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corrosive water and emulsion separation
Accepted Manuscript Flame-retardant superhydrophobic coating derived from fly ash on polymeric foam for efficient oil/corrosive water and emulsion separation Jintao Wang, Hongfei Wang, Guihong Geng PII: DOI: Reference:
S0021-9797(18)30454-5 https://doi.org/10.1016/j.jcis.2018.04.069 YJCIS 23534
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
25 March 2018 14 April 2018 17 April 2018
Please cite this article as: J. Wang, H. Wang, G. Geng, Flame-retardant superhydrophobic coating derived from fly ash on polymeric foam for efficient oil/corrosive water and emulsion separation, Journal of Colloid and Interface Science (2018), doi: https://doi.org/10.1016/j.jcis.2018.04.069
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Flame-retardant superhydrophobic coating derived from fly ash on polymeric foam for efficient oil/corrosive water and emulsion separation Jintao Wang a,*, Hongfei Wang b, Guihong Geng a,* a
College of Materials Science and Engineering, North Minzu University, Yinchuan
750021, P.R. China b
Suzhou Wuwei Environmental Technology Co., Ltd., Suzhou 215100, P.R. China
Corresponding author. E-mail addresses: [email protected] (J.T. Wang), [email protected] (G.H. Geng).
ABSTRACT Developing novel oil sorbents with both superhydrophobicity and flame resistance reveals an enticing prospect for oil/water separation. In this study, superhydrophobic foam with superior flame retardancy and sorption capability is reported through a simple one-step fabrication route in alkaline water/ethanol system containing dopamine, fly ash (FA) and dodecanethiol (DT). The introduction of FA endows the foam with excellent flame retardancy, and the as-prepared foam reveals improved flame resistance compared with original and polydopamine (PDA) coated foams, The obtained foams can quickly absorb various types of oils up to 34-47 times of their own weight, and the absorbed oils can be repeatedly recovered by a simple vacuum filtration process. The foams can also maintain their high hydrophobicity after long term immersion in different corrosive solutions and oils, and are able to be used for removing the oils from corrosive high-temperature water. More importantly, the foams with FA coating can effectively separate a broad range of oil-in-water emulsions with high efficiency (>93.0%). The outstanding separation property of the as-prepared foams and their eco-friendly, low-energy, and inexpensive fabrication process imply the great potential for oily wastewater treatment and oil spill cleanup.
Keywords: Fly ash; PU foam; Flame retardancy; Sorption; Harsh condition; Emulsion
1. Introduction In recent years, water contamination resulting from the spillage of oil products and industrial organic contaminants has posed a detrimental impact on the global ecological environment and human health [1,2]. To address this problem, various approaches including sorption [3-5], filtration membrane [6,7], in-situ burning [8], bioremediation [9], and dispersant [10], have been developed to remove oils or organic solvents from water. Among these reported methods, the sorption removal of oil using sorbent is considered to be one of the most effective methods owing to the recyclability of oils and organic solvents. So far, a variety of oil sorbents, such as nanosilica modified cotton fibers [11], carbon aerogels from poplars catkins [12], reduced graphene aerogel [13], polyvinyl alcohol/cellulose nanofibril hybrid aerogel microspheres [14], electrospun cellulose acetate nanofibrous mats [15], and melamine-derived carbon sponges [16], have been prepared for the sorption and removal of oil pollutants. However, high preparation cost, complex preparation process, and poor recyclability of the aforementioned sorbents restrict their large-scale production and applications. Therefore, it is of importance to develop novel sorbents with low cost, good recyclability, and superb sorption performance to address the global-scale of water contamination arising from spilled oil. Foam, a type of cheap three-dimensional porous material, is a potential substrate for oil/water separation [17]. In recent years, superhydrophobic and superoleophilic foams have aroused wide concern owing to their fast and highly efficient cleanup of oil and organic contaminants from water. For instance, Gao et al. prepared a
hydrophobic/oleophilic carbon soot coated melamine foam for oil cleanup via a dip-coating method, and the carbon soot was synthesized by an ethylene-oxygen combustion flame [18]. Wang et al. developed superhydrophobic carbon nanotubes reinforced polyurethane (PU) foam for selective oil/water separation [19]. Zhang et al. synthesized MnO2 nanowires via hydrothermal method and then MnO2 nanowires/PU foam composites for absorbing oils from water surface were fabricated though foaming technology [20]. Cao et al. firstly prepared superhydrophobic nanodiamonds via
coating
polydopamine
and
subsequent
reaction
with
1H,1H,2H,2H-perfluorodecanethiol, and then superhydrophobic foam was obtained via decoration of the modified nanodiamonds onto the skeleton of commercial PU foam [21]. Although these studies have demonstrated that superhydrophobic foams with various synthesized nanoparticles-based coating are ideal candidates for oil cleanup operation, none of the aforementioned foams have been proven to possess flame resistance. In addition, the reported foam-based sorbents can only be utilized for removing oil layer from water and cannot effectively separate emulsified oil/water mixture. Hence, it is very urgent to construct foam-derived sorbent that possess flame retardant nature, inexpensive superhydrophobic coating, and effective separation performance for emulsified oil/water emulsion. FA is a waste substance from the burning of fossil fuel, municipal waste, coal, etc. that is found in abundance in the world. In recent years, the utilization of FA has received much attention in public and industry, which will be conductive to reduce the environmental burden. Despite available approaches, only a small amount of FA has
been used in commercial application, the rest being buried in landfills or stored in holding ponds pending burial [22]. Fly ash appears as a relatively fine powder with rounded particle diameters mainly between 0.1 and 400 μm [23]. The assembly of micro/nano-scale particles can form hierarchical coating on the surface of substrates. Nevertheless, up to now, FA has received little attention on exploring its potential applications in the formation of hierarchical coating with flame retardancy. In our previous paper, superhydrophobic fiber and fabric have been prepared via creating hierarchical coating composed by ZnO nanoneedles, irregular ZnO nanosheet, or Cu nanoparticles onto the substrate surfaces under hydrothermal or chemical reduction conditions [24-26]. In this study, a type of cost-effective superhydrophobic PU foam was developed though a simple one-step reaction in alkaline water/ethanol medium of containing dopamine, FA and DT. By using solid waste FA as coating material, the as-prepared foam shows improved flame-retardant property as compared to the original and PDA coated foams. The decorated foam can maintain its superhydrophobicity under extremely harsh (highly acidic, alkaline, salty solutions) and turbulent water environment. The superhydrophobic foam is able to absorb a wide range of oils and organic solvents from both pure water and corrosive solutions with high selectivity, and the harvested foam can maintain its superhydrophobicity after repeating the separation process for 15 cycles. More importantly, the coated foam is able to separate various oil-in-water emulsions with a separation efficiency of higher than 93.0%. Thus, the as-fabricated foam is a promising candidate for wastewater treatment related to oil pollution.
2. Experimental 2.1. Materials PU foam was purchased from market, Yinchuan, China. Fly ash was provided by Shenhua Ningxia Coal Industry Group, Yinchuan, China. Dopamine hydrochloride (98%) was supplied by Nanjin Aoduo Biotechnology Co. Ltd., China. Toluene (99.9%), n-hexane (99.9%), chloroform (99.9%), kerosene (95%), NH3·H2O (25%) were obtained from Ningxia Yaoyi Chemical Reagent Co. Ltd., China. Dodecanethiol (DT, 97%) was provided by Nanjin Chengong Organosilicon Co. Ltd, China. Gasoline and diesel came from the local market, Yinchuan, China. 2.2. Preparation process of PDA/FA/DT coated foams Before using, PU
rinsed with ethanol FA (0.2 wt.%) and dopamine hydrochloride (2 mg/mL)
were added into the mixture of
to vigorously stir
(1000 rpm) at 30 ℃ for 30 min. Then, DT (2mM) and PU foams (the mass ratio of foam, dopamine hydrochloride and DT, 5:20:2) were placed into the above dispersion. After the
ammonia aqueous
solution, the system was stirred for 16 h at 30 ℃. Finally, the obtained foams were removed from the mixture, washed with plenty of distilled water, and dried to constant weight at 80 ℃. The schematic of the fabrication process of PDA/FA/DT decorated foam is also displayed in Fig. 1a. 2.3. Measurements of oil sorption capacity A piece of PDA/FA/DT coated foam weighed beforehand was put into a beaker
containing oil at room temperature. The foam was removed from the oil after reaching sorption saturation. The oil sorption capacities of the foam were measured by weighing the foams before and after the sorption, and calculated by the following equation: Q = (mt – mi ) / mi where Q is the oil sorption capacity (g/g) of the sorbent calculated as grams of oil per gram of sample, mt is the weight of saturated sorbent (g), mi is the initial weight of sorbent (g). 2.4. Characterizations Water contact angle measurements were carried out using a Krüss DSA 100 (Krüss Company, Ltd., Germany) apparatus at 30 ℃, and the volumes of probing liquids in the measurements were approximately 5 μL. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet NEXUS FTIR spectrometer using KBr pellets. The KBr pellet was prepared via pressing the ground mixture of sample (2 mg) and KBr powder (100 mg) at 25 ℃ on hydraulic machine (10 Mpa). The micrographs of samples were examined using SEM (JSM–5600LV, JEOL) at an acceleration voltage of 5.0 kV. Before SEM observation, all samples were fixed on aluminum studs and coated with gold. 2. Results and discussion 3.1. Wettability The surface wettability of the obtained PDA/FA/DT coated foam is illustrated in Fig 1b. Obviously, the blue-colored water droplet on the surface of decorated foam
maintain a stable spherical shape with a water contact angle of 161°, whereas it sinks into the surface of original foam to form a large spreading radius, implying the wettability transition of the foam from hydrophilicity to superhydrophobicity. The superior hydrophobicity of the as-prepared foam should be attributed to the micro/nano roughness created by FA microparticles, PDA nanoparticles, and macroscopic voids of the three-dimensional foam. When the red-colored oil (kerosene) is dropped onto the surface of the coated foam, it can be completely wetted by the oil within 1 s and the oil contact angle is measured to be 0°, confirming excellent superoleophilicity of the foam. As illustrated in Fig. 1c, when the PDA/FA/DT coated foam is cut into two pieces, the exposed new surface still possesses good water repellence with a water contact of over 155°, suggesting that the internal surface of as-prepared foam is also modified completely rather than just the external surface. In addition, when the coated foam is put into water, it can float on the water surface owing to its high water-repellence, but the original PU foam submerges into the water because of its hydrophlicity (Fig. 1d). When the PDA/FA/DT coated foam is forced to enter into the water, a silver mirror-like thin film on the foam surface can be observed owing to the trapped air layer between the rough surface and water (Fig. 1e). Once the external force is released, the decorated foam would rise to the water surface immediately and the weighing result reveals almost no water is absorbed by the removed foam. [Fig. 1] The effect of FA concentration on water contact angle and oil sorption capacity of
the decorated foam was systematically investigated (Fig. 2). As displayed in Fig. 2a, the water contact angle increases with the increase of FA concentration, and it reaches the maximum of 161° at the FA concentration of 0.2 wt.%, implying the ultrahigh hydrophobic property of the coated foam. With the further increase of the FA concentration, an obvious decrease in the water contact angles can be found. The relationship between the FA concentration of preparing samples and oil sorption capacity is illustrated in Fig. 2b. It can be seen that the sorption capacity of the coated foam for kerosene increases with the FA concentration, and it achieves the optimal value of up to 34.7 times the coated foam weight at the FA concentration of 0.2 wt.%. The aforementioned results reveal that the enhancement of hydrophobicity via increasing the FA loading ratio is beneficial to the improvement of oil sorption capacity. However, excessive content of FA bring about a decrease of oil sorption capacity. This may be ascribed to the fact that excessive FA particles in the foam will act as the filler to reduce the porosity of the foam and increase the weight of the coated foam in a unit volume [18]. Accordingly, the oil sorption capacity of the obtained foam is decreased. [Fig. 2]
3.2. FTIR spectra analysis FTIR spectra of original PU foam and PDA/FA/DT foam are presented in Fig. 3. The original PU foam displays absorption bands at 3135, 2920-2850, 1491-1441, 1173-1005 cm-1, which are attributed to the N-H stretching vibration, C-H stretch vibration of -CH3- and -CH2-, C-H deformation vibration of -CH3- and -CH2-, and
C-O stretching vibration, respectively [4]. All of these characteristic absorption bands reveal that the foam is a kind of PU-based material. In contrast to the original foam, the absorption band intensity of PDA/FA/DT foam at 2920-2850 cm-1 exhibits a remarkable enhancement, suggesting the introduction of DT on the foam skeleton. In addition, the increase of absorption peak and the appearance of new absorption peak at 795-607 cm-1 can be clearly observed, which is ascribed to the symmetrical stretching and bending vibration of Si-O-Si in FA. Hence, the aforementioned analyses indicate that the PU foam has been successfully decorated by FA and DT. [Fig. 3] 3.3. SEM and element mapping analysis The surface appearance of original foam and as-fabricated PDA/FA/DT foam were observed by SEM. The SEM images of original and PDA/FA/DT foams with different magnifications are displayed in Fig 4. It can be found that two kinds of foams have three dimensional macroporous structure (Fig 4a and 4e). The magnified images of original foam demonstrate a relatively flat skeleton surface (Fig 4b-d). In the case of PDA/FA/DT
foam,
the
skeleton
surface
becomes
rougher
and
micro/nano-agglomerates are completely attached to the foam framework, as shown in Fig 4f and 4g. It is believed that the self-polymerization of dopamine offers a strong impetus for the establishment of FA on the large interconnected pore surface of PU foam [2,11]. In Fig 4h, SEM image with higher magnification illustrates the skeleton surface of PU foam possesses the hierarchical substructure composed of FA and PDA particles, clearly differing from original PU foam in Fig 4d. The hierarchical
PDA/FA coating and microporous foam structure synergistically contribute to the hydrophobicity of the as-obtained foam. The element mapping images of PDA/FA/DT foam are shown in Fig 4i. EDS elemental mapping analysis confirms the co-existence of C, Al, Si, and S as primary elements and their uniformly dispersion, implying that FA and DT distribute onto the surface of the foam. [Fig. 4] 3.4. Flame resistance In recent years, a large amount of superhydrophobic PU foams based on different coatings have been developed for oil sorption. Nevertheless, these reported PU foams are easily ignited and turn into ashes in a short time. Meanwhile, oils and organic solvents are also flammable or highly flammable. The flammability of both sorbent and adsorbate undoubtedly increases the risk of combustion and explosion in dealing with oil accidents, Hence, the development of porous oil sorbents with both superhydrophobicity and flame resistance is urgent for practical applications [27]. Unfortunately, there are few works on endowing the superhydrophobic oil sorbents with flame resistance. Herein, a type of superhydrophobic foam with flame retardancy was prepared via a facile one-step route. A simple combustion experiment is used to evaluate the flame retardancy of the PDA/FA/DT coated foam. For comparing, original PU foam and the foam only modified by PDA/DT without FA are also subjected to the test of burning behavior. As displayed in Fig. 5(a-a4), the original foam burns rapidly after being ignited and almost all foam burns up thoroughly in 9 s, leaving only a small amount of residue at the end. Compared with the original foam, the PDA/DT modified foam exhibits an improved flame resistance. As shown in Fig.
5(b-b4), although the PDA/DT modified foam also is easily ignited and burnt to ashes, the burning process can last 25 s, which is nearly three times longer than the original foam. For the PDA/FA/DT decorated foam, an entirely different phenomenon is observed as illustrated in Fig. 5(c-c4). The superhydrophobic foam with FA coating burns with slight flame and the combustion stops at 9 s. After the extinction of the flame, the foam retains its basic skeleton and most of the foam does not burn at all, demonstrating the improved flame retardancy. This may be explained by the fact the incombustible FA layer isolates the flammable PU foam skeleton. In addition, the PDA/FA/DT decorated foam is used to absorb kerosene and the burning behavior of the toluene-loaded foam was investigated. As shown in Fig. 5(d-d4), the foam with kerosene can burn for 175 s, and a large amount of foam still can be left after the flame extinguishes. The above results further confirm that the superhydrophobic foam with FA coating has good flame-retardant property. As a result, the probability of fire can be decreased via using the as-prepared foam as sorbent for removing oils and organic solvents. [Fig. 5] 3.5. Oil sorption capability The sorption capacity of the PDA/FA/DT foam prepared at the FA concentration of 0.2 wt.% for a broad range of oils and organic solvents was investigated. As displayed in Fig. 6a, the as-prepared foam shows sorption capacities ranging from 34 to 47 times their initial weight for various oils and organic solvents, and the difference in the sorption capacity is mainly dependent on the viscosity and density of the oils and
organic solvents [4,28]. For instance, due to high density of chloroform and high viscosity of diesel, the PDA/FA/DT coated foam demonstrates sorption capacity of 47 g/g and 37.8 g/g in chloroform and diesel, respectively. The sorption capacities are also observably superior to those of porous materials having been reported recently, such as magnetic porous carbon aerogel converted from biorenewable popcorn (<10.8 g/g) [29], CTAB-modified polymethylsilsesquioxane aerogels (<4.7 g/g) [30], fluorinated polydopamine/chitosan/reduced graphene oxide composite aerogel (<16 g/g) [31], cupric stearate coated sponge
(<26.6 g/g) [32], and comparable to those of
lignin/agarose/poly(vinyl alcohol)-based honeycomblike aerogel (<40 g/g) [33] and polyurethane foams composites modified with MnO2 nanowires (<40.2 g/g) [20]. Although the as-prepared foam exhibits lower sorption capacity than that of carbon aerogels from poplars catkins (<161 g/g) [12], the process for fabricating PDA/FA/DT coated foam is simpler, cheaper and more environmentally friendly. The recyclability of the as-prepared foam was also evaluated using toluene and kerosene as model oils. In the recycling tests, the absorbed oils are removed from the coated foam via two approaches: vacuum filtration and mechanical squeezing. The vacuum filtration cannot cause severe damage to the porous structure of the fabricated foam compared with mechanical squeezing. The recycle times for sorption versus the sorption capacity for the two oils is displayed in Fig. 6b,c. In either oil-removal method, the sorption capacity of the coated foam does not present significant decrease after 15 cycles of sorption-desorption process, and approximately 91.7% and 85.7% of the initial sorption capacity is retained for vacuum filtration and mechanical
squeezing removal manner, respectively, suggesting the good recyclability. The result also implies the FA coating is firmly immobilized onto the skeleton of foam and it cannot be easily destroyed by the strongly external force as mechanical squeezing. [Fig. 6] 3.6. Durability The durability of the PDA/FA/DT coated foams was investigated via water contact angle measurement after immersing into corrosive aqueous solutions and oils for 12 h. As illustrated in Fig 7a, the droplets (dyed with methylene blue) of 1 M HCl, 1 M NaOH, 1 M NaCl solutions and hot water (90 ℃) exhibit spherical shape on the surface of the coated foam, and the water contact angles are greater than 151°. Moreover, the water contact angles of the coated foam are always higher than 146° after being immersed into 1 M HCl, 1 M NaOH, 1 M NaCl aqueous solutions for different times (Fig. 7b). Especially, after the coated foams are immersed in salty solutions (1 M NaCl) with different pH values for 24 h, no obvious decrease in water contact angles is observed (Fig. 7c). The wettability of the foam was also evaluated after being immersed in different oils at 30 ℃ for 10 min. It is obvious that the resultant foams can maintain their high hydrophobicity after being immersed by various oils (Fig. 7d). The above results indicate the as-prepared foams possess outstanding resistance to corrosive solutions and oils, and can be utilized for removing oil under harsh water environments. [Fig. 7] 3.7. Oil/corrosive water separation performance
Due to porous structure, light weight, and ultrahigh hydrophobicity, the PDA/FA/DT coated foams were applied to the selective sorption of oils from corrosive aqueous solutions. To study the oil removal behavior under harsh conditions, three corrosive water, including 1 M HCl, 1 M NaOH, and 1 M NaCl solutions, are prepared to simulate various practical wastewater. As displayed in Fig. S1, no matter what type of corrosive solution is used, once the coated foam is put into contact with the kerosene layer (dyed with oil red O), all of oil will be immediately and completely absorbed into the coated foam. After the removal of oil-loaded foam, the water surface becomes clear. Moreover, the underwater high-density oil (chloroform) can also be quickly and selectively removed from these corrosive solutions via using the as-prepared foams (Fig. S2). During the sorption of oil pollutants both on the surface of corrosive solutions and in the bottom of the solutions, no water uptake is observed, indicating superior sorption selectivity of the coated foams. In general, industrial oily wastewater and oil spillage needs to be treated on the site under corrosive high-temperature water environments. Hence, the oil/water separation capability of the coated foam was also evaluated under the conditions of simulating industrial oily wastewater environments. To form corrosive high-temperature water environments, kerosene (dyed with oil red O) is chose to mix with hot acidic, alkaline, and salty aqueous solutions (80 ℃). As displayed in Fig. 8(a-c), for all the aforementioned kerosene/corrosive hot solution mixtures, kerosene can be absolutely absorbed and separated by the foam within several seconds. For evaluating the removal effect of the coated foam toward the oil under corrosive hot solutions, the chloroform dyed by oil
red O is used to mixed with hot (80 ℃) 1 M HCl/1 M NaCl and 1 M NaOH/1 M NaCl solutions, respectively. As displayed in Fig. 8d,e, after the PDA/FA/DT coated foam is placed into the two mixtures, the underwater oil can also be completely absorbed. These results suggest the as-prepared foam has potential for use in the separation of industrial oily wastewater. [Fig. 8] 3.8. Oil-in-water emulsion separation ability Because of the selective oil sorption capability, the PDA/FA/DT coated foam was applied to harvest oil microdroplets from surfactant-free n-hexane, toluene, chloroform, kerosene, gasoline, and diesel emulsified in water (Voil:Vwater = 1:10). A piece of coated foam is forced to contact with oil-in-water emulsions under vigorous stirring, and then the emulsion will become clear and transparent slowly within several minutes. Fig. 9a reveals the separation effect of the foam for different oil-in-water emulsions in the form of digital images. In contrast to the original milky emulsions (Fig. 9a, top), the obtained filtrates are relatively clear and transparent after the separation (Fig 9a. bottom). Moreover, taking kerosene-in-water emulsion as an example, the optical microscope images of the emulsion and the harvested filtrate are presented. Obviously, the two images show observable distinction. For the emulsion without separation, a large number of droplets can be found under optical microscope (Fig. 9b, top), and for the harvested filtrate, no obvious oil droplets are observed in the display areas (Fig. 9b, bottom), implying the effectiveness of the PDA/FA/DT decorated foam for removing dispersed oil microdroplets in water. In the separation
process, the intermolecular forces between nonpolar oil droplets emulsified in water and superhydrophobic foam is the main driving force for emulsion breaking [4]. In addition, the oil content in the emulsions before and after the separation was tested by using infrared spectrometer oil content analyzer, and the separation efficiency was calculated by the following formula: E = (1 － Cb/Ca) × 100%, where Cb is the oil content in the harvested filtrates after the separation and Ca is the oil content in the emulsions before the separation. As displayed in Fig. 9c, it can be found that the separation efficiency for six kinds of oil-in-water emulsions is greater than 93.0%. Compared with other four types of emulsions, the relatively poor separation effect for gasoline-in-water and diesel-in-water may be attributed to the complicated constituents of the two oils, particularly soluble species in the oils. Furthermore, the removal kinetics of the coated foam for toluene and kerosene droplets was investigated. Obviously, almost all of oil droplets can be removed from the two emulsions within 2-3 min (Fig. 9d). [Fig. 9] On the other hand, taking toluene-in-water emulsion as the sample for separation, the effect of FA concentration on the separation efficiency was also evaluated. As illustrated in Fig. 10a, all the filtrates become clear and transparent after finishing the separation for toluene-in-water emulsion using the coated foams prepared at different FA concentration. The separation efficiency of the coated foam increases with the FA concentration increasing from 0 wt.% to 0.8 wt.% but decreases slowly with further increase in the FA concentration (Fig. 10b). The optical microscopic images also
indicate no oil droplets can be observed after the separation of toluene-in-water emulsion by using the coated foam prepared at the FA concentration of 0.2 wt.% (Fig. 10c). As it is well-known, for the separation of oil/water emulsions by porous materials, the effective pore size of separation material is required to be small enough to remove oil droplets [34]. But, the PU foam used in this study has interconnected macroporous structure, which is not conductive to the capture of oil microdroplets in the separation process. By loading certain amount of FA onto the foam skeleton, the pores of the obtained foam can be reduced to some extent, resulting in the improvement of separation performance. Therefore, the PDA/FA/DT coated foam with appropriate amount of FA loading ratio is a good sorbent for the removal of oil microdroplets from water. [Fig. 10] 4. Conclusions In conclusion, a kind of novel superhydrophobic/superoleophilic and flame retardant foam was successfully developed via a simple one-step fabrication of PDA/FA/DT functionalized coating. Because of the introduction of fly ash coating on PU foam surface, the foam demonstrates improved flame resistance compared with original foam, PDA coated foams, and other reported polymeric foam-based oil sorbent [4,28]. In addition, the as-prepared foams reveal high sorption capacity up to 34-47 times of their own weight, excellent recyclability, good superhydrophobic stability under harsh conditions (corrosive solution and oils), and highly selective separation performance for the oils in pure water. Moreover, the coated foams can
separate the oil from various corrosive hot water. The modified foam also is capable of realizing the effective separation of surfactant-free oil-in-water emulsions with separation efficiency higher than 93.0%. Due to the excellent flame retardancy, simple preparation procedure, inexpensive raw materials, and superior separation performance, the coated foams are good candidates for the disposal of industrial wastewater and oil spills. Acknowledgments This research was supported by the Scientific Research Project of Ninxia High School (No. NGY2017145). References [1] D. Jung, J.A. Kim, M.S. Park, U.H. Yim, K. Choi, Human health and ecological assessment programs for Hebei Spirit oil spill accident of 2007: Status, lessons, and future challenges, Chemosphere 173 (2017) 180–189. [2] J.T. Wang, G.H. Geng, X. Liu, F.L. Han, J.X. Xu, Magnetically superhydrophobic kapok fiber for selective sorption and continuous separation of oil from water, Chem. Eng. Res. Des. 115 (2016) 122–130. [3] M.J. Alessandrello, M.S.J. Tomás, E.E. Raimondo, D.L. Vullo, M.A. Ferrero, Petroleum oil removal by immobilized bacterial cells on polyurethane foam under different temperature conditions, Mar. Pollut. Bull. 122 (2017) 156–160. [4] J.T. Wang, Y.A. Zheng, Oil/water mixtures and emulsions separation of stearic acid-functionalized sponge fabricated via a facile one-step coating method, Sep. Purif. Technol. 181 (2017) 183–191.
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Figure captions: Fig. 1. (a) Schematic representation of fabricating PDA/FA/DT foam, (b) water and droplet on the PDA/FA/DT foam and original foam as well as oil droplet on PDA/FA/DT foam, (c) water droplet on the dissected coated foam, (d) photograph of PDA/FA/DT foam (black color) and original (white color) foam after being placed in water, (e) PDA/FA/DT foam immersed in water by an external force. Fig. 2. The effect of FA concentration for (a) water contact angle and (b) oil sorption capacity. Fig. 3. FTIR spectra of original PU foam and PDA/FA/DT foam. Fig. 4. SEM images of (a-d) PU foam and (e-h) PDA/FA/DT foam, (i) elemental mappings of PDA/FA/DT foam. Fig. 5. The combustion process of (a-a4) original PU foam, (b-b4) PDA/DT foam, (c-c4) PDA/FA/DT foam, and (d-d4) PDA/FA/DT foam with kerosene. Fig. 6. (a) Sorption capacity of PDA/FA/DT foam for different oils and organic solvents, recyclability of PDA/FA/DT foam using (b) vacuum filtration and (c) mechanical squeezing. Fig. 7. (a) Digital images of 1 M HCl, 1 M NaOH, 1 M NaCl solutions and hot water on the coated foam, (b) water contact angles of PDA/FA/DT foams after immersion into (b) various corrosive solutions for different time periods, (c) 1M NaCl solutions with different pH, and (d) oils and organic solvents. Fig. 8. Photographs of kerosene removal process from the surface of hot (a) 1 M HCl, (b) 1 M NaOH, (c) 1 M NaCl solutions and chloroform removal process from the
bottom of hot (d) 1 M HCl/1 M NaCl and (e) 1 M NaOH/1 M NaCl solutions by PDA/FA/DT foam. Fig. 9. (a) Digital images of various oil-in-water emulsion before and after the separation using PDA/FA/DT foam prepared at the FA concentration of 0.2 wt.%, (b) optical microscope images of kerosene-in-water emulsion and the corresponding filtrate, (c) separation efficiency for different emulsions, (d) separation efficiency of the two emulsion as a function of time. Fig. 10. (a) Digital images of toluene-in-water emulsion and various filtrates separated by the coated foams prepared at the different FA concentration, (b) separation efficiency for toluene-in-water emulsion by the coated foams prepared at the different FA concentration, (c) optical microscope images of the emulsion and the filtrate after the separation by the coated foam prepared at the FA concentration of 0.2%,
Fig. 1
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Graphical abstract
S0021-9797(18)30454-5 https://doi.org/10.1016/j.jcis.2018.04.069 YJCIS 23534
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
25 March 2018 14 April 2018 17 April 2018
Please cite this article as: J. Wang, H. Wang, G. Geng, Flame-retardant superhydrophobic coating derived from fly ash on polymeric foam for efficient oil/corrosive water and emulsion separation, Journal of Colloid and Interface Science (2018), doi: https://doi.org/10.1016/j.jcis.2018.04.069
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Flame-retardant superhydrophobic coating derived from fly ash on polymeric foam for efficient oil/corrosive water and emulsion separation Jintao Wang a,*, Hongfei Wang b, Guihong Geng a,* a
College of Materials Science and Engineering, North Minzu University, Yinchuan
750021, P.R. China b
Suzhou Wuwei Environmental Technology Co., Ltd., Suzhou 215100, P.R. China
Corresponding author. E-mail addresses: [email protected] (J.T. Wang), [email protected] (G.H. Geng).
ABSTRACT Developing novel oil sorbents with both superhydrophobicity and flame resistance reveals an enticing prospect for oil/water separation. In this study, superhydrophobic foam with superior flame retardancy and sorption capability is reported through a simple one-step fabrication route in alkaline water/ethanol system containing dopamine, fly ash (FA) and dodecanethiol (DT). The introduction of FA endows the foam with excellent flame retardancy, and the as-prepared foam reveals improved flame resistance compared with original and polydopamine (PDA) coated foams, The obtained foams can quickly absorb various types of oils up to 34-47 times of their own weight, and the absorbed oils can be repeatedly recovered by a simple vacuum filtration process. The foams can also maintain their high hydrophobicity after long term immersion in different corrosive solutions and oils, and are able to be used for removing the oils from corrosive high-temperature water. More importantly, the foams with FA coating can effectively separate a broad range of oil-in-water emulsions with high efficiency (>93.0%). The outstanding separation property of the as-prepared foams and their eco-friendly, low-energy, and inexpensive fabrication process imply the great potential for oily wastewater treatment and oil spill cleanup.
Keywords: Fly ash; PU foam; Flame retardancy; Sorption; Harsh condition; Emulsion
1. Introduction In recent years, water contamination resulting from the spillage of oil products and industrial organic contaminants has posed a detrimental impact on the global ecological environment and human health [1,2]. To address this problem, various approaches including sorption [3-5], filtration membrane [6,7], in-situ burning [8], bioremediation [9], and dispersant [10], have been developed to remove oils or organic solvents from water. Among these reported methods, the sorption removal of oil using sorbent is considered to be one of the most effective methods owing to the recyclability of oils and organic solvents. So far, a variety of oil sorbents, such as nanosilica modified cotton fibers [11], carbon aerogels from poplars catkins [12], reduced graphene aerogel [13], polyvinyl alcohol/cellulose nanofibril hybrid aerogel microspheres [14], electrospun cellulose acetate nanofibrous mats [15], and melamine-derived carbon sponges [16], have been prepared for the sorption and removal of oil pollutants. However, high preparation cost, complex preparation process, and poor recyclability of the aforementioned sorbents restrict their large-scale production and applications. Therefore, it is of importance to develop novel sorbents with low cost, good recyclability, and superb sorption performance to address the global-scale of water contamination arising from spilled oil. Foam, a type of cheap three-dimensional porous material, is a potential substrate for oil/water separation [17]. In recent years, superhydrophobic and superoleophilic foams have aroused wide concern owing to their fast and highly efficient cleanup of oil and organic contaminants from water. For instance, Gao et al. prepared a
hydrophobic/oleophilic carbon soot coated melamine foam for oil cleanup via a dip-coating method, and the carbon soot was synthesized by an ethylene-oxygen combustion flame [18]. Wang et al. developed superhydrophobic carbon nanotubes reinforced polyurethane (PU) foam for selective oil/water separation [19]. Zhang et al. synthesized MnO2 nanowires via hydrothermal method and then MnO2 nanowires/PU foam composites for absorbing oils from water surface were fabricated though foaming technology [20]. Cao et al. firstly prepared superhydrophobic nanodiamonds via
coating
polydopamine
and
subsequent
reaction
with
1H,1H,2H,2H-perfluorodecanethiol, and then superhydrophobic foam was obtained via decoration of the modified nanodiamonds onto the skeleton of commercial PU foam [21]. Although these studies have demonstrated that superhydrophobic foams with various synthesized nanoparticles-based coating are ideal candidates for oil cleanup operation, none of the aforementioned foams have been proven to possess flame resistance. In addition, the reported foam-based sorbents can only be utilized for removing oil layer from water and cannot effectively separate emulsified oil/water mixture. Hence, it is very urgent to construct foam-derived sorbent that possess flame retardant nature, inexpensive superhydrophobic coating, and effective separation performance for emulsified oil/water emulsion. FA is a waste substance from the burning of fossil fuel, municipal waste, coal, etc. that is found in abundance in the world. In recent years, the utilization of FA has received much attention in public and industry, which will be conductive to reduce the environmental burden. Despite available approaches, only a small amount of FA has
been used in commercial application, the rest being buried in landfills or stored in holding ponds pending burial [22]. Fly ash appears as a relatively fine powder with rounded particle diameters mainly between 0.1 and 400 μm [23]. The assembly of micro/nano-scale particles can form hierarchical coating on the surface of substrates. Nevertheless, up to now, FA has received little attention on exploring its potential applications in the formation of hierarchical coating with flame retardancy. In our previous paper, superhydrophobic fiber and fabric have been prepared via creating hierarchical coating composed by ZnO nanoneedles, irregular ZnO nanosheet, or Cu nanoparticles onto the substrate surfaces under hydrothermal or chemical reduction conditions [24-26]. In this study, a type of cost-effective superhydrophobic PU foam was developed though a simple one-step reaction in alkaline water/ethanol medium of containing dopamine, FA and DT. By using solid waste FA as coating material, the as-prepared foam shows improved flame-retardant property as compared to the original and PDA coated foams. The decorated foam can maintain its superhydrophobicity under extremely harsh (highly acidic, alkaline, salty solutions) and turbulent water environment. The superhydrophobic foam is able to absorb a wide range of oils and organic solvents from both pure water and corrosive solutions with high selectivity, and the harvested foam can maintain its superhydrophobicity after repeating the separation process for 15 cycles. More importantly, the coated foam is able to separate various oil-in-water emulsions with a separation efficiency of higher than 93.0%. Thus, the as-fabricated foam is a promising candidate for wastewater treatment related to oil pollution.
2. Experimental 2.1. Materials PU foam was purchased from market, Yinchuan, China. Fly ash was provided by Shenhua Ningxia Coal Industry Group, Yinchuan, China. Dopamine hydrochloride (98%) was supplied by Nanjin Aoduo Biotechnology Co. Ltd., China. Toluene (99.9%), n-hexane (99.9%), chloroform (99.9%), kerosene (95%), NH3·H2O (25%) were obtained from Ningxia Yaoyi Chemical Reagent Co. Ltd., China. Dodecanethiol (DT, 97%) was provided by Nanjin Chengong Organosilicon Co. Ltd, China. Gasoline and diesel came from the local market, Yinchuan, China. 2.2. Preparation process of PDA/FA/DT coated foams Before using, PU
rinsed with ethanol FA (0.2 wt.%) and dopamine hydrochloride (2 mg/mL)
were added into the mixture of
to vigorously stir
(1000 rpm) at 30 ℃ for 30 min. Then, DT (2mM) and PU foams (the mass ratio of foam, dopamine hydrochloride and DT, 5:20:2) were placed into the above dispersion. After the
ammonia aqueous
solution, the system was stirred for 16 h at 30 ℃. Finally, the obtained foams were removed from the mixture, washed with plenty of distilled water, and dried to constant weight at 80 ℃. The schematic of the fabrication process of PDA/FA/DT decorated foam is also displayed in Fig. 1a. 2.3. Measurements of oil sorption capacity A piece of PDA/FA/DT coated foam weighed beforehand was put into a beaker
containing oil at room temperature. The foam was removed from the oil after reaching sorption saturation. The oil sorption capacities of the foam were measured by weighing the foams before and after the sorption, and calculated by the following equation: Q = (mt – mi ) / mi where Q is the oil sorption capacity (g/g) of the sorbent calculated as grams of oil per gram of sample, mt is the weight of saturated sorbent (g), mi is the initial weight of sorbent (g). 2.4. Characterizations Water contact angle measurements were carried out using a Krüss DSA 100 (Krüss Company, Ltd., Germany) apparatus at 30 ℃, and the volumes of probing liquids in the measurements were approximately 5 μL. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet NEXUS FTIR spectrometer using KBr pellets. The KBr pellet was prepared via pressing the ground mixture of sample (2 mg) and KBr powder (100 mg) at 25 ℃ on hydraulic machine (10 Mpa). The micrographs of samples were examined using SEM (JSM–5600LV, JEOL) at an acceleration voltage of 5.0 kV. Before SEM observation, all samples were fixed on aluminum studs and coated with gold. 2. Results and discussion 3.1. Wettability The surface wettability of the obtained PDA/FA/DT coated foam is illustrated in Fig 1b. Obviously, the blue-colored water droplet on the surface of decorated foam
maintain a stable spherical shape with a water contact angle of 161°, whereas it sinks into the surface of original foam to form a large spreading radius, implying the wettability transition of the foam from hydrophilicity to superhydrophobicity. The superior hydrophobicity of the as-prepared foam should be attributed to the micro/nano roughness created by FA microparticles, PDA nanoparticles, and macroscopic voids of the three-dimensional foam. When the red-colored oil (kerosene) is dropped onto the surface of the coated foam, it can be completely wetted by the oil within 1 s and the oil contact angle is measured to be 0°, confirming excellent superoleophilicity of the foam. As illustrated in Fig. 1c, when the PDA/FA/DT coated foam is cut into two pieces, the exposed new surface still possesses good water repellence with a water contact of over 155°, suggesting that the internal surface of as-prepared foam is also modified completely rather than just the external surface. In addition, when the coated foam is put into water, it can float on the water surface owing to its high water-repellence, but the original PU foam submerges into the water because of its hydrophlicity (Fig. 1d). When the PDA/FA/DT coated foam is forced to enter into the water, a silver mirror-like thin film on the foam surface can be observed owing to the trapped air layer between the rough surface and water (Fig. 1e). Once the external force is released, the decorated foam would rise to the water surface immediately and the weighing result reveals almost no water is absorbed by the removed foam. [Fig. 1] The effect of FA concentration on water contact angle and oil sorption capacity of
the decorated foam was systematically investigated (Fig. 2). As displayed in Fig. 2a, the water contact angle increases with the increase of FA concentration, and it reaches the maximum of 161° at the FA concentration of 0.2 wt.%, implying the ultrahigh hydrophobic property of the coated foam. With the further increase of the FA concentration, an obvious decrease in the water contact angles can be found. The relationship between the FA concentration of preparing samples and oil sorption capacity is illustrated in Fig. 2b. It can be seen that the sorption capacity of the coated foam for kerosene increases with the FA concentration, and it achieves the optimal value of up to 34.7 times the coated foam weight at the FA concentration of 0.2 wt.%. The aforementioned results reveal that the enhancement of hydrophobicity via increasing the FA loading ratio is beneficial to the improvement of oil sorption capacity. However, excessive content of FA bring about a decrease of oil sorption capacity. This may be ascribed to the fact that excessive FA particles in the foam will act as the filler to reduce the porosity of the foam and increase the weight of the coated foam in a unit volume [18]. Accordingly, the oil sorption capacity of the obtained foam is decreased. [Fig. 2]
3.2. FTIR spectra analysis FTIR spectra of original PU foam and PDA/FA/DT foam are presented in Fig. 3. The original PU foam displays absorption bands at 3135, 2920-2850, 1491-1441, 1173-1005 cm-1, which are attributed to the N-H stretching vibration, C-H stretch vibration of -CH3- and -CH2-, C-H deformation vibration of -CH3- and -CH2-, and
C-O stretching vibration, respectively [4]. All of these characteristic absorption bands reveal that the foam is a kind of PU-based material. In contrast to the original foam, the absorption band intensity of PDA/FA/DT foam at 2920-2850 cm-1 exhibits a remarkable enhancement, suggesting the introduction of DT on the foam skeleton. In addition, the increase of absorption peak and the appearance of new absorption peak at 795-607 cm-1 can be clearly observed, which is ascribed to the symmetrical stretching and bending vibration of Si-O-Si in FA. Hence, the aforementioned analyses indicate that the PU foam has been successfully decorated by FA and DT. [Fig. 3] 3.3. SEM and element mapping analysis The surface appearance of original foam and as-fabricated PDA/FA/DT foam were observed by SEM. The SEM images of original and PDA/FA/DT foams with different magnifications are displayed in Fig 4. It can be found that two kinds of foams have three dimensional macroporous structure (Fig 4a and 4e). The magnified images of original foam demonstrate a relatively flat skeleton surface (Fig 4b-d). In the case of PDA/FA/DT
foam,
the
skeleton
surface
becomes
rougher
and
micro/nano-agglomerates are completely attached to the foam framework, as shown in Fig 4f and 4g. It is believed that the self-polymerization of dopamine offers a strong impetus for the establishment of FA on the large interconnected pore surface of PU foam [2,11]. In Fig 4h, SEM image with higher magnification illustrates the skeleton surface of PU foam possesses the hierarchical substructure composed of FA and PDA particles, clearly differing from original PU foam in Fig 4d. The hierarchical
PDA/FA coating and microporous foam structure synergistically contribute to the hydrophobicity of the as-obtained foam. The element mapping images of PDA/FA/DT foam are shown in Fig 4i. EDS elemental mapping analysis confirms the co-existence of C, Al, Si, and S as primary elements and their uniformly dispersion, implying that FA and DT distribute onto the surface of the foam. [Fig. 4] 3.4. Flame resistance In recent years, a large amount of superhydrophobic PU foams based on different coatings have been developed for oil sorption. Nevertheless, these reported PU foams are easily ignited and turn into ashes in a short time. Meanwhile, oils and organic solvents are also flammable or highly flammable. The flammability of both sorbent and adsorbate undoubtedly increases the risk of combustion and explosion in dealing with oil accidents, Hence, the development of porous oil sorbents with both superhydrophobicity and flame resistance is urgent for practical applications [27]. Unfortunately, there are few works on endowing the superhydrophobic oil sorbents with flame resistance. Herein, a type of superhydrophobic foam with flame retardancy was prepared via a facile one-step route. A simple combustion experiment is used to evaluate the flame retardancy of the PDA/FA/DT coated foam. For comparing, original PU foam and the foam only modified by PDA/DT without FA are also subjected to the test of burning behavior. As displayed in Fig. 5(a-a4), the original foam burns rapidly after being ignited and almost all foam burns up thoroughly in 9 s, leaving only a small amount of residue at the end. Compared with the original foam, the PDA/DT modified foam exhibits an improved flame resistance. As shown in Fig.
5(b-b4), although the PDA/DT modified foam also is easily ignited and burnt to ashes, the burning process can last 25 s, which is nearly three times longer than the original foam. For the PDA/FA/DT decorated foam, an entirely different phenomenon is observed as illustrated in Fig. 5(c-c4). The superhydrophobic foam with FA coating burns with slight flame and the combustion stops at 9 s. After the extinction of the flame, the foam retains its basic skeleton and most of the foam does not burn at all, demonstrating the improved flame retardancy. This may be explained by the fact the incombustible FA layer isolates the flammable PU foam skeleton. In addition, the PDA/FA/DT decorated foam is used to absorb kerosene and the burning behavior of the toluene-loaded foam was investigated. As shown in Fig. 5(d-d4), the foam with kerosene can burn for 175 s, and a large amount of foam still can be left after the flame extinguishes. The above results further confirm that the superhydrophobic foam with FA coating has good flame-retardant property. As a result, the probability of fire can be decreased via using the as-prepared foam as sorbent for removing oils and organic solvents. [Fig. 5] 3.5. Oil sorption capability The sorption capacity of the PDA/FA/DT foam prepared at the FA concentration of 0.2 wt.% for a broad range of oils and organic solvents was investigated. As displayed in Fig. 6a, the as-prepared foam shows sorption capacities ranging from 34 to 47 times their initial weight for various oils and organic solvents, and the difference in the sorption capacity is mainly dependent on the viscosity and density of the oils and
organic solvents [4,28]. For instance, due to high density of chloroform and high viscosity of diesel, the PDA/FA/DT coated foam demonstrates sorption capacity of 47 g/g and 37.8 g/g in chloroform and diesel, respectively. The sorption capacities are also observably superior to those of porous materials having been reported recently, such as magnetic porous carbon aerogel converted from biorenewable popcorn (<10.8 g/g) [29], CTAB-modified polymethylsilsesquioxane aerogels (<4.7 g/g) [30], fluorinated polydopamine/chitosan/reduced graphene oxide composite aerogel (<16 g/g) [31], cupric stearate coated sponge
(<26.6 g/g) [32], and comparable to those of
lignin/agarose/poly(vinyl alcohol)-based honeycomblike aerogel (<40 g/g) [33] and polyurethane foams composites modified with MnO2 nanowires (<40.2 g/g) [20]. Although the as-prepared foam exhibits lower sorption capacity than that of carbon aerogels from poplars catkins (<161 g/g) [12], the process for fabricating PDA/FA/DT coated foam is simpler, cheaper and more environmentally friendly. The recyclability of the as-prepared foam was also evaluated using toluene and kerosene as model oils. In the recycling tests, the absorbed oils are removed from the coated foam via two approaches: vacuum filtration and mechanical squeezing. The vacuum filtration cannot cause severe damage to the porous structure of the fabricated foam compared with mechanical squeezing. The recycle times for sorption versus the sorption capacity for the two oils is displayed in Fig. 6b,c. In either oil-removal method, the sorption capacity of the coated foam does not present significant decrease after 15 cycles of sorption-desorption process, and approximately 91.7% and 85.7% of the initial sorption capacity is retained for vacuum filtration and mechanical
squeezing removal manner, respectively, suggesting the good recyclability. The result also implies the FA coating is firmly immobilized onto the skeleton of foam and it cannot be easily destroyed by the strongly external force as mechanical squeezing. [Fig. 6] 3.6. Durability The durability of the PDA/FA/DT coated foams was investigated via water contact angle measurement after immersing into corrosive aqueous solutions and oils for 12 h. As illustrated in Fig 7a, the droplets (dyed with methylene blue) of 1 M HCl, 1 M NaOH, 1 M NaCl solutions and hot water (90 ℃) exhibit spherical shape on the surface of the coated foam, and the water contact angles are greater than 151°. Moreover, the water contact angles of the coated foam are always higher than 146° after being immersed into 1 M HCl, 1 M NaOH, 1 M NaCl aqueous solutions for different times (Fig. 7b). Especially, after the coated foams are immersed in salty solutions (1 M NaCl) with different pH values for 24 h, no obvious decrease in water contact angles is observed (Fig. 7c). The wettability of the foam was also evaluated after being immersed in different oils at 30 ℃ for 10 min. It is obvious that the resultant foams can maintain their high hydrophobicity after being immersed by various oils (Fig. 7d). The above results indicate the as-prepared foams possess outstanding resistance to corrosive solutions and oils, and can be utilized for removing oil under harsh water environments. [Fig. 7] 3.7. Oil/corrosive water separation performance
Due to porous structure, light weight, and ultrahigh hydrophobicity, the PDA/FA/DT coated foams were applied to the selective sorption of oils from corrosive aqueous solutions. To study the oil removal behavior under harsh conditions, three corrosive water, including 1 M HCl, 1 M NaOH, and 1 M NaCl solutions, are prepared to simulate various practical wastewater. As displayed in Fig. S1, no matter what type of corrosive solution is used, once the coated foam is put into contact with the kerosene layer (dyed with oil red O), all of oil will be immediately and completely absorbed into the coated foam. After the removal of oil-loaded foam, the water surface becomes clear. Moreover, the underwater high-density oil (chloroform) can also be quickly and selectively removed from these corrosive solutions via using the as-prepared foams (Fig. S2). During the sorption of oil pollutants both on the surface of corrosive solutions and in the bottom of the solutions, no water uptake is observed, indicating superior sorption selectivity of the coated foams. In general, industrial oily wastewater and oil spillage needs to be treated on the site under corrosive high-temperature water environments. Hence, the oil/water separation capability of the coated foam was also evaluated under the conditions of simulating industrial oily wastewater environments. To form corrosive high-temperature water environments, kerosene (dyed with oil red O) is chose to mix with hot acidic, alkaline, and salty aqueous solutions (80 ℃). As displayed in Fig. 8(a-c), for all the aforementioned kerosene/corrosive hot solution mixtures, kerosene can be absolutely absorbed and separated by the foam within several seconds. For evaluating the removal effect of the coated foam toward the oil under corrosive hot solutions, the chloroform dyed by oil
red O is used to mixed with hot (80 ℃) 1 M HCl/1 M NaCl and 1 M NaOH/1 M NaCl solutions, respectively. As displayed in Fig. 8d,e, after the PDA/FA/DT coated foam is placed into the two mixtures, the underwater oil can also be completely absorbed. These results suggest the as-prepared foam has potential for use in the separation of industrial oily wastewater. [Fig. 8] 3.8. Oil-in-water emulsion separation ability Because of the selective oil sorption capability, the PDA/FA/DT coated foam was applied to harvest oil microdroplets from surfactant-free n-hexane, toluene, chloroform, kerosene, gasoline, and diesel emulsified in water (Voil:Vwater = 1:10). A piece of coated foam is forced to contact with oil-in-water emulsions under vigorous stirring, and then the emulsion will become clear and transparent slowly within several minutes. Fig. 9a reveals the separation effect of the foam for different oil-in-water emulsions in the form of digital images. In contrast to the original milky emulsions (Fig. 9a, top), the obtained filtrates are relatively clear and transparent after the separation (Fig 9a. bottom). Moreover, taking kerosene-in-water emulsion as an example, the optical microscope images of the emulsion and the harvested filtrate are presented. Obviously, the two images show observable distinction. For the emulsion without separation, a large number of droplets can be found under optical microscope (Fig. 9b, top), and for the harvested filtrate, no obvious oil droplets are observed in the display areas (Fig. 9b, bottom), implying the effectiveness of the PDA/FA/DT decorated foam for removing dispersed oil microdroplets in water. In the separation
process, the intermolecular forces between nonpolar oil droplets emulsified in water and superhydrophobic foam is the main driving force for emulsion breaking [4]. In addition, the oil content in the emulsions before and after the separation was tested by using infrared spectrometer oil content analyzer, and the separation efficiency was calculated by the following formula: E = (1 － Cb/Ca) × 100%, where Cb is the oil content in the harvested filtrates after the separation and Ca is the oil content in the emulsions before the separation. As displayed in Fig. 9c, it can be found that the separation efficiency for six kinds of oil-in-water emulsions is greater than 93.0%. Compared with other four types of emulsions, the relatively poor separation effect for gasoline-in-water and diesel-in-water may be attributed to the complicated constituents of the two oils, particularly soluble species in the oils. Furthermore, the removal kinetics of the coated foam for toluene and kerosene droplets was investigated. Obviously, almost all of oil droplets can be removed from the two emulsions within 2-3 min (Fig. 9d). [Fig. 9] On the other hand, taking toluene-in-water emulsion as the sample for separation, the effect of FA concentration on the separation efficiency was also evaluated. As illustrated in Fig. 10a, all the filtrates become clear and transparent after finishing the separation for toluene-in-water emulsion using the coated foams prepared at different FA concentration. The separation efficiency of the coated foam increases with the FA concentration increasing from 0 wt.% to 0.8 wt.% but decreases slowly with further increase in the FA concentration (Fig. 10b). The optical microscopic images also
indicate no oil droplets can be observed after the separation of toluene-in-water emulsion by using the coated foam prepared at the FA concentration of 0.2 wt.% (Fig. 10c). As it is well-known, for the separation of oil/water emulsions by porous materials, the effective pore size of separation material is required to be small enough to remove oil droplets [34]. But, the PU foam used in this study has interconnected macroporous structure, which is not conductive to the capture of oil microdroplets in the separation process. By loading certain amount of FA onto the foam skeleton, the pores of the obtained foam can be reduced to some extent, resulting in the improvement of separation performance. Therefore, the PDA/FA/DT coated foam with appropriate amount of FA loading ratio is a good sorbent for the removal of oil microdroplets from water. [Fig. 10] 4. Conclusions In conclusion, a kind of novel superhydrophobic/superoleophilic and flame retardant foam was successfully developed via a simple one-step fabrication of PDA/FA/DT functionalized coating. Because of the introduction of fly ash coating on PU foam surface, the foam demonstrates improved flame resistance compared with original foam, PDA coated foams, and other reported polymeric foam-based oil sorbent [4,28]. In addition, the as-prepared foams reveal high sorption capacity up to 34-47 times of their own weight, excellent recyclability, good superhydrophobic stability under harsh conditions (corrosive solution and oils), and highly selective separation performance for the oils in pure water. Moreover, the coated foams can
separate the oil from various corrosive hot water. The modified foam also is capable of realizing the effective separation of surfactant-free oil-in-water emulsions with separation efficiency higher than 93.0%. Due to the excellent flame retardancy, simple preparation procedure, inexpensive raw materials, and superior separation performance, the coated foams are good candidates for the disposal of industrial wastewater and oil spills. Acknowledgments This research was supported by the Scientific Research Project of Ninxia High School (No. NGY2017145). References [1] D. Jung, J.A. Kim, M.S. Park, U.H. Yim, K. Choi, Human health and ecological assessment programs for Hebei Spirit oil spill accident of 2007: Status, lessons, and future challenges, Chemosphere 173 (2017) 180–189. [2] J.T. Wang, G.H. Geng, X. Liu, F.L. Han, J.X. Xu, Magnetically superhydrophobic kapok fiber for selective sorption and continuous separation of oil from water, Chem. Eng. Res. Des. 115 (2016) 122–130. [3] M.J. Alessandrello, M.S.J. Tomás, E.E. Raimondo, D.L. Vullo, M.A. Ferrero, Petroleum oil removal by immobilized bacterial cells on polyurethane foam under different temperature conditions, Mar. Pollut. Bull. 122 (2017) 156–160. [4] J.T. Wang, Y.A. Zheng, Oil/water mixtures and emulsions separation of stearic acid-functionalized sponge fabricated via a facile one-step coating method, Sep. Purif. Technol. 181 (2017) 183–191.
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Figure captions: Fig. 1. (a) Schematic representation of fabricating PDA/FA/DT foam, (b) water and droplet on the PDA/FA/DT foam and original foam as well as oil droplet on PDA/FA/DT foam, (c) water droplet on the dissected coated foam, (d) photograph of PDA/FA/DT foam (black color) and original (white color) foam after being placed in water, (e) PDA/FA/DT foam immersed in water by an external force. Fig. 2. The effect of FA concentration for (a) water contact angle and (b) oil sorption capacity. Fig. 3. FTIR spectra of original PU foam and PDA/FA/DT foam. Fig. 4. SEM images of (a-d) PU foam and (e-h) PDA/FA/DT foam, (i) elemental mappings of PDA/FA/DT foam. Fig. 5. The combustion process of (a-a4) original PU foam, (b-b4) PDA/DT foam, (c-c4) PDA/FA/DT foam, and (d-d4) PDA/FA/DT foam with kerosene. Fig. 6. (a) Sorption capacity of PDA/FA/DT foam for different oils and organic solvents, recyclability of PDA/FA/DT foam using (b) vacuum filtration and (c) mechanical squeezing. Fig. 7. (a) Digital images of 1 M HCl, 1 M NaOH, 1 M NaCl solutions and hot water on the coated foam, (b) water contact angles of PDA/FA/DT foams after immersion into (b) various corrosive solutions for different time periods, (c) 1M NaCl solutions with different pH, and (d) oils and organic solvents. Fig. 8. Photographs of kerosene removal process from the surface of hot (a) 1 M HCl, (b) 1 M NaOH, (c) 1 M NaCl solutions and chloroform removal process from the
bottom of hot (d) 1 M HCl/1 M NaCl and (e) 1 M NaOH/1 M NaCl solutions by PDA/FA/DT foam. Fig. 9. (a) Digital images of various oil-in-water emulsion before and after the separation using PDA/FA/DT foam prepared at the FA concentration of 0.2 wt.%, (b) optical microscope images of kerosene-in-water emulsion and the corresponding filtrate, (c) separation efficiency for different emulsions, (d) separation efficiency of the two emulsion as a function of time. Fig. 10. (a) Digital images of toluene-in-water emulsion and various filtrates separated by the coated foams prepared at the different FA concentration, (b) separation efficiency for toluene-in-water emulsion by the coated foams prepared at the different FA concentration, (c) optical microscope images of the emulsion and the filtrate after the separation by the coated foam prepared at the FA concentration of 0.2%,
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Graphical abstract