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superoleophilic natural fibres for continuous oil-water separation and interfacial dye-adsorption
Separation and Purification Technology 233 (2020) 116062
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Superhydrophobic/superoleophilic natural fibres for continuous oil-water separation and interfacial dye-adsorption
T
Md J. Nine , Shervin Kabiri, Achia K. Sumona, Tran T. Tung, Mahmoud M. Moussa, Dusan Losic ⁎
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School of Chemical Engineering and Advanced Materials, The University of Adelaide, Adelaide, SA 5005, Australia
ARTICLE INFO
ABSTRACT
Keywords: Biowaste Microfibres Superoleophilicity Separation Adsorption
The inconsistent wettability of biological superwetting materials, due to aging, morphological change, structural fragility and biodegradation, limit their practical use for highly demanding applications such as oil-water separation and dye adsorption. Herein, we present a new source of superwetting materials harvested from waste chestnut-shell. The material is in the form of micro-fibres which are intrinsically oleophilic/hydrophobic, chemically stable, lightweight and structurally robust. The harvested microfibres, laying between inner-liner and outer shell of the chestnut, are naturally enriched with aliphatic and aromatic hydrocarbon that results in their high oleophilicity. We demonstrated that these superoleophilic fibre-networks could be used as oil-absorbent exhibiting outstanding absorption efficiency with a maximum capacity of ~94% of their own weight. Afterwards, an efficient filtration membrane was engineered using these micro-fibres showing their ability for continuous oil-water separation process for a series of organic solvents (toluene, canola oil, engine oil, hexane, turpentine oil, petrol and olive oil) co-existing with water. Furthermore, the fibres were realized to be capable of adsorbing organic dyes at oil-water interfaces in both static (slow adsorption) and dynamic (instant adsorption) condition suggesting their multifunctionality in wastewater treatments. A small amount of fibres (0.75 g/L) could efficiently remove water miscible dyes of Rhodamine-B and Methylene blue with a maximum removal efficiency of 88% and ~91%, respectively. These low-cost natural fibres from biowaste with outstanding oilwater separation and organic dye-adsorption capacity have considerable advantages compared to other low-cost materials reported earlier for industrial wastewater-treatment and environmental remediation.
1. Introduction Superwetting materials found in nature are the unique sources of scientific inspirations to mimic their properties on target substrates for versatile applications such as oil-water separation, dye adsorption, selfcleaning, anti-bacterial, anti-fogging, de-icing, water-harvesting, and corrosion resistance [1–5]. However, the expensive mimicry of these properties would not be necessary if those naturally abundant biological materials could directly be exploited. Unfortunately, the biological sources of superwetting materials including tree leaves, insects or fish-skin (e.g. clover leaves, lotus leave, poplar leaf, rice leaf, gecko Foot, butterfly wing) grow, die and decompose over time that eventually results in disappearance of their intrinsic surface properties and exceptional wettability [3,6–8]. The inconsistent wettability, fragility, inappropriate size and shape of these natural materials make them unfit for direct use in many engineering applications such as oil-water separation and dye adsorption. The discovery of new source of superwetting materials with consistent wettability would greatly benefit the
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existing technologies for environmental remediation. The increased demand of such special wettability materials is obvious for their versatile applications in the field of environmental remediation, protective coatings (e.g. self-cleaning, antibacterial, deicing, anticorrosion) and water harvesting technology; hence the topic is of great interest in both academia and industry [3–5]. Oil and dye as major industrial pollutants contaminate water and cause hindering of rehydration, light penetration and reduction of dissolved oxygen in water that eventually costs enormous loss of aquatic lives in the marine food chain [9–12]. Therefore, prior to the discharge of industrial effluents into the environment, it is essential to achieve an acceptable purification level using low-cost and eco-friendly sorbent-materials to stop possible extinction of many aqueous species. To address the challenge of oil- and dye-contaminated wastewater treatment, various types of materials with different wettability and porosity have been employed over the years such as carbon nanomaterials (e.g. carbon black, graphene, carbon nanotube, carbon fibre) [13–17], metal oxide based sorbents [18–20], metal organic framework [21], chemically
Corresponding authors. E-mail addresses: [email protected] (M.J. Nine), [email protected] (D. Losic).
https://doi.org/10.1016/j.seppur.2019.116062 Received 9 July 2019; Received in revised form 4 September 2019; Accepted 10 September 2019 Available online 10 September 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
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modified foams [22,23], synthetic fibres [24,25], nanocellulose [26] and other nanomaterials [27]. These materials in the form of powder, 3D porous structures, and containment boom often modified with organosilane based alkyl and fluorinated chemicals which are expensive, corrosive, flammable and toxic [28–34]. Undoubtedly, the rapid development of the colloid and interface science in engineering of superwetting materials paved the way for creating new technology based on the lessons obtained from nature. Despite significant development in the field, many of these synthetic procedures are not sustainable, and scalable as the fabrication methods are complex and expensive. Hence, there is a growing demand for new superwetting materials based on abundant and inexpensive natural source for industrial applications. In this article, we report a new source of highly stable and robust superwetting (superhydrophobic/superoleophilic) fibres extracted from waste chestnut-shell. The fibres were further converted into separation membrane without any chemical and thermal modification for oilwater separation. These microfibrillar materials have unique combination of chemical and physical properties to offer extreme wettability. We demonstrated the use of microfibres for continuous oil-water separation and dye adsorption in static and dynamic oil-water interface. The reusability of fibres was also evident for several organic oils and dyes. These fibrous biowaste have considerable potential to be used as low-cost resource of high-value superwetting sorbent materials in industrial waste-water treatment technology.
at ambient condition. The flexibility of the fibrous membrane with a diameter of 2.54 cm provides a quick and facile installation on a mesh support for continuous oil-water separation. 2.2. Specific surface area (SSA) and porosity measurement The SSA of fibres was measured using methylene blue adsorption method by UV–Vis spectroscopy.[35] A known mass of fibres was added to 50 ml of methylene blue solution (0.02 g L−1). The mixed suspension of fibres and methylene blue solution was agitated for 24 hr in a shaker followed by a centrifugation to separate fibres from solution. Subsequently, the concentration of methylene blue in the solution was measured through UV–vis spectroscopy at a wavelength of 665 nm and determined based on the standard calibration curve for the methylene blue solution. The value of SSA can be calculated from the amount of adsorbed methylene blue by fibres according to the following equation Eq. (1):
SSA =
NA AMB (C 0 Ce)V MMB mS
(1) 23
where NA is Avogadro number (6.022 × 10 ), AMB is the covered area by a methylene blue molecule which is assumed to be 1.35 nm2, C0 and Ce are the initial and equilibrium concentrations of methylene blue, V is the volume of solution, MMB is the molecular mass of methylene blue and ms is the mass of fibrous. For porosity measurement, a series of optical images of the membrane were taken to process with ImageJ software following standard procedure reported earlier [36]. The imported image were sequenced for thresholding before analysis (e.g. set measurement and batch process measure). The thresholding was carried out using default setting and manual adjusting to incorporate the maximum degree of observed porosity. The analysed porosity percentage value for each image was saved to obtain the resultant mean porosity of the whole batch of images.
2. Materials and methods 2.1. Fibre extraction and membrane preparation The raw chestnuts were collected from grocery shop in South Australia to extract fibres. The fibres were found stacked between hard outer shell and papery-like inner linear as shown in Fig. 1a. The harvesting process involves (a) peeling off chestnut shell to expose fibre enriched area and, (b) extracting stacked fibres from fibre enriched area between inner wall and outer layer of the chestnut shell. Approximately ~0.1 g of fibres was harvested from a single chestnut. The weighted amount of extracted fibres was directly dispersed in organic solvent (i.e. toluene) and deposited on a metal mesh to prepare a thin fibre-film which resembles a freestanding fibrous membrane after a complete dry
2.3. Oil absorption, separation and reusability of chestnut fibres Batch absorption and continuous oil-water separation were performed for different organic solvents and vegetable oils. The oil Fig. 1. Fibre harvesting process and their interaction with oil, water, and dye. (a) The steps of the fibre extraction from waste chestnut-shell. (b) Adhesive superhydrophobicity of the fibrous surface, (c) Oil (Red colour-from Sudan III) and water (nocolour) droplet on fibrous surface. (d) Extracted fibres dispersed in oil phase (red colour) in a bottle, (e) The change in colour of fibres indicates their affinity to organic dyes (Sudan III). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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absorption by fibres is an adhesion of the organic solvents (adsorbate) onto the surface of a fibres (adsorbent) which eventually reach at a saturated state. The saturated adsorption capacity of fibres (total absorption) was examined for various oils and organic solvents at room temperature. The weighted amount (0.05 g) of dry fibres was introduced to the organic solvents (oils) co-existing with water on a Petridish to allow continuous adsorption for at least three times. According to literature, saturated adsorption capacity is defined as weight of oil or organic solvent adsorbed by per unit of weighted materials (g/g) and calculated by the following Eq. (2);
Q=
Wt1 Wt 0 Wt 0
The dye removal efficiency at equilibrium was calculated using the following Eq. (4),
E(%) =
m1 × 100 m0
Ceq) Co
× 100
(4)
where E (%) is the removal efficiency, C0 (µM) and Ceq (µM) represent the initial and equilibrium dye concentrations, respectively. 2.5. Materials characterisation
(2)
The morphological analysis of extracted fibres was performed using a scanning electron microscope (SEM-FEI Quanta 450, Japan) in a low vacuum chamber at an accelerating voltage of 5 kV. The presence of specific functional groups on fibre structure was studied by analysing vibrational and stretching modes of different molecular bonds using a Fourier transform infrared spectroscopy (FTIR) (Nicolet 6700, Thermo Fisher, USA) within a spectral range between 400 cm−1 and 4000 cm−1. Thermogravimetric analysis (TGA) was used to examine thermal decomposition of the compounds to identify the major composition of chestnut fibres using a TA Instruments (Q-500, Tokyo, Japan). The temperature was raised from 30 °C to 600 °C at a rate of 5 °C/min for the decomposition in nitrogen environment. The wettability of chestnut fibres was studied following a method of sessile drop water contact angle (WCA) using an Attension theta optical tensiometer (KSV instruments, Finland). The optical image of the membrane was captured by a Nikon Optical Petrographic Microscope (LV100 POL, USA) to analyse the porosity using ImageJ software (Version 1.47, National Institutes of Health, USA). All the supporting movies of different experimental demonstrations were recorded using a high definition video camera (Sony HDR-PJ260).
where Q is the saturated adsorption capacity (total absorption) of fibres and, Wt0 and Wt1 are the weight of fibres before and after saturated absorption, respectively. To determine the reusability of fibres, the saturated samples were squeezed, and oven dried at vacuum condition before introducing for further absorption tests. The as-prepared free-standing fibrous filter was soaked with 1 ml of oil before placing on a specially made metal mesh chamber in the filtration tube. The specially made filtration tube was installed at an inclination of 5° that allows a gravity driven separation through filtration tube (Fig. 5a). The filtrate was collected and compared with the weight before and after the separation to calculate separation efficiency (S (%)) using the following Eq. (3);
S(%) =
(Co
(3)
where m0 and m1 are the weight of the target filtrate liquid before and after the separation, respectively. The oil-water separation was performed introducing 5 M NaCl solutions with hexane to demonstrate chemical stability of the fibres in saline water. The stability of fibres was further tested by agitating them for 48 hr in acid (1 M HCl) and basic (1 M NaOH) solutions using a shaker at a speed of 200 rpm. To demonstrate the reusability of the used material, the fibres were heated up to the boiling point of organic solvents to evaporate them and used for the next oil/organic removal. The cycle of the total absorption and recovery was repeated 5 times to characterise reusability performance. Furthermore, a vacuum system setup used to reveal the absorption performance of the fibre in continuous manure for oil-water removal following the method used in our previous work [35]. Briefly, the fibres were packed in the plastic vials with a diameter of 1.5 cm and height of 6 cm. The vial was punched up to 5 cm in height with small holes (1 mm in diameter) to allow sorption of oil through the fibres. The top surface of fibres in the vial was covered with stainless steel wiremesh to prevent the uptake of fibres under vacuum pressure. Separated oil was collected in a flask connected through a tube to the vial.
3. Results and discussion To probe the initial properties of isolated fibres, the interaction of fibres with oil, water and dye were examined as presented in Fig. 1. A fibrous-layer (prior to the extraction) anchored with hard shell displays superhydrophobic behaviour as displayed in Fig. 1b and Movie S1, which holds the droplet even the shell is upside down indicating adhesive superhydrophobicity [2]. The adhesive state of superhydrophobicity is directly related to water impregnation into the micro/nano roughness on the surface to increase solid-liquid adhesion which is identical to rose petal effect [37]. The fibrous shell is further examined to investigate interaction with organic solvents. The primary outcome reveals that the surface has strong affinity to a series of organic solvents (e.g. toluene, hexane, turpentine) while repelling water droplet (Fig. 1c and Movie S2). The oil-water interfacial interaction of these fibres was also explored by placing them in an oil-water containing glass bottle (Fig. 1d). A gentle shake makes the fibres well dispersed in oil phase (red-colour containing organic dye Sudan III) on a clear water phase (Movie S2). However, a change in colour of oildispersed fibres was observed when the fibre was taken out of the oil revealing affinity to dyes (Sudan-III dye in oil phase) as shown in Fig. 1e. The water repellence and affinity to both oil and dye is an interesting visual outcome that motivated us to apply these extracted fibres for oil-water separation and dye adsorption application.
2.4. Dye adsorption test The adsorption of dyes was primarily examined at static condition to allow oil-water interfacial adsorption, wherein the adsorbent was dispersed in oil, and dyes (Methylene Blue and Rhodamine B) were mixed in water. The static adsorption process started with a little shake to make adsorbent perfectly placed at the interface in a 20 ml glass bottle which was monitored undisturbed for 60 hr. The digital photos taken at different time intervals to record the visual adsorption of dye at the interface. For quantitative study of dye removal, two aqueous suspensions of dyes were prepared using Methylene Blue and Rhodamine B with a concentration of ~15.34 µM and ~11.80 µM, respectively. Then the fibres at different dosage were introduced in the suspension at a neutral pH subjected to a continuous shaking at a speed of 100 rpm for 5 hr to facilitate dye adsorption. The suspension after each test were taken under UV–vis spectroscopy (Shimadzu UV-1601 (200–1100 cm−1) to capture the absoprtion spectra to identify the concentration change using standard calibration curve of these dyes.
Movie S1. Superhydrophobicity of the fibrous surface. 3
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Fig. 2. Characterisation of fibres. (a) SEM image of fibres, (b) Water and oil contact angle on fibrous surface, (c) FTIR of fibres, (d) Thermogravimetric analysis of fibres in nitrogen atmosphere, (e) Proposed mechanism of superwetting properties of chestnut fibres.
Fig. 2c. The appearance of bands at 2942–2920 cm−1 are related to CeH and CH2e vibration of aliphatic hydrocarbons, while the peaks at 1720–1706 cm−1 belong to carbonyl (C]O) groups from dimerized saturated aliphatic acids. The spectra in the range between 1650 and 1450 cm−1 are due to the aromatic ring stretching vibration. The presence of carbohydrate contents is also evident which is corresponding to the bands between 1200 and 1000 cm−1, however the OeH in plane deformation at 1420–1330 cm−1 is usually week for chestnut shell [38,39]. Thermogravimetric analysis was performed to examine thermal decomposition profile of fibres in nitrogen atmosphere (Fig. 2d). The first stage mass loss is due to moisture in the fibre structure which is as low as 7 wt%. Other major compounds of fibres were found identical to chestnut shell which is composed of cellulose, hemicellulose, lignin, and monosaccharides with other constituents of polyphenol compounds [38,40]. Lignin decomposes slower than cellulose and the hemicellulose components over a broader temperature range between 200 °C and 500 °C [41,42]. A second stage mass loss of 59.42 wt% between 180 °C and 350 °C is mostly attributed to the decomposition of cellulose, and hemicellulose, as well as partial decomposition of lignin [43]. Third stage mass loss is also observed which is due to the rest of the lignin
Movie S2. Oil-water interaction with fibres.
3.1. Characterisation of fibres The fibres were found flexible and bendable with an estimated extracted length of 2 ± 0.5 mm and a diameter of 15 ± 2 μm as shown in Fig. 2a. The rippled textures on fibre surface (inset photo, Fig. 2a) were also visible. The contact angle measurement on fibrous surface using water and oil droplet clearly indicates superhydrophobicity and superoleophilicity as shown in Fig. 2b. The nano-textures combining with low surface energy compounds on fibre is anticipated to create such superhydrophobicity. To identify corresponding functional groups related to low surface energy and oil-loving behaviour, the extracted fibres were examined by Fourier-transform infrared spectroscopy (FTIR) as shown in 4
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decomposition left at the end of second stage [42]. Polysaccharides such as starch and cellulose in chestnut fibres crosslinked by lignin which creates an obstacle for water absorption through fibre surface [44]. The lignin enriched area combining with the micro-ripple texture on the fibre surface are greatly responsible to absorb oil compounds as illustrated in Fig. 2e. Along with the aliphatic functional groups (Fig. 2c), the slow thermal decomposition of the lignin over a broad temperature range (Fig. 2d) is evidence of the lignin enrichment on fibre surface which is significantly responsible for superoleophilic properties. In biomass, polysaccharides in the cell walls are protected by lignin as their decay is carried out by microbial consumption as a food source, whereas lignin is transferred to soil as carbon storage [45]. Based on the finding in literature, it is anticipated that the lignin enriched stacked layer of chestnut fibres between inner papery linear and outer shell is to protect chestnut from microbial (e.g. fungus) attack as well as to control moisture level in the core of chestnuts.
Table 1 Viscosity and surface tension value of the oils at 21 ± 1 °C with corresponding adsorption capacity and saturated adsorption rate exhibited by chestnut fibres for batch absorption [35,47]. Organic solvents and Oils
Viscosity (cP or mPa s)
Saturated adsorption capacities (g/g)
Saturated adsorption rate (g/ g)/s
Toluene Canola oil Engine oil (SAE 10 W-40) Petrol Hexane Turpentine oil Olive oil
0.58 63.50 208.89
44.13 93.83 67.62
9.81 5.86 2.41
0.60 0.31 1.37 74.10
41.26 36.00 52.22 72.33
3.75 24.00 7.54 3.81
was the resultant surface adsorption and absorption in the interfibrillar gap. Later, the fibres were examined under a repetitive absorption test for at least 5 cycles wherein a slight decrease in absorption capacity for 5th cycle was recorded than the fresh fibres as shown in Fig. 3c. The comparison of the absorption capacity by different low-cost hydrophobic absorbents for a range of oils has been shown in Fig. 3d [46]. The absorption capacity of chestnut fibres is 2–10 times better than many other previously reported biowaste such as Cotton grass, Kapok fibres, Barely straw, Coconut coir etc [46]. The difference in absorption capacity among the materials is probably due to the dissimilarity in their SSA, surface textures and chemical composition. The high SSA combining with unique surface properties enabled chestnut fibres to outperform other low-cost absorbents presented in Fig. 3d.
3.2. Oil absorption and separation performance Prior to the demonstration of oil-water separation using fibrous membrane, the fibres were examined to investigate their SSA and total oil absorption capacity (saturated adsorption). Using a standard method of dye adsorption test (described in Section 2.2), the SSA of fibres were found to be 190.8 ± 4.5 m2/g. A certain amount of fibres (0.05 g) was introduced to oil co-existing with water in a glass petri-dish to realize the action of oil absorption as shown Fig. 3a and Movie S3. However, the test revealed that the absorption capacity is relatively lower for the oil with low viscosity than the one with higher viscosity as tabulated in Table 1 and presented in Fig. 3b. The saturation adsorption rate is found viscosity dependent of oils which was higher for relatively less viscous oil than that with high viscosity. The saturated adsorption rate for hexane is as high as ~24 gg−1/s whereas the rate is 2.4 gg−1/s for highly viscous engine oil. The fibres were readily saturated by low viscous oil due to their quick oil-uptake capability through the surface of fibres instead of the micro-pores among the cluster of fibres. The total absorption capacity of low viscous oil was mostly influenced by the well textured lignin enriched micro-ripples on the surface. On the other hand, the slow adsorption rate with higher absorption capacity of highly viscous oil
Movie S3. Batch Oil-absorption using chestnut fibres; Movie S4: Oil-water separation using membrane. Fig. 3. Oil absorption performance of chestnut fibres. (a) Saturated oil adsorption test for different type of oils using chestnut fibre-networks. (b) Repetitive performance of oil absorption for several cycles. (c) Oil absorption capacity. (d) Comparison of the absorption capacity by different low-cost hydrophobic absorbents [46].
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Fig. 4. Membrane set-up, porosity and stability measurement. (a) The dimension of fibrous membrane. (b) Mesh support for placing membrane in filtration tube. (c) Membrane placed on mesh support. (d) High resolution optical image of membrane. (e) Porosity analysis of membrane using ImageJ. (f) Examining chemical stability of fibres in acidic (1 M HCl) and basic solution (1 M NaOH) where the inset photo (bottom) showed no change in wettability of fibres.
Afterwards, with an approach to prepare a low-cost but novel membrane for oil-water separation technology, we deposited a layer of fibres on a metal mesh for continuous oil-water separation illustrated in Fig. 4a–c. The 1 mm thick membrane had a diameter of 2.54 cm placed on a metal mesh support. The porosity of the prepared membrane was further analysed using ImageJ software as displayed in Fig. 4d and e which was measured to be 44.5%. The chemical stability of the fibrous cluster was tested in acidic and basic solution with a pH range between ~0 and 14 scale using 1 M HCl and 1 M NaOH, respectively, as shown in Fig. 4f. The regain of superhydrophobicity of chestnut fibres after 48 h of agitation in acidic and basic solutions indicates their ability to perform in adverse environment. A series of organic solvents as well as vegetable oils were used to perform oil-water separation through filtration tube as presented in Fig. 5a. It was observed that the oil with very low viscosity provides instant separation while the oil with high viscosity (e.g. engine oil, olive oil, canola oil) requires longer period for separation (Movie S4) due to their resistance in mobility. However, the separation efficiency for all different type of oils were more than 92%, which is approximately 99% for the oils with very low viscosity (e.g. Toluene, hexane, petrol, turpentine) as shown in Fig. 5b. The oil-water separation was also performed introducing 5 M NaCl solutions co-existing with hexane to demonstrate chemical stability of the fibres in saline water (Movie S4). In addition, no change in wettability was observed (Fig. 4f) when the fibres were agitated in acidic and basic solution which indicates noticeable chemical stability of extracted fibres.
The continuous and pressure-driven separation of organic solvents was performed using a vacuum pump employing another experimental set-up as shown in Fig. 5c. As shown, the fibres packed in a plastic porous vial was connected to a vacuum pump through a hose and filtering flask. Being dominantly hydrophobic, the fibre-loaded porous tube does not allow any water inside the fibre-network to pass through the hose which were tested before demonstrating suction induced oilwater separation as shown in Movie S5. An artificial turbulence was created (resembling realistic condition) in oil-water mixture to justify whether the system can selectively separate oil in turbulence under external suction force as shown in Fig. 5d and Movie S5. Remarkably, the oil was separated from an oil-water mixture in continuous manner with higher efficiency which was simultaneously collected in the filtering flask. The dominant oleophilic properties (due to the presence of aliphatic and aromatic hydrocarbon in the structure) of fibres only allows oil to be penetrated through the fibre network under suction leaving the water behind in the beaker. The as prepared fibrous membrane and suction induced oil-water separation provides distinct advantages of these fibres over the commonly used synthetic materials reported for wastewater treatment.
Movie S5. Oil-water separation using pump.
Movie S4. Oil-water separation using membrane.
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Fig. 5. Continuous oil-water separation. (a) Continuous separation of toluene (red)-water (blue) under gravity in filtration tube. (b) Separation efficiency of fibrous membrane for various oils cos-existing with water. (c) Experimental set-up for continuous and high-speed separation using suction pump. (d) Separation of oil-water in turbulence. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.3. Dye-adsorption performance
demonstrate that it is not necessarily very important for materials to be hydrophilic to decolorize water. The recovered fibres from the oil-water interface are closely observed and found to display low contact angle (CA 72 ± 1°) than the unused fibres that implies that some segments of the fibres are still water loving (sticky). Furthermore, a dynamic dyeadsorption was performed with rigorous shake as shown in Movie S6. The dye adsorption under turbulence were demonstrated to observe whether a fast adsorption is achievable or not. The outcome in the presence of fibres at turbulent mode is impressive which exhibited an instant declaration capability of dye-contaminated water in a few seconds as shown in Fig. 6b and Movie S6.
As discussed, the change in colour of fibres was observed (Fig. 1e) which was primarily due to the adsorption of azo dyes (Sudan III) dispersed in oil. This phenomenon motivated to further investigate dye adsorption behaviour of fibres for water soluble cationic dyes (Methylene blue (MB) and Rhodamine B). We choose cationic dyes as they are used in various industries (e.g. wood, paper, textile, leather, medical, paper) and often released with industrial effluents [48]. Usually, the increase in initial dye concentration provides an increase in the capacity of the adsorbent due to the high driving force for mass transfer at a high initial dye concentration [30]. Therefore, we used low concentration of dyes to observe how these fibres perform to remove dyes even at very low concentration. Fig. 6a presents static adsorption behaviour of fibres at oil-water interface with respect to time. The fibres (in oil) are found to be highly capable to bind these cationic dyes (in water) from oil-water interface. The uptake of both Rhodamine B and Methylene blue dyes from water to fibres were obvious and nearly removed in 60 hr of adsorption period as shown in Fig. 6a. These results
Movie S6. Instant dye adsorption using chestnut fibres. 7
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Fig. 6. Static (slow interfacial adsorption) and dynamic (instant adsorption in turbulence) dye adsorption using chestnut fibres from water. (a) Dye adsorption by chestnut-fibres from oil-water interface in static mode, where the dyes are Rhodamine B (Pink) and Methylene blue (Blue) in water. (b) Instant (dynamic) dye adsorption in oil-water turbulence. (c) UV–vis absorption spectra. (d) Dosage dependent dye removal efficiency. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The qualitive results with both static and dynamic dye adsorption using chestnut fibres are very promising which are further evaluated by quantitative outcome presented in Fig. 6c and d. The individual UV–vis spectra of the aqueous dispersion containing Rhodamine B (~11.80 µM) as well as Methylene Blue (~15.35 µM) were captured before and after adsorption by different dosages of fibres as shown in Fig. 6c. The characteristic peaks of Rhodamine B and Methylene Blue were identified at the wavelength of 554 nm and 665 nm, respectively. The increase in fibre dosage decreases the adsorption peak intensity of the dispersion (conversely, increases dye removal effect). Approximately 90% dye-removal were achieved using a maximum fibre dosage of 0.75 g/L (88.1% removal for Rhodamine B and 91.5% removal of Methylene Blue) as presented in Fig. 6d. With an increased dosage of fibres, the removal efficiency of dyes gradually increases. The dosagedependent dye removal efficiency of different low-cost adsorbent
materials is tabulated in Table 2 for the comparison with chestnut fibres. The chestnut fibres were found to outperform other low-cost adsorbents to achieve maximum removal efficiency using least amount of dosage. Mechanism of interfacial dye adsorption properties of these hydrophobic fibres might be due to the combination of surface chemistry and roughness. Despite possessing aliphatic functional groups (which is responsible for hydrophobicity), the fibres also contain negatively charged carbonyl (CO) and hydroxyl (OH) functional groups as confirmed by FTIR (Fig. 2a) which might influence to generate adhesive superhydrophobicity (cassie-state) as shown in Fig. 1b and Movie S1. To elucidate the mechanism of dye adsorption, the micro- or nanostructure with a variable functional group including aliphatic hydrocarbon and oxygen functional groups can be impregnated by water or air. The regimes with high solid–water adhesion allows penetrating 8
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Table 2 Comparison of the dosage dependent dye removal efficiency by low-cost adsorbents. Materials
Dosage (g/L)
Dye name
Percentage of removal range (%)
Chestnut fibres (present work) Kaolin [49] Modified sawdust [50] Fly-ash [51] Tea waste [52] Activated carbon of corn-husk [53]
0.25–0.75
Methylene blue Rhodamine B Crystal violet Methylene blue Methylene blue Basic yellow 2 Methylene blue
56.2–91.5 60.5–88.1 75–97 34.4–96.6 45.16–96 19–60 ~67.5–98
0.25–4 1.5–5 8–20 2–20 0.1–0.5
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Mitropoulos, G.Z. Kyzas, Use of nanoparticles for dye adsorption: review, J. Dispersion Sci. Technol. 39 (2018) 836–847. [28] M.O. Adebajo, R.L. Frost, J.T. Kloprogge, O. Carmody, S. Kokot, Porous materials for oil spill cleanup: a review of synthesis and absorbing properties, J. Porous Mater. 10 (2003) 159–170. [29] R.K. Gupta, G.J. Dunderdale, M.W. England, A. Hozumi, Oil/water separation techniques: a review of recent progresses and future directions, J. Mater. Chem. A 5 (2017) 16025–16058. [30] M.T. Yagub, T.K. Sen, S. Afroze, H.M. Ang, Dye and its removal from aqueous solution by adsorption: a review, Adv. Colloid Interface Sci. 209 (2014) 172–184. [31] Z. Wang, M. Elimelech, S. Lin, Environmental applications of interfacial materials with special wettability, Environ. Sci. Technol. 50 (2016) 2132–2150. [32] Z. Weifeng, L. Na, C. Yingze, L. Xin, L. Yanan, F. Lin, Superwetting porous materials for wastewater treatment: from immiscible oil/water mixture to emulsion
water into the microstructure [37]. This impregnation of water is the bridge between dye and fibres due to partial negative charges in the structure. Binding sites of the basic dyes used in this experiment could be those oxygen fictional groups (O-H, C = O) as Rhodamine B and Methylene blue are cationic by nature. From roughness point of view, the micro wrinkles containing dip and rise have variable affinity to water due the presence of both oxygen functional groups (OeH, C]O) and hydrophobic aliphatic groups (e.g. eCH2e, eCH3) [2]. The nanoripples with dip and sharp rise, consists of a complex of lignin and polysaccharide, are the bridge between water and oil phase to create an uptake of dyes from water to oil as shown in Fig. 2e. 4. Conclusion We presented, for the first time, superwetting microfibres isolated from chestnut-shell for continuous oil-water separation and dye adsorption. Owing to the presence of aliphatic and aromatic hydrocarbon in the structure (lignin enriched surface), the fibres were intrinsically oleophilic with an excellent oil absorption capacity of ~94 g/g. Furthermore, the fibres exhibited exceptional pliability to fabricate a flexible membrane for filtration process which was hardly achievable using other bio-waste. Results showed that these membranes have an outstanding oil separation efficiency (99%) subjected to gravity or an external suction force for the oils with low viscosity. In addition, the dye removal efficiency of fibres for two model dyes (Methylene blue and Rhodamine B) were found as high as ~91%. The partial negative charges (oxygen functional groups) on fibres were assumed to be responsible for adhesive hydrophobicity and cationic dye adsorption in oil-water interrace. This newly discovered bio-sorbent is low-cost, nontoxic and ready for use without chemical treatments which provides many advantages over other synthetic superwetting materials (e.g. carbon and polymer-based sorbents) introduced for oil- and dye contaminated wastewater treatment. Declaration of Competing Interest The authors declare no conflict of interest. Acknowledgements The authors thank the support of the Australian Research Council (IH 150100003 ARC Research Hub for Graphene Enabled Industry Transformation). Furthermore, we thank The University of Adelaide for providing access to necessary research facilities and supports. References [1] M.J. Nine, M.A. Cole, L. Johnson, D.N.H. Tran, D. Losic, Robust superhydrophobic graphene-based composite coatings with self-cleaning and corrosion barrier properties, ACS Appl. Mater. Interfaces 7 (2015) 28482–28493. [2] M.J. Nine, T.T. Tung, F. Alotaibi, D.N.H. Tran, D. Losic, Facile adhesion-tuning of superhydrophobic surfaces between “lotus” and “petal” effect and their influence on icing and deicing properties, ACS Appl. Mater. Interfaces 9 (2017) 8393–8402.
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M.J. Nine, et al. separation, Adv. Mater. Interfaces 4 (2017) 1600029. [33] S.M.R. Razavi, J. Oh, S. Sett, L. Feng, X. Yan, M.J. Hoque, A. Liu, R.T. Haasch, M. Masoomi, R. Bagheri, N. Miljkovic, Superhydrophobic surfaces made from naturally derived hydrophobic materials, ACS Sustain. Chem. Eng. 5 (2017) 11362–11370. [34] F. Wang, S. Lei, M. Xue, J. Ou, W. Li, In situ separation and collection of oil from water surface via a novel superoleophilic and superhydrophobic oil containment boom, Langmuir 30 (2014) 1281–1289. [35] S. Kabiri, D.N.H. Tran, T. Altalhi, D. Losic, Outstanding adsorption performance of graphene–carbon nanotube aerogels for continuous oil removal, Carbon 80 (2014) 523–533. [36] J. Buckman, A.S. Bankole, S. Zihms, H. Lewis, G. Couples, W.M.P. Corbett, Quantifying porosity through automated image collection and batch image processing: case study of three carbonates and an aragonite cemented sandstone, Geosciences 7 (2017). [37] B. Bhushan, M. Nosonovsky, The rose petal effect and the modes of superhydrophobicity, Philos. Trans. R. Soc. A: Math., Phys. Eng. Sci. 368 (2010) 4713–4728. [38] G. Vázquez, E. Fontenla, J. Santos, M.S. Freire, J. González-Álvarez, G. Antorrena, Antioxidant activity and phenolic content of chestnut (Castanea sativa) shell and eucalyptus (Eucalyptus globulus) bark extracts, Ind. Crops Prod. 28 (2008) 279–285. [39] G. Xu, Z. Shi, Y. Zhao, J. Deng, M. Dong, C. Liu, V. Murugadoss, X. Mai, Z. Guo, Structural characterization of lignin and its carbohydrate complexes isolated from bamboo (Dendrocalamus sinicus), Int. J. Biol. Macromol. 126 (2019) 376–384. [40] W. Chin-San, H. Yi-Chiang, L. Hsin-Tzu, Y. Fu-San, W. Chen-Yu, H. Chia-Tsc, Characterization and biocompatibility of chestnut shell fiber–based composites with polyester, J. Appl. Polym. Sci. 131 (2014). [41] H. Kawamoto, Lignin pyrolysis reactions, J. Wood Sci. 63 (2017) 117–132. [42] C.V. Mihai Brebu, Thermal degradation of lignin – a review, Cellulose Chem. Technol. 44 (2010) 353–363.
[43] M.J. Nine, D.N.H. Tran, T.T. Tung, S. Kabiri, D. Losic, Graphene-borate as an efficient fire retardant for cellulosic materials with multiple and synergetic modes of action, ACS Appl. Mater. Interfaces 9 (2017) 10160–10168. [44] Y. Zeng, S. Zhao, S. Yang, S.-Y. Ding, Lignin plays a negative role in the biochemical process for producing lignocellulosic biofuels, Curr. Opin. Biotechnol. 27 (2014) 38–45. [45] M. Thevenot, M.-F. Dignac, C. Rumpel, Fate of lignins in soils: a review, Soil Biol. Biochem. 42 (2010) 1200–1211. [46] S. Sabir, Approach of cost-effective adsorbents for oil removal from oily water, Critical Rev. Environ. Sci. Technol. 45 (2014) 1916–1945. [47] S.N. Sahasrabudhe, V. Rodriguez-Martinez, M. O’Meara, B.E. Farkas, Density, viscosity, and surface tension of five vegetable oils at elevated temperatures: measurement and modeling, Int. J. Food Prop. 20 (2017) 1965–1981. [48] V.K. Gupta, D. Mohan, S. Sharma, M. Sharma, Removal of basic dyes (Rhodamine B and Methylene Blue) from aqueous solutions using bagasse fly ash, Sep. Sci. Technol. 35 (2000) 2097–2113. [49] B.K. Nandi, A. Goswami, A.K. Das, B. Mondal, M.K. Purkait, Kinetic and equilibrium studies on the adsorption of crystal violet dye using kaolin as an adsorbent, Sep. Sci. Technol. 43 (2008) 1382–1403. [50] W. Zou, H. Bai, S. Gao, K. Li, Characterization of modified sawdust, kinetic and equilibrium study about methylene blue adsorption in batch mode, Korean J. Chem. Eng. 30 (2013) 111–122. [51] K.V. Kumar, V. Ramamurthi, S. Sivanesan, Modeling the mechanism involved during the sorption of methylene blue onto fly ash, J. Colloid Interface Sci. 284 (2005) 14–21. [52] E. Khosla, S. Kaur, P.N. Dave, Tea waste as adsorbent for ionic dyes, Desalin. Water Treat. 51 (2013) 6552–6561. [53] M. Khodaie, N. Ghasemi, B. Moradi, M. Rahimi, Removal of methylene blue from wastewater by adsorption onto ZnCl2 activated corn husk carbon equilibrium studies, J. Chem. 2013 (2013) 6.
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Superhydrophobic/superoleophilic natural fibres for continuous oil-water separation and interfacial dye-adsorption
T
Md J. Nine , Shervin Kabiri, Achia K. Sumona, Tran T. Tung, Mahmoud M. Moussa, Dusan Losic ⁎
⁎
School of Chemical Engineering and Advanced Materials, The University of Adelaide, Adelaide, SA 5005, Australia
ARTICLE INFO
ABSTRACT
Keywords: Biowaste Microfibres Superoleophilicity Separation Adsorption
The inconsistent wettability of biological superwetting materials, due to aging, morphological change, structural fragility and biodegradation, limit their practical use for highly demanding applications such as oil-water separation and dye adsorption. Herein, we present a new source of superwetting materials harvested from waste chestnut-shell. The material is in the form of micro-fibres which are intrinsically oleophilic/hydrophobic, chemically stable, lightweight and structurally robust. The harvested microfibres, laying between inner-liner and outer shell of the chestnut, are naturally enriched with aliphatic and aromatic hydrocarbon that results in their high oleophilicity. We demonstrated that these superoleophilic fibre-networks could be used as oil-absorbent exhibiting outstanding absorption efficiency with a maximum capacity of ~94% of their own weight. Afterwards, an efficient filtration membrane was engineered using these micro-fibres showing their ability for continuous oil-water separation process for a series of organic solvents (toluene, canola oil, engine oil, hexane, turpentine oil, petrol and olive oil) co-existing with water. Furthermore, the fibres were realized to be capable of adsorbing organic dyes at oil-water interfaces in both static (slow adsorption) and dynamic (instant adsorption) condition suggesting their multifunctionality in wastewater treatments. A small amount of fibres (0.75 g/L) could efficiently remove water miscible dyes of Rhodamine-B and Methylene blue with a maximum removal efficiency of 88% and ~91%, respectively. These low-cost natural fibres from biowaste with outstanding oilwater separation and organic dye-adsorption capacity have considerable advantages compared to other low-cost materials reported earlier for industrial wastewater-treatment and environmental remediation.
1. Introduction Superwetting materials found in nature are the unique sources of scientific inspirations to mimic their properties on target substrates for versatile applications such as oil-water separation, dye adsorption, selfcleaning, anti-bacterial, anti-fogging, de-icing, water-harvesting, and corrosion resistance [1–5]. However, the expensive mimicry of these properties would not be necessary if those naturally abundant biological materials could directly be exploited. Unfortunately, the biological sources of superwetting materials including tree leaves, insects or fish-skin (e.g. clover leaves, lotus leave, poplar leaf, rice leaf, gecko Foot, butterfly wing) grow, die and decompose over time that eventually results in disappearance of their intrinsic surface properties and exceptional wettability [3,6–8]. The inconsistent wettability, fragility, inappropriate size and shape of these natural materials make them unfit for direct use in many engineering applications such as oil-water separation and dye adsorption. The discovery of new source of superwetting materials with consistent wettability would greatly benefit the
⁎
existing technologies for environmental remediation. The increased demand of such special wettability materials is obvious for their versatile applications in the field of environmental remediation, protective coatings (e.g. self-cleaning, antibacterial, deicing, anticorrosion) and water harvesting technology; hence the topic is of great interest in both academia and industry [3–5]. Oil and dye as major industrial pollutants contaminate water and cause hindering of rehydration, light penetration and reduction of dissolved oxygen in water that eventually costs enormous loss of aquatic lives in the marine food chain [9–12]. Therefore, prior to the discharge of industrial effluents into the environment, it is essential to achieve an acceptable purification level using low-cost and eco-friendly sorbent-materials to stop possible extinction of many aqueous species. To address the challenge of oil- and dye-contaminated wastewater treatment, various types of materials with different wettability and porosity have been employed over the years such as carbon nanomaterials (e.g. carbon black, graphene, carbon nanotube, carbon fibre) [13–17], metal oxide based sorbents [18–20], metal organic framework [21], chemically
Corresponding authors. E-mail addresses: [email protected] (M.J. Nine), [email protected] (D. Losic).
https://doi.org/10.1016/j.seppur.2019.116062 Received 9 July 2019; Received in revised form 4 September 2019; Accepted 10 September 2019 Available online 10 September 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
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modified foams [22,23], synthetic fibres [24,25], nanocellulose [26] and other nanomaterials [27]. These materials in the form of powder, 3D porous structures, and containment boom often modified with organosilane based alkyl and fluorinated chemicals which are expensive, corrosive, flammable and toxic [28–34]. Undoubtedly, the rapid development of the colloid and interface science in engineering of superwetting materials paved the way for creating new technology based on the lessons obtained from nature. Despite significant development in the field, many of these synthetic procedures are not sustainable, and scalable as the fabrication methods are complex and expensive. Hence, there is a growing demand for new superwetting materials based on abundant and inexpensive natural source for industrial applications. In this article, we report a new source of highly stable and robust superwetting (superhydrophobic/superoleophilic) fibres extracted from waste chestnut-shell. The fibres were further converted into separation membrane without any chemical and thermal modification for oilwater separation. These microfibrillar materials have unique combination of chemical and physical properties to offer extreme wettability. We demonstrated the use of microfibres for continuous oil-water separation and dye adsorption in static and dynamic oil-water interface. The reusability of fibres was also evident for several organic oils and dyes. These fibrous biowaste have considerable potential to be used as low-cost resource of high-value superwetting sorbent materials in industrial waste-water treatment technology.
at ambient condition. The flexibility of the fibrous membrane with a diameter of 2.54 cm provides a quick and facile installation on a mesh support for continuous oil-water separation. 2.2. Specific surface area (SSA) and porosity measurement The SSA of fibres was measured using methylene blue adsorption method by UV–Vis spectroscopy.[35] A known mass of fibres was added to 50 ml of methylene blue solution (0.02 g L−1). The mixed suspension of fibres and methylene blue solution was agitated for 24 hr in a shaker followed by a centrifugation to separate fibres from solution. Subsequently, the concentration of methylene blue in the solution was measured through UV–vis spectroscopy at a wavelength of 665 nm and determined based on the standard calibration curve for the methylene blue solution. The value of SSA can be calculated from the amount of adsorbed methylene blue by fibres according to the following equation Eq. (1):
SSA =
NA AMB (C 0 Ce)V MMB mS
(1) 23
where NA is Avogadro number (6.022 × 10 ), AMB is the covered area by a methylene blue molecule which is assumed to be 1.35 nm2, C0 and Ce are the initial and equilibrium concentrations of methylene blue, V is the volume of solution, MMB is the molecular mass of methylene blue and ms is the mass of fibrous. For porosity measurement, a series of optical images of the membrane were taken to process with ImageJ software following standard procedure reported earlier [36]. The imported image were sequenced for thresholding before analysis (e.g. set measurement and batch process measure). The thresholding was carried out using default setting and manual adjusting to incorporate the maximum degree of observed porosity. The analysed porosity percentage value for each image was saved to obtain the resultant mean porosity of the whole batch of images.
2. Materials and methods 2.1. Fibre extraction and membrane preparation The raw chestnuts were collected from grocery shop in South Australia to extract fibres. The fibres were found stacked between hard outer shell and papery-like inner linear as shown in Fig. 1a. The harvesting process involves (a) peeling off chestnut shell to expose fibre enriched area and, (b) extracting stacked fibres from fibre enriched area between inner wall and outer layer of the chestnut shell. Approximately ~0.1 g of fibres was harvested from a single chestnut. The weighted amount of extracted fibres was directly dispersed in organic solvent (i.e. toluene) and deposited on a metal mesh to prepare a thin fibre-film which resembles a freestanding fibrous membrane after a complete dry
2.3. Oil absorption, separation and reusability of chestnut fibres Batch absorption and continuous oil-water separation were performed for different organic solvents and vegetable oils. The oil Fig. 1. Fibre harvesting process and their interaction with oil, water, and dye. (a) The steps of the fibre extraction from waste chestnut-shell. (b) Adhesive superhydrophobicity of the fibrous surface, (c) Oil (Red colour-from Sudan III) and water (nocolour) droplet on fibrous surface. (d) Extracted fibres dispersed in oil phase (red colour) in a bottle, (e) The change in colour of fibres indicates their affinity to organic dyes (Sudan III). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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absorption by fibres is an adhesion of the organic solvents (adsorbate) onto the surface of a fibres (adsorbent) which eventually reach at a saturated state. The saturated adsorption capacity of fibres (total absorption) was examined for various oils and organic solvents at room temperature. The weighted amount (0.05 g) of dry fibres was introduced to the organic solvents (oils) co-existing with water on a Petridish to allow continuous adsorption for at least three times. According to literature, saturated adsorption capacity is defined as weight of oil or organic solvent adsorbed by per unit of weighted materials (g/g) and calculated by the following Eq. (2);
Q=
Wt1 Wt 0 Wt 0
The dye removal efficiency at equilibrium was calculated using the following Eq. (4),
E(%) =
m1 × 100 m0
Ceq) Co
× 100
(4)
where E (%) is the removal efficiency, C0 (µM) and Ceq (µM) represent the initial and equilibrium dye concentrations, respectively. 2.5. Materials characterisation
(2)
The morphological analysis of extracted fibres was performed using a scanning electron microscope (SEM-FEI Quanta 450, Japan) in a low vacuum chamber at an accelerating voltage of 5 kV. The presence of specific functional groups on fibre structure was studied by analysing vibrational and stretching modes of different molecular bonds using a Fourier transform infrared spectroscopy (FTIR) (Nicolet 6700, Thermo Fisher, USA) within a spectral range between 400 cm−1 and 4000 cm−1. Thermogravimetric analysis (TGA) was used to examine thermal decomposition of the compounds to identify the major composition of chestnut fibres using a TA Instruments (Q-500, Tokyo, Japan). The temperature was raised from 30 °C to 600 °C at a rate of 5 °C/min for the decomposition in nitrogen environment. The wettability of chestnut fibres was studied following a method of sessile drop water contact angle (WCA) using an Attension theta optical tensiometer (KSV instruments, Finland). The optical image of the membrane was captured by a Nikon Optical Petrographic Microscope (LV100 POL, USA) to analyse the porosity using ImageJ software (Version 1.47, National Institutes of Health, USA). All the supporting movies of different experimental demonstrations were recorded using a high definition video camera (Sony HDR-PJ260).
where Q is the saturated adsorption capacity (total absorption) of fibres and, Wt0 and Wt1 are the weight of fibres before and after saturated absorption, respectively. To determine the reusability of fibres, the saturated samples were squeezed, and oven dried at vacuum condition before introducing for further absorption tests. The as-prepared free-standing fibrous filter was soaked with 1 ml of oil before placing on a specially made metal mesh chamber in the filtration tube. The specially made filtration tube was installed at an inclination of 5° that allows a gravity driven separation through filtration tube (Fig. 5a). The filtrate was collected and compared with the weight before and after the separation to calculate separation efficiency (S (%)) using the following Eq. (3);
S(%) =
(Co
(3)
where m0 and m1 are the weight of the target filtrate liquid before and after the separation, respectively. The oil-water separation was performed introducing 5 M NaCl solutions with hexane to demonstrate chemical stability of the fibres in saline water. The stability of fibres was further tested by agitating them for 48 hr in acid (1 M HCl) and basic (1 M NaOH) solutions using a shaker at a speed of 200 rpm. To demonstrate the reusability of the used material, the fibres were heated up to the boiling point of organic solvents to evaporate them and used for the next oil/organic removal. The cycle of the total absorption and recovery was repeated 5 times to characterise reusability performance. Furthermore, a vacuum system setup used to reveal the absorption performance of the fibre in continuous manure for oil-water removal following the method used in our previous work [35]. Briefly, the fibres were packed in the plastic vials with a diameter of 1.5 cm and height of 6 cm. The vial was punched up to 5 cm in height with small holes (1 mm in diameter) to allow sorption of oil through the fibres. The top surface of fibres in the vial was covered with stainless steel wiremesh to prevent the uptake of fibres under vacuum pressure. Separated oil was collected in a flask connected through a tube to the vial.
3. Results and discussion To probe the initial properties of isolated fibres, the interaction of fibres with oil, water and dye were examined as presented in Fig. 1. A fibrous-layer (prior to the extraction) anchored with hard shell displays superhydrophobic behaviour as displayed in Fig. 1b and Movie S1, which holds the droplet even the shell is upside down indicating adhesive superhydrophobicity [2]. The adhesive state of superhydrophobicity is directly related to water impregnation into the micro/nano roughness on the surface to increase solid-liquid adhesion which is identical to rose petal effect [37]. The fibrous shell is further examined to investigate interaction with organic solvents. The primary outcome reveals that the surface has strong affinity to a series of organic solvents (e.g. toluene, hexane, turpentine) while repelling water droplet (Fig. 1c and Movie S2). The oil-water interfacial interaction of these fibres was also explored by placing them in an oil-water containing glass bottle (Fig. 1d). A gentle shake makes the fibres well dispersed in oil phase (red-colour containing organic dye Sudan III) on a clear water phase (Movie S2). However, a change in colour of oildispersed fibres was observed when the fibre was taken out of the oil revealing affinity to dyes (Sudan-III dye in oil phase) as shown in Fig. 1e. The water repellence and affinity to both oil and dye is an interesting visual outcome that motivated us to apply these extracted fibres for oil-water separation and dye adsorption application.
2.4. Dye adsorption test The adsorption of dyes was primarily examined at static condition to allow oil-water interfacial adsorption, wherein the adsorbent was dispersed in oil, and dyes (Methylene Blue and Rhodamine B) were mixed in water. The static adsorption process started with a little shake to make adsorbent perfectly placed at the interface in a 20 ml glass bottle which was monitored undisturbed for 60 hr. The digital photos taken at different time intervals to record the visual adsorption of dye at the interface. For quantitative study of dye removal, two aqueous suspensions of dyes were prepared using Methylene Blue and Rhodamine B with a concentration of ~15.34 µM and ~11.80 µM, respectively. Then the fibres at different dosage were introduced in the suspension at a neutral pH subjected to a continuous shaking at a speed of 100 rpm for 5 hr to facilitate dye adsorption. The suspension after each test were taken under UV–vis spectroscopy (Shimadzu UV-1601 (200–1100 cm−1) to capture the absoprtion spectra to identify the concentration change using standard calibration curve of these dyes.
Movie S1. Superhydrophobicity of the fibrous surface. 3
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Fig. 2. Characterisation of fibres. (a) SEM image of fibres, (b) Water and oil contact angle on fibrous surface, (c) FTIR of fibres, (d) Thermogravimetric analysis of fibres in nitrogen atmosphere, (e) Proposed mechanism of superwetting properties of chestnut fibres.
Fig. 2c. The appearance of bands at 2942–2920 cm−1 are related to CeH and CH2e vibration of aliphatic hydrocarbons, while the peaks at 1720–1706 cm−1 belong to carbonyl (C]O) groups from dimerized saturated aliphatic acids. The spectra in the range between 1650 and 1450 cm−1 are due to the aromatic ring stretching vibration. The presence of carbohydrate contents is also evident which is corresponding to the bands between 1200 and 1000 cm−1, however the OeH in plane deformation at 1420–1330 cm−1 is usually week for chestnut shell [38,39]. Thermogravimetric analysis was performed to examine thermal decomposition profile of fibres in nitrogen atmosphere (Fig. 2d). The first stage mass loss is due to moisture in the fibre structure which is as low as 7 wt%. Other major compounds of fibres were found identical to chestnut shell which is composed of cellulose, hemicellulose, lignin, and monosaccharides with other constituents of polyphenol compounds [38,40]. Lignin decomposes slower than cellulose and the hemicellulose components over a broader temperature range between 200 °C and 500 °C [41,42]. A second stage mass loss of 59.42 wt% between 180 °C and 350 °C is mostly attributed to the decomposition of cellulose, and hemicellulose, as well as partial decomposition of lignin [43]. Third stage mass loss is also observed which is due to the rest of the lignin
Movie S2. Oil-water interaction with fibres.
3.1. Characterisation of fibres The fibres were found flexible and bendable with an estimated extracted length of 2 ± 0.5 mm and a diameter of 15 ± 2 μm as shown in Fig. 2a. The rippled textures on fibre surface (inset photo, Fig. 2a) were also visible. The contact angle measurement on fibrous surface using water and oil droplet clearly indicates superhydrophobicity and superoleophilicity as shown in Fig. 2b. The nano-textures combining with low surface energy compounds on fibre is anticipated to create such superhydrophobicity. To identify corresponding functional groups related to low surface energy and oil-loving behaviour, the extracted fibres were examined by Fourier-transform infrared spectroscopy (FTIR) as shown in 4
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decomposition left at the end of second stage [42]. Polysaccharides such as starch and cellulose in chestnut fibres crosslinked by lignin which creates an obstacle for water absorption through fibre surface [44]. The lignin enriched area combining with the micro-ripple texture on the fibre surface are greatly responsible to absorb oil compounds as illustrated in Fig. 2e. Along with the aliphatic functional groups (Fig. 2c), the slow thermal decomposition of the lignin over a broad temperature range (Fig. 2d) is evidence of the lignin enrichment on fibre surface which is significantly responsible for superoleophilic properties. In biomass, polysaccharides in the cell walls are protected by lignin as their decay is carried out by microbial consumption as a food source, whereas lignin is transferred to soil as carbon storage [45]. Based on the finding in literature, it is anticipated that the lignin enriched stacked layer of chestnut fibres between inner papery linear and outer shell is to protect chestnut from microbial (e.g. fungus) attack as well as to control moisture level in the core of chestnuts.
Table 1 Viscosity and surface tension value of the oils at 21 ± 1 °C with corresponding adsorption capacity and saturated adsorption rate exhibited by chestnut fibres for batch absorption [35,47]. Organic solvents and Oils
Viscosity (cP or mPa s)
Saturated adsorption capacities (g/g)
Saturated adsorption rate (g/ g)/s
Toluene Canola oil Engine oil (SAE 10 W-40) Petrol Hexane Turpentine oil Olive oil
0.58 63.50 208.89
44.13 93.83 67.62
9.81 5.86 2.41
0.60 0.31 1.37 74.10
41.26 36.00 52.22 72.33
3.75 24.00 7.54 3.81
was the resultant surface adsorption and absorption in the interfibrillar gap. Later, the fibres were examined under a repetitive absorption test for at least 5 cycles wherein a slight decrease in absorption capacity for 5th cycle was recorded than the fresh fibres as shown in Fig. 3c. The comparison of the absorption capacity by different low-cost hydrophobic absorbents for a range of oils has been shown in Fig. 3d [46]. The absorption capacity of chestnut fibres is 2–10 times better than many other previously reported biowaste such as Cotton grass, Kapok fibres, Barely straw, Coconut coir etc [46]. The difference in absorption capacity among the materials is probably due to the dissimilarity in their SSA, surface textures and chemical composition. The high SSA combining with unique surface properties enabled chestnut fibres to outperform other low-cost absorbents presented in Fig. 3d.
3.2. Oil absorption and separation performance Prior to the demonstration of oil-water separation using fibrous membrane, the fibres were examined to investigate their SSA and total oil absorption capacity (saturated adsorption). Using a standard method of dye adsorption test (described in Section 2.2), the SSA of fibres were found to be 190.8 ± 4.5 m2/g. A certain amount of fibres (0.05 g) was introduced to oil co-existing with water in a glass petri-dish to realize the action of oil absorption as shown Fig. 3a and Movie S3. However, the test revealed that the absorption capacity is relatively lower for the oil with low viscosity than the one with higher viscosity as tabulated in Table 1 and presented in Fig. 3b. The saturation adsorption rate is found viscosity dependent of oils which was higher for relatively less viscous oil than that with high viscosity. The saturated adsorption rate for hexane is as high as ~24 gg−1/s whereas the rate is 2.4 gg−1/s for highly viscous engine oil. The fibres were readily saturated by low viscous oil due to their quick oil-uptake capability through the surface of fibres instead of the micro-pores among the cluster of fibres. The total absorption capacity of low viscous oil was mostly influenced by the well textured lignin enriched micro-ripples on the surface. On the other hand, the slow adsorption rate with higher absorption capacity of highly viscous oil
Movie S3. Batch Oil-absorption using chestnut fibres; Movie S4: Oil-water separation using membrane. Fig. 3. Oil absorption performance of chestnut fibres. (a) Saturated oil adsorption test for different type of oils using chestnut fibre-networks. (b) Repetitive performance of oil absorption for several cycles. (c) Oil absorption capacity. (d) Comparison of the absorption capacity by different low-cost hydrophobic absorbents [46].
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Fig. 4. Membrane set-up, porosity and stability measurement. (a) The dimension of fibrous membrane. (b) Mesh support for placing membrane in filtration tube. (c) Membrane placed on mesh support. (d) High resolution optical image of membrane. (e) Porosity analysis of membrane using ImageJ. (f) Examining chemical stability of fibres in acidic (1 M HCl) and basic solution (1 M NaOH) where the inset photo (bottom) showed no change in wettability of fibres.
Afterwards, with an approach to prepare a low-cost but novel membrane for oil-water separation technology, we deposited a layer of fibres on a metal mesh for continuous oil-water separation illustrated in Fig. 4a–c. The 1 mm thick membrane had a diameter of 2.54 cm placed on a metal mesh support. The porosity of the prepared membrane was further analysed using ImageJ software as displayed in Fig. 4d and e which was measured to be 44.5%. The chemical stability of the fibrous cluster was tested in acidic and basic solution with a pH range between ~0 and 14 scale using 1 M HCl and 1 M NaOH, respectively, as shown in Fig. 4f. The regain of superhydrophobicity of chestnut fibres after 48 h of agitation in acidic and basic solutions indicates their ability to perform in adverse environment. A series of organic solvents as well as vegetable oils were used to perform oil-water separation through filtration tube as presented in Fig. 5a. It was observed that the oil with very low viscosity provides instant separation while the oil with high viscosity (e.g. engine oil, olive oil, canola oil) requires longer period for separation (Movie S4) due to their resistance in mobility. However, the separation efficiency for all different type of oils were more than 92%, which is approximately 99% for the oils with very low viscosity (e.g. Toluene, hexane, petrol, turpentine) as shown in Fig. 5b. The oil-water separation was also performed introducing 5 M NaCl solutions co-existing with hexane to demonstrate chemical stability of the fibres in saline water (Movie S4). In addition, no change in wettability was observed (Fig. 4f) when the fibres were agitated in acidic and basic solution which indicates noticeable chemical stability of extracted fibres.
The continuous and pressure-driven separation of organic solvents was performed using a vacuum pump employing another experimental set-up as shown in Fig. 5c. As shown, the fibres packed in a plastic porous vial was connected to a vacuum pump through a hose and filtering flask. Being dominantly hydrophobic, the fibre-loaded porous tube does not allow any water inside the fibre-network to pass through the hose which were tested before demonstrating suction induced oilwater separation as shown in Movie S5. An artificial turbulence was created (resembling realistic condition) in oil-water mixture to justify whether the system can selectively separate oil in turbulence under external suction force as shown in Fig. 5d and Movie S5. Remarkably, the oil was separated from an oil-water mixture in continuous manner with higher efficiency which was simultaneously collected in the filtering flask. The dominant oleophilic properties (due to the presence of aliphatic and aromatic hydrocarbon in the structure) of fibres only allows oil to be penetrated through the fibre network under suction leaving the water behind in the beaker. The as prepared fibrous membrane and suction induced oil-water separation provides distinct advantages of these fibres over the commonly used synthetic materials reported for wastewater treatment.
Movie S5. Oil-water separation using pump.
Movie S4. Oil-water separation using membrane.
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Fig. 5. Continuous oil-water separation. (a) Continuous separation of toluene (red)-water (blue) under gravity in filtration tube. (b) Separation efficiency of fibrous membrane for various oils cos-existing with water. (c) Experimental set-up for continuous and high-speed separation using suction pump. (d) Separation of oil-water in turbulence. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.3. Dye-adsorption performance
demonstrate that it is not necessarily very important for materials to be hydrophilic to decolorize water. The recovered fibres from the oil-water interface are closely observed and found to display low contact angle (CA 72 ± 1°) than the unused fibres that implies that some segments of the fibres are still water loving (sticky). Furthermore, a dynamic dyeadsorption was performed with rigorous shake as shown in Movie S6. The dye adsorption under turbulence were demonstrated to observe whether a fast adsorption is achievable or not. The outcome in the presence of fibres at turbulent mode is impressive which exhibited an instant declaration capability of dye-contaminated water in a few seconds as shown in Fig. 6b and Movie S6.
As discussed, the change in colour of fibres was observed (Fig. 1e) which was primarily due to the adsorption of azo dyes (Sudan III) dispersed in oil. This phenomenon motivated to further investigate dye adsorption behaviour of fibres for water soluble cationic dyes (Methylene blue (MB) and Rhodamine B). We choose cationic dyes as they are used in various industries (e.g. wood, paper, textile, leather, medical, paper) and often released with industrial effluents [48]. Usually, the increase in initial dye concentration provides an increase in the capacity of the adsorbent due to the high driving force for mass transfer at a high initial dye concentration [30]. Therefore, we used low concentration of dyes to observe how these fibres perform to remove dyes even at very low concentration. Fig. 6a presents static adsorption behaviour of fibres at oil-water interface with respect to time. The fibres (in oil) are found to be highly capable to bind these cationic dyes (in water) from oil-water interface. The uptake of both Rhodamine B and Methylene blue dyes from water to fibres were obvious and nearly removed in 60 hr of adsorption period as shown in Fig. 6a. These results
Movie S6. Instant dye adsorption using chestnut fibres. 7
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Fig. 6. Static (slow interfacial adsorption) and dynamic (instant adsorption in turbulence) dye adsorption using chestnut fibres from water. (a) Dye adsorption by chestnut-fibres from oil-water interface in static mode, where the dyes are Rhodamine B (Pink) and Methylene blue (Blue) in water. (b) Instant (dynamic) dye adsorption in oil-water turbulence. (c) UV–vis absorption spectra. (d) Dosage dependent dye removal efficiency. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The qualitive results with both static and dynamic dye adsorption using chestnut fibres are very promising which are further evaluated by quantitative outcome presented in Fig. 6c and d. The individual UV–vis spectra of the aqueous dispersion containing Rhodamine B (~11.80 µM) as well as Methylene Blue (~15.35 µM) were captured before and after adsorption by different dosages of fibres as shown in Fig. 6c. The characteristic peaks of Rhodamine B and Methylene Blue were identified at the wavelength of 554 nm and 665 nm, respectively. The increase in fibre dosage decreases the adsorption peak intensity of the dispersion (conversely, increases dye removal effect). Approximately 90% dye-removal were achieved using a maximum fibre dosage of 0.75 g/L (88.1% removal for Rhodamine B and 91.5% removal of Methylene Blue) as presented in Fig. 6d. With an increased dosage of fibres, the removal efficiency of dyes gradually increases. The dosagedependent dye removal efficiency of different low-cost adsorbent
materials is tabulated in Table 2 for the comparison with chestnut fibres. The chestnut fibres were found to outperform other low-cost adsorbents to achieve maximum removal efficiency using least amount of dosage. Mechanism of interfacial dye adsorption properties of these hydrophobic fibres might be due to the combination of surface chemistry and roughness. Despite possessing aliphatic functional groups (which is responsible for hydrophobicity), the fibres also contain negatively charged carbonyl (CO) and hydroxyl (OH) functional groups as confirmed by FTIR (Fig. 2a) which might influence to generate adhesive superhydrophobicity (cassie-state) as shown in Fig. 1b and Movie S1. To elucidate the mechanism of dye adsorption, the micro- or nanostructure with a variable functional group including aliphatic hydrocarbon and oxygen functional groups can be impregnated by water or air. The regimes with high solid–water adhesion allows penetrating 8
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Table 2 Comparison of the dosage dependent dye removal efficiency by low-cost adsorbents. Materials
Dosage (g/L)
Dye name
Percentage of removal range (%)
Chestnut fibres (present work) Kaolin [49] Modified sawdust [50] Fly-ash [51] Tea waste [52] Activated carbon of corn-husk [53]
0.25–0.75
Methylene blue Rhodamine B Crystal violet Methylene blue Methylene blue Basic yellow 2 Methylene blue
56.2–91.5 60.5–88.1 75–97 34.4–96.6 45.16–96 19–60 ~67.5–98
0.25–4 1.5–5 8–20 2–20 0.1–0.5
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water into the microstructure [37]. This impregnation of water is the bridge between dye and fibres due to partial negative charges in the structure. Binding sites of the basic dyes used in this experiment could be those oxygen fictional groups (O-H, C = O) as Rhodamine B and Methylene blue are cationic by nature. From roughness point of view, the micro wrinkles containing dip and rise have variable affinity to water due the presence of both oxygen functional groups (OeH, C]O) and hydrophobic aliphatic groups (e.g. eCH2e, eCH3) [2]. The nanoripples with dip and sharp rise, consists of a complex of lignin and polysaccharide, are the bridge between water and oil phase to create an uptake of dyes from water to oil as shown in Fig. 2e. 4. Conclusion We presented, for the first time, superwetting microfibres isolated from chestnut-shell for continuous oil-water separation and dye adsorption. Owing to the presence of aliphatic and aromatic hydrocarbon in the structure (lignin enriched surface), the fibres were intrinsically oleophilic with an excellent oil absorption capacity of ~94 g/g. Furthermore, the fibres exhibited exceptional pliability to fabricate a flexible membrane for filtration process which was hardly achievable using other bio-waste. Results showed that these membranes have an outstanding oil separation efficiency (99%) subjected to gravity or an external suction force for the oils with low viscosity. In addition, the dye removal efficiency of fibres for two model dyes (Methylene blue and Rhodamine B) were found as high as ~91%. The partial negative charges (oxygen functional groups) on fibres were assumed to be responsible for adhesive hydrophobicity and cationic dye adsorption in oil-water interrace. This newly discovered bio-sorbent is low-cost, nontoxic and ready for use without chemical treatments which provides many advantages over other synthetic superwetting materials (e.g. carbon and polymer-based sorbents) introduced for oil- and dye contaminated wastewater treatment. Declaration of Competing Interest The authors declare no conflict of interest. Acknowledgements The authors thank the support of the Australian Research Council (IH 150100003 ARC Research Hub for Graphene Enabled Industry Transformation). Furthermore, we thank The University of Adelaide for providing access to necessary research facilities and supports. References [1] M.J. Nine, M.A. Cole, L. Johnson, D.N.H. Tran, D. Losic, Robust superhydrophobic graphene-based composite coatings with self-cleaning and corrosion barrier properties, ACS Appl. Mater. Interfaces 7 (2015) 28482–28493. [2] M.J. Nine, T.T. Tung, F. Alotaibi, D.N.H. Tran, D. Losic, Facile adhesion-tuning of superhydrophobic surfaces between “lotus” and “petal” effect and their influence on icing and deicing properties, ACS Appl. Mater. Interfaces 9 (2017) 8393–8402.
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