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Preparation of a new superhydrophobic/superoleophilic corn straw fiber used as an oil absorbent for selective absorption of oil from water

Preparation of a new superhydrophobic/superoleophilic corn straw fiber used as an oil absorbent... Background: Oil leakages frequently occur during oil product development and oil transportation. These incidents are a vital factor in water contamination, thus leading to serious environmental destruction. Therefore, superhydro‑ phobic/superoleophilic material is one of the solutions to treat oily wastewater. Results: This study aimed to develop a simple, fast and low‑ cost method to treat oily wastewater by synthesizing a new superhydrophobic/superoleophilic corn straw fiber via conventional impregnation. The corresponding results illustrate that abundant homogeneous silica (SiO ) granules evenly accreted on the surface of the prepared fiber were conducive to high surface roughness. Meanwhile, (Heptadecafluoro ‑ 1,1,2,2‑ tetradecyl) trimethoxysilane, a sort of silane coupling agent, could greatly reduce surface free energy by grafting with SiO particles on the corn straw fiber surface. The obtained superhydrophobic/superoleophilic corn straw fiber exhibited a water contact angle of 152° and an oil contact angle of 0° for various oils, strongly demonstrating its considerable application as an oil absorbent that can be applied for oil cleanup. In addition, the prepared fiber displayed a great chemical stability and environmental durability. Conclusions: Due to its high absorption capacity and absorption efficiency, the prepared fiber has great potential as a new oil absorbent for treatment of oily water. Keywords: Oil absorption, Corn straw, Superhydrophobic, Superoleophilicity, SiO particles (Angelova et  al. 2011; Howarter and Youngblood 2007), Background chemical processing methods and biological treatment Oil-spill pollution can significantly destroy the human (Boopathy et  al. 2012) to cleanup oil-bearing wastewa- living environment and poses a severe threat to eco- ter have been reported. Among them, physical adsorp- logical systems (Cojocaru et  al. 2011; Deng et  al. 2013; tion techniques, which employ hydrophilic materials to Schaum et  al. 2010). At present, physical methods remove waste oil from water, is considered as the most economic and valid method due to its high absorption capacity and low cost. However, most of the traditional *Correspondence: wangcy@nefu.edu.cn; stephen6949@msn.com; stephen6949@hit.edu.cn oil absorbents generally encounter problems, such as low Yang Xu and Haiyue Yang contributed equally to this work and share first separation efficiency, high cost, poor absorption capac - authorship 1 ity and non-biodegradable characteristics (Karakasi and Key Laboratory of Bio‑based Material Science and Technology, Ministry of Education, Northeast Forestry University, Harbin 150040, People’s Moutsatsou 2010). Thus, fabrication of new environmen - Republic of China tal-friendly oil absorbing material with a higher absorp- State Key Laboratory of Urban Water Resource and Environment, School tion ability and lower production cost is required and of Environment, Harbin Institute of Technology, Harbin 150090, People’s Republic of China urgent (Yao et al. 2011). Using waste biomass materials as Full list of author information is available at the end of the article © The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Xu et al. Bioresour. Bioprocess. (2018) 5:8 Page 2 of 11 an oil absorbent is of great interest for oil–water separa- providing new insight into producing a sustainable tion and is becoming an area of intense research because high-efficiency oil absorbent from agricultural waste. they are cheap, accessible, and biodegradable (Vlaev et al. 2011; Zang et al. 2016). Methods Recently, superhydrophobic surfaces have attracted Materials and chemicals attention because of their unique properties, including Corn straw from a farm in Harbin Jiangbei District was self-cleaning, anti-sticking, water proof, chemical stabil- used in this study. Ethanol (99.7%), sodium hydroxide ity, and oil recovery (Yuan et  al. 2013; Zang et  al. 2015). (96.0%), hydrogen peroxide (30.0%), hydrochloric acid In theory, construction of a hierarchical rough structure (37%), ammonium hydroxide (25.0%), tetraethoxysilane with low surface energy are two indispensable param- (98.0%) and glacial acetic acid (99.5%) were purchased eters for the formation of superhydrophobic surfaces, from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tian- which are characterized by a water contact angle greater jin, China). (Heptadecafluoro-1,1,2,2-tetradecyl) trimeth - than 150° and a water sliding angle less than 10° (Kim oxysilane (99.5%), employed to decorate SiO granules, et  al. 2011; Li et  al. 2010; Wang et  al. 2011a). To date, was purchased from Aladdin Chemicals Co., Ltd. (Shang- multiple technologies and various materials have been hai, China). All chemicals were used as received with- proposed to achieve superhydrophobic and superoleo- out any purification. Diesel oil, gasoline, crude oil, bean philic surfaces, such as the solution-immersion process, oil, n-hexane, octane, toluene, and chloroform, supplied chemical etching, layer-by-layer assembly, laser treating, by Sigma-Aldrich Co., Ltd. (St. Louis, MO, USA), were and ultrasound irradiation (Liang and Guo 2013; Zhang used for oil contact angle testing and absorption capacity et al. 2013; Zhou et al. 2013). measurements. Corn straw fiber, a natural and biodegradable bio - mass from agricultural waste, is usually discarded and Pre‑treatment of the corn straw fiber burned on the spot, thus resulting in severe air pollu- Peeled corn straw was placed in a pulveriser to obtain tion. In fact, efficient utilization of straw resources is straw fibers, and then sieved through 60 and 80 mesh vital to solving the serious air pollution caused by straw standard screens to collect fibers (250–425 μm). In addi - burning. Zang et  al. (2016) removed oil from water tion, corn straw fibers were ultrasonically rinsed three using superhydrophobic/superoleophilic corn straw fib - times with deionized water, ethanol and deionized water. ers that they prepared with ZnO particles via conven- The corn straw was then placed in an aqueous solution tional impregnation. ZnO particles are hollow spheres of NaOH (100 mL, 0.5 wt%) and 30% H O (3.5 mL) with 2 2 with an average diameter approximately 5  μm, and a stirring at ambient temperature for 14  h. This helped in hexadecyltrimethoxysilane (HDTMOS) chemical agent removing the resins and additional impurities present in is employed to act as a hydrophobic modifier. In this the corn straw and exposed the hydroxy groups. Next, study, we exploited corn straw fibers as a raw material the pH of above solution was adjusted to 6.5–7.0 with to prepare a high-efficiency oil absorbent that exhibits HCl (6  mol/L). After washing several times with deion- great chemical stability and environmental friendli- ized water to remove chemical residues, the pretreated ness. The superhydrophobic/superoleophilic properties corn straw was dried at 40  °C for 24  h until its weigh of the prepared corn straw fiber arose from the com - remained constant. bined effects from the deposition of homogeneous SiO inorganic particles, with an average particle size about Synthesis of SiO particles 40–50 nm using the sol–gel method, and the hydropho-The SiO particles were fabricated by a sol–gel process. bic embellishment of (Heptadecafluoro-1,1,2,2-tetrade - Briefly, 10 mL NH OH was added dropwise into a beaker cyl) trimethoxysilane (PTES), thus giving the product containing 180  mL ethanol, 20  mL tetraethoxysilane the ability to efficiently dislodge oils from oily waste - and 20 mL deionized water. The solution was vigorously water. Compared to HDTMOS, the amount of PTES is stirred for 2  h at room temperature. Next, the mixture less; therefore, the production cost is reduced. In view was left for 10  h to produce a white suspension. Homo- of its intrinsic water-repellence, high absorption capac-geneous SiO particles were dissociated from the white ity, chemical stability and environmental friendliness, suspension by centrifugation in ethanol and then dried at the prepared corn straw fiber floats on the surface of 60 °C for 6 h. the water after absorbing the oil, allowing it to be easily transported and recycled. The information obtained in Fabrication of superhydrophobic/superoleophilic corn this study demonstrates that the prepared superhydro- straw fiber phobic/superoleophilic corn straw fiber can be widely In detail, 0.1 g SiO was added to a solution of 10 mL anhy- applied in the treatment of oily wastewater, thereby drous ethanol, 0.03 mL PTES, 0.025 mL H O, and 0.005 mL 2 Xu et al. Bioresour. Bioprocess. (2018) 5:8 Page 3 of 11 acetic acid. Subsequently, 0.2 g corn straw was added into photoelectron spectroscopy (XPS, PHI Thermo Fisher the mixture. The reaction was maintained under stirring at Scientific, USA) and energy-dispersive X-ray analysis room temperature for 5  h, and then placed in an oven at (EDX, Quantax70, USA). Water contact angle (WCA) 65 °C for 3 h. Finally, the superhydrophobic/superoleophilic and oil contact angle (OCA) measurements were car- corn straw fibers were acquired by drying the corn straw at ried out on a contact angle instrument (CA-A, Hitachi, 50 °C until its weight was consistent. Japan) by dropping a 5 μL deionized water or oil onto five In this study, the PTES chemical agent acted as a hydro- or more different positions on the corn straw specimens. phobic modifier, with its modification mechanism as fol - The values of the WCA and OCA were determined as lows (Fig. 1): silicon hydroxyl groups, which were generated averages of those of five measurements. from a hydrolysis reaction with the PTES reagent, reacted with hydroxyl groups on the surfaces of SiO particles Evaluation of absorption capability and oil removal and pristine corn straw fibers. Therefore, the hydrophobic efficiency heptadecafluoro-decylalkyl of PTES was introduced onto An absorption capability test was performed in pure the surface of the fibers to induce the low surface energy of oil by suspending a nylon net bag containing 0.5  g corn superhydrophobic/superoleophilic corn straw. straw into a beaker with 150 mL oil at room temperature. After 5  h, the nylon bag was taken out from the oil and Characterization of pristine corn straw fiber let to stand for 10  min. Absorption capability calculated and superhydrophobic/superoleophilic corn straw fiber using Eq. (1): Scanning electron microscope (SEM) images were col- q = (m − m )/m 2 1 1 (1) lected on a Hitachi TM3030 tabletop microscope. The chemical composition of the prepared corn straw prod- where q is the absorption capability (g/g); m is the uct was detected by Fourier transform infrared spec- weight of corn straw fibers after the absorption; and m is troscopy (FTIR, Magna-IR 560, Nicolet, USA), X-ray the initial weight of corn straw fibers prior to absorption. Fig. 1 a Synthesis of superhydrophobic/superoleophilic corn straw and a water droplet on the resulting corn straw fiber surface. b Schematic illustration for the modification of SiO particles with PTES. c Chemical structure of (Heptadecafluoro ‑1,1,2,2‑tetradecyl) trimethoxysilane (PTES) 2 Xu et al. Bioresour. Bioprocess. (2018) 5:8 Page 4 of 11 Oil removal efficiency experiments were performed in five measurements. A p value  ≥  0.95 for the Student’s t an oil–water system, which was similar to the oil absorp- test demonstrates reliability of the experimental data. tion capability test. Nylon net bags containing 0.5 g corn straw were placed in 150 mL oil–water mixtures with dif- Results and discussion ferent mass ratios, and then stirred (500  r/min) at room Micro‑structure analysis of a novel superhydrophobic/ temperature. After 5  h, the nylon bags were removed superoleophilic corn straw fiber from the oil and let to stand for 10 min. The oil removal It is known that surface microtopography is primarily efficiency was calculated using Eq. (2): responsible for establishing superhydrophobic surface, similar to the lotus leaf’s self-cleaning property that is k = (w − w − w )/(w − w ) 3 2 1 3 2 (2) associated with its micro/nanostructure (Wang et  al. where k is the oil removal efficiency (%); w is the weight 2015; Autumn et al. 2000; Ju et al. 2012; Wei et al. 2010; of the corn straw fibers after absorption; w is the initial Gao et al. 2009; Zhang et al. 2008). Therefore, it is crucial weight of the corn straw fibers prior to absorption; and to survey the surface micro-profiles of corn straw fiber w is the weight of water absorbed in the absorbents. by SEM. The corresponding surface morphology results In this study, one-way analysis of variance (AVONA) of raw corn straw fiber and superhydrophobic/superoleo - was used to analyze the reliability of the experimental philic corn straw fiber are shown in Fig.  2. At low SEM data. All experimental data are presented as averages of magnifications (Fig.  2a, b), there was no marked differ - ence in the fibrous shape between untreated sample and Fig. 2 SEM images of the raw corn straw fiber (a, b) and superhydrophobic/superoleophilic corn straw fiber (c, d) at different magnifications. Com‑ pared to raw corn straw fiber, SiO particles compactly deposit on the superhydrophobic/superoleophilic corn straw fiber surface 2 Xu et al. Bioresour. Bioprocess. (2018) 5:8 Page 5 of 11 modified product, indicating that the product retained characteristics. For raw corn straw fiber, a water con - the characteristics of the original corn straw fiber. In con - tact angle of 0° was visible on fiber surface, which is trast with the smooth fiber surface of pristine corn straw, ascribed to massive hydroxyl groups on a pristine fiber the high magnification SEM image of the prepared supe - surface (Fig.  3a). In contrast, a spherical water droplet rhydrophobic/superoleophilic corn straw fiber showed a was observed on the prepared fiber, with a water contact surface layer of solid spherical granules, with an average angle of 152° (Fig.  3c), indicating its superior superhy- diameter of 40–50  nm (Fig.  2c), which was attributed to drophobicity. Moreover, when dripped on the surface of the compact deposition of the SiO particles. the raw corn straw fiber and prepared corn straw fiber, oil droplets instantly spread, indicating an oil contact Surface wettability of superhydrophobic/superoleophilic angle of 0° (Fig. 3b, d). Uniform coverage of sub-microm- corn straw fibereter SiO microspheres, coupled with micron-sized corn The principle of surface wettability can usually be dem - straw fiber, while using the PTES function as a modifier onstrated by the Young equation, as follows (Wang et al. to ornament SiO particles would facilitate low surface 2015): energy. The combination of a particularly hierarchical rough structure and low surface energy is regarded as γ − γ sv sl cos θ = an indispensable condition when constructing special (3) lv superhydrophobic and superoleophilic material from the where γ , γ and γ are the interfacial free energy of resulting corn straw fiber surface. Because air was cap - sv sl lv solid/vapor, solid/liquid, and liquid/vapor, respectively; tured and trapped, while falling onto corn straw fiber and θ is the contact angle. In general, the value of the surface, by the abundant cavities and interspaces among contact angle is an essential to measuring the surface SiO particles on the fiber surface, a water droplet could wettability of a superhydrophobic material. Hence, the contact the trapped air to manifest a non-wetting phe- water/oil contact angles of corn straw fiber were investi - nomenon. As soon as water was dropped on the prepared gated to determine its superhydrophobic/superoleophilic corn straw fiber surface, it was repelled without leaving a Fig. 3 Images of a water droplet (a) and an oil droplet on raw corn straw fiber (b); a water droplet (c) and an oil droplet (d) on prepared superhy‑ drophobic/superoleophilic corn straw fiber. Compared to raw corn straw fiber, the resulting corn straw fiber indicates its superior superhydropho ‑ bicity Xu et al. Bioresour. Bioprocess. (2018) 5:8 Page 6 of 11 trail, which demonstrates the great waterproof character- superhydrophobic/superoleophilic corn straw fiber are −1 istic of the resulting product (Wang et al. 2011b). Accord- listed in Fig.  4. The absorption peak at 955  cm was ingly, it could be deduced that the wettability of the corn ascribed to stretching vibration of isolated Si–OH, which straw fiber was transformed from superhydrophilicity to was perceptible only in the case of bare silica (Fig.  4a) superhydrophobicity. Taken together, these results dem- (Kulkarni et al. 2008). Moreover, corresponding to Si–O– onstrate the superhydrophobic and superoleophilic prop- Si asymmetric stretching and symmetric stretching, the −1 erties of the prepared corn straw fiber. bands at 1056 and 795 cm were also observable (Hsieh In this study, the combination of numerous SiO par- et al. 2010; Vinogradova et al. 2006). Compared with pris- −1 ticle aggregates and surface modification by PTES could tine corn straw fiber, the band at 804  cm was due to prevent water from wetting the treated fiber surface and Si–O–Si symmetric stretching (Hsieh et al. 2010) and the −1 result in water droplets on the obtained corn straw fiber absorption peak at 1203  cm was a typical characteris- surface rolling off without leaving a trace, thereby dem - tic of the C–F stretching vibration of PTES (Zhou et  al. onstrating a novel non-wetting material. Hence, the pre- 2013), which proves that the SiO particle deposition and pared superhydrophobic/superoleophilic corn straw fiber PTES organic chemistry reagent were observed on the absorbs only oil while completely repelling water. prepared superhydrophobic/superoleophilic corn straw fiber surface (Fig. 4b). Surface chemical component analysis In this study, SiO particles were prepared using the Stö- ber method, where tetraethoxysilane and ammonium hydroxide acted as a precursor and a catalyst, respec- tively. The synthesis process of SiO particles was divided into two stages, which included the hydrolysis of tetra- ethoxysilane and condensation polymerization of the hydrolyzed intermediate in the presence of the ammonia catalyst. The concrete forming process of the SiO parti- cles was as follows (Wang et al. 2011b): (1) Hydrolysis : Si − OC H + 4H O [ ] 2 5 2 → Si − (OH) + 4C H OH. 2 5 (2) Alcohol condensation: Si − (OH) + Si − [OC H ] 2 5 4 4 →≡ Si − O − Si ≡+4C H OH. 2 5 (3) Water condensation: Si − (OH) + Si − (OH) 4 4 →≡ Si − O − Si ≡ +4H O. Large amounts of hydroxyl groups on the surface of the silica particles are critical for the preparation of superhy- drophobic/superoleophilic corn straw fibers (Wang et al. 2011b). In addition, due to the great influence of silica size distribution on the generation of a superhydropho- bic surface, we strictly abided to the well-known Stöber method for fabrication of silica particles. FTIR, XPS and EDX were used to analyze the sur- Fig. 4 a FTIR spectra of SiO particles; b FTIR spectra of raw corn face chemical composition of superhydrophobic/ straw fiber (i) and prepared superhydrophobic/superoleophilic corn superoleophilic corn straw fiber. The relevant FTIR straw fiber (ii) spectra of SiO particles, pristine corn straw fiber and 2 Xu et al. Bioresour. Bioprocess. (2018) 5:8 Page 7 of 11 The XPS spectra of pristine corn straw fiber and supe - observed. By contrast, the XPS spectra of superhydro- rhydrophobic/superoleophilic corn straw fiber are shown phobic/superoleophilic corn straw fiber demonstrated in Fig.  5. With regards to raw corn straw fiber (Fig.  5a), four new peaks including Si2p, Si2s, F1s and F KLL, only peaks corresponding to C1s and O1s elements were which accounted for the generation of SiO particles and PTES on the prepared fiber surface. However, the peak intensity of C1s and O1s in curve b was weaker than that in curve a. This can be attributed to the addition of SiO2 particles and PTES, thereby decreasing the relative mass ratio of C1s and O1s. Apart from FTIR and XPS characterizations, the elemental composition of superhydrophobic/supero- leophilic corn straw fiber was investigated via energy- dispersive X-ray analysis (EDX). The oxygen (O) peak and the carbon (C) peak were observed in corn straw fiber (Fig. 6). In comparison with the raw fiber, there were two new peaks of silica (Si) and fluorine (F) induced by SiO and PTES in the prepared superhydrophobic/superoleo- philic corn straw fiber, thus providing evidence for the presence of SiO particles and PTES on the obtained corn straw fiber surface. Taken together, these results show that SiO particles were successfully modified by Fig. 5 XPS spectra of pristine corn straw fiber (a) and superhydro ‑ PTES and truly existed on the surface of the superhydro- phobic/superoleophilic corn straw fiber (b) phobic/superoleophilic corn straw fiber. Fig. 6 The wt% of each element and EDX spectra of pristine corn straw fiber (a) and the prepared superhydrophobic/superoleophilic corn straw fiber (b) Xu et al. Bioresour. Bioprocess. (2018) 5:8 Page 8 of 11 Environmental durability and application in water–oil oil of the superhydrophobic/superoleophilic corn straw separation fiber were monitored over time at ambient temperature Considering the importance of environment durabil- and humidity (Fig. 7b). After 150 days, there were no dis- ity and chemical stability for prepared materials used in tinct changes in the water and oil contact angles of the practical application, it is necessary to investigate these prepared corn straw fiber, which clearly demonstrates properties with respect to superhydrophobic/superoleo- that the superhydrophobic/superoleophilic corn straw philic corn straw fiber to confirm its potential as an oil fiber obtained in this study possess excellent environ - absorbent. The effects of acidic and alkaline conditions ment stability. on the wettability of superhydrophobic/superoleophilic Because the prepared corn straw exhibited favorable corn straw fiber were systematically investigated. Contact superhydrophobic/superoleophilic performance in both angle measurements were performed by pipetting 5  μL acidic solutions and under ambient conditions, these pre- aqueous solution, from pH 0–14, onto fiber surfaces in pared fibers could be utilized as highly selective absorp - order to evaluate chemical stability and  durability of the tion materials to achieve effective separation of oil–water prepared material (Fig.  7a). The measured water con - mixtures. Oil absorption performance is measured by tact angle ranged from 152° to 150°, while the oil contact the absorption capacity (g/g), as well as the oil removal angle remained constant at 0°, implying that the result- efficiency (%). To determine the maximum absorption ing fiber surface still maintained outstanding superhy - capacity of superhydrophobic/superoleophilic corn straw drophobicity and superoleophilicity properties, even in fiber, experiments were performed in a variety of pure strong acid and strong alkaline conditions. To evaluate its oils and organic solvents. Figure  8a presents the absorp- environmental stability, the contact angles of water and tion capacities of raw corn straw fiber, pretreated corn Fig. 7 a Variation of water contact angle and oil contact angle of Fig. 8 a The absorption capacities of raw corn straw fiber, pretreated superhydrophobic/superoleophilic corn straw surfaces in aqueous corn straw fiber and prepared superhydrophobic/superoleophilic solutions with different pH values. b The relationship between con‑ corn straw fiber for various oils and organic solvents; b absorption tact angles of the resulting superhydrophobic/superoleophilic corn efficiency of superhydrophobic/superoleophilic corn straw with dif‑ straw fibers and days of storage in air environment ferent mass ratios of water to oil Xu et al. Bioresour. Bioprocess. (2018) 5:8 Page 9 of 11 straw fiber and the prepared superhydrophobic/supero - Table 1 Comparison of the oil absorption capacity of some recently reported oil sorbent and the samples prepared leophilic corn straw fiber in various oils and organic sol - in this study taking diesel oil for example vents. The adsorption capacity of raw corn straw fiber was very low and was always less than 10 g/g. In contrast Oil Oil sorbent Oil adsorption capacity References (g/g) with the raw fiber, the pretreated corn straw fiber exhib - ited a higher absorption capacity for all oils and organic Diesel Corn straw 17.5 This study solvents, with its value almost 3 times higher than pris- fiber tine corn straw fiber. Moreover, the oil absorption quan - Peat sorb 2.7 (Ribeiro et al. 2003) tity of the prepared superhydrophobic/superoleophilic Sugi bark 16.5 (Saito et al. 2003) corn straw fiber for diesel oil, crude oil, bean oil, and Cotton 41 (Liu et al. 2014) chloroform was relatively large at 17.5, 20.3, 22.6, and Corn straw 18 (Zang et al. 2016) fiber 27.8 times their own quality, respectively; however, the oil absorption quantity of the prepared superhydropho- Sawdust 11.5 (Gan et al. 2016) bic/superoleophilic corn straw fiber for gasoline, n-hex - ane, octane, and toluene was relatively small at 15.5, 13.5, 15.1, and 16.3 times their own quality, respectively. The Conclusions reason is that the oil absorption capacity is relevant to In this study, we successfully developed a prepara- the viscosity and density of the oil products and organic tion process for a novel superhydrophobic/superoleo- solvents, with a higher viscosity and density resulting in philic corn straw fiber by attachment of PTES-modified a saturated absorption capacity of prepared corn straw SiO particles onto the fiber surface via the sol–gel and fiber (Guo et  al. 2015). For the same oil or organic sol - impregnation method. The prepared corn straw fiber vents, the absorption capacity of the prepared superhy- exhibited outstanding properties of superhydrophobicity drophobic/superoleophilic corn straw fiber was slightly and simultaneous superoleophilicity with a water contact higher than that of the pretreated corn straw fiber, indi - angle of 152° and an oil contact angle of 0° for different cating that the superhydrophobic modification can fur - oils. In addition, the microtopography, wetting property, ther enhance the oil capacity, which is beneficial for chemical composition and oil absorption performance practical application. In addition to the absorption capac- were comprehensively studied. Results revealed that SiO ity, the absorption efficiency of the prepared corn straw granules successfully modified by PTES were robustly fiber was studied to ascertain the potential of superhy - attached to the fiber surface, resulting in a hierarchical drophobic/superoleophilic corn straw fiber in oil/water structure and low surface energy, thus giving rise to the separation. In theory, the superhydrophobic/superoleo- significant phenomenon of both superhydrophobicity philic corn straw fibers absorb very little water; however, and superoleophilicity. Moreover, the prepared super- there are errors in the oil absorption efficiency. When hydrophobic/superoleophilic corn straw fiber displayed the water content of the oil–water mixture increased, great chemical stability and environmental durabil- the prepared corn straw fiber absorbed a small amount ity. Most importantly, the prepared superhydrophobic/ of water during magnetic stirring. The oil removal effi - superoleophilic corn straw fiber possessed an excel - ciency of the resulting fiber for diesel oil and crude oil lent absorption capacity and high absorption efficiency. varied from 100 to 99%, with different mass ratios of Taken together, these results demonstrate that the pre- water-to-oil (Fig.  8b). The main cause of this phenome - pared fiber obtained in this study exhibits a high applica - non is that there was a small amount of water absorbed at tion potential to effectively separate oil/water mixtures. the same time that the prepared corn straw fibers absorb oil (Wang et al. 2013), which indicates that the prepared Abbreviations PTES: (Heptadecafluoro ‑1,1,2,2‑tetradecyl) trimethoxysilane; FTIR: Fourier trans‑ fiber can be widely applied for oil removal from water. formation infrared spectroscope; XPS: X‑ray photoelectron spectroscopy; EDX: Taken together, because of the high absorption capacity energy‑ dispersive X‑ray analysis; WCA: water contact angle; OCA: oil contact and oil removal efficiency, we clearly demonstrate that angle; SEM: scanning electron microscope. the novel superhydrophobic/superoleophilic corn straw Equation parameters fiber obtained in this study can be regarded as a high- γ , γ and γ : solid–vapor, solid–liquid and liquid–vapor interfacial tensions, sv sl lv efficient oil absorbent with great chemical stability and respectively; θ: contact angle; q: sorption capability (g/g); m : the weight of corn straw fibers after absorption; m : the initial weight of corn straw fibers environmental durability. Moreover, it has a higher oil before absorption; k: oil removal efficiency (%); w : the weight of corn straw adsorption capacity, compared to other biomass-based fibers after absorption; w : the initial weight of corn straw fibers before absorbents (Table 1). absorption; w : the weight of water absorbed in the absorbents. 1 Xu et al. Bioresour. Bioprocess. (2018) 5:8 Page 10 of 11 Authors’ contributions Guo P, Zhai S, Xiao Z, An Q (2015) One‑step fabrication of highly stable, YX and HY designed the study, performed experiments, analyzed data, and superhydrophobic composites from controllable and low‑ cost PMHS/ prepared the manuscript. DZ, FL and XH contributed to the discussion. CY, TEOS sols for efficient oil cleanup. J Colloid Interface Sci. 446(Supplement SHH, JSC and YZ reviewed the results, helped in data analysis, and edited the C):155–162. https://doi.org/10.1016/j.jcis.2015.01.062 manuscript. All authors read and approved the final manuscript. 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Zang D, Liu F, Zhang M, Niu X, Gao Z, Wang C (2015) Superhydrophobic coat‑ org/10.1002/adma.201204520 ing on fiberglass cloth for selective removal of oil from water. Chem Eng J Zhou X, Zhang Z, Xu X, Guo F, Zhu X, Men X, Ge B (2013) Robust and durable 262:210–216. https://doi.org/10.1016/j.cej.2014.09.082 superhydrophobic cotton fabrics for oil/water separation. ACS Appl Zang D, Zhang M, Liu F, Wang C (2016) Superhydrophobic/superoleophilic Mater Interfaces 5(15):7208–7214. https://doi.org/10.1021/am4015346 corn straw fibers as effective oil sorbents for the recovery of spilled oil. J Chem Technol Biotechnol 91(9):2449–2456. https://doi.org/10.1002/ jctb.4834 Zhang Y, Wang H, Yan B, Zhang Y, Yin P, Shen G, Yu R (2008) A rapid and effi‑ cient strategy for creating super‑hydrophobic coatings on various mate ‑ rial substrates. J Mater Chem 18(37):4442–4449. https://doi.org/10.1039/ B801212A http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png "Bioresources and Bioprocessing" Springer Journals

Preparation of a new superhydrophobic/superoleophilic corn straw fiber used as an oil absorbent for selective absorption of oil from water

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Springer Journals
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2018 The Author(s)
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2197-4365
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10.1186/s40643-018-0194-8
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Abstract

Background: Oil leakages frequently occur during oil product development and oil transportation. These incidents are a vital factor in water contamination, thus leading to serious environmental destruction. Therefore, superhydro‑ phobic/superoleophilic material is one of the solutions to treat oily wastewater. Results: This study aimed to develop a simple, fast and low‑ cost method to treat oily wastewater by synthesizing a new superhydrophobic/superoleophilic corn straw fiber via conventional impregnation. The corresponding results illustrate that abundant homogeneous silica (SiO ) granules evenly accreted on the surface of the prepared fiber were conducive to high surface roughness. Meanwhile, (Heptadecafluoro ‑ 1,1,2,2‑ tetradecyl) trimethoxysilane, a sort of silane coupling agent, could greatly reduce surface free energy by grafting with SiO particles on the corn straw fiber surface. The obtained superhydrophobic/superoleophilic corn straw fiber exhibited a water contact angle of 152° and an oil contact angle of 0° for various oils, strongly demonstrating its considerable application as an oil absorbent that can be applied for oil cleanup. In addition, the prepared fiber displayed a great chemical stability and environmental durability. Conclusions: Due to its high absorption capacity and absorption efficiency, the prepared fiber has great potential as a new oil absorbent for treatment of oily water. Keywords: Oil absorption, Corn straw, Superhydrophobic, Superoleophilicity, SiO particles (Angelova et  al. 2011; Howarter and Youngblood 2007), Background chemical processing methods and biological treatment Oil-spill pollution can significantly destroy the human (Boopathy et  al. 2012) to cleanup oil-bearing wastewa- living environment and poses a severe threat to eco- ter have been reported. Among them, physical adsorp- logical systems (Cojocaru et  al. 2011; Deng et  al. 2013; tion techniques, which employ hydrophilic materials to Schaum et  al. 2010). At present, physical methods remove waste oil from water, is considered as the most economic and valid method due to its high absorption capacity and low cost. However, most of the traditional *Correspondence: wangcy@nefu.edu.cn; stephen6949@msn.com; stephen6949@hit.edu.cn oil absorbents generally encounter problems, such as low Yang Xu and Haiyue Yang contributed equally to this work and share first separation efficiency, high cost, poor absorption capac - authorship 1 ity and non-biodegradable characteristics (Karakasi and Key Laboratory of Bio‑based Material Science and Technology, Ministry of Education, Northeast Forestry University, Harbin 150040, People’s Moutsatsou 2010). Thus, fabrication of new environmen - Republic of China tal-friendly oil absorbing material with a higher absorp- State Key Laboratory of Urban Water Resource and Environment, School tion ability and lower production cost is required and of Environment, Harbin Institute of Technology, Harbin 150090, People’s Republic of China urgent (Yao et al. 2011). Using waste biomass materials as Full list of author information is available at the end of the article © The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Xu et al. Bioresour. Bioprocess. (2018) 5:8 Page 2 of 11 an oil absorbent is of great interest for oil–water separa- providing new insight into producing a sustainable tion and is becoming an area of intense research because high-efficiency oil absorbent from agricultural waste. they are cheap, accessible, and biodegradable (Vlaev et al. 2011; Zang et al. 2016). Methods Recently, superhydrophobic surfaces have attracted Materials and chemicals attention because of their unique properties, including Corn straw from a farm in Harbin Jiangbei District was self-cleaning, anti-sticking, water proof, chemical stabil- used in this study. Ethanol (99.7%), sodium hydroxide ity, and oil recovery (Yuan et  al. 2013; Zang et  al. 2015). (96.0%), hydrogen peroxide (30.0%), hydrochloric acid In theory, construction of a hierarchical rough structure (37%), ammonium hydroxide (25.0%), tetraethoxysilane with low surface energy are two indispensable param- (98.0%) and glacial acetic acid (99.5%) were purchased eters for the formation of superhydrophobic surfaces, from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tian- which are characterized by a water contact angle greater jin, China). (Heptadecafluoro-1,1,2,2-tetradecyl) trimeth - than 150° and a water sliding angle less than 10° (Kim oxysilane (99.5%), employed to decorate SiO granules, et  al. 2011; Li et  al. 2010; Wang et  al. 2011a). To date, was purchased from Aladdin Chemicals Co., Ltd. (Shang- multiple technologies and various materials have been hai, China). All chemicals were used as received with- proposed to achieve superhydrophobic and superoleo- out any purification. Diesel oil, gasoline, crude oil, bean philic surfaces, such as the solution-immersion process, oil, n-hexane, octane, toluene, and chloroform, supplied chemical etching, layer-by-layer assembly, laser treating, by Sigma-Aldrich Co., Ltd. (St. Louis, MO, USA), were and ultrasound irradiation (Liang and Guo 2013; Zhang used for oil contact angle testing and absorption capacity et al. 2013; Zhou et al. 2013). measurements. Corn straw fiber, a natural and biodegradable bio - mass from agricultural waste, is usually discarded and Pre‑treatment of the corn straw fiber burned on the spot, thus resulting in severe air pollu- Peeled corn straw was placed in a pulveriser to obtain tion. In fact, efficient utilization of straw resources is straw fibers, and then sieved through 60 and 80 mesh vital to solving the serious air pollution caused by straw standard screens to collect fibers (250–425 μm). In addi - burning. Zang et  al. (2016) removed oil from water tion, corn straw fibers were ultrasonically rinsed three using superhydrophobic/superoleophilic corn straw fib - times with deionized water, ethanol and deionized water. ers that they prepared with ZnO particles via conven- The corn straw was then placed in an aqueous solution tional impregnation. ZnO particles are hollow spheres of NaOH (100 mL, 0.5 wt%) and 30% H O (3.5 mL) with 2 2 with an average diameter approximately 5  μm, and a stirring at ambient temperature for 14  h. This helped in hexadecyltrimethoxysilane (HDTMOS) chemical agent removing the resins and additional impurities present in is employed to act as a hydrophobic modifier. In this the corn straw and exposed the hydroxy groups. Next, study, we exploited corn straw fibers as a raw material the pH of above solution was adjusted to 6.5–7.0 with to prepare a high-efficiency oil absorbent that exhibits HCl (6  mol/L). After washing several times with deion- great chemical stability and environmental friendli- ized water to remove chemical residues, the pretreated ness. The superhydrophobic/superoleophilic properties corn straw was dried at 40  °C for 24  h until its weigh of the prepared corn straw fiber arose from the com - remained constant. bined effects from the deposition of homogeneous SiO inorganic particles, with an average particle size about Synthesis of SiO particles 40–50 nm using the sol–gel method, and the hydropho-The SiO particles were fabricated by a sol–gel process. bic embellishment of (Heptadecafluoro-1,1,2,2-tetrade - Briefly, 10 mL NH OH was added dropwise into a beaker cyl) trimethoxysilane (PTES), thus giving the product containing 180  mL ethanol, 20  mL tetraethoxysilane the ability to efficiently dislodge oils from oily waste - and 20 mL deionized water. The solution was vigorously water. Compared to HDTMOS, the amount of PTES is stirred for 2  h at room temperature. Next, the mixture less; therefore, the production cost is reduced. In view was left for 10  h to produce a white suspension. Homo- of its intrinsic water-repellence, high absorption capac-geneous SiO particles were dissociated from the white ity, chemical stability and environmental friendliness, suspension by centrifugation in ethanol and then dried at the prepared corn straw fiber floats on the surface of 60 °C for 6 h. the water after absorbing the oil, allowing it to be easily transported and recycled. The information obtained in Fabrication of superhydrophobic/superoleophilic corn this study demonstrates that the prepared superhydro- straw fiber phobic/superoleophilic corn straw fiber can be widely In detail, 0.1 g SiO was added to a solution of 10 mL anhy- applied in the treatment of oily wastewater, thereby drous ethanol, 0.03 mL PTES, 0.025 mL H O, and 0.005 mL 2 Xu et al. Bioresour. Bioprocess. (2018) 5:8 Page 3 of 11 acetic acid. Subsequently, 0.2 g corn straw was added into photoelectron spectroscopy (XPS, PHI Thermo Fisher the mixture. The reaction was maintained under stirring at Scientific, USA) and energy-dispersive X-ray analysis room temperature for 5  h, and then placed in an oven at (EDX, Quantax70, USA). Water contact angle (WCA) 65 °C for 3 h. Finally, the superhydrophobic/superoleophilic and oil contact angle (OCA) measurements were car- corn straw fibers were acquired by drying the corn straw at ried out on a contact angle instrument (CA-A, Hitachi, 50 °C until its weight was consistent. Japan) by dropping a 5 μL deionized water or oil onto five In this study, the PTES chemical agent acted as a hydro- or more different positions on the corn straw specimens. phobic modifier, with its modification mechanism as fol - The values of the WCA and OCA were determined as lows (Fig. 1): silicon hydroxyl groups, which were generated averages of those of five measurements. from a hydrolysis reaction with the PTES reagent, reacted with hydroxyl groups on the surfaces of SiO particles Evaluation of absorption capability and oil removal and pristine corn straw fibers. Therefore, the hydrophobic efficiency heptadecafluoro-decylalkyl of PTES was introduced onto An absorption capability test was performed in pure the surface of the fibers to induce the low surface energy of oil by suspending a nylon net bag containing 0.5  g corn superhydrophobic/superoleophilic corn straw. straw into a beaker with 150 mL oil at room temperature. After 5  h, the nylon bag was taken out from the oil and Characterization of pristine corn straw fiber let to stand for 10  min. Absorption capability calculated and superhydrophobic/superoleophilic corn straw fiber using Eq. (1): Scanning electron microscope (SEM) images were col- q = (m − m )/m 2 1 1 (1) lected on a Hitachi TM3030 tabletop microscope. The chemical composition of the prepared corn straw prod- where q is the absorption capability (g/g); m is the uct was detected by Fourier transform infrared spec- weight of corn straw fibers after the absorption; and m is troscopy (FTIR, Magna-IR 560, Nicolet, USA), X-ray the initial weight of corn straw fibers prior to absorption. Fig. 1 a Synthesis of superhydrophobic/superoleophilic corn straw and a water droplet on the resulting corn straw fiber surface. b Schematic illustration for the modification of SiO particles with PTES. c Chemical structure of (Heptadecafluoro ‑1,1,2,2‑tetradecyl) trimethoxysilane (PTES) 2 Xu et al. Bioresour. Bioprocess. (2018) 5:8 Page 4 of 11 Oil removal efficiency experiments were performed in five measurements. A p value  ≥  0.95 for the Student’s t an oil–water system, which was similar to the oil absorp- test demonstrates reliability of the experimental data. tion capability test. Nylon net bags containing 0.5 g corn straw were placed in 150 mL oil–water mixtures with dif- Results and discussion ferent mass ratios, and then stirred (500  r/min) at room Micro‑structure analysis of a novel superhydrophobic/ temperature. After 5  h, the nylon bags were removed superoleophilic corn straw fiber from the oil and let to stand for 10 min. The oil removal It is known that surface microtopography is primarily efficiency was calculated using Eq. (2): responsible for establishing superhydrophobic surface, similar to the lotus leaf’s self-cleaning property that is k = (w − w − w )/(w − w ) 3 2 1 3 2 (2) associated with its micro/nanostructure (Wang et  al. where k is the oil removal efficiency (%); w is the weight 2015; Autumn et al. 2000; Ju et al. 2012; Wei et al. 2010; of the corn straw fibers after absorption; w is the initial Gao et al. 2009; Zhang et al. 2008). Therefore, it is crucial weight of the corn straw fibers prior to absorption; and to survey the surface micro-profiles of corn straw fiber w is the weight of water absorbed in the absorbents. by SEM. The corresponding surface morphology results In this study, one-way analysis of variance (AVONA) of raw corn straw fiber and superhydrophobic/superoleo - was used to analyze the reliability of the experimental philic corn straw fiber are shown in Fig.  2. At low SEM data. All experimental data are presented as averages of magnifications (Fig.  2a, b), there was no marked differ - ence in the fibrous shape between untreated sample and Fig. 2 SEM images of the raw corn straw fiber (a, b) and superhydrophobic/superoleophilic corn straw fiber (c, d) at different magnifications. Com‑ pared to raw corn straw fiber, SiO particles compactly deposit on the superhydrophobic/superoleophilic corn straw fiber surface 2 Xu et al. Bioresour. Bioprocess. (2018) 5:8 Page 5 of 11 modified product, indicating that the product retained characteristics. For raw corn straw fiber, a water con - the characteristics of the original corn straw fiber. In con - tact angle of 0° was visible on fiber surface, which is trast with the smooth fiber surface of pristine corn straw, ascribed to massive hydroxyl groups on a pristine fiber the high magnification SEM image of the prepared supe - surface (Fig.  3a). In contrast, a spherical water droplet rhydrophobic/superoleophilic corn straw fiber showed a was observed on the prepared fiber, with a water contact surface layer of solid spherical granules, with an average angle of 152° (Fig.  3c), indicating its superior superhy- diameter of 40–50  nm (Fig.  2c), which was attributed to drophobicity. Moreover, when dripped on the surface of the compact deposition of the SiO particles. the raw corn straw fiber and prepared corn straw fiber, oil droplets instantly spread, indicating an oil contact Surface wettability of superhydrophobic/superoleophilic angle of 0° (Fig. 3b, d). Uniform coverage of sub-microm- corn straw fibereter SiO microspheres, coupled with micron-sized corn The principle of surface wettability can usually be dem - straw fiber, while using the PTES function as a modifier onstrated by the Young equation, as follows (Wang et al. to ornament SiO particles would facilitate low surface 2015): energy. The combination of a particularly hierarchical rough structure and low surface energy is regarded as γ − γ sv sl cos θ = an indispensable condition when constructing special (3) lv superhydrophobic and superoleophilic material from the where γ , γ and γ are the interfacial free energy of resulting corn straw fiber surface. Because air was cap - sv sl lv solid/vapor, solid/liquid, and liquid/vapor, respectively; tured and trapped, while falling onto corn straw fiber and θ is the contact angle. In general, the value of the surface, by the abundant cavities and interspaces among contact angle is an essential to measuring the surface SiO particles on the fiber surface, a water droplet could wettability of a superhydrophobic material. Hence, the contact the trapped air to manifest a non-wetting phe- water/oil contact angles of corn straw fiber were investi - nomenon. As soon as water was dropped on the prepared gated to determine its superhydrophobic/superoleophilic corn straw fiber surface, it was repelled without leaving a Fig. 3 Images of a water droplet (a) and an oil droplet on raw corn straw fiber (b); a water droplet (c) and an oil droplet (d) on prepared superhy‑ drophobic/superoleophilic corn straw fiber. Compared to raw corn straw fiber, the resulting corn straw fiber indicates its superior superhydropho ‑ bicity Xu et al. Bioresour. Bioprocess. (2018) 5:8 Page 6 of 11 trail, which demonstrates the great waterproof character- superhydrophobic/superoleophilic corn straw fiber are −1 istic of the resulting product (Wang et al. 2011b). Accord- listed in Fig.  4. The absorption peak at 955  cm was ingly, it could be deduced that the wettability of the corn ascribed to stretching vibration of isolated Si–OH, which straw fiber was transformed from superhydrophilicity to was perceptible only in the case of bare silica (Fig.  4a) superhydrophobicity. Taken together, these results dem- (Kulkarni et al. 2008). Moreover, corresponding to Si–O– onstrate the superhydrophobic and superoleophilic prop- Si asymmetric stretching and symmetric stretching, the −1 erties of the prepared corn straw fiber. bands at 1056 and 795 cm were also observable (Hsieh In this study, the combination of numerous SiO par- et al. 2010; Vinogradova et al. 2006). Compared with pris- −1 ticle aggregates and surface modification by PTES could tine corn straw fiber, the band at 804  cm was due to prevent water from wetting the treated fiber surface and Si–O–Si symmetric stretching (Hsieh et al. 2010) and the −1 result in water droplets on the obtained corn straw fiber absorption peak at 1203  cm was a typical characteris- surface rolling off without leaving a trace, thereby dem - tic of the C–F stretching vibration of PTES (Zhou et  al. onstrating a novel non-wetting material. Hence, the pre- 2013), which proves that the SiO particle deposition and pared superhydrophobic/superoleophilic corn straw fiber PTES organic chemistry reagent were observed on the absorbs only oil while completely repelling water. prepared superhydrophobic/superoleophilic corn straw fiber surface (Fig. 4b). Surface chemical component analysis In this study, SiO particles were prepared using the Stö- ber method, where tetraethoxysilane and ammonium hydroxide acted as a precursor and a catalyst, respec- tively. The synthesis process of SiO particles was divided into two stages, which included the hydrolysis of tetra- ethoxysilane and condensation polymerization of the hydrolyzed intermediate in the presence of the ammonia catalyst. The concrete forming process of the SiO parti- cles was as follows (Wang et al. 2011b): (1) Hydrolysis : Si − OC H + 4H O [ ] 2 5 2 → Si − (OH) + 4C H OH. 2 5 (2) Alcohol condensation: Si − (OH) + Si − [OC H ] 2 5 4 4 →≡ Si − O − Si ≡+4C H OH. 2 5 (3) Water condensation: Si − (OH) + Si − (OH) 4 4 →≡ Si − O − Si ≡ +4H O. Large amounts of hydroxyl groups on the surface of the silica particles are critical for the preparation of superhy- drophobic/superoleophilic corn straw fibers (Wang et al. 2011b). In addition, due to the great influence of silica size distribution on the generation of a superhydropho- bic surface, we strictly abided to the well-known Stöber method for fabrication of silica particles. FTIR, XPS and EDX were used to analyze the sur- Fig. 4 a FTIR spectra of SiO particles; b FTIR spectra of raw corn face chemical composition of superhydrophobic/ straw fiber (i) and prepared superhydrophobic/superoleophilic corn superoleophilic corn straw fiber. The relevant FTIR straw fiber (ii) spectra of SiO particles, pristine corn straw fiber and 2 Xu et al. Bioresour. Bioprocess. (2018) 5:8 Page 7 of 11 The XPS spectra of pristine corn straw fiber and supe - observed. By contrast, the XPS spectra of superhydro- rhydrophobic/superoleophilic corn straw fiber are shown phobic/superoleophilic corn straw fiber demonstrated in Fig.  5. With regards to raw corn straw fiber (Fig.  5a), four new peaks including Si2p, Si2s, F1s and F KLL, only peaks corresponding to C1s and O1s elements were which accounted for the generation of SiO particles and PTES on the prepared fiber surface. However, the peak intensity of C1s and O1s in curve b was weaker than that in curve a. This can be attributed to the addition of SiO2 particles and PTES, thereby decreasing the relative mass ratio of C1s and O1s. Apart from FTIR and XPS characterizations, the elemental composition of superhydrophobic/supero- leophilic corn straw fiber was investigated via energy- dispersive X-ray analysis (EDX). The oxygen (O) peak and the carbon (C) peak were observed in corn straw fiber (Fig. 6). In comparison with the raw fiber, there were two new peaks of silica (Si) and fluorine (F) induced by SiO and PTES in the prepared superhydrophobic/superoleo- philic corn straw fiber, thus providing evidence for the presence of SiO particles and PTES on the obtained corn straw fiber surface. Taken together, these results show that SiO particles were successfully modified by Fig. 5 XPS spectra of pristine corn straw fiber (a) and superhydro ‑ PTES and truly existed on the surface of the superhydro- phobic/superoleophilic corn straw fiber (b) phobic/superoleophilic corn straw fiber. Fig. 6 The wt% of each element and EDX spectra of pristine corn straw fiber (a) and the prepared superhydrophobic/superoleophilic corn straw fiber (b) Xu et al. Bioresour. Bioprocess. (2018) 5:8 Page 8 of 11 Environmental durability and application in water–oil oil of the superhydrophobic/superoleophilic corn straw separation fiber were monitored over time at ambient temperature Considering the importance of environment durabil- and humidity (Fig. 7b). After 150 days, there were no dis- ity and chemical stability for prepared materials used in tinct changes in the water and oil contact angles of the practical application, it is necessary to investigate these prepared corn straw fiber, which clearly demonstrates properties with respect to superhydrophobic/superoleo- that the superhydrophobic/superoleophilic corn straw philic corn straw fiber to confirm its potential as an oil fiber obtained in this study possess excellent environ - absorbent. The effects of acidic and alkaline conditions ment stability. on the wettability of superhydrophobic/superoleophilic Because the prepared corn straw exhibited favorable corn straw fiber were systematically investigated. Contact superhydrophobic/superoleophilic performance in both angle measurements were performed by pipetting 5  μL acidic solutions and under ambient conditions, these pre- aqueous solution, from pH 0–14, onto fiber surfaces in pared fibers could be utilized as highly selective absorp - order to evaluate chemical stability and  durability of the tion materials to achieve effective separation of oil–water prepared material (Fig.  7a). The measured water con - mixtures. Oil absorption performance is measured by tact angle ranged from 152° to 150°, while the oil contact the absorption capacity (g/g), as well as the oil removal angle remained constant at 0°, implying that the result- efficiency (%). To determine the maximum absorption ing fiber surface still maintained outstanding superhy - capacity of superhydrophobic/superoleophilic corn straw drophobicity and superoleophilicity properties, even in fiber, experiments were performed in a variety of pure strong acid and strong alkaline conditions. To evaluate its oils and organic solvents. Figure  8a presents the absorp- environmental stability, the contact angles of water and tion capacities of raw corn straw fiber, pretreated corn Fig. 7 a Variation of water contact angle and oil contact angle of Fig. 8 a The absorption capacities of raw corn straw fiber, pretreated superhydrophobic/superoleophilic corn straw surfaces in aqueous corn straw fiber and prepared superhydrophobic/superoleophilic solutions with different pH values. b The relationship between con‑ corn straw fiber for various oils and organic solvents; b absorption tact angles of the resulting superhydrophobic/superoleophilic corn efficiency of superhydrophobic/superoleophilic corn straw with dif‑ straw fibers and days of storage in air environment ferent mass ratios of water to oil Xu et al. Bioresour. Bioprocess. (2018) 5:8 Page 9 of 11 straw fiber and the prepared superhydrophobic/supero - Table 1 Comparison of the oil absorption capacity of some recently reported oil sorbent and the samples prepared leophilic corn straw fiber in various oils and organic sol - in this study taking diesel oil for example vents. The adsorption capacity of raw corn straw fiber was very low and was always less than 10 g/g. In contrast Oil Oil sorbent Oil adsorption capacity References (g/g) with the raw fiber, the pretreated corn straw fiber exhib - ited a higher absorption capacity for all oils and organic Diesel Corn straw 17.5 This study solvents, with its value almost 3 times higher than pris- fiber tine corn straw fiber. Moreover, the oil absorption quan - Peat sorb 2.7 (Ribeiro et al. 2003) tity of the prepared superhydrophobic/superoleophilic Sugi bark 16.5 (Saito et al. 2003) corn straw fiber for diesel oil, crude oil, bean oil, and Cotton 41 (Liu et al. 2014) chloroform was relatively large at 17.5, 20.3, 22.6, and Corn straw 18 (Zang et al. 2016) fiber 27.8 times their own quality, respectively; however, the oil absorption quantity of the prepared superhydropho- Sawdust 11.5 (Gan et al. 2016) bic/superoleophilic corn straw fiber for gasoline, n-hex - ane, octane, and toluene was relatively small at 15.5, 13.5, 15.1, and 16.3 times their own quality, respectively. The Conclusions reason is that the oil absorption capacity is relevant to In this study, we successfully developed a prepara- the viscosity and density of the oil products and organic tion process for a novel superhydrophobic/superoleo- solvents, with a higher viscosity and density resulting in philic corn straw fiber by attachment of PTES-modified a saturated absorption capacity of prepared corn straw SiO particles onto the fiber surface via the sol–gel and fiber (Guo et  al. 2015). For the same oil or organic sol - impregnation method. The prepared corn straw fiber vents, the absorption capacity of the prepared superhy- exhibited outstanding properties of superhydrophobicity drophobic/superoleophilic corn straw fiber was slightly and simultaneous superoleophilicity with a water contact higher than that of the pretreated corn straw fiber, indi - angle of 152° and an oil contact angle of 0° for different cating that the superhydrophobic modification can fur - oils. In addition, the microtopography, wetting property, ther enhance the oil capacity, which is beneficial for chemical composition and oil absorption performance practical application. In addition to the absorption capac- were comprehensively studied. Results revealed that SiO ity, the absorption efficiency of the prepared corn straw granules successfully modified by PTES were robustly fiber was studied to ascertain the potential of superhy - attached to the fiber surface, resulting in a hierarchical drophobic/superoleophilic corn straw fiber in oil/water structure and low surface energy, thus giving rise to the separation. In theory, the superhydrophobic/superoleo- significant phenomenon of both superhydrophobicity philic corn straw fibers absorb very little water; however, and superoleophilicity. Moreover, the prepared super- there are errors in the oil absorption efficiency. When hydrophobic/superoleophilic corn straw fiber displayed the water content of the oil–water mixture increased, great chemical stability and environmental durabil- the prepared corn straw fiber absorbed a small amount ity. Most importantly, the prepared superhydrophobic/ of water during magnetic stirring. The oil removal effi - superoleophilic corn straw fiber possessed an excel - ciency of the resulting fiber for diesel oil and crude oil lent absorption capacity and high absorption efficiency. varied from 100 to 99%, with different mass ratios of Taken together, these results demonstrate that the pre- water-to-oil (Fig.  8b). The main cause of this phenome - pared fiber obtained in this study exhibits a high applica - non is that there was a small amount of water absorbed at tion potential to effectively separate oil/water mixtures. the same time that the prepared corn straw fibers absorb oil (Wang et al. 2013), which indicates that the prepared Abbreviations PTES: (Heptadecafluoro ‑1,1,2,2‑tetradecyl) trimethoxysilane; FTIR: Fourier trans‑ fiber can be widely applied for oil removal from water. formation infrared spectroscope; XPS: X‑ray photoelectron spectroscopy; EDX: Taken together, because of the high absorption capacity energy‑ dispersive X‑ray analysis; WCA: water contact angle; OCA: oil contact and oil removal efficiency, we clearly demonstrate that angle; SEM: scanning electron microscope. the novel superhydrophobic/superoleophilic corn straw Equation parameters fiber obtained in this study can be regarded as a high- γ , γ and γ : solid–vapor, solid–liquid and liquid–vapor interfacial tensions, sv sl lv efficient oil absorbent with great chemical stability and respectively; θ: contact angle; q: sorption capability (g/g); m : the weight of corn straw fibers after absorption; m : the initial weight of corn straw fibers environmental durability. Moreover, it has a higher oil before absorption; k: oil removal efficiency (%); w : the weight of corn straw adsorption capacity, compared to other biomass-based fibers after absorption; w : the initial weight of corn straw fibers before absorbents (Table 1). absorption; w : the weight of water absorbed in the absorbents. 1 Xu et al. Bioresour. Bioprocess. (2018) 5:8 Page 10 of 11 Authors’ contributions Guo P, Zhai S, Xiao Z, An Q (2015) One‑step fabrication of highly stable, YX and HY designed the study, performed experiments, analyzed data, and superhydrophobic composites from controllable and low‑ cost PMHS/ prepared the manuscript. DZ, FL and XH contributed to the discussion. CY, TEOS sols for efficient oil cleanup. J Colloid Interface Sci. 446(Supplement SHH, JSC and YZ reviewed the results, helped in data analysis, and edited the C):155–162. https://doi.org/10.1016/j.jcis.2015.01.062 manuscript. All authors read and approved the final manuscript. 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Journal

"Bioresources and Bioprocessing"Springer Journals

Published: Dec 1, 2018

Keywords: Biochemical Engineering; Environmental Engineering/Biotechnology; Industrial and Production Engineering

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