although the Al sludge-sawdust biochar released Al into the alkaline drinking water and filtrate. Overall, this study proved co-pyrolyzing sawdust and Fe-rich biosolids to be an effective approach to activate sawdust biochar for enhanced removal of PFOS while recycling wastewater treatment residuals and sawdust.

Treatment of aqueous per- and poly-fluoroalkyl substances: A review of biochar adsorbent preparation methods

Per- and poly-fluoroalkyl substances (PFAS) are synthetic chemicals widely used in everyday products, causing elevated concentrations in drinking water and posing a global challenge. While adsorption methods are commonly employed for PFAS removal, the substantial cost and environmental footprint of commercial adsorbents highlight the need for more cost-effective alternatives. Additionally, existing adsorbents exhibit limited effectiveness, particularly against diverse PFAS types, such as short-chain PFAS, necessitating modifications to enhance adsorption capacity. Biochar can be considered a cost-effective and eco-friendly alternative to conventional adsorbents. With abundant feedstocks and favorable physicochemical properties, biochar shows significant potential to be applied as an adsorbent for removing contaminants from water. Despite its effectiveness in adsorbing different inorganic and organic contaminants from water environments, some factors restrict its effective application for PFAS adsorption. These factors are related to the biochar properties, and characteristics of PFAS, as well as water chemistry. Therefore, some modifications have been introduced to overcome these limitations and improve biochar's adsorption capacity. This review explores the preparation conditions, including the pyrolysis process, activation, and modification techniques applied to biochar to enhance its adsorption capacity for different types of PFAS. It addresses critical questions about the adsorption performance of biochar and its composites, mechanisms governing PFAS adsorption, challenges, and future perspectives in this field. The surge in research on biochar for PFAS adsorption indicates a growing interest, making this timely review a valuable resource for future research and an in-depth exploration of biochar's potential in PFAS remediation.

Transformative Dynamics: Self-Assembly of Iron Oxide Hydroxide Nanorods into Iron Oxide Microcubes for Enhanced Perfluoroalkyl Substance Remediation

We report the controlled synthesis of iron oxide microcubes (IOMCs) through the self-assembly arrays of ferric oxide hydroxide nanorods (NRs). The formation of IOMCs involves a complex interplay of nucleation, self-assembly, and growth mechanisms influenced by time, thermal treatment, and surfactant dynamics. The self-assembly of vertically aligned NRs into IOMCs is controlled by dynamic magnetism properties and capping agents like cetyltrimethylammonium bromide (CTAB), whose concentration and temperature modulation dictate growth kinetics and structural uniformity. These controlled structural growths were obtained via a hydrothermal process at 120 °C at various intervals of 8, 16, 24, and 32 h in the presence of CTAB as the capping agent. In this hydrothermal method, the formation of vertically oriented NR arrays was observed without the presence of ligands, binders, harsh drying techniques, and solvent evaporation. The formation of the self-assembly of NRs to IOMCs is obtained with an increase in saturated magnetization to attain the most stable state. The synthesized IOMCs have a uniform size, quasi-shape, and excellent dispersion. Due to its excellent magnetic and catalytic properties, IOMCs were employed to remove the various emerging pollutants known as per- and polyfluorinated substances (PFAS). Various microscopic and spectroscopic techniques were employed for the characterization and interaction studies of IOMCs with various PFAS. The interaction between IOMCs and perfluoroalkyl substances (PFAS) was investigated, revealing strong adsorption tendencies facilitated by electrostatic interactions, as evidenced by UV-vis and FT-IR spectroscopic studies. Furthermore, the higher magnetic and positive surface charge of IOMCs is responsible for an effective remediation eliminating any secondary pollution with ease of recovery after the sorption interaction studies, thereby making it practically worthwhile.

Development of ionic-liquid-impregnated activated carbon for sorptive removal of PFAS in drinking water treatment

Adsorption of per- and poly-fluoroalkyl substances (PFAS) on activated carbon (AC) is considerably hindered by the surface water constituents, degrading the ability of the AC adsorption process to remove PFAS in drinking water treatment. Herein, we developed ionic-liquid-impregnated AC (IL/AC) as an alternative to AC for PFAS sorption and demonstrated its performance with real surface water for the first time. Ionic liquids (ILs) of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (IL(C2)) and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (IL(C6)) were selected from among 272 different ILs using the conductor-like screening model for realistic solvents (COSMO-RS) simulation. Impregnation of the ILs in AC was verified using various analytical techniques. Although the synthesized IL/ACs were less effective than pristine AC in treating PFAS in deionized water, their performances were less impacted by the surface water constituents, resulting in comparable or sometimes better performances than pristine AC for treating PFAS in surface water. The removal efficiencies of 10 wt% IL(C6)/AC for six PFAS were 1.40-1.96 times higher than those of pristine AC in a surface water sample containing 2.6 mg/L dissolved organic carbon and millimolar-level divalent cation concentration. PFAS partitioning from the surface water to ILs was not hindered by dissolved organic matter and was enhanced by the divalent cations, indicating the advantages of IL/ACs for treating significant amounts of PFAS in water. The synthesized IL/ACs were effective at treating coexisting pharmaceutical and personal-care products in surface water, showcasing their versatility for treating a broad range of water micropollutants.

PFAS promotes disinfection byproduct formation through triggering particle-bound organic matter release in drinking water pipes

Loose deposit particles in drinking water distribution system commonly exist as mixtures of metal oxides, organic materials, bacteria, and extracellular secretions. In addition to their turbidity-causing effects, the hazards of such particles in drinking water are rarely recognized. In this study, we found that trace per- and polyfluoroalkyl substances (PFASs) could dramatically promote the formation of disinfection byproducts (DBPs) by triggering the release of particle-bound organic matter. Carboxylic PFASs have a greater ability to increase chloroacetic acid than sulfonic PFASs, and PFASs with longer chains have a greater ability to increase trichloromethane release than shorter-chain PFASs. Characterization by organic carbon and organic nitrogen detectors and Fourier transform ion cyclotron resonance mass spectrometry revealed that the released organic matter was mainly composed of proteins, carbohydrates, lignin, and condensed aromatic structures, which are the main precursors for the formation of DBPs, particularly highly toxic aromatic DBPs. After the release of organic matter, the particles exhibit a decrease in surface functional groups, an increase in surface roughness, and a decrease in particle size. The findings provide new insights into the risks of loose deposits and PFASs in drinking water, not only on PFASs per se but also on its effect of increasing toxic DBPs.

Adsorption of perfluoroalkyl substances on deep eutectic solvent-based amorphous metal-organic framework: Structure and mechanism

Perfluoroalkyl substances (PFASs) are a class of emerging organic pollutants characterized by high toxicity, environmental persistence, and widespread detection in water sources. The removal of PFASs from water is a matter of global concern, given their detrimental impact on both the environment and public health. Many commonly used PFAS adsorbents demonstrate limited adsorption capacities and/or slow adsorption kinetics. Therefore, there is an urgent need for the development of efficient adsorbents. For the first time, this work systematically investigated the performance of a deep eutectic solvent (DES)-based amorphous metal-organic framework (MOF) for the adsorption of PFASs with different carbon-chain lengths under the state of the mixture in aquatic environments. The adsorption mechanism was probed by a suite of adsorption kinetics studies, adsorption isotherm profiling, spectral characterization, and ab initio molecular dynamics (AIMD) simulations, revealing that PFAS adsorption is driven by synergistic capturing effects including acid/base coordination, CF-π (carbon-fluorine-π), hydrogen bonding, and hydrophobic interactions. Furthermore, the adsorption processes of short-chain and long-chain targets were found to involve different rate-controlling steps and interaction sites. Hydrophobic interactions facilitated the swift arrival of long-chain PFASs at the coordinatively interacting sites between carboxyl termini and Lewis acid Zr unsaturated sites, thanks to their lower reaction barriers. On the other hand, the adsorption of short-chain PFASs primarily relied on a Zr hydroxyl-based ligand exchange force, which would take place at Brønsted acid sites. The existence of massive structural disorder in amorphous UiO-66 led to the development of larger pores, thus improving the accessibility of abundant adsorption sites and facilitating adsorption and diffusion. The presence of multiple types of interactions and flexible structure in defect-rich amorphous UiO-66 significantly increased the exposure of functional groups to the adsorbates. Additionally, this material possessed outstanding regeneration efficiency and outperformed other MOF-based adsorbents with high affinity for targets. It enhances our understanding of the adsorption performances and mechanisms of amorphous materials toward PFASs, thereby paving the way for designing more efficient PFAS adsorbents.

Adsorption and leaching of fluorotelomer compounds and perfluoroalkyl acids in aqueous media by activated carbon prepared from municipal biosolids

Perfluoroalkyl acids (PFAAs) are ubiquitous in nature and pose serious health risks to humans and animals. Limiting PFAA exposure requires novel technology for their effective removal from water. We investigated the efficacy of biosolid-based activated carbon (Bio-SBAC) in removing frequently detected PFAAs and their precursor fluorotelomer compounds at environmentally relevant concentrations (∼50 μg/L). Batch experiments were performed to investigate adsorption kinetics, isotherms, and leachability. Bio-SBAC achieved >95% removal of fluorotelomeric compounds, indicating that the need for PFAA removal from the environment could be minimised if the precursors were targeted. Kinetic data modelling suggested that chemisorption is the dominant PFAA adsorption mechanism. As evidenced by the isotherm modelling results, Freundlich adsorption intensity, n-1, values of <1 (0.707-0.938) indicate chemisorption. Bio-SBAC showed maximum capacities for the adsorption of perfluorooctanoic acid (1429 μg/g) and perfluorononanoic acid (1111 μg/g). Batch desorption tests with 100 mg/L humic acid and 10 g/L NaCl showed that Bio-SBAC effectively retained the adsorbed PFAA with little or no leaching, except perfluorobutanoic acid. Overall, this study revealed that Bio-SBAC is a value-added material with promising characteristics for PFAA adsorption and no leachability. Additionally, it can be incorporated into biofilters to remove PFAAs from stormwater, presenting a sustainable approach to minimise biosolid disposal and improve the quality of wastewater before discharge into receiving waters.

PFAS remediation in soil: An evaluation of carbon-based materials for contaminant sequestration

The presence of per- and poly-fluoroalkyl substances (PFAS) in soils is a global concern as these emerging contaminants are highly resistant to degradation and cause adverse effects on human and environmental health at very low concentrations. Sequestering PFAS in soils using carbon-based materials is a low-cost and effective strategy to minimize pollutant bioavailability and exposure, and may offer potential long-term remediation of PFAS in the environment. This paper provides a comprehensive evaluation of current insights on sequestration of PFAS in soil using carbon-based sorbents. Hydrophobic effects originating from fluorinated carbon (C-F) backbone "tail" and electrostatic interactions deriving from functional groups on the molecules' "head" are the two driving forces governing PFAS sorption. Consequently, varying C-F chain lengths and polar functional groups significantly alter PFAS availability and leachability. Furthermore, matrix parameters such as soil organic matter, inorganic minerals, and pH significantly impact PFAS sequestration by sorbent amendments. Materials such as activated carbon, biochar, carbon nanotubes, and their composites are the primary C-based materials used for PFAS adsorption. Importantly, modifying the carbon structural and surface chemistry is essential for increasing the active sorption sites and for strengthening interactions with PFAS. This review evaluates current literature, identifies knowledge gaps in current remediation technologies and addresses future strategies on the sequestration of PFAS in contaminated soil using sustainable novel C-based sorbents.

Progress and perspectives on carbon-based materials for adsorptive removal and photocatalytic degradation of perfluoroalkyl and polyfluoroalkyl substances (PFAS)

Per- and polyfluoroalkyl substances (PFAS) (also known as 'forever chemicals') have emerged as trace pollutants of global concern, attributing to their persistent and bio-accumulative nature, pervasive distribution, and adverse public health and environmental impacts. The unregulated discharge of PFAS into aquatic environments represents a prominent threat to the wellbeing of humans and marine biota, thereby exhorting unprecedented action to tackle PFAS contamination. Indeed, several noteworthy technologies intending to remove PFAS from environmental compartments have been intensively evaluated in recent years. Amongst them, adsorption and photocatalysis demonstrate remarkable ability to eliminate PFAS from different water matrices. In particular, carbon-based materials, because of their diverse structures and many exciting properties, offer bountiful opportunities as both adsorbent and photocatalyst, for the efficient abatement of PFAS. This review, therefore, presents a comprehensive summary of the diverse array of carbonaceous materials, including biochar, activated carbon, carbon nanotubes, and graphene, that can serve as ideal candidates in adsorptive and photocatalytic treatment of PFAS contaminated water. Specifically, the efficacy of carbon-mediated PFAS removal via adsorption and photocatalysis is summarised, together with a cognizance of the factors influencing the treatment efficiency. The review further highlights the neoteric development on the novel innovative approach 'concentrate and degrade' that integrates selective adsorption of trace concentrations of PFAS onto photoactive surface sites, with enhanced catalytic activity. This technique is way more energy efficient than conventional energy-intensive photocatalysis. Finally, the review speculates the cardinal challenges associated with the practical utility of carbon-based materials, including their scalability and economic feasibility, for eliminating exceptionally stable PFAS from water matrices.

Recent advances in the application of magnetic materials for the management of perfluoroalkyl substances in aqueous phases

Perfluoroalkyl substances (PFASs) are a class of artificially synthesised organic compounds extensively used in both industrial and consumer products owing to their unique characteristics. However, their persistence in the environment and potential risk to health have raised serious global concerns. Therefore, developing effective techniques to identify, eliminate, and degrade these pollutants in water are crucial. Owing to their high surface area, magnetic responsiveness, redox sensitivity, and ease of separation, magnetic materials have been considered for the treatment of PFASs from water in recent years. This review provides a comprehensive overview of the recent use of magnetic materials for the detection, removal, and degradation of PFASs in aqueous solutions. First, the use of magnetic materials for sensitive and precise detection of PFASs is addressed. Second, the adsorption of PFASs using magnetic materials is discussed. Several magnetic materials, including iron oxides, ferrites, and magnetic carbon composites, have been explored as efficient adsorbents for PFASs removal from water. Surface modification, functionalization, and composite fabrication have been employed to improve the adsorption effectiveness and selectivity of magnetic materials for PFASs. The final section of this review focuses on the advanced oxidation for PFASs using magnetic materials. This review suggests that magnetic materials have demonstrated considerable potential for use in various environmental remediation applications, as well as in the treatment of PFASs-contaminated water.

Removal of perfluoroalkyl acids from aqueous media by surfactant-modified clinoptilolites

Perfluoroalkyl and polyfluoroalkyl substances (PFASs) are environmentally persistent, bioaccumulating, and toxic compounds that have attracted global attention. It is challenging to reduce the residual concentrations of these compounds to safe discharge limits. In this study, batch experiments were performed to evaluate natural clinoptilolite and clinoptilolites modified (MC) with cetylpyridinium chloride (CPC-MC), didodecyldimethylammonium bromide (DDAB-MC), hexadecyltrimethylammonium bromide (HDTMA-MC), and tetramethylammonium chloride (TMA-MC) as cost-effective aqueous PFAS adsorbents. The removal capacities of the adsorbents for the majority of the PFASs decreased in the following order: DDAB-MC > CPC-MC ≫ modified natural clinoptilolite with hexadecyltrimethyl ammonium bromide (HDTMA-MC) ≫ modified natural clinoptilolite with tetramethylammonium chloride (TMA-MC) ≈ natural clinoptilolite modified with NaCl (NC). In particular, CPC-MC and DDAB-MC reduced PFASs concentration in 50 μg/L by up to 98% for perfluorooctane sulphonate. Within 30 min, CPC-MC (30.5 μg/L) and DDAB-MC (32.1 μg/L) met the PFOS water quality criterion of 36 μg/L in inland surface waters. Both adsorbents met this criterion at the highest solution volume (40 mL) and 0.125 g/L (solid-to-liquid ratio of 1:8). PFASs with short hydrocarbon chains competed more for adsorption. PFASs with sulphonate functional groups were also adsorbed more than carboxyl groups in single- and multi-PFAS solutions. The modified surfaces of clinoptilolites controlled PFAS adsorption through hydrophobic and electrostatic interactions. PFAS removal with surfactant-modified clinoptilolites is cost-effective and protects aquatic environments by using surplus natural materials.

Substitution and Orientation Effects on the Crystallinity and PFAS Adsorption of Olefin-Linked 2D COFs

Solid phase adsorbents with high removal affinity for per- and polyfluoroalkyl substances (PFAS) in aqueous environments are sought. We report the synthesis and investigation of COF-I, a new covalent organic framework (COF) with a good affinity for PFAS adsorption. COF-I was synthesized by the condensation reaction between 2,4,6-trimethyl-1,3,5-triazine and 2,3-dimethoxyterephthaldehyde and fully characterized. In addition to the high crystallinity and surface area, COF-I showed high hydrolytic and thermal stability. Further, we converted its hydrophobic surface to a hydrophilic surface by converting the ortho-methoxy groups to hydroxyl derivatives and produced a new hydrophilic olefin-linked two-dimensional (2D) COF. We experimentally measured the crystallinity of both COFs by X-ray diffraction and used atomistic simulations coupled with cross-polarization/magic angle spinning solid-state nuclear magnetic resonance (CP/MAS ssNMR) to determine the relative amounts of AA-stacking and AB-stacking present. COF-I, with its hydrophobic surface and methoxy groups in the ortho positions, showed the best PFAS adsorption. COF-I reduced the concentration of perfluorooctanoic acid from 20 to 0.069 μg L-1 and to 0.052 μg L-1 for perfluorooctanesulfonic acid. These amounts are lower than the U.S. Environmental Protection Agency advisory level (0.070 μg L-1). High efficiency, fast kinetic adsorption, and reusability of COF-I are advantages of COF-I for PFAS removal from water.

Sorption of per- and poly-fluoroalkyl substances and their precursors on activated carbon under realistic drinking water conditions: Insights into sorbent variability and PFAS structural effects

Recent stringent drinking water quality standards create challenges for water utilities to meet these standards. Advanced treatment techniques will have to be applied on many drinking water production locations to meet these quality standards. This study investigated the sorption of per- and polyfluorinated-alkyl substances (PFAS) onto granular activated carbon (GAC). The study was performed at environmentally relevant PFAS concentrations and a realistic water-to-GAC ratio, providing a realism often overlooked in existing studies. Three different forms of GAC were evaluated, differing in micropore and mesopore structures. Tap water spiked with 5 ng/L of each of 31 PFAS was used in the sorption experiments, i.e. perfluorocarboxylic acids (C4-C12), perfluorosulfonic acids (PFSA, C5-C10) including linear and branched isomers, and three groups of PFAS precursors (per-/polyfluoroalkyl ether acids, sulfonamides, and sulfonamide acetic acids). The three studied GAC did not exhibit distinct differences in PFAS sorption. The removal of PFAS was below 50 % for most studied PFAS, except for the short-chain PFAS precursors. Sorption was affected by both the carbon chain length and functional groups for PFAS, while this was not observed for PFAS precursors. The presence of ether linkages and sulfonamide groups notably enhanced sorption. Linear and branched PFSA demonstrated similar sorption behavior, whereas branched isomers of the sulfonamide acetic acid precursors exhibited significantly higher sorption. This indicates that sorption was determined by both hydrophobic and electrostatic interactions. Given the relatively low PFAS removal by GAC under environmentally relevant test conditions, further improvements in sorbents are required to ensure that PFAS concentrations in produced drinking water comply with drinking water standards.

Assessing construction and demolition wood-derived biochar for in-situ per- and polyfluoroalkyl substance (PFAS) removal from landfill leachate

With regulations for per-and polyfluoroalkyl substances (PFAS) impending, the abundance of these chemicals of emerging concern in municipal solid waste (MSW) landfill leachate increasingly challenges landfill operators to seek on-site leachate pre-treatment options. This two-staged study explores the potential reuse of biochar derived from construction and demolition debris (CDD) wood as an in-situ PFAS sorbent for application within MSW landfill leachate collection systems. Batch leaching tests were first used to examine the feasibility of capturing PFAS from landfill leachate using two sources of CDD-wood-derived biochar. Then, columns were used to test the in-situ sorption capabilities of the same biochars under simulated landfill conditions. All leachates were characterized for pH, chemical oxygen demand, ammonia-nitrogen, and 92 PFAS. Seventeen PFAS were detected in the batch leaching experiment, and nine PFAS were detected in column leachates. In the batch leaching scenario, Biochar 1 achieved a maximum of 29% PFAS reduction compared to controls. Columns containing Biochar 1 generated leachates with PFAS concentrations 50% to 80% higher than those in control columns for the duration of the experiment. Columns containing Biochar 2 generated leachates with PFAS concentrations 44% less than controls in week 1 and similar concentrations in weeks 2, 3, and 4. In this study, PFAS removal from landfill leachate using biochar derived from CDD wood was not significant. Further research on biochar derived from CDD wood is needed before it can be recommended as an in-situ landfill leachate pre-treatment method.

Metal-Organic Framework-Based Composites for the Adsorption Removal of Per- and Polyfluoroalkyl Substances from Water

The increasing health risks posed by per- and polyfluoroalkyl substances (PFASs) in the environment highlight the importance of implementing effective removal techniques. Conventional wastewater treatment processes are inadequate for removing persistent organic pollutants. Recent studies have increasingly demonstrated that metal-organic frameworks (MOFs) are capable of removing PFASs from water through adsorption techniques. However, there is still constructive discussion on the potential of MOFs in adsorbing and removing PFASs for large-scale engineering applications. This review systematically investigates the use of MOFs as adsorbents for the removal of PFAS in water treatment. This primarily involved a comprehensive analysis of existing literature to understand the adsorption mechanisms of MOFs and to identify factors that enhance their efficiency in removing PFASs. We also explore the critical aspects of regeneration and stability of MOFs, assessing their reusability and long-term performance, which are essential for large-scale water treatment applications. Finally, our study highlights the challenges of removing PFASs using MOFs. Especially, the efficient removal of short-chain PFASs with hydrophilicity is a major challenge, while medium- to long-chain PFASs are frequently susceptible to being captured from water by MOFs through multiple synergistic effects. The ion-exchange force may be the key to solving this difficulty, but its susceptibility to ion interference in water needs to be addressed in practical applications. We hope that this review can provide valuable insights into the effective removal and adsorption mechanisms of PFASs as well as advance the sustainable utilization of MOFs in the field of water treatment, thereby presenting a novel perspective.

Adsorption of per- and polyfluoroalkyl substances (PFAS) from water with porous organic polymers

Per- and polyfluoroalkyl substances (PFAS) are man-made environmental contaminants causing increasing global concern due to their adverse effect on environmental and human health. Conventional treatment methods are ineffective in removing short-chain PFAS because they are persistent and recalcitrant to treatment. This study evaluated the performance of a structurally-tunable and chemically-stable porous organic polymer (POP) for PFAS removal under realistic environmental conditions. The POP demonstrated an exceptionally high removal efficiency (>95%) within 15 min when the initial PFAS concentration was approximately 400 ng/L. The adsorption of PFAS on the POP was not significantly affected by changes in solution pH within the range of 5-9. The common co-contaminants in water competed with short-chain PFAS for active sites during the adsorption process following the order of natural organic matter (NOM), long-chain PFAS, and Cl-. The Freundlich-type model could predict the multicomponent interactions well with a R2 value above 0.91. The spent POP was effectively regenerated using a mixture of the 10% NaCl and 30% methanol solution and the PFAS removal maintained at 90% through five adsorption and desorption cycles. The characteristics of the designed POP make it a highly promising and stable absorbent. It enables fast and effective removal of short-chain PFAS.

Graphene Oxide and Its Derivatives as Adsorbents for PFOA Molecules

Effective, low-cost adsorbents are needed to remove perfluoroalkyl and polyfluoroalkyl substances (PFAS) from water sources. Carbon-based materials are promising PFAS adsorbents. Here, we explore the potential of graphite oxide (GO) and its derivatives as PFAS adsorbents by studying the adsorption of perfluorooctanoic acid (PFOA), a model PFAS molecule, on GO surfaces with O/C ratios up to 16.7% using molecular dynamics simulations. An adsorption free energy of approximately -30 kJ/mol (or -310 meV) is obtained for pristine graphene in pure water, and adsorbed PFOA molecules diffuse rapidly. As the O/C ratio increases, hydrophobic interactions' contribution to PFOA adsorption diminishes, but that by electrostatic interactions becomes important. Overall, adsorption is weakened, but favorable adsorption still occurs at an O/C ratio of 16.7%. The in-plane diffusion coefficient of adsorbed PFOA molecules decreases by more than 45 times as the O/C ratio increases to 8.3% but increases significantly when the O/C ratio increases further to 16.7%. Adding salt improves the adsorption owing to the salting-out and screening effects but slows the diffusion of adsorbed PFOA molecules, and these effects are more pronounced at low O/C ratios. These results show that GOs are effective PFOA adsorbents. Such effectiveness, along with GO's potentially low cost and the possibility of regenerating spent GO by removing adsorbed PFOA molecules through a mild electrical potential, makes GO a promising adsorbent for PFOA and similar molecules. The insights from the present study can help the rational design of GOs to realize their full potential.

Environmental Potential of Carbonized MOF-5/PANI Composites for Pesticide, Dye, and Metal Cations-Can They Actually Retain Them All?

The environmental application of the carbonized composites of the Zn-containing metal-organic framework MOF-5 and polyaniline (PANI) in its emeraldine salt and base forms (C-(MOF-5/PANI)) was investigated for the first time. Textural properties and particle size distributions revealed that composites are dominantly mesoporous and nanoscale in nature, while Raman spectroscopy revealed the ZnO phase beneath the carbon matrix. Adsorption of pesticide, dye, and metal cation on C-(MOF-5/PANI) composites in aqueous solutions was evaluated and compared with the behavior of the precursor components, carbonized MOF-5 (cMOF), and carbonized PANIs. A lower MOF-5 content in the precursor, a higher specific surface area, and the pore volume of the composites led to improved adsorption performance for acetamiprid (124 mg/g) and Methylene Blue (135 mg/g). The presence of O/N functional groups in composites is essential for the adsorption of nitrogen-rich pollutants through hydrogen bonding with an estimated monolayer capacity twice as high as that of cMOF. The proton exchange accompanying Cd2+ retention was associated with the Zn/Cd ion exchange, and the highest capacity (9.8 mg/g) was observed for the composite synthesized from the precursor with a high MOF-5 content. The multifunctionality of composites was evidenced in mixtures of pollutants where noticeably better performance for Cd2+ removal was found for the composite compared to cMOF. Competitive binding between three pollutants favored the adsorption of pesticide and dye, thereby hindering to some extent the ion exchange necessary for the removal of metal cations. The results emphasize the importance of the PANI form and MOF-5/PANI weight ratio in precursors for the development of surface, porosity, and active sites in C-(MOF-5/PANI) composites, thus guiding their environmental efficiency. The study also demonstrated that C-(MOF-5/PANI) composites retained studied pollutants much better than carbonized precursor PANIs and showed comparable or better adsorption ability than cMOF.

Magnetic Fe3O4/bamboo-based activated carbon/UiO-66 composite as an environmentally friendly and effective adsorbent for removal of Bisphenol A

The magnetic Fe3O4/bamboo-based activated carbon/Zr-based metal-organic frameworks composite (Fe3O4/BAC/UiO-66) was prepared by hydrothermal method. The as-prepared material was analyzed via TEM, XRD, FT-IR, BET-BJH, VSM and XPS techniques, the results showed that it had good dispersion and magnetic separation capacity (Ms = 44.06 emu∙g-1). Then, the adsorption properties of materials for bisphenol A (BPA) were studied. The results revealed that the removal efficiency of 50 mg·L-1 BPA by 0.1 g of adsorbent can reach 87.18-95% in a wide pH range. Langmuir isotherm model and pseudo-second-order kinetic well fitted the adsorption data. The thermodynamic data indicated that the adsorption process was spontaneous and endothermic. Moreover, BAC as a supporter and UiO-66 as the functional part in the ternary composite may have a synergistic effect, which was beneficial for the removal of contaminants. The Fe3O4/BAC/UiO-66 can be simply separated from the water using its strong magnetism after finish adsorption process, which effectively avoids secondary contamination.

Revealing the factors resulting in incomplete recovery of perfluoroalkyl acids (PFAAs) when implementing the adsorbable and extractable organic fluorine methods

Per- and polyfluoroalkyl substances (PFASs) are environmental contaminants of concern. Techniques that quantify total organic fluorine (TOF) such as the adsorbable organic fluorine (AOF) and extractable organic fluorine (EOF) methods are important for PFAS risk assessments. The objective of this study was to systematically evaluate each step of the AOF (loading, washing, combustion) and EOF (loading, washing, elution, combustion) methods for the recovery of ten ultrashort-, short-, and long-chain unsubstituted perfluoroalkyl acids (PFAAs). We measured the overall recovery of fluoride for each method for each PFAA, and the recovery of each PFAA around the loading, washing, and elution steps. We also measured the combustion efficiency of each PFAA by direct combustion. The overall AOF and EOF recovery ranged from 9.3%-103.3% to 21.0%-108.1%, respectively, with higher recoveries measured for PFAAs with increasing chain length in both methods. The three ultrashort-chain PFAAs (trifluoroacetic acid, perfluoropropionic acid, and perfluoropropanesulfonic acid) exhibited the lowest overall recoveries from 9.3-25.2% for AOF and 21.0-51.5% for EOF. We found that decreases in the overall recovery are the result of losses of ultrashort- and short-chain PFAAs during the washing step and the incomplete mineralization of perfluoroalkyl sulfonic acids during combustion for AOF and incomplete elution of short- and long-chain PFAAs and the loss of ultrashort-chain PFAAs during the washing step for EOF. Our data suggest that the EOF method is more appropriate than the AOF method for measuring TOF in samples containing ultrashort- and short-chain PFAAs and that methodological improvements are possible with a focus on the washing, elution, and combustion steps.

Toward systematic understanding of adsorptive removal of legacy and emerging per-and polyfluoroalkyl substances (PFASs) by various activated carbons (ACs)

Per-and polyfluoroalkyl substances (PFASs) have received great attention due to their persistence, bioaccumulation and toxicity. Various activated carbons (ACs) exhibit wide variability in adsorptive performance towards PFASs. In order to gain a systematic understanding of adsorptive removal of legacy and emerging PFASs by ACs, the adsorption of ten PFASs on various ACs was comprehensively investigated. Results showed that granular activated carbon-1 (GAC-1) and powdered activated carbon-1 (PAC-1) removed more than 90% of all target PFASs. Particle size, surface charge, and micropores quantity of ACs were closely related to their performance for PFASs removal. Electrostatic interaction, hydrophobic interaction, surface complexation and hydrogen bonding were the adsorption mechanisms, with hydrophobic interaction being the predominant adsorptive force. Physical and chemical adsorption were both involved in PFAS adsorption. The removal rates of PFASs by GAC-1 decreased from 93%-100% to 15%-66% in the presence of 5 mg/L fulvic acid (FA). GAC was able to remove more PFASs under acidic medium, whereas PAC removed hydrophobic PFASs better under the neutral medium. The removal rates of PFASs by GAC-3 increased significantly from 0%-21% to 52%-97% after being impregnated with benzalkonium chlorides (BACs), demonstrating the superiority of this modification method. Overall, this study provided theoretical support for removing PFASs from water phase with ACs.

An overview on human exposure, toxicity, solid-phase microextraction and adsorptive removal of perfluoroalkyl carboxylic acids (PFCAs) from water matrices

Perfluoroalkyl carboxylic acids (PFCAs) are sub-class of perfluoroalkyl substances commonly detected in water matrices. They are persistent in the environment, hence highly toxic to living organisms. Their occurrence at trace amount, complex nature and prone to matrix interference make their extraction and detection a challenge. This study consolidates current advancements in solid-phase extraction (SPE) techniques for the trace-level analysis of PFCAs from water matrices. The advantages of the methods in terms of ease of applications, low-cost, robustness, low solvents consumption, high pre-concentration factors, better extraction efficiency, good selectivity and recovery of the analytes have been emphasized. The article also demonstrated effectiveness of some porous materials for the adsorptive removal of the PFCAs from the water matrices. Mechanisms of the SPE/adsorption techniques have been discussed. The success and limitations of the processes have been elucidated.

Impact of effluent organic matter on perfluoroalkyl acid removal from wastewater effluent by granular activated carbon and alternative adsorbents

Occurrence of perfluoroalkyl acids (PFAAs) in wastewater effluent coupled with increasingly stringent regulations has increased the need for more effective sorption-based PFAA treatment approaches. This study investigated the impact of ozone (O3)- biologically active filtration (BAF) as integral components of non-reverse osmosis (RO)-based potable reuse treatment trains and as a potential pretreatment option to improve adsorptive PFAA removal from wastewater effluent by nonselective (e.g., granular activated carbon (GAC) and selective (e.g., anionic exchange resins (AER) and surface-modified clay (SMC)) adsorbents. For nonselective GAC, O3 and BAF resulted in similar PFAA removal improvements, while BAF alone performed better than O3 for AER and SMC. O3-BAF in tandem resulted in the highest PFAA removal performance improvement among pretreatments investigated for selective and nonselective adsorbents. Side by side evaluation of the dissolved organic carbon (DOC) breakthrough curves and size exclusion chromatography (SEC) for each pretreatment scenario suggested that despite the higher affinity of selective adsorbents towards PFAAs, the competition between PFAA and effluent organic matter (EfOM) (molecular weights (MWs): 100-1000 Da) negatively impacts the performance of these adsorbents. The SEC results also demonstrated that transformation of hydrophobic EfOM to more hydrophilic molecules during O3 and biotransformation of EfOM during BAF were the dominant mechanisms responsible for alleviating the competition between PFAA and EfOM, resulting in PFAA removal improvement.

Destruction and defluorination of PFAS matrix in continuous-flow supercritical water oxidation reactor: Effect of operating temperature

Cleanup and disposal of stockpiles and waste streams containing per- and polyfluoroalkyl substances (PFAS) require effective end-of-life destruction/mineralization technologies. Two classes of PFAS, perfluoroalkyl carboxylic acids (PFCAs) and perfluoroalkyl sulfonic acids (PFSAs), are commonly found in legacy stockpiles, industrial waste streams, and as environmental pollutants. Continuous flow supercritical water oxidation (SCWO) reactors have been shown to destroy several PFAS and aqueous film-forming foams. However, a direct comparison of the SCWO efficacy for PFSAs and PFCAs has not been reported. We show the effectiveness of continuous flow SCWO treatment for a matrix of model PFCAs and PFSAs as a function of operating temperature. PFSAs appear to be significantly more recalcitrant than PFCAs in the SCWO environment. The SCWO treatment results in a destruction and removal efficiency of 99.999% at a T > 610 °C and at a residence time of ∼30 s. Fluoride recovery lags destruction PFAS at 510 °C and reaches >100% above 610 °C, confirming the formation of liquid and gaseous phase intermediate product during lower temperature oxidation. This paper establishes the threshold for destroying PFAS-containing liquids under SCWO conditions.

Investigating the Electrochemically Driven Capture and Release of Long-Chain PFAS by Redox Metallopolymer Sorbents

The remediation of perfluoroalkyl substances (PFAS) is an urgent challenge due to their prevalence and persistence in the environment. Electrosorption is a promising approach for wastewater treatment and water purification, especially through the use of redox polymers to control the binding and release of target contaminants without additional external chemical inputs. However, the design of efficient redox electrosorbents for PFAS faces the significant challenge of balancing a high adsorption capacity while maintaining significant electrochemical regeneration. To overcome this challenge, we investigate redox-active metallopolymers as a versatile synthetic platform to enhance both electrochemical reversibility and electrosorption uptake capacity for PFAS removal. We selected and synthesized a series of metallopolymers bearing ferrocene and cobaltocenium units spanning a range of redox potentials to evaluate their performance for the capture and release of perfluorooctanoic acid (PFOA). Our results demonstrate that PFOA uptake and regeneration efficiency increased with more negative formal potential of the redox polymers, indicating possible structural correlations with the electron density of the metallocenes. Poly(2-(methacryloyloxy)ethyl cobaltoceniumcarboxylate hexafluorophosphate) (PMAECoPF₆) showed the highest affinity toward PFOA, with an uptake capacity of more than 90 mg PFOA/g adsorbent at 0.0 V vs Ag/AgCl and a regeneration efficiency of more than 85% at -0.4 V vs Ag/AgCl. Kinetics of PFOA release showed that electrochemical bias greatly enhanced the regeneration efficiency when compared to open-circuit desorption. In addition, electrosorption of PFAS from different wastewater matrices and a range of salt concentrations demonstrated the capability of PFAS remediation in complex water sources, even at ppb levels of contaminants. Our work showcases the synthetic tunability of redox metallopolymers for enhanced electrosorption capacity and regeneration of PFAS.

A review of the occurrence, transformation, and removal technologies for the remediation of per- and polyfluoroalkyl substances (PFAS) from landfill leachate

Per- and polyfluoroalkyl substances (PFAS) are persistent organic pollutants (POPs) that pose significant environmental and human health risks. The presence of PFAS in landfill leachate is becoming an increasingly concerning issue. This article presents a comprehensive review of current knowledge and research gaps in monitoring and removing PFAS from landfill leachate. The focus is on evaluating the effectiveness and sustainability of existing removal technologies, and identifying areas where further research is needed. To achieve this goal, the paper examines the existing technologies for monitoring and treating PFAS in landfill leachate. The review emphasizes the importance of sample preparation techniques and quality assurance/quality control measures in ensuring accurate and reliable results. Then, this paper reviewed the existing technologies for removal and remediation of PFAS in landfill leachates, such as adsorption, membrane filtration, photocatalytic oxidation, electrocatalysis, biodegradation, and constructed wetlands. Additionally, the paper summarizes the factors that exhibit the performance of various treatment technologies: reaction time, experimental conditions, and removal rates. Furthermore, the paper evaluates the potential application of different remediation technologies (i.e., adsorption, membrane filtration, photocatalytic oxidation, electrocatalysis, biodegradation, and constructed wetlands, etc.) in treating landfill leachate containing PFAS and its precursors, such as fluorotelomeres like FTOH and FTSs. The review highlights the importance of considering economic, technical, and environmental factors when selecting control measures. Overall, this article aims to provide guidance for promoting environmental protection and sustainable development in the context of PFAS contamination in landfill leachate.

Enhanced adsorption of short-chain perfluorobutanoic acid by functionalized periodic mesoporous organosilica: Performance and mechanisms

Removal of short-chain per- and polyfluoroalkyl substances (PFAS) represents a unique challenge in comparison to the long-chain homologs. In this study, a series of functionalized periodic mesoporous organosilica (PMO) materials with tunable molar ratio of fluoroalkyl to amine functional groups were developed and used as platform adsorbents to investigate the adsorption behavior of short-chain PFAS, with a focus on perfluorobutanoic acid (PFBA). Modification with fluoroalkyl group substantially enhanced the adsorption affinity of PFBA with the functionalized PMO materials. Adsorption free energy analysis suggested that although electrostatic interactions were more predominant in PFBA adsorption, modification of PMOs with increased fluoroalkyl group loadings increased the non-electrostatic interactions with PFBA, resulting in more favorable PFBA adsorption. The optimal functionalized PMO showed fast PFBA adsorption kinetics, excellent PFBA removal efficiency in various water chemistry conditions, and can be regenerated and reused for numerous cycles with methanol/water mixture containing 500-mM NH₃·H₂O as regenerant. Furthermore, the optimal functionalized PMO showed robust performance for the removal of PFAS mixtures under complex natural water matrix. Results of this study suggested the important role of non-electrostatic interactions in enhancing the removal of short-chain PFAS and can provide mechanistic insights into guiding the design of improved adsorbents for PFAS removal.

A review of PFAS adsorption from aqueous solutions: Current approaches, engineering applications, challenges, and opportunities

Per- and polyfluoroalkyl substances (PFAS) have drawn great attention due to their wide distribution in water bodies and toxicity to human beings. Adsorption is considered as an efficient treatment technique for meeting the increasingly stringent environmental and health standards for PFAS. This paper systematically reviewed the current approaches of PFAS adsorption using different adsorbents from drinking water as well as synthetic and real wastewater. Adsorbents with large mesopores and high specific surface area adsorb PFAS faster, their adsorption capacities are higher, and the adsorption process are usually more effective under low pH conditions. PFAS adsorption mechanisms mainly include electrostatic attraction, hydrophobic interaction, anion exchange, and ligand exchange. Various adsorbents show promising performances but challenges such as requirements of organic solvents in regeneration, low adsorption selectivity, and complicated adsorbent preparations should be addressed before large scale implementation. Moreover, the aid of decision-making tools including response surface methodology (RSM), techno-economic assessment (TEA), life cycle assessment (LCA), and multi criteria decision analysis (MCDA) were discussed for engineering applications. The use of these tools is highly recommended prior to scale-up to determine if the specific adsorption process is economically feasible and sustainable. This critical review presented insights into the most fundamental aspects of PFAS adsorption that would be helpful to the development of effective adsorbents for the removal of PFAS in future studies and provide opportunities for large-scale engineering applications.

Removal of Low Trace ppb-Level Perfluorooctanesulfonic Acid (PFOS) with ZIF-8 Coatings Involving Adsorbent Degradation

For the first time, low trace-level removal of perfluorooctanesulfonic acid (PFOS), i.e., 20-500 μg/L (ppb), from aqueous solutions using zeolitic imidazolate framework-8 (ZIF-8)-coated copper sheet (ZIF-8@Cu) composite is reported here. In comparison with different commercial activated carbon (AC) and all-silica zeolites, the composite showed the highest removal rate of 98%, which remained consistent over a wide range of concentrations. Additionally, no adsorbent leaching from the composite was noticed, which eradicated pre-analysis steps such as filtration and centrifugation, unless needed for other adsorbents studied here. The composite displayed fast uptake with saturation reaching within 4 h, irrespective of the initial concentration. However, the morphological and structural characterization revealed surface degradation of ZIF-8 crystals, along with a decline in the crystal size. The adsorption of PFOS on ZIF-8 crystals was linked to chemisorption, as the surface degradation surges with an increase in PFOS concentration or with cyclic exposure at low concentrations. Methanol seemingly removed surface debris (partially), thus providing access to ZIF-8 beneath the surface debris. Overall, the findings demonstrate that at low trace ppb-level PFOS concentrations ZIF-8 can be considered as a possible candidate for PFOS removal, even though it suffers slow surface degradation, it also removes efficiently PFOS molecules from aqueous solutions.

Two-in-one platform based on conjugated polymer for ultrasensitive ratiometric detection and efficient removal of perfluoroalkyl substances from environmental water

Continuous emergence of persistent organic pollutants perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) in various water bodies around the world poses a serious threat to the global ecosystem. The exploration of advanced detection/removal techniques to monitor/treat such type of toxicants is urgently required. Herein, we unveiled a donor-acceptor type conjugated polymer PF-DBT-Im as a first-of-its-kind ratiometric fluorescent probe for visual, amplified, and specific monitoring of PFOA and PFOS with ultra-low detection limits of 6.12 nM (PFOA) and 14.3 nM (PFOS), respectively. PF-DBT-Im undergoes strong aggregation after binding with PFOA/PFOS as evident by transmission electron microscopy, zeta potential measurements, and dynamic light scattering studies. This promotes interchain Förster resonance energy transfer process to endorse an obvious emission color change from blue-to-magenta under ultraviolet lamp excitation. Consequently, a smartphone-integrated portable device is fabricated for realizing rapid and on-site detection of PFOA/PFOS. Besides, a new class of magnetic adsorbent Fe₃O₄@NH₂&F₁₃ is also prepared and used in combination with PF-DBT-Im to remove PFOA/PFOS from the environmental water effectively and rapidly as confirmed by liquid chromatography-mass spectrometry analysis. Thus, utilizing the excellent signal amplification property of PF-DBT-Im and the remarkable magnetic separation capability of Fe₃O₄@NH₂&F₁₃, a multifunctional system is developed for step-wise recognition and separation of PFOA/PFOS from the environmental water proficiently and rapidly.

Effect of different co-foaming agents on PFAS removal from the environment by foam fractionation

Per- and poly-fluoroalkyl substances (PFAS) are recalcitrant, synthetic chemicals that are ubiquitous in the environment because of their widespread use in a variety of consumer and industrial products. PFAS contamination has become an increasing issue in recent years, which needs to be urgently addressed. Foam fractionation is emerging as a potential remediation option that removes PFAS by adsorption to the surface of rising air bubbles which are removed from the system as a foam. PFAS concentrations in the environment are often not sufficient to allow for formation of a foam by itself and often a co-foaming agent is required to be added to enhance the foamability of the solution. In this study, the effect of different classes of co-foaming agents, anionic, non-ionic, zwitterionic and cationic surfactants on the removal of PFAS with varying fluorocarbon chain length from 3 to 8 in a foam fractionation process have been investigated. Evaluation of the air-water interface partitioning coefficient (k') in addition with surface tension and PFAS removal results support the contention that using a co-foaming agent with the opposite charge to the PFAS in question significantly facilitates the adsorption of PFAS to the air-water interface, enhancing the efficiency of the process. Using the non-ionic surfactant (no headgroup electrostatic interaction with PFAS), as a reference, it was observed, in terms of PFAS separation and rate of PFAS removal, that anionic co-surfactant performed worst, zwitterionic was better, and cationic co-surfactant performed best. All of the PFAS species were able to be removed below the limit of detection (0.05 µg/L) after 45 minutes of foaming time with the cationic surfactant.

Curbing per- and polyfluoroalkyl substances (PFASs): First investigation in a constructed wetland-microbial fuel cell system

The presence of per- and polyfluoroalkyl substances (PFASs) in water environments has been linked to a slew of negative health effects in both animals and humans, but the green and eco-sustainable removal technologies remain largely unknown. Constructed wetland coupled microbial fuel cell (CW-MFC) is termed a "green process" to control pollutants and recover energy. However, so far, no study has investigated the removal of PFASs and their effects on the performance of the CW-MFC systems. Here, we investigated the removal performance of PFOA and PFOS in the CW-MFC systems both in the absence and presence of electricity circuit, and explored the distribution and fate of PFASs and their interactions with other elements in the systems. Our findings demonstrated excellent removal efficiency of >96% PFOA and PFOS in CW-MFC systems. PFOA and PFOS were distributed throughout the system via wastewater flow, while electrode material and plants are the main enrichment sites in which MFC enhanced up to 10% PFASs removal. However, a loss of 7.2-13.5% of nitrogen removal and a decrease of 7.3% in bioelectricity output were observed when PFASs were introduced in the system. The driven force led to the loss of nitrogen removal and bioelectricity generation lies in the accumulation of PFASs in system composition, which affected microbial activity and community composition, damaging the health of the plant, and in turn reducing CW-MFC's functioning. No doubt, CW-MFC systems provide an alternative technique for PFASs removal, alleviating some limitations to the physical and chemical techniques, but further investigation is highly needed.

Effect of two sorbents on the distribution and transformation of N-ethyl perfluorooctane sulfonamido acetic acid (N-EtFOSAA) in soil-soybean systems

The broad application of perfluoroalkyl acid (PFAA) precursors has led to their occurrence in soil, resulting in potential uptake and bioaccumulation in plants. In this study, we investigated the effect of powdered activated carbon (PAC) and montmorillonite on the distribution and transformation of a perfluorooctanesulfonic acid (PFOS) precursor, N-ethyl perfluorooctane sulfonamido acetic acid (N-EtFOSAA), in soil-plant systems. The results showed that N-EtFOSAA at 300 μg/kg was taken up by soybean roots and shoots together with its transformation products (i.e., perfluorooctane sulfonamide (PFOSA), PFOS), while decreasing the biomass of shoots and roots by 47.63% and 61.16%, respectively. PAC amendment significantly reduced the water leachable and methanol extractable N-EtFOSAA and its transformation products in soil. In the presence of soybean and after 60 days, 73.5% of the initially spiked N-EtFOSAA became non-extractable bound residues. Compared to the spiked controls, the PAC addition also decreased the total plant uptake of N-EtFOSAA by 94.96%. In contrast, montmorillonite showed limited stabilization performance for N-EtFOSAA and its transformation products and was ineffective to lower their bioavailability. Overall, the combination of PAC and soybean was found to be effective in immobilizing N-EtFOSAA in soil.

Poly- and Perfluoroalkyl Substances (PFAS) in Landfills: Occurrence, Transformation and Treatment

Landfills have served as the final repository for > 50 % municipal solid wastes in the United States. Because of their widespread uses and persistence in the environment, per- and polyfluoroalkyl substances (PFAS) (>4000 on the global market) are ubiquitously present in everyday consumer, commercial and industrial products, and have been widely detected in both closed (tens ng/L) and active (thousands to ten thousands ng/L) landfills due to disposal of PFAS-containing materials. Along with the decomposition of wastes in-place, PFAS can be transformed and released from the wastes into leachate and landfill gas. Consequently, it is critical to understand the occurrence and transformation of PFAS in landfills and the effectiveness of landfills, as a disposal alternative, for long-term containment of PFAS. This article presents a state-of-the-art review on the occurrence and transformation of PFAS in landfills, and possible effect of PFAS on the integrity of modern liner systems. Based on the data published from 10 countries (250 + landfills), C4-C7 perfluoroalkyl carboxylic acids were found predominant in the untreated landfill leachate and neutral PFAS, primarily fluorotelomer alcohols, in landfill air. The effectiveness and limitations of the conventional leachate treatment technologies and emerging technologies were also evaluated to address PFAS released into the leachate. Among conventional technologies, reverse osmosis (RO) may achieve a high removal efficiency of 90-100 % based on full-scale data, which, however, is vulnerable to the organic fouling and requires additional disposal of the concentrate. Implications of these knowledge on PFAS management at landfills are discussed and major knowledge gaps are identified.

A juxtaposed review on adsorptive removal of PFAS by metal-organic frameworks (MOFs) with carbon-based materials, ion exchange resins, and polymer adsorbents

The removal of poly- and perfluoroalkyl substances (PFAS) from the aquatic environment is a universal concern due to the adverse effects of these substances on both the environment and public health. Different adsorbents, including carbon-based materials, ion exchange resins, biomaterials, and polymers, have been used for the removal of short-chain (C < 6) and long-chain (C > 7) PFAS from water with varying performance. Metal-organic frameworks (MOFs), as a new generation of adsorbents, have also been recently used to remove PFAS from water. MOFs provide unique properties such as significantly enhanced surface area, structural tunability, and improved selectivity compared to conventional adsorbents. However, due to various types of MOFs, their complex chemistry and morphology, different PFAS compounds, lack of standard adsorption test, and different testing conditions, there are inconclusive and contradictory findings in the literature. Therefore, this review aims to provide critical analysis of the performance of different types of MOFs in the removal of long-chain (C > 7), short-chain (C < 6), and ultra-short-chain (C < 3) PFAS and comprehensively study the efficiency of MOFs for PFAS removal in comparison with other adsorbents. In addition, the adsorption mechanisms and kinetics of PFAS components on different MOFs, including Materials of Institute Lavoisier (MIL), Universiteit of Oslo (UiO), Zeolitic imidazolate frameworks (ZIFs), Hong Kong University of Science and Technology (HKUST), and other hybrid types of MOF were discussed. The study also discussed the effect of environmental factors such as pH and ionic strength on the adsorption of PFAS on MOFs. In addition to the adsorption process, the reusability and regeneration of MOFs in the PFAS removal process are discussed. Finally, challenges and future outlooks of the utility of MOFs for PFAS removal were discussed to inspire future critical research efforts in removing PFAS.