Letter pubs.acs.org/journal/ascecg
Oil Absorbents Based on Melamine/Lignin by a Dip Adsorbing Method Yu Yang, Huan Yi, and Chaoyang Wang* Research Institute of Materials Science, South China University of Technology, Guangzhou 510640, China S Supporting Information *
ABSTRACT: Effective removal of oils and leakage chemicals from water is of significance in oceanography, environmental protection, and industrial production. Materials that can reduce environmental pollution are in high demand. Herein, we have developed a facile synthesis of ultralight, highhydrophobic, and superoleophilic sponges (UHS sponges) through a dip adsorbing process based on lignin and commercially available melamine sponges. The obtained UHS sponges consist of an interconnected structure with high porosity and ultralow density (6.4 mg cm−3). As the hydrophobic carbon coating of the skeleton and its microstructure trapping the air, the UHS sponge exhibits high-hydrophobicity and superoleophilicity, which are beneficial to its applications in oil−water separation. Besides lignin, other biomass like tannin is also suitable as the modification agent to prepare UHS sponges via a dip adsorbing method. As a result, this novel sponge exhibits excellent oil/water separation performance such as high selectivity, good recyclability, and oil absorption capacities up to 217 times of its own weight or 99 vol % of its own volume. We believe that this dip adsorbing method resultant sponge is highly promising as an ideal oil absorbent in oil spill recovery and environmental protection. KEYWORDS: Lignin, Melamine sponge, Oil absorbent, Dip adsorbing, Oil spill cleanup
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INTRODUCTION Three-dimensional (3D) porous materials are a kind of solid materials containing pores with an in-filling fluid (liquid or gas) at the interfaces.1−3 Owing to their exceptional high porosity, large specific surface area, easily accessible pore structures, and adjustable surface properties, these materials have shown great application potential in storage, catalysis, separation, tissue engineering, and water purification.4−8 Furthermore, endowing some specific properties to the 3D porous materials could largely enhance and extend their practical application performances, such as magnetism, conductivity, compressibility, selfcleaning property, fire-resistance, catalytic activity, and photothermal effect.9−12 Very recently, incorporating hydrophobicity and oleophilicity to the 3D porous materials received extensive attention because the obtained architectures could effectively demonstrate selective absorption of oils from water, making it the most promising technique to counter increasing oil spills and chemical leakages.13−27 Traditionally, strategies for the hydrophobization of porous materials include (i) use of hydrophobic nanoblocks to form 3D porous materials, like carbon nanotube sponges fabricated via the chemical vapor deposition method;28 (ii) introduce low surface tension groups to the material surface, like grafting fluorine-containing groups or long alkyl groups on an existing porous material surface;29,30 (iii) construct a hierarchical multiscale porous structure on the surface, like lotus-leafinspired hydrophobic porous materials;31 (iv) remove the hydrophilic groups off the material surface, like chemical © XXXX American Chemical Society
reducing the graphene oxide aerogel or pyrolyzing cellulose aerogel in inert atmosphere at high temperature to eliminate the oxygen-containing groups.32,33 Abundant hydrophobic porous materials were successfully synthesized based on the above principles and exhibited remarkable oil selective absorption ability. However, the use of a large amount of expensive modifying agents, organic solvents, high energy cost, and the requirement of complicated and unusual fabricated devices seriously restricts their industrialization. Thus, great effort should be dedicated to developing a facile, cost-effective, and eco-friendly approach to prepare hydrophobic and oleophilic porous materials with excellent oil/water selective absorption ability. Recently, Nguyen et al. fabricated a superhydrophobic and superoleophilic graphene-based sponge through a novel hydrophobization approach by covering a hydrophobic coating on the porous material skeleton surface, namely a dip coating method.34 In this case, the dip coating times need to be carefully controlled to optimize surface wettability. Following the idea of this excellent work, several hydrophobic porous materials have been prepared.35 However, this method involves multiple operations, large doses of organic solvents, and generation of a large amount of acidic waste during the preparation of graphene oxide. Thus, it still remains Received: April 21, 2015 Revised: October 26, 2015
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DOI: 10.1021/acssuschemeng.5b01187 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Figure 1. Synthetic pathway of UHS sponges and their morphologies. The fabrication process of UHS sponges. By dipping melamine sponges (a1) into lignin aqueous solution for absorbing lignin (a2) and pyrolyzing in N2 atmosphere at 400 °C, the UHS sponges (a3) were obtained. Panels b1, b2, b3 and c1, c2, c3 are the corresponding different magnifying SEM images of the melamine sponges (a1), melamine/lignin hybrid sponges (a2), and obtained UHS sponges (a3), respectively. The scale bars in panels b1, b2, b3 and c1, c2, c3 are 200 and 5 μm, respectively.
promising as an ideal oil absorbent in the spill oil recovery applications and environmental protections.
a big challenge to fabricate hydrophobic 3D porous oil absorbents via a facile and eco-friendly process. Nowadays, there is a trend to produce 3D porous materials based on biomass resources, as their easiness to obtain, lowcost, environmental friendliness, and renewable properties, etc.36 Lignin, as a type of biomass material, is the most abundant natural phenolic polymer, and can be produced in large amounts by the paper industry.37,38 Here, we report a facile dip adsorbing route to construct an ultralight, highhydrophobic, and superoleophilic sponge (UHS sponge), derived from lignin and commercial available melamine sponge, as an ideal oil absorbent with excellent oil−water separation performance, high oil absorption capacity, and good oil recyclability. From the viewpoint of material design, the commercial melamine sponge is a desirable template for fabricating 3D porous materials, as it is composed of a micrometer-scale interconnected framework and possesses easily surface modification, high thermal stability, and good compressibility. Ruan et al. prepared a superhydrophobic sponge with excellent absorbency and flame retardancy from melamine sponges.29 Chen et al. successfully fabricated elastic carbon foam via direct carbonization of melamine sponge at 1800 °C, and the obtained architecture exhibited excellent performance as flexible electrodes and organic chemical absorbents. This remarkable work provides enlightened inspiration to realize the production of oil absorbents.39 Pham et al. also prepared robust, superhydrophobic sponges through the silanization of melamine sponges via a toluene solution-immersion process.30 Inspired by these achievements, we have developed a facile synthesis of UHS sponges through a dip adsorbing process based on lignin and commercially available melamine sponges. The obtained UHS sponges exhibited excellent oil/water separation performance such as high selectivity, good recyclability, and great oil absorption capacity. We believe that this dip adsorbing method resultant UHS sponge is highly
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EXPERIMENTAL SECTION
Materials. Melamine sponges were obtained from the local supermarket. Furfural residues were purchased from Tian Guan Furfural Co. Ltd. (China) and used with further cleaning with water and drying. Lignin was obtained from the Kraft pulping process that was reported in our previous work.38 The hydroxyl group of lignin used in this work is 2.189 mmol/g. Preparation of UHS Sponges. Melamine sponge was cut into pieces of 5.0 × 4.0 × 3.0 cm in size, and then cleaned with alcohol and distilled water for three times. After the the melamine sponge was dried in an oven at 60 °C, it was immersed into the alkaline solution of lignin (pH = 11) with a concentration of 1.0 wt % for 10 min; afterward, the lignin-adsorbed sponge was squeezed and dried in an oven at 60 °C to constant weight. Finally, the obtained lignin/ melamine hybrid sponge was pyrolyzed at 400 °C for 1 h under nitrogen atmosphere and the UHS sponge was achieved. Oils Remove Test. First, we measured the absorption capacity of the UHS sponge for various organic solvents and oils. The oil or organic solvent was poured into a beaker. UHS sponges were forced into the organic liquids for about 1 min and then picked out for measurements. To avoid evaporation of absorbed organic liquids, weight measurements were done quickly. The UHS sponge weights before and after absorption were recorded for calculating the values of weight gain. Regeneration of UHS Sponges. The regeneration of oiladsorbed UHS sponges includes absorption/squeezing and absorption/distillation ways. In adsorption/squeezing cycles, the oil-absorbed UHS sponges were employed for directly squeezing for recovering the oil, and then the no-oil UHS sponges were used in the next cycle. In absorption/distillation cycles, the oil-adsorbed UHS sponges were regenerated by heat treatment at a selected temperature around the boiling point of the oil, and then the dried UHS sponges were used in the further cycle. The weights of UHS sponges were recorded before and after each cycle to determine the absorption capacity. Materials Characterization. The sample structure was observed by scanning electron microscopy (SEM) images that were taken with a Zeiss EVO 18 scanning electron microscope equipped with a field B
DOI: 10.1021/acssuschemeng.5b01187 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering emission electron gun. The wetting properties of different samples were evaluated through contact angle tests, which were performed by the CAST2.0 contact angle analysis system at room temperature (OCA20LHT-TEC700-HTFC1500, Dataphysics, Germany). TGA was performed using a NETZSCH TG 209F3 instrument under a nitrogen atmosphere and a heating rate of 10 °C min−1 from 30 to 700 °C.
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RESULTS AND DISCUSSION As shown in Figure 1a, melamine sponge was cut into different shapes to imitate various practical requirements. Subsequently, the sponges were immersed into a lignin aqueous solution with a concentration of 1.0 wt %. Because of the positive charge on the sponge surface, melamine sponge could adsorb the negative lignin molecules in aqueous solution. After a saturated adsorption with an adsorption capacity of 0.205 g/g, a lignin layer was coated on the sponge skeleton surface resulting in melamine/lignin hybrid sponges. Finally, by pyrolyzing the asprepared hybrid sponge at 400 °C for 1 h, the UHS sponges were obtained (Figure S1). From the optical images of the sponges in different positions, the shape of the sponges was successfully maintained, except that the sizes of obtained UHS sponges had been scaled down. Each dimension decreased to be 72.9% of the original length, and the volume of the UHS sponge remained about 38.7 vol % of the pristine melamine sponge. It was demonstrated that the size and shape of the UHS sponges could be finely tuned by precisely adjusting the original melamine sponges. Furthermore, the microscopic change of the materials was observed by SEM (Figure 1b,c). It was revealed that lignin layers on the skeleton surface were generated as expected after the dip adsorbing process; after pyrolysis treatment, the 3D interconnected porous framework was successfully maintained and the lignin layers were carbonized, leading to carbon membranes coating on the skeleton surface. Furthermore, the robustness of the carbon coating was measured by ultrasonication in ethanol. SEM images showed that the carbon coatings were able to resist ultrasonication for 10 min without any peeling (Figure S2), revealing the existence of the strong adhesive force between carbon coating and sponge skeleton. As the 3D porous framework with abundant open-cell pores was maintained, the obtained UHS sponge successfully inherits the intrinsic ultralight property of the original melamine sponge (Figure 2a,b). The density was measured to be as low as approximately 6.4 mg cm−3, making the UHS sponge one of the lightest 3D porous materials with a density that is comparable to those of ultralight silica-, carbon-, polymer-, and metal-based structures.40−44 Therefore, a UHS sponge piece of about 77 mg and 12 cm3 could effortlessly stand on a grass stalk (Figure 2a). However, other than the hydrophilicity of the melamine sponge, the surface wettability of the UHS sponge had dramatically changed to be hydrophobic because of the pyrolysis treatment. To investigate the effect of temperature on the surface wettability of the obtained pyrolysis products, different temperatures were applied. The water contact angle (WCA) test revealed that the WCA of the sponges pyrolyzed at 300 °C was 0° and the WCA of the sponges pyrolyzed at 350 °C was 131.7° (Figure S3), whereas the sponge obtained at 400 °C exhibited great water repellence with a WCA of 142.5° (Figure 2c,d). As a result, the water droplet stayed as quasispheres on the surface of the UHS sponge. Enhancing the pyrolysis temperature did not have an obvious improvement to the sponge hydrophobicity (Figure S4). Therefore, we chose
Figure 2. Physical properties of UHS sponges. (a) Photograph of an as-prepared UHS sponge (mass = 77 mg) with a size of 2 × 2 × 3 cm standing on a piece of Marsilea quadrifolia grass. (b) Optical image of UHS sponge on the centimeter scale. The left one is a piece of freshly cut UHS sponge, showing that the hydrophobization was even all through this thick sample. (c) Water contact angle (left, 142.5°) and the toluene contact angle (right, 0°) of the UHS sponge. (d) Water droplet stayed as quasi-sphere on the surface of the UHS sponge, whereas toluene was readily absorbed.
the UHS sponge generated from melamine/lignin hybrid sponge pyrolyzed at 400 °C as the optimal sample. Furthermore, the high-hydrophobic mechanism of UHS sponge was further analyzed by a control experiment on a slice, which was prepared by compressing the carbonized lignin on a piece of adhesive tape. The water contact angle of the slice turned out to be only 65.2° (Figure S5), indicating that the surface carbon coating itself is not the sole contributor to the hydrophobicity of the UHS sponges (with a WCA of 142.5° and a sliding angle of 7.3°). According to the Cassie model, the microstructure could make a flat hydrophobic surface to be more hydrophobic as the trapped air cushion beneath the water droplet and more oleophilic owing to the capillary effect.45,46 In the case of highly porous UHS sponge, its 3D interconnected porous structure determines that the surface wettability is contributed by the hydrophobic carbon coatings on the skeleton surface and the trapped air in the micropores simultaneously. In terms of the Cassie equation,47 85.4% of the water droplet contact area on the sponge surface is occupied by air (Figure S5). Furthermore, the products achieved by pyrolyzing melamine sponge at 400 °C did not exhibit water repellences with a low water contact angle (0°), thus confirming that the carbon coating derived from carbonized lignin is also an essential factor for the hydrophobicity of the UHS sponge as it can effective prevent the water capillary penetration. In a word, the hydrophobicity is traced to the hydrophobic carbon coating on the skeleton surface and the trapped air that is due to its highly porous open-cell structure: The hydrophobic carbon coating prevents the water capillary penetration, and the trapped air enhanced its water repellence. It was demonstrated that the dip adsorbing method based on lignin is a facile approach to produce hydrophobic 3D porous materials. Besides the lignin, can other biomass agents also be successfully applied in this melamine sponge dip adsorbing system? As the above discussion, the key factor of the hydrophobization of sponge is giving a hydrophobic coating to the skeleton surface while maintaining its microstructure for the air-trapping. It could be concluded that water-soluble, C
DOI: 10.1021/acssuschemeng.5b01187 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Figure 3. (a) Fabrication process of fabrication of the hydrophobic sponges via dip absorbing method based on tannin and melamine sponge. (b) Optical image of a water droplet on the surface of the sponge staying as quasi-sphere, exhibiting great water repellence. (c) SEM images of the obtained sponges in different magnification times. Energy disperse spectroscopy (EDS) analysis revealed that the obtained UHS sponges consisted of 43% of carbon, 46% of nitrogen, and 11% of oxygen.
Figure 4. Oil−water separation performance of UHS sponges. (a) Remove toluene (dyed with Sudan I) from water surface by UHS sponge and followed by simply squeezing the oil-absorbed sponge to recover the absorbed toluene. (b) Collection of chloroform (dyed with Sudan I) from the bottom of water by UHS sponge. (c) Mass-based and (d) volume-based oil-absorption capacities of the sponge. Absorption recyclability of the UHS sponge by using (e) squeezing method to recover oils with high boiling points, and (f) distillation method to recover oils with low boiling points.
negative-charged, carbonizable molecules are satisfied, such as tannin, gallate acid, gallogen, quercetin, citexin, and so on. Here tannin was used as an example (Figure 3). Just as in the lignin case, tannin/melamine hybrid sponges were prepared by the dip adsorbing process. Followed by the pyrolysis treatment, hydrophobic and superoleophilic sponges were achieved as
expected, demonstrating that this hydrophobic modification strategy has a certain universal applicability. Recently, owing to the severe environmental and ecological issues arising from oil spills and toxic chemical leakage, the separation of oil from water presents a worldwide challenge to save the endangered environment. The UHS sponges, as the D
DOI: 10.1021/acssuschemeng.5b01187 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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UHS sponge through the corresponding recycling process: absorption/distillation cycling or absorption/squeezing cycling means. And this recycle ability is comparable to most those of previous oil absorbents.52,53
above confirmed, possessing high porosity, high-hydrophobic surface, and 3D porous interconnected structure, were expected as an ideal oil absorbent for the effective removal of the spilled oils from water. First, the selective oil sorption capability of the UHS sponge was systematically demonstrated: By forcing a piece of UHS sponge to the toluene layer on a water surface, the oil was soaked up in several seconds (Figure 4a, Movie S1). In addition, the sponge was immersed into the water for quick absorbing of chloroform that sank to the bottom of water (Figure 4b, Movie S2). It indicated that no matter oils with densities lower or higher than that of water, the UHS sponges could rapidly separate them out of the water phase. In addition, various oils and organic solvents were applied for separation test to investigate the comprehensive oil absorption capacities of the UHS sponges, such as hexane, gasoline, toluene, sunflower oil, hexadecane, which are common oily pollutants in daily life or the chemical industry. The results revealed that the mass-based absorption capacities of UHS sponges ranged from 98 to 217 times of its own weight (Figure 4c), depending on viscosity, density, and surface tension of the absorbed liquids. As a result, it is much higher than other previous oil sorbents such as conjugated microporous polymers (6−23 times),13 graphene/R-FeOOH aerogel (12−27 times),48 PU sponges (13−26 times),49 and nanocellulose aerogels (20−40 times),50 but are close to carbon nanotube sponges (80−180 times),28 and twisted carbon fiber aerogels (50−192 times),51 indicating that the present UHS sponges exhibit as one of the absorbents with the highest mass-based absorption values. Although the mass-based absorption capacity highly depends on the density of absorbed oils and absorbents, volume-based absorption capacity (Voil/Vabsorbent) might be a more appropriate parameter to characterize the absorbent absorptive capability in practical applications. As shown in Figure 4d, the volume-based oil absorption capacities of the UHS sponges for various oils are higher than 93%, which is comparable to the sponge porosity (>99%), indicating that almost all the pores in the sponge are used for oil storage. Besides the water−oil separation ability, recyclability is also a key criterion to characterize the performance of oil removers in oil cleanup applications. In the case of the present UHS sponges, oils or organic solvents could be harvested through manually squeezing the sponge or distillation process. In the absorption-squeezing cycling test, especially for the recovering oils with high boiling points, hexadecane with a high boiling point of 287 °C was applied as an example (Figure 4e). The oil absorption capacity of the second cycle turned out to be about 76.7% of the first cycle, which could be traced to the residual oils inside the sponges and the incomplete recovery of the compressed sponges. After the second cycle, the oil absorption capacity decreases slightly, as the loss is only ascribed to the volume contraction of the sponge under the compression treatment. After five cycles, the oil absorption capacity of the UHS sponge still kept about 59.7% of the original value, exhibiting good recyclability (Figure 4e and Figure S6). Although oils or organic solvents with low boiling points could be desorbed by distillation methods, in which the absorbed sample was heated at the boiling point to release the absorbed liquid and collected the vapor of the liquid for recycling. As shown in Figure 4f, even when the sorption− evaporation process was repeated for five times, no obvious change of absorption capacity was observed, indicating excellent recyclability. That is to say, oils with low or high boiling points could be effectively recovered by our present
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CONCLUSIONS In summary, we have developed a facile synthesis of ultralight high-hydrophobic and superoleophilic sponges through a dip adsorbing process based on lignin and commercially available melamine sponges. The obtained UHS sponges consist of an interconnected structure with high porosity and ultralow density (6.4 mg cm−3). As the hydrophobic carbon coating of the skeleton and its microstructure trapping the air, the UHS sponge exhibits high-hydrophobicity and superholeophilicity, which is beneficial to its applications in oil−water separation. Besides lignin, other biomass like tannin was also suitable as the modification agent to prepare UHS sponges via a dip adsorbing method, demonstrating the general applicability of our dip adsorbing method. The commercial and natural available source and simple preparation method make the UHS sponge costeffective for possible industrial application. As a result, this novel sponge exhibits excellent oil/water separation performance such as high selectivity, good recyclability, and oil absorption capacities up to 217 times its own weight or 99 vol % its own volume, which is comparable to the previous oil absorbents with the maximum absorption capacities. Therefore, we believe that this dip adsorbing method resultant UHS sponge is highly promising as ideal oil absorbents in spill oil recovery applications and environmental protection.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01187. Details of synthesis and characterization of UHS sponge; TGA spectra of lignin and melamine sponge; SEM images of the ultrasonic-treated UHS sponge; WCA of sponges obtained from different temperature; WCA of the carbonized lignin slice (PDF). Process of the UHS sponge absorbing toluene (dyed with Sudan I) from the water surface (AVI). Process of the UHS sponge absorbing chloroform (dyed with Sudan I) from the bottom of water (AVI).
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AUTHOR INFORMATION
Corresponding Author
*C. Wang. E-mail:
[email protected]. Author Contributions
The paper was written through contributions of all authors. All authors have given approval to the final version of the paper. Funding
National Natural Science Foundation of China (21274046 and 21474032); National Basic Research Program of China (973 Program, 2012CB821500); Natural Science Foundation of Guangdong Province (S20120011057) Notes
The authors declare no competing financial interest. E
DOI: 10.1021/acssuschemeng.5b01187 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21274046 and 21474032), the National Basic Research Program of China (973 Program, 2012CB821500) and the Natural Science Foundation of Guangdong Province (S20120011057).
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ABBREVIATIONS 3D, three-dimensional EDS, energy disperse spectroscopy SEM, scanning electron microscopy UHS sponge, ultralight, high-hydrophobic, and superoleophilic sponge WCA, water contact angle
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DOI: 10.1021/acssuschemeng.5b01187 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Letter
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DOI: 10.1021/acssuschemeng.5b01187 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
