Green Construction of Oil-Water Separator at Room Temperature and

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Green Construction of Oil-Water Separator at Room Temperature and Its Promotion to Adsorption Membrane Ye Xiong, Lulu Xu, Kangchen Nie, Chunde Jin, Qingfeng Sun, and Xijin Xu Langmuir, Just Accepted Manuscript • Publication Date (Web): 05 Aug 2019 Downloaded from pubs.acs.org on August 5, 2019

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Green Construction of Oil-Water Separator at Room Temperature and Its Promotion to Adsorption Membrane Ye Xiong,† Lulu Xu,† Kangchen Nie,† Chunde Jin,† Qingfeng Sun,‡* Xijin Xu,§* †School

of Engineering, Zhejiang A&F University, Hangzhou, Zhejiang Province, 311300, P.R.

China §School

of Physics and Technology, University of Jinan, Shandong Province, 250022, P.R.

China KEYWORDS: oil-water separator, green construction, water treatment, adsorption membrane.

ABSTRACT: Underwater superoleophobic membranes as an effective means of resisting oil stains are often subjected to cumbersome modification procedures, limited stability and difficult expansion of assembly. To develop simple, green, stable and scalable underwater superoleophobic films, herein, cellulose-based oil-water separators with high-efficiency oil purification was constructed by using commercial carboxymethocel (CMC) as a solute and DMSO-modified ionic liquid as a solvent. Owing to the superior dissolution, regenerability, and gelation of CMC, metal mesh and gauze can be imparted with an excellent oleophobic ability

ACS Paragon Plus Environment

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through simple dipping, spraying, and coating of CMC solution. As results, these modified functionalized devices exhibit a purification capacity of more than 99.5% for various oil-water mixtures. Unexpectedly, the CMC gel coat also shields the gloves from organic solvents. Significantly, when the CMC solution is applied to an adsorption membrane, it not only endows the film excellent oil-water separation characteristics but also enhances the adsorption amount and rate of the adsorbent. Therefore, CMC base oleophobic materials can be widely developed and applied to a variety of fields that require oleophobic properties.

INTRODUCTION Oily wastewater caused by crude oil extraction, offshore oil spill, industrial wastewater, and domestic sewage has become a major challenge of water treatment.1-3 It can cause serious water and soil ecological damage, various diseases, fire accidents, bridges and ship corrosion, and more.4-5 Membrane separation with low energy consumption, high single-stage efficiency, flexible and convenient process, low environmental pollution and high universality is regarded as a promising treatment method for oily water treatment.6-8 The inherent challenge of oil-water separation is the design of interfacial surface energy, and special wetted surfaces are generally considered to be a versatile method for optimizing oil-water separation.9-12 Conventional oilwater separation membranes include superhydrophobic superlipophilic membranes and superhydrophilic superoleophobic membranes.13-15 They are capable of selectively and effectively purifying oily water. The method of allowing oil to pass through the membrane while intercepting water is called “oil-removing”.16 Some oil-removing type of materials has been successfully developed, such as hydrophobic aerogels, nanoporous polydivinylbenzene materials, polytetrafluoroethylene coating mesh, silica alkylation modified films and cross-linked


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oil-absorbing polymer gels.1, 17-20 However, due to the intrinsic lipophilicity of superlipophilic membranes, oil droplets and other impurities tend to adsorb irreversibly on the membrane surface, leading to severe membrane fouling and drastic decay of flux.21 Moreover, the contaminated film is difficult to clean, resulting in poor circulation and secondary pollution.22 Another alternative approach is called “water-removing”, where water can easily pass through the membrane while oil cannot.23 Compared with "oil-removing", the superhydrophilic superoleophobic film with opposite design concept reveals the advantages of excellent pollution resistance, low-energy, high service life, and superior efficiency.24-25 Therefore, it is considered to be a promising development trend of oil-water separation devices. For a superoleophobic surface in air, in addition to further reducing surface free energy through rigorous chemical modification, sufficiently rough micro/nanostructures and re-entrant surface curvature are also required.26-27 Recently, inspired by fish scales, an underwater superoleophobic concept is put forward and confirmed by Jiang’s group.21, 28-29 Nevertheless, those underwater superoleophobic surfaces also suffer from tedious modification procedures and limited stability in a complex oily wastewater environment. In addition, the limitations of scalable assembly prevent their further use. Therefore, it is of great significance to develop stable underwater superoleophobic separators that are simple, economical, green and scalable way. Carboxymethocel (CMC), a common commercially available material, has been widely used in petroleum, food, medicine, textile, and paper industries due to its high water retention, strong conglutination, easy film forming, etc.30 Moreover, CMC is capable of dissolving and regenerating to form a cellulose hydrogel having a crosslinked network.31 Owing to its excellent water-absorbing and water-retaining capacities, the cellulose-based hydrogel can be served as a promising candidate for designing novel water-removing materials for oil-water separation.32 In


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this work, we report a green scalable preparation of superhydrophilic and underwater superoleophobic membrane at room temperature for excellent oil-water separation, which can simply obtain via CMC solution dissolved by DMSO modified ionic liquid (IL). As a result, selfcrosslinked CMC hydrogel membrane displays a superior selectivity and efficiency (>99.5%) for various oil-water mixtures involving cyclohexane, gasoline, dichloromethane, petroleum ether, and even high viscosity silicone oil without an additional driving force. Moreover, CMC hydrogel also realizes the oil-water separation ability of the adsorption-type functional film and optimizes the adsorption performance of the membrane. On the other hand, CMC gel-coated gloves unexpectedly exhibit excellent resistance to organic solvents. EXPERIMENTAL SECTION Materials.

Bamboo-based

Carboxymethocel

(bamboo-based

CMC),

1-allyl-3-

methylimidazolium chloride (AMIMCl), Dimethyl sulfoxide (DMSO), Nitrate Pentahydrate (Bi(NO3)3·5 H2O), Sodium Iodide (NaI), Ethylene Glycol (EG), Ethanol (EtOH), Hydrochloric Acid (HCl), and Sodium Hydroxide (NaOH) were analytical grade and purchased from Aldrich. Deionized water (DIW) was used throughout. Preparation of CMC Functioned Membranes. First, 500 mg of bamboo-based CMC was added to 99.95 g of AMIMCl solution containing 25 wt% DMSO and reacted at room temperature for 1 h to obtain a transparent 0.5 wt% CMC solution. Prior to gelation, the metal mesh and gauze were washed several times with 1 M HCl solution, ethanol and deionized water in sequence. Then, the CMC solution was infiltrated onto the cleaned metal mesh and gauze by simple dipping, spraying, and coating. Finally, the films impregnated with the CMC solution


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were transferred to a water/ethanol coagulation bath with a volume ratio of 3:1 for 30 s to obtain a CMC gelled metal mesh, gauze, and filter paper. Preparation of CMC Modified Adsorption-Type Functional Film. The δ-Bi2O3 film (BF) as previously reported was selected as the adsorption-type functional film.33 Typically, 18 mg newly synthesized δ-Bi2O3 was uniformly dispersed to 4 mL of the above-prepared cellulose solution and strongly stirred for 60 min. Next, a pure δ-Bi2O3 film could be obtained by simple vacuum filtration and drying. Finally, the obtained δ-Bi2O3 film was immersed into the previously configured CMC solution and aged in water for 30 seconds to obtain the desired CMC modified adsorption-type functional film (CMC-BF). Preparation of oil-water mixture. Typically, mixing an equal volume of DIW (10 g) with a specific oil to synthesize the desired oil-water mixture wherein oil samples were chosen from cyclohexane, gasoline, dichloromethane, petroleum ether, and silicone oil. Further, to mark the oil, 100 mg of Sudan III was added to the above mixture and sonicated for 10 minutes. As for oil-doped iodide ion mixture, except that the pure water was replaced with 0.5 ~ 4 mmol L-1 NaI aqueous solutions, the rest remained unchanged. Treatment of oil-water mixtures. Simply, the prepared CMC hydrogel coated mesh or gauze was firstly placed in the detachable filter device having a capacity of 20 mL. Then, 15 mL of oilwater mixing was added to the upper part of the separator and separated under the action of gravity. The separation efficiency was calculated by the water obtained in the lower portion of the filter, while the water flux was determined by the flux time for water. Disposal of oil-doped iodide solution. A piece of fabricated CMC-BF was used as sorption beds to purify and separate 10 ml of the above fabricated oil-doped iodide ion mixture.


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Correspondingly, the blank comparison test was performed with pure BF. The adsorption kinetics curve of the adsorbed membrane was performed at a liquid flow rate of 1 mL min-1. The filtrate was also collected to ascertain the concentration of iodine anions. The saturated sorption capacity (qe, mmol g-1) of the adsorbent membrane was determined by Eq. 1:

qe 

(C 0  Ce )  V m

(1)

Where: C0 (mg L-1) was the initial concentration of iodine ions, Ce (mg L-1) was the equilibrium concentration, V (L) was the volume of the testing solution, and m (g) was the weight of the sorbents. The relative removal rate (R%) of iodide ion was calculated by Eq. 2.

R% 

qt 

qt 100% qe

(2)

C0  Ct V m

(3)

Where: qt (mmol g-1) and Ct (mg L-1) were the sorption capacity and the iodine ions concentration at the end of the reaction. Selective Adsorption Experiments. The selective uptake of I- anions by the fabricated CMC-BF adsorbent was investigated in the presence of high concentrations of Cl-, NO3-, SO42-, and CO32anions. Briefly, a piece of CMC-BF was used as an adsorption bed to purify 10 ml of a competing ionic solution containing different molar ratios of Mx-/I- (Mx- = Cl-, NO3-, SO42-, and CO32- anions, the concentration of I-was set as 1.0 mmol L-1).


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Characterizations. Contact angle measurements for underwater oil were examined via an Optical Contact Angle Meter (Dataphysics-OCA20, Germany). FTIR was measured on a NICOLET 5700 FT-IR Spectrometer. SEM images were taken with a JEOL JSM-6700F scanning electron microscope. LSCM pictures were conducted on Laser Scanning Confocal Microscopy (LSM 780 DUO, Carl Zeiss Micro Imaging GmbH).

Figure 1. Diagrammatic drawing of the manufacture of CMC hydrogel functional meshes for oil-water separation.

RESULTS AND DISCUSSION Preparation and characterization. The construction process and the oil-water separation mechanism of the CMC hydrogelled functional membrane were described in Figure 1. Surface CMC hydrogel functionalized film was obtained by aging the initial device of CMC solution


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infiltration in a water/alcohol coagulation bath. Firstly, the addition of appropriate non-polar DMSO enabled the enhancement of the dissolution of biomass material into AMIMCl solution, which was allowed for room temperature preparation of the CMC solution.34-35 On the other hand, attributed to the excellent adhesion of DMSO dissolved CMC solution as well as the lower viscosity, CMC-impregnated functional film was able to be realized by simple dipping, spraying or coating. As compared in Figure S1, the stirring rate of the CMC solution prepared from the DMSO-modified ionic liquid reached 240 rpm, however, that of the jelly-like CMC solution fabricated by the pure ionic liquid solution at the same power was only 110 rpm. Additionally, owing to the superior solubility regeneration property, the coated CMC solution was able to be completely gelatinized within several seconds.36 By means of the abundant carboxyl, hydroxyl, and ether group functional groups of CMC, a hydration layer would be formed on the CMC hydrogel surface when applied to oil-water separation, thus resulting in a high-efficient and selective separation of the oil-water hybrid.37 Significantly, due to the biocompatibility and degradability of CMC and the high safety of AMIMCl, the post-treatment of the product would not pose a threat to the natural environment.38 Compared to other materials that typically took longer or other conditions such as UV irradiation or chemical induction, the strategy proposed in this paper that greatly simplified the gelation construction was more convenient, economical, and green.39


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Figure 2. (a-d) SEM images and (e-h) LSCM images of the pure metal and CMC hydrogel coated metal mesh.

Transformation of various devices before and after gelation was recorded by SEM. As shown in Figure 2a, the original metal mesh accompanied by a great number of pores could be clearly observed. As expected, a CMC hydrogel coating was clearly exhibited on the mesh after simple impregnation, spraying, and coating of the CMC solution and corresponding gelation process (Figure 2b-d). This result indicated that CMC hydrogel had been successfully anchored on the surface of the metal mesh and the anchoring methods were diverse. Figure 2e-h exhibited the LSCM images of the metal mesh before and after CMC hydrogel deposited. Obviously, CMC hydrogel was covered on the metal network after gelation treatment, which further proved the diversity of the hydrogel layer construction process. Correspondingly, a similar phenomenon was found in gauze impregnated with CMC solution. As revealed in Figure S2, it could be observed that the opening of the gauze was also occupied by the generated CMC hydrogel coating. To


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further verify the creation of CMC hydrogel on those samples, FTIR was introduced. As displayed in Figure S3, the strong absorption bands occurring at 1600 cm-1 and 1420 cm-1 were attributed to CH2COO groups of the CMC gel coating.40 Accordingly, the absorption peak emerging at 3482 cm-1 was assigned to the O-H stretching vibration of CMC hydrogel or the adsorbed water molecules.41 Moreover, the presence of C-O-C stretching vibration of CMCcoated metal mesh at 1113 cm-1 further demonstrated the generation of CMC maskant.40 Combined with the above discussion, the CMC hydrogel coating was effectively self-crosslinked on the metal mesh and gauze.

Figure 3. (a-b) Oil-repellent/washing-rinsing examine of silicone oil on CMC coated metal mesh and gauze. (c-f) Underwater penetration test of dichloromethane: (c and e) CMC hydrogel coated metal mesh and gauze; (d and f)


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the original metal mesh and gauze. (g-h) Underwater dichloromethane droplets at the interface with CMC decorated metal mesh and gauze.

Oil repellency of CMC hydrogel modified metal mesh and gauze. Sudan III dyed high viscosity and low-density silicone oil (density was relative to water) was employed to investigate the oil repellence behavior of CMC modified metal mesh and gauze. As revealed in Figure 3a-b, the CMC hydrogel coated iron mesh and gauze stained with red silicone oil were quickly cleaned after being placed in deionized water. Those results indicated that the prepared CMC hydrogel coated web had excellent oil repellency and easy cleanability, which was evidence of its low oil adhesion. These features might be stemmed from the superior water absorption and water retention capabilities of cellulose-based hydrogels, which was deemed to be promising candidates for designing eco-friendly oil-water separation devices.42 When the CMC geldecorated mesh contacted with water, a hydration layer would form on its surface, resulting in reduced or even isolated contact of the oil droplets with the mesh. Moreover, these attributes could endow the raw material oil-water separation and long-term use and/or recyclability. However, as basic equipment of the laboratory, latex glove was easily corroded by corrosive solvents such as dichloromethane, toluene, acetone, etc. As evidenced in Figure S4 and Movie S1, the lower portion of the glove modified by the CMC hydrogel was intact when operating in a dichloromethane solvent (Sudan III dyed), while the upper portion unprotected by the CMC gel was corroded. This simple and environmentally friendly construction strategy made it possible to use latex gloves in complex environments. In addition, relatively low viscosity and high-density dichloromethane were used to further verify the superior underwater oil resistance of the CMC hydrogel functionalized films. As represented in Figure 3c and 3e, the red droplets quickly penetrated the grid when the Sudan III red-wat dichloromethane was shot through a syringe to


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the pristine metal mesh and gauze. Nevertheless, the red dichloromethane was completely repelled by the CMC hydrogel-coated metal mesh and gauze and slipped away from its surface (Figure 3d and 3f). At the same time, similar results were found when using low viscosity and low-density cyclohexane test (Figure S5). All of the outcomes declared that the CMC gel coating was beneficial for the material to stay away from oil contamination.

Figure 4. (a) OCAs of the CMC hydrogel decorated metal mesh. (b) Oil-water separation device and the physical pictures of the separation for cyclohexane/water mixture. (c) Cycling test of CMC coated metal mesh for oil-water separation. (d) Acid and alkali resistance detection of CMC coating.

Correspondingly, underwater oil droplets (dichloromethane and cyclohexane) at the interface with CMC decorated metal mesh and gauze were clearly exhibited in Figure 3g-h and Figure S6. All of the pictures revealed high underwater oil contact angles (OCA), suggesting its superior oil resistance. In fact, the static OCA values reported for cyclohexane, gasoline, dichloromethane,


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petroleum ether, and silicone oil in Figure 4a (metal mesh) and Figure S7 (gauze) were above 153°, both of them met the superoleophobic standard. Even more surprisingly, the OCA of dichloromethane on CMC coated gauze was as high as 166.4°, achieving the effect of nonsticking of the oil (Movie S2). Figure 4b displayed the progress of the CMC-modified metal mesh to separate the cyclohexane/water mixture (an appropriate amount of Cu ions was added to the water to highlight the water) under gravity drive. Obviously, water was immediately separated from the hybrid and penetrated the membrane, while cyclohexane was intercepted by the metal film (Movie S3). Moreover, none oil drops were detected in the separated lower filter liquor, demonstrating the efficient separation for the oil-water mixed composite. Recyclability was one of the important indicators for detecting the economic and green color of separation membranes. In Figure 4c, the water flux maintained at 2400 L m-2 h-1 and the separation efficiency stayed above 99.5% in the eight cycles, which implied the great reusability and antifouling ability of the constructed CMC hydrogel coating. Here, oil-water separation efficiency was calculated by the mass percentages of the purified aqueous solution, and the membrane flux was obtained based on the amount of fluid passing through the unit membrane area per unit time. In addition, the stability of the hydrogel also determined its availability in complex oily wastewater environments such as strong acidic or alkaline conditions. As revealed in Figure 4d, the OCAs of the CMC modified metal mesh and gauze was examined by the newly prepared NaOH and HCl solution. Luckily, both of the OCAs in acidic or alkaline environments were as high as 154 ± 2°, confirming the great stability of CMC coating and was capable of challenging the complex environments.


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Figure 5. (a) Protective verification test of CMC hydrogel coating on BF adsorption performance. (b) Adsorption equilibrium and (c) adsorption kinetics experiments of CMC-BF and pure BF.

Versatility of CMC hydrogel improved water purifier. A δ-Bi2O3 film (BF) applied for capturing radioactive iodine anions was selected as the blank adsorption membrane. Correspondingly, the CMC hydrogel modified δ-Bi2O3 film (CMC-BF) was obtained by simply immersing and aging the CMC solution. A mixture of cyclohexane and iodine ionic solution was prepared for the subsequent adsorption contrast test, and a BF membrane partially coated with CMC gel (the upper) was used to verify the promotion of BF adsorption membrane by CMC protective layer. As shown in Figure 5a and Movie S4, when the composite membrane was


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immersed in Sudan III-dyed cyclohexane, the unprotected lower portion was found to be contaminated with cyclohexane, while the upper part was not eroded. This consequence announced that CMC hydrogel coating might be advantageous for adsorption membrane disposing of the wastewater containing some oil contaminations. Further, when this hybrid membrane was transferred to the iodide ion solution, it was observed that the upper portion quickly became yellow, and the polluted lower portion was not significantly changed. This evidenced that CMC-BF achieved efficient capture of iodide ions without oil contamination. Adsorption equilibrium and adsorption kinetics experiments were also performed to demonstrate the promotion of CMC hydrogels on the treatment of oily sewage by adsorption materials. As exhibited in Figure 5b, the maximum adsorption of CMC-BF (1.59 mmol g-1) was slightly higher than that of the pristine BF (1.47 mmol g-1). At the same time, the adsorption rate for CMC-BF was quicker than the normal BF. As demonstrated in Movie S5, when CMC-BF and pure BF were simultaneously applied to a 4 mmol L-1 iodine solution, CMC-BF turned yellow faster (when the iodide ion was captured by δ-Bi2O3, it would produce bright yellow Bi4O5I2).43 As illustrated in Figure 5c, CMC-BF was capable of achieving 100% of iodine anion removal when I- concentration was ≤ 2.5 mmol L-1, and approximately 82% of I- was treated when iodine anions concentration was 4.0 mmol L-1. However, pure BF just realized a 98% purification when iodine concentration was set to 1.5 mmol L-1, and even only 58.2% disposal rate was reached when the iodine anions concentration was as high as 3.5 mmol L-1. The selective adsorption of I- anions by CMC-BF was shown in Figure S8. Apparently, the presence of, Cl-, SO42-, and CO32- had a great influence on I- uptake, whereas NO3- did not compete for Ianion adsorption. And, the bigger of Mx-/I- molar ratios, the lower adsorption of I- adsorption. In addition, the competitiveness followed the order CO32- > SO42- > Cl-, which was closely related


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to the Guinness free energy for the reaction to I- anion.43 The Gibbs energy for the reaction between δ-Bi2O3 and I- anions was lower than that for the reaction between δ-Bi2O3 and Cl-, but higher than that for the reaction between δ-Bi2O3 and CO32−. Thus, the CMC-BF adsorbent displayed higher selectivity adsorption of I- ions under competitive Cl- than CO32−. Based on the above results, it could conclude that the construction of CMC hydrogel coating helped to achieve the efficient application of BF in oily wastewater.

Figure 6. Acceleration mechanism of the capture of iodide ion by CMC hydrogel coated BF adsorption membrane.

Correspondingly, the acceleration mechanism of CMC hydrogel coated BF adsorption membrane capturing iodine ions was described in Figure 6. On the one hand, CMC self-crossing network was rich in carboxyl, hydroxyl, and ether-based oxygen-containing groups, which could attract halogen ions under hydrogen bonding interaction, thereby inducing a rapid mass accumulation of iodide ions to the surface or inside of the membrane.44-45 Therefore, the adsorption speed of CMC-BF on iodide ions was accelerated. On the other hand, part of the


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iodide ions was able to be fixed inside the surface gel of CMC-BF under hydrogen bonding, thus increasing the adsorption capacity of BF.46-47 In addition, in view of the carboxyl, hydroxyl, and ether group have the ability to adsorb and aggregate radioactive or heavy metal cations via electrostatic adsorption, the fabricated CMC solution was also suitable for the improvement of the cationic adsorption film.48-49 CONCLUSIONS In summary, the green and large scale preparation of carboxymethocel solution at room temperature was achieved by DMSO-modified ionic liquids. Owing to the outstanding biocompatibility of CMC solution, different multifunctional materials having an oleophobic function could be constructed by simple dipping, spraying, and coating of CMC solution. For example, CMC-modified wire mesh and gauze had an oil-water separation ability of up to 99.5%, CMC-decorated gloves possessed excellent organic solvent resistance and the like. More importantly, attributing the strong hydrogen bond interaction, it not only gifted the adsorptive membrane superior oil removal capabilities but also optimized its inherent adsorption ability. In addition, due to the inherent corrosion resistance of CMC to strong acids and bases solutions, all of these CMC hydrogel-based materials possessed excellent environmental stability. In short, this work provided a simple, economical, effective and environmentally friendly universal approach to the various materials that required oil repellent treatment.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxxxxx/acs.langmuir.xxxxx. Additional information as noted in the text including seven figures (PDF). AUTHOR INFORMATION Corresponding Author *[email protected] and [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The work is partially supported the Zhejiang Provincial Natural Science Foundation for Distinguished Young Scholars of China (Grant No. LR19C160001), the National Natural Science Foundation of China (Grant No. 51672109), and the Zhejiang Provincial Collaborative Innovation Center for Bamboo Resources and High-efficiency Utilization (Grant No. 2017ZZY2-07).


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