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Materials & Design

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Cotton aerogels and cotton-cellulose aerogels from environmental waste foroil spillage cleanup

Hanlin Cheng, Bowen Gu, Mark P. Pennefather, Thanh X. Nguyen, Nhan Phan-Thien,Hai M. Duong⁎

Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, EA-07-08, 117575, Singapore

A R T I C L E I N F O

Keywords:Cotton fibersCellulose fibersAerogelOil absorptionEnvironmental waste

A B S T R A C T

For the first time, successful fabrication of the cotton aerogels and cotton-cellulose aerogels is achieved usingrecycled fibers from environmental waste for oil absorption. The pure cotton and cotton-cellulose aerogels areobtained using a cost-effective mixing-blending method with polyamide-epichlorohydrin as strengtheningadditives. The obtained aerogels are silanized using methyltrimethoxysilane via a facile chemical vapordeposition to endow aerogels with hydrophobic surface. Effects of fiber concentrations and cotton-to-cellulosemass ratio on oil absorption performance in various solvents are also investigated. The cotton aerogel with aninitial concentration of 0.25 wt% presents the highest oil absorption capacity over 100 g g−1. Besides, thecotton/cellulose aerogels also demonstrate good absorption capacity in different pollutant organics. Theabsorption kinetics of the aerogels with different cotton concentrations are also investigated using pseudofirst-order model. Both equilibrium absorption and absorption kinetics demonstrate cotton/cellulose aerogels aspromising materials for oil absorption and environmental pollution treatment.

1. Introduction

Oil removal and separation are becoming important due to acci-dental oil leakage and oil spillage from the ever-increasing oil industry.The untreated oil waste and contaminant not only cause the unneces-sary economic loss but are dangerous to the environment and theecology system. To date, various technologies including chemicaltreatments [1], bioremediation [2,3] and physical skimming [4–6]have been developed for the oil absorption and separation. Amongthese methods, using absorbents to directly remove the oil from thecontaminated area is the most efficient and cost-effective [7–9]. More-over, it also brings great convenience if the absorbed oil can be recycledupon simple squeezing [10], distillation [11] or extraction [12].

Commercial sorbents including polypropylene mats with remark-able hydrophobicity and oil absorption capacity of 15 g g−1 [13] havebeen widely used in the oil pollutant treatment. Additionally, othersynthetic polymers such as polypyrrole sponge [14], polyurethane [15],polydimethylsiloxane [16], and polysiloxane [17] have been used forthe oil absorption. However, their applications are greatly limited notonly because of their low absorption capacity but also their poorenvironmental compatibility [13]. The synthetic polymers stay longtime in environment due to its difficulty to degrade and theiraccumulation in the ecosystem makes this problem more serious [18].

To further improve the oil absorption capacity, nano-designed carbonmaterials have been developed [19–21]. Carbon nanotube spongederived from the chemical vapor deposition method (CVD) can achievean ultrahigh absorption capacity of 120 g g−1 with remarkable me-chanical reversibility [20]. Ultralight graphene aerogel with an absorp-tion capacity of 200 g g−1 in chloroform was also used for thecontaminant absorption [21]. Although pure cotton fibers suffer alow capacity below 30 g g−1 [22], a high temperature pyrolysis canconvert commercial cottons directly to carbon foam with good mechan-ical stability for oil absorption applications [23]. However, this hightemperature treatment and the small-scale production make them costinefficient for oil removal applications [24]. Besides, the carbonnanomaterials are not bio-degradable and may cause potential healthissues [25,26].

In order to overcome these challenges, great effort has been focusedon cellulose derived materials due to its rich natural abundance andenvironmental benignity [27–29]. Unlike synthetic polymers and nano-carbon materials, cellulose can be easily decomposed by certainbacterial in environment [30]. Methods such as oleophilic TiO2 coating[28] and silanization routine using methyltrimethoxysilane (MTMS)[29] can functionalize cellulose fibers with hydrophobic surface torender them good candidates for oil absorption. Among different typesof cellulose [8,20–23], nanocellulose-based products have achieved

http://dx.doi.org/10.1016/j.matdes.2017.05.082Received 19 February 2017; Received in revised form 26 May 2017; Accepted 27 May 2017

⁎ Corresponding author.E-mail address: mpedhm@nus.edu.sg (H.M. Duong).

Materials & Design 130 (2017) 452–458

Available online 28 May 20170264-1275/ © 2017 Elsevier Ltd. All rights reserved.

MARK

high adsorption capacity [10,31]. However, the fabrication process ofnanocellulose is expensive and time-consuming. To understand the oilabsorption process, various models have been proposed [32–34] forkinetics calculation. Among them pseudo-first model is widely used forthe oil absorption [35].

To reduce the environmental burden and establish green productionrecycle, investigations on cellulose absorbent from waste paper havebeen reported in our previous research [36,37]. It was found that the oilabsorption capacity was merely around 50 g g−1. The further increaseof the oil absorption capacity by reducing the material densityinevitably leads to the poor mechanical performance. Meanwhile,cotton is a commercial available cellulose based material, which iscost-effective and environmental friendly. Its packed cellulose structureis believed to increase the stability of the aerogels. Therefore, in thiswork, we successfully develop pure cotton (PC) and cotton/cellulose(CC) aerogels from the paper waste for the oil absorption applicationwith good absorption capacity and mechanical handleability.

2. Experimental section

2.1. Materials

Recycled cellulose fibers from paper waste and polyamide-epichlor-ohydrin (PAE, Kymene 557H) were obtained from Insul-DekEngineering Pte. Ltd. (Singapore) and Ashland (Taiwan), respectively.The cotton pads consisting of cotton fibers were purchased in theFairprice (Singapore). Motor oil (5w40) and Singer machine oil werepurchased from commercial market. Analytical grade MTMS, ethanol,acetone, hexane and dichloromethane were obtained from the Sigma-Aldrich (Singapore). All chemicals were used without further purifica-tion.

2.2. Synthesis of pure-cotton (PC) and cotton-cellulose (CC) aerogels

The synthesis process of the PC and CC aerogels is illustrated inFig. 1. The cotton pads were cut into small strands (0.5 × 4 cm) andthen were mixed with the cellulose fibers recycled from the paper wastein 200 ml deionized water. The cotton-to-cellulose mass ratios werefixed at 1:0 (pure cotton), 1:1, 1:2 and 1:4 and the fiber-to-water massconcentration in aqueous dispersion was controlled with 0.25 wt%,0.5 wt% and 0.75 wt%, respectively Then, the mixture dispersion washomogenized using a juice blender (Tefal 400W) for 15 min. After-wards, 66.6 μL PAE solution was added into the above dispersion,which later went through a sonication process (Hielscher UltrasoundTechnology) at 140 W for 5 min. The homogenized dispersion wasfrozen at −18 °C for 24 h and then dried in vacuum at −98 °C for 96 hto obtain the monolith aerogels. During the freeze, both cotton andcellulose fiber was squeezed due to the volume expansion from water toice. Finally, these aerogels were cured at 120 °C for another 3 h.

2.3. Hydrophobic functionalization of the aerogels

In order to obtain the hydrophobic aerogels for oil absorption, theobtained aerogels were placed in the glass chamber with four poly-tetrafluoroethylene vials with each containing 1.5 ml MTMS and heatedat 70 °C for 12 h to undergo a silanization process. The silanizationprocess was carried out based on the reaction between hydroxyl groupsand alkoxy groups in MTMS [38]. The PC aerogels were prepared usingthe same method without the addition of the cellulose fibers. The PCaerogels with different fiber-to-water concentrations and the CC aero-gels with different cotton-to-cellulose mass ratios are summarized inTable 1, where the parameters changed are marked with bold font.

2.4. Characterizations

Sample morphologies were investigated by a scanning electronmicroscope (SEM, JSM-6010 of Japan). Before the testing, sampleswere sputtered with a thin layer gold via JEOL sputter (JFC-1200) at20 mA for 30 s to enhance their electrical conductivity.

Sample weight was measured by a digital microbalance withaccuracy of 0.01 mg. Water contact angles measurements were carriedusing a syringe system (VCA Optima goniometer, AST Products Inc.USA) with each droplet of 0.5 μL. The bulk density of the samples wasobtained by measuring the mass and volume of the cylinder-shapedaerogels. The porosity, Φ, can be calculated by:

⎛⎝⎜

⎞⎠⎟ϕ

ρρ

= 100 1 − a

c (1)

Where ρa is the density of the aerogel and ρc (1.5 g cm−3) is the densityfor both cellulose and cotton fiber since they process similar density[37,39].

Equilibrium oil and water absorption tests were also conducted.Each sample (~10 mg) was immersed into the certain oil or water for30 min to reach the equilibrium and then was drained for another20 min to determine the weight. The absorption capability can becalculated by:

Q m mm

= −r

w d

d (2)

Where Qr (g g−1) is the absorption capability, md (g) and mw (g) are the

Fig. 1. Schematic illustration of the synthesis of PC and CC aerogels.

Table 1Properties of pure cotton (PC) and cotton-cellulose (CC) aerogels with different cottonconcentrations and cotton-cellulose mass ratios.

Aerogels Concentration (wt%)

Mass ratios ofcotton-cellulose

Density(mg cm−3)

Porosity (%)

PC25 0.25 1:0 5.13 ± 0.29 99.66PC50 0.50 1:0 6.85 ± 0.33 99.54PC75 0.75 1:0 8.22 ± 0.49 99.45CC1-1 0.50 1:1 8.50 ± 0.33 99.43CC1-2 0.50 1:2 6.40 ± 0.08 99.57CC1-4 0.50 1:4 6.19 ± 0.33 99.59

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aerogel weight before and after the oil absorption, respectively. Theerror range including the standard deviation was determined by theAnalysis of Variance in software “Originlab”.

To determine the oil absorption kinetics, samples with identical size(cylinder shape with diameter of 6 cm and height of 3 cm) wasimmersed into the singer machine oil at different time to measure itsweight change. The kinetic constant, k1 was determined by [40]:

QQ Q

k tln−

=m

m t1

(3)

Where Qmis the saturated absorption capacity (g g−1), Qtis the absorp-tion capacity (g g−1) at time t (s).

For the distillation absorption cycling experiment, after the absorp-tion of ethanol, aerogels were placed into a flask and heated at 100 °Cwith a condenser to collect the recycled ethanol. For the squeezingabsorption cycling experiment, after the absorption of ethanol, theaerogel was squeezed to one fourth of height original to release theethanol.

3. Results and discussion

The PC and CC aerogels possess good handleability. Various shapeand size of the developed aerogels can be controlled using molding.Fig. 2a shows a large-scale CC aerogel having 1-cm thickness, 0.50 wt%and the cotton-cellulose ratio of 1-1 can be fabricated using the A4-sized tray. The good handleability of the CC aerogel can be found with aheight change less than 80% when placing a 100 ml glass beakerweighted ~53 g, which is near 100 times of aerogel (Fig. 2b). Fig. 2cexhibits the MTMS-coated CC aerogel does not absorb small waterdroplets (dyed with blue ink) placed on its top surface, confirming itshydrophobicity.

The results of water contact angle tests of different PC and CCaerogels are shown in Fig. 3. A large contact angle over 130° of all PCand CC aerogels indicates their hydrophobic properties. The contactangle slightly decreases with the increase of the cotton concentration.This occurs because the homogenous silanization process may be lesseffective. Compared with the 0.50 wt% PC aerogel, the CC aerogelswith the same 0.50 wt% have a larger contact angle up to 142.8o. It canbe explained that more cellulose fibers can add more hydroxyl groupsexposed to the MTMS vapor, which are replaced by the alkoxy groupsand make surface of the CC aerogel more hydrophobic [41,42]. Thefurther increase of cellulose content also brings the decrease of thewater contact angle, possibly due to a less-efficient coating. Althoughsome reports achieved superhydrophobic cellulose with constant angleabove 150 °C, our work has merit of a low-cost raw materials andsimple functionalization method [43–45].

The PC and CC aerogels present macropores with diameter largerthan 50 μm estimated from the SEM images in Fig. 4. For the PCaerogels, the increase of the cotton fiber concentration causes morepacked structure as seen in Fig. 4a–c. For the CC aerogels in Fig. 4d–f,the increase of the cellulose fibers also causes more packed structure. Itmay be explained that the cellulose fibers can be dispersed better than

the cotton fibers. The SEM image of the MTMS-coated CC aerogel ispresented in Fig. S1 (Data in Brief) and there is no apparent morphologydifference between the uncoated samples due to the very small amountof MTMS usage.

Fig. 5 demonstrates the absorption process of machine oil (dyedusing Sudan Red) using the CC1-1 aerogel with the size of2 × 2 × 0.5 cm. The aerogel initially floats on the oil and thengradually sinks down upon the absorption of the oil. The absorptionis completed in 180 s. Besides, the CC aerogels can be shaped into smallpellets to treat and remove the oil from the water as shown in the movieS1. The aerogel pellets can help on storage space reduction and easytransportation.

Fig. 6 exhibits the machine oil absorption capabilities of the PC andCC aerogels. Samples with different fiber concentration clearly showdifferent absorption performance analyzed by F-test using one-wayAnova. As can be seen in Fig. 6a, the PC aerogel with a lowconcentration of 0.25 wt% has the highest absorption capacity over100 g g−1, much larger than those of the commercial sorbents. Theincrease of the cotton fiber concentration from 0.25 wt% to 0.75 wt%decreases the absorption capability of the PC aerogels due to theirraised density and lower porosity as listed in Table 1. For the CCaerogels in Fig. 6b, both CC1-1 and CC1-2 aerogels present competitiveoil absorption compared to the PC aerogel with same fiber concentra-tion of 0.50 wt%. It is also important to note that during the drainingperiod to remove the oil absorbed, the CC aerogels show little shapechange while the PC aerogel shrink to the 80% of the original volume.This provides an indirect evidence of the better mechanical stability ofthe composite design over the pure cotton aerogel. With the furtherincrease of the cellulose fibers leads to the decrease of the absorptioncapability despite of its smaller density. Two reasons can be used forexplanation: (i) the dense structure in Fig. 4f can prevent the oildiffusion and entrap the great air inside the aerogels, (ii) the higherpercentage of the cellulose fiber leads to an inefficient silanizationprocess due to more exposed hydroxy groups while the amount ofMTMS used for coating is fixed [46].

Water absorption capability of the MTMS-uncoated PC and CCaerogels are illustrated in the Fig. S2 (Data in Brief). Interestingly, wefind that although the water absorption capacity of PC aerogels is muchhigher than our previous pure cellulose aerogel (~20 g g−1) [37], it ismuch smaller than the absorption capacity of machine oil, regardless ofthe larger density of the water. Both cotton aerogels and cotton/cellulose aerogel were swelled during the water absorption tests.However only cotton aerogels were swelled during the oil absorptiontests. Also, it is found that during the draining process after the waterabsorption capability, the prepared cotton aerogels undergo an ob-servable shrink (~60% volume change) once taken out from the water.Meanwhile this volume shrinkage is much smaller in the oil absorptiontests. This can be explained by the high viscosity of the machine oilcompared with the water. It is worth noting that this difference is muchsmaller in the CC aerogels, indicating their improved mechanicalstrength. Although some reports [23,28] have achieved higher absorp-

Fig. 2. (a) A large-scale CC1-1 aerogel, (b) The CC1-1 aerogel can stand an empty 100 ml beaker (~53 g) place on the top, and (c) MTMS-coated CC1-1 aerogel does not absorb blue waterdroplets.

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tion values, they either suffer a complex fabrication process usingnanocellulose [31] or an environmental hazardous pyrolysis process[23].

The absorption kinetic results are shown in the Fig. 6c. The PC50aerogel has the highest absorption speed compared with the CCaerogels. But the CC1-4 aerogel has the lowest absorption capabilityand the sluggish absorption kinetic, possibly due to its least hydro-phobicity. The initial absorption kinetic constants are obtained byfitting the results according to the notable first-order pseudo Eq. (3).The kinetic constants of the PC50, CC1-1, CC1-2, and CC1-4 aerogels inFig. S3. (Data in Brief) are 0.217, 0.094, 0.143 and 0.189, respectively.

Besides the machine oil, the aerogels are also tested with otherorganic solvents. As can be seen in the Fig. 7, the CC1-1 aerogelpresents the highest absorption capacity for dichloromethane and theleast for hexane. Generally, the absorption capacity increases with thedensity of the solvent. However, the absorption capacity for the acetoneis higher than that for the ethanol regardless of their close density(Fig. 7b). The absorption capacities of the PC and CC aerogelsdeveloped in this work are higher than those of conjugated polymers[24], nanowire membrane [47], exfoliated graphite [48], carbonaerogel [49] and competitive to spongy CNT and graphene [50,51].We also plot the theoretical absorption ratio based on the solvent

density and sample porosity (porosity × ρliquid / ρaerogel). Current ab-sorption capacities of the CC1-1 aerogels for the different solvents arenear the line of the theoretical absorption capacity. It is believed withincreased pressure, higher absorption capacity can be achieved.

More importantly, the absorbed liquids can be re-collected throughthe distillation. Fig. 8 shows the absorption-distillation cycles of theethanol. Both PC50 and CC1-1 aerogels undergo minor shrinkageduring the distillation process due to the capillary force during theliquid-vapor phase transition [52]. After 5 cycles of liquid recovery, thePC50 and CC1-1 aerogels yield the absorption capacities of 38 ad40 g g−1, respectively. The CC1-1 aerogel presents a slightly betterperformance because the strengthening effects of two different cottonand cellulose fibers [53]. Compared with the poor absorption cyclingcapacity in squeeze-cycling method (Fig. S4, Data in Brief), thedistillation method clearly presents a better performance.

4. Conclusions

In summary, we have successfully prepared the cotton/cellulosecomposite aerogels using commercial cotton and cellulose fiber frompaper waster. The functionalized aerogels with hydrophobicity demon-strate a good contaminant absorption with 72.3 g g−1 in machine oil

Fig. 3. Contact angles of the MTMS-coated PC aerogels with different concentrations: (a) PC25, (b) PC50, (c) PC75 of the cotton fibers and MTMS-coated CC aerogels having the same0.50 wt% of the fibers with different cotton-cellulose ratios: (d) CC1-1, (e) CC1-2, and (f) CC1-4.

200 µm

Fig. 4. FESEM images of the PC aerogels with different concentrations of (a) PC25, (b) PC50, (c) PC75 and the CC aerogels with different cotton-cellulose ratios of (d) CC1-1 (e) CC1-2, (f)CC1-4.

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Fig. 5. Oil absorption process of the CC1-1 aerogel in 180 s.

Fig. 6. Machine oil absorption capacities of (a) the PC aerogels with different cotton fiber concentrations (0.25–0.75 wt%), (b) CC aerogels with different cotton-cellulose ratios (1-1 to 1-4), and (c) absorption kinetics of PC50 aerogel and CC aerogels with different cotton-cellulose ratios (Figure inset is the absorption result in initial 30 s with fitting lines).

Fig. 7. Various solvent absorption capacity of the (a) CC1-1 aerogel and (b) absorption capacity marked with liquid density.

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and 94.3 g g−1 for dichloromethane. The cotton/cellulose compositeaerogel presents a slightly better performance because the synergisticeffects of two different cotton and cellulose fibers.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.matdes.2017.05.082.

Acknowledgement

We'd like to thanks FB Fund C-265-000-049-001 for the fundingsupport and the lab officers in Materials Division, Department ofMechanical Engineering, NUS for their significant help on characteriza-tions.

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