Easy preparation of wood-base membrane with fouling resistance in complex environments for efficient oil–water emulsion separation

Oily wastewater causes a serious threat to the ecological environment and human health, how to effectively treat oily wastewater is a big concern. In recent years, the treatment of oil–water emulsions has considerably advanced through the development of separation membranes with special wettability, However, these membranes involve problems, such as complex preparation processes and material contamination, so developing an economical and environmentally friendly, high-performance membrane is a significant challenge. In this work, a wood-based membrane was easily prepared by a simple dipping process using aramid nanofibers (ANFs) to modify the surface of wood. Compared to synthetic hydrogel membranes, the wet ANF/wood membrane exhibits higher tensile strength (1.69 ± 0.32 MPa). More importantly, the membrane presents underwater superoleophobic properties and fouling resistance under complex environmental conditions (acid, alkali, seawater, and high temperature) and effectively separates various oil–water emulsions with high separation efficiency (> 99.3%) and flux (> 227 L m−2 h−1). More excitingly, the membrane retains its original separation properties after 13 cycles of oil–water emulsion separation. Therefore, the inexpensive, environmentally friendly and easily prepared ANF/wood membrane is well tolerated under extreme conditions, presents excellent separation performance and provides a material basis for the treatment of actual oily wastewater.


Introduction
With rapid economic and social development, large amounts of domestic and industrial wastewater are randomly discharged without treatment, which can directly affect the balance of the ecosystem and pose a threat to human health (Cao et al. 2017;Chen et al. 2022;Li et al. 2021a;Yang et al. 2023;Zhang et al. 2022b).To address this practical challenge, a wide variety of techniques including skimmer, air flotation, in-situ burning and coagulation have been adopted to separate immiscible oil/water mixtures with satisfactory results (Han et al. 2019;Kong et al. 2023;Li et al. 2021b;Zhang et al. 2023a).However, for surfactant-stabilized oil-water emulsions with small droplet sizes (< 20 µm), the above-mentioned technologies suffer from a dramatic decline in separation rates, high energy consumption, and failure in complex environments (Wang et al. 2021c;Zhang et al. 2022a).Relative to these traditional process, membrane separation technology has been identified as one of the most effective and promising methods for oil-water emulsion purification with the advantages of straightforward process, high separation efficiency and low cost (Guo et al. 2020).Currently, chemically modified hydrophilic polymeric membranes (polypropylene, PP; poly (vinylidene fluoride), PVDF; polytetrafluoroethylene, PTFE) have been extensively exploited and demonstrated excellent separation performance for emulsified oil/water mixtures (Dong et al. 2022;Zhang et al. 2022a).For example, Tripathi et al. fabricated underwater superoleophobic PVDF membranes by a molecular grafted process, revealing > 99% oil rejection (Nayak et al. 2021).Wang et al. prepared PP membranes grafted with poly (2-dimethylaminoethyl methacrylate) and poly(oligo(ethylene glycol) methacrylate) via ultraviolet (UV)-initiated polymerization, presenting high separation efficiency and pollutant resistance (Wu et al. 2018).Wang et al. obtained underwater oleophobic PTFE membranes with high efficiency (> 95%), excellent anti-fouling properties and long-term stability (Wei et al. 2017).However, these modified membranes remain unsatisfactory for several domains of materials used and industrial applications; non-environmentally friendly raw materials, such as monomers without polymerization, nanoparticles that are easily dislodged and non-biodegradable polymers, inevitably lead to human diseases and environmental pollution (Li et al. 2020;Wang et al. 2021a;Yan et al. 2020;Yang et al. 2022).Sophisticated preparation processes are timeconsuming, cost-effective and cannot easily meet the requirements for practical applications.Therefore, the ability to prepare oil-water separation membranes using green and environmentally friendly raw materials through facile processes is highly desirable (Deng et al. 2020).
To date, many green materials (wood, egg shell and corn stalk, etc.) have been utilized as oil-water separation membranes (Ahmad et al. 2023;Bai et al. 2019;Fu et al. 2018;Shi et al. 2020;Wang et al. 2022a;Zhang et al. 2019).Among these green materials, wood has attracted our interest due to its porous microstructures, high mechanical properties, cost effectiveness and renewability.For example, Yong et al. obtained a porous wood membrane by a simple mechanical drilling process to separate oil/water mixtures with high separation efficiency (Yong et al. 2018).Fu et al. used delignified wood compounded with epoxy resin to prepare superlipophilic/superhydrophobic separation membranes for the adsorption of organic matter (Fu et al. 2018).Kim et al. utilized micron-sized pores of wood membranes to purify emulsions by chemically removing lignin and hemicellulose from wood (Kim et al. 2020).However, wood-based membranes still present challenges in terms of complex treatment processes and the inability to scale up production for oil-water emulsion purification.
Vol.: (0123456789) In this work, an easy-to-prepare, scalable ANF/ wood separation membrane with underwater superoleophobicity is designed and prepared by a simple dip coating process for separating oil-water emulsions with small droplet sizes (< 15 µm) (Scheme 1).The wood serves as a support layer and water transport layer, and a crosslinked ANF hydrogel layer acting as an oil repelling layer was introduced onto the wood surface.The ANF/wood membrane displays excellent mechanical properties (strength: 1.69 ± 0.32 MPa, strain: 6.48 ± 0.86%).More importantly, the membrane shows underwater superoleophobicity with oil contact angles (OCAs) of approximately 150° and antifouling properties even under complex conditions.More excitingly, the ANF/wood membrane exhibits separation efficiency for various oil-water emulsions (above 99.3%) in conjunction with a high separation flux of ~ 227 L m −2 h −1 .The separation efficiency and the flux remains basically constant even after being recycled 13 times, revealing the high cycling stability.

Preparation of ANF/DMSO dispersion
A stable, dark red ANF/DMSO dispersion was prepared through a deprotonation process in the KOH/ DMSO system.Briefly, 6 g KOH and 4 g aramid fibre were added to 196 g DMSO solution, and then mechanically stirred at 1200 rpm for 96 h to obtain a homogeneous ANF/DMSO dispersion with a concentration of 2 wt%.

Preparation of the ANF/wood separation membrane
The diluted ANF/DMSO dispersion (30 mL, 0.1 wt%) was poured into a beaker.Immediately afterwards, a piece of balsa wood (50 × 50 × 1 mm) was completely immersed in the above-mentioned solution for 24 h.
Vol:. ( 1234567890) Then, the wood modified with the ANF dispersion was placed in deionized water for 24 h.Finally, the ANF/wood separation membrane was prepared.

Preparation of various emulsions
Carbon tetrachloride, dichloroethane, n-hexane, n-dodecane and diesel were separately mixed with deionized water in a volume ratio of 1:99.SDS surfactant (0.2 mg/mL) was added into the above-mentioned oil-water mixtures, stirred for 40 min at 800 rpm, and then sonicated for 20 min to obtain various well-dispersed emulsions.

Emulsion separation process
The ANF/wood membranes with a diameter of 1.6 cm were prewetted with deionized water and then sandwiched between a glass tube and flask.Various emulsions with a volume of 20 mL were poured into the glass tube in a single pass, and then the filtrates were collected in the flask under a vacuum pressure of 0.93 MPa.After a single pass, the ANF/wood membrane was removed and washed with deionized water.Subsequently, the cleaned membrane was sandwiched between the glass tube and the flask, and then the next separation was performed under vacuum.The ANF/wood membrane was heated at 90°C for 20 min, cooled to room temperature and then sandwiched between the glass tube and the flask to perform hightemperature separation experiments.

Characterizations
Microstructure was characterized by scanning electron microscopy (SEM Regulus 8100, Hitachi, Japan).X-ray photoelectron spectra (XPS) analysis was carried out by a Thermo Scientific K-Alpha (US).FT-IR spectra of the samples were recorded with a Thermo Scientific Nicolet iS20 (US).The underwater oil contact angles were measured by a contact angle measuring instrument (LSA60, LAUDA Scientific, Germany).Stress-strain curves were tested by an electronic universal testing machine (KXWW-05C, Chengde Kebiao Testing Instrument Manufacturing Co., Ltd., China).The oil content of the filtrate was detected by infrared oil meter (OIL 460).Olympus (U-RFL-T) microscope was employed to obtain microscopic images of the emulsions and filtrate samples.The three dimensional structure of the membrane surface was verified by a laser microscope imaging system (VK-150K, Keyence, Japan).Transmission electron microscope (TEM, JEM-2100, JEOL, Japan) was utilized to observe the morphological structures of ANF.

Exfoliation and preparation of aramid nanofibers
The ANFs covered the wood surface to form a hydrogel film layer, and the resulting nanopores could simultaneously attract water and repel oil.Among them, ANFs are chemically exfoliated from yellow and short aramid fibres that form the words "AF" (Fig. 1a).The surface morphology of the aramid fibres with a diameter of 10.2-14.9μm shows smooth and uniform characteristics according to SEM analyses (Fig. 1b-d).The specific stripping process is as follows: the aramid fibres were immersed in a DMSO/KOH solution system followed by mechanical stirring to undergo deprotonation (i.e., the hydrogen in the amide group on the polymer chain was removed) (Su et al. 2022;Wang et al. 2022b).The hydrogen bonding interactions in the polymer chains tend to be attacked by hydroxide ions and show negative electricity; thus, electrostatic repulsion are generated between chains, resulting in the exfoliation of aramid fibres (Fig. 1i) (Yang et al. 2011).A dark red ANF dispersion with a concentration of 2 wt% was obtained (Fig. 1e).As determined by TEM characterizations (Fig. 1f), exfoliated ANFs with diameters ranging from 10 to 40 nm were generated with a branched micromorphology (Fig. S1).The ANF solution was subjected to a typical sol-gel conversion process to obtain a yellow 'ANF' hydrogel (Fig. 1g).The ANF aerogel was transformed from the hydrogels using freeze-drying technology to reveal a 3D interconnected network with a nanopore structure (Fig. 1h), which was similar to the TEM structure.These results indicate that the exfoliation of the aramid fibre was successful in this experiment.

Structure and mechanical properties of the materials
Natural Balsa wood (Fig. 2a) containing cellulose, hemicellulose and lignin is environmentally friendly Deprotonation and biocompatible with many hydroxyl groups, which endows it with hydrophilic properties (Jia et al. 2017).Extensive vascular structures with a diameter of 134-318 μm inside the wood provide fast transport pathways for water (Fig. 2b, c).In addition, the surface of wood and voids of vascular bundles are covered with a substantial amount of wood chips produced by the cutting process, as observed in the SEM image, which greatly increased the roughness of the wood surface.This rough structure helps to reform the combination of wood and ANF hydrogel and thus improves the mechanical properties of the ultrathin ANF hydrogel layer.On the other hand, these debris can be well filled into the voids of the vascular bundles, reducing the pore size and simultaneously providing good support to the ANF hydrogel layer during the oil-water purification process.Wood was immersed in the ANF dispersion at a concentration of 0.1 wt% to form the ANF/wood membrane.Compared with other green materials, ANF/wood membranes are prepared in a more environmentally friendly, inexpensive and simple process (Table.S1).After the sol-gel conversion approach, ANF hydrogel wrapped around wood.Be-implementing part of the hydrogen bonding interactions between ANFs in appearance contributes to a change in wood colour from yellow to light yellow through the ANF reprotonation process (Fig. 2e).The surface of wood was well filled by the ANF, creating a relatively smooth surface morphology (Fig. 2f).The surface structure of the ANF/wood membrane was further identified using laser confocal microscopy, revealing a similar surface micromorphology to that of the SEM image (Fig. 2g).A comparison of the cross-sectional images of the wood before and after the modification of ANF also displayed that the ANF hydrogel layers only cover the surface of the wood relying on hydrogen bonding interactions (Fig. 2d, h).To demonstrate the presence of hydrogen bonding interactions, Fourier transform infrared (FTIR) spectra and X-ray photoelectron spectroscopy (XPS) of the pure ANF membrane, wood membrane and ANF/wood membrane were measured.The ANF membrane exhibits characteristic peak at 1644 cm −1 corresponding to the -C=O groups and the other characteristic peaks at 3322 and 1541 cm −1 are associated with -N-H groups and the N-H/C-N coupled mode, respectively (Songfeng et al. 2023;Liu et al. 2023) (Fig. 2i).For the ANF/ wood membrane, the -C=O stretching peak changed from 1644 cm −1 (ANF) to 1651 cm −1 , demonstrating that hydrogen bond interactions formed between ANF and wood (Fig. S2) (Chen et al. 2021;Zeng et al. 2020).XPS spectroscopy was used to further verify the hydrogen bond interactions (Fig. 2j-m).The characteristic peaks at 284.8, 286.4,287.9 eV and 288.4 eV correspond to C-C, C-O, C-N and C=O for the ANF/wood membrane, respectively.The binding energy of the C=O characteristic peak at 290.8 eV for ANF shifts to 288.4 eV for the ANF/wood membrane.The above-mentioned results further demonstrate that hydrogen bond interactions are formed between ANF and wood.The XPS wide-scan spectra of pure ANF, wood membrane and ANF/wood membrane are shown in Fig. 2j.We can see that the wood membrane contains C and O (Table S2).After modification, a new N element appears for the ANF/wood membrane in the XPS spectrum.High-resolution XPS spectra for C 1s of the ANF/wood membrane and wood membrane are displayed in Fig. 2k and m.Compared with the wood membrane, a new C-N group appears for the ANF/wood membrane.N and the C-N group originate from the ANF coatings.These results indicate that the ANF hydrogel layer was successfully introduced into the wood surface.
In addition, the proportion of wood in the overall ANF/wood membrane was calculated to be 89.4 wt% maximum; in other words, most membrane materials can be naturally degraded, which significantly reduces the environmental residue of the waste (Fig. S3).Mechanical performance is an important indicator in the actual oil-water separation process to meet the demands of high water pressure.In this experiment, the mechanical properties of the ANF/wood membrane in different directions were investigated comprehensively using a universal mechanical stretching machine.The formed ANF/wood membrane has a tensile strength of 0.92 ± 0.06 MPa, 1.69 ± 0.32 MPa and strains of 12.35 ± 0.81% and 6.48 ± 0.86% in the TR and LR directions, respectively (Fig. S4), and the values remain much higher than those of most previously reported synthetic separation membranes, demonstrating obvious superiority in terms of mechanics (Fig. 2n).

Wettability characterization of the membrane
To efficiently separate emulsions, apart from mechanical properties, the interfacial wettability of ANF/ wood separation membranes must be different for the oil and water phases (Cai et al. 2022).Natural wood is hydrophilic and oleophilic in air (Fig. S5), and its OCA reaches 151° under water.The ANF-modified wood that exhibits an underwater OCA of up to 157° displays stable underwater superoleophobicity, mainly depending on an effect caused by the highly porous structure of polymeric layer on the wetting behavior of underwater oil droplets (Jiang et al. 2023) (Fig. S6).The ANF/wood membrane displays excellent separation performance when exposed to extreme conditions (acid, alkali, salt solution and high temperature), which is an important prerequisite for realizing practical applications.The underwater OCAs of the ANF/wood separation membrane for five types of oils, including carbon tetrachloride, dichloroethane, n-hexane, n-dodecane, and diesel under corrosive aqueous solutions (3 M HCl, 0.1 M NaOH, seawater) and high temperature (maintaining temperature 90 °C for 20 min) were thoroughly investigated via a contact angle instrument.Amazingly, the values are up to approximately 150° (Fig. 3a-e), demonstrating excellent ability to repel oil underwater and prominent durability.The excellent ability to repel corrosive liquids and hot water makes it suitable for a variety of complex environments.Furthermore, the ANF/ wood separation membrane exhibits an excellent self-cleaning performance.As shown in Fig. S7, the soil covering the membrane surface could be easily washed away, and the surface of the separation membrane was spread over by carbon tetrachloride solution coloured with oil red; this surface was restored to its original state after being washed with clean water (Fig. 3f).In addition, no noticeable changes in the wettability were observed under different treatment conditions (Fig. S8).

Emulsion separation
The above-mentioned ANF/wood separation membranes, which exhibit fantastic mechanical properties and special wettability, have been exploited to separate oil-water emulsions, including carbon tetrachloride-in-water, dichloroethane-in-water, n-hexanein-water, n-dodecane-in-water and diesel-in-water (Fig. 4).Macroscopically, these emulsions are milky white, opaque dispersions (Fig. 4a1-e2 left) and basically remain their original state after long storage times (24 h) (Fig. S9), showing good dispersion stability.In addition, the microstructures characterized using optical microscopy reveal droplets with varying sizes distributed in the above emulsions (Fig. 4a1-e 1 ), and the corresponding lateral size of the emulsions is less than 15 μm based on data statistics (Fig. 4a4-e4), which strongly demonstrated the presence of massive oil-in-water emulsions with high stability.During the separation process, the membranes were prewetted with deionized water and then sandwiched between a glass tube and flask.The emulsions were poured into the glass tube and the filtrates were collected in the flask under a vacuum pressure of 0.93 MPa.It was found that water could penetrate the membranes, whereas emulsified oil droplets were retained on the membrane surface with almost no penetration.The colour of the filtrates became colourless and transparent (Fig. 4a2-e2 right), providing initial evidence that the emulsions were successfully separated through the ANF/wood separation membrane.The filtrates were further examined under an optical microscope and no micron-sized oil droplets were found because the pore size of the separation membrane is smaller than that of the oil droplets (Fig. 4a3-e3).More importantly, the oil-in-water emulsions formed by emulsified n-hexane in different environments (3 M HCl, 0.1 M NaOH seawater and high temperature) using the same experimental procedure as above can efficiently separate oil-water emulsions; this was determined by comparing the optical microscopy images before and after separation (Fig. S10).In addition, natural wood as a separation membrane could selectively separate water from various oil/water mixtures (Fig. S11).
To accurately assess the separation behaviour, the oil content before and after separation was measured separately.Separation efficiency (SE) can be calculated with the following Eq.( 1) (Wang et al. 2021b): where C 0 represents the amount of oil in the oil-in- water emulsion and C s is the amount of oil in the fil- trate after emulsion separation.
The separation flux can be calculated by Formula (2) (Gao et al. 2021): where J is the water flux, V represents the volume of the solution, A is the valid separation area and Δ t is the separation time.
As shown in Fig. 5a, the SE values of the separation membrane for surfactant-stabilized dichloroethane-in-water, n-hexane-in-water, n-dodecane-inwater and diesel-in-water were up to 99.99%, 99.99%, 99.37% and 98.87%, respectively.In addition, the filtration fluxes for the four different emulsions are 422 L m −2 h −1 , 435 L m −2 h −1 , 363 L m −2 h −1 and 321 L m −2 h −1 , respectively, thereby indicating its superior separation ability for oil-in-water emulsions.More excitingly, the ANF/wood membrane possesses high SE (> 99.38%) for three corrosive emulsions and high temperature with the flux reaching more than 227 L m −2 h −1 (Fig. 5b), indicating that the ANF/wood membranes are chemically stable even in extremely harsh environments.In addition, the long-term separating performance of the membranes was investigated by repeating the separation procedures 13 times.The membranes with no changeable morphology exhibited excellent cycle stability, as the separation efficiencies remained basically unchanged (up to 99.12%) and the filtration flux remained higher than 5c, S12).Compared with other types of separation membranes, the ANF/wood membrane reveals good integrated properties (Fig. 5d).

Oil-water separation process and mechanisms
The separation mechanism of the ANF/wood membrane for oil-in-water emulsions is shown in Fig. 6.  et al. 2020;Xu et al. 2023;Zhang et al. 2019Zhang et al. , 2023b) ) The hydration layer generated by a large amount of hydrophilic groups on the surface of the separation membrane achieves macroscopic oil-repelling performance.Microscopically, the layer can preferentially bind the water molecules in the emulsion and reject the oil molecules inside, breaking the emulsion (Fig. 6a).The oil-in-water emulsion separation was mainly determined by the pore size of the separation matrix and breakthrough pressure (Kim et al. 2020).Among them, breakthrough pressure refers to the maximum penetrating pressure exerted on the ANF/ wood membrane surface when the liquid can invade the pore space (Mosadegh-Sedghi et al. 2014) and can be calculated as follows: where R represents the radius of the membrane pore, γ is the surface tension of the liquid-gas interface, and θ refers to the essential contact angle of the liquid.As illustrated by Eq. ( 3), the maximum penetrating pressure decreased as the pore size increased when > 90° and ΔP > 0. Thus, the surface of the membrane can withstand a certain amount of pressure, creating a repulsion of the liquid so that it cannot pass through the membrane (Fig. 6b).In contrast, when < 90° and ΔP < 0, the liquid can quickly pass through the film (Fig. 6c).Therefore, the oil drop is blocked by the ANF/wood membrane, and the water passes through quickly, thus successfully achieving emulsion separation.

Conclusion
In summary, the ANF/wood membrane was prepared by a facile dip coating process.The ANF/wood membrane exhibits a high strength of 1.69 ± 0.32 MPa along with superoleophobic properties under water and fouling resistance under complex conditions (acid, alkali, seawater and high temperature).In particular, the as-prepared membrane could separate various oil-water emulsions (carbon tetrachloridein-water, dichloroethane-in-water, n-hexane-in-water, n-dodecane-in-water and diesel-in-water) with high separation efficiency (> 99.3%) and permeation flux (> 227 L m −2 h −1 ) even after being treated by corrosive emulsions and high temperature.More importantly, the membrane retains high separation efficiency and permeation flux after 13 cycles, presenting satisfactory recyclability.Therefore, the ANF/wood membrane, which is scalable, contamination-resistant, eco-friendly and can withstand extreme environments, provides the material basis for the treatment of oily wastewater.

Fig. 1 a
Fig. 1 a Optical photo of aramid fiber with the word of "AF".b, c SEM images of aramid fiber.d Diameter statistics of aramid fiber.e Optical photo of ANF dispersion with the concentration of 2 wt%.f TEM image of ANF revealing a branched structure.g After sol-gel conversion, the hydrogel with the 'ANF' letters was shown.h SEM image of the ANF aerogel.i Schematic illustration of preparation of ANF from macroscopic aramid fiber

Fig. 2 a
Fig. 2 a Optical photo of wood.b, c Top-view SEM images of wood.d Side-view SEM image of wood.e Optical photo of ANF/wood.f Top-view SEM image of ANF/wood.g Laser confocal scanning microscope image of ANF/wood surface morphology.h Side-view SEM image of ANF/wood.i FTIR spectra of the ANF, wood membrane and ANF/wood membrane.j XPS wide-scan spectra of the ANF, wood membrane and ANF/wood membrane.k-m High-resolution XPS spectra

Fig. 3
Fig. 3 OCAs of ANF/wood membrane under a water, b 3 M HCl, c 0.1 M NaOH, d seawater and e high temperature.f Photographic images revealing self-cleaning property of the ANF/wood membrane

Fig. 5 a
Fig. 5 a Separation efficiency and flux of the ANF/wood membranes for various oil-water emulsions.b Separation efficiency and flux of the ANF/wood membrane for surfactant-stabilized n-hexane-in-corrosive solution emulsions.c Recycla-