Juncus Pith: A Versatile Material for Automatic and Continuous

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Juncus pith: A versatile material for automatic and continuous separation of various oil-water mixtures Fatang Liu, Rui Lu, and Qinmin Pan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02232 • Publication Date (Web): 31 Oct 2016 Downloaded from http://pubs.acs.org on November 6, 2016

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ACS Sustainable Chemistry & Engineering

Juncus pith: A versatile material for automatic and continuous separation of various oil-water mixtures †

†

Fatang Liu, Rui Lu, Qinmin Pan*,

†

†

School of Chemistry and Chemical Engineering, Harbin Institute of Technology, No. 92 West Dazhi Street,

Nangang District, Harbin 150001, P. R. China

1

ACS Paragon Plus Environment


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ABSTRACT It is extremely important to develop a facile and versatile strategy for effectively separating various oil-water mixtures. Here we report that the pith of juncus (which is also known as common rushes) can sever as a versatile material for oil-water separation. Under the action of capillary force and gravity, a piece of pith automatically and continuously separates not only immiscible oil-water mixtures but also surfactant-stabilized water-in-oil emulsions with high selectivity and desirable flux. The separation strategy is cost-effective and energy-efficient because it avoids the switch of wettability, the input of additional energy, and the use of low-surface-energy chemicals or special equipment. The unique capability of the pith is mainly attributed to the presence of three-dimensionally (3D) reticular texture with highly hydrophobic and superoleophilic properties. Owing to its easy availability and high effectiveness, juncus might be a promising material aiming for oil-water separation, water treatment, and purification of solvent or fuel, and so on.

KEYWORDS: Juncus pith, oil-water separation, emulsion, 3D reticular texture, capillary force

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INTRODUCTION Effective separation of various oil-water mixtures, either immiscible mixture or emulsion, is extremely important for petroleum industry, chemical engineering, and environmental protection.1,2 In the past decades, bio-inspired

functional

materials

with

superantiwetting

and

superwetting

properties,3-6

including

superhydrophobicity7-17 and underwater superoleophobicity,18-24 have been widely investigated for oil-water separation because of high selectivity25,26 and efficiency. Thanks to their specific repellence to water or oils, these bio-inspired materials selectively separated oil-water mixtures through an absorption or filtration process. Unfortunately, a single superhydrophobic or superhydrophilic material could not separate different types of oil-water mixtures. To overcome the limitation, researchers have proposed smart surfaces with switchable wettabilities. The switch of wettability could be achieved via stimuli such as temperature,27,28 pH,29,30 light irradiation31 or ammonia gas.32 By reversible switch between superhydrophobicity and superhydrophilicity, the smart surfaces effectively separated oil-water mixtures no matter they were stabilized by surfactant or not.33,34 However, these strategies involved the elaborate control of both microstructure and composition of separating materials, or the use of expensive fluorinated chemicals. Moreover, the strategies also required external energy input and separating devices like funnels or pumps.35,36 Therefore, it is highly desirable to develop a general, facile, energy and cost effective strategy for separating a wide range of oil-water mixtures, especially surfactant-stabilized emulsions. Plants often offer inspirations for the development of novel separation strategy. Juncus, commonly known as rushes, is a kind of herbaceous plants widespread in temperate and montane-tropical regions. In ancient time, the pith of juncus was widely used as a wick for oil lamps or candles because it could transport oil through capillary force. According to capillary phenomenon, the climbing height (H) of liquids, either water or oils, on a juncus pith is determined by 3



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H = 2γcosθ/ρd

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(1)

where θ, γ, ρ, d are contact angle, surface tension, density of liquid, and diameter of capillary tube, respectively. If a liquid has high affinity to the pith (i.e., θ ≈ 0°), it can climb along the pith because of a positive H. On the contrary, the climbing is forbidden for the liquid exhibiting a high contact angle on the pith (e.g., θ > 90°, H < 0). Therefore, it is possible to separate oil from water by juncus pith if the liquids display dramatically different H. Herein, we demonstrate that juncus piths can automatically, continuously and effectively separate a wide range of oil-water mixtures, including immiscible oil-water mixtures and even surfactant-stabilized water-in-oil emulsions. The separation is simply achieved by capillary force and gravity (Scheme 1), which avoids the switch of wettability, and the use of low-surface-energy chemicals, special equipment or external energy input. Considering that juncus piths are readily available and inexpensive, our investigations offer a facile, versatile, energy-efficient and cost-effective route for oil-water separation. Potential application of this bio-inspired strategy might be found in chemical engineering, petroleum industry, and environmental protection, etc.

Scheme 1. The separation process of water-oil mixture by juncus pith.

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EXPERIMENTAL SECTION Materials: Juncus pith (Juncus effusus L.) was purchased from Haozhou Tongjide Health Pavilion (Anhui province, China) and used as received. Toluene, n-hexane, n-octane, n-dodecane and span80 (HLB = 4.3) were provided by Aladdin Industrial Corporation (China). Dichloromethane (CH2Cl2), chloroform (CHCl3) and carbon tetrachloride (CCl4) were supplied by Tianjin Kermel Chemical Reagent Co., Ltd (China). The fluorescent agent (yellow green) was provided by Kunsan Wanfukai Chemical Co., Ltd (China). Preparation of water-in-oil emulsions: Span80 (75 mg) was added to 15 g of toluene, n-octane or n-dodecane containing 1 wt% of water, respectively. The resulting mixtures were stirred by a homogenizer (FJ200-SH) at a rate of 8000 r.p.m. for 45 min to obtain milk-like water-in-toluene (W/T), water-in-octane (W/O) and water-in-dodecane (W/D) emulsions. W/D emulsion containing 2 wt% of water was also prepared by the similar process. All the emulsions were stable for at least 5 h without demulsification. Oil-water separation experiments: Typically, one end of a piece of juncus pith was immersed in a mixture of oil-water, and the other end was placed in a glass vial. The height from the mixture surface to the top height of juncus pith was kept about 2.2 cm during the separation process (Scheme 1). The oil-water separation proceeded automatically and continuously until the oil phase was completely collected. Then the above process was repeated four times for good reproducibility. The permeation flux (L m−2 h−1) of juncus pith was estimated by the formula Flux = V/(At)

(2)

where V is the volume of efflux, A is the cross-section area of juncus pith, t is the separation time. After the separation of the water-in-dodecane (W/D) emulsion, the resulting pith was frozen at -18 oC and then cut into slices of ~1 mm in thickness. The distribution of water droplets on the slices was observed by a fluorescent microscope (Nikon Eclipse 80i).

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Characterizations: The morphology of juncus pith was observed on a scanning electron microscope (SEM, FEI Quanta200). Contact angles (CA) were measured by an OCA20 (Dataphysics). Optical microscopy images were recorded by a PH50-1B43L-A/PL (Phenix, China). FT-IR spectra were recorded by a Thermo-Nicolet Avatar 360 spectrometer. Water content was measured by a coulometer (SFY-3000, Haifen Instrument Co., Ltd. China). Dynamic light scattering (DLS) measurements were conducted on a Zetasizer Nano ZSP (Malvern, UK). Fluorescent images were recorded by a Nikon Eclipse 80i microscope. Stress-strain curves were measured on a universal electromechanical testing machine CMT 8102.

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RESULTS AND DISCUSSION (c)

(b)

(a)

(e)

Air

(f)

(d)

Air

Air

(g)

n-hexane

CHCl3 Water

148.9 ± 1.5

Water o

1 ms

o

153.9 ± 1

Figure 1. (a) Side view and (b-c) cross-section images of a juncus pith. (d) is the cross-section image of a microtube. Wetting behavior of the pith to (e) a water droplet and (f) a CHCl3 droplet in air. (g) Water CA of the pith immersed in n-hexane. The droplets had a volume of 3 µL. At first, the microstructure and wettability of a piece of juncus pith were investigated by scanning electron microscopy (SEM) and contact angle (CA) measurements, respectively. As shown in Figure 1a-d, the pith displays a 3D reticular texture, whose basic structure is triangle meshes constructed by interconnected microtubes. These microtubes have a diameter of ~5 microns and a wall-thickness of ~890 nm. SEM observation implies that the pith has a porous structure at micrometer scale. Figure 1e and 1f show the wetting behavior of the pith to a water and CHCl3 droplet in air, respectively. The pith exhibits a nearly zero oil contact angle (CA) and ~148.9° water CA (Figure 1e), indicating superoleophilic and highly hydrophobic properties. Actually the oil droplet spread out and penetrated into the pith within 1 ms (Figure 1f). The special wettability of the pith is ascribed to its unique chemical composition and 3D reticular structure. The pith contains not only hydrophilic groups such as –OH and –COOH37,38 but also many aromatic derivatives (Figure S1),39,40 which endows its hydrophobicity and oleophilicity. The hierarchical roughness of the 3D reticular texture further enhances the hydrophobicity and oleophilicity of the pith,9,31-46 and thus greatly reduces the actual contact area to the water droplet. Notably, the 7



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pith showed superhydrophobicity when it was immersed in oils. For example, it had a water CA of ~153.9° in n-hexane (Figure 1g). The under-oil superhydrophobicity is due to the infiltration of n-hexane into the reticular texture. The trapped hexane acts as a repulsive layer for a water droplet, thus forming a pith/hexane/water interface. Owing to the presence of the 3D reticular texture, the trapped n-hexane greatly reduces the contact area between the pith and the water droplet. Since the pith exhibits different wettability to water and oil, it is possible to separate oil from water by taking advantage of different capillary heights. To confirm the possibility, a piece of juncus pith (2.7 mm in diameter and 7.0 cm in length) was used to separate immiscible hexane-water mixture. One end of the pith was immersed in the mixture and the other end was placed in a glass vial (Figure S2a). Almost immediately the pith was wetted by the yellow-dyed hexane, and the wetting region quickly spread to the vial end. Only a few seconds later, yellow hexane dropped into the vial beneath it. The collection proceeded automatically and continuously until the hexane layer was completely removed. No hexane leaked from the wall of the pith during the separation process, suggesting that the oil was mainly transported along the pith via capillary force and desorbed under the gravity. We also collected CHCl3 and crude oil from water using the same strategy (Figure S2b and S2c). To exclude the possible transportation of water along the pith, the water layer of the CHCl3-water mixture was labeled by fluorescent yellow-green. The separation process was then monitored by a digital camera under ultraviolet (UV, 254 nm) illumination, as shown in Figure S2d. Although the pith was immersed in both water and CHCl3 layers, no water transported to the vial during the separation process, even after the oil phase was completely removed from the water. This is because CHCl3 first infiltrated into the reticular texture of the pith, and the trapped oil acted as an effective repulsive layer for water. Indeed the CHCl3-wetted pith displayed a water CA of 74.1 ± 4° in air (the inset of Figure S2b), which means that water is difficult to climb along the CHCl3-wetted pith. As a result, CHCl3 was separated from the mixture with a high purity of 99.98% (Table S1), 8


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respectively, demonstrating high separation efficiency of our pith. The above results offer an alternative strategy to automatic, continuous and selectively separate oils from water without using additional energy input and low-surface-energy chemicals. (a) Hexane

CHCl3

(b)

CH2Cl2

CCl4

CHCl3

(c) hexane water

Figure 2. Separation of (a) ternary hexane/water/CHCl3 mixture, (b) quaternary CH2Cl2/CHCl3/CCl4/water mixture, (c) hexane-water mixture stored in a curly tube of 5 mm in diameter. The CH2Cl2, CHCl3, CCl4 and hexane were dyed with green, red 24 and yellow 14, respectively. Interestingly, the pith was able to separate multiple oil-water mixtures without switching its wettability. Firstly, the pith was used to separate ternary n-hexane/water/CHCl3 mixture. In this case, the hexane layer was firstly collected by one pith, while the CHCl3 layer was eluted subsequently via another pith (Figure 2a). Moreover, both oils could be collected in one step through a single pith (Figure S3). By using several piths, we were able to separate more complicated quaternary oil-water mixtures. Here, oil droplets of CH2Cl2 (green), CHCl3 (red) and CCl4 (orange) were placed at the bottom of water. These droplets could be separately collected by the piths 9



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without fusion (Figure 2b). To our knowledge, few studies had separated complicated immiscible oil-water mixtures by a single hydrophobic material without changing its wettability. Owing to its flexibility, the pith could separate the oil-water mixture stored in a complex-shaped vessel, which is not accessible to conventional separating materials. Here we injected a mixture of n-hexane and water into a curly tube with a diameter of only 5 mm. The mixture is difficult to be separated through a conventional absorption or filtration process. Once a piece of pith slowly entered the tube and then contacted with the mixture, automatic separation and collection took place shortly (Figure 2c). The process continued until the hexane was completely drained from the tube. The collection strategy is very useful for separating trace amount of oil-water mixture stored in a complex-shaped vessel or under harsh conditions. It was found that successful separation of oil-water mixtures was dependent on the limit capillary height of oils on the pith (Figure S4). Interestingly, the piths could be bundled to separate oils from water surface. Here several piths were filled in a tube of 1.5 cm in diameter and ~16 cm in length. Figure S5a of Supporting Information shows that the mixture of n-dodecane and water can be effectively separated by the bundled piths. The separation rate is much higher than that of individual pith. The results suggest that although a pith has the tensile strength of only1.6 kPa (Figure S5b, Supporting Information), it is still possible to effectively improve the separating capability by simply bundling piths together. Clearly bundled juncus piths have potential application for oil-water separation on large scale. More interestingly, the piths could even separate surfactant-stabilized water-in-oil emulsions. Here, span80-stabilized water-in-dodecane (W/D) emulsion was prepared to investigate the unique capability. Figure 3a shows the separation results of the emulsion. Clearly, the collected n-dodecane efflux (right) is transparent, unlike its original milky emulsion (left). The phase composition of the efflux was further evaluated by an optical microscopy. As shown in Figure 3a, almost no droplets are observed in the image of the efflux. In contrast, the image of the original emulsion contains droplets ranging from ~1 µm to ~16 µm. The significant change in phase 10


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composition suggests that most of micron water droplets have been removed from the efflux. Dynamic light scattering (DLS) measurements further revealed that the largest droplet size of the efflux is only ~342 nm (Figure 3b). Then the purity of the collected n-dodecane was analyzed by a FT-IR spectrometer (Thermo-Nicolet Avatar 360) and a Karl Fischer moisture titrator (SFY-3000). The results revealed that no vibration of water was detected in the FT-IR spectrum (Figure 3c) and the water content was only 23.8 ppm (Table S2), indicating high efficiency and selectivity of our strategy. Similar separating efficiency was also achieved for span80-stabilized water-in-toluene (W/T) and water-in-octane (W/O) emulsions (Figure S6). The high purity implies that most water droplets (> 340 nm) of these emulsions did not transport along the piths in the separation process. Although the juncus cannot remove water drops smaller than 340 nm, it still offers a possibility to separate surfactant-stabilized emulsion without using conventional membranes. (a) efflux

feed

20 µm

20 µm

25

(b)

(c)100

(d)

Transmittance / %

20

Intensity / %

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


15

10

5

80

1373.8

718.5

1462.7

60

40 2958.3

2851.9

20 2921.6

0

10

100

1000

0 4000

10000

3500

3000

2500

2000

1500

1000

500

-1

Size (d.nm)

Wavenumber / cm

(f)

(e)

100 µm

(g)

100 µm

100 µm

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Figure 3. (a) Photographs and microscopic images of the span80-stabilized water-in-dodecane (W/D) emulsion before (left) and after (right) separation by a juncus pith. (b) Drop size distribution and (c) FT-IR spectrum of the efflux. (d) Schematic illustration of the cutting of the resulting pith into slices. (e-g) Fluorescent images of the corresponding slices. To confirm the above speculation, we labeled the water phase of the W/D emulsion with fluorescent yellow-green. After the separation, the resulting pith was frozen at -18 oC and cut into slices of ~1 mm in thickness according to Figure 3d. Then the distribution of water droplets on the slices was observed by a fluorescent microscope (Nikon Eclipse 80i). For the slice near the emulsion, the fluorescent image of Figure 3e shows that some water droplets are embedded in the pores formed by interconnected microtubes. It is reasonable that these water droplets originate from the penetration of trace emulsion during the separation. The droplet density of the slice is much lower than that of the original emulsion (the left microscopic image of Figure 3a), indicating that most water droplets of the emulsion were effectively filtrated by the reticular texture. Notably, the size of the embedded water droplets (15-100 µm) is much larger than that of the feed emulsion (1-16 µm). The enlarged size is ascribed to the coalescence or demulsification of water droplets on the reticular texture. In contrast, very few water droplets are observed on the slices far away from the emulsion (Figure 3f-g), implying that the embedded water droplets did not transport along the pith in the process of separation. The results revealed that most water droplets of the emulsion were not carried by the mobile n-dodecane during the separation process, although they were stabilized by span80. According to the above investigations, the separation mechanism of the span80-stabilized W/D emulsion can be understood by capillary phenomenon and 3D reticular microstructure. Once the pith was immersed in the emulsion, its reticular texture was preferentially wetted by excessive n-dodecane because of superoleophilic property. The high affinity of n-dodecane (nearly zero oil contact angle, θ ≈ 0) means that it displayed a positive H 12


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and could climb on the pith. In contrast, most droplets of the emulsion were filtered by the reticular texture during the climbing process.20 For the water droplets penetrated into the pith, they coalesced on the reticular texture to form larger droplets. Since the reticular texture trapped a large amount of n-dodecane, the larger droplets exhibited a spherical shape on its surface (Figure 3e). From a microscopic standpoint, the pith was still unwettable for the larger droplets and thus they showed a minus H in this case. Consequently, n-dodecane was allowed to transport along the pith through capillary force, while the process was forbidden for the larger droplets. (a)

Dodecane Toluene Octane

600 500

(b) 500

W/D W/T W/O

400 -1

Flux / (L m h )

400

-2

Flux / (L m-2 h-1)

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


300 200 100 0

1

2

3

4

300 200 100 0

1

Number of Cycles

2

3

4

Number of Cycles

Figure 4. Cycling permeation flux of juncus piths for (a) pure organic solvents and (b) span80 stabilized water-in-oil emulsions. The permeability of the piths was comprehensively investigated. On the whole, the piths exhibited a flux in the range of 204-500 L m−2 h−1 for pure oils (Figure 4a). The value is comparable to those of conventional porous and membrane-based materials driven by gravity.9,47,48 However, the flux decreased to 183-414 L m−2 h−1 for the span80 stabilized water-in-oil emulsions (Figure 4b), implying that water droplets partly blocked the transportation of oils. Moreover, our piths also showed good recyclability for oil-water separation. After separation for 4 cycles, the piths did not significantly decrease their flux. The results confirm that although the piths separate oil-water mixture through capillary force, they still show desirable permeability.

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CONCLUSIONS In summary, we demonstrated that juncus piths could automatically, continuously and effectively separate various oil-water mixtures no matter water was continuous phase or dispersed phase. This versatile separation strategy was simply operated by capillary force and gravity, which is facile, energy-efficient and cost-effective. Although the mechanism for the unique separating capability deserves further investigation, we believe that it is related to the 3D reticular texture with hydrophobicity and superoleophilicity. The structural characteristics allow oils and water to exhibit different capillary heights on the piths. Our findings offer a bio-inspired strategy for oil-water separation by taking advantage of capillary phenomenon and 3D reticular microstructure. On the basis of this bio-inspired strategy, novel separating materials might be developed aiming for emulsion separation, water treatment, and purification of solvent or fuel, and so on.

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ASSOCIATED CONTENT Supporting Information FT-IR spectra and possible components of juncus pith; Separation of oil-water mixture; Water content of oil; Climbing height of oil on juncus pith; Photographs and microscopic images of emulsions before and after separation.

AUTHOR INFORMATION Corresponding author *E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The work was financially supported by a self-planned task of the State Key Laboratory of Robotics and Systems of the Harbin Institute of Technology (SKLRS201604C), National Natural Science Foundation of China (51473041), and the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (51521003).

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Juncus pith

Juncus pith can automatically, continuously and effectively separate various oil-water mixtures through capillary force and gravity.

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Juncus Pith: A Versatile Material for Automatic and Continuous

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