System, method and bicontinuous polyelectrolyte complex-layered membrane for selective recovery of volatile fatty acids

Abstract

A bicontinuous polyelectrolyte complex (PEC)-layered membrane for selective recovery of volatile fatty acids (VFAs), the bicontinuous PEC-layered membrane including: a membrane, and a PEC layer disposed on the membrane, the PEC layer including a hydrophobically modified cationic polyelectrolyte and an anionic polyelectrolyte, wherein the PEC layer includes a bicontinuous structure due to complexation-induced phase separation driven by hydrophobicity contrast between the hydrophobically modified cationic polyelectrolyte and the anionic polyelectrolyte.

Description

SYSTEM, METHOD AND BICONTINUOUS POLYELECTROLYTE COMPLEX-LAYEREDMEMBRANE FOR SELECTIVE RECOVERY OF VOLATILE FATTY ACIDSRELATED APPLICATION

[0001] The present patent document claims the benefit of priority under 35 U.S.C. 119(e) to U.S. Provisional Patent Application 63 / 678,830, which was filed on August 2, 2024, and is hereby incorporated by reference in its entirety.TECHNICAL FIELD

[0002] The present disclosure is related generally to membrane separations, and more specifically to a bicontinuous polyelectrolyte complex (PEC)-layered membrane for selective recovery of volatile fatty acids (VFAs) and a redox-mediated electrodialysis (redox-ED) system including the same.BACKGROUND

[0003] VFAs are aliphatic carboxylic acids with carbon numbers of 2 to 5 and important platform molecules for fine chemical production. VFAs can be produced through the anaerobic fermentation of organic waste, which offers a sustainable and promising alternative to the catalytic processes that rely on petrochemical feedstocks. Redox-ED systems have been investigated for selective VFA separation due to higher energy efficiency, modular configuration, and less chemical footprint. However, previous studies have primarily focused on the recovery of a single organic species and removal of inorganic salts from organic ions, and there has been limited exploration into designing efficient and selective recovery of VFAs from other organic acids having similar physicochemical properties (e.g., lactic acid and longer-chain fatty acids).BRIEF DESCRIPTION OF THE DRAWINGS

[0004] FIG. 1A is a schematic illustration of a tailored PEC layer for selective recovery of VFAs.

[0005] FIG. IB is a schematic illustration of a tailored PEC layer that permits the passage of VFAs but blocks medium chain fatty acid or lactic acid.

[0006] FIG. 1C is a schematic illustration of selective recovery of VFAs using an exemplary redox-mediated electrodialysis (redox-ED) using a tailored PEC layer.

[0007] FIG. 2A is a schematic illustration of a layer-by-layer (LBL) deposition of protonated poly(ethylene imine (PEI) or S-PEI and sodium poly(styrene sulfonate) (PSS) on polyamide (PA) nanofiltration (NF) membrane support.

[0008] FIG. 2B shows a chemical equation of an aza-Michael addition of PEI with a vinyl sulfone to synthesize a sulfonyl poly(ethylene imine) (S-PEI) and the partition coefficient (Log P) values of various vinyl sulfones estimated by Virtual Computational Chemistry Laboratory software.

[0009] FIG. 2C is a schematic illustration of three different morphologies of a PEC: nanoparticular PEC network, bicontinuous PEC layer, and macroscale PEC aggregates.

[0010] FIG. 2D is a schematic illustration of complexation-induced phase separation behaviors of hydrophobically modified cationic polyelectrolyte and unmodified cationic poly electrolyte.

[0011] FIG. 3A shows conversion rates of aza-Michael addition reactions of PEI with methyl vinyl sulfone (MVS), ethyl vinyl sulfone (EVS), and divinyl sulfone (DVS) in deuterated water estimated by1H-nuclear magnetic resonance (1H-NMR) spectroscopy.

[0012] FIG. 3B shows average hydrodynamic diameter of PEI, PEI-MVS, PEI-EVS, and PEI-DVS at different pHs, monitored by Dynamic Light Scattering (DLS) analysis.

[0013] FIG. 3C shows a phase diagram of PSS / PEI complexes

[0014] FIG. 3D shows a phase diagram of PSS / PEI- EVS complexes.

[0015] FIG. 3E shows tie line analyses of complexation-induced phase separation behaviors of PSS / PEI (Point A) and PSS / PEI-EVS complexes (Point B), with straight lines connecting the polymer concentrations of each coexisting phase.

[0016] FIG. 4A shows liquid atomic force microscopy (AFM) images of a pristine NF membrane, a 5PSS / 5PEI / NF membrane, and a 5PSS / 5PEI-EVS / NF membrane.

[0017] FIG. 4B shows a cross-sectional Scanning Electron Microscopy (SEM) image of a 5PSS / 5PEI-EVS / NF membrane.

[0018] FIG. 4C shows a cross-sectional Transmission Electron Microscopy (TEM) image of a microtomed 5PSS / 5PEI-EVS / NF membrane.

[0019] FIG. 4D shows an AFM height profile of a 5PSS / 5PEI-EVS / PA layer detached from a 5PSS / 5PEI-EVS / NF membrane.

[0020] FIG. 5A shows attenuated total reflectance-Fourier transform infrared (ATR- FTIR) spectra of a pristine NF membrane, a 5PSS / 5PEI-EVS / NF membrane, a 10PSS / 10PEI- EVS / NF membrane, and a 15PSS / 15PEI-EVS / NF membrane.

[0021] FIG. 5B shows the variation in the quantities of PSS in a PSS / PEI-EVS / NF membrane and a PSS / PEI / NF membrane with increasing number of LBL deposition, estimated from the characteristic ATR-FTIR peak of sulfonate group.

[0022] FIG. 5C include graphs showing high-resolution X-ray Photoelectron Spectroscopy (XPS) spectra of a 5PSS / 5PEI-EVS / NF membrane.

[0023] FIG. 5D shows the variation in water contact angle (WCA) of a PSS / PEI-EVS / NF membrane and a PSS / PEI / NF membrane with increasing number of LBL deposition.

[0024] FIG. 5E shows the variation in surface zeta potential (mV) of a PSS / PEI-EVS / NF membrane and a PSS / PEI / NF membrane with increasing number of LBL deposition.

[0025] FIG. 6A is a schematic illustration of an example continuous redox-ED system.

[0026] FIG. 6B shows changes in molar ion flux of organic acids (C3, C4, and C6) through a PSS / PEI-EVS / NF membrane with increasing number of LBL deposition.

[0027] FIG. 6C shows molar ion flux ratio of VFA (C3 + C4) / C6 versus the molar ion flux of VFA (C3 + C4).

[0028] FIG. 6D shows molar flux ratio of C3 / C6, C4 / C6, C3 / C4 of a double-layered pristine NF, a double-layered 5PSS / 5PEI / NF membrane, and a double-layered 5PSS / 5PEI- EVS / NF membrane.

[0029] FIG. 6E shows flow-through ion conductivity of organic acids (C3, C4, C6) through a PSS / PEI-EVS / NF membrane.

[0030] FIG. 6F shows molar flux ratio of C3 / C6, C4 / C6, C3 / C4 of a 5PSS / 5PEFNF membrane, a 5PSS / 5PEI-MVS / NF membrane, a 5PSS / 5PEI-EVS / NF membrane, and a 5PSS / 5PEI-DVS / NF membrane.

[0031] FIG. 7A is a schematic illustration of an example continuous redox-ED system.

[0032] FIG. 7B shows concentration of VFAs accumulated in the accumulation tank after operation of redox-ED using a synthetic anaerobic digestion effluent.

[0033] FIG. 7C shows specific energy consumption of redox-ED integrated with 5PSS / 5PEI-EVS / NF membrane and commercial AEM for synthetic effluent separation.

[0034] FIG. 7D shows organic acid distribution in an accumulation tank over time during operation of the batch mode redoc-ED using a 5PSS / 5PEI-EVS / NF membrane.

[0035] FIG. 7E shows a comparison of specific energy consumption for VFA separation using different ED systems.DETAILED DESCRIPTION

[0036] Described in this disclosure are a bicontinuous PEC-layered membrane for the selective recovery of VFAs and an energy-efficient redox-ED VFA separation system that incorporates the bicontinuous PEC-layered membrane. A method of selectively recovering VFAs from a multicomponent feed solution is also described.

[0037] Referring to FIG. 2A, the bicontinuous PEC-layered membrane 10 includes a membrane 12 and a PEC layer 14 disposed on the membrane 12, where the PEC layer 14 includes a hydrophobically modified cationic polyelectrolyte and an anionic poly electrolyte. The PEC layer 14 may have a bicontinuous structure (e.g., see FIG. 2C) due to complexation-induced phase separation driven by hydrophobicity contrast between the hydrophobically modified cationic polyelectrolyte and the anionic polyelectrolyte.

[0038] The term “bicontinuous structure” may be understood to refer to the structure of a layer having at least two distinct phases interwoven in a continuous manner throughout the layer, where each phase may form a continuous pathway from one surface of the layer to another surface of the layer (e.g., see FIG. 2C). In the context of a PEC layer, the first phase may be a domain predominantly composed of cationic polyelectrolytes, and the second phase may be a domain predominantly composed of anionic poly electrolytes. The bicontinuous structure described in this disclosure may account for at least 80 % of the PEC layer 14, or preferably at least 90% of the PEC layer 14. More preferably, the PEC layer 14 may consist of the bicontinuous structure; that is, the bicontinuous structure may account for 100%, or up to 100%, of the PEC layer 14.

[0039] The hydrophobically modified cationic polyelectrolyte may comprise a product of an aza-Michael addition reaction between a cationic polyelectrolyte having amine groups, hydroxyl groups, or thiol groups as a Michael receptor and an acrylate derivative, an acrylamide derivative, or a vinyl sulfone amide derivative as a Michael acceptor. The hydrophobically modified cationic polyelectrolyte may comprise sulfonyl polyethylene imine (S-PEI). For example, the hydrophobically modified cationic poly electrolyte may comprise polyethylene imine-methyl vinyl sulfone (PEI-MVS), polyethylene imine-ethyl vinyl sulfone (PEI-EVS), or polyethylene imine-divinyl sulfone (PEI-DVS), preferably PEI-MVS or PEI- EVS, more preferably PEI-EVS. The anionic polyelectrolyte may comprise polystyrene sulfonate (PSS). In one example, the PEC layer comprises PSS / PEI-EVS complex.

[0040] Referring to FIG. 1A, molecular level modification of polyelectrolytes is found to facilitate complexation and interfacial phase separation behaviors, enabling uniformformation of the nanoscale bicontinuous PEC layer 14 on top of the membrane 12. It is believed that the interplay between the transition of complexation-induced phase separation mechanism and surface-confined layer-by-layer (LBL) deposition is a key to realizing bicontinuous structures having a controlled morphology. The morphology-controlled PEC layer 14 provides ion conduction pathways that enhance the transfer of VFA anions, as illustrated in FIG. IB, simultaneously exhibiting enhanced VFA selectivity among organic acids due to steric hindrance and hydrophobicity of the membrane 10.

[0041] Referring to FIG. 2A, a facile polyelectrolyte modification method to modulate the complexation-induced phase separation behaviors, and LBL deposition to confine the structure formation at the nanoscale on the membrane surface are the keys in developing of a bicontinuous PEC layer 14. In an example of FIG. 2A, an asymmetric polyamide (PA) NF membrane is used as the membrane 12 because of its lower cost than commercial ionexchange membranes as well as its moderate resistance and residual surface charges for LBL deposition.

[0042] LBL deposition involves sequentially depositing alternative layers of cationic polyelectrolytes and anionic polyelectrolytes onto a membrane. During the LBL assembly process, the polyelectrolyte complexation is induced by interaction between the oppositely charged polyelectrolytes, followed by phase separation. The progress of phase separation is limited due to the hindered diffusion of polyelectrolyte chains that are bound to a membrane surface or confined within the alternatively deposited cationic / anionic polyelectrolyte layers. Surface-confined LBL deposition enables the creation of a PEC layer 14 with tailored surface properties which is inaccessible through bulk complexation.

[0043] Referring to FIG 2C, the complexation-induced phase may follow two different mechanisms (associative and segregative) depending on the characteristics of the poly electrolytes. A bicontinuous structure is likely to form when the molecular modification balances the associative interaction (e.g., electrostatic attraction and hydrogen bonding) and segregative interaction between the poly electrolytes.

[0044] A balance between associative and segregative interactions may be controlled by modifying hydrophobicity of the polyelectrolytes because the phase separation is driven by hydrophobicity contrast between cationic polyelectrolytes and anionic polyelectrolytes forming the PEC layer 14 (e.g., see FIG. 2D).

[0045] To modify the hydrophobicity of polyelectrolytes and molecularly control the intrinsic phase separation behaviors, aza-Michael addition reaction may be used. Specifically,the hydrophobically modified cationic polyelectrolyte may be prepared via aza-Michael addition reaction of a cationic polyelectrolyte having amine groups, hydroxyl groups, or thiol groups as a Michael receptor with an acrylate derivative, an acrylamide derivative, or a vinyl sulfone amide derivative as a Michael acceptor. Referring to FIG. 2B, as an example, the hydrophobically modified cationic polyelectrolyte may be prepared via aza-Michael addition reaction of branched PEI as a Michael receptor with vinyl sulfones as a Michael acceptor. Selecting a Michael acceptor with an appropriate hydrophobicity allows for easy adjustment of complexation-induced phase separation behaviors and the transition from associative to segregative separation mechanism to form a PEC layer 14 exhibiting a bicontinuous structure. The resulting bicontinuous PEC layers have tailored surface properties that improve ion transport and separation performance.

[0046] Referring to FIG. 1C, the bicontinuous PEC-layered membrane 10, 110 may be integrated into a redox-ED system 100. As shown in FIG. 1C, one example of a redox-ED system 100 includes a first electrode (e.g., a positive electrode) 102, a second electrode (e.g., negative electrode) 104 positioned in opposition to the first electrode 102, and a first and second cation exchange membranes (CEMs) 106, 108 positioned between the first electrode 102 and the second electrode 104. The system 100 further includes the bicontinuous PEClayered membrane 110 (e.g., VFA-selective NF) disposed between the first and the second CEMs 106, 108.

[0047] Referring now to FIGs. 6A and 7A, a feed channel (FC) 112 extends between the PEC-layered membrane 110 and the second CEM 108 for delivery of a multicomponent feed solution including organic acids (e.g., synthetic anaerobic digestion effluent or cow manure anaerobic digestion effluent). An accumulating channel 114 extends between the bicontinuous PEC-layered membrane 110 and the first CEM 106 for collecting charged organic species separated from the multicomponent feed solution. The system 100 also includes a redox channel 116 that contains the first and second electrodes 102, 104 and is configured for continuous circulation of a redox fluid or solution; that is, the redox channel 116 may form a closed loop. The redox solution may include ferri- / ferrocyanide ([Fe(CN)6]3- / ([Fe(CN)6]4-) as redox-active mediators that lower the minimum energy requirement by replacing water splitting reaction. Flow through each of the channels (feed, accumulating and redox channels 112, 114, 116) may be independently controllable, e.g., by one or more pumps. The system 100 may include a power supply connected to the first and second electrodes for application of a suitable voltage (potential).

[0048] A redox-ED system 100 using the bicontinuous PEC-layered membrane 110 described in this disclosure may exhibit enhancement of both ion permeability and selectivity toward VFAs, with notable reduction of energy consumption by up to 80 % compared to conventional electrodialysis (see FIG. 7E). Thus, the redox-ED system 100 may be used in a multichannel flow-system for the recovery of VFAs from various multicomponent feedstock solutions, such as, for example, real cow manure fermentation broths. Treatment of synthetic and cow manure fermentation effluents described below showcases 2 to 4-fold enrichment of VFAs and simultaneous removal of co-existing organic acids, with an energy consumption as low as 1.5 kWh kg'1.

[0049] EXAMPLES

[0050] Example 1: Synthesis of s-PEI via aza-Michael addition reaction

[0051] Branched poly(ethyleneimine) (PEI) (average Mw - 25 kDa by light scattering, average Mn - 10 kDa) was used as a Michael receptor, and methyl vinyl sulfone (MVS), ethyl vinyl sulfone (EVS), and divinyl sulfone (DVS) were used as a Michael acceptor.

[0052] A PEI solution (2 g L'1) was prepared by dissolving branched PEI in deionized water. Into 18 mL of PEI solution 0.749 mmol of vinyl sulfones (0.079 g for MVS and 0.09 g for EVS) was added. For DVS, 0.375 mmol (0.044 g) was added considering its two vinyl sulfone functional groups. The solution was stirred for 24 h at room temperature. The resultant S-PEIs were named as PEI-MVS, PEI-EVS, and PEI-DVS, respectively.

[0053] Referring to FIG. 3A, the synthesis of PEI-MVS, PEI-EVS, and PEI-DVS was monitored during the reaction of PEI and vinyl sulfones in deuterated water by1H- NMR spectroscopy using Varian U500 spectrometer (500 MHz). The available quantity of primary amines was estimated from elemental analysis and NMR spectroscopy because the reaction maintains the number of amine groups used for the following protonation and complex formation with anionic poly electrolytes. As shown in FIG. 3 A, the conversion of PEI to PEI- MVS and PEI-EVS reached a plateau close to 50 % after 24 h of reaction, which corresponds to the conversion of available primary amine groups of the pristine PEI. By contrast, only 18 % of amine groups reacted with DVS due to the low solubility of PEI-DVS in water which may slow down the reaction.

[0054] Example 2: Phase behaviors of S-PEI / PSS complex

[0055] Referring to FIG. 3B, DLS analysis was carried out at different solution pHs using a model Zetasizer Nano (Malvern Panalytical) to examine the effect of alkyl sulfonyl modification on the hydrophobicity of the S-PEI species. The hydrodynamic diameter of thepristine PEI ranged in 3-6 nm at overall pH ranges. In contrast, the hydrodynamic diameter of PEI- MVS particles dramatically decreased from about 200 nm to 5-6 nm at pHs lower than pH = 6, as well as change from a translucent to transparent solution. The amine groups are protonated at pHs lower than pKa, which may range in 8.2-9.5 as for linear PEI, increasing the solubility of the PEI-MVS in water. The PEI-EVS solution became transparent at pHs lower than pH = 4, indicating that PEI-EVS requires a higher degree of protonation to be solubilized because of its longer alkyl chains than PEI-MVS. In contrast, PEI-DVS formed a transluscent dispersion of 200 nm-sized particles. The poor solubility may limit its use for generating uniform active layers by aqueous-phase LBL deposition.

[0056] Referring to FIGs. 3C and 3D, the alkyl sulfonyl modification also significantly changed the complexation behaviors of PSS and PEI (FIG. 3C), and PSS and PEI-EVS (FIG. 3D). Bulk polyelectrolyte complexes were obtained by directly mixing protonated PEI or PEI-EVS solution (pH = 4) with PSS solution at different concentration ratios and the resultant phases were summarized in the phase diagrams. The molar concentration of the polyelectrolytes (PEI, PEI-EVS, and PSS) was used to compare the phase behaviors given the equivalent amount of amine groups.

[0057] Referring to FIG. 3C, the PEI / PSS complexes exhibited a broad range of different phases depending on the poly electrolyte concentrations. At relatively low PEI concentrations (< [PEI] = 6.0xl0'5M), mixing PSS led to formation of transparent nanoparticulate dispersion (sol) (see FIG. 2C), and rapid precipitation at much higher PSS concentrations. The transparent dispersion region extended following the increase of PEI concentration, and PEI / PSS complex formed opaque microparticle suspensions. Interestingly, transparent complex coacervate droplets also formed at higher PSS concentrations ([PSS] = 1.4xl0'4M). The observed complexation of PSS and PEI at low ionic strength was expected to follow an association-dominant phase separation mechanism, which forms PEC phases that are rich in both cationic and anionic polyelectrolytes by their association. Referring to FIG. 3E, phase composition analysis of Point A revealed that the dispersed particulate phase was concentrated with PEI and PSS, whereas the solution phase contained only a few amounts of the poly electrolytes.

[0058] In comparison, referring to FIG. 3D, the PSS / PEI-EVS complexes exhibited a different phase diagram and typically formed white precipitates within a high polymer concentration range (> [PEI-EVS] = 1.8xl0'4M and [PSS] = 7.0xl0'5M). The overall phase behaviors seem to follow a segregative phase separation mechanism, as supported by thephase composition analysis result of Point B in FIG. 3E, at which the mixed polyelectrolytes tended to separate into a PSS-rich phase and a PEI-EVS rich phase. The unique phase behaviors can be ascribed to less favorable interaction between PSS and PEI-EVS compared to PSS and PEI due to the hydrophobic ethyl sulfonyl groups that increases the segregation strength of PEI-EVS in water.

[0059] Example 3: Preparation of example PEC-layered NF membranes

[0060] A pristine NF membrane was assembled on a petri dish with a custom-made mask for depositing polyelectrolytes on the active layer side of the NF membrane (5 X 5 cm2). 2 g L'1PEI stock solution and 3.33 g L'1PEI-EVS stock solution were diluted with an acetate buffer (0.1 M, pH = 4) in a 50:50 volume ratio to prepare coating solutions.

[0061] On top of the exposed NF membrane surface, 5 mL of the PEI-EVS solution or the PEI solution was first poured to coat the surface and kept for 5 min. The membrane was then thoroughly washed by submerging in deionized water for 1 min and wetted with PSS solution (1 g L'1) for 5 min. This procedure was repeated in a LBL manner until the desired number of LBL deposition is achieved. The LBL deposition process was terminated by coating PSS. The PEC-layered NF membranes are designated as mPSS / nPEI / NF and mPSS / nPEI-EVS / NF, where m and n refer to the number of each layer.

[0062] Example 4: Morphological characterization of example PEC-layered NF membranes

[0063] Referring to FIG. 4A, the surface morphologies of the example PEC-layered NF membranes were studied by liquid- AFM in water. Liquid- AFM was performed with CypherTM AFM (Oxford Instruments Asylum Research). The membrane examples for liquid AFM imaging were glued to metal specimen discs and wetted with deionized water for at least 10 min prior to image acquisition. The tapping mode liquid AFM imaging was conducted in the presence of deionized water.

[0064] Coating at least 5 bilayers of PSS / PEI-EVS was necessary to form a uniform PEC layer and cover the whole surface of the NF membrane. The surface of 5PSS / 5PEFNF membrane showed a network of spherical nanoparticles (particle diameter = 80-220 nm) formed by complexation of PSS and PEI. Further depositions up to 15 bilayers yielded almost identical surface structures, with the root- mean- square (RMS) roughness parameter ranging from 5 to 6 nm independent of the number of LBL deposition. In contrast, the surface of 5PSS / 5PEI-EVS / NF membrane exhibited a nanoscale bicontinuous morphology with a polymeric domain size close to 50 nm. The overall domain size increased up to hundreds ofnanometers with the deposition of more PSS / PEI-EVS bilayers, while preserving the bicontinuous connectivity of the PEC layer in line with the increase of RMS roughness.

[0065] Referring to FIG. 4B, the cross-sectional SEM images of the 5PSS / 5PEI- EVS / NF membrane also supports the formation of a smooth and nanoscale PEC layer on a NF membrane. SEM was performed using a Hitachi S-4800 High Resolution Field emission scanning electron microscope. Sample films for cross-section analysis were fractured in liquid nitrogen and dried for 12 h under vacuum at 80 °C. The examples were then coated with 5 nm of gold using a sputter coater prior to SEM imaging.

[0066] Referring to FIG. 4C, the cross-sectional TEM image of the microtomed 5PSS / 5PEI-EVS / NF membrane revealed the deposition of PSS / PEI-EVS complex layer with the thickness of about 20 nm on top of a polyamide (PA) active layer of NF. TEM was performed using JEOL-2100200 kV LaB6 transmission electron microscope. The nonwoven polyester backing of the NF membrane was removed by dipping in ethyl acetate and the detached top layer was cured with epoxy resin. The thickness of the microtomed sample was approximately 70 nm and the image was obtained after staining with RuCE for higher contrast.

[0067] Referring to FIG. 4D, the PEC layer thickness obtained from the AFM height profile of the detached PEC / PA layer (19.9 ± 11.5 nm) also corresponds with the TEM result. The AFM imaging was conducted in air atmosphere.

[0068] Deposition of more PSS / PEI-EVS complexes formed a thicker but rougher PEC layer as shown for the 10PSS / 10PEI-EVS / NF membrane (PEC layer thickness = 21.3 ± 33.8 nm and RMS roughness - 30 nm). Notably, such bicontinuous morphologies were not observed when PEI-EVS and PSS solutions at the same concentrations were directly mixed, but rather formed non-uniform particulate aggregates. In addition, increasing the polyelectrolyte concentrations used for PSS / PEI-EVS complexes resulted in generation of micron-scale interconnected fibrous structures, which is distinct from the macroscale spherical aggregates of PSS / PEI complexes. The interconnected feature reflects the different phase separation mechanism of PSS / PEI-EVS complexes, while the phase- separated morphology could not remain at nanoscale in case of bulk complexation.

[0069] The overall results show that the nanoscale and bicontinuous structural features of the PSS / PEI- EVS / NF membranes originate from the surface-confined growth of poly electrolyte complexes upon the LBL deposition. The chain mobility of the polyelectrolytes would decrease when electrostatically bound to the oppositely chargedmembrane surface, which limits the degree of complexation-induced phase separation and fast growth of the domain sizes. The branched architecture of PEI-EVS helps create such nanoscale micro structures, like those seen in star-shaped polyelectrolytes assembled in layers. Macroscale phase separation can be hindered by a fast polyelectrolyte buildup owing to a higher mobility and more compact structure compared to linear counterparts.

[0070] On the other hand, the PEC-layered NF membranes prepared with other S-PEI species such as PEI-MVS and PEI-DVS exhibited surface morphologies formed by spherical aggregates in different sizes. The surface of 5PSS / 5PEI-MVS / NF was covered by similar nanoparticle network structures to 5PSS / 5PEVNF, despite the smaller particle size of PSS / PEI-MVS (particle diameter = 60-100 nm) and slightly lower RMS roughness (5.3 ± 0.3 nm for 5PSS / 5PEI-MVS / NF; 5.8 ± 0.2 nm for 5PSS / PEVNF). In case of 5PSS / 5PEI- DVS / NF, non-uniform macroscale aggregates were deposited due to the large particle size of PEI-DVS (particle diameter = 200-600 nm). The overall results show that a nanoscale bicontinuous PEC layer can be formed by the synergistic interplay of hydrophobic modification of polyelectrolytes to control their phase behaviors and confinement of complexation-induced phase separation to the substrate surface.

[0071] Example 5: Surface characterization of PEC-layered NF membranes.

[0072] Referring to FIG. 5A, the ATR-FTIR spectra of the PSS / PEI-EVS / NF membrane confirmed deposition of PEC layers on the NF membrane. ATR-FTIR spectra were recorded with a range of 4,000 - 400 cm-1by a NicoletTM iS50 FTIR spectrometer (Thermo ScientificTM) equipped with a built-in diamond ATR. The PEC-layered NF membranes show characteristic peaks at 1036 cm'1(SCE' symmetric stretching), 1126 cm'1(aromatic C-H stretching), and 1679 cm'1(amide I band, C=O stretching), as well as increasing sulfonate group intensity with higher number of LBL deposition.

[0073] Referring to FIG. 5B, the LBL deposition behavior can be traced from the peak intensity change of sulfonate groups, which proved to be different for PEI and PEI-EVS. In FIG. 5B, the nomalized amount of sulfonate group is represented relative to the PSS quantity of 1PSS / 1PEFNF. For PSS / PEI / NF membranes, the nomalized amount of sulfonate group linearly increases, independent of the number of depositions up to 15 bilayers. For PSS / PEI- EVS / NF membranes, the nomalized amount of sulfonate group dramatically increased after deposition of 7 bilayers, following an exponentially fitted trend.

[0074] Referring to FIG. 5C, the XPS characterization using Thermo Scientific K- Alpha also confirmed the deposition of PEC layer formation on the NF membrane surface. Newpeaks appeared in the high resolution XPS spectra in Nls and S2p region, which correspond to the protonated ammonium groups (Nls = 401.6 eV) and sulfonate groups of PSS (S2pi / 2 = 168.8 eV and S2p3 / 2 = 167.6 eV). The PSS / PEI-EVS / NF membrane showed the distinct binding energy for sulfone (SO2) groups at 169.7 eV and higher sulfur composition (5.52 atomic %) than the PSS / PEI / NF membranes (2.57 atomic %), indicating successful incorporation of PEI-EVS within the PEC layer.

[0075] Referring to FIG. 5D, the wettability of PSS / PEI-EVS / NF membranes differed from that of the PSS / PEI / NF membranes as shown from their WCA changes upon LBL deposition. The WCA increased as the number of LBL deposition increases and reached a plateau after 5 deposition cycles for both membranes. The higher WCAs of the PSS / PEI- EVS / NF membranes (WCA - 50°) reflect the relatively hydrophobic nature of the ethyl sulfonyl groups embedded in PEI-EVS. Coating with PEI- MVS yielded a less hydrophobic surface (WCA = 36.9°) due to the shorter methyl sulfonyl groups (not shown in FIG. 5D). A PEI-DVS deposited membrane showed a WCA only slightly higher than that of pristine NF (WCA = 23.7° and 18.2° respectively) (not shown in FIG. 5D), which may be ascribed to the incomplete coating of the surface as observed in the AFM image.

[0076] Ion transport flux and permselectivity are critically affected by the membrane surface charge. To investigate the membrane surface charge, surface zeta potential of the PEC-layered membranes were measured as shown in FIG. 5E. The surface zeta potential varied between positive and negative values upon increasing the number of layers, reflecting the typical characteristics of LBL deposition driven by electrostatic complexation. Referring to FIG. 5E, the surface zeta potential of the PSS -terminated membranes increased (e.g., shifted from -47.0 ± 5.5 eV up to -31.0 ± 1.4 eV for PSS / PEI / NF membrane), which indicates successful shielding of the charges coming from the residual carboxylate groups of the polyamide layer that can prohibit faster transport of anions. The higher but still negative surface charges are beneficial in reducing membrane fouling caused by organic foulants, which are typically negatively charged in the fermentation broth.

[0077] Example 6: Redox-ED VFA separation with PSS / S-PEI / NF membrane and PSS / PEI / NF membrane

[0078] The example PEC-layered NF membranes were integrated into the optimized redox-ED to evaluate the performance for VFA recovery.

[0079] Referring to FIG. 6A, the redox-ED system comprised of four flow channels-a feed channel (FC) 112, an accumulation channel (AC) 114, and two redox channels (RC) 116-as well as a PEC-layered membrane 110 (or a pristine NF membrane) and two cation CEM 106, 108 separating the channels. A solution of ferri- / ferrocyanide redox couple was circulated in the anodic and cathodic redox channels.

[0080] The redox-ED cell was assembled with a pristine NF membrane or PEC-layered NF membrane. The modified membrane surface was oriented to face the feed channel side. An equimolar ternary solution (75 mM in total) of VFA (C3 + C4) and C6, and sodium chloride electrolyte solution (75 mM) were used for experiments. The solutions were permeated through the flow cell by a peristaltic pump at a flow rate of 0.5 mL min'1. 100 mM Na4Fe(CN)e and 100 mM K3Fe(CN)e solution were prepared respectively. The same volume of the solution was then mixed 1 : 1 volumetric ratio to prepare a fresh redox channel solution (20 mL).

[0081] The electrical potential applied to the redox-ED system drives the ionic migration and accumulates organic acids in the accumulation channel receiving tank (or an accumulation tank). It was revealed that CEMs (Selemion CMV-N) may prevent the loss of recovered organic acids when used for separating a model ternary solution of VFA (C3 + C4) and C6. In addition, the redox-ED significantly reduced specific energy consumption to 0.48 kWh kg'1, an 80 % decrease compared to a conventional ED system (2.3 kWh kg'1).

[0082] Considering this energetic advantage, the redox-ED system was first optimized with a pristine NF membrane at different applied cell voltages (0.5 V to 1.5 V).

[0083] The molar ion flux was higher in the order of C3 > C4 > C6 at low cell voltage of 0.5 V (C3 = 0.40, C4 = 0.29, and C6 = 0.20; xl0'7mol m'2s'1), as can be expected from their chain lengths. Increasing the cell voltages led to an increase in ion flux for all carboxylates, with a trade-off of lower molar ion flux ratio (or ion selectivity) of C3 / C6, C4 / C6, and C3 / C4. For example, the C3 flux increased from 0.40 to 0.55 (xl0'7mol m'2s'1) at 1.5 V, whereas the selectivity of C3 / C6 = 1.98 decreased to 1.27. Notably, the specific energy consumption was 0.13 kWh mol'1at 0.8 V, but dramatically increased beyond this voltage, reaching a plateau of about 0.30 kWh mol'1at 1.5 V. The cell voltage was set to 0.8 V as an optimum cell voltage for the following VFA separation experiments.

[0084] The PEC-layered NF membranes were then integrated into the optimized redox- ED to evaluate the performance for VFA recovery.

[0085] Referring to Figure 6B, the molar flux of each organic acid anions (C3, C4, and C6) through the PSS / PEI-EVS / NF membrane dramatically changed depending on the number of PEC layers. The molar flux steadily increased for all carboxylates up to 5 bilayers,reaching C3 = 0.99, C4 = 0.71, and C6 = 0.52 (xl0'7mol m'2s'1). The C3 and C6 flux were 2.6 and 1.7 times higher than those of the pristine NF membrane, showing the preferential transport of shorter chain carboxylate. The molar flux then decreased upon the deposition of more bilayers (up to 15 bilayers).

[0086] Referring to FIG. 6C, the PSS / PEI-EVS / NF membranes exhibited both higher molar flux and selectivity between VFA (C3+C4) and C6, in comparison with the pristine NF and PSS / PEI / NF membranes. For the 5PSS / 5PEI-EVS / NF, the VFA ion flux (1.6 xl0'7mol nr2s'1) was enhanced by about 1.9 times compared to the pristine NF (0.82 xl0'7mol m'2s'1), as well as the increase of VFA selectivity from 1.38 to 1.66. The VFA ion flux through the 5PSS / 5PEENF (1.1 xl0'7mol m'2s'1) was significantly lower, despite exhibiting a similar VFA selectivity of 1.65.

[0087] Referring to FIG. 6D, the bicontinuous PEC layered-NF membrane enabled higher organic acid selectivity and molar flux than pristine NF membrane by stacking multiple membrane sheets. Two sheets of each membrane (pristine NF membrane, 5PSS / 5PEENF membrane, and 5PSS / 5PEI-EVS / NF membrane) were stacked facing the same direction in the redox-ED cell. As shown in FIG. 6D, when two 5PSS / 5PEI / NF sheets were stacked, the selectivity of C3 / C6 can be further increased to 2.9xl0'7mol m'2s'1from 1.9xl0'7mol m'2s'1for one sheet. The stacking inevitably decreased the molar flux of C3 but it was still higher than the flux through the pristine NF membrane (0.60x10'7mol m'2s'1and 0.30xl0'7mol m'2s'1, respectively).

[0088] Referring to FIG. 6E, the selectivity for the organic anions was further investigated by measuring the flow-through ion conductivity using a direct current method with a four-electrode set-up. The example PSS / PEI-EVS / NF membrane was positioned between two chambers, each filled with an organic acid solution of equal concentration (200 mM) to minimize the effect of the difference in salt concentration. The ion conductivity of C3 steadily increased upon the deposition of PSS / PEI-EVS layers up to 5 bilayers, which was similar to the performance obtained by redox-ED (FIG. 6B). In contrast, the conductivity of C4 and C6 anions decreased following the increase of the number of deposition cycles and reached the minimum at 5 bilayers. The different conductivity trends between C3 and C4 (or C6) anions indicate that the ion selectivity, particularly between C3 and C6, arises primarily from the slower migration of longer-chain C6 molecules due to the deposition of PEC layers.

[0089] Referring to FIG. 6F, the ion selectivity among different PSS / S-PEI / NF membranes were compared to examine the effect of hydrophobic modification and differentS-PEI species. The PSS / PEI-EVS layer showed the highest C3 / C6 selectivity (1.91), while that of PSS / PEI-MVS was slightly lower (1.81). Notably, the molar flux of C3 through the PSS / PEI-EVS / NF membrane was 1.6 times higher than that through the PSS / PEI-MVS / NF membrane (0.63xl0'7mol m'2s'1). PSS / PEI-DVS / NF membrane exhibited a much lower C3 / C6 selectivity of 1.65, as can be expected from the non-uniform and aggregated PSS / PEI- DVS layer structures.

[0090] The higher organic ion transport rate through the PEC-layered NF membranes than that of pristine NF membrane can be attributed to the charge screening effect of the PEC layer, as observed in the surface zeta potential measurement (FIG. 5E). The variance in ion flux and selectivity among the S-PEI species can be ascribed to the different microstructures of the PEC layers and their hydrophobic modification. The bicontinuous morphologies of the PSS / PEI-EVS layer seem to play important roles in promoting the organic ion transfer, given the similar estimated ion exchange capacities and PEC layer thickness of the PSS / PEI- EVS / NF and PSS / PEI / NF membrane NF membranes. Such interconnected ion transport networks have shown more efficient ion transport (higher ion conductivity) than the disordered and isolated morphologies, which have highly tortuous pathways inhibiting faster ion migration. The lower molar ion fluxes through the PSS / PEI / NF and PSS / PEI-MVS / NF membranes can be attributed to their disordered nanoparticulate network structures, with the variations in nanoparticle size that would result in the differences in ion flux. In addition, the selectivity between VFAs and C6 mainly comes from the steric hindrance posed by the deposited PEC layers. The hydrophobic alkyl sulfone groups are also likely to hinder the permeation of longer chain carboxylate anions (C6) by interacting with them favorably compared to more hydrophilic VFA anions, resulting in higher VFA selectivity.

[0091] Example 7: Selective VFA recovery from synthetic and cow manure anaerobic digestion effluent

[0092] To demonstrate the potential of bicontinuous PEC-layered membranes within a redox-ED system, a lab-scale separation of VFAs from synthetic and cow manure anaerobic digestion effluent was conducted (FIG. 7A). Referring to FIG. 7A, the FC and AC solution were recirculated to concentrate the stripped VFAs through the membranes. The synthetic effluent consisted of C3, C4, C6 (C3:C4:C6 = 20 mM:10 mM:10 mM), and inorganic salts in a concentration range close to the previously reported fermentation effluents. The redox-ED system was operated in a batch mode with a different solution volume ratio (FC:AC = 500 mL:200 mL) to up-concentrate the extracted VFAs in an accumulation tank.

[0093] As shown in FIG. 7B, the redox-ED system utilizing 5PSS / 5PEI-EVS / NF membrane exhibited steady enrichment of organic acids up to 2 times after 72 h of operation. The final concentration ratio of VFAs (C3+C4) and C6 also increased by 4.9 times to 1.56 from 0.32 of the initial feed, indicating preferential up-concentration of VFAs. The membrane selectivity, expressed by a concentration ratio of VFAs / C6 normalized by the initial ratio, became consistent within 1.9-2.0, which shows the robust performance of the membrane even in the presence of salts. With the redox-ED system utilizing commercial AEM, in contrast, ATR-FTIR analysis of the AEM showed that the membrane suffered from significant fouling by the organic anions, resulting in negligible up-concentration in the accumulation tank. Besides, with the redox-ED system utilizing commercial AEM, the ion selectivity between VFA and C6 was not consistent and decreased to 0.62 over time, which may be due to the organic anions fouled onto the AEM. Correspondingly, as shown in FIG. 7C, the specific energy consumption to recover VFAs by a redox-ED system rapidly increased over time for AEM, while the bicontinuous PEC-layered membrane exhibited a steady value of 1.5 KWh kg'1.

[0094] The effluent was obtained from anaerobic fermentation of glucose using cow manure inoculum with a custom-designed bioreactor, followed by centrifugation and microfiltration. To create representative concentrations of the organic acids, the VFAs were added into the as-prepared anaerobic digestion effluent for proof-of-concept. It must be noted that this particular raw effluent was rich in lactic acid as a metabolic intermediate and did not contain C6. The redox-ED system was run for 168 h in a closed-loop batch mode with the optimized 5PSS / 5PEI-EVS / NF membrane, and the organic acid distribution in the accumulation tank was monitored over time.

[0095] As shown in FIG. 7D, the total VFA concentration was 4 times higher than the feed, in which acetic acid (C2) was enriched by 5.2 times and it was particularly higher than C3 (3.8 times) and C4 (2.3 times). In addition, the lactic acid content notably decreased by 40 % after 48 h of operation, reaching 15 % of the total organic acid distribution from the initial 55 %. The depletion of lactate was also observed for separation of a model quaternary mixture containing VFAs (C2, C3, C4) and lactate. The 5PSS / 5PEI-EVS / NF membrane exhibited higher VFAs / lactate selectivity of 1.3 compared to the pristine NF (-1.05). The lower lactic acid content in the accumulation tank can be ascribed to the combined effect of concentration polarization at the membrane surface, hydrogen-bonding capability of the PEC-layers that potentially slows the transport of lactate, and the intrinsic characteristics of anaerobic digestion effluents.

[0096] Furthermore, the C6 content decreased from -1 % to < 0.1 % of the total organic acid content after continuously operating the redox-ED system for 168 h with 5PSS / 5PEI- EVS / NF membrane. This contrasts with the result of pristine NF that showed the initial decrease of the relative C6 content within a short operation time (< 6 h) but increased to 2 % of organic acids after 48 h, due to its poor organic ion selectivity. To reduce the C6 content within a level close to 5PSS / 5PEI-EVS / NF membrane, we had to run at least two sequential stages of redox-ED system with the pristine NF membranes. The energy consumption of the sequential cascade was 8.4 kWh kg'1in comparison with 7.6 kWh kg'1for a single stage using 5PSS / PEI-EVS / NF, which could reduce 10 % of the energy requirement to remove C6.

[0097] Referring to FIG. 7E, the energy consumption of redox-ED system with the example bicontinuous PEC-layered membrane was lower than or comparable with other ED systems for VFA recovery, despite the difficulties in direct comparison due to the different feed conditions and target VFAs. Finally, the bicontinuous structure of the PEC-layered NF membranes remained intact after running redox-ED system with cow manure fermentation effluents, as shown from the AFM images obtained in liquid and dried conditions. ATR-FTIR analysis also supported the retention of the bicontinuous PEC layer after prolonged exposure to the effluents. The overall result highlights that our redox-ED system integrated with bicontinuous PEC-layered NF membranes presents a robust, energy-efficient, and cost- effective platform for VFA separation.

[0098] The results demonstrated the facile synthesis of a bicontinuous PEC-layered NF membrane and its deployment for the selective recovery of VFAs in a redox-electrodialysis system. The aza-Michael addition reaction of a cationic polyelectrolyte having amine groups, hydroxyl groups, or thiol groups with an acrylate derivative, an acrylamide derivative, or a vinyl sulfone amide derivative to branched PEI easily adjusted complexation-induced phase separation behaviors and the transition from associative to segregative separation mechanism.

[0099] A bicontinuous PEC layer, approximately 20 nm thick, was uniformly formed on the membrane surface by employing a LBL deposition technique. In contrast to direct mixing, the LBL processing enabled surface-confined complexation-induced phase separation and precise tailoring of surface properties. The optimized bicontinuous PEClayered NF membranes exhibited about 1.9 times higher VFA fluxes with about 20 % increase in selectivity compared to a pristine NF. A pristine NF here refers to the same typeof membrane used in the PEC-layered membrane being compared, but without any additional layers or modifications. This unique behavior was attributed to the formation of a bicontinous ion transport pathway and hydrophobic modification providing ionic selectivity. It was further demonstrated the selective recovery of VFAs using a redox-ED system with bicontinuous PEC-layered NF membrane. The redox-ED system reduced energy consumption by 80 % compared to conventional ED through a reversible reaction of redox mediators. Additionally, the PEC-layered NF membrane exhibited steady flux and selectivity towards VFAs in a salt-rich synthetic effluent, whereas commercial AEM performance deteriorated due to significant organic fouling. A proof-of-concept recovery of VFAs from cow manure anaerobic digestion effluent using the redox-ED system achieved up to 4 times enrichment of VFAs while simultaneously reducing C6 and lactate concentrations. The bicontinuous PEClayered NF membrane also reduced specific energy consumption by 10 % compared to pristine NF membranes, which required an additional stage for higher purity.

[0100] Overall, the disclosure provided a robust and energy-efficient electrochemical platform for separating structurally similar organic ion species through molecular design of PEC-layered membranes. It is envisioned this PEC-layered membrane design strategy can be extended to a wide range of Michael acceptors and polyelectrolytes to achieve higher molecular selectivity and permeability. Furthermore, the detailed molecular understanding of bicontinuous polyelectrolyte complex formation will offer insights into surface-confined phase separation of complex coacervates beyond applications in separations.

[0101] The subject matter of this disclosure also includes the following aspects:

[0102] A first aspect relates to a bicontinuous PEC-layered membrane for selective recovery of VFAs, the bicontinuous PEC-layered membrane comprising: a membrane, and a PEC layer disposed on the membrane, the PEC layer comprising a hydrophobically modified cationic polyelectrolyte and an anionic polyelectrolyte, wherein the PEC layer comprises a bicontinuous structure due to complexation-induced phase separation driven by hydrophobicity contrast between the hydrophobically modified cationic polyelectrolyte and the anionic poly electrolyte.

[0103] A second aspect relates to the bicontinuous PEC-layered membrane of the preceding aspect, wherein the hydrophobically modified cationic polyelectrolyte has a predetermined hydrophobicity to balance associative and segregative interactions between the hydrophobically modified cationic polyelectrolyte and the anionic polyelectrolyte within the PEC layer during complexation-induced phase separation.

[0104] A third aspect relates to the bicontinuous PEC-layered membrane, wherein the hydrophobically modified cationic polyelectrolyte comprises a product of an aza-Michael addition reaction between a cationic polyelectrolyte having amine groups, hydroxyl groups, or thiol groups as a Michael receptor and an acrylate derivative, an acrylamide derivative, or a vinyl sulfone amide derivative as a Michael acceptor.

[0105] A fourth aspect relates to the bicontinuous PEC-layered membrane of any preceding aspect, wherein the hydrophobically modified cationic polyelectrolyte comprises sulfonyl polyethylene imine (S-PEI).

[0106] A fifth aspect relates to the bicontinuous PEC-layered membrane of any preceding aspect, wherein the hydrophobically modified cationic polyelectrolyte comprises polyethylene imine-methyl vinyl sulfone (PEI-MVS), polyethylene imine-ethyl vinyl sulfone (PEI-EVS), or polyethylene imine-divinyl sulfone (PEI-DVS), preferably PEI-MVS or PEI- EVS, more preferably PEI-EVS.

[0107] A sixth aspect relates to the bicontinuous PEC-layered membrane of any preceding aspect, wherein the anionic polyelectrolyte comprises polystyrene sulfonate (PSS).

[0108] A seventh aspect relates to the bicontinuous PEC-layered membrane of any preceding aspect, wherein the PEC layer comprises PSS / PEI-EVS complex.

[0109] An eighth aspect relates to the bicontinuous PEC-layered membrane of any preceding aspect, wherein the PEC layer is formed by alternatively depositing a layer comprising hydrophobically modified cationic polyelectrolyte and a layer comprising anionic poly electrolyte.

[0110] A ninth aspect relates to the bicontinuous PEC-layered membrane of any preceding aspect, wherein the PEC layer is formed by alternatively depositing a layer comprising hydrophobically modified cationic polyelectrolyte and a layer comprising anionic poly electrolyte to form 5 to 50 bilayers, preferably 5 to 15 bilayers.

[0111] A tenth aspect relates to the bicontinuous PEC-layered membrane of any preceding aspect, wherein the PEC layer has a thickness of about 5 nm to about 100 nm, preferably about 5 nm to about 50 nm layers.

[0112] An eleventh aspect relates to the bicontinuous PEC-layered membrane of any preceding aspect, wherein the membrane comprises a nano filtration (NF) membrane.

[0113] A twelfth aspect relates to the bicontinuous PEC-layered membrane of any preceding aspect, wherein the membrane comprises a polyamide (PA) NF membrane.

[0114] A thirteenth aspect relates to the bicontinuous PEC-layered membrane of any preceding aspect, wherein the membrane has a molecular weight cut-off of about 150 to about 300 Da.

[0115] A fourteenth aspect relates to a method of preparing a bicontinuous PEC-layered membrane for selective recovery of VFAs, the method comprising: alternatively depositing a layer comprising a hydrophobically modified cationic polyelectrolyte and a layer comprising an anionic polyelectrolyte on a membrane, thereby forming a PEC layer comprising a bicontinuous structure due to complexation-induced phase separation driven by hydrophobicity contrast between the hydrophobically modified cationic polyelectrolyte and the bicontinuous PEC-layered membrane.

[0116] A fifteenth aspect relates to the method of preparing a bicontinuous PEC-layered membrane of any preceding aspect, further comprising: preparing the hydrophobically modified cationic polyelectrolyte via aza-Michael addition reaction between a cationic poly electrolyte having amine groups, hydroxyl groups, or thiol groups as a Michael receptor and an acrylate derivative, an acrylamide derivative, or a vinyl sulfone amide derivative as a Michael acceptor.

[0117] A sixteenth aspect relates to the method of preparing a bicontinuous PEC-layered membrane of any preceding aspect further comprising: selecting a Michael acceptor with an appropriate hydrophobicity to achieve a predetermined hydrophobicity of the hydrophobically modified cationic polyelectrolyte to balance associative and segregative interactions between the hydrophobically modified cationic polyelectrolyte and the anionic polyelectrolyte within the PEC layer during complexation-induced phase separation.

[0118] A seventeenth aspect relates to the method of preparing a bicontinuous PEClayered membrane of any preceding aspect, wherein the aza-Michael addition reaction is a catalyst-free reaction carried out in an aqueous phase.

[0119] An eighteenth aspect relates to the method of preparing a bicontinuous PEClayered membrane of any preceding aspect, wherein the step of alternatively depositing a layer comprising a hydrophobically modified cationic polyelectrolyte and a layer comprising an anionic polyelectrolyte on a membrane is repeated until forming 5 to 50 bilayers, preferably 5 to 15 bilayers, on a side of the membrane.

[0120] A nineteenth aspect relates to a redox-ED system for selective recovery of VFAs, the system comprising: a first electrode; a second electrode positioned in opposition to the first electrode; a first and second CEMs disposed between the first and second electrodes; anda bicontinuous PEC-layered membrane disposed between the first and the second CEMs, wherein the bicontinuous PEC-layered membrane comprises: a membrane, and a PEC layer disposed on the membrane, the PEC layer comprising a hydrophobically modified cationic polyelectrolyte and an anionic polyelectrolyte, wherein the PEC layer comprises a bicontinuous structure due to complexation-induced phase separation driven by hydrophobicity contrast between the hydrophobically modified cationic polyelectrolyte and the anionic poly electrolyte.

[0121] A twentieth aspect relates to the redox-ED system of any preceding aspect wherein the hydrophobically modified cationic polyelectrolyte has a predetermined hydrophobicity to balance associative and segregative interactions between the hydrophobically modified cationic polyelectrolyte and the anionic polyelectrolyte within the PEC layer during complexation-induced phase separation.

[0122] A twenty-first aspect relates to the redox-ED system of any preceding aspect, wherein the hydrophobically modified cationic polyelectrolyte comprises a product of an azaMichael addition reaction between a cationic polyelectrolyte having amine groups, hydroxyl groups, or thiol groups as a Michael receptor and an acrylate derivative, an acrylamide derivative, or a vinyl sulfone amide derivative as a Michael acceptor.

[0123] A twenty-third aspect relates to the redox-ED system of any preceding aspect, comprising from 2 to 5 bicontinuous PEC-layered membranes stacked on one another, preferably 2 bicontinuous PEC-layered membranes.

[0124] A twenty-second aspect relates to the redox-ED system of any preceding aspect, further comprising: a feed channel extending between the bicontinuous PEC-layered membrane and the second CEM for delivery of a multicomponent feed solution; an accumulating channel extending between the bicontinuous PEC-layered membrane and the first CEM for collecting VFAs separated from the multicomponent feed solution; and a redox channel containing the first and second electrodes and being separated from the feed and accumulating channels by the first and second CEMs.

[0125] A twenty-fourth aspect relates to the redox-ED system of any preceding aspect, wherein the first electrode is configured to become positively charged upon application of a voltage, and the second electrode is configured to become negatively charged upon application of a voltage.

[0126] A twenty-fifth aspect relates to a method of selective recovery of VFAs from a multicomponent feed solution, comprising: exposing a bicontinuous PEC-layered membraneto a multicomponent feed solution in a redox-ED system, the multicomponent feed solution comprising a plurality of organic acids, and the bicontinuous PEC-layered membrane comprising a membrane and a PEC layer disposed on the membrane, the PEC layer comprising a hydrophobically modified cationic polyelectrolyte and an anionic polyelectrolyte, wherein the PEC layer comprises a bicontinuous structure due to complexation-induced phase separation driven by hydrophobicity contrast between the hydrophobically modified cationic polyelectrolyte and the anionic polyelectrolyte; and selectively removing VFAs from the multicomponent feed solution.

[0127] A twenty- sixth aspect relates to the method of selective recovery of the preceding aspect, wherein the redox-ED system comprises: a first electrode; a second electrode positioned in opposition to the first electrode; a first second CEMs disposed between the first and second electrodes; and the bicontinuous PEC-layered membrane disposed between the first and second cation exchange membranes, wherein a feed channel extends between the bicontinuous PEC-layered membrane and the second CEM for delivery of the multicomponent feed solution; wherein an accumulating channel extends between the bicontinuous PEC-layered membrane and the first CEM for collecting the VFAs separated from the multicomponent feed solution, and wherein a redox channel separated from the feed and accumulating channels by the first and second CEMs contains the first and second electrodes.

[0128] A twenty- seventh aspect relates to the method of selective recovery of any preceding aspect, wherein the VFAs selectively pass through the bicontinuous PEC-layered membrane and into the accumulating channel; and further comprising collecting the VFAs in an accumulation tank.

[0129] A twenty-eighth aspect relates to the method of selective recovery of any preceding aspect, wherein the VFAs pass through the bicontinuous PEC-layered membrane at a flux at least about 1.9 times higher than through a pristine membrane.

[0130] A twenty-ninth aspect relates to the method of selective recovery of any preceding aspect, wherein, during the exposure of the bicontinuous PEC-layered membrane to the multicomponent feed solution, a voltage is applied to the redox-ED system, whereby the first electrode becomes positively charged and the second electrode becomes negatively charged, and redox reactions occur in the redox channel.

[0131] Unless otherwise explained, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which a disclosed disclosure belongs.

[0132] The singular terms “a,” “an,” and “the” include plural referents unless context clearly indicates otherwise. Similarly, the word “or” is intended to include “and” unless the context clearly indicates otherwise.

[0133] The term "about" means that the characteristic, item, quantity, parameter, property, or term so qualified encompasses a range of plus or minus ten percent above and below the value of the stated characteristic, item, quantity, parameter, property, or term. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the specification and attached claims are approximations that may vary. For instance, as mass spectrometry instruments can vary slightly in determining the mass of a given analyte, the term "about" in the context of the mass of an ion or the mass / charge ratio of an ion refers to + / -0.50 atomic mass unit. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical indication should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

[0134] The term “comprising” means “including”; hence, “comprising A or B” means “including A” or “including B” or “including A and B.” All references cited herein are incorporated by reference. The term “consisting of’ excludes any element, step, or ingredient not specified in the aspect element. As used herein, “consisting essentially of’ does not exclude materials or steps that do not materially affect the basic and novel characteristics of the aspect. In each instance herein any of the terms "comprising", "consisting essentially of" and "consisting of" may be replaced with either of the other two terms. The disclosure illustratively described herein suitably may be practiced in the absence of any element or elements, limitation or limitations which is not specifically disclosed herein.

[0135] The disclosure may be further understood by the following non-limiting examples. All references cited herein are hereby incorporated by reference to the extent not inconsistent with the disclosure herewith. Although the description herein contains many specificities, these should not be construed as limiting the scope of the disclosure but as merely providing illustrations of some of the presently preferred embodiments of the disclosure. For example, thus the scope of the disclosure should be determined by the appended aspects and their equivalents, rather than by the examples given.

[0136] While the present disclosure can take many different forms, for the purpose of promoting an understanding of the principles of the disclosure, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the disclosure is thereby intended. Any alterations and further modifications of the described embodiments, and any further applications of the principles of the disclosure as described herein are contemplated as would normally occur to one skilled in the art to which the disclosure relates.

[0137] All references throughout this application, for example patent documents including issued or granted patents or equivalents; patent application publications; and nonpatent literature documents or other source material; are hereby incorporated by reference herein in their entireties, as though individually incorporated by reference, to the extent each reference is at least partially not inconsistent with the disclosure in this application (for example, a reference that is partially inconsistent is incorporated by reference except for the partially inconsistent portion of the reference).

[0138] The terms and expressions which have been employed herein are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the disclosure. Thus, it should be understood that although the present disclosure has been specifically disclosed by preferred embodiments, exemplary embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this disclosure as defined by the appended aspects. The specific embodiments provided herein are examples of useful embodiments of the present disclosure and it will be apparent to one skilled in the art that the present disclosure may be carried out using a large number of variations of the devices, device components, methods steps set forth in the present description. As will be obvious to one of skill in the art, methods and devices useful for the present methods can include a large number of optional composition and processing elements and steps.

[0139] When a group of substituents is disclosed herein, it is understood that all individual members of that group and all subgroups, including any isomers, enantiomers, and diastereomers of the group members, are disclosed separately. When a Markush group or other grouping is used herein, all individual members of the group and all combinations andsubcombinations possible of the group are intended to be individually included in the disclosure. When a compound is described herein such that a particular isomer, enantiomer or diastereomer of the compound is not specified, for example, in a formula or in a chemical name, that description is intended to include each isomers and enantiomer of the compound described individual or in any combination. Additionally, unless otherwise specified, all isotopic variants of compounds disclosed herein are intended to be encompassed by the disclosure. For example, it will be understood that any one or more hydrogens in a molecule disclosed can be replaced with deuterium or tritium. Isotopic variants of a molecule are generally useful as standards in assays for the molecule and in chemical and biological research related to the molecule or its use. Methods for making such isotopic variants are known in the art. Specific names of compounds are intended to be exemplary, as it is known that one of ordinary skill in the art can name the same compounds differently.

[0140] Many of the molecules disclosed herein contain one or more ionizable groups [groups from which a proton can be removed (e.g., -COOH) or added (e.g., amines) or which can be quaternized (e.g., amines)]. All possible ionic forms of such molecules and salts thereof are intended to be included individually in the disclosure herein. With regard to salts of the compounds herein, one of ordinary skill in the art can select from among a wide variety of available counterions those that are appropriate for preparation of salts of this disclosure for a given application. In specific applications, the selection of a given anion or cation for preparation of a salt may result in increased or decreased solubility of that salt.

[0141] Every formulation or combination of components described or exemplified herein can be used to practice the disclosure, unless otherwise stated.

[0142] Whenever a range is given in the specification, for example, a temperature range, a time range, or a composition or concentration range, all intermediate ranges and subranges, as well as all individual values included in the ranges given are intended to be included in the disclosure. It will be understood that any subranges or individual values in a range or subrange that are included in the description herein can be excluded from the aspects herein.

[0143] All patents and publications mentioned in the specification are indicative of the levels of skill of those skilled in the art to which the disclosure pertains. References cited herein are incorporated by reference herein in their entirety to indicate the state of the art as of their publication or filing date and it is intended that this information can be employed herein, if needed, to exclude specific embodiments that are in the prior art. For example, when compositions of matter are disclosed, it should be understood that compounds known andavailable in the art prior to Applicant's disclosure, including compounds for which an enabling disclosure is provided in the references cited herein, are not intended to be included in the composition of matter aspects herein.

[0144] One of ordinary skill in the art will appreciate that starting materials, biological materials, reagents, synthetic methods, purification methods, analytical methods, assay methods, and biological methods other than those specifically exemplified can be employed in the practice of the disclosure without resort to undue experimentation. All art-known functional equivalents, of any such materials and methods are intended to be included in this disclosure. The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention that in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the disclosure. Thus, it should be understood that although the present disclosure has been specifically disclosed by preferred embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this disclosure as defined by the appended aspects.

[0145] Although the present disclosure has been described with reference to certain embodiments thereof, other embodiments are possible without departing from the present disclosure. The spirit and scope of the appended aspects should not be limited, therefore, to the description of the preferred embodiments contained herein. All embodiments that come within the meaning of the aspects, either literally or by equivalence, are intended to be embraced therein. Furthermore, the advantages described above are not necessarily the only advantages of the disclosure, and it is not necessarily expected that all of the described advantages will be achieved with every embodiment of the disclosure.

Claims

CLAIMS1. A bicontinuous polyelectrolyte complex (PEC)-layered membrane for selective recovery of volatile fatty acids (VFAs), the bicontinuous PEC-layered membrane comprising: a membrane, and a PEC layer disposed on the membrane, the PEC layer comprising a hydrophobically modified cationic polyelectrolyte and an anionic polyelectrolyte, wherein the PEC layer comprises a bicontinuous structure due to complexation- induced phase separation driven by hydrophobicity contrast between the hydrophobically modified cationic polyelectrolyte and the anionic polyelectrolyte.

2. The bicontinuous PEC-layered membrane of claim 1, wherein the hydrophobically modified cationic polyelectrolyte has a predetermined hydrophobicity to balance associative and segregative interactions between the hydrophobically modified cationic polyelectrolyte and the anionic polyelectrolyte within the PEC layer during complexation-induced phase separation.

3. The bicontinuous PEC-layered membrane of claim 1, wherein the hydrophobically modified cationic polyelectrolyte comprises a product of an aza-Michael addition reaction between a cationic polyelectrolyte having amine groups, hydroxyl groups, or thiol groups as a Michael receptor and an acrylate derivative, an acrylamide derivative, or a vinyl sulfone amide derivative as a Michael acceptor.

4. The bicontinuous PEC-layered membrane of claim 1, wherein the hydrophobically modified cationic polyelectrolyte comprises sulfonyl polyethylene imine (S- PEI).

5. The bicontinuous PEC-layered membrane of claim 4, wherein the hydrophobically modified cationic polyelectrolyte comprises polyethylene imine-methyl vinyl sulfone (PEI-MVS), polyethylene imine-ethyl vinyl sulfone (PEI-EVS), or polyethylene imine-divinyl sulfone (PEI-DVS), preferably PEI-MVS or PEI-EVS, more preferably PEI- EVS.

6. The bicontinuous PEC-layered membrane of claim 1 , wherein the anionic polyelectrolyte comprises polystyrene sulfonate (PSS).

7. The bicontinuous PEC-layered membrane of claim 1, wherein the PEC layer comprises PSS / PEI-EVS complex.

8. The bicontinuous PEC-layered membrane of claim 1, wherein the PEC layer is formed by alternatively depositing a layer comprising hydrophobically modified cationic polyelectrolyte and a layer comprising anionic polyelectrolyte.

9. The bicontinuous PEC-layered membrane of claim 1, wherein the PEC layer is formed by alternatively depositing a layer comprising hydrophobically modified cationic polyelectrolyte and a layer comprising anionic polyelectrolyte to form 5 to 50 bilayers, preferably 5 to 15 bilayers.

10. The bicontinuous PEC-layered membrane of claim 1, wherein the PEC layer has a thickness of about 5 nm to about 100 nm, preferably about 5 nm to about 50 nm layers.

11. The bicontinuous PEC-layered membrane of claim 1, wherein the membrane comprises a nano filtration (NF) membrane.

12. The bicontinuous PEC-layered membrane of claim 1, wherein the membrane comprises a polyamide (PA) NF membrane.

13. The bicontinuous PEC-layered membrane of claim 1, wherein the membrane has a molecular weight cut-off of about 150 to about 300 Da.

14. A method of preparing a bicontinuous PEC-layered membrane for selective recovery of VFAs, the method comprising: alternatively depositing a layer comprising a hydrophobically modified cationic polyelectrolyte and a layer comprising an anionic polyelectrolyte on a membrane, thereby forming a PEC layer comprising a bicontinuous structure due to complexation-induced phase separation driven by hydrophobicity contrast between the hydrophobically modified cationic poly electrolyte and the anionic poly electrolyte.

15. The method of claim 14, further comprising: preparing the hydrophobically modified cationic polyelectrolyte via aza-Michael addition reaction between a cationic polyelectrolyte having amine groups, hydroxyl groups, or thiol groups as a Michael receptor and an acrylate derivative, an acrylamide derivative, or a vinyl sulfone amide derivative as a Michael acceptor.

16. The method of claim 15, further comprising: selecting a Michael acceptor with an appropriate hydrophobicity to achieve a predetermined hydrophobicity of the hydrophobically modified cationic polyelectrolyte to balance associative and segregative interactions between the hydrophobically modified cationic polyelectrolyte and the anionic polyelectrolyte within the PEC layer during complexation-induced phase separation.

17. The method of claim 15, wherein the aza-Michael addition reaction is a catalyst-free reaction carried out in an aqueous phase.

18. The method of claim 14, wherein the step of alternatively depositing a layer comprising a hydrophobically modified cationic polyelectrolyte and a layer comprising an anionic polyelectrolyte on a membrane is repeated until forming 5 to 50 bilayers, preferably 5 to 15 bilayers, on a side of the membrane.

19. A redox-electrodialysis (redox-ED) system for selective recovery of VFAs, the system comprising: a first electrode; a second electrode positioned in opposition to the first electrode; a first and second cation exchange membrane (CEMs) disposed between the first and second electrodes; and a bicontinuous PEC-layered membrane disposed between the first and the second CEMs, wherein the bicontinuous PEC-layered membrane comprises: a membrane, and a PEC layer disposed on the membrane, the PEC layer comprising a hydrophobically modified cationic polyelectrolyte and an anionic polyelectrolyte,wherein the PEC layer comprises a bicontinuous structure due to complexation-induced phase separation driven by hydrophobicity contrast between the hydrophobically modified cationic polyelectrolyte and the anionic polyelectrolyte.

20. The redox-ED system of claim 19, wherein the hydrophobically modified cationic poly electrolyte has a predetermined hydrophobicity to balance associative and segregative interactions between the hydrophobically modified cationic polyelectrolyte and the anionic polyelectrolyte within the PEC layer during complexation-induced phase separation.

21. The redox-ED system of claim 19, wherein the hydrophobically modified cationic polyelectrolyte comprises a product of an aza-Michael addition reaction between a cationic polyelectrolyte having amine groups, hydroxyl groups, or thiol groups as a Michael receptor and an acrylate derivative, an acrylamide derivative, or a vinyl sulfone amide derivative as a Michael acceptor.

22. The redox-ED system of claim 19, comprising from 2 to 5 bicontinuous PEClayered membranes stacked on one another, preferably 2 bicontinuous PEC-layered membranes.

23. The redox-ED system of claim 19, further comprising: a feed channel extending between the bicontinuous PEC-layered membrane and the second CEM for delivery of a multicomponent feed solution; an accumulating channel extending between the bicontinuous PEC-layered membrane and the first CEM for collecting VFAs separated from the multicomponent feed solution; and a redox channel containing the first and second electrodes and being separated from the feed and accumulating channels by the first and second CEMs.

24. The redox-ED system of claim 19, wherein the first electrode is configured to become positively charged upon application of a voltage, and the second electrode is configured to become negatively charged upon application of a voltage.

25. A method of selective recovery of VFAs from a multicomponent feed solution, comprising: exposing a bicontinuous PEC-layered membrane to a multicomponent feed solution in a redox-ED system, the multicomponent feed solution comprising a plurality of organic acids; and selectively removing VFAs from the multicomponent feed solution, wherein the bicontinuous PEC-layered membrane comprises a membrane and a PEC layer disposed on the membrane, the PEC layer comprising a hydrophobically modified cationic polyelectrolyte and an anionic polyelectrolyte, and wherein the PEC layer comprises a bicontinuous structure due to complexation-induced phase separation driven by hydrophobicity contrast between the hydrophobically modified cationic polyelectrolyte and the anionic poly electrolyte.

26. The method of claim 25, wherein the redox-ED system comprises: a first electrode; a second electrode positioned in opposition to the first electrode; a first second CEMs disposed between the first and second electrodes; and the bicontinuous PEC-layered membrane disposed between the first and second CEMs, wherein a feed channel extends between the bicontinuous PEC-layered membrane and the second CEM for delivery of the multicomponent feed solution; wherein an accumulating channel extends between the bicontinuous PEC-layered membrane and the first CEM for collecting the VFAs separated from the multicomponent feed solution, and wherein a redox channel separated from the feed and accumulating channels by the first and second CEMs contains the first and second electrodes.

27. The method of claim 25, wherein the VFAs selectively pass through the bicontinuous PEC-layered membrane and into the accumulating channel; and further comprising collecting the VFAs in an accumulation tank.

28. The method of claim 25, wherein the VFAs pass through the bicontinuous PEC-layered membrane at a flux at least about 1.9 times higher than through a pristine membrane.

29. The method of claim 25, wherein, during the exposure of the bicontinuous PEC-layered membrane to the multicomponent feed solution, a voltage is applied to the redox-ED system, whereby the first electrode becomes positively charged and the second electrode becomes negatively charged, and redox reactions occur in the redox channel.

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