Shrimp Shell-inspired Antifouling Chitin Nanofibrous Membrane for

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Shrimp Shell-inspired Antifouling Chitin Nanofibrous Membrane for Efficient Oil/Water Emulsion Separation with in Situ Removal of Heavy Metal Ions Liyu Yan, Peiyuan Li, Weikang Zhou, Zhiguo Wang, xianmou fan, Mingfeng Chen, Yan Fang, and Haiqing Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04511 • Publication Date (Web): 21 Dec 2018 Downloaded from http://pubs.acs.org on December 21, 2018

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ACS Sustainable Chemistry & Engineering

Shrimp Shell-inspired Antifouling Chitin Nanofibrous Membrane for Efficient Oil/Water Emulsion Separation with in Situ Removal of Heavy Metal Ions Liyu Yan,

‡

Peiyuan Li,

‡

Weikang Zhou, Zhiguo Wang, Xianmou Fan, Mingfeng

Chen, Yan Fang,* Haiqing Liu*

Fujian Provincial Key Laboratory of Polymer Materials, College of Chemistry and

Materials Science, Fujian Normal University, Fujian 350007, China

Correspondence

to: Prof. Haiqing Liu (E-mail: [email protected]); Dr. Yan Fang (E-mail: [email protected])

‡

Peiyuan Li and Liyu Yan contributed equally to this paper

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ABSTRACT: In nature, shrimp has an outstanding antifouling behavior since its shell

possesses underwater superoleophobicity. As a major component of the shrimp shell, chitin might has the potential of self-cleaning and can be exploited for dealing with oily water. Hence, novel filtration membranes derived from shellfish wastes were constructed via filtration of chitin nanofiber suspension. The resultant chitin nanofibrous membrane (CNM) was evaluated as a highly efficient oil/water emulsion separation material for the first time. Similar to shrimp shell, CNM with excellent superhydrophilicity and underwater superoleophobicity displayed extremely low underwater oil adhesion and self-cleaning performance. With controllable thickness and nanoscale pore size, CNM could effectively separate micrometer- and nanometersized oil/water emulsions with a high separation efficiency (> 95%) and water flux (> 1500 L∙m-2∙h-1∙bar-1). Furthermore, CNM displayed multifunctional water remediation characteristics, i.e. it could simultaneously remove heavy metal ions from water phase in the course of oil/water emulsion separation. The CNM also exhibited excellent mechanical strength, recyclable performance, thermal stability, and pH-resistance. Therefore, CNM with unique characteristics e.g. sustainability, chemical and thermal resistance, multifunctionality and excellent oil/water separation efficiency has the high potential in the practical oily water treatment. Keywords:

chitin,

nanofiber,

superhydrophilic,

underwater

antifouling, oil/water emulsion separation

2


superoleophobic,

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INTRODUCTION

Nowadays, numerous oil pollutions from our daily life and industries have imparted serious environmental threats to ecology and our life.[1-3] Traditional separation technologies, such as skimming,[4] oil-adsorbing materials,[5] air flotation,[6] and chemical coagulation,[7] have been developed for oil/water separation. However, these methods either ineffectively treat oil/water emulsions,[8] or demulsify the emulsion by adding electric field or chemicals. The latter would inevitably result in depletion of resources.[9] Designing an efficient means for dealing with oil/water emulsions, especially nanometer-scale oil/water emulsions, is still a big challenge. The membrane filtration has been acknowledged as the broadly applicable technology for oily water treatment as it primarily functions based on a “size sieving” effect.[10, 11] For the substrate, porous materials including ceramic filtration membrane, polymer dominated filtration membrane, and nanomaterial-based membrane have been developed.[12,

13]

Among them, the ceramic filtration membrane displays excellent

mechanical properties with high chemical and thermal resistance,[14, 15] but it suffers from severe fouling problem, resulting from the adhesion of oil droplets on outside and internal surface of membrane. [16] This causes a fast decline in separation efficiency in the course of oil/water separation.[17, 18] On the other hand, polymer-based membranes e.g. polyvinylidene fluoride (PVDF) have been designed for handling various oil/water emulsions.[19-22] Unfortunately, because of the oleophilic feature of most polymers, a series of complicated and high cost fabrication procedures involving blending with hydrophilic components, coating, and surface modification, have to be used to enhance their hydrophilicity and anti-oil adhesive properties. This would certainly restrict their 3



wide applications.[21, 23-29] Even worse, most of them are only applicable for separation of micrometer-scale emulsions, but fail to handle with nanometer-scale emulsions,[21, 28, 29]

as their micrometer-scale pore size is larger than that of nanoemulsions.

Particularly, these nanoemulsions are thermodynamically and kinetically stable in the real wastewater, bringing a long-time threat to environment and human life. Recently, the construction of nanomaterial-based filtration membrane has aroused considerable interests in the application of oil/water separation.[30, 31] Thanks to their high surface area, adjustable pore size and ultrathin active separation layer, the nanomaterial-based membrane shows high-efficient separation and high flux for both micrometer and nanometer-sized emulsions. Whereas, with the superior properties, they also suffered from complicated preparation process and expensive raw materials e.g. carbon nanotubes. Thus, a simple, economical method for constructing filtration membranes with eco-friendliness, antifouling, high liquid flux, and high separation efficiency need to be designed. Fortunately, nature enlightens us in the selection of raw materials. Shrimp has an outstanding antifouling properties in water because its shell is hydrophilic, endowing it with underwater superoleophobicity property.[32] As a major component of the shrimp shell, chitin might also have the potential of self-cleaning, depending on the existence of numerous hydrophilic hydroxyl and acetylamino groups on chitin. Notably, native chitin derived from shrimp shells consists of bundles of crystalline chitin nanofibers and can be easily and sufficiently disintegrated into individual chitin nanofibers.[33] Typically, the chitin nanofibers with extremely high specific surface area can easily 4


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form a highly porous mesh.[34] Such a porous material would be beneficial to water permeation and oil/water emulsions separation of. Furthermore, chitin nanofibers with high Young’s modulus and high fracture possess superior mechanical performance.[35] Undoubtedly, the combination of hydrophilic feature and nanostructure of chitin nanofiber would be an advantageous strategy to fabricate oil/water emulsion separation membrane with antifouling, high efficiency, and high flux properties. Until now, chitin has been rarely studied as oil/water separation materials, except for hydrophobic chitin sponges derived from modification of chitin by methyltrichlorosilane.[36] However, such chitin sponges could only absorb oil from layered oil/water mixture, but it is not suitable for separation of surfactant-stabilized oil/water emulsion. As the second most abundant biomass after cellulose, chitin has long been thrown away as seafood waste. Herein, inspired by the self-cleaning efficiency of shrimp shell, we attempt to utilize shellfish wastes as raw materials to construct novel oil/water emulsion separation membranes via a simple vacuum-assisted filtration of chitin nanofiber suspensions without any further chemical modifications. The wettability and antifouling behavior of the CNM are investigated. Their mechanical strength, stability, durability, as well as oil/water separation efficiency are studied. Additionally, their performance in the adsorption behavior of heavy metal ions is also evaluated in the course of the oil/water separation. It is anticipated that this work not only deal with oil/water emulsion separation, but also facilitate the utilization of shrimp/crab wastes, and thus develop functional materials from the sustainable marine resources.

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EXPERIMENTAL SECTION Materials. Ground shrimp shell powder was provided from Golden-Shell Pharmaceutical Co. Ltd (Zhejiang, China). Nylon filter papers were supplied from Xinya Purification Co. Ltd (Shanghai, China). 2 mol∙L-1 sodium hypochlorite solution was bought from Alfa Aesar (Beijing, China). TEMPO, sodium bromide were purchased from Aladdin (Zhejiang, China).

Fabrication of CNM. To remove impurities such as protein and minerals, the raw shrimp shell powder was sequentially treated with 5 wt% NaOH and 5 wt% HCl aq. solutions at room temperature, each for 24 h, and thoroughly washed with deionized water to neutral, which was finally dried in the oven at 60 oC for 24 h. The yield and the degree of acetylation (DA) of chitin was evaluated to be 35% and 95%, respectively, where DA was measured to by the reported method.[37] Subsequently, chitin nanofibers were produced by TEMPO-mediated oxidation method.[38] Briefly, 10 g of purified chitin powder was added into the mixture of sodium phosphate buffer (0.16 g, 10 mmol), TEMPO (0.16 g, 10 mmol) and sodium bromide (1 g, 10 mmol) and the resultant suspension was mechanically stirred at room temperature for 0.5 h. Afterwards, 172.13 g NaClO was dropwise adding into the suspension, in which the pH was kept at 10.5 by continuous addition of 0.1 M NaOH aq. solution. When the pH of slurry did not change, the reaction was stopped. Then the obtained suspension was centrifuged at 8000 gravity for 10 min. The sediment was dispersed in deionized water and then the turbid liquid was treated by ultrasonication in ice water for decomposition on an 6


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ultrasonic cell disruptor (JY98-DN, Ningbo Scientz Biotechnology Co., Ltd., China). Finally, the resultant chitin nanofibers suspension was dialyzed in the deionized water for a week and then saved it in the fridge at 4 oC. The concentration of the chitin nanofiber suspension was calculated by the mass ratio of chitin nanofiber/suspension after freeze-drying. The carboxylate content of the TEMPO-oxidized chitin nanofibers was evaluated to be 0.175 mmol∙g-1 by electrical conductively titration method.[38] The CNM was prepared according to the reported method.[39] Chitin nanofiber suspensions were carefully poured onto a vacuum filtration equipment installed with a nylon filter membrane. After filtration at 0.5 MPa, chitin nanofibrous-based membrane was obtained.

Oil/water emulsion separation. Five kinds of oil (chloroform, toluene, n-hexane, kerosene, dodecane) were used for preparing oil-in-water emulsions. For preparing nanoemulsions, the mixture including 1 mL oil, 0.1 g Tween and 0.1 g Span 80 were added into 99 mL deionized water. The resultant heterogeneous mixture was mechanical stirred for 12 h, followed by ultrasonication. For oil-in-water microemulsions, the mixture including 30 mL oil, 1.4 g Tween were blended with 70 mL deionized water, followed by mechanical stirring for 12 h at room temperature. To measure oil/water emulsion separation of CNM, it was equipped on a vacuum filter equipment. After CNM was prewetted by deionized water, the as-prepared emulsions were carefully poured onto the vacuum filter apparatus. The separation process was performed under vacuum at 0.5 MPa, the separated filtrate was collected. 7



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The fluxes J (L∙m−2 ∙h−1 ∙bar−1) for CNM was determined by calculating the volume of the filtrate after permeating for 5 min. The oil/water separation efficiency was evaluated by oil rejection R (%), which was calculated according to the eq 1: R = (1 − Cf /C0) × 100%

(1)

where Cf (mg∙mL−1) and C0 (mg∙mL−1) are the concentration of oil before and after filtration, respectively, which were evaluated by total organic carbon analyzer (TOC, Aurora 1030W, USA).

Removal of heavy metal ions. Four heavy metal ions (Cu2+, Ni2+, Cd2+, Zn2+) were used as representatives to evaluate the removal efficiency of noble metal ions. Typically, 1 mL of hexane was mixed with 99 mL of water containing 150 ppm CuSO4 and the resultant heterogeneous mixture was mechanical stirred for 12 h. Finally, as-prepared heavy metal ion contaminated nanoemulsions were poured onto the CNM, the filtrate was collected for determining the Cu2+ ions concentration by an inductively coupled plasma optical emission spectroscopy (PerKinElmer Optima 8000, USA). Oil/water nanoemulsions containing Zn2+ ions was applied as an example to evaluate the adsorption-desorption ability of the CNM, and to test the durability of the membrane. After separating the oil/water nanoemulsions containing 150 ppm Zn2+ ions, the Zn2+ ions loaded CNM was soaked in 0.1 mol∙L-1 HCl aq. solution to desorb the Zn2+ ions, then the CNM was sequentially treated with 1.0 mol∙L-1 NaCl aq. solution, 0.1 mol∙L-1 NaOH aq. solution, and deionized water. The desorption efficiency was calculated as the ratio of the amount of the desorbed Zn2+ ions to that of the adsorbed Zn2+ ions. The 8


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adsorption–desorption cycle was repeated three times by using the same CNM and the same feed (hexane-in-water nanoemulsion containing 150 ppm ZnSO4).

Characterization. The chitin structure before and after Tempo-oxidation was analyzed by FT-IR (Perkin-Elmer Co. USA. model 1600). The morphology of chitin nanofibers was observed by a transmission electron microscopy (JEM-2010, JEOL, Japan). The microstructure of CNM was characterized by scanning electron microscopy (JEOL7500LV, Japan). The effective pore size of the CNM was determined by permeationrejection testing of PEO with different mass average molar weigh (Mw).[39] The surface wettability of CNM and shrimp shells were characterized on DSA 100 (Krüss, Germany). The morphology of microemulsions before and after filtration was observed on EX20 Instec H601. The droplets sizes of nanoemulsions were measured using Zetasizer Nano (Malvern, UK). The mechanical strength of CNM was tested on a universal testing machine (CMT6350, Shenzhen SANs, China).

RESULTS AND DISCUSSION Preparation and Structure of CNM. The strategy for fabricating CNM is schematically illustrated in Fig. 1. The native chitin powder derived from shrimp shells was sufficiently disintegrated into individual nanofibers by TEMPO-mediated oxidation. The obtained chitin nanofibers displayed 200-400 nm of length and 10-20 nm of width, respectively (Fig. S1). Subsequently, chitin nanofibers were assembled 9



into a tightly packed membrane on the nylon support by vacuum filtration. The resultant membrane has a randomly oriented and extensively entangled nanofibrous network structure with nanometer-sized pores. The TEMPO-mediated oxidation treatment would inevitably slightly change the chemical structure of chitin. In the FT-IR spectrum, a weak band at 1730 cm-1 corresponding to the carboxylic group appears for both the chitin nanofibers and the CNM, ascribed to the fact that the primary C6 hydroxyl groups of chitin are partially oxidized to carboxylate groups after TEMPO-mediated oxidation, while such a signal is absent for the original chitin powder (Fig. S2). The content of carboxylate groups of oxidized chitin was determined to be 0.175 mmol ∙ g-1 by the electrical conductively titration method. It’s well known that the formation of C6 carboxylate groups is vital during the preparation of chitin nanofibers, as the electrostatic repulsion between the negatively charged chitin nanofibers help these chitin nanofibers disperse well in water during disintegration.[40] Though the slightly oxidation changes the intrinsic structure of chitin, the appearance of carboxylate groups on chitin nanofiber surfaces unexpectedly improves the hydrophilicity of chitin.

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Figure 1 Illustration of the preparation of chitin nanofibrous membranes Wettability and Antifouling of CNM. In nature, shrimp has excellent underwater antioil-fouling feature. As shown in Fig 2a, when shrimp shell was soaked into the crude oil, the crude oil easily adhered on the surface of shrimp shell but easily wiped off underwater. The unique self-cleaning behavior of shrimp shell should be closely related to its special wettability, which was measured by the test of water and oil contact angle (CA) on the surface of shrimp shell. As shown in Fig. 2c, the shrimp shell exhibited hydrophilic and oleophilic wettability in air, with water CAs of 28o (Fig. 2b) and oil (chloroform) CAs of 32o, respectively. On the contrary, it exhibited superoleophobic property with an oil (chloroform) contact angle of 158o underwater (Fig. 2d). The unique behavior of shrimp shell is closely related to their hydrophilic chemical composition, where water is easily trapped on the hydrophilic surface to generate a water layer, resulting in oil repellency. As a major component of shrimp shells, chitin may have a potential of self-cleaning, according to the existence of numerous hydrophilic hydroxyl/amino groups on chitin chain. To verify this hypothesis, we 11



immersed the CNM into crude oil and then took it out, followed by placing it into clean water (Fig. 2e). As expected, the crude oil was spontaneously washed off and CNM became clean again, indicating that CNM has superior capability of underwater antifouling, same to the shrimp shell. Furthermore, when water and oil (chloroform) were dripped on CNM surface, they both quickly and completely permeated across CNM with CA of 0o in air (Fig. 2f, g). Similar to shrimp shell, CNM exhibited hydrophilic and oleophilic wettability in air, facilitating the permeation of water and oil through the nanoporous structure of CNM (Fig. 2i). Inversely, after CNM was soaked into water, the oil droplet kept well on the surface of CNM without changing spherical shape with a static CA of 168o (Fig. 2h). Analogously, CAs of dodecane, hexane, kerosene, and toluene were all higher than 150o (Fig. S3). These results showed that the surface of CNM has underwater superoleophobicity. Moreover, the dynamic underwater oil-adhesion measurement on CNM was carried out (Fig. 2k). After compressing the chloroform droplet on the underwater CNM, the chloroform droplet deformed from a spherical to ellipsoidal shape. After lifted up the oil droplet from the surface of CNM, the oil droplet displayed no obvious deformation, suggesting low underwater oil-adhesion. All these aforementioned results demonstrate that the CNM can maintain the inherent wettability of shrimp shell, and shows excellent superhydrophilicity and underwater superoleophobicity with low underwater oil adhesion, allowing it to separate oil/water emulsions into oil and water.

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Figure 2 (a) Underwater antifouling behavior of the shrimp shell. The special wettability of the shrimp shell: (b) water droplet and (c) chloroform droplet with CA = 28o and 32o in air, respectively. (d) Underwater oil droplet standing on the shrimp shell with CA = 152o. (e) Underwater self-cleaning behavior of the CNM. (f, g, and i) Both water and oil droplets completely spread across the CNM in air, (h, j) underwater chloroform droplets stand on the CNM with CA = 168o. (k) Dynamic underwater oil (chloroform)-adhesion measurements on CNM. Additionally, most of oil/water separation materials fail to maintain their wettability in harsh environments.[41-44] Such as, organic polymer could be destroyed at high temperatures;[41] Meanwhile, ceramic membranes can be corroded by strong acidic (e.g., pH < 3).[44] However, the practical oil/water separation always occurs in complicated circumstance e.g. in strong acidic, basic solutions or high salinity water, and high temperature. In this work, the wettability of our CNMs was measured under above mentioned circumstance. As shown in Fig. S4, CNM maintained the underwater superoleophobicity with CAs of over 150o, when soaked into aqueous solutions with 13



temperature of 20 - 80 oC, as well as upon exposure to solutions with pH value of 1.0 and 13.0, and to a salty solution with NaCl conc. of 1.0 mol ∙ L⁻1. This indicates its stability against harsh environments. Such superior chemical- and thermal-resistance may be ascribed to its high crystalline structure with high binding energy,[34] showing great potential in the practical application of oil/water separation. Separation of Oil/Water Emulsions. Besides selective wettability, an ideal membrane for separation of oil/water emulsions should satisfy two essential requirements i.e. high porosity and a thin separation layer, ensuring effective rejection of oil droplets and the permeation of water.[45] Therefore, the thickness and the effective pore size should be optimized, as they are critical factors influencing the permeation flux and oil rejection. The thickness and the effective pore size of CNM can be controlled by regulating the dosage of chitin nanofibers to be filtered. Fig. 3a shows the curves of the average thickness and water flux of CNM versus usage of chitin nanofibers. Obviously, the thickness increased from 0.05 to 18.80 μm with increasing usage of chitin nanofiber from 0.36 to 24.70 g∙m-2. On the contrary, water flux sharply declined from 5843 to 85 L∙m-2∙h-1∙bar-1. To accurately evaluate effective pore size of CNMs, the permeate-reject testing of PEO with different Mw was measured. Three kinds of PEO with Mw of 100 kg∙mol-1, 300 kg∙mol-1 and 600 kg∙mol-1 have corresponding hydrodynamic diameter for 24 nm, 48 nm, and 70 nm,[39] respectively. Fig. 3b shows the relationship between the rejections of PEO with usage of chitin nanofibers. When the usage of chitin nanofiber was 0.36 g∙m-2, the rejection for 600 kg∙mol-1 PEO (d = 70 nm) was only 39.5%, but it increased to 91% after the usage of chitin nanofiber was increased to 1.10 14


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g ∙ m-2. When the usage of chitin nanofiber is further increased, PEO with smaller diameter is almost rejected. Hence, the effective pore size of CNM could be rationally tuned via regulating the usage of chitin nanofiber at nanoscale level. For example, when the usage of chitin nanofiber was less than 1.43 g∙m-2, the pore size of the membrane was larger than 70 nm. While the usage of chitin nanofiber ranged from 1.40 to 7.90 g ∙m-2, the pore size of CNMs was between 48 and 70 nm. As a consequence, such a membrane with precisely tuned ultrathin thickness and nanopore size would possibly endow it with high permeation flux and high separation efficiency.

Figure 3 (a) The relationship between the thickness and water flux of CNM with usage of chitin nanofibers. (b)The PEO with different Mw (100 kg∙mol-1, 300 kg∙mol-1, and 600 kg∙mol-1) rejections of CNMs as a function of usage of chitin nanofibers.

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Figure 4 Optical images and photographs of (a) hexane-in-water microemulsion and (b) water-in-hexane microemulsion before and after filtration. (c) photographs and size distribution of hexane-in-water nanoemulsion before and after filtration. To evaluate the efficiency separation of oil/water emulsions, the membrane with thickness of ca. 730 nm was selected as an example. As well known, surfactantsstabilized emulsions are the most troublesome. Herein, a series of micro-and nano-sized emulsions stabilized by surfactants were applied to test the performance of oil/water emulsion separation. Fig. 4a displays the photographs and optical images of surfactantstabilized (Tween 80) hexane-in-water microemulsion before and after separation. After filtrated by CNM, the numerous oil droplets with diameter of 5 - 25 μm disappeared, corresponding to the phenomenon that the milky white emulsions became 16


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transparent in the collected filtrate. For surfactant-stabilized (Tween 80) water-inhexane separation (Fig. 4b), the turbid emulsion also became clear after filtration, suggesting that the water droplets were successfully separated by the CNM. Moreover, the CNM can also deal with the most troublesome nanoemulsion. The hexane-in-water nanoemulsion with size of around 120 nm were muddy (Fig. 4c), showing typical Tyndall phenomenon. After filtration, Tyndall phenomenon disappeared in the filtrate, in which only signal at 8 nm was detected by DLS. The existence of signal at 8 nm was attributable to the surfactant micelles instead of oil droplets.[31] This result suggests that CNM can effectively separate the troublesome nanoemulsion. To quantitatively evaluate the separation efficiency of CNM, the oil rejection (R) of CNM with thickness of ca. 730 nm was measured. The R values of the CNM for nanoemulsions (dodecane-in-water, hexane-in-water, kerosene-in-water, and toluenein-water) were greater than 96.8%, except for chloroform-in-water nanoemulsions (R = 92.1%) (Fig. 5a). Furthermore, CNM displayed higher oil rejection for microemulsions, which were above 98% (Fig. S6). Hence, CNM can effectively deal with micrometer and nanometer-sized oil/water emulsions. Additionally, the water flux (J) is also an important index in the application of oil/water separation. As shown in Fig. 5a, CNM exhibited high flux with 1203±23.2, 1527±45.8, 1439±64, 1732±89, and 1563±46 L∙m-2∙h-1∙bar-1 for chloroform-in-water, dodecane-in-water, hexane-in-water, kerosenein-water, and toluene-in-water nanoemulsions, respectively. These values are higher than that (J < 300 L∙m-2 ∙h-1 ∙bar-1) of some commercial ultrafiltration membranes with similar permeation properties.[46, 47] 17



Though various superwetting materials have been reported for efficient separation of the surfactant-stabilized emulsions, most of them are only effective for separation of micrometer-scale emulsions rather than nanometer-scale emulsions, as their pore size is larger than the size of nanoemulsion.[28, 29, 48] However, our CNMs with adjustable pore size can highly efficiently separate not only microemulsion but also nanoemulsion. Moreover, the values of R (> 95%) and J (1200-1700 L∙m-2∙h-1∙bar-1) of the CNM are higher than most of polymer- or ceramic- based filter membranes which have nanometer-scale pore size (Table 1). For example, the PVA/graphene oxide membrane with an average pore diameter of 10.2 - 23.8 nm displayed an oil rejection of 93.5% and a flux of 102 - 610 L∙m-2∙h-1∙bar-1.[49] The excellent separation efficiency of CNM should be ascribed to its nanoporous architecture with high surface area and ultrathin active separation layer at the nanometer scale, which is beneficial to reject oil droplets but accelerate the permeation of water. Furthermore, it is also associated with the superhydrophilic feature of the CNM. Once the oil droplets touched CNM surface, water can be trapped in the superhydrophilic surface, leading to fast reduction of the contact area between the oil droplet and the solid surface. Along with water permeating through the CNM, these emulsions gradually aggregate and coalesce, which may accelerate the demulsification process.

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Figure 5 (a) Water flux and the separation efficiency of CNMs for separation of various oil-in-water nanoemulsions. (b) Recyclable performance of the CNM.

Table 1 Comparison of various materials for oil/water emulsion separation. 19



Filtration membranes Polysulfone/PEG

Flux (L∙m-2∙h-1 )

Pore size (nm)

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Rejection

Sources

36 - 116

120 (1 bar)

>95%

[22]

Hydrogel coated-PVDF

220

650 - 1000

>98%

[27]

Dopamine/PEI/SiO2

99%

[50]

Porous PVDF membrane

93.5%

[49]

PVA/graphene oxide

10.4 - 23.8

102 - 610 (1 - 5 bar)

Ceramic (α-Al2O3-ZrO2)

220

40 - 80 (1 -5 bar)

>90%

[15]

SWCNTs/dopamine/PEI

12

1500 - 3000 (0.5 bar)

>94%

[31]

Cellulose nanosheet

96.5%

[52]

48-70

1203 - 1734 (0.5 bar)

>95%

This work

Chitin

Durability and Stability of CNM. In general, the filtration membrane always suffer from oil-fouling issues, which seriously restricts its long-term application. Hence, the reusability and anti-oil fouling properties of CNM were measured by the cyclic filtration tests. After each cycle, the CNM was simply washed with deionized water and reused for next cycle. The values of R and J for CNM showed no obvious reduction with increasing cycle numbers even up to 30 cycles (Fig. 5b), indicating their excellent antifouling behavior and reusability. Besides, the filtration membrane should possess excellent mechanical properties, as it usually suffer from hydraulic pressure during oil/water emulsion separation, Fortunately, the tensile strength of the CNM was up to ca. 85.0 ± 8.0 MPa, indicating excellent mechanical strength. The high mechanical strength of the CNM results from the fact that these chitin nanofibers extensively entangle with each other and their hydrogen bonds substantially reform after filtration 20


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and drying. To visually feel the mechanical properties of CNM in water, a 200 g weight was put on a water-soaked CNM with the thickness of 7.6 μm. As shown in Fig S7b, the obtained CNM could well support the weight well in the water, suggesting the CNM could maintain excellent mechanical properties in water. Therefore, the CNM with excellent durability and high mechanical strength can be applied for oil/water separation under harsh conditions.

Figure 6 (a) Photographs of the filtration system and a CNM after separating hexanein-water nanoemulsion with 150 ppm CuSO4. (b) XPS spectra of fully scan and (c) N1s for CNM before and after Cu2+ ions absorption. (d) Concentration of various heavy metal ions in the filtrate before and after the oil/water separation process. (e) The adsorption-desorption cycle of Zn2+ ions by CNM.

Multifunctional Water Treatment. In some cases, oil polluted water also contains heavy metal ions. Therefore, Wiping out heavy metal ions is imperative in the course 21



of the oil/water separation. Impressively, Our CNM could simultaneously adsorb heavy metals ions from water phase. Fig. 6a shows the separation of hexane-in-water nanoemulsion containing 150 ppm CuSO4 by the CNM. The blue oil/water emulsion became clear and colorless, and the color of the CNM changed to blue after filtration. Meanwhile, the Cu2+ ions concentration in the solution decreased from 150 ppm to 19.23 ppm in the filtrate (Fig. 6c). This result suggests that Cu2+ ions were significantly intercepted and adsorbed by the CNM after one single filtration process. To clarify the adsorption mechanism, the structure of the CNM before and after filtration was characterized by XPS. Fig. 6b shows the fully scanned spectra of the CNM before and after filtration. Obviously, C, O, N, and Cu co-exist in the membrane after filtration. On the contrary, just C, O, and N appear in the original membrane. It indicates that Cu2+ ions were adsorbed on the membrane after filtration. Specially, the N1s peak of the CNM shifts from 398.10 to 398.33 eV after filtration (Fig. 6c), while there are no obvious peak shift for C1s and O1s of the CNM before and after filtration (Fig. S8). This implies that N atom of amine or acetylamino on CNM acts as anchor for Cu2+ ion, i.e. the pair of electrons in the nitrogen atom donates to the coordination bond between N and Cu2+ ion. This reduces the electron cloud density of the nitrogen atom and thus resulting in higher binding energy.[53, 54] Meanwhile, the CNM also exhibited excellent adsorption for other heavy metal ions such as Cr2+, Zn2+, and Ni2+. Particularly, the removal efficiency for Zn2+ ions is about 91% (Fig. 6c). The removal efficiency of heavy metal ions of CNM was much higher than the reported oil/water separation materials with the same properties.[55, 56] For example, the removal efficiency of Cu2+ 22


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by the waste glass-based mesh was just about 45%,[56] compared to up to 87% by the CNM. Furthermore, the adsorbed metal ions on the CNM could be easily desorbed by 0.1 mol∙L-1 HCl solution and the resultant CNM could be reused. The adsorptiondesorption cycle was repeated three times by using the same CNM and the same feed (hexane-in-water nanoemulsion containing 150 ppm ZnSO4). As shown in Fig. 6e, after three cycles of adsorption-desorption, the adsorption capacity was regained completely and the desorption efficiency of CNM was maintained at around 95%. Furthermore, the CNM after adsorption-desorption could separate oil/water emulsion as usual, as shown in Fig. S8. These R values of CNM for nanoemulsion (hexane-in-water) were all higher than 94%. These results suggest that the CNM can be repeatedly used for simultaneous adsorption of heavy metal ions during oil/water emulsion separation.

CONCLUSION. In summary, novel filter membranes composed of chitin nanofibers were successfully fabricated via a simple vacuum filtration. Similar to shrimp shell, CNM with superhydrophilicity and underwater superoleophobicity showed excellent antifouling property. Furthermore, the thickness and the pore size of the CNM could be rationally tuned by regulating the usage of chitin nanofibers. Combination of antifouling property and nanoprorous structure of CNM endowed it with the capacity to deal with various emulsions including micrometer and nanometer-sized emulsions. Furthermore, CNM displayed high separation efficiency (>95%) and water flux (>1500 L ∙ m-2 ∙ h-1) for the troublesome nanoemulsions. Meanwhile, the CNMs weaved with chitin nanofibers not only exhibited excellent mechanical strength, but also displayed 23



thermal-and chemical-stability, making it possibility in the practical oil/water separation. The CNM could remove heavy metal ions from water simultaneously during oil/water emulsion separation. Therefore, the CNM is a promising material for oily water purification.

ACKNOWLEDGEMENTS This work is supported by Fujian Provincial Health Education Joint Research Project (WKJ2016-2-29), the Social Development of Instructive Program of Fujian Province (2017Y0025), Natural Science Foundation of China (51773038, 21805036), Natural Science Foundation of Fujian Province (2017J01408), and the Scientific Research Innovation Team Construction Program of Fujian Normal University (IRTL1702). SUPPORTING INFORMATION FTIR spectra of original chitin, chitin nanofiber and CNM. CA of several different oils on the surface of CNM underwater, insets: photosgraphs of the corresponding oil droplets standing on the CNM under water. Stability of the wettability CNM in rugged circumstances. SEM images of CNM with chitin usage of 0 g∙m-2 (a, a-1), 0.43 g∙m-2 (b, b-1), 5.21 g∙m-2 (c, c-1). The separation efficiency of CNMs for (a) various waterin-oil microemulsions and (b) oil-in-water microemulsions. Mechanical properties for CNMs with thickness of 7.6 μm (a) and the photograph of CNM with thickness of 7.6 μm supporting a 200 g weight in water (b). The oil rejection of CNM during the adsorption-desorption cycle. XPS spectra of (a) O1s and (b) C1s for the CNM before and after filtration. This information is available free of charge via the internet at 24


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For Table of Contents Use Only

A novel shrimp shell-inspired filtration membrane composed of chitin nanofibers from shrimp shell showed excellent antifouling behavior. It can effectively separate surfactant-stabilized oil/water emulsions and even remove heavy metal ions contained in the emulsion.

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Shrimp Shell-inspired Antifouling Chitin Nanofibrous Membrane for

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