Micro-dissolved fabrication of robust superhydrophilic and underwater superoleophobic membranes based on cotton fabrics for oil/water separation

The superhydrophilic and underwater superoleophobic membranes based on textile possessed excellent separation efficiency as well as outstanding anti-oil fouling property, while the poor stability caused by nanoparticle (NP) shedding had severely limited the practical application in oily water separation. In this paper, a feasible method of fabricating the robust membranes based on cotton fabric (CF), which was decorated with TiO2 NP and citric acid (CA) through micro-dissolution method, was reported. Firstly, the CF was slightly dissolved in the NaOH/Urea solution at low temperature, accelerating the destruction of the cellulose macromolecule on the superficial layers of the fabrics. Then, the vacuum filtration process promoted the TiO2 NP to distribute uniformly on the CF surface. In the subsequent coagulation process, the TiO2 NP were firmly anchored on the fabric surface because of self-glue effect of micro-dissolved CF. Finally, the esterification reaction between CA and CF was carried out to enhance the hydrophilic property of CF. The prepared membranes were characterized by fourier transform infrared spectroscopy, scanning electron microscopy (SEM), X-ray diffraction and thermogravimetric analysis. Additionally, the separation performance and stability of membranes were systematically evaluated. The results of SEM indicated that the TiO2 NP were uniformly anchored on the membrane surface. Especially, the prepared membranes could not only separate traditional oil/water mixture but also treat complicated oil-in-water (O/W) emulsion with excellent separation efficiency. What’s more, the membranes could withstand various harsh condition and exhibit excellent application stability. In terms of separation performance and stability, the CA/TiO2 decorated cotton fabrics have the potential to be used in the practical oily water separation.

between CA and CF was carried out to enhance the hydrophilic property of CF. The prepared membranes were characterized by fourier transform infrared spectroscopy, scanning electron microscopy (SEM), X-ray diffraction and thermogravimetric analysis. Additionally, the separation performance and stability of membranes were systematically evaluated. The results of SEM indicated that the TiO 2 NP were uniformly anchored on the membrane surface. Especially, the prepared membranes could not only separate traditional oil/water mixture but also treat complicated oil-in-water (O/W) emulsion with excellent separation efficiency. What's more, the membranes could withstand various harsh condition and exhibit excellent application stability. In terms of separation performance and stability, the CA/TiO 2 decorated cotton fabrics have the potential to be used in the practical oily water separation.
Abstract The superhydrophilic and underwater superoleophobic membranes based on textile possessed excellent separation efficiency as well as outstanding anti-oil fouling property, while the poor stability caused by nanoparticle (NP) shedding had severely limited the practical application in oily water separation. In this paper, a feasible method of fabricating the robust membranes based on cotton fabric (CF), which was decorated with TiO 2 NP and citric acid (CA) through micro-dissolution method, was reported. Firstly, the CF was slightly dissolved in the NaOH/Urea solution at low temperature, accelerating the destruction of the cellulose macromolecule on the superficial layers of the fabrics. Then, the vacuum filtration process promoted the TiO 2 NP to distribute uniformly on the CF surface. In the subsequent coagulation process, the TiO 2 NP were firmly anchored on the fabric surface because of self-glue effect of microdissolved CF. Finally, the esterification reaction

Introduction
In recent years, membrane separation technology for the oily water separation had aroused the widespread interest in the academia and industry (Xue et al. 2011). Various available materials, such as metal mesh , filter paper (Wang et al. 2010), fabric (Sun et al. 2020), were used to design mechanically flexible membranes with energy saving, simple operation, and environmentally friendly properties . Current membranes with superhydrophilicity and superoleophilicity were recognized as the promising materials in oily water separation, because of their water-removing and oilremoving properties, respectively. As for superoleophilic membranes, bottlenecks still remained including oil fouling (Singh et al. 2019) and low permeation flux in the practical application . In contrast, superhydrophilic membranes particles became unstable when exposed to high temperature and corrosive conditions due to the lack of strong bond between particles and membrane surface. Given the importance of hierarchical structure, numerous efforts had been made to improve adhesion of particle on membrane surface. Among these outstanding researches, the application of binder was one of the most effective ways to enhance the anchoring stability. Dopamine, which could self-polymerize and adhere on various substrates surface, had been used to directly graft TiO 2 NP on the PVDF membrane surface (Shi et al. 2016). To further improve adhesion between inorganic NP and substrate, two commercial silane coupling agents (TTOP-12, KH550) were applied during the pretreatment process. It was obvious that the prepared membrane exhibited excellent stability even under harsh conditions, such as mechanical abrasion, high temperature, corrosion and UV radiation (Gao et al. 2018). Lin and coworkers prepared a robust waterborne superhydrophobic coating by using inorganic binder (aluminum orthophosphate) to enhance the interfacial binding between the substrate and the coating, which exhibited excellent abrasion resistance due to the formation of phosphate networks ). Very recently Bai et al. reported a fabrication of superhydrophobic cotton fabric using natural coating material through incorporation of nano-TiO 2 and oriental lacquer, which was collected from lacquer trees and used as natural binder material for thousands of years. The resulting cotton was sufficiently to tolerate harsh conditions, such as strong acidic and strong alkaline solutions (Bai et al. 2020). However, the presence of the binder may have negative effect on separation performance of membranes due to the clogging of pores in the membranes. What's more, the using of binder will increase the cost of preparation process and may cause damages to the environment (Liu et al. 2012).
Thus, it is necessary to investigate a feasible way to prepare membranes, which can maintain their separation properties in harsh conditions and also reduce the harm to the environment. According to the previous researches (Cai and Zhang 2005;Lu et al. 2016), CF could be slightly dissolved in the NaOH/Urea (7 wt%/12 wt%) mixture solution at − 12 °C. In the NaOH/Urea aqueous solution, the cellulose macromolecules were interrupted by NaOH hydrates and Urea hydrates, which could destroy the hydrogen-bond networks of macromolecules at low temperature. In additional, the extent of cellulose dissolution could be controlled by regulating the experimental parameters. When the microdissolved cellulose was introduced into Na 2 SO 4 (5 wt%)/H 2 SO 4 (5 wt%) coagulation bath, the dissolved superficial layers could work like the glue to adhere on the surface of fabrics. Inspired by these meaningful results, a variety of nanoparticles such as TiO 2 (Fan et al. 2017), Fe 3 O 4 (Zhao et al. 2020) and carboxylated multiwall carbon nanotubes  were embedded on the CF surface to prepare functional materials by using micro-dissolution method. The as-prepared composites exhibited satisfactory stability even after 2 h washing.
In the paper, we prepared superhydrophilic and underwater superoleophobic membranes based on CF using the micro-dissolution method. Firstly, in order to achieve roughness structure, the CF was slightly dissolved in NaOH/ Urea mixture solution at − 12 °C for 2 h. Then, the TiO 2 NP were directly deposited on the fabric surface using vacuum filtration method. Owing to the self-glue effect of dissolved CF in the coagulation bath, the TiO 2 NP were firmly anchored on the fabric surface. To further improve the hydrophilic properties of membrane, CF was modified with CA which possessed three carboxylic groups and could be irreversibly bound to CF through esterification reaction. Finally, the CA/TiO 2 decorated membranes were applied for separating various oil/water mixture and O/W emulsions. Moreover, the water flux, oil rejection ratio, stability and recyclability of prepared fabrics membranes were systematically evaluated.
Preparation of superhydrophilic and underwater superolephobic CA/TiO 2 decorated membranes The superhydrophilic and underwater superolephobic CA/TiO 2 decorated membranes were prepared by using micro-dissolution method. Firstly, the CF was immersed in 100 mL 7 wt% NaOH/14 wt% Urea aqueous solution for 2 h at − 12 °C. The mixed solution containing 0.02 g TiO 2 , 7 wt% NaOH/14 wt% Urea was ultrasonicated for 45 min to obtain a welldispersed TiO 2 solution. Then, the mixed solution containing TiO 2 passed through the fabric by using vacuum filtration method with the vacuum degree of − 0.1 MPa. Subsequently, the coagulation process of micro-dissolved cotton fabrics was carried out in the coagulation bath containing 5 wt% H 2 SO 4 /5 wt% Na 2 SO 4 for 30 min. The load content of TiO 2 on fabrics was 2.1%. After that, the cotton fabrics were dipped into 5 wt% CA/5 wt% SHP solution and nipped with a padder (MU3C5T, China) to obtain 80% pickup. Finally, as-prepared CA/TiO 2 decorated membranes were obtained by drying in oven at 80 ℃ and curing at 180 °C for 2 min. After being washed with deionized water and drying in the oven, the weight gain of fabric was about 3.6%.

Characterization
The morphologies of treated CF were observed by a SEM analyzer (Quanta-250, FEI, USA). The chemical compositions and elemental compositions were analyzed by a FTIR (Alpha, Brooke Germany) and an elemental dispersive spectrometry (EDS, Ametek, USA), respectively. The crystal structure of CF was measured by XRD (TD-3500, China) using Cu Kα 1 radiation (λ = 0.15406 nm) at 30 kV and 20 mA. Additionally, the angle was from 5° to 60° in steps of 0.02°. The thermostability of samples was studied by a TG analyzer (TG 209, Netzsch, Germany) with a heating rate of 10 ℃/min. The water contact angle (WCA) and underwater oil contact angle (UOCA) were evaluated by a contact angle meter (OCA200, dataphysics, Germany) using 4 µL of water or oil. The obtained values were the average measurement of five random regions of the membranes. To comprehensively evaluate the oil adhesion properties, the oil droplet (chloroform) was pressed on the surface of samples by external force, and then was lifted. The droplet size of O/W emulsion was measured by nanometer particle size meter (Zetasizer Lab, Malvern Panalytical, UK).

Oil/water separation experiments
The CA/TiO 2 decorated membranes were firstly wetted with 10 g/L NaOH solution for 30 min at 50 ℃ and then fixed on a filter element with a diameter of 2.5 cm, which was placed between two glass tubes. The oil/water mixture was prepared by mixing oil (xylene, n-hexane and soybean oil) with water at volume ratio of 1:1 to obtain 200 mL solution. Then the mixture solution was poured into the upper tube, and the water rapidly passed through the fabric while the oil was restrained. Moreover, a series of O/W emulsions were prepared by mixing oil (hexane, petroleum ether, trichloromethane) and water in the ratio of 1:100 v/v and shearing under a speed of 12,000 rpm for 15 s, the concentration of surfactant (SDS, OP-10, CTAB) was 0.2 g/L. The emulsions were stable for serval hours before they were used for separation. What's more, all the separations were carried out under gravity. The oil/water separation efficiency defined as oil rejection coefficient (R) and water flux (F) were calculated according to the Eq. (1) and Eq. (2), respectively.
where V 0 and V 1 were the volume of water before and after oil/water separation, respectively. V was the volume of solution passed through the prepared fabric and S was effective separation area of membrane. Additionally, t represented the passing time of separation. Each separation test was repeated for five times and the average result was taken as the experimental result. (1)

Stability of membranes
The stability of the coating on fabric membranes was determined by the measurements of water flux, oil rejection coefficient and UOCA after the treatments of samples with a variety of experiments such as chemical stability, thermal stability, abrasion stability and washing stability with repetitions. The chemical stability of prepared membranes was characterized by soaking in a series of pH = 1-14 solutions at 25 °C for 2 h. As for the evaluation of abrasion stability, the modified side of membranes was pressed on a sandpaper (1200 mesh) under a weight of 100 g and moved straightly for 20 cm. To assess the thermal stability, the prepared membranes were immersed in 100 mL aqueous solution at various temperature ranging from 20 to 100 °C. The samples were also subjected through a washing test (ISO 105-C10 2006 A) where the fabrics were soaked in a solution containing 5 g/L detergent and rotated for 30 min at 40 °C. The washing test was repeated several times and corresponding testing results were recorded after each experiment.

Structural characterization
The preparation process of CA/TiO 2 decorated membranes consisted of three steps. In the first step, the CF, which was slightly dissolved in NaOH/Urea solution, could sever as a loading platform. In the second step, the TiO 2 particles were distributed on the fabric surface by using vacuum filtration method. Then, the re-coagulate process in the Na 2 SO 4 /H 2 SO 4 aqueous solution facilitated the immobilization of NP on the surface of CF. In the third step, the CA was grafted on cotton fabric by esterification reaction to construct superhydrophilic coating on membrane surface. Compared to the CF, a new absorption peak at 1760 cm −1 , which was assigned to C=O bond, could be clearly observed in the CA/TiO 2 /CF (Fig. 1a), indicating that CA was successfully grafted on the CF. During the separation process, the negatively charged cotton fabrics were activated by alkali treatment, leading to absorb abundant water due to the hydration effect of sodion presented in -COONa groups. Additionally, the TG analysis of samples after modification were conducted under nitrogen atmosphere with a heating rate of 10 °C/min. As shown in Fig. 1b, in the first stage, the two TG curves were similar and the weight lost was around 5% in the temperature ranged from 50 to 100 °C, which was ascribed to the dehydration of fiber (Zhou et al. 2019). The onset degradation temperature of second stage, namely the degradation of cellulose, decreased from 378 to 362 °C after the introduction of CA, which was attributed to that the presence of organic acid facilitated the destruction of the polymer chains of cellulose during the heating process. Furthermore, the char yield of CA/TiO 2 /CF at 600 °C could reach 21%, which was much higher than that of CF (1.2%), indicating that the TiO 2 NP were firmly embedded on the surface of cellulose. The crystal structure of samples was investigated by XRD analysis. For the CF, the appearance of peaks at approximately 14.8°, 16.5°, 22.8°, and 34.1° were assigned to the (1 -1 0), (1 1 0), (2 0 0) and (0 0 4) crystal faces of cellulose, respectively. The locations of these peaks for TiO 2 /CF and CA/TiO 2 /CF did not change distinctly, implying that micro-dissolution process and esterification process had no crucial effect on the fabrics. Moreover, five new peaks at 25.11°, 37.74°, 47.92°, 53.87° and 54.86° were detected for TiO 2 /CF and CA/TiO 2 /CF (Fig. 1c), which were corresponded to the (1 0 1), (0 0 4), (2 0 0), (1 0 5) and (2 1 1) crystal faces of anatase TiO 2 . The appearance of the typical TiO 2 peaks demonstrated that the TiO 2 particles were anchored on the surface of CF. The surface morphologies of samples were visualized by SEM. From Fig. 2a, it was clear that CF had relatively clean and smooth surface. After micro-dissolution process, the surface of TiO 2 / CF was distinctly rough and the TiO 2 particles were trapped in the ravine along the length of the fiber. Vacuum filtration method was applied to construct uniform TiO 2 NP coating on the substrate fabric with a tunable deposition amount. With further increasing the amount of TiO 2 (Fig. 2d), the surface of CA/ TiO 2 /CF became coarser, indicating that the TiO 2 particles were uniformly distributed on the surface of cotton fabrics. Form the EDS results (Fig. 1d), a certain amount of Ti from the surface of the CA/TiO 2 / CF can be clearly verified, which also confirmed that TiO 2 particles were successfully anchored on the fabric surface.

Surface wettability analysis
The surface wettability was essential to the permeation property of the membranes. The CF coated with TiO 2 NP and CA were compared to investigate effect of deposition materials on the permeation of water droplet through the membrane. For the CF decorated with TiO 2 NP, the WCA decreased instantly to 0° in 134 ms (Fig. 3b), indicating the presence of TiO 2 nanoparticles notably enhanced the hydrophilic property of CF. More superior results were also observed for the membranes coated with TiO 2 NP and CA (Fig. 3c). The difference in the water spread on the surface of membranes originated from the synergistic effect of NP and hydrophilic materials (Zhang et al. 2014). On one hand, the hydrophilic layer with numerous anionic groups gave rise to proper water wettability, ensuring the water droplets quickly spread on the membrane surfaces ). The as-prepared membrane surfaces could be spread by water due to the high affinity between water and fabric surface, which was modified with CA. On the other hand, the presence of TiO 2 NP endowed the membrane surface with hierarchical structure, resulting in further improving hydrophilic feature of membranes, which was in accordance with Cassie-Baxter wetting theory (Lou et al. 2022). As a consequence, Fig. 1 a FTIR spectra of cotton fabric and CA/TiO 2 decorated CF (CA/TiO 2 /CF), b TG and DTG analysis of CF and CA/TiO 2 /CF under nitrogen condition, c XRD spectra of CF, TiO 2 decorated CF (TiO 2 / CF) and CA/TiO 2 /CF, d EDS analysis of CA/TiO 2 /CF the water spreading speed of membrane decorated with TiO 2 NP and CA was much faster than pristine CF, providing a chance that water droplets rapidly permeated through membranes.
The occurrence of membrane fouling by oil was one of the most difficult problems during the oily water separation process. This problem could be handled by lowering the oil adhesion on the membrane surface. To evaluate the underwater oleophobicity of as-prepared membranes, n-hexane, octane, xylene, 1, 2-dichloroethane and chloroform were used to test the UOCA, as shown in Fig. 4. The UOCAs of CA/ TiO 2 /CF membrane reached to 150°, revealing that the membranes displayed underwater superoleophobic properties. As the oil droplets contacted the membrane surface, the hydrophilic materials and hierarchical structure hindered the spread of oil droplets on the surface. As a result, the shape of oil droplets maintained an almost sphere, leading to the reduction in the contact area between oil and membrane and the enhancement in the antifouling property of membrane. Further work demonstrated the underwater-oil adhesion property of the prepared membranes. As depicted in Fig. 5c, the oil droplets could be easily lifted from the surface of CA/TiO 2 /CF membrane and kept the initial shape even after being compressed. It was difficult to lift the oil droplets from surface of the CF (Fig. 5a) and TiO 2 /CF (Fig. 5b). On account of its superhydrophilic property, a stable water film with higher polarity could be tightly absorbed on the membrane to repel the various oils with lower polarity (Miao et al. 2022). As a consequence, the above results indicated that the CA/TiO 2 /CF membrane have the potential to exhibit outstanding superhydrophilic and underwater superoleophobic properties, which will be beneficial to prevent the occurrence of membrane fouling by oils. The separation properties of prepared membranes under gravity condition were deeply evaluated in terms of water flux and oil rejection ratio. In the separation process, three types of oil/water mixture (soybean oil/water, n-hexane oil/water, xylene oil/ water) were used. At the beginning of separation process, the fabric membranes were wetted with water and then fixed on a glass funnel. The purpose for the wetting of fabric membrane was to expel air bubbles from membrane and ensure the feasibility of the separation (Yang et al. 2016). As showed in Fig. 6a, the oil rejection rate of membranes exceeded 97%. When the oily water was poured into the upper container, water rapidly passed through the membranes, while the oil remained over the membrane surface, as depicted in Fig. 6b. The presence of TiO 2 NP and CA promoted the separation of oil/water mixture with water flux of 1201 L m −2 h −1 , which was approximately twice as much as the CF membrane with water flux of 646.4 L m −2 h −1 . The excellent separation performance of prepared fabric membrane could be attribute to the synergistic effect of presence of hierarchical structure and hydrophilic material, which made the water spread quickly on the membrane surface.
Furthermore, a series of O/W emulsions containing n-hexane, petroleum ether and chloroform were prepared to evaluate the universality of membranes decorated by CA/TiO 2 , concerning the separation of different oils with diverse density in large-scale application. Figure 8a showed that a muddy emulsion  Fig. 6 The oil/water mixture separation performance of CA/TiO 2 /CF membranes. a the water flux and oil rejection ratio of separation, the photograph b displays the separation process became transparent after filtration by gravity. From the optical microscope images, there was no emulsion droplets in the filtrate, while a large number of emulsion droplets presented in the original mixture. As depicted in Fig. 8b, the high flux of 876 L m −2 h −1 could be achieved for petroleum ether-in-water stabilized by SDS, which was higher than that of some reported separation materials, such as tannic acid/polyethyleneimine/dopamine modified polypropylene (489 ± 24 L m −2 h −1 ) (Fang et al. 2020), regenerated cellulose/SiO 2 coating based polypropylene fabric (380.6 L m −2 h −1 ) (Wang et al. 2022), acid-base/squeeze treated rice straw separation layer (> 146 L m −2 h −1 ) (Feng et al. 2021). It was interesting to observe that the highest water flux was for that emulsion prepared with SDS (negative charge), while the CTAB-stabilized emulsion (positive charge) displayed the lowest water flux (Fig. 8b). This phenomenon was attributed to the charge-screening effect . During the separation process, the electrostatic repulsion between same charges hindered the approach of emulsion droplets to membrane surface, which facilitated the water penetration through the membrane. On the contrary, electrostatic attraction between positive charge presented in CTAB and negative charge presented in CA accelerated the demulsification on the surface of membrane, leading to occupation of membrane surface by oils or emulsion droplets, thus obstructing the penetration of water droplets. The mechanism of demulsification was briefly depicted in Fig. 7. Therefore, the charged CA coating was beneficial to promote an effective separation for the O/W emulsion with similar charge features.
Taking the oil density into consideration, the light oil (n-hexane, petroleum ether) exhibited more superior separation flux than that of heavy oil (chloroform). This behavior was ascribed to that the oil droplets of chloroform after demulsification tended to go down and aggregate on the membrane surface because of its high density, leading to a low separation flux (Hu et al. 2015).
To study the effect of emulsion size on water flux, three types of oils stabilized by CTAB were selected to evaluate the separation performance. The measured average size was 5620 nm, 685 nm and 435 nm for petroleum ether, n-hexane and chloroform emulsions, respectively. It was clearly seen that the larger the droplets were, the faster was the separation (Fig. 8d). The reason for the higher separation flux of petroleum ether could be explained as follows. As the emulsion droplets were aggregated on the membrane surface by electrostatic attraction (Wang et al. 2016), the existence of interspace between the bigger emulsions droplets facilitated the water to penetrate through membranes, resulting in improving the separation flux. The above results indicated that the CA/TiO 2 decorated membrane could separate both conventional oil/water mixture and complicated emulsion, thus exhibiting great potential for application in practical oily wastewater separation.

Stability of CA/TiO 2 coated membrane
Although detail measurement results revealed that the wettability and separation performance of prepared membranes were outstanding, there still were lots of technical challenges in application stability of membrane that must be overcome before those products could be applied in the practical oily water separation (Li et al. 2017). Therefore, the mechanical abrasion stability and corrosion resistance of prepared fabric membrane in harsh condition were worth to pay attention. In order to evaluate the anti-abrasion stability of as-prepared membranes, the modified side of membranes were pressed on a sandpaper under a weight of 100 g and moved straightly for 20 cm. As exhibited in Fig. 9a, the separation flux and oil rejection ration of membranes changed slightly after 20 cycles friction, suggesting that the membranes possessed satisfying Fig. 7 The demulsification mechanism of CA/TiO 2 /CF membrane mechanical stability, which was attribute to the strong adhesion between TiO 2 NP and membrane surface . During the preparing process, the TiO 2 particles were tightly anchored on the fiber surface due to the self-glue effect of CF in the coagulation bath. In addition, the CA was grafted on fabric through covalent bond. The corrosion resistance of fabric membranes was investigated by soaking the membranes in the different acid or alkali solutions for 2 h under room temperature. The separation flux and rejection ratio of each membrane were shown in Fig. 9b. It was observed that there were negligible variations in the flux with the pH more than 8. Further decrease of pH value (pH < 7) could lead to the slight decline in the water flux, which was ascribed to the protonation of carboxyl group, resulting in reducing the affinity of membrane surface to water. What's more, the separation performance of fabric membrane under high temperature condition was also assessed, as shown in Fig. 9c. Apparently, the separation flux increased with increasing the temperature, and oil rejection ratio still remained above 98%. The improvement in separation flux of fabric membrane at high temperature may be attributed to the decrease of fabric pore size due to the swelling of fiber. With decreasing pore size of the fabrics, the water-holding properties decreased, leading to facilitate the water penetrate through the membrane (Zheng et al. 2015). The washing stability of decorated fabrics was assessed by measuring water flux, rejection ratio and UOCA after many times of washing. From Fig. 10a, it could be seen that after 20 times washing, the water flux of fabric almost remained above 1100 L m −2 h −1 and the UOCA of fabric slight decreased. In addition, the TiO 2 NP were also firmly anchored on the fabric surface even after 20 times washing, as depicted in the Fig. 10b. Thus, the prepared membrane exhibited excellent washing durability, ascribing to the strong covalent bonding and the stable adhesive property.
In practical application, the clogging of membrane caused by oil pollution was always occurred, resulting in the decline in the separation performance. Therefore, the reusability of prepared fabric membrane was one of the important factors to determine the separation performance of membrane. The oil/water mixture separation experiments were carried out for ten cycles. As could be seen from Fig. 9d, although the water flux slightly decreased with increasing the number of separation cycle, the separation flux kept above 1059 L m −2 h −1 , and the oil rejection ratio kept above 97%. Additionally, the UOCA of fabrics membranes still kept above 146° after 10 cycles of separation. From the results above, it could be concluded that the CA/TiO 2 decorated membrane exhibited superior stability for long-term usage.

Conclusion
In this study, an efficient and versatile method was proposed to prepare a robust superhydrophilic and underwater superoleophobic membranes decorated with TiO 2 NP and CA, which could effectively separate oily wastewater by gravity. Nanoparticle-stacked surface roughness, along with retention of abundant hydrophilic groups conferred the membranes with excellent superhydrophilicity property. The characterization results revealed that the TiO 2 NP were uniformly distributed on the surface of fabric by using vacuum filtration. More importantly than high efficiency oily wastewater separation, the CA/TiO 2 decorated membranes exhibited excellent application stability after even being applied in harsh conditions. This success in fabricating membranes based on CF for oily wastewater treatment suggested that the surface micro-dissolution method was benefited for immobilizing the TiO 2 NP on the surface of fabrics. In view of the separation efficiency and application stability, the membranes based on cotton fabrics could be used as commercial membranes and display the potential of its application on an industrial scale.