Efficient PFOA removal from drinking water by a dual-functional mixed-matrix-composite nanofiltration membrane

doi:10.21203/rs.3.rs-2815368/v1

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Efficient PFOA removal from drinking water by a dual-functional mixed-matrix-composite nanofiltration membrane

https://doi.org/10.21203/rs.3.rs-2815368/v1

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published 07 Nov, 2023

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Drinking-water contamination by per- and polyfluorinated alkyl substances (PFAS) is a global concern. Nanofiltration is a promising PFAS removal technology due to its scalability and cost-effectiveness. However, nanofiltration cannot typically reduce PFAS concentrations below current drinking-water recommendations. To enhance PFAS removal, we developed mixed-matrix-composite nanofiltration (MMCNF) membranes - an active nanofiltration layer on porous adsorptive support that synergetically combines filtration and adsorption. We synthesized MMCNF membranes comprising thin polyelectrolyte multilayer films deposited on thick (~400 µm) polyethersulfone supports incorporating β-cyclodextrin microparticles. These membranes achieved near complete removal (>99.9%) of model PFAS (perfluorooctanoic acid) for significantly longer filtration times compared to a control membrane without β-cyclodextrin, but otherwise identical. The spent MMCNF membrane was regenerated using ethanol, showing consistent performances in three filtration cycles. Perfluorooctanoic acid was concentrated 38-fold in the ethanol eluent. Further concentration by evaporation is straightforward and can enable eluent recycling and effective PFAS destruction.

Physical sciences/Engineering/Chemical engineering

Earth and environmental sciences/Environmental sciences/Environmental chemistry/Pollution remediation

Forever Chemicals

Organic micropollutants treatment

Adsorption

filtration

desorption

regeneration

In the last few decades, per and polyfluoroalkyl substances (PFAS), have been detected in drinking water at varying concentrations^1–3resulting from various industrial and commercial sources (e.g., nonstick coating, fire-fighting foams, etc.)³. PFAS are persistent organic micropollutants which are considered potentially harmful to human health at very low concentrations (in the range of parts per trillion)^2,4. They were shown to affect the immune system and are associated with a higher incidence of infectious diseases and varying cancer species⁵. To minimize the risk of PFAS exposure through drinking water, different regulatory agencies are developing regulations for maximum levels of PFAS in drinking water (as specific compounds and when addressed as the total PFAS concentration), For example, according to the United States Environmental Protection Agency (US EPA) the maximum concentrations of Perfluorooctanoic acid (PFOA) and perfluorobutane sulfonic acid (PFOS) in drinking water should be less than 70 ng/L and according to the EU 2020 advisory for PFOA and PFOS is 100 ng/L ⁶⁷. The maximum allowed concentration was recently modified (US EPA 2023) to a stricter value of 4 ng/l for PFOA and PFOA⁸. Unfortunately, the most common technologies used by water treatment facilities (e.g., coagulation, sand filtration, etc.) have an insignificant impact on PFAS concentrations ^9,10.

Various technologies have been developed and demonstrated for PFAS removal from aqueous systems^1,4,11–14, e.g., adsorption, oxidation, photolysis, UV degradation, and membrane separation. Among these technologies, membrane separation and adsorption stand out as practical and cost-effective^1,15. Reverse Osmosis (RO) and nanofiltration (NF) membranes can remove and concentrate most PFAS species. RO was shown to remove more than 99 % of PFAS, while NF was shown to remove only 70-99 %, depending upon membrane characteristics, type of PFAS, and operating conditions.^1,16–19 ^20,21Although effective, membrane separation has critical limitations. While RO membranes present higher PFAS rejection, they require more energy (higher applied pressure) due to their higher density and salt rejection (higher osmotic pressure gradient)^3,6. Conventional polyamide-based NF membranes typically remove 70 to 90 %, often insufficient to meet drinking water quality regulation^6,17,18,24. The widely studied and recently commercialized tunable, chemically stable, and antifouling NF membranes – based on multilayer assembly of polyelectrolytes- also give similar insufficient PFAS rejection (60 to 90 %)²⁵.

In addition to rejection by membranes, PFAS adsorption was performed using a range of adsorbers, including activated carbon, ion exchange resins, etc.^20,21. Recently, cross-linked β-cyclodextrin-based adsorbent ^22,23 was reported to exhibit high removal capacity (~ 89.50 % removal from groundwater sample having PFOA concentration of 218.6 ng/L), an exceptionally high uptake rate, and good regeneration potential. Currently, the main restriction of using β-cyclodextrin for PFAS removal is the competition with organic matter and the need for frequent sorbent regeneration or replacement²⁶. In this work, we overcome the limitations of both filtration and adsorption by hybridizing these technologies.

Heretofore, coupling of adsorption with membrane filtration was studied mainly as separate process steps^12,25,2 or by incorporating the adsorbent in the membrane's active layer^27,28. A recent study used mixed matrix composite nanofiltration membranes containing poly-N-isopropylacrylamide (PNIPAm) functionalized microfiltration support layer²⁹. This dual-functional adsorptive nanofiltration membrane showed a rejection of only 70 % for PFOA. Moreover, PFOA desorption and recovery were not explored. Thus, the development of regenerable mixed-matrix nanofiltration membranes with very high PFAS removal is in need to treat drinking water according to the increasing regulations. Various studies have introduced mixed matrix membrane/hybrid membranes to remove different organic micropollutants through selective adsorption. However, due to early saturation of the adsorbent's active sites, difficulty in regeneration, dependence on feed pH, and low residence time, these membranes were not found commercially viable^30–32.

Here, we fabricated a novel mixed-matrix composite NF membrane (MMCNF). The membrane included an asymmetric polyethersulfone (PES) support layer wherein β-cyclodextrin adsorbent microparticles were embedded. On top of mixed matrix support, a multilayer polyelectrolyte NF thin film was deposited. We selected PFOA as a model for long-chain PFAS as it is found in most contaminated groundwater^3,5,9,17 and is addressed in US and European regulations⁶. Compared to the control, PFOA removal by the MMCNF was dramatically enhanced and remained very high through several filtration-regeneration cycles. We found strong indications that PFOA adsorption on the MMCNF surface improved PFOA rejection through electrostatic effects. We used ethanol²³ to extract the adsorbed PFOA and regenerate the MMCNF. The extracted PFOA was at a higher concentration in the ethanol solution, facilitating more economical disposal or destruction of these chemicals.

2.1 Membranes synthesis and characterization

SEM) images (Fig. 1) confirm the successful synthesis of MMCNF membranes, prepared with three different mass loading of β-cyclodextrin (Table 1), 0% (M0), 6% (M6), and 8% (M8).

Table. 1 Composition and other properties of M0, M6, and M8

Membrane	PES (wt % of dope solution)	Dexsorb (wt % of dope solution)	NMP solvent (%)
M0	17	0	83
M6	17	6	77
M8	17	8	75

Loadings greater than 8% resulted in a non-usable brittle support layer due to the size of the adsorbent particle (see below) and the high viscosity of the casting mixture. Plan view image of the M0 surface (Fig. 1a) showed a smooth surface typical to a layer-by-layer assembly of polyelectrolytes³³,and the cross-section image of M0 (Fig. 1b) revealed the asymmetric sponge structure typical to PES ultrafiltration membranes (through non-solvent-induced phase separation). No particles appear on either image. In contrast, plan view and cross-section SEM images of membranes M6 and M8 (Fig. 1c-f) revealed distinct particles on the membrane surface and within the porous support. The embedded particles are uniformly distributed, and their observed density was higher for the membrane with the higher β-cyclodextrin loading (M8) as expected. Furthermore, the particle sizes observed with SEM match the particle diameter range of 1-20 µm, measured independently in solution via electrical zone sensing (Fig. 2S).

The incorporation of β-cyclodextrin particles affected the properties of the PES support layer and the active NF surface layer. In M0, the SEM imaging revealed macro voids underneath the skin layer (Fig. 1b), while M6 and M8 had a more unidirectional micropore structure (Fig. 1d & 1f). This structural difference may be attributed to the delayed mass transfer and solvent and nonsolvent demixing during the MMCNF membrane casting process^18,19. A decrease in void volume was confirmed by measuring the bulk porosity (Fig. 2a), which was slightly lower for the MMCNF membranes compared to the pure PES. Surface-bound β-cyclodextrin particles affected the MMCNF surface roughness and charge. The surface roughness of M8 was higher than M0 and had a greater variance, as indicated by the root mean square roughness (R_q) measured using AFM (Fig. 2c). Lower negative surface charge (zeta potential) was recorded for M0 compared to M6 and M8 (Fig. 2b) at ambient pH range (5-9). This loss of negative charge is due to the commercial sorbent being a mixture of positively charged and neutral β-cyclodextrin particles.

The embedded β-cyclodextrin particles also affected the filtration performances of the MMCNF membranes. The pure water permeability increased with increased particle mass load (Fig. 2d), reaching a ~28% increase for M8 compared to M0. The increased permeability suggests a more loose structure of the polyelectrolyte multilayer, which may be attributed to the increased surface roughness (Fig. 2b) caused by near-surface particles. The MWCO of the NF membranes also increased with particle loading (Fig. 2e), indicating increased effective pore size and affirming a more loose active layer structure for MMCNF membranes. Further support was obtained from the decrease in Na₂SO₄ rejection with increased particle loading (Fig. 2e), which may be partly attributed also to the smaller negative surface charge (Fig. 2b) in the MMCNF membranes. Compared to M0, M8 had ~5% higher MWCO and 6% lower Na₂SO₄ rejection. In summary, the β-cyclodextrin particles embedded in the support significantly improved membrane permeability while only slightly affecting neutral and charged solute removal.

2.2 PFOA retention improved upon increasing adsorbent loading

Preliminary experiments with high PFOA feed concentrations (161-345 ppb) revealed that higher loadings of β-cyclodextrin significantly increased PFOA removal efficiency (Fig. 3a). Filtration experiments were carried out with loadings of 0 % (M0), 6 % (M6) and 8 % (M8) w/w (Table 1), using a typical filtration setup (Fig. 1S) operating in a full recirculation mode. During the first 3h of filtration, the control (M0) membrane achieved PFOA removal of ~97%, but afterward, the rejection declined, reaching about 73% after 20 h. Using the M6 membrane, PFOA concentration in the permeate was below the detection limit (1 µg/L) for 10 hours, represented as 100% removal (Fig. 3a). After 20 hours, PFOA was detected in the permeate at a low concentration corresponding to ~99.7% removal and the removal declined to ~99% after 33 hrs. For the membrane with the highest loading, M8, the removal was greater than 99.9% throughout the experimental time (33h). These preliminary results pointed to the potential of the MMCNF concept as an effective barrier for PFAS.

Adsorption and saturation of PFOA in the membranes partly explain the PFOA removal trends (Fig. 3a). As illustrated in Fig. 3b, PFOA removal occurs via two mechanisms: (1) rejection by the NF layer; and (2) adsorption either in the NF layer or the porous support. Consequently, the rejection data form a 'breakthrough curve' shape that depends on the NF layer base-level rejection, adsorption capacity, and kinetics. Batch adsorption results (Table 1S & Fig. 6S) revealed that M8 adsorption capacity for PFOA (5368 µg / g_membrane) is higher than that of M0 (5.99 µg / g_membrane) by a factor of almost 1000. In M8, PFOA was mainly adsorbed on the β-cyclodextrin embedded in the PES support. In contrast, in M0 (which has no β-cyclodextrin), only the pDADMAC-PSS active layer or the PES could provide adsorption sites for PFOA^34–36. Accordingly, PFOA saturation occurred faster for M0, leading to the early drop in the PFOA removal rate until reaching a steady state at ~73%. In contrast, M8 achieved almost complete removal throughout the experiment. For M6, PFOA removal declined earlier than M8 due to its lower β-cyclodextrin loading but higher than M0, supporting the filtration-adsorption explanation. The fact that the removal rate remains high in the MMCNF membranes is addressed at the end of this section. Following these results, the M8 membrane was selected for further investigations and is referred to as MMCNF below.

2.3 The MMCNF membrane achieved high PFOA removal in several filtration-regeneration cycles

In the filtration experiments described in the previous section, the high PFOA feed concentration (161-345 µg/L) enabled us to observe breakthrough behavior in practical experimental times and to compare the composite membranes accordingly. In the current sections, we tested the MMCNF performances under lower feed concentrations (~45 µg/L in milli-Q water), such as those found in highly PFOA-contaminated groundwater¹. Feed concentration may affect removal efficiency depending on the adsorption isotherm and kinetics. Demonstrating high removal at different feed concentrations during longer operation times is imperative.

The highly effective PFOA removal by the M8 MMCNF membrane was maintained in four consecutive cycles during the entire filtration time (Fig. 4). We conducted four filtration-regeneration cycles using the same membrane. Ethanol was used for PFOA desorption due to its lower toxicity than methanol used in previous studies³⁷. PFOA removal was consistently greater than 99.8% during the 1^st and 2^nd cycles for 160 and 185 hours, respectively (Fig, 4a-b). In the 3^rd and 4^th cycles (fig. 4C), PFOA removal slightly decreased but remained very high (99.25-99.9%). Due to the high removal rate, PFOA permeate concentrations were below the EU and EPA 2020 recommendations (0.1 & 0.07 µg/L, respectively), despite the high PFOA feed concentration (45 µg/L, much higher than in typical for contaminated groundwater). A decreasing trend in the PFOA removal rate in the final stages of filtration was not observed in any cycle, indicating that vacant adsorption sites remained. Extrapolation of these results to a more typical (yet still high) groundwater PFOA concentration (e.g., 0.5 µg/L) suggests an effective removal for over two years can be achieved before regeneration is needed.

High PFOA removal efficiency was maintained during filtration and after regeneration throughout all cycles, indicating that the MMCNF was stable in water and ethanol (Fig. 4a-c). In the 4^th cycle, the initial removal rate was lower but later recovered. Filtration experiments using (i) milli-Q water and (ii) 1 mM Na₂SO₄ after each cycle showed a decline in pure water flux (~25%), whereas the salt rejection did not show a significant trend. Interestingly, immersing the membrane in ethanol for four days resulted in an opposite trend, i.e., a 25% increase in pure water flux (Fig. 7S.). Ethanol was previously found to affect the interactions in PSS-PDADMAC complexes³⁸. The salt rejection was decreased by 9%, consistent with the filtration experiments. Fluctuations in water permeability and salt rejection were previously recorded for polyelectrolyte multilayers and can be ascribed only to the thin NF layer³⁹. NF layer stability will be tested for longer times in the future and can be further improved by, e.g., cross-linking.

2.4 High PFOA removal was maintained for spiked tap water

When using the MMCNF to treat PFOA-spiked tap water, the removal rate was very high in three consecutive filtration-regeneration cycles, despite competing inorganic ions⁴⁰ and other trace organics (Fig. 5a-b). A reproducible breakthrough behavior, typical to adsorption, was observed after ~45h for the 1^st and 3^rd cycles and ~32h for the 2^nd cycle. The breakthrough behavior can be related to the higher PFOA feed concentration (~500 µg/L) used in tap water, which was ~10-fold higher than in the DI experiments depicted in Fig. 4. The first 45h, in which the MMCNF membrane achieved 99.9% PFOA removal, translates to ~712 L of treated drinking water for every m² of the membrane (Fig. 5a) before regeneration is needed. Extrapolating to a more typical PFOA concentration in contaminated groundwater (0.5 µg/L) translates into several years of operation at a typical flux (15-25

Low eluent volume in the desorption step resulted in a concentrated PFOA-in-ethanol solution. Concentrating PFOA from a diluted source could facilitate cheaper and more effective destruction or disposal. Similar to the DI experiments, regeneration after spiked tap-water filtration fully restored the membrane adsorption capacity. PFOA removal rate returned to the high initial value after each regeneration step (Fig. 3b), indicating successful desorption, further affirmed by the high PFOA concentration in the ethanol eluent (Fig. 3c). The regeneration step did not significantly affect the salt rejection. At the same time, pure water permeability decreased by ~30% (Fig. 8S), whereas the average water flux during the filtration steps decreased only by 12% (Fig. 5C). mass balance revealed that PFOA was concentrated in ethanol by a factor of 10-38 compared to the feed, with a 29% PFOA surplus in ethanol after the 2^nd cycle (Fig 9Sa) and a 19% deficit after the 3^rd cycle (Fig 9Sb). These discrepancies may be related to adsorption/desorption from tubing or analytical issues. We expect much higher concentration factors would be attainable for a lower eluent-volume to membrane-surface area ratio and lower PFOA feed concentration, which requires long-duration experiments and a higher membrane surface area than reported here. Nevertheless, the results indicate that MMCNF can remove PFOA from a diluted source into a concentrated solution (retentate or eluent) in a reusable manner.

2.5 PFOA adsorption enhances its rejection by affecting the membrane surface charge

PFOA removal by the MMCNF occurs through adsorption and rejection by the NF layer (Fig. 3b). Distinguishing between these two mechanisms is imperative for optimizing the MMCNF membrane and process. Using feed (Fig. 10Sc) and permeate (Fig. 4C) PFOA concentrations from a spiked Milli-Q water filtration experiment (cycle 4), we performed a mass balance to estimate PFOA rejection by the NF layer (assuming that the PFOA mass reduced from the feed was added to the permeate – see Table 2S and associated explanations). The mass balance (Fig. 7a) resulted in lower-than-expected PFOA rejection in the first 10 hrs (35%). We attribute this result to the adsorption of PFOA from feed water not passing the membrane, either on the membrane surface or tubing. As more PFOA adsorbed to the membrane, the estimated rejection by the NF layer gradually increased to 77% and fluctuated within the range of 60-73%, which is similar to the steady state removal of the control NF (M0, Fig. 3a). The results thus suggest that in long-term filtration, ~70% of PFOA will remain in the concentrate, ~30% will be adsorbed and ~0.1% will pass to the permeate. Optimizing the active layer for higher PFAS rejection can further prolong the time between regeneration steps.

We propose that the increasing rejection of PFOA by the NF active layer (Fig. 7a) is due to enhanced negative surface charge induced by PFOA adsorption (Fig. 7c). PFOA molecules are negatively charged at ambient pH²⁶ and can be electrostatically adsorbed to the positively charged quaternary ammonium groups in the polyelectrolyte layer (the polyDADMAC's charge that the PSS did not neutralize). PFOA adsorption, therefore, induces excess negative charge, which is confirmed by zeta potential measurements of unused and spent M0 (Fig. 7b). In the MMCNF membrane, PFOA also adsorbs to the surface-bound Dexsorb sorbent (Fig. 1c&e). The latter is a mix of positively charged and neutral particles, which adsorbs PFOA through electrostatic and hydrophobic interactions. Therefore, the increase in surface charge following PFOA adsorption is more prominent for M8 (Fig. 7b), likely contributing to higher electrostatic repulsion¹⁶ of anions.

2.6 Significance and prospects

In previous studies, commercial and lab-made nanofiltration membranes achieved 80 - 99 % PFOA rejection, depending on membrane characteristics and other operating conditions ¹⁷^,¹⁸^,^24,^29,²⁵ (Table. 3). The MMCNF membrane developedin this study showed markedly improved PFOA removal (> 99.9) due to its dual functionality. The removal was in-par with removal by reverse-osmosis membranes with the advantage of generating a lower salinity retentate, thus saving energy and brine treatment costs. The MMCNF developed here also significantly exceeds the performance of a dual-functional membrane suggested recently²⁹, which achieved 70% PFOA removal. Moreover, the membrane was reusable after a simple regeneration cycle. Therefore, our dual-functional MMCNF membrane is a significant step forward in PFAS removal technology from drinking water.

Table. 3 Comparison of the current work with similar recently published work on PFOA removal by reverse osmosis and nanofiltration membranes.

Membrane Type^ref	MWCO/pore diameter	Filtration Experiment Unit	Feed PFOA (µg/L)	Water permeability (Lm^-2h^-1bar^-1)	Water Matrix and maximum PFOA removal (%)
Polyamide NF (lab)¹⁸ NF 270 (commercial)¹⁸	1.2 nm 0.8nm	Crossflow Crossflow	1000 1000	~ 12 ~ 21	DI water- 90 DI water- 90
Fully aromatic polyamide advanced composite membrane (commercial)¹⁷	200 Da	Crossflow	5, 50, 100	~ 5	DI water 97.3- 99.85 Spiked groundwater 99.54
SiO₂/CMWCNT/PMIA hollow fiber NF (Lab)²⁴	661 Da	Crossflow	25-100	-	DI Water- 95.3 to 98.3
Poly-N-isopropylacrylamide (PNIPAm) pore-functionalized microfiltration support-based NF (Lab)²⁹	-	Crossflow	70	~12	DI water + 2 mM CaCl₂- 70
Polyelectrolyte (PDADMAC, PSS) Multilayer Nanofiltration Membranes²⁵	-	Crossflow	1000	~12.5	DI water- ∼ 90 DI water+ combined salts- ∼ 90
Mixed Matrix Composite Nanofiltration membrane (This study)	346 Da	Crossflow	45-500	~9	~ 99.9

The composite mixed-matrix membrane synthesis approach presented in this work can be further optimized and adjusted for treating a wide range of microcontaminants. Porous PES filters (used in this study) support the thin active film in most commercial composite membranes and many novel ones. Therefore, the active layer can be modified based on separation needs, e.g., denser NF / RO layers for short-chain PFAS or looser NF layer, or even no layer for larger PFAS or other microcontaminants. Specifically, polyelectrolyte multilayer deposition (used here) is a flexible and tunable approach that can be optimized to achieve desired membrane performance. Further performance enhancements can be expected from optimizing the embedded sorbent. Furthermore, using nanoparticles instead of microparticles could improve adsorption capacity and kinetics, allowing even higher contaminant removal for longer filtration cycles. Beyond that, the sorbent type can be adjusted to target different organic and inorganic contaminants.

3.1 Materials and chemicals

High molecular weight polyether sulfone (PES, Ultrason E6020P with MW 58000 g/mol) flakes and N-Methyl-2-pyrrolidone (NMP) solvent were used to prepare the ultrafiltration support layer. A mixture of cross-linked β-cyclodextrin polymer-based neutral and positively charged adsorbent Dexsorb (50 % Dexsorb and 50 % Dexsorb +) with particle size < 45 µm was provided by CycloPure, Encinitas, CA. The Dexsorb was further sieved (Arilevy, Petah-Tikva, Israel) to obtain a lower particle size (< 20 µm). 20 % Poly diallyldimethylammonium chloride (PDADMAC) with molecular weight 400,000 to 500,000 and poly (sodium 4- styrenesulfonate) (PSS) with a molecular weight of 100,000 kDa (Sigma-Aldrich) were used to prepare polyelectrolyte multilayers. Sodium Metabisulfite was used to prepare the membrane storage solution. Methanol (Bruker), Ammonium Acetate (Sigma Aldrich), Milli-Q water (LCMS grade), 24 native compound mix (PFAC-24PAR), and labeled compound mix (MPFAC-24ES) (Wellington Laboratories, Greyhound, UK), PFOA 40% (Sigma-Aldrich, XX) was used for PFOA measurements.

3.2 Preparation of MMCNF membrane and crossflow setup

3.2.1 Preparation of Mixed-matrix ultrafiltration support layer

A (~400 µm) PES UF support layer loaded with varying concentrations of commercially available β-cyclodextrin based Dexsorb adsorbent was fabricated using the phase inversion method. The UF membrane casting solution was prepared by dissolving PES (17 wt %) in NMP at room temperature. PES casting solution with 0, 6, and 8 wt % of Dexsorb (from the total casting solution weight) was used to form the mixed matrix UF support. Briefly, Dexorb was added into the NMP solution while stirring at 800 rpm for 20 min. Next, PES was added to the solutions in small amounts over 2h. The solution was stirred at 300 rpm for 24 h and left untouched for a few hours to remove air bubbles.

Afterward, the casting solution was poured onto a non-woven fabric on a glass plate, and a casting blade was used to cast a ~ 400 μm thick membrane (thickness excluding non-woven support). The glass plate with the membrane was transferred into a water bath at 25 °C. After 2 h, the membrane was kept in Milli-Q water with 0.25 wt % sodium metabisulfite solution. The prepared support layers were named UF0, UF6, and UF8, having dexsorb loading of 0.0, 6, and 8 wt %, respectively.

3.2.2 Fabrication of negatively charged six-bilayer MMCNF

Positively charged PDADMAC (1 g/L in 0.5 M NaCl solution) and negatively charged PSS (1 g/L in 0.5 M NaCl solution) were used to prepare a polyelectrolyte nanofiltration thin active layer on the mixed matrix ultrafiltration support layer. PDADMAC and PSS were selected for the preparation of the NF active layer as they are strong polyelectrolytes, stable in a wide pH range, and proven to be the appropriate pair for nanofiltration active layer preparation⁴¹. The NF layer was fabricated on the top of UF support layers (UF0, UF6, UF8) through the deposition of six PDADMAC and PSS bilayers, thus forming the MMCNF with a negative surface charge. The MMCNF were named M0 (0 % Dexsorb loading), M6 (6 % Dexsorb loading), and M8 (8 % Dexsorb loading) following the support UF membranes used. Only six polyelectrolyte bilayers were deposited for fabricating the NF active layer because, due to the deposition of more than six bilayers, excess positive charge accumulates on the membrane's surface, resulting in a lower negative surface charge⁴².

3.2.3 Crossflow membrane setup

A crossflow membrane setup with an effective membrane area of 22 cm² was used to conduct the filtration experiments in full recirculation mode (Fig.1S). The permeate and concentrate were recirculated back to the feed tank. All filtration experiments were carried out at low operating pressure (at 2 bar to maximize PFOA adsorption on the support layer by increasing the contact time between PFOA and Dexsorb particles) and crossflow velocity of 11.20 cm/sec.

3.3 Characterization of membranes

M0, M6, and M8 membranes were characterized to investigate the following properties. Surface and cross-sectional Field Emission Scanning Electron Microscope (FESEM) micrographs were obtained using a JSM-IT200 instrument (JEOL, Japan). Furthermore, the top surface morphology of the PES support and MMCNF membranes were compared using Atomic Force Microscopy (AFM) imaging by NanoWizard 4 microscope (JPK Instruments, Bruker Nano GmbH, Berlin, Germany). The membranes' Molecular weight cutoff (MWCO) was obtained using 1g/L of Glucose (180 Da), Sucrose (354 Da), Raffinose hydrate (494 Da), and PEG 1000^43,44. The membranes' water permeabilities were calculated as in Kamp et al. (2021) ³⁹.

The bulk porosity (P) of membranes was obtained using eq. 1.

Where w₁ and w₂ are the weights of wet and dry membranes, respectively, while ρ, A, and l are the density of water, area, and thickness of the membranes, respectively. The Zeta potential of the membranes at varying pH values was measured using a SurPASS electrokinetic analyzer (Anton Paar, Graz, Austria). During measurement, 10 mM KCl electrolyte solution was passed through an adjustable gap cell (at 100±7 µm gap) at 400 mbar pressure. Sodium sulfate rejection was measured using the following equation,

C_f and C_p are the feed and permeate conductivity (µS/cm), respectively.

3.4 PFOA adsorption, rejection, and membrane regeneration

Initially, PFOA removal performances of three MMCNF membranes, M0, M6, and M8, were compared to identify the best dexsorb loading. The best-performing membrane (M8) was used for further PFOA adsorption, rejection, and regeneration experiments. Other experimental conditions, such as transmembrane pressure (2 bar), temperature (25 °C), and feed flow rate (22 L/h), were kept constant for all filtration experiments.

Batch adsorption experiments were carried out at different PFOA concentrations (100 µg/L; 250 µg/L; 500 µg/L; 1000 µg/L; 2000 µg/L; 5000 µg/L) using only PES support layer (UF0) and mixed matrix support with optimized loading (UF8) with similar area and mass (9 cm² of membrane weighing 0.15 g). All experiments were conducted at a stirring speed of 200 rpm, 48 h contact time, and a temperature of 25 ^oC. The experimental data were fitted to adsorption isotherms (see supplementary information) to estimate the PFOA adsorption capacity of the mixed-matrix membranes (in both mg/g and mg/m2).

Rejection experiments were carried out in crossflow at recirculation mode using Milli-Q water spiked with ~45 µg/L PFOA and with tap water spiked with ~500 µg/L PFOA. Experimental conditions such as operating pressure, crossflow rate, and feed temperature were kept as described earlier in this section. According to equipment PFOA measurement capacity (LC-MS-MS), to estimate the permeate concentration in ppt (only for spiked Milli-Q), permeate samples were concentrated 20 times by reducing the liquid volume at 38 ^oC.

The stability of MMCNF towards regeneration was evaluated using ethanol (99%) at static and dynamic filtration conditions. At static conditions, M8 membranes were kept in an ethanol solution for up to 4 days. Afterward, membrane permeability and salt rejection were examined and compared with the initial flux and salt rejection values. Stability tests in dynamic filtration conditions were performed by four consecutive filtration cycles (each about 180 h) at 2.0 bar operating pressure with a feed containing milli-Q water spiked with ~45 µg/L of PFOA. Following each filtration cycle, the feed was replaced by ethanol for 6 hours (similar filtration conditions). Following regeneration, the properties of the membranes were assessed (i.e., permeability and salt rejection).

MMCNF membrane regeneration experiments were carried out up to three consecutive cycles for PFOA spiked Tap water using the same procedure used during the membrane stability test in dynamic filtration mode. PFOA spiked Tap water (Composition: Ca²⁺: 36 mg/L, Mg²⁺: 8 mg/L, K⁺: 1.7 mg/L, Na⁺: 46.2 mg/L, Cl^-: 77 mg/L, S: 10 mg/L, pH: 7.7, Conductivity: 500 µS/cm, TOC: ~ 2 mg/L) collected from Midreshet Ben Gurion, Israel was used as feed during PFOA rejection and membrane regeneration experiments.

3.5 PFOA measurements

PFOA was analyzed using an Agilent Technologies LCMSMS, comprised of a 1260 Infinity II pump (model G7111B) coupled to a triple quadrupole (model G6465B) mass spectrometer and ESI source (Agilent Technologies Inc., Santa Clara, CA, USA). The system was adjusted to PFAS measurements using polypropylene tubes and aluminum filters for the eluent bottles and polypropylene vials with PFAS-free septas. A delay column was installed before the injector. The analytical column was ZORBAX RRHD Eclipse Plus C18, 2.1 × 100 mm,1.8 μm (Part No. 959757-302). The binary solvent gradient consisted of (a) Milli-Q water with 5 mM Ammonium acetate and (b) 95% MeOH 5% water with 5 mM Ammonium-acetate. The gradient method conditions were: 0 minutes 10% B, 1.5 minutes 10% B, 3 minutes 30% B, 15 minutes 95% B, 15.5 minutes 100% B, 16.5 minutes 10% B, run ended at 21 minutes and had 7.5 minutes post-time. The flow rate was 0.4 mL/min, and the column temperature was 50°C. Seven-point calibration curves were developed for each target analyte by diluting calibration stock at concentrations ranging from 1 to 100 ng/mL. All concentration levels were spiked with the internal standard at 10 ng/mL; the injection volume was 30 μL. Standards and samples were prepared in water. PFOA in experiment samples was identified by comparing chromatographic peak retention times and MRM parameters (specific qualifier and quantifier ions) with those of analytical standards. Quantification was done based on the response of PFOA in the sample relative to the internal-standard responses. Measurements were done on a negative ESI mode, with the gas temperature at 300°C, gas flow at 15 L/min, nebulizer pressure at 15 psi, and capillary voltage at 2500 V. The ion source conditions for PFOA measurement included 69 V fragmentor and precursor ion m/z 413 for internal standard M8PFOA precursor ion m/z 421. Product ions for PFOA were m/z 369 and 169, with collision energies 3V and 9V, respectively. For the M8, the PFOA product ion was m/z 376 with collision energy 3V.

Data Availability

Data generated or analyzed during this work is included in this manuscript and its supplementary information file. Raw data can be made available upon request from the authors.

Acknowledgments

The authors thank the German Federal Ministry of Education and Research (BMBF; 02WIL1660A and 02WIL1660B) and the Israeli Ministry of Science and Technology (MOST) for funding within the German-Israeli Water Technology Cooperation Program. M.C. acknowledges the Kreitman School of Advance Studies for providing his fellowship. We thank CycloPure, Encinitas, CA, for providing the Dexsorb powder for this research work.

Author Contributions

MC contributed to the development and synthesis of the mixed-matrix nanofiltration membranes, filtration experiments design, conducting the experiments, writing the draft, and data analysis. MSA contributed to developing the PFOA analysis methodology using LCMSMS, performing the analysis, quality assurance, and writing the methodology. AR contributed to methodology development, experimental design, review & editing, and funding acquisition. ON contributed to creating the concept, developing the methodology, experimental design, supervision, review & editing, writing the draft, and funding acquisition. All authors read and approved the final manuscript.

Competing Interests

The authors declare no competing interests.

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Efficient PFOA removal from drinking water by a dual-functional mixed-matrix-composite nanofiltration membrane

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