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Journal of Environmental Management 114 (2013) 158e167

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Journal of Environmental Management

journal homepage: www.elsevier .com/locate/ jenvman

Populus seed fibers as a natural source for production of oil super absorbents

Marko Likon a,*, Maja Rem�skar b, Vilma Ducman c, Franc �Svegl d

a Insol Ltd, Cankarjeva 16 a, SI-6230 Postojna, Sloveniab Jozef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Sloveniac Slovenian National Building and Civil Engineering Institute, Dimi�ceva ulica 12, SI-1000 Ljubljana, SloveniadAmanova Ltd, Tehnolo�ski Park 18, SI-1000 Ljubljana, Slovenia

a r t i c l e i n f o

Article history:Received 9 September 2011Received in revised form16 March 2012Accepted 21 March 2012Available online 18 April 2012

Keywords:Oil spillNatural absorbentsPoplar seed fibers

* Corresponding author. Tel.: þ386 0 41 648 377; fE-mail address: marko.likon@telemach.net (M. Lik

0301-4797/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.jenvman.2012.03.047

a b s t r a c t

The genus Populus, which includes poplars, cottonwoods and aspen trees, represents a huge natural sourceof fiberswith exceptional physical properties. In this study, the oil absorptionproperties of poplar seed hairfibers obtained from Populus nigra italica when tested with high-density motor oil and diesel fuel arereported. Poplar seed hair fibers are hollow hydrophobic microtubes with an external diameter between 3and 12 mm, an average length of 4� 1 mm and average tube wall thickness of 400� 100 nm. The solidskeleton of the hollow fibers consists of lignocellulosic material coated by a hydrophobic waxy coating.The exceptional chemical, physical and microstructural properties of poplar seed hair fibers enable super-absorbent behavior with high absorption capacity for heavy motor oil and diesel fuel. The absorptionvalues of 182e211 g heavy oil/g fiber and 55e60 g heavy oil/g fiber for packing densities of 0.005 g/cm3

and 0.02 g/cm3, respectively, surpass all known natural absorbents. Thus, poplar seed hair fibers obtainedfrom Populus nigra italica and other trees of the genus Populus are an extremely promising natural sourcefor the production of oil super absorbents.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The extraction, refining, production, transportation, storage,and usage of oil and its derivatives pose continuous danger to theenvironment. Effective means for removal of oil from contaminatedsurfaces are absorbents that are produced from different natural andsynthetic organic and inorganicmaterials. Commercially available oilabsorbents are non-biodegradable synthetic organic products (e.g.polypropylene and polyurethane) and may cause serious disposalproblems after their use. The inorganic absorbents based on porousmineral materials such as perlite, exfoliated graphite, vermiculite,organoclay, zeolite, silica, bentonite, fly ash, and diatomite have poorbuoyancy and oil absorption capacity. The use of paper mill sludge asan oil absorbent presents a sustainable solution for the conversion ofindustrial waste into a valuable oil absorbent (Likon et al., 2011).However, due to inadequate hydrophobicity, the microstructure ofpaper mill sludge may collapse because of water absorption.

The limitations of mineral products and organic synthetic prod-ucts have triggered an intensive development of alternative oilabsorbents based on organic natural materials. These include straw,sawdust, corncob, rice, coconut husk, cotton, wood, wool, kenaf,

ax: þ386 0 59 959 047.on).

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cattail and kapok fibers (Mungasatkit, 2004). Agricultural remainslike rice straw, corncob, peat moss, cotton, cotton grass, barks,milkweed and kenaf, exhibit good oil sorbency (Adebajo et al., 2003;Haussard et al., 2003; Saito et al., 2003; Suni et al., 2004). It has beenobserved that rice straw, corncob, and wood fibers have somedrawbacks like poor buoyancy, relatively lowoil absorption capacity,and low hydrophobicity (Wei et al., 2003). Whereas milkweed andcotton have greater potential for oil spill cleanup, absorbing signif-icantly more oil than commercial synthetic absorbent materials(Choi, 1996; Sun et al., 2003). The morphology of these agriculturalfibers reveals shapes in the form of ropes or ribbons. Conversely, thekapok (Ceiba petandra L.Gaertn.) is consisting offibers in the shape ofhollow tubes. Because of this cylindrical geometry, the oil absorbentproperties of kapok fibers are superior to agricultural remainsmentioned above (Hori et al., 2000; Lim and Huang, 2007a,b;Voumbo et al., 2010). The surface of kapok fibers is covered by waxycoating which makes them highly hydrophobic and resistant toswelling. Recent studies of kapok fibers as an oil-absorbent materialhave shown excellent oil absorption characteristics (Lim and Huang,2007a,b; Abdullah et al., 2010).

The fibers produced from poplar seeds have a similar morpho-logical structure to kapok fibers. Poplar and aspen trees areangiosperm plants from the Salicaceae family. On average, thepoplar tree produces 35 liters of fresh fruits that yield from 280,000to 14,850,000 seeds (0.9 kg) depending on species, location and

M. Likon et al. / Journal of Environmental Management 114 (2013) 158e167 159

type of the tree (Cooper and Van Haverbeke, 2011). Presently, thefibrous Poplar seeds are treated as waste or, at best, used as a cheapfertilizer. The microtube morphological structure and high hydro-phobicity indicate that poplar seed fibers may serve as a valuableindustrial raw material for the production of oil absorbent (Likonand Likon, 2010).

The aim of this study was to assess the physicochemical and oilabsorption characteristics of fibers produced from poplar seeds(Populus nigra italica) as a raw material for production of an oilsuper absorbent.

2. Experimental

2.1. Fiber preparation

The seed fibers of Populus nigra italica trees were collected duringthe blooming period inMay. Hand-harvestedfibers contaminated byforeign matter (remnants of envelopes and leaves, dust and debris)were placed in a glass container (14 cm in diameter) that wasequipped with an anchor stirrer with sharpened edges (approx.13.9 cm in diameter). The fiberswere processed for 60 s at 5600 rpmand then passed through a vibrating sieve (300 rpm, amplitude ofapprox.1 cm) withmesh 16. The rate of fiber purificationwas higherthan 95%, as determined by weighing.

2.2. Morphology determination

The processed fibers were placed on carbon adhesive tape andexamined with a field-emission scanning electron microscope (FE-SEM, Supra 35 VP, Carl Zeiss). Image processing was performed withthe software package Media Cybernetics Image-Pro Plus version6.0.0.260.

2.3. Bulk and absolute density determination

The bulk density of the poplar seed fibers was determinedaccording to the procedure described by Mwaikambo and Ansell(2001). A bundle of fibers was weighed on an electronic balanceto the nearest 1 mg. The weight of the bundle was recorded asWfa. The fibers were then immersed in benzene solvent (densityrs¼ 875 kg/m3) and left to soak. The weight of soaked fibers wasrecorded as Wfs. The bulk density (rf) of the fibers was calculatedusing Eq. (1).

rf ¼ rsWfaWfa �Wfs

(1)

All measurements were determined at 21 �C.The absolute density of poplar seed fiberswasmeasured using an

automatic gas pycnometer (Ultrapyc 1200e, Quantchrome Instru-ments). A sample of fibers (0.5 g in mass) was placed into a 135-cm3

container and purgedwith helium to open the free lumen and pores.The absolute density of the poplar seed fibers was calculated as anarithmetic average of ten consecutive measurements.

2.4. IR analysis of chemical structure

The chemical structure of processed poplar seed fibers wasexamined with infrared reflection and absorption spectroscopy. Abundle of processed fibers (0.5 g in mass) was divided into twoequal portions. The first portion (0.25 g in mass) was frozen inliquid nitrogen and crushed into powder in an agate mortar. Thesecond portion was left as prepared. The crushed and uncrushedsamples were dried overnight at 105 �C. The infrared spectra (IR) ofboth samples were measured using Perkin Elmer FTIR Spectrum BX

system equipped with Pike Technologies MIRacle ATR attachment.Spectra were collected over 128 scans at 1 cm�1 resolution andprocessed with Spectrum GX v.5.3.1 software.

The chemical composition of fibers was evaluated fromcomparison of the absorption peak ratios I1595/I1105 and I1735/I1105for the stretching modes of CeOeC ether bonds at 1595 cm�1 andthe C]O group in the carbonyl ester group of lignin at 1735 cm�1

(Sun et al., 2003; Lim and Huang, 2007a,b; Abdullah et al., 2010),with a normal mode of glycosidic bonds at 1105 cm�1 in the cellu-lose and the hemicellulose (Garside andWyeth, 2003), respectively.The results were compared with the data in the literature for kapokfibers (Hori et al., 2000).

The solid remains after the extraction of fibers in differentsolvents were analyzed by measuring the ATR (attenuated totalreflection) infrared spectra of samples collected from thewall of theevaporation flask.

2.5. X-ray diffraction analysis

X-ray diffraction (XRD) analysis of poplar seed fibers was per-formed on samples pressed into pellets. The diffractionpatternswerecollected at room temperature with a D4 Endeavor diffractometer(Bruker AXS) using a quartzmonochromator Cu Ka1 radiation source(l¼ 0.1541 nm) and a Sol-X energy dispersive detector with a 2Qrange from 6� to 73�, a step scan of 0.04�, and a collection time of 4 s.The samples were rotated during measurements at 6 rpm.

2.6. Durability and stability testing

The chemical stability of poplar seed fibers was tested by twoextraction processes in different organic solvents and water. Beforeextraction, the fibers were dried in an oven at 105 �C untila constant weight was attained. The dry fibers (1 g in mass) wereplaced into a glass thimble (2.8-cm in diameter and 6-cm in length)attached to Soxhlet apparatus.

The first extraction process was accomplished by using a succes-sion of solvents with increasing polarity (hexane, petrol ether,tetrahydrofuran, ethyl acetate, acetone, methanol and a 1:1 mixtureof methanol and water). The duration of each extraction step (onesolvent) was 24 hours.

The second extraction process started by the extraction stepwith chloroform for 8 hours followed by sequential extractionswith diethyl ether for 3 hours, alcoholebenzene mixture (1:2) for 8hours and hot water for 24 hours.

After each extraction step, the sample of fibers was dried in anoven at 105 �C for 24 hours. The dry samples were analyzed by ATRinfrared spectroscopy. A small portion of fibers taken for infraredanalysis was returned to the original sample before next extractionstep was performed.

The decrease in weight measured before and after the completeextraction process indicated the extent of the solubility or chemicaldecomposition of poplar seed fibers in different solvents.

The remaining solvent in the extract after completion of theextraction process was evaporated on a rotary evaporator. The solidresidue was analyzed by infrared spectroscopy.

2.7. Determination of contact angle

The contact angle between the solidwall of the poplar seed fibersand the liquid phase was measured using static digital methoddescribed in the standard D5725-97 (ASTM, 2003). The fibers (0.1 gin mass) pressed into pellets (1-cm in diameter) were placed ona lifting table mounted of an optical microscope (Olympus SZ e STU2) equipped with a digital camera (Olympus Camedia C-3040)supported by the software package ACDsee 5.0. A syringe filled with

Fig. 1. SEM images of (A) poplar seed fibers, (B) kapok fibers, (C) cotton, and (D) expanded polypropylene fibers (magnification 500�).

M. Likon et al. / Journal of Environmental Management 114 (2013) 158e167160

liquid (25 ml of heavy motor oil, diesel fuel or water) was mountedapproximately 5 mm above the sample pellet. The droplet of liquidwas carefully placed onto the surface of the pellet. A digital photo ofthe contact surface between the solid and liquid phaseswas acquiredat a 25.6� magnification on the microscope and 3� optical magni-fication on the digital camera. Digital images of droplets on thesurface of the pellets were captured at time intervals of 0, 1, 3 5, 10and 15 s. Time zero (0 s) was taken as the initial point when thedroplet touched the surface of the pellet. The images were analyzedusing the software package Media Cybernetics Image-Pro Plusversion 6.0.0.260. The contact angle was calculated using Eq. (2):

Q ¼ 2� arctg�2hD

�(2)

where D and h are the droplet’s width and height, respectively.

2.8. Determination of oil absorption capacity and retention

The oil absorption capacity and retention of poplar seeds fibersin their loose and packed forms were determined according to thestandard procedure “Standard Test Method for Absorbent Perfor-mance of Adsorbents” described in ASTM 726-06. The loose fibers(0.5 g in mass) in their natural form (bulk density of 0.0036 g/cm3)were placed on a clean, round metal wire mesh. Another sample offibers (1.3 g in mass) was packed together with a metal wire meshinto a ball shaped package (5-cm in diameter) to test the oilabsorption performance at defined packing density (0.02 g/cm�3).The volume of 400 ml of a hydrophobic liquid (motor oil (SAE15W-40) or diesel fuel (OMV-D2)) was poured into a 600-ml grad-uated glass beaker. The samples of loose and packaged fibers wereimmersed into the hydrophobic liquid and left to soak for 15 min and24 h for the determination of absorptionperformance and saturationlimit, respectively. After soaking, the assemblies were taken out ofthe liquid and allowed to drain to remove any loosely attached liquid.The samplewas allowed to drain for 60 s. The absorption capacity (R)

of the material was calculated by dividing the difference in weightbetween the soaked (Ws) and non-soaked (Wns) absorbent sample bythe weight of the sample before absorption, according to Eq. (3).

R ¼ Ws �Wns

Wns(3)

The additional absorption test was performed for oil floating onwater surface. The rate of oil removal from water surface wasdetermined by placing 2.82 g of processed fibers into a vertical glasstube (2-cm in diameter and 10-cm in length) equipped with wiremesh at the bottom. The packing density of fibers was 0.02 g/cm3.The tube was filled at the top, above the fiber package, with 10%suspension of SAE 15W-40 motor oil in water (100 ml in volume).The oilewater suspension permeated through the packed fibers atthe rate of 1 ml/min. The efficiency of oil removal from water wasabove 97.5% determined by centrifugation of effluent taken at thebottom of the tube.

The hydrophobic liquids used for testing were high-density OMVclassic motor oil (SAE 15W-40) and diesel fuel (OMV-D2). Thedensities of the high-density oil and diesel fuel were determinedaccording to the standard procedure described in EN ISO 12185using a Paar density meter DMA 48 at 15 �C. The densities of thehigh-density oil and diesel fuel were 0.883 g/cm3 and 0.832 g/cm3,respectively. The kinematic viscosities of the high-density oil anddiesel fuel were determined according to the standard proceduredescribed in EN ISO 3104 using a viscosity meter (Dema type BTE).The kinematic viscosities of the high-density oil and diesel fueldetermined at 40 �C were 4250 mm2/s and 110 mm2/s, respectively.The surface tensions measured at 20 �C of the high-density oil anddiesel fuel were 0.030 N/m and 0.025 N/m, respectively.

2.9. Degradation and buoyancy testing

The degradation and buoyancy of poplar seed fibers were testedaccording to the “Standard Test Method for Sorbent Performance of

Table 1Morphological characteristics of different fibers used for oil absorbent production.

Fibers Morphologicalstructure

Externaldiameter[mm]

Activesurface (BET)[m2/g fibers]

Poplar seed fibers Microtube 8.7� 5.7 2.42� 0.16Kapok fibers Microtube 16.5� 2.4 3.07� 0.14Cotton fibers Twisted ribbon 23.9� 16.0 1.46� 0.04Sulfate cellulose fibers Twisted ribbon 22.0� 14.0 1.00� 0.04Silanized cellulose fibers Twisted ribbon 22.0� 14.0 1.05� 0.04Viscose fibers Twisted ribbon 9.1� 1.1 1.16� 0.02Expanded polypropylene

fibersStraight rope 9.1� 3.7 0.99� 0.07

Paper mill sludge fibers Ropeþ particles 23.9� 14.6 2.66� 0.03

M. Likon et al. / Journal of Environmental Management 114 (2013) 158e167 161

Adsorbents” described in ASTM 726-06. A glass beaker in volume of5 liters (18 cm in diameter) was filled with 2 l of water. The volumeof 10-ml of SAE 15W-40 heavy motor oil was slowly added on thetop of the water surface in the beaker. In the middle of the oil slicka sample of poplar seed fibers (1-g in mass) was carefully placed.Subsequently, the beaker containing oil, water and fibers wasshaken for 15 min (frequency of 150 rpm and amplitude of 3 cm) onan automatic shaker. The buoyancy test was positive if more than90% of the fibers remained floating on the water surface and theturbidity of the water was below 10%, measured by Agilent 8453UVeVIS spectrometer. The degradation and buoyancy testing wasperformed for loose fibers and packed fibers with densities of0.0036 g/cm3 and 0.02 g/cm3, respectively.

3. Results and discussion

3.1. Microstructure, morphology and density of poplar seed fibers

Themorphology of the poplar seed fibers (Populus nigra italica) issimilar to that of the kapok fibers produced from the fruit of the treeCeiba pentandra L. Gaertn. The average length of the poplar seedfibers (4�1 mm) is significantly shorter than that of kapok fibers(25� 2.4 mm). More detailed morphological characteristics ofpoplar seed fibers and comparison with the morphology of otherfibers used for the production of oil absorbents are shown in Fig. 1and summarized in Table 1. The poplar seed fibers consist ofhydrophobic microtubes with an external diameter of 8.7� 5.7 mmand an average tube wall thickness of 400�100 nm (Fig. 2A). Bycomparison, kapok fibers have double as big external diameter(16.5� 2.4 mm) and four times thicker tube wall (average thicknessof tubewall 2.0 mm) (Lim and Huang, 2007a,b). The density of kapokfiber is low (0.384 g/cm3) in comparison to other known natural oilabsorbent materials because 77% of the fiber is hollow (emptylumen) (Lim and Huang, 2007a,b). However, the density of poplar

Fig. 2. SEM images of (A) natural poplar seed fibers (at a magnification of 35,500�) and(at magnification 22,660�).

seed fiber is even considerably lower (0.36 g/cm3) because 89% ofthe fiber total volume is empty lumen.

3.2. Chemical structure and composition of poplar seed fibers

3.2.1. XRD analysisThe XRD diffraction pattern of pressed poplar seed fibers (Fig. 3)

revealed that most of the fiber structure has considerable periodicorder. The diffraction lines at the 2q positions 14.7�, 17.2�, 18.8�,22.0�, and 25.8� indicate the presence of microcrystalline cellulose Iand lignin. The diffraction lines at 14.7� and at 22.0� can be assignedto diffraction at the 110 and 200 planes, respectively, which istypical for pure microcellulose I structure as reported recently byDriemeier and Calligaris (2011). The most intense lines at 17� and26� agree with the calculated but not clearly resolved lines forcellulose I reported by Oh et al. (2005).

Extraction of the poplar seed fibers with diethyl ether, a mixtureof ethanol/benzene (1/2) and hot water decreased the degree ofcrystallinity in the fibers. Only three broadened lines are wellresolved in the XRD diffraction pattern of the amorphous back-ground, i.e., lines centered at 16.5�, 22.3�, and at 42�. All three peakscan be assigned to cellulose I. The intensity ratio of the first twolines for the extracted fibers is in agreement with the results re-ported by Driemeier and Calligaris (2011) indicating that the purecellulose I phase is present in the form of small crystallites.

3.2.2. IR analysisThe infrared spectra of the poplar seed, kapok and cotton fibers

are presented in Fig. 4.Comparison of the IR spectra confirmed the chemical similarity of

kapok and poplar seed fibers. The IR spectrum of cotton exhibitscharacteristic vibrational modes of cellulose and hydrocarbons(resulting from wax). Same absorption bands appear also in thespectra of kapok and poplar seed fibers. In addition to this the newabsorption bands appear in the region between2000 and1150 cm�1,which indicate the presence of aliphatic aldehydes, esters, ketonesand lignin. The IR spectra of the poplar seed fibers exhibit absorptionbands at 2921 cm�1 and 2849 cm�1 corresponding to the asym-metric and symmetric stretching of aliphatic CH2 and CH3 groupsindicating the presence of plant wax (Wi�sniewska et al., 2003; Limand Huang, 2007a,b). This kind of wax consists of n-alkanes anda smaller portion of alcohols, fatty acids, aldehydes, ketones and n-alkyl esters (Abdullah et al., 2010; Tulloch,1976; Baker,1982; Bianchi,1995; Velí�sek, 2006). The absorption bands at 1733 and 1237 cm�1

are associated with the vibrational modes of carbonyl groups. TheC]O stretching vibrations are associated with the aliphatic alde-hydes, esters and ketones of the kapok fibers (Abdullah et al., 2010;Rengasamy et al., 2010). Four intensive absorption bands at 1593,

(B) after sequential extraction in diethyl ether, ethanol/benzene (1:2) and hot water

Fig. 3. XRD pattern of raw poplar seed fibers (left) and extracted (extraction 2) poplar seed fibers (right).

M. Likon et al. / Journal of Environmental Management 114 (2013) 158e167162

1505, 1457 and 1422 cm�1 correspond to the stretching modes ofCeO and C]C bonds in lignin (Hergert, 1971; Mohebby, 2008). TheIR absorption band at 1033 cm�1 belongs to the stretching of CeObonds in cellulose, hemicellulose and lignin (Adebajo and Frost,2004). The IR spectra of poplar seed fibers differ distinctively fromother natural absorbentmaterials in the appearance of an absorptionband at 770 cm�1 associated with the vibrational modes of theparaben structure (Venverloo, 1971). The evaluation of peak inten-sity ratios of absorption bands at 1595 and 1105 cm�1 for the poplarseed fibers and comparison with measured and published data forthe kapok fibers (Hori et al., 2000) showed that the poplar seedfibers contain approximately 29% lignin. Additionally, the degree ofacetylation of the polysaccharides present in the poplar seed fiberstructure is approximately 9%, which is in agreement with the datafor poplar woods (Ramirez, 2005).

Drying of poplar seed fibers at elevated temperatures decreasedthe intensity of the absorption band at 2851 cm�1 (Fig. 5) associatedwith stretching modes of aliphatic CH2 and CH3 groups, which indi-cates that small portion of lower alkanes and alcohols that are highlyvolatile got released from thewaxy coating of the fibers. Furthermore,the shape of the absorption band between 810e860 cm�1, charac-teristic for C-H modes in isolated aromatics (see Fig. 5 e insert), andthe changes in intensity of absorption bands between 700 and

Fig. 4. ATR-FTIR spectra of (A) poplar se

800 cm�1 typical for CeH modes in adjacent aromatics of ligninindicate increased condensation with heating, i.e., a decrease in thenumber of adjacent aromatic CeH bonds and an increase in thenumber of isolated aromatic CeH bonds (Lora and Wayman, 1980).These results suggest that the lignin in the poplar seed fibers ispredominantly of the p-hydroxyphenyl/guaiacyl type as observed inother poplar tissues (Christiernin, 2006).

In general, the results of this study show that poplar seedfibers consist of 41e44% cellulose, 19e21% hemicellulose (mainly asxylan), approximately 29% lignin (predominantly p-hydroxyphenyl/guaiacyl) and 4e9% extractives (mainly wax).

3.3. Absorption capacity and buoyancy of poplar seed fibers

The results of the average oil absorption capacity measurementsfor different natural organic and synthetic absorbent materials forhigh-density oil and diesel fuel are presented in Figs. 6 and 7. The oilabsorption capacity of loose poplar seed fibers (bulk density of0.0036 g/cm3) surpassed by almost 70% the second-highest valuemeasured for loose kapok fibers. The efficiency of the absorptionprocess depends on the packing density of the fibers. Themeasurements showed that the absorption capacity of the poplarseed fibers decreased considerably from 182e211 g oil/g fiber to

ed, (B) kapok and (C) cotton fibers.

Fig. 5. ATR-FTIR absorption spectra of raw poplar seed fibers (A) and fibers dried at 150 �C for 12 h (B). The insert shows absorption bands between 860 cm�1 to 810 cm�1 assignedto CeH modes of aromatics.

M. Likon et al. / Journal of Environmental Management 114 (2013) 158e167 163

55e60 g oil/g fiber at packing densities 0.005 g/cm3 and 0.02 g/cm3, respectively. Published data have shown the same trend forkapok fibers (Lim and Huang, 2007b; Abdullah et al., 2010;Rengasamy et al., 2010). Fiber assemblies with lower packingdensity are more porous and have a large hydraulic diameter ofinter-fiber pores. The poplar seed fibers are fine enough to enhancethe absorption capacity, especially with the high porosity of thefiber assembly as evidenced by the Washburn Equation.

Fig. 6. Average absorption capacity of d

h* ¼ 2glvcos qrgr

(4)

When liquid rises inside a circular pore of radius r, thegravitational force g balances the capillary forces and the liquidceases to rise beyond the equilibrium wicking height h.Furthermore, the absorption depends on the surface tension glv,the capillary diameter r, the contact angle between solid and

ifferent fibers for high-density oil.

Fig. 7. Average absorption capacity of different fibers for diesel fuel.

M. Likon et al. / Journal of Environmental Management 114 (2013) 158e167164

liquid interfaces q, the density of the liquid r and the accelerationdue to gravity g.

The absorption capacity of poplar seed fibers at standardpacking density of 0.02 cm3 was more than 10% higher than that ofkapok and almost doubled in comparison with cotton, silanizedcellulose fibers, viscose fibers and expanded polypropylene fibers.A comparison of the results for expanded polypropylene fibersand poplar seed or kapok fibers suggests that the capillary effectof microtubes has an important role in the absorption process.The poplar seed fibers and the kapok fibers, with hollow tubestructures, exhibit a much higher absorption capacity (almost twiceas high) than ropelike polypropylene fibers.

Considering the similar chemical and morphological propertiesof the poplar seed and kapok fibers, a comparable absorptioncapacity was expected. The higher oil absorption capacity of packedpoplar seed fibers results from shorter fibers, better fineness,greater porosity and smaller microtubes. The density of the fibersand liquids used influences the absorption capacity, expressed interms of mass ratio. All of the fiber assemblies showed higherabsorption capacity for high-density oil compared to the diesel fuel.The explanation for this is the higher viscosity of the high-densityoil that could not drain out as easily from the pores of the fiberassembly and hence higher absorption capacity was observed.Similar observations have been published for kapok and other oilabsorption materials (Abdullah et al., 2010; Rengasamy et al., 2010).

As described in the literature (Abdullah et al., 2010), the drainingprocess happens because of the instantaneous dripping of oil fromthe kapok fiber assemblies and the draining of the extra-lumenliquids. The rate of draining is directly related to the difference inpressure across the interface between the fiber surface and the oil.The draining starts when the capillary pressure is insufficient tohold the weight of the oil entrapped in the inter-pore system of thefiber assembly. Loosely packed poplar seed fibers allow greateroil drainage because of the hollow lumen network with largerinter-fiber distances that destabilize the liquid bridges between

adjacent fibers. This can be explained by the hydraulic pore diam-eter of an inter-fiber pore using Eq. (5) from the model developedby Rengasamy et al. (2010) for a parallel fiber assembly.

Dh ¼ 1doð1� 3Þ

�3d2o � d2i

�(5)

where df and d0 are the inner and outer diameters of the hollowfiber, 3is the porosity of the fiber assembly, and Dh is the hydraulicdiameter of an inner fiber pore.

The inner fiber pore diameter is zero when the porosity of poplarseed fibers assembly is below 86%, corresponding to the porosityof individual fibers themselves. Similarly to the kapok fibers, thisindicates that the poplar seed fibers cannot be packed in a fiberassembly with porosity lower than these values without com-pressing them or collapsing the hollow lumen of the fibers. Oilpenetration into larger capillaries present in the loosely packedassemblies depends on the balance between the capillary forces andretarding gravitational forces due to the weight of the oil in thecapillaries. Liquid bridges, developed within the structures, stabilizeheavy oil trapped inside the inter-fiber pores. With packed assem-blies, most of the oil is trapped inside the inter- and intra-fiber poresas shown in Fig. 8a. The draining of the oil is assisted by viscosity.Accordingly, the diesel fuel drains more quickly because of its lowerviscosity. Similarly to the kapok fibers, the high absorption capacityof the poplar seed fibers is a result of the Van der Waals forces andhydrophobic interactions. These interactions are coupled with theavailability of the hollow lumen and the oleophilicity of the poplarseed fibers. The chemical compatibility between the liquid phase(oil and diesel fuel) and the poplar seed fiber surface wax leads toa minimum energy barrier for the liquid to penetrate the hollowtubular fiber structure (Fig. 8b and c). After the minimum energybarrier is overcome, the void fraction inside the fiber assembly isfilled with liquid and predominantly affects the absorption capacityof the fibers. It has been shown that the higher the amount ofeffective free space in the fiber assembly increases the oil absorption

Fig. 8. (a) The influence of void effects on the absorption of used motor oil after buoyancy testing. Capillary action of motor oil (b) and diesel fuel (c) into the hollow tubes of poplarseed fibers.

M. Likon et al. / Journal of Environmental Management 114 (2013) 158e167 165

capacity, which is achievable at low packing densities. Large inter-fiber pores are not favorable to obtain a high absorption capacityif the capillary forces are not sufficient to counteract the gravita-tional forces. The maximum absorption capacity of the poplar seedfibers is reached at packing density 0.005 g/cm3 (rather loosepacking) that indicates strong capillary forces still prevail thegravitational draining. At higher packing densities (>0.03 g/cm3), itbecomes harder for oil to penetrate, which reduces the absorptioncapacity. At loose packing densities, the gravitational force prevailsover the capillary force in the form of liquid bridges leading to heavydraining of oil from the loosely packed assembly structure.

The results of saturation time measurements of the fiberswith packing densities of 0.005, 0.02 and 0.04 g/cm3 showed that indensely packed poplar seed fiber assemblies, longer times wereneeded (w220 s (0.005 g/cm3), w300 s (0.02 g/cm3) and w530 s(0.04 g/cm3) for high density oil and w30 s (0.005 g/cm3), w50 s(0.02 g/cm3) and w180 s (0.04 g/cm3) for diesel fuel) to reachthe saturation. A significant reduction of oil flow was caused by theconstriction of the channel flow sizes. Generally, the rate of oilpenetration into a capillary is inversely proportional to its viscosity,which explains the longer saturation times measured for high-viscosity engine oil compared to diesel fuel.

The temperature has a significant effect on the absorptionprocessmainly through the change in viscosity of the oil. The measured oilabsorption capacity values for temperatures 50 and 70 �C were 33.2and 25.8 g oil/ g fibers (package density w0.02 g/cm3), respectively.Reduction in absorption capacity at elevated temperatures isexplained by a speed up of Brownian motion of oil molecules thusreducing the possibility of adsorption of oil molecules on the surfaceof the poplar seed fibers. The oil viscosity decreases as the temper-ature rises, reducing the possibility of oil removal. In addition, thehigher the temperature, the fasterwill the already separated oil drainoff the packed poplar seed fiber assembly. Finally, as temperatureincreases, the oil solubility increases as well, decreasing the separa-tion of the oil fromwater. The decrease in temperature increased theoil viscosity and reduced the rate of absorption process. Moredetailed kinetic studies of oil absorption process are in progress andsurpass the scope of this paper.

The results of buoyancy and stability testing of the fiber assemblywith a packing density of 0.02 g/cm3 showed that the inter-fiberliquid bridges were sufficiently strong to hold the absorbed oilwithin the structure of the poplar seed assembly (Fig. 8a). Theamount of diesel fuel retained after 30 min of shaking ranged from96% to 99% and suggests that, despite its lower viscosity, diesel doesnot readily drain out of a packing density of 0.02 g/cm3, compared toheavy motor oil. The oil retention was more than 90% at 0.02 g/cm3

packing density, which is considerably higher aswith kapok fibers atsimilar packing (Abdullah et al., 2010).

Poplar seeds and kapok fibers have excellent buoyancy becauseof their high hydrophobicity, which is comparable to expanded

polypropylene fibers. Silanized cellulose also showed good buoy-ancy. Cotton, sulfate cellulose and viscose do not float on the watersurface, and they are not suitable as absorbents for water surfacecleaning. Paper mill sludge is conditionally useful as an absorbentfor water surface cleaning because of the slow degradation of itsstructure inwater. Comparison of the absorption capacity of sulfateand silanized cellulose fibers (Figs. 6 and 7) shows that the surfacescovered with long and branched hydrocarbon chains have a stronginfluence on the absorption and buoyancy of the material.

3.4. Extraction of poplar seed fibers in different solvents

The IR spectra of poplar seed fibers after extraction in solventswith increasing polarity (hexane, petrol ether, tetrahydrofuran, ethylacetate, acetone,methanol and a 1:1mixture ofmethanol andwater)are presented in Fig. 9aef. The results have shown that extraction inthese solvents did not considerably affect the chemical compositionand stability of the fibers. Minor differences were observed only in IRspectra of fibers treated with the mixture of methanol and water(Fig. 9f) indicating hydroxylation of the fiber surface.

The IR spectra of solid residue in the extract left after theextraction process provided more information on the effect ofsolvent on chemical stability of the poplar seed fibers. The nonpolarsolvents (hexane, petrol ether) did not affect the fibers’ chemicalstructure. The extracts of semi-polar solvents (tetrahydrofuran,ethyl acetate, acetone) contained mainly waxes in small quantities,and the extracts of polar solvents contained hemicelluloses. In theextract of the methanol/water (1:1) mixture, a xylanewas detected.The decrease in weight was greatest after extraction in the meth-anol/water (1:1) mixture; however, the weight decreased only 3%after the complete extraction process.

The second extraction process was more harmful to chemicalstability of poplar seed fibers. The IR spectra of residues obtainedafter the extraction steps with chloroform for 8 hours, diethyl etherfor 3 hours, alcohol-benzenemixture (1:2) for 8 hours and hotwaterfor 24 hours are shown in Fig. 10. The chloroformwas chosen as theprimary extracting solvent because of its hydrophobicity, similar tothe poplar seed fibers’waxy surface. The IR spectra of the extracts ofpoplar seed fibers exhibited similar characteristic absorptionbands as the extracts of kapok fibers (Abdullah et al., 2010). Theappearance of absorption bands at 2920 cm�1 and 2850 cm�1 cor-responding to the asymmetric and symmetric stretching of aliphaticCH2 and CH3 indicated certain removal of wax from the surface ofthe poplar seed fibres by chlorofom. As shown in Fig. 2, theextraction in chloroform removed indeed a considerable amount ofwax from the fiber surface, leaving the cellulose microtubularstructure intact. The treatment of fibers in alcohol-benzene mixtureand hot water affected primarily the lignin and cellulose structure.The broad absorption band between 3500 and 3100 cm�1 indicatedthe hydroxylation of the fiber structure. The evaluation of peak

Fig. 9. ATR-FTIR spectra of poplar seed fibers (a) untreated, after extraction in hexane (b), petrol ether (c), tetrahydrofuran (d), ethyl acetate (e), and in the mixture ofmethanol/water (1:1) (f).

M. Likon et al. / Journal of Environmental Management 114 (2013) 158e167166

intensity ratios of absorption bands at 1595 and 1105 cm�1 fortreated poplar seed fibers in alcohol-benzenemixture and hot watershowed decrease in lignin content from 29 to 6%.

The decrease inweight of the poplar seed fibers and kapok fibersafter extraction 1 in chloroform and extraction 2 in diethyl ether,ethanol/benzene and water is presented in Table 2. These results

Fig. 10. Absorbance FTIR spectra of the solid residue after the extraction of poplar seed fiber

indicate a rather thick hydrophobicwax coating covering the surfaceof the poplar seed fibers. This makes poplar seed fibers resilient andallows them to retain their hydrophobicity after treatment ina hydrophobic medium under harsh conditions. The hydrophobicityof the poplar seed fiber wall structure was not affected by extractionas shown by high contact angle presented in Table 3.

s in (a) chloroform; (b) diethyl ether, (c) mixture ethanol/benzene (1:2), and (d) water.

Table 2The decrease inweight expressed inwt. % of dry poplar seed and kapok fibers beforeand after complete extraction in chloroform (extraction 1) and in diethyl ether,ethanol/benzene and water (extraction 2).

Decrease in weight in wt. % of dry poplar

Extraction 1 Extraction 2

Chloroform Diethyl ether Ethanol/benzene Water Total

Poplar seed fibers 4.62 5.49 3.55 10.73 19.77Kapok fibers 1.46 3.18 3.05 9.38 15.61

Table 3Hydrophobicity of the fibers after the extraction process measured by the contactangle between the liquid and fiber surfaces.

Fiber Contact angle Q in �

Water Oil SAE 15-40 W

Poplar seed fibers (Populus nigra italica) 150 35Kapok fibers 117 27

M. Likon et al. / Journal of Environmental Management 114 (2013) 158e167 167

The absorption capacity of the poplar seed fiber assemblies afterthe extraction with chloroform and the subsequent extractionwith diethyl ether, mixture ethanol/benzene and hot water wasdecreased by 3.48%. This value was slightly higher than the 2.13%measured in the kapok fibers. Generally, the decrease in theabsorption capacity of the poplar seed fibers after the extractionprocess was low and within the measurement error.

4. Conclusions

Poplar seed fibers obtained from the trees of Populus nigra italicaare generally treated as waste material or, at best, as low-qualityfertilizer. The fibers are extremely light, hydrophobic, possess largeactive specific surface area and float onwater surfaces without long-term degradation, even when soaked with hydrophobic liquids. Themicrotubular morphology and the resistant and resilient chemicalstructure of the tube walls make poplar seed fibers an extremelypromising natural source for the production of an oil super absor-bent. The use of poplar seed fibers for the production of oil absor-bents is sustainable, has a low carbon footprint, has a low energydemand and is very clean. At the end of its life cycle, when an oilabsorbent based on poplar seed fibers becomes a waste product, itcan be used as a high-energy fuel, burning without the emission ofnoxious fumes.

Acknowledgement

The authors would like to thank S. Skapin for the x-raymeasurements and M. Cesarek for help with the literature survey.The authors also thank theMinistry for High Education, Science andTechnology of the Republic of Slovenia, the Public Agency of theRepublic of Slovenia for Entrepreneurship and Foreign Investments,the European Commission and company Fibranet Ltd. for technicaland financial support.

Appendix A. Supplementary material

Supplementary material associated with this article can befound, in the online version, at doi:10.1016/j.jenvman.2012.03.047.

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