Palygorskite Nanorod

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Robust tunicate cellulose nanocrystal/palygorskite nanorod membranes for multifunctional oil/water emulsion separation Hui Zhan, Tao Zuo, Rongjun Tao, and Chunyu Chang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02137 • Publication Date (Web): 13 Jun 2018 Downloaded from http://pubs.acs.org on June 16, 2018

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ACS Sustainable Chemistry & Engineering

Robust tunicate cellulose nanocrystal/palygorskite nanorod membranes for multifunctional oil/water emulsion separation Hui Zhan,† Tao Zuo,‡ Rongjun Tao,† and Chunyu Chang*, †, §, ǁ

†

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China ‡

Yangtze Valley Water Environment Monitoring Center, Wuhan, 430010, China

§

Suzhou Institute of Wuhan University, Wuhan University, Suzhou, 215123, China

ǁ

State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, 510640, China

Email address: [email protected] (C. Chang) ORCID: 0000-0002-3531-5964 (C. Chang)

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Nanoporous membranes with superhydrophilic and underwater superoleophobic surfaces have shown excellent performance in oil/water emulsion separation due to their low oil adhesion and good oil/water selectivity. However, fabrication of membranes from low cost raw materials through green and sustainable routes for multifunctional separation of oily water still remains a challenge. Herein, we reported a facile strategy to prepare robust membranes from renewable tunicate cellulose nanocrystal (TCNC) and low-cost palygorskite (PGS) for multifunctional oil/water emulsion separation. The TCNC/PGS membranes possess nanoporous structure with tunable thickness, and superhydrophilic and underwater superoleophobic surface, which could effectively separate micro/nanoemulsions with high water flux and oil rejection. Moreover, the resulting membranes exhibited high mechanical strength, excellent recyclability, and good stability under harsh conditions. More importantly, TCNC/PGS membranes could remove water soluble contaminants (dye or heavy ions) during the oil/water separation process, leading to multifunctional water purification. Our works provided a fast and economical strategy for the fabrication of robust membranes from inexpensive and renewable nanomaterials, which would be suitable for demanding multifunctional oil/water separation.

Keywords: Membrane, multifunctional, oil/water emulsion separation, tunicate cellulose nanocrystal, palygorskite


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INTRODUCTION Owing to the rapid development of economy, pollution caused by frequent oil spill accidents and oily effluent discharges from industrial process and daily life have seriously threatened the safety of environment and human health.1-2 Traditional oil/water separation techniques such as skimming, air floatation, bioremediation, ultrasonic separation, centrifuging, and adsorption, are useful to treat the layered oil/water mixture, but not effective for surfactant-stabilized oil/water emulsions because of the small size of oil droplets.3 Thus, research and development of novel materials for efficient separation of emulsified oil/water emulsions, especially nanoemulsions (typical droplet size: 20~200 nm), has become an urgent task in both academic and industrial fields.4 Benefitting from the principle of size exclusion, nanoporous membrane was considered as the most promising candidate to cope with the challenge of complete separation of oil nanoemulsion.5 Recently, various materials with superwettablity have been developed for the selective oil/water separation due to their opposite affinity towards water and oil.6 Superhydrophobic and superoleophilic membranes were firstly reported in the selective oil/water separation through “oil-removing” process.7 Unfortunately, the membranes were easily fouled by oils while the continuous oil permeation was easily blocked during filtration process.8-10 Therefore, the development of membranes with superhydrophilic and underwater superoleophobic surface properties have received intensive attention.11-13 During the “water-removing” filtration process, water molecules could continuously permeate through membrane and form a thin water



layer which is repulsive to the wetting by oil droplets and prevents the oil fouling.14 However, fabrication of membranes from low cost raw materials through green and sustainable routes for multifunctional separation of oily water still remains a challenge.

15-20

On the other hand, all the membranes developed so far could only

separate emulsions into oil and water phase but did not possess the function of removing contaminants from water phase during the separation process. Therefore, it is critical need to explore a low cost, environmentally friendly, and sustainable route for the fabrication of multifunctional membranes, which can not only efficiently separate oil/water emulsions but also remove contaminants simultaneously. Palygorskite (PGS) is an inexpensive and chemical inert clay mineral with ribbon-layer microstructure and a rod-like crystal morphology, which could be used as adsorbents for removal of dyes and heavy ions from solution due to its porous structure, adsorbed cations, and large specific surface area.21-23 However, PGS existed in the form of aggregates because the strong hydrogen-bonding and Van der Waals’ interaction among nanorods.24 Tunicate cellulose nanocrystals (TCNCs) which were isolated from the mantles of sessile sea creatures by sulfuric acid hydrolysis well dispersed in water due to the presence of negative charges on their surfaces.25 Therefore, the dispersity of PGS could be significantly improved by adding TCNCs into the suspension. Herein, we demonstrated superhydrophilic and underwater superoleophobic TCNC/PGS membranes which were fabricated by a simple vacuum assisted filtration of TCNC/PGS suspension on a supporting membrane. The water flux and wettability of membrane could be simultaneous tailored by the weight ratio


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of TCNC and PGS, while the effective pore size and thickness of membrane were well controlled by the dosage of suspension. The as-prepared TCNC/PGS membrane showed

excellent

performance

in

the

selective

separation

of

various

surfactant-stabilized oil/water emulsions with high water flux and oil rejection. Moreover, they exhibited good reusability, high mechanical strength, and excellent stability under high acidic, salty, and alkaline environments. Importantly, TCNC/PGS membrane could also efficiently remove dyes and heavy ions during the separation of oil/water emulsions.

EXPERIMENTAL Materials Palygorskite (PGS, 99.8%) was purchased from Jiuchuan Technology Co. (Jiangsu China). Tunicate (Halocynthia roretzi Drasche) was purchased from Weihai Evergreen Marine science and technology Co. Ltd (Shandong, China) and tunicate cellulose nanocrystals (TCNCs) were isolated by sulfuric acid hydrolysis according to our previous method.25 Cellulose ester membrane (cut-oﬀ: 0.22 µm; diameter: 50 mm) was obtained from Xinya purification Co. Ltd (Shanghai, China). Poly(ethylene oxide) (PEO, 300 kDa/ 600 kDa) were purchased from Sigma Aldrich, and methylene blue, sudan III, chromium chloride hexahydrate (CrCl3·6H2O), manganese chloride tetrahydrate (MnCl2·4H2O), iron chloride anhydrous (FeCl3), nickel chloride hexahydrate (NiCl2·6H2O), copper sulfate anhydrous (CuSO4) were purchased from Sinopharm Chemical Reagent Co., Ltd., respectively. The other reagents are all



analytically pure and used without purification. Fabrication of TCNC/PGS membranes TCNC/PGS membranes were fabricated by vacuum-assisted filtration of TCNC/PGS suspensions onto a cellulose ester membrane. Typically, a certain amount of PGS powders were added to TCNC suspension (250 mL), and then the mixed suspension was ultra-sonicated for 1 h at 25 °C. After filtration under 0.5 bar pressure, the suspensions (1 mL) were converted into composite membranes after dried in air with TCNC/PGS weight ratios of 0, 0.5, 1.0, and 2.0. To control the thickness of TCNC/PGS membranes, the dosages of TCNC/PGS suspensions (TCNC/PGS weight ratio: 1.0) used for the fabrication of membranes were varied from 0.17 (1 mL) to 21.76 g m-2 (128 mL). Characterization Scanning electron micrograph (SEM) measurements were carried out on a HITACHI 5-4800 microscope (Hitachi, Japan) at an accelerating voltage of 5 kV. The wet membranes were frozen in liquid nitrogen, fractured immediately, and then freeze dried. The surface and cross-section of samples were sputtered with gold, and then observed and photographed. Atomic force microscopy (AFM) was performed on an Asylum Research Cypher system (Oxford, UK) to characterize the thickness of membranes. The concentration and hydrodynamic radius of PEO were determined by the combination system of size exclusion chromatography (SEC), multi-angle static light scattering (DAWN HELEOS-II), refractometer (Optilab T-rEX), and viscometer (ViscoStar II) (Wyatt Technology Co., US). The effective pore sizes of membranes


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were determined by the hydrodynamic diameter of PEO when more than 90% of standard polymers were rejected by membrane. The contact angle was measured by DSA100 (Krüss, Germany). The size oil droplets in microemulsions and nanoemulsions were measured by EX20 optical microscopy (Sunny, China) and dynamic light scattering (Malvern, UK), respectively. The oil concentration of the filtrates was analyzed by an infrared spectrometer oil content analyzer (Beijing ChinaInvent Instrument Tech. Co., Ltd., China). Mechanical properties of the membranes with width of 2 mm and thickness of 0.02 mm were tested by using an Electromechanical Universal Testing Machine CMT6503 (Xinsansi, China). The measurements were conducted at 25 °C with an elongation rate of 2 mm·min-1. Oil/water emulsion separation Four kinds of oil (soybean oil, pump oil, hexane, and isooctane) were used for the fabrication of emulsions, including microemulsions (the diameter of oil droplet: 1~100 µm) and nanoemulsions (the diameter of oil droplet: 90% (Fig. 2b). The dosages of TCNC/PGS located in the pore size of 42–60 nm were 1.36 g m-2 and 2.72 g m-2. To get higher water flux, TCNC/PGS membrane with a dosages of 1.36 g m-2 was used to separate


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nanoemulsions. Under the same rule, TCNC/PGS suspension with a dosage of 0.34 g m-2 was applied to construct membrane for the separation of microemulsion, because 0.17 g m-2 TCNC/PGS could not completely cover the supporting membrane. The influence of TCNC/PGS dosages on the pore size of membrane was also confirmed by the SEM images (Fig. S4). As the dosage of TCNC/PGS increased, the pore size of membrane sharply decreased. When the TCNC/PGS dosage was as low as 0.08 g m-2, the morphology of cellulose ester membrane could be clearly observed, indicating that TCNC/PGS did not completely cover the supporting membrane. When the TCNC/PGS dosage reached 0.34 g m-2, the morphology of supporting membrane disappeared and TCNC/PGS membrane showed porous architecture. With the increment of TCNC/PGS dosages, the pore size of membranes gradually decreased to achieve nanoscale.

Fig. 3. Photographs of a water droplet on the TCNC/PGS membrane in air (a),



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various oil droplets on TCNC/PGS membrane underwater (b, hexane; c, isooctane; d, pump oil; and e, soybean oil), and a soybean oil droplet on pure PGS membrane underwater (f). Dynamic approach-compress-detach soybean oil adhesion test of TCNC/PGS membrane (g).

Wettability of TCNC/PGS membranes It is well known that the surface wetting behavior of membrane is determined by the chemical composition and architecture of their surface. Owing to the intrinsic hydrophilicities and nanoscale dimensions of TCNCs and PGS (Fig. 1a, b), the as-prepared membrane exhibited desired superwetting properties, which were evaluated by the apparent water contact angles in air and apparent oil contact angles underwater. As shown in Fig. 3a, the water contact angle on the TCNC/PGS membrane in air was about 0°, because the surface of membrane processed a large amount

of

hydroxyl

groups

and

high

surface

energy,

indicating

the

superhydrophilicity of membrane that would be beneficial to the permeation of water though membrane during the separation process. On the other hand, TCNC/PGS membrane could not be wetted by various oil underwater with oil contact angles of 156.1° for hexane (Fig. 3b), 161.7° for isooctane (Fig. 3c), 164.4° for pump oil (Fig. 3d), and 164.7° for soybean oil (Fig. 3e), respectively, confirming its underwater superoleophobicity. In contrast, pure PGS membrane exhibited a lower underwater oil contact angle of 139.4° (Fig. 3f), revealing that addition of TCNCs in the membranes could improve their underwater superoleophobicity (Fig. S1e). We should emphasize


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that the supporting membrane was easily fouled by oil with low underwater oil contact angles from 98.2° to 124.2° (Fig. S5), which could not be used for separation of oil/water emulsions. Moreover, the underwater oil adhesion of TCNC/PGS membrane was measured by an approach-compress-detach experiment (Fig. 3g). A soybean droplet approached and contacted with TCNC/PGS membrane, and then was compressed to become elliptical in shape. As the oil droplet was gradually removed from the membrane, it could easily overcome the adhesion force to detach without obvious deformation, indicating that very low oil-adhesiveness of TCNC/PGS membrane. Furthermore, our TCNC/PGS membrane also consistently showed low oil adhesiveness underwater for other oil droplets (Fig. S6). For high oil adhesion materials, the deformation of oil droplet usually appeared because the adhesion force between the oil droplet and membrane surface generated vertical tensile stress when oil droplet was detached.15 Therefore, the underwater low oil adhesion behavior of TCNC/PGS membrane was attributed to the presence of abundant hydroxyl groups and hierarchical nanostructure on the surface of membrane. These results confirmed that TCNC/PGS membranes were superhydrophilic and underwater superoleophobic with extremely low underwater oil adhesion, which would be promising materials for the separation of oily water. Separation of oil/water emulsions Considering that the TCNC/PGS membrane featured the superhydrophilicity, and underwater superoleophobicity, as well as tunable nanometer-sized pores, we prepared



various surfactant-stabilized oil/water emulsions to evaluate its separation performance. Fig. 4a shows the photographs and microscope images of soybean oil/water microemulsion before and after filtration. The as-prepared microemulsion was milky white due to the presence of oil droplets with diameters of 5-20 µm. Once the emulsion was filtrated through TCNC/PGS membrane, the collected filtrate became transparent and no droplet could be observed by optical microscope, revealing that the oily water was effectively separated. For hexane/water nanoemulsion, it appeared semitransparent with a droplet size of 50-200 nm, showing an apparent Tyndall phenomenon (Fig. 4b). After filtration, a transparent liquid with a droplet size of ~8 nm was obtained and the Tyndall phenomenon disappeared in the filtrate. This result could be explained by the fact that hexane droplets was essentially rejected as the nanoemulsion contacted the underwater superoleophobic membrane, but a small amount of surfactant permeated through the membrane during the filtration process. Furthermore, our TCNC/PGS membrane was also effective to separate other surfactant-stabilized oil/water emulsions prepared by other kinds of oil. (Fig. S7)


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Fig. 4. Oil/water emulsions separated by TCNC/PGS membrane: (a) Photographs and microscope images of soybean oil/water microemulsion before and after filtration, (b) Photographs and size distribution of hexane/water nanoemulsion before and after filtration.

The water flux (J) and oil rejection (R) of TCNC/PGS membrane for various microemulsions are shown in Fig. 5. For oil/water microemulsions, the J values of membranes were 140.8 ± 7.2 L m-2 h-1 bar-1, 156.6 ± 6.8 L m-2 h-1 bar-1, 335.9 ± 6.8 L m-2 h-1 bar-1, and 287.1 ± 15.3 L m-2 h-1 bar-1 for soybean oil, pump oil, hexane, and isooctane, respectively (Fig. 5a). The variation in J values among the four emulsions could be attributed to the different viscosities of the emulsions. In the separation of nanoemulsions, the J values of membranes were 1765.9 ± 61.5 L m-2 h-1 bar-1, 1633.5 ± 87.0 L m-2 h-1 bar-1, 2264.4 ± 190.0 L m-2 h-1 bar-1, and 2050.9 ± 115.2 L m-2 h-1 bar-1, for soybean oil, pump oil, hexane, and isooctane, respectively (Fig. 5b). This result indicated that the resulting membranes exhibited higher J values for nanoemulsions



than those for microemulsions. On the other hand, the oil rejection (R) of TCNC/PGS membrane for soybean oil, pump oil, hexane, and isooctane were 99.87 ± 0.02%, 99.2 ± 0.06%, 99.86 ± 0.01%, and 99.89 ± 0.01%, respectively, indicating that the membrane almost separate these microemulsions completely (Fig. 5a). Moreover, high oil rejection of TCNC/PGS membrane for nanoemulsions were also obtained (＞ 99%), as shown in Fig. 5b. These results demonstrated that TCNC/PGS membrane could effectively separate oil/water emulsions with a wide size range of oil droplets, due to their controllable pore size and superwettability. In comparison with oil/water microemulsions, the TCNC/PGS membrane showed higher separation fluxes for nanoemulsions, because oil droplets in microemulsions easily form oil layer on the surface of membrane, which leads to the lower J values of the composite membranes.

Fig. 5. Water flux and oil rejection of TCNC/PGS membranes in separation of various microemulsions (a) and nanoemulsions (b).

Durability and stability of TCNC/PGS membranes The durability and stability of filtration membrane are important for practical


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oil/water separation, especially in harsh environments, such as mechanical scratch, highly acidic, alkaline, and salty conditions.28-29 Fig. 6a shows the water flux (J) and oil rejection (R) of TCNC/PGS membrane in 10 cycles. After 10 cycles, the J and R values of the membrane maintained at 1651.1 ± 148.1 L m-2 h-1 bar-1 and 99.60 ± 0.46%, respectively, demonstrating good recyclability of TCNC/PGS membrane. Additionally, in oil/water separation, membrane usually undergo large hydrostatic pressure or even violent mechanical vibrations from pump, so mechanical strength of TCNC/PGS membrane is also vital in practical application. As given in Fig. 6b, the tensile strength of membrane was as high as 153.5 MPa due to the high modulus of TCNCs, indicating its excellent mechanical property. The underwater oil (soybean oil) contact angles of TCNC/PGS membrane were 156.4°, 154.2°, and 146.6°, in HCl (1M), NaCl (1M), and NaOH (1M) aqueous solution, respectively (Fig. 6c), suggesting that the TCNC/PGS membrane could tolerate acidic, salty, and alkaline environments and still maintain their underwater superoleophobicity. Because the harsh environment hardly changed wettability of membrane, TCNC/PGS membrane wound effectively separate the oil/water emulsions. Therefore, we demonstrated that TCNC/PGS membrane with excellent reusability and durability could be used for long-term in oil/water separation.



Fig. 6. Cycling performance (a) and stress-strain curve (b) of TCNC membranes, and (c) the contact angles of soybean oil on the TCNC/PGS membranes under HCl (1 M), NaCl (1 M), and NaOH (1 M) aqueous solution.

Multifunctional water treatment For the application of TCNC/PGS membrane in multifunctional water treatment, the removal of water contaminants, such as methylene blue (MB) and heavy metal ions, during the oil/water separation process was conducted (Fig. 7). The layered oil/water mixture could be easily distinguished by the different colors, where upper layer was isooctane dyed by Sudan III (red) and lower layer was water contained MB (blue), as shown in Fig. 7a. After filtration by TCNC/PGS membrane, the filtrate was colorless and the red layer could not penetrate the membrane (Fig. 7b), indicating that MB was effectively adsorbed and oil phase was effectively rejected by the membrane during oil/water separation process. However, the supporting membrane (cellulose


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ester) could barely remove MB during the separation of oil/water mixture (Fig. S8), suggesting that TCNC/PGS membrane dominated removal of MB from water. Moreover, beside MB, some toxic heavy metal ions could also be partly removed by TCNC/PGS membrane during the oil/water separation. Fig. 7c shows the removal efficiency of various water contaminants by TCNC/PGS membrane. In the oil/water separation process, the removal rate of MB, Cr3+, Mn2+, Fe3+, Ni2+, and Cu2+ were 97.63% ± 0.03%, 55.91% ± 0.34%, 62.13% ± 0.84%, 77.00% ± 0.44%, 61.39% ± 0.38%, and 88.72% ± 0.28%, respectively. These results revealed that our TCNC/PGS membrane could also simultaneous reduce the concentrations of various contaminants in water phase during the oil/water separation.

Fig. 7. Photographs of filtrating equipment and membranes before (a) and after (b) oil/water mixture separation, where oil (isooctane) and water were dyed by Sudan III and methylene blue, respectively. (c) Removal of MB and various metal ions for TCNC/PGS membrane during the oil/water separation process.

CONCLUSIONS ACS Paragon Plus Environment


In summary, robust membranes with superhydrophilic and underwater superoleophobic surfaces have been fabricated from renewable TCNCs and inexpensive PGS by a facile vacuum assisted filtration without adding other reagents. The thickness and effective pore size of TCNC/PGS membrane were tuned by varying the weight ratio and dosage of TCNC/PGS. The resulting membrane could effectively separate various surfactant-stabilized emulsions with high water flux and oil rejection, and maintained good recyclability. Moreover, TCNC/PGS membrane exhibited excellent stability under harsh environments, which made it a promising candidate in practical oil/water separation. Impressively, the composite membrane could also remove toxic dye and heavy ions from solution during the oil/water separation, indicating multifunctional characteristics. This work provided a facile strategy to convert low cost resources to valuable materials for multifunctional oily water treatment.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publish website at DOI: … … Preparation of oil/water emulsions; SEM images of TCNCs and PGS nanorods; SEM image, the water flux, and oil contact angles of composite membranes with different TCNC/PGS weight ratio; AFM images of the composite membranes; SEM image of TCNC/PGS membranes with different dosages; Dynamic approach-compress-detach


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oil-adhesion test of TCNC/PGS membrane; Separation of emulsions by TCNC/PGS membrane; Photographs for the separation process of oil/water mixture.

AUTHOR INFORMATION Corresponding author *

C.C. e-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (21304021), Jiangsu Province Science Foundation for Youths (BK20150382), Pearl River S&T Nova Program of Guangzhou (201506010101), State Key Laboratory of Pulp and Paper Engineering (201824), and the Fundamental Research Funds for the Central Universities (2042015kf0028 and 2042018kf0213).

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Table of Contents

Multifunctional membranes from renewable TCNCs and low cost PGS were constructed for oil/water emulsion separation and simultaneous removal of contaminants.


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Palygorskite Nanorod

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