Recyclable, Fire-Resistant, Superhydrophobic, and Magnetic Paper

Jul 9, 2018 - (39−45). Herein, we report a new kind of free-standing, recyclable, magnetic, .... Figure 2. SEM micrographs and digital images: (a, d...
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Recyclable, Fire-Resistant, Superhydrophobic, and Magnetic Paper Based on Ultralong Hydroxyapatite Nanowires for Continuous Oil/ Water Separation and Oil Collection Ri-Long Yang,†,‡ Ying-Jie Zhu,*,†,‡ Fei-Fei Chen,†,‡ Dong-Dong Qin,†,‡ and Zhi-Chao Xiong*,† †

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State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding-Xi Road, Shanghai 200050, P. R. China ‡ University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, P. R China S Supporting Information *

ABSTRACT: The frequent occurrence of oil spilling and industrial oily wastewater have serious negative effects on the marine ecosystem and human health. For this reason, high performance, effective, and continuous oil adsorption materials are highly desirable. Herein, a new kind of recyclable, magnetic, fire-retardant, and superhydrophobic large-sized paper based on environmental friendly inorganic ultralong hydroxyapatite nanowires is reported. The ultralong hydroxyapatite nanowires not only act as building blocks for binding magnetic nanoparticles and coating a polydimethylsiloxane layer to form the free-standing composite paper but also endow the paper with high thermal stability, excellent fire resistance, porous structure, and high permeation flux. Moreover, the scaled-up production of ultralong hydroxyapatite nanowires and a large-sized composite paper has been demonstrated. The as-prepared composite paper exhibits a high separation selectivity (>99.0%), high permeation flux (2924.3 L m−2 h−1), and good recycling ability (at least 10 times). More importantly, a mini-boat made from the composite paper acting as an oil-collecting device is fabricated for continuous oil collection. The mini-boat can be magnetically driven to the oil-polluted region, and the selective collection, convenient transportation, and recovery of oil from water are achieved. The oil-collecting device shows a high separation efficiency (>99.2%) and good reusability (at least 10 times). The present work may inspire the development and application of high-performance paper-based oil-collecting devices for continuous oil/water separation and oily contaminant collection in the field of oily wastewater treatment. KEYWORDS: Fire-resistant paper, Magnetism, Superhydrophobicity, Oil/water separation, Hydroxyapatite nanowires

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and multiscale surface roughness.16 Various methods have been developed for modifying the substrates including cotton fabrics,17,18 metal meshes,19,20 sponges and foam,21,22 filter paper,23 and nanofibrous membranes.24,25 By regulating the wettability difference of the surface toward water and oil, high separation efficiencies of superhydrophobic or superoleophilic materials can be prepared. However, low adsorption capacity and slow separation hinder their further applications. Currently, the development of vessel-type oil-collecting devices based on oil adsorption materials have aroused considerable research interests.26−29 The superhydrophobic walls of oilcollecting devices selectively allow the oily solvents to penetrate into the vessel, and the collected oil can be easily recovered. In this case, the processes of oil separation from water, oil collection, oil recovery, and oil cleanup can be simultaneously realized.30,31

INTRODUCTION In recent years, water pollution caused by accidental oil leakage and industrial oily wastewater has become a severe global environmental problem.1−3 The frequent oil spilling accidents, for example, 2010 Gulf of Mexico and 2014 Galveston Bay oil spillings, resulted in serious and long-term threats to the ecological environment and human health.4−7 To address this issue, some oily water treatment technologies, such as floatation, combustion, oil containment boom and skimming, and oil adsorbents, have been developed for the oil/water separation. Nevertheless, these methods still suffer from the limitations of poor selectivity and low adsorption capacity.8,9 Therefore, there is an urgent demand to develop new technologies and high-performance materials for continuous oil/water separation and oil contaminant collection. The rapid development of the interface sciences and bionics, especially for superhydrophobic/superoleophilic surfaces and underwater superoleophobic surfaces, have offered brand new ideas for designing and fabricating highly efficient oil/water separation materials.10−15 Superhydrophobic/superoleophilic surfaces can be fabricated by the appropriate surface chemistry © XXXX American Chemical Society

Received: March 31, 2018 Revised: May 26, 2018

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DOI: 10.1021/acssuschemeng.8b01463 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 1. Schematic illustration of the fabrication process of the HAP@Fe3O4@PDMS paper (a) and its application as a filter paper for oil/water separation and magnetically driven oil collection (b).

oil-polluted region, where it automatically and selectively adsorbs floating oil on the water surface, and then is transported by a magnet and recovered. The high recovery efficiency and excellent recycling ability indicate the great potential of the HAP@Fe3O4@PDMS paper for applications in oil/water separation and oily wastewater treatment.

Recently, oil adsorption materials with multifunctional properties have attracted extensive attention. For example, to facilitate the handling of oil/water separation, oil collection, and transportation, it is highly desirable to endow materials with the remote control ability. For this purpose, one feasible approach is the integration of the magnetic constituent.32 The introduction of a magnetic constituent imparts the materials with an on−off switch ability by using an external magnetic field, which is promising for fabricating intelligent oil-collecting devices. A variety of magnetic composites and magnetismcontrolled devices have been prepared.33−37 By using an external magnetic field, the magnetic oil-collecting device can be driven to the oil-polluted region, and selective oil collection from a mixture of oil and water, rapid transportation, and highly efficient recovery of oil can be achieved. Moreover, since most oily contaminants and solvents are highly flammable and extremely dangerous if ignited, it is necessary to develop the adsorbent materials with high thermal stability and fire resistance.38 In comparison with organic materials, inorganic materials are ideal candidates because of their unique properties such as high thermal stability, fire resistance, and rarely releasing toxic gas and smoke in fire.39−45 Herein, we report a new kind of free-standing, recyclable, magnetic, fire-retardant, and superhydrophobic large-sized paper based on environmental friendly inorganic ultralong hydroxyapatite (HAP) nanowires and a fabricated oilcollecting device for oil/water selective separation and magnetically controlled oil collection. Magnetic Fe3O4 nanoparticles are decorated on the surface of ultralong hydroxyapatite nanowires, which endow the paper with a magnetic property for realizing a romote controllability. In addition, the paper is coated by a polydimethylsiloxane (PDMS) layer to achieve the superhydrophobic property. In this way, the freestanding, recyclable, magnetic, fire-retardant, and superhydrophobic HAP@Fe3O4@PDMS paper was obtained. Different from the traditional cellulose paper, the as-prepared HAP@Fe3O4@PDMS paper shows a high thermal stability and excellent fire resistance. Moreover, the porous structure and superhydrophobicity endow the HAP@Fe3O4@PDMS paper with a selective oil infiltration, high permeation flux, and excellent recycling ability. Furthermore, a mini-paper boat made from the HAP@Fe3O4@PDMS paper as an oil-collecting device is fabricated, and it can be magnetically driven to the

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EXPERIMENTAL SECTION

Materials. Triethylene glycol (TEG), ethyl acetate, acetone, isopropanol, methanol, n-hexanol, cyclohexane, ethylene glycol, chloroform, toluene, iron(III) acetylacetonate (Fe(acac)3), and isooctane were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Soybean oil, petroleum ether, vacuum pump oil, oil red, and methyl blue were purchased from Aladdin Industrial Corporation (Shanghai, China). A polydimethylsiloxane (PDMS, Sylgard 184) prepolymer and curing agent were purchased from Dow Corning Corporation, USA. All chemical reagents were used as received without further purification. Preparation of Multifunctional HAP@Fe3O4@PDMS Paper. The preparation process of the HAP@Fe3O4@PDMS paper is shown in Figure 1a. First, ultralong hydroxyapatite nanowires were synthesized according to the calcium oleate precursor solvothermal method reported previously by our group.41,42,46−51 In brief, 150 mL of an aqueous solution containing 10.500 g of NaOH was added into a mixture of 135 mL of deionized water, 105 mL of oleic acid, and 60 mL of methanol under mechanical stirring. After stirring for 30 min, 120 mL of an aqueous solution containing 3.330 g of CaCl2 and 180 mL of an aqueous solution containing 9.360 g of NaH2PO4·2H2O were added into the mixture. After stirring for 5 min, the resulting mixture was transferred into a 1 L Teflon-lined stainless steel autoclave and solvothermally treated at 180 °C for 24 h. The product was washed with ethanol and deionized water three times, respectively. The product was vacuum freeze-dried for further experiments. Here, 100 mg of the as-prepared ultralong HAP nanowires and 100 mg of Fe(acac)3 were dispersed in 30 mL of TEG by ultrasound and subsequent magnetic stirring. The homogeneous suspension was transferred into a 50 mL Teflon-lined stainless steel autoclave, sealed, and heated at 220 °C for 12 h. Similarly, the scaled-up preparation of HAP@Fe3O4 nanowires was achieved by thermal treatment of the mixture containing 2 g of ultralong HAP nanowires, 2 g of Fe(acac)3, and 600 mL of TEG in a 1 L Teflon-lined stainless steel autoclave at 220 °C for 12 h. The obtained HAP@Fe3O4 nanowires were washed with ethanol three times, and a free-standing HAP@Fe3O4 paper with a diameter of 4 or 20 cm was prepared from an ethanol suspension of HAP@Fe3O4 nanowires by vacuum filtration. Finally, the HAP@ Fe3O4 paper was immersed in a mixture of ethyl acetate, PDMS B

DOI: 10.1021/acssuschemeng.8b01463 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Then, a simple vacuum filtration process is employed to prepare the HAP@Fe3O4 magnetic paper. Finally, after immersing the HAP@Fe3O4 magnetic paper into a mixture of ethyl acetate, PDMS prepolymer, and curing agent followed by thermal curing, the magnetic, fire-retardant, and superhydrophobic HAP@Fe3O4@PDMS paper is obtained. Benefiting from the PDMS coating and increased surface roughness by Fe3O4 nanoparticles, the HAP@Fe3O4@PDMS paper has a superhydrophobic property. Furthermore, the HAP@Fe3O4@PDMS paper has a porous structure. The HAP@Fe3O4@PDMS paper can be used as the filter paper for the oil/water selective separation (Figure 1b). In addition, the free-standing and highly flexible HAP@Fe3O4@PDMS paper can be folded into a mini-paper boat. Owing to the unique magnetic response, the mini-paper boat can be manipulated by a magnet. The mini-paper boat acts as an oil-collecting device that can achieve integrated magnetic-driven oil collection from a mixture of oil and water, transportation, and recovery of oil. SEM micrographs and digital images of the as-prepared HAP nanowire paper, HAP@Fe3O4 paper, and HAP@Fe3O4@ PDMS paper are shown in Figure 2. The HAP nanowire

prepolymer, and curing agent at a mass ratio of 100:10:1 in a glass culture dish for 30 min, and then cured at 100 °C for 4 h. Finally, the magnetic, fire-retardant, and superhydrophobic HAP@Fe3O4@PDMS paper was obtained. Characterization. Scanning electron microscopy (SEM) micrographs were recorded on a field-emission scanning electron microscope (Hitachi S-4800, Japan). Transmission electron microscopy (TEM) micrographs were taken with a transmission electron microscope (HITACHI H-800, Japan). Fourier transform infrared (FTIR) spectra were obtained using a FTIR spectrometer (FTIR7600, Lambda Scientific, Australia). The water contact angle tests were carried out with an optical contact angle system (Model SL200B), and each sample was tested at more than three different locations. X-ray diffraction (XRD) patterns were measured using an X-ray diffractometer (Rigaku D/max 2550 V, Cu Kα radiation, λ = 1.54178 Å). The magnetic property was measured on a physical property measurement system (PPMS, Quantum Design, USA) at room temperature. Fe content was determined by inductively coupled plasma (ICP)-optical emission spectrometry (JY 2000-2, HORIBA Scientific). Thermogravimetric (TG) curves were measured on a simultaneous thermal analyzer (STA 409PC, Netzsch, Germany) with a heating rate of 10 °C min−1 in flowing air. Atomic force microscopy (AFM) images were recorded on an atomic force microscopy (NTEGRA, Russia). Resistance to Mechanical Damage and Chemical Corrosion Tests. Finger wipe, adhesive tape peeling, knife scratching, and sandpaper abrasion were used in the mechanical damage tests for the as-prepared HAP@Fe3O4@PDMS paper. In the finger wipe test, a finger was wiped across the surface of the HAP@Fe3O4@PDMS paper repeatedly. The adhesive tape was attached to the surface of the HAP@Fe3O4@PDMS paper, and then, the adhesive tape was peeled off the paper in the adhesive tape peeling tests. A knife was used to scratch the HAP@Fe3O4@PDMS paper in the knife scratching tests. In the sandpaper abrasion, the HAP@Fe3O4@PDMS paper was abraded using sandpaper. To investigate the chemical corrosion resistance, the HAP@Fe3O4@PDMS paper was soaked in several different organic solvents including acetone, cyclohexane, isopropanol, methanol, ethanol, n-hexanol, and ethylene glycol, and aqueous solutions with different pH values for 1 h each. Then, the HAP@ Fe3O4@PDMS paper was withdrawn and dried at 60 °C for 1 h. Then, water contact angles of the HAP@Fe3O4@PDMS paper were measured. Oil/Water Separation and Continuous Oil Collection Tests. A piece of the HAP@Fe3O4@PDMS paper was used as a filter paper in a vacuum filtration apparatus. A mixture containing 10 mL of deionized water dyed with methyl blue and 10 mL of chloroform dyed with oil red was poured onto the HAP@Fe3O4@PDMS paper. The separation efficiency of the HAP@Fe3O4@PDMS paper was measured.52 For the continuous oil collection test, a piece of the HAP@Fe3O4@PDMS paper was folded into a mini-paper boat with sizes of 1.8 cm × 1.8 cm × 0.6 cm. The oily solvent (cyclohexane, isooctane, toluene, petroleum ether, soybean oil, or vacuum pump oil) was poured onto the water surface. Subsequently, the mini-paper boat was put onto the water surface and driven by a magnet to the oilpolluted region. The selective oil adsorption, oil collection, transportation, and recovery of oily solvent were achieved, as shown in Figure 1b.

Figure 2. SEM micrographs and digital images: (a, d) HAP nanowire paper, (b, e) HAP@Fe3O4 magnetic paper, (c, f) HAP@Fe3O4@ PDMS paper. The insets in panels a−c are the digital images of the corresponding paper sheets with a diameter of 4 cm. (g−i) HAP@ Fe3O4@PDMS paper possesses high flexibility and good processability. (j) As-prepared HAP@Fe3O4@PDMS paper is tightly absorbed on a magnet. (k) Digital image of the HAP@Fe3O4@ PDMS paper with a diameter of 20 cm.

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RESULTS AND DISCUSSION Preparation and Characterization of Multifunctional HAP@Fe3O4@PDMS Paper. The multifunctional and intelligent oil adsorbents and oil-collecting devices are promising for oil pollution-related environmental remediation. In this work, a new kind of free-standing, recyclable, magnetic, fireretardant, superhydrophobic, large-sized paper based on hydroxyapatite nanowires is designed and fabricated, as shown in Figure 1a. Briefly, the ultralong HAP nanowires are loaded with Fe3O4 nanoparticles by the solvothermal method.

paper is composed of ultralong HAP nanowires with diameters of 10−20 nm and lengths of several hundred micrometers (Figure 2a and d). The high-aspect ratio HAP nanowires selfassemble into bundles, and therefore, many HAP nanowire bundles in addition to single nanowires are observed. The ultralong HAP nanowires and nanowire bundles interweave C

DOI: 10.1021/acssuschemeng.8b01463 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 3. Characterization of the as-prepared paper sheets: (a) FTIR spectra, (b) TG curves, (c) XRD patterns, (d) water contact angles: (i) HAP nanowire paper, (ii) HAP@Fe3O4 magnetic paper, and (iii) magnetic, fire-retardant, superhydrophobic HAP@Fe3O4@PDMS paper.

Figure 4. Characterization of the as-prepared paper sheets: (a−d) TEM micrographs: (a) ultralong HAP nanowires, (b) HAP@Fe3O4-1 sample, (c) HAP@Fe3O4-2 sample, (d) HAP@Fe3O4-3 sample; (e−h) SEM micrographs: (e) HAP@PDMS paper, (f) HAP@Fe3O4@PDMS-1 paper, (g) HAP@Fe3O4@PDMS-2 paper, (h) HAP@Fe3O4@PDMS-3 paper. Insets are the digital images of the corresponding paper sheets with a diameter of 4 cm. (i) Water contact angles. (j) Room-temperature magnetic hysteresis loops.

nanowires. After the immersing and curing process, a PDMS coating is formed on the surface of the HAP@Fe3O4 paper, as illustrated in Figure 2c and f. Additionally, the color of the HAP@Fe3O4@PDMS paper changes to brown. After the PDMS coating, the porous structure can be well preserved. Moreover, the HAP@Fe3O4@PDMS paper pos-

with each other to form a three-dimensional porous network structure. After loading with Fe3O4 nanoparticles, the color of the paper changes from white to gray (insets in Figure 2a and b), indicating the successful loading of Fe3O4 nanoparticles. Fe3O4 nanoparticles with an average particle size of about 10 nm are well dispersed and decorated on the surface of HAP D

DOI: 10.1021/acssuschemeng.8b01463 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering sesses a high flexibility and excellent processability, and it can be tailored, folded, or rolled readily into various desired shapes without apparent damage (Figure 2g−i). When a magnetic field is applied, the HAP@Fe3O4@PDMS paper can be tightly absorbed on the magnet, indicating good magnetic response property (Figure 2j). More importantly, the scaled-up fabrication of the HAP@Fe3O4@PDMS paper can be realized. The scaled-up synthesis of ultralong HAP nanowires and HAP@Fe3O4 nanowires can be realized in the laboratory by using a 1 L stainless steel autoclave (Figure S1, Supporting Information). In addition, a large-sized HAP@Fe3O4 paper is fabricated using a commercial paper sheet former by vacuum filtration (Figure S2, Supporting Information). After coating with a PDMS layer, the HAP@Fe3O4@PDMS paper with a diameter of 20 cm is obtained, as shown in Figure 2k. Figure 3 shows the characterization results of the asprepared paper sheets, including FTIR spectra, TG curves, XRD patterns, and water contact angles. As shown in Figure 3a, the typical absorption peaks of the PO43− group (1097, 1030, 962, 603, and 563 cm−1), hydroxyl group (3571 cm−1), and adsorbed water (3429 and 1635 cm−1) are observed in the FTIR spectrum of the HAP nanowire paper. The characteristic peak of the Fe−O band at 575 cm−1 overlaps with the absorption peaks of the PO43− group, and it can be hardly distinguished from the FTIR spectrum of the HAP@Fe3O4 magnetic paper. In addition, in the FTIR spectrum of the HAP@Fe3O4@PDMS paper, the characteristic peaks of the Si−CH3 vibration (1262 and 801 cm−1) and C−H stretching bands (2963 and 2904 cm−1) appear, indicating successful coating of PDMS on the HAP@Fe3O4 magnetic paper. The PDMS content in the HAP@Fe3O4@PDMS paper is estimated to be about 15 wt %, as shown in the TG curve (Figure 3b). Figure 3c shows that the diffraction peaks in the XRD patterns can be indexed to the hexagonal crystal phase of hydroxyapatite (JCPDS No. 09-0432).44 After decorating with Fe3O4 nanoparticles, three new diffraction peaks (30.1°, 35.4°, and 57.0°) appear, corresponding to the (220), (311), and (511) crystal planes of Fe3O4 (JCPDS No. 87-2334).53,54 In comparison with the HAP@Fe3O4 magnetic paper, the diffraction peaks in the XRD pattern of the HAP@Fe3O4@ PDMS paper are similar. The water contact angle measurements were carried out to characterize the surface wettability of the as-prepared paper sheets. As shown in Figure 3d, the water contact angles of the HAP nanowire paper, HAP@Fe3O4 paper, and HAP@Fe3O4@PDMS paper are measured to be 0°, 0°, and 152.1 ± 3.4°, respectively, revealing the superhydrophilic surface of the HAP nanowire paper and HAP@Fe3O4 paper and the superhydrophobic surface of the HAP@Fe3O4@PDMS paper. We have found that the loading amount of the Fe3O4 nanoparticles not only influences the magnetization but also the hydrophobic property of the paper. The loading amount of Fe3O4 nanoparticles can be controlled by the initial amount of iron(III) acetylacetonate. Figure 4a−d shows the TEM micrographs of the HAP nanowire paper and three kinds of HAP@Fe3O4 paper prepared using 50, 100, and 200 mg of iron(III) acetylacetonate. It is obvious that the amount of Fe3O4 nanoparticles increases with an increasing initial amount of iron(III) acetylacetonate. Fe3O4 nanoparticles are relatively uniformly dispersed in the HAP@Fe3O4-1 paper and HAP@ Fe3O4-2 paper, while Fe3O4 nanoparticles agglomerate in the HAP@Fe3O4-3 paper. The Fe contents of the three samples

are measured by ICP to be 6.37, 12.3, and 19.0 wt %, respectively. Figure 4e−h shows the SEM images of the as-prepared paper sheets. It is clearly observed that the surfaces of three kinds of HAP@Fe3O4@PDMS paper sheets are more rough compared with the HAP@PDMS paper. With increasing the loading amount of Fe3O4 nanoparticles, the rough surface of the paper is more obvious. The HAP@Fe3O4@PDMS paper sheets exhibit different colors from light brown to dark brown, depending on the loading amount of the Fe3O4 nanoparticles (insets in Figure 4e−h). The water contact angles of the paper sheets are shown in Figure 4i. The water contact angles of the HAP@Fe3O4@PDMS-1 paper, HAP@Fe3O4@PDMS-2 paper, and HAP@Fe3O4@PDMS-3 paper are measured to be 145.6 ± 2.8°, 152.1 ± 3.4°, and 154.8 ± 1.5°, respectively, which are higher than that of the HAP@PDMS paper (138.5 ± 2.2°). The superhydrophobicity of the HAP@Fe3O4@PDMS paper can be achieved after loading a sufficient amount of Fe3O4 nanoparticles and PDMS coating. The decorated Fe3O4 nanoparticles on the surface of ultralong HAP nanowires generate surface roughness at the nanoscale. As shown in Figure S3 in the Supporting Information, the average surface roughness values of the HAP@PDMS paper and HAP@ Fe3O4@PDMS-2 paper are measured to be 178 and 207 nm, respectively. The combination of the multiscale roughness and hydrophobicity of the PDMS coating leads to the superhydrophobicity of the HAP@Fe3O4@PDMS paper. In addition, the magnetic performance of the HAP@ Fe3O4@PDMS paper was investigated. According to the magnetic hysteresis loops in Figure 4j, the as-prepared samples exhibit superparamagnetic behavior. The saturation magnetizations of the HAP@Fe3O4@PDMS-1 paper, HAP@Fe3O4@ PDMS-2 paper, and HAP@Fe3O4@PDMS-3 paper are measured to be 4.1, 7.6, and 11.5 emu g−1, respectively. It can be concluded that the initial amount of iron(III) acetylacetonate influences the loading amount of Fe3O4 nanoparticles and subsequently affects the magnetic performance and the superhydrophobic property of the as-prepared HAP@Fe3O4@PDMS paper. Magnetic Property, Self-Cleaning Ability, Thermal Stability, Fire Resistance, and Superhydrophobicity of the HAP@Fe3O4@PDMS Paper. The as-prepared HAP@ Fe3O4@PDMS paper can be facilely controlled and actuated by a magnetic field owing to its unique magnetic property. Figure 5a shows that a simple magnet provides reversible and large bending deformation, and the HAP@Fe3O4@PDMS paper can be drawn to the magnet when it is released. In addition to the water contact angle tests, the water sliding angle tests were also carried out to further verify the superhydrophobicity of the HAP@Fe3O4@PDMS paper. As shown in Figures S4 and S5 in the Supporting Information, the HAP@Fe3O4@PDMS paper was glued onto a glass slide with an inclined angle of about 5°, and a water droplet rolled down once it dripped on the paper surface. The HAP@Fe3O4@ PDMS paper exhibits low adhesion to water (Figure 5b). The water droplet can easily depart from the paper surface even after being pressed by a syringe needle. The superhydrophobicity and low water adhesion property endow the HAP@ Fe3O4@PDMS paper with an excellent self-cleaning ability. The dirt can be completely cleaned away by water droplets when they roll down the paper surface, as shown in Figure 5c. Other unique properties of the HAP@Fe3O4@PDMS paper are the high thermal stability and nonflammable features. It is E

DOI: 10.1021/acssuschemeng.8b01463 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 5. Digital images demonstrating magnetic response and selfcleaning property of the HAP@Fe3O4@PDMS paper. (a) HAP@ Fe3O4@PDMS paper can be facilely controlled and actuated by a magnet. (b) Digital images of water adhesion behavior on the HAP@ Fe3O4@PDMS paper surface. (c) Self-cleaning test for the HAP@ Fe3O4@PDMS paper.

Figure 6. (a) Water contact angle measurement results of the HAP@ Fe3O4@PDMS paper after treatment at different temperatures for 1 h. (b) Fire-resistance tests of the common cellulose printing paper (first row) and the HAP@Fe3O4@PDMS paper (second row). (c) Digital images of the common cellulose printing paper (first row) and the HAP@Fe3O4@PDMS paper (second row) after the fire resistance test.

noteworthy that the HAP@Fe3O4@PDMS paper possesses excellent resistance to high temperatures. As shown in Figure 6a, the HAP@Fe3O4@PDMS paper can well maintain its superhydrophobicity after treatment in a wide temperature range from −198 to 250 °C. However, when the treating temperature is increased to 300 °C, the water contact angle is reduced to 133.24 ± 3.72°, indicating that the HAP@Fe3O4@ PDMS paper loses its superhydrophobicity and is only hydrophobic due to the decomposition of PDMS. These experimental results indicate the excellent stability of the superhydrophobicity and its potential applications under harsh conditions. To test the fire-resistant performance, the HAP@Fe3O4@ PDMS paper was directly heated in an ethanol flame, and a common cellulose printing paper was chosen for comparison. As illustrated in Figure 6b, the common cellulose printing paper is burned to ash when it is exposed to the ethanol flame for only 4 s. In contrast, the HAP@Fe3O4@PDMS paper can well maintain its whole shape even after heating by the ethanol flame for 2 min (Figure 6c). The excellent nonflammable feature of the HAP@Fe3O4@PDMS paper is due to the inorganic framework of ultralong HAP nanowires with a high thermal stability. It is known that many oily contaminants and solvents are highly flammable and extremely dangerous if being

ignited; development of adsorbent materials with high thermal stability and excellent fire resistance is necessary. Combining the high flexibility, superhydrophobility, magnetic property, high thermal stability, nonflammability, and porous structure, it is expected that the HAP@Fe3O4@PDMS paper is promising for various applications such as oil/water separation, oil collection, and oil recovery. Superhydrophobicity Stability of the HAP@Fe3O4@ PDMS Paper. The stability of superhydrophobic materials is important for practical applications. The mechanical damage and chemical corrosion may lead to the loss of the superhydrophobic function. In this research work, several mechanical damage tests and chemcial corrision tests were performed to evaluate the stability of the HAP@Fe3O4@ PDMS paper, including finger wipe, adhesive tape peeling, knife scratching, sandpaper abrasion, and chemical corrosion. As shown in Figure 7, the HAP@Fe3O4@PDMS paper is able to well preserve its superhydrophobicity after different mechanical damages. The water droplets on the surface of the damaged HAP@Fe3O4@PDMS paper are still nearly spherical, and the corresponding contact angles are all above 151°, indicating that the superhydrophobic HAP@Fe3O4@ PDMS paper has a high mechanical durability. F

DOI: 10.1021/acssuschemeng.8b01463 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 7. High stability of superhydrophobicity of the HAP@Fe3O4@ PDMS paper after different physical damages: (a1) finger wipe, (b1) adhesive tape peeling, (c1) knife scratching, (d1) sandpaper abrasion. (a2, b2, c2, d2) Digital images of water droplets on the HAP@ Fe3O4@PDMS paper after different physical damages.

Figure 8. Water contact angle measurement results of the HAP@ Fe3O4@PDMS paper after soaking in different organic solvents (a) and aqueous solutions with different pH values (b) for 1 h each.

The chemical stability of the HAP@Fe3O4@PDMS paper was also investigated. The HAP@Fe3O4@PDMS paper was soaked in several different organic solvents and aqueous solutions with different pH values for 1 h each, and water contact angles were measured. As shown in Figure 8a, the water contact angles of the HAP@Fe3O4@PDMS paper after soaking in different organic solvents including acetone, cyclohexane, isopropanol, methanol, ethanol, n-hexanol, and ethylene glycol are all higher than 150°, indicating the high superhydrophobicity stability of the HAP@Fe3O4@PDMS paper in these organic solvents. In addition, the pH tolerance of the HAP@Fe3O4@PDMS paper was also tested (Figure 8b). In a wide range of pH values ranging from 2.35 to 12.98, the HAP@Fe3O4@PDMS paper can well maintain its superhydrophobic property, and water contact angles are all higher than 150°. However, the HAP@Fe3O4@PDMS paper loses its superhydrophobicity under the extremely acidic conditions (e.g., pH 1.46). Oil/Water Separation and Continuous Oil Collection Performance of the HAP@Fe3O4@PDMS Paper. The asprepared HAP@Fe3O4@PDMS paper can be used as a filter paper for oil/water separation. As shown in Figure 9, chloroform dyed with oil red can permeate rapidly through

the HAP@Fe3O4@PDMS paper solely by gravity, while water is rejected to go through and stay on the upper surface of the HAP@Fe3O4@PDMS paper. The separation efficiency was measured according to a previously reported method.52 The separation efficiency is calculated to be 99.6%, indicating the excellent oil/water separation performance of the HAP@ Fe3O4@PDMS paper. More importantly, the HAP@Fe3O4@ PDMS paper exhibits good chemical stability in ethanol; it can be washed and recycled using ethanol. Additionally, the recycling performance is tested, and the separation efficiencies are all higher than 99.0% for 10 cycles (Figure 10a). Moreover, the permeation flux is another important parameter for oil/water separation materials. Benefiting from the three-dimensional porous structure formed by ultralong HAP nanowires, the HAP@Fe3O4@PDMS paper exhibits a high permeation flux of 2835.9 L m−2 h−1. In addition, the high permeation flux can be well preserved even after 10 cycles (Figure 10b), indicating the excellent recyclability of the HAP@Fe3O4@PDMS paper. Furthermore, in order to simulate the oil spillage in real life, the soil was dispersed in the oil/water mixture (Figure S6a, Supporting Information). Then, the HAP@Fe3O4@PDMS paper was used as a filter paper to separate the oil from water containing soil. The oil G

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Figure 9. Digital images of the oil (chloroform dyed with oil)/water (dyed with methylene blue) separation process using the HAP@Fe3O4@ PDMS paper as a filter paper.

absorption of oil from oily water; then, the absorbed oil was collected by squeezing the foams.21 This approach is simple and efficient, but the process is time consuming. Also, the mechanical strength and superhydrophobic property of the foams are usually reduced in the repetitive squeeze process. Furthermore, the developed methods combine the superhydrophobic materials with a vacuum system.22,55 In this way, the oil can be separated from oily water in a continuous manner, but this way has complicated operations and needs extra energy. For continuous in situ remediation of the oil-contaminated water, the HAP@Fe3O4@PDMS paper is used to make a vessel-type device with a magnetic property. A mini-paper boat is designed and fabricated using the HAP@Fe3O4@PDMS paper for automatic absorption, collection, transportation, and recovery of oil from oily water under the magnetically driven manipulation (Figure 11a). Owing to its high flexibility, the HAP@Fe3O4@PDMS paper can be folded into a mini-paper boat. The superhydrophobic property prevents water from permeating into the mini-paper boat and makes the mini-paper boat float on the water surface. More importantly, the minipaper boat can be magnetically driven to the oil (cyclohexane dyed with oil red)-polluted region. As shown in Figure 11b, oil can be absorbed and collected automatically without extra driving force by the mini-paper boat. After the oil collection, the mini-paper boat loaded with oil is actuated by a magnet and transported to the edge of the glass dish. The oil loaded in the mini-paper boat can be easily recovered, and the minipaper boat can also be recycled for repeated use. In addition, the separation efficiency of the mini-paper boat for different types of oil were investigated. As shown in Figure 12a, the separation efficiencies for five kinds of oil (isooctane, toluene, petroleum ether, soybean oil, and vacuum pump oil) are all higher than 99.2%, revealing the good applicability for various oils. In addition, the mini-paper boat exhibits excellent recyclability, which is important for practical applications. The mini-paper boat can be washed with ethanol and dried. The high separation efficiency of the mini-paper boat for cyclohexane is well preserved even after 10 cycles (Figure 12b). The operation processes of automatic absorption, collection, transportation, and recovery of oil from oily water using the HAP@Fe3O4@PDMS superhydrophobic magnetic paper are simple, facile, and energy saving. Moreover, the HAP@ Fe3O4@PDMS paper exhibits excellent recyclability and can

Figure 10. Oil (chloroform)/water separation performance of the HAP@Fe3O4@PDMS paper: (a) separation efficiencies for 10 cycles and (b) permeation fluxes for 10 cycles.

quickly passes through the HAP@Fe3O4@PDMS paper, and the soil and water are retained above the HAP@Fe3O4@ PDMS paper. Finally, the clean oil is successfully separated and collected from the oil/water mixture containing soil (Figure S6b, Supporting Information). The separation efficiency is calculated to be 97.8 ± 1.1%, and permeation flux is 2034.6 ± 262.2 L m−2 h−1. Although the filtration process is an effective way to separate oil from oily water, this procedure is limited for the application in large-scale oil spillage because the collection and transportation of a large volume of oil-contaminated water are difficult. It is ideal to realize in situ separation and collection of spilled oil. For this purpose, some technologies have been developed. For instance, the flexible three-dimensional foams with superhydrophobicity were used for the selective H

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Figure 12. (a) Recovery efficiencies of the mini-paper boat made from the HAP@Fe3O4@PDMS superhydrophobic magnetic paper. (b) Recovery efficiencies of the mini-paper boat for cyclohexene over 10 cycles.

good recycling ability (at least 10 times). In addition, a minipaper boat as a continuous oil collection device has been designed and prepared using the HAP@Fe3O4@PDMS superhydrophobic magnetic paper, which can realize automatic absorption, collection, transportation, and recovery of oil from oily water. Because of the unique magnetic response, the minipaper boat can be magnetically driven to the oil-polluted region, and highly efficient oil collection (>99.2%), rapid transportation, and recycling (at least 10 times) can be achieved. Considering the unique advantages demonstrated above, the as-prepared multifunctional HAP@Fe3O4@PDMS superhydrophobic magnetic paper based on ultralong hydroxyapatite nanowires is promising for various applications such as oil-collecting devices, continuous oil/water separation, oily wastewater treatment, and environmental protection.

Figure 11. Automatic absorption, collection, transportation, and recovery of oil from oily water using the HAP@Fe3O4@PDMS superhydrophobic magnetic paper under the magnetically driven manipulation. (a) Schematic illustration of the superhydrophobic mini-paper boat made from the HAP@Fe3O4@PDMS paper for absorption, collection, transportation, and recovery of oil from oily water under the magnetically driven manipulation. (b) Digital images of the process for selective oil absorption, collection, transportion, and recovery of oil from oily water by the mini-paper boat using a magnet.

be reused by a simple washing process. In consideration of its advantages, the HAP@Fe3O4@PDMS superhydrophobic magnetic paper is promising for various applications such as in situ remediation of the oil-contaminated water.

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ASSOCIATED CONTENT

S Supporting Information *

CONCLUSION In summary, a novel kind of free-standing, recyclable, superhydrophobic, fire-retardant, magnetic, large-sized paper based on environmentally friendly ultralong hydroxyapatite nanowires and an oil-collecting device for magnetically controlled oil/water separation have been developed. Ultralong hydroxyapatite nanowires act as a free-standing framework for the immobilization of Fe3O4 nanoparticles, providing high flexibility, porous structure, magnetic property, high thermal stability, and excellent fire resistance of the as-prepared paper. The decorated Fe3O4 nanoparticles on ultralong hydroxyapatite nanowires increase the surface roughness and endow the paper with the superhydrophobic property in combination with a PDMS coating. The as-prepared HAP@Fe3O4@PDMS paper has been demonstrated as an highly efficient filter paper for oil/water separation with high separation efficiencies (up to >99.0%), high permeation flux (up to 2924.3 L m−2 h−1), and

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b01463. Digital images of the Teflon-lined stainless steel autoclave, commercial paper sheet former, water sliding angle test, oil/water separation test from oily water containing some soil, and AFM images. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: 0086-21-52412616. Fax: 0086-21-52413122 (Y.-J. Zhu). *E-mail: [email protected] (Z.-C. Xiong). ORCID

Ying-Jie Zhu: 0000-0002-5044-5046 Zhi-Chao Xiong: 0000-0002-0216-5699 I

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (21601199), the Science and Technology Commission of Shanghai (15JC1491001, 18ZR1445200), and the Shanghai Sailing Program (16YF1413000) is gratefully acknowledged.

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