Fabrication of hydrolytically stable magnetic core-shell aminosilane nanocomposite for the adsorption of PFOS and PFOA

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

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

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.

Unveiling nano-empowered catalytic mechanisms for PFAS sensing, removal and destruction in water

Per- and polyfluoroalkyl substances (PFAS) are organofluorine compounds used to manufacture various industrial and consumer goods. Due to their excellent physical and thermal stability ascribed to the strong CF bond, these are ubiquitously present globally and difficult to remediate. Extensive toxicological and epidemiological studies have confirmed these substances to cause adverse health effects. With the increasing literature on the environmental impact of PFAS, the regulations and research have also expanded. Researchers worldwide are working on the detection and remediation of PFAS. Many methods have been developed for their sensing, removal, and destruction. Amongst these methods, nanotechnology has emerged as a sustainable and affordable solution due to its tunable surface properties, high sorption capacities, and excellent reactivities. This review comprehensively discusses the recently developed nanoengineered materials used for detecting, sequestering, and destroying PFAS from aqueous matrices. Innovative designs of nanocomposites and their efficiency for the sensing, removal, and degradation of these persistent pollutants are reviewed, and key insights are analyzed. The mechanistic details and evidence available to support the cleavage of the CF bond during the treatment of PFAS in water are critically examined. Moreover, it highlights the challenges during PFAS quantification and analysis, including the analysis of intermediates in transitioning nanotechnologies from the laboratory to the field.

Sorption and diffusion of per-polyfluoroalkyl substances (PFAS) in high-density polyethylene geomembranes

The objective of this study is to investigate the fate and transport of per-polyfluoroalkyl substances (PFAS) through a high-density polyethylene (HDPE) geomembrane (GM) that is commonly used in landfill composite liner systems. Tests were conducted to measure the sorption and diffusion of per-polyfluoroalkyl substances (PFAS) with varying number of carbons in chain and functional groups on HDPE GM. Perfluoroalkyl carboxyl acids (PFCAs), perfluoroalkyl sulphonic acids (PFSAs), alkyl-sulfonamidoacetic acids (FOSAAs), fluorotelomer sulfonic acids (FtSAs), alkane sulfonamides (FOSA) and ether carboxylic acids (Gen X) were investigated in this study. The partition coefficients (Kd) on HDPE GM ranged from 3.8 to 98.3 L/kg. PFAS with amide and sulfonic functional groups showed stronger sorption than that of PFAS with carboxylic acid functional groups. Molecular weight directly affected the Kd for long-chained PFAS whereas the Kd of short-chained PFAS was not sensitive to molecular weight. The diffusion coefficients (Dg) of PFCAs and PFSAs through 0.1-mm HDPE GM were found to be in the orders of 10-18 to 10-17 m2/s. The Dg decreased with increasing molar mass and were also observed to be dependent on the functional group. Dg of PFSAs was lower than that of PFCAs for similar number of carbons in the chain. The estimated mass flux for PFAS in an intact 1.5-mm HDPE GM varied from 38.7 to 2080.8 ng/m2/year whereas the estimated diffusive breakthrough time for PFAS in intact 1.5-mm HDPE was 1526 years or longer.

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 perfluorooctanoic acid via polyethyleneimine modified graphene oxide: Effects of water matrices and understanding mechanisms

This research aimed to evaluate the adsorption behaviors and mechanisms of perfluorooctanoic acid (PFOA) onto polyethyleneimine modified graphene oxide (GO-PEI) from aqueous solutions. The adsorption capacity was significantly improved by doping polyethyleneimine (PEI) onto graphene oxide (GO). The Brunauer-Emmett-Teller (BET) isotherm model was considered as the best isotherm model in describing the PFOA adsorption onto GO-PEI3 (w_PEI/w_GO = 3). GO-PEI3 exhibited high adsorption capacity (q_e = 368.2 mg/g, calculated from BET isotherm model) and excellent stability. The maximum monolayer amount of PFOA adsorption onto GO-PEI3 (q_m = 231.2 mg/g) was successfully evaluated. The calculated saturated concentration (C_s = 169.9 mg/L) of PFOA on GO-PEI3 closely agrees with its critical micelle concentration (CMC = 157.0 mg/L), suggesting the formation of multilayer hemi-micelles or micelles PFOA structures on the surface of GO-PEI3. PFOA adsorption onto GO-PEI3 was inhibited by several factors including: the presence of humic acid (HA) by competing with the adsorption sites, background salts through the double-layer compression effect, and the competition from soluble ions for the amine or amide functional groups on GO-PEI3. Finally, both the FT-IR and XPS results confirmed that the adsorption of PFOA onto GO-PEI3 was through electrostatic attraction and hydrophobic interaction (physical adsorption), but not chemical adsorption. This work provides fundamental knowledge both in understanding the adsorption behavior through the BET isotherm model and in developing a stable adsorbent for PFOA adsorption. In addition, the findings highlight the potential of PFOA remediation from wastewater systems using GO-PEI in engineering applications.

Adsorption of persistent organic pollutants (POPs) from the aqueous environment by nano-adsorbents: A review

The intensification of urbanisation and industrial activities significantly exacerbates the distribution of toxic contaminations into the aqueous environment. Persistent organic pollutants (POPs) have received considerable attention in the past few decades because of their persistence, long-distance migration, potential bioaccumulation, latent toxicity for humans and wildlife. There is no doubt that POPs cause serious effects on the global ecosystem. Therefore, it is necessary to develop a simple, safe and sustainable approach to remove POPs from water bodies. Among other conventional techniques, the adsorption process has proven to be a more effective method for eliminating POPs and to a larger extent meet discharge regulations. Nanomaterials can effectively adsorb POPs from aqueous solutions. For most POPs, a >70% adsorptive removal efficiency was achieved. The major mechanisms for POPS uptake by nano-adsorbents includes electrostatic interaction, hydrophobic (van der Waals, π-π and electron donor-acceptor) interaction and hydrogen bonding. Nano-adsorbent can sustain a >90% POPs adsorptive removal for about 3 cycles and reuseable for up to 10 cycles. Challenges around adsorbent ecotoxicity and safe disposal were also discussed. The present review evaluated recent research outcomes on nanomaterials that are employed to remove POPs in water systems.

Ecotoxicological effects of per- and polyfluoroalkyl substances (PFAS) and of a new PFAS adsorbing organoclay to immobilize PFAS in soils on earthworms and plants

A novel adsorptive organoclay (Intraplex A®) was developed for the in situ immobilization of per- and polyfluoroalkyl substances (PFAS) in the vadose zone. We provide the first evaluation of the effects of Intraplex A® on earthworms and plants in a PFAS-contaminated soil. Ecotoxicological tests were carried out on control soil with and without Intraplex A® (C + I and C, respectively) and PFAS-contaminated soil with and without Intraplex A® (PFAS + I and PFAS, respectively). We investigated the acute ecotoxicological effects of PFAS and Intraplex A® on the growth, reproduction and survival of earthworms (Eisenia fetida) and on plant growth (oat - Avena sativa and turnip - Brassica rapa L. silvestris). Earthworm lethality was 7.6 lower in PFAS + I than in PFAS soil. Earthworms avoided 100% C + I and PFAS + I soils, and reduced earthworms' reproduction was observed in both these soils. For both plant species, the PFAS + I soil yielded less fresh and dry shoot biomass than the PFAS soil, while root growth remained unaffected (all tests: p < 0.05). Soils with Intraplex A® had some negative effects on plants and earthworms, which must be balanced with its benefits as an in situ PFAS adsorbent.

Rapid removal of PFOA and PFOS via modified industrial solid waste: Mechanisms and influences of water matrices

Emerging perfluoroalkyl and polyfluoroalkyl substances contaminate waters at trace concentrations, thus rapid and selective adsorbents are pivotal to mitigate the consequent energy-intensive and time-consuming issues in remediation. In this study, coal combustion residuals-fly ash was modified (FA-SCA) to overcome the universal trade-off between high adsorption capacity and fast kinetics. FA-SCA presented rapid adsorption (t_eq = 2 min) of PFOX (perfluorooctanoic acid and perfluorooctanesulfonic acid, collectively), where the dynamic adsorption capacity (q_dyn = q_m/t_eq) was 2-3 orders of magnitude higher than that of benchmark activated carbons and anion-exchange resins. Investigated by advanced characterization and kinetic models, the fast kinetics and superior q_dyn are attributed to (1) elevated external diffusion driven by the submicron particle size; (2) enhanced intraparticle diffusion caused by the developed mesoporous structure (V_meso/V_micro = 8.1); (3) numerous quaternary ammonium anion-exchange sites (840 μmol/g), and (4) appropriate adsorption affinity (0.031 L/μmol for PFOS, and 0.023 L/μmol for PFOA). Since the adsorption was proven to be a synergistic process of electrostatic and hydrophobic interactions, effective adsorption ([PFOX]_ini = 1.21 μM, concentration levels of highly-contaminant-sites) was obtained at conventional natural water chemistries. High selectivity (>85.4% removal) was also achieved with organic/inorganic competitors, especially compounds with partly similar molecular structures to PFOX. In addition, >90% PFOX was removed consistently during five cycles in mild regeneration conditions (pH 12 and 50 °C). Overall, FA-SCA showed no leaching issues of toxic metals and exhibits great potential in both single-adsorption processes and treatment train systems.

Enhanced adsorption of perfluorooctanoate (PFOA) onto low oxygen content ordered mesoporous carbon (OMC): Adsorption behaviors and mechanisms

The pollution of perfluorooctanoic acid (PFOA) in water bodies has been a serious threat to environment and human health. Ordered mesoporous carbons (OMCs) with different oxygen contents were prepared and first used for adsorbing PFOA from aqueous solutions. The OMC-900 with a lower oxygen content has a higher PFOA adsorption capacity than the oxygen-rich OMC-700. OMCs require a much shorter time to reach the adsorption equilibrium comparing with other adsorbents reported in literature. The mesopores play an important role in this rapid adsorption kinetics. The pseudo-second-order model better fitted the kinetic data. The multilayers adsorption was proposed for the adsorption of PFOA onto OMCs since the Freundlich isotherm model fits the experimental data well. The micelle or hemi-micelle structures may be formed during the adsorption. Various background salts showed a positive effect on PFOA adsorption due to the salting-out and divalent bridge effects. The humic acid can lead to a discernible reduction in PFOA adsorption by competing for adsorption sites on OMCs. The hydrophobic interaction and electrostatic interaction adsorption mechanisms were proposed and verified by the adsorption data. The high adsorption capacity and fast adsorption kinetics of the OMC make it a potential adsorbent for PFOA removal in engineering applications.

Electrochemical adsorption of perfluorooctanoic acid on a novel reduced graphene oxide aerogel loaded with Cu nanoparticles and fluorine

Perfluorooctanoic acid (PFOA) is widely concerned because its serious toxicity to the environment and ecosystems. In order to effectively and conveniently remove PFOA from aqueous solutions, reduced graphene oxide aerogel modified by Cu nanoparticles and fluorine (Cu/F-rGA) was prepared by the microbubble template method as an electrode in electrosorption. The removal capacity of Cu/F-rGA electrode to PFOA was 489% and 45.9% higher at + 0.8 V than that of open circuit and unmodified electrode, respectively. These significant improvements can be attributed to the advantages of Cu/F-rGA in ligand exchange reaction and electrostatic attraction under voltage assistance. The regeneration of Cu/F-rGA electrode maintained 75.51% capacity after 10 times electrosorption-desorption by applying reverse voltage. These properties provided potential for the reuse and application of Cu/F-rGA electrode. The electrosorption isotherm and model results showed that PFOA tended to be parallel to the adsorption site at low temperature and perpendicular at high temperature. The number of PFOA molecules connected to each adsorption site was 0.72-1.76, and the number of adsorption layers of PFOA on the electrode was between 1.46 and 2.87. Findings from this study provide a green and effective strategy to remove PFOA from aqueous solutions with low energy consumption.

Copper Nanoparticle Loading and F Doping of Graphene Aerogel Enhance Its Adsorption of Aqueous Perfluorooctanoic Acid

Perfluorooctanoic acid (PFOA) persists in the environment for a long time due to its stable physical and chemical properties, and it is harmful to the environment and biological system. In order to effectively remove PFOA from aqueous solution, Cu nanoparticles and fluorine-modified graphene aerogel (Cu/F-rGA) were fabricated by the microbubble template method. Compared with unmodified aerogels (rGA), the adsorption rate of PFOA on Cu/F-rGA was enhanced 2.68-fold. These significant improvements were assumed to benefit from the ligand exchange reaction and hydrophobic and F-F interactions. The regeneration of Cu/F-rGA maintained 73.26% with ethanol as the desorption solvent after 10 times adsorption-desorption. The fitting results of the statistical physics model showed that PFOA tended to be parallel to the adsorption site at low temperature and perpendicular at high temperature. The number of PFOA molecules connected to each adsorption site was 0.53 to 1.41, and the number of adsorption layers of PFOA on the Cu/F-rGA was between 1.63 and 2.51. Compared with the response surface methodology and artificial neural network, an adaptive neuro-fuzzy inference system had more accurate analysis and prediction results. These results provide an effective and alternative strategy to remove PFOA from aqueous solution with environment-friendly consumption.

Modulating hierarchically microporous biochar via molten alkali treatment for efficient adsorption removal of perfluorinated carboxylic acids from wastewater

This work presented a three-dimensional (3D) hierarchically microporous biochar (HMB) via molten alkali treatment that achieved efficient adsorption of perfluorinated carboxylic acids (PFCAs), which was a significant environment concern due to the global distribution and potential health risks. The systematic optimization of fabrication process rendered the HMB large surface area and uniform microporous structure, leading to a high adsorption capacity and adsorption rate of 1269 mg/g and 197 mg/(g·min), respectively, when perfluorooctanoic acid (PFOA) was as a representative. The adsorption mechanisms were explored via controlling the interaction between PFCAs and the HMB900-2.4. Specifically, hydrophobic effect was verified by the enhanced adsorption performance with the increase of the PFCAs homologues hydrophobicity. The observed highly pH-dependent adsorption capacity additionally suggested the dominant contribution of electrostatic interaction. For long-chain PFCAs (C_nF_2n+1COOH, n > 5), the HMB900-2.4 presented a high removal efficiency (> 90%) within 30 min. Even for short-chain PFCAs (C_nF_2n+1COOH, n = 4-5), the removal efficiency reached to over 60%. The synthesized HMB900-2.4 exhibited high stability during recycling experiments and superior performance over commercial adsorbents, suggested a promise of utilizing it to remove PFCAs from wastewater.