Electric Field-Assisted Nanofiltration for PFOA Removal with Exceptional Flux, Selectivity, and Destruction

PFAS removal via adsorption: A synergistic review on advances of experimental and computational approaches

Per- and polyfluoroalkyl substances (PFAS), commonly known as "forever chemicals", have become a major focus of current research due to their toxicity and persistence in the environment. These synthetic compounds are notoriously difficult to degrade, accumulating in water systems and posing long-term health and environmental risks. Adsorption is one of the most investigated technologies for PFAS removal. This review comprehensively reviewed the PFAS adsorption process, focusing not only on the adsorption itself, but also on the behavior of PFAS in the aquatic environment prior to adsorption because these behaviors directly affect PFAS adsorption. Significantly, this review summarized in detail the advances made in PFAS adsorption from the computational approach and emphasized the importance of integrated experimental and computational studies in gaining molecular-level understanding on the adsorption mechanisms of PFAS. Toward the end, the review identified several critical research gaps and suggested key interdisciplinary research needs for further advancing our understanding on PFAS adsorption.

Highly selective copper recovery from industrial wastewater via electric field-enhanced ultrafiltration assisted with a picolyl-modified polyelectrolyte

Copper-containing industrial wastewater, characterized by strong acidity, high ionic strength, and various competing metals, presents significant challenges for Cu(II) recovery. To address these issues, an electric field-enhanced ultrafiltration process was developed, assisted with a functional polyelectrolyte with high selectivity for Cu(II). The polyelectrolyte, termed PPEI, was synthesized by grafting picolyl groups onto polyethyleneimine (PEI), enhancing its affinity for Cu(II). The captured Cu(II) was subsequently recovered through electrolysis, demonstrating a sustainable approach for both Cu recovery and PPEI recycling. The synthesis and stability of PPEI were confirmed through infrared spectroscopy, particle size analysis, and dialysis validation, ensuring its reliability in practical applications. The incorporation of picolyl groups onto PPEI enhances its selectivity for Cu(II) via coordination with two amines and four pyridyl groups per copper ion. Under acidic conditions, the maximum loading ratio of copper to PPEI is 1:4 with loading capacity of 119.4 mg/g, which increases to 1.5:4 (i.e., 179.1 mg/g) under neutral to alkaline conditions due to the deprotonation of excess amines. PPEI effectively removes Cu(II) from solutions under various harsh conditions at the loading ratio of 4, maintaining 92-98 % removal efficiency in the presence of high salt concentrations (up to 1 M NaCl) and pH as low as 1, and approximately 85 % removal in solutions with competing metal ions at concentrations up to 50 times higher than Cu(II). Scanning electron microscopy and membrane flux changes indicated that the application of a positive electric field significantly reduces membrane fouling and enhances Cu(II) selectivity. The application of a +0.2 V voltage to the membrane side reduced the flux decline rate by 58 %, significantly improving membrane performance while maintaining a Cu(II) removal efficiency of over 95 %. Electrolysis optimized at a current density of ≤0.004 A/cm2 achieved an 80 % copper recovery while allowing PPEI to be released for recycling. Tests conducted using two types of real industrial wastewater demonstrated a copper removal rate of ∼95 %, with a recovery rate of ∼80 %. This study provides a novel and highly selective approach for the efficient recovery of valuable metals from industrial wastewaters.

Electrochemical destruction of PFAS at low oxidation potential enabled by CeO2 electrodes utilizing adsorption and activation strategies

The persistence and ecological impact of per- and poly-fluoroalkyl substances (PFAS) in water sources necessitate effective and energy-efficient treatment solutions. This study introduces a novel approach using cerium dioxide (CeO2) electrodes enhanced with oxygen vacancy (Ov) to catalyze the defluorination of PFAS. By leveraging the unique affinity between cerium and fluorine-containing species, our approach enables adsorptive preconcentration and catalytic degradation at low oxidation potentials (1.37 V vs. SHE). Demonstrating high removal and defluorination efficiencies of perfluorooctanoic acid (PFOA) at 94.0 % and 73.0 %, respectively, our approach also proves effective in the environmental matrix. It minimizes the impacts of co-existing natural organic matter and chloride ions, crucial benefits of operating at lower oxidation potentials. The role of Ov in CeO2 is validated by both experimental results and density functional theory modeling, demonstrating that these sites can activate the C-F bond and substantially reduce the energy barriers for defluorination. Consequently, our CeO2-based method not only achieves defluorination efficiencies comparable to more energy-intensive techniques but does so while requiring less than 0.62 kWh/m3 per order. This positions our approach as a promising, cost-effective alternative for the remediation of PFAS-contaminated waters, emphasizing its relevance and effectiveness in environmental remediation scenarios.

Enhanced removal of perfluorooctanoic acid by aluminum-based metal-organic frameworks prepared by bauxite residue

Upcycling solid waste into advanced adsorbents is a sustainable approach in the field of waste valorization and wastewater treatment. In this study, we developed a phase-controlled synthesis method for a single phase of an aluminum-based metal-organic framework (MOF) using an aluminum source (Al3+) in red mud (RM), and demonstrated its potential for aqueous perfluorooctanoic acid (PFOA) removal. By optimizing the pre-treatment process, the selective extraction of aluminum ion from RM was achieved. Subsequently, three distinct aluminum-based MOFs (i.e., MIL-53(Al), MIL-96(Al), and MIL-100(Al)) were synthesized by controlling the hydrothermal synthesis conditions and using specific organic linkers (terephthalic acid and trimesic acid). For MOFs based on trimesic acid, the initial Al3+: trimesic acid ratio and duration of hydrothermal synthesis exerted an observable influence on the formation of the second building unit of the MOF. By manipulating these factors, we could precisely control isolated MIL-96(Al) and MIL-100(Al). The PFOA adsorption results revealed a remarkable increase in the adsorption capacity (Qmax: 131.58 mg/g) on MIL-100(Al) compared with that on MIL-96(Al). This was due to its large surface area (1189.15 m2/g) and the presence of numerous hydrophilic sites favorable for interaction with the carboxylic group of PFOA. Furthermore, a computational investigation revealed that in addition to direct Lewis acid-base interaction between PFOA and aluminum sites, the major mechanism involved the formation of a complex induced by ion exchange between coordinated NO3- and PFOA anions.

State of the science and regulatory acceptability for PFAS residual management options: PFAS disposal or destruction options

This systematic review covers the urgent challenges posed by per- and polyfluoroalkyl substances (PFAS) in managing residuals from municipal, industrial, and waste treatment sources. It covers regulatory considerations, treatment technologies, residual management strategies, and critical conclusions and recommendations. A rigorous methodology was employed, utilizing scientific search engines and a wide array of peer-reviewed journal articles, technical reports, and regulatory guidance, to ensure the inclusion of the most relevant and up-to-date information on PFAS management of impacted residuals. The increasing public and regulatory focus underscores the persistence and environmental impact of PFAS. Emerging technologies for removing and sequestrating PFAS from environmental media are evaluated, and innovative destruction methods for addressing the residual media and the concentrated waste streams generated from such treatment processes are reviewed. Additionally, the evolving regulatory landscape in the United States is summarized and insights into the complexities of PFAS in residual management are discussed. Overall, this systematic review serves as a vital resource to inform stakeholders, guide research, and facilitate responsible PFAS management, emphasizing the pressing need for effective residual management solutions amidst evolving regulations and persistent environmental threats.

Elucidating the Mechanism of Electro-Adsorption on Electrically Conductive Ultrafiltration Membranes via Modified Poisson-Boltzmann Equation

Electrically conductive membranes (ECMs) were prepared by coating porous ethylenediamine-modified polyacrylonitrile (PAN-EDA) UF membranes with an ultrathin layer of platinum (Pt) nanoparticles through magnetron sputtering. These ECMs were used in electrofiltration to study the removal of brilliant blue dye from an aqueous solution under positive electrical potentials (0-2.5 V). Negative electrical potentials (-1.0--2.5 V) were also investigated to regenerate the membrane by desorbing the dye from the ECM surface. At +0 V, the EC PAN-EDA membrane adsorbed the dye due to its intrinsic positive charge. Application of -2.0 V resulted in a maximum of 39% desorption of the dye. A modified Poisson-Boltzmann (MPB) model showed that -2.0 V created a repulsive force within the first 24 nm of the membrane matrix, which had a minimal effect on dye ions adsorbed deeper within the membrane, thus limiting the electro-desorption efficiency to 39%. Moreover, increasing positive potentials from +0.5 V to +2.5 V led to increased dye electro-adsorption by 9.5 times, from 132 mg/m2 to 1112 mg/m2 at pH 8 (equivalent to the membrane's isoelectric point). The MBP simulations demonstrated that increasing electro-adsorption loadings are related to increasing attractive force, indicating electro-adsorption induced by attractive force is the dominant mechanism and the role of other mechanisms (e.g., electrochemical oxidation) is excluded. At pH 5, electro-adsorption further increased to 1390 mg/m2, likely due to the additional positive charge of the membrane (zeta potential = 9.2 mV) compared to pH 8. At pH 8, complete desorption of the dye from the ECM surface was achieved with a significant repulsive force at -2.0 V. However, as pH decreased from 8 to 5, the desorption efficiency decreased by 3.9% due to the membrane's positive charge. These findings help elucidate the mechanisms of electro-adsorption and desorption on ECMs using dye as a model for organic compounds like humic acids.

Fouling behavior of nanofiltration membrane during the refining treatment of morphlines-dominant reverse osmosis concentrate

Nanofiltration (NF) has been proven to be with great potential for the separation of morpholines with molecular weight less than 200 Da in refining reverse osmosis concentrate (ROC), but its application is significantly restricted by the membrane fouling, which can reduce the rejection and service time. To enable the long-term operation stability of nanofiltration, this work focuses on the fouling behavior of each substance in the hydrosaline organic solution on nanofiltration membrane, aiming to give insight into the fouling mechanism. To this end, in this work, the effects of salts (i.e NaCl and Na2SO4), organic substances (including N-(2-hydroxypropyl)morpholine(NMH) and 4-morpholineacetate(MHA)) and representative divalent ions (Ca2+ and Mg2+) on the performance and physicochemical properties of DK membrane were systematically investigated. The results show that both salts and organics can induce DK membrane swelling, leading to an increase of the mean effective pore size. After the filtration of Na2SO4-NaCl-H2O, the mean pore size increased by 0.002 nm, resulting in the decrease of the removal ratio of NMH and MHA for 3.82% and 13.10%, respectively. With static adsorption of NMH and MHA, the mean pore size of DK membrane increased by 0.005 and 0.003 nm. The swelling slowed the entrance of more organic molecules into membrane pores. Among them, MHA led to the terrible irreversible pore blocking. As the concentration of Ca2+ increased, gypsum scaling was formed on the membrane surface. During this process, NMH and MHA played different roles, i.e. NMH accelerated the CaSO4 crystallization while MHA inhibited. As a conclusion, the fouling behavior of substances in the high saline organic wastewater on DK membrane were systematically revealed with the fouling mechanisms proposed, which could provide an insightful guidance for membrane fouling control and cleaning in the treatment of high salinity and organic wastewater.

Application of electron beam technology to decompose per- and polyfluoroalkyl substances in water

The widespread detection of per- and polyfluoroalkyl substances (PFAS) in environmental compartments across the globe has raised several health concerns. Destructive technologies that aim to transform these recalcitrant PFAS into less toxic, more manageable products, are gaining impetus to address this problem. In this study, a 9 MeV electron beam accelerator was utilized to treat a suite of PFAS (perfluoroalkyl carboxylates: PFCAs, perfluoroalkyl sulfonates, and 6:2 fluorotelomer sulfonate: FTS) at environmentally relevant levels in water under different operating and water quality conditions. Although perfluorooctanoic acid and perfluorooctane sulfonic acid showed >90% degradation at <500 kGy dose at optimized conditions, a fluoride mass balance revealed that complete defluorination occurred only at/or near 1000 kGy. Non-target and suspect screening revealed additional degradation pathways differing from previously reported mechanisms. Treatment of PFAS mixtures in deionized water and groundwater matrices showed that FTS was preferentially degraded (∼90%), followed by partial degradation of long-chain PFAS (∼15-60%) and a simultaneous increase of short-chain PFAS (up to 20%) with increasing doses. The increase was much higher (up to 3.5X) in groundwaters compared to deionized water due to the presence of PFAS precursors as confirmed by total oxidizable precursor (TOP) assay. TOP assay of e-beam treated samples did not show any increase in PFCAs, confirming that e-beam was effective in also degrading precursors. This study provides an improved understanding of the mechanism of PFAS degradation and revealed that short-chain PFAS are more resistant to defluorination and their levels and regulation in the environment will determine the operating conditions of e-beam and other PFAS treatment technologies.

Membrane processes enhanced by various forms of physical energy: A systematic review on mechanisms, implementation, application and energy efficiency

Membrane technologies in water and wastewater treatment have been eagerly pursued over the past decades, yet membrane fouling remains the major bottleneck to overcome. Membrane fouling control methods which couple membrane processes with online in situ application of external physical energy input (EPEI) are getting closer and closer to reality, thanks to recent advances in novel materials and energy deliverance methods. In this review, we summarized recent studies on membrane fouling control techniques that depend on (i) electric field, (ii) acoustic field, (iii) magnetic field, and (iv) photo-irradiation (mostly ultraviolet or visible light). Mechanisms of each energy input were first reported, which defines the applicability of these methods to certain wastewater matrices. Then, means of implementation were discussed to evaluate the compatibility of these fouling control methods with established membrane techniques. After that, preferred applications of each energy input to different foulant types and membrane processes in the experiment reports were summarized, along with a discussion on the trends and knowledge gaps of such fouling control research. Next, specific energy consumption in membrane fouling control and flux enhancement was estimated and compared, based on the experimental results reported in the literature. Lastly, strength and weakness of these methods and future perspectives were presented as open questions.