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Contents lists available at ScienceDirect

Industrial Crops and Products

jo ur nal home p age: www.elsev ier .com/ locate / indcrop

comprehensive approach for obtaining cellulose nanocrystal fromoconut fiber. Part I: Proposition of technological pathways

iego M. do Nascimento a,∗, Jessica S. Almeida b, Maria do S. Vale c, Renato C. Leitão c,elli R. Muniz c, Maria Clea B. de Figueirêdo c, João Paulo S. Morais d,orsyleide de F. Rosa c,∗

Federal University of Ceará (UFC), Department of Chemical, Bloco 940, CEP 60455-760 Fortaleza, Ceará, BrazilState University of Ceará (UECE), Campus do Itapery, CEP 60455-900 Fortaleza, Ceará, BrazilEmbrapa Tropical Agroindustry, Rua Dra Sara Mesquita 2270, Planalto do Pici, CEP 60511-110 Fortaleza, Ceará, BrazilEmbrapa Cotton, Rua Oswaldo Cruz 1143, Centenário, CEP 58428-095 Campina Grande, Paraíba, Brazil

r t i c l e i n f o

rticle history:eceived 30 October 2015eceived in revised form1 December 2015ccepted 28 December 2015vailable online xxx

eywords:ellulose nanocrystal

a b s t r a c t

The high lignin content in the unripe coconut fiber limits the use of this biomass as a cellulose nanocrys-tal source compared to other cellulose-rich materials. The aim of this study was to obtain lignin andbiomethane, and evaluate different approaches for extracting cellulose nanocrystal from unripe coconutcoir fiber. The environmental evaluation of these approaches is presented in the second part of thispaper. Lignin was extracted by acetosolv pulping and cellulose by alkaline hydrogen peroxide bleachingrespectively. Were evaluated the biochemical methane potential of the effluents resulting from aceto-solv pulping as well as the lignin concentration. Cellulose nanocrystals were prepared from cellulosepulp via four methods: acidic hydrolysis with high acid concentration, acidic hydrolysis with low acid

anotechnologyigninreen chemistryiorefinery

concentration, ammonium persulfate oxidation, and high-power ultrasound. The cellulose nanocrystalswere analyzed by FTIR spectroscopy, X-ray diffraction, transmission electron microscopy, and TG anal-ysis. Using these methods, the whole coconut fiber could be used to produce cellulose nanocrystals andlignin. Among the proposed methods, high-power ultrasound showed the highest efficiency in cellulosenanocrystal extraction.

© 2016 Elsevier B.V. All rights reserved.

. Introduction

Cellulose nanocrystal is an abundant potential nanomaterialhat can be extracted from many renewable sources (Dufresne,013). This nanostructure has attracted attention for application ineveral different areas because of its extraordinary physical proper-ies, biodegradability, biocompatibility, and low cytotoxicity (Jorfind Foster, 2015; Rojo et al., 2015). Cellulose nanocrystals can bebtained from wood, non-wood fibers, algae, tunicates, and agroin-ustrial biomass, among other sources (Li et al., 2015).

Among the sources, lignocellulosic agroindustrial byproductsre the most promising because of their low cost and availability.xamples include sugarcane bagasse (Li et al., 2012a; Mandal and

Please cite this article in press as: Nascimento, D.M.d., et al., A comprehfiber. Part I: Proposition of technological pathways. Ind. Crops Prod. (2

hakrabarty, 2014), corn straw (Huntley et al., 2015), palm-pressedesocarp fiber (Souza et al., 2015), sisal (Rodrigues et al., 2015),

ineapple leaves (Deepa et al., 2015), cotton linter (Morais et al.,

∗ Corresponding author.E-mail address: morsyleide@gmail.com (M.d.F. Rosa).

ttp://dx.doi.org/10.1016/j.indcrop.2015.12.078926-6690/© 2016 Elsevier B.V. All rights reserved.

2013), banana pseudostem (Pereira et al., 2014), banana peel, andunripe coconut husk (Fahma et al., 2011; Nascimento et al., 2014;Rosa et al., 2010). However, the profit ability of new biobased indus-tries highly depends on integrating biomass conversion processesto produce a range of fuels, power, materials and chemicals.

Worldwide coconut production was estimated to be approxi-mately 62 million tons in 2013, of which nearly 3 million tonswere harvested in Brazil (FAOStat, 2015). Coconut water is the mainproduct of coconut crops. It is usually consumed directly from thefruit in coastal cities or is bottled for shipping to inland locations.Both processes generate large amounts of unripe coconut coir thatif not correctly collected and disposed of, causes environmentalproblems, reducing the useful lifetime of landfills or causing waterpollution.

Unripe coconut fibers can be extracted and used to manufac-ture several products, such as reinforced polymeric composites,

ensive approach for obtaining cellulose nanocrystal from coconut016), http://dx.doi.org/10.1016/j.indcrop.2015.12.078

reinforced cement and concrete, geotextile fabrics and screens,and wood-replacement fiberboards. Other alternatives for addingvalue and reducing the disposal issues of such biomass involvethe extraction the cellulose nanocrystals. However, there are

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wo main limitations to performing this fractionation: highower and reagent consumption to remove lignin and lowield of cellulose nanocrystals from the original raw materialFigueirêdo et al., 2012). In this sense, all feasible approachesor obtaining cellulose nanocrystals must also add value to other

acromolecules, such as lignin and hemicellulose.Lignin is traditionally considered to be a troublesome waste and

s typically burned in mill boilers to generate power (Cotana et al.,014). Despite this traditional use, lignin has gained attention for

ndustrial applications because of its amorphous nature and highlyromatic molecular structure. Thus, lignin can be used as a rawaterial for producing bulk and fine chemicals, such as vanillin,

allic acid, oils, phenols, acetic acid, films, polyurethanes, carbonbers, and other materials (Norgren and Edlund, 2014). Strass-erger et al. (2014) report that the low price of natural gas andhe diversity of alternatives for lignin conversion is an atractive sce-ario to use lignin as a chemical instead of a simple fuel to be burned

n the boilers. The versatility of fine chemicals that can be producedrom lignin makes this biomacromolecule as lower carbon foot-rint alternative to oil, reducing environmental impacts related tolimate change (McDevitt and Grigsby, 2014). In addition to ligninnd cellulose, the coconut fiber contains high hemicellulose levelshat typically are hydrolyzed during the extraction processes. Suchompounds are present in the effluents of cellulose pulp and cane used as carbon sources for anaerobic fermentation and methaneroduction. Methane has a calorific power (∼50 MJ/kg) higher than

ignin (∼21 MJ/kg) and hemicelluloses (∼16 MJ/kg). The gas can beurned to generate power for the nanocrystal extraction process,educing biogas production and emission in the effluents.

The aim of the present study was to develop new approach fornripe coconut, allowing for the sustainable extraction of celluloseanocrystal and to add value to the extraction byproducts. Ligninas recovered and pulping effluents were fermented to produceethane. Here, the properties of the extracted cellulose nanocrys-

al are reported. In the Part II, the environmental impacts of thextraction methods are evaluated with respect to life cycle analysis.

. Experimental

.1. Materials

Unripe coconut fiber was provided by Embrapa Agroindústriaropical (Fortaleza, CE, Brazil). All chemicals were of analytical

Please cite this article in press as: Nascimento, D.M.d., et al., A comprehfiber. Part I: Proposition of technological pathways. Ind. Crops Prod. (2

rade and were used without further purification: 97% (w/w) NaOH,0% (w/w) H2O2, 99.7% (w/w) CH3COOH, 80% (w/w) NaClO2, and8% (w/w) H2SO4 (Vetec Química Fina Ltda/Sigma–Aldrich—Duquee Caxias, RJ, Brazil).

Fig. 1. General flowchart of coc

PRESSs and Products xxx (2016) xxx–xxx

2.2. Coconut fiber fractioning

The fractionation flowchart is presented in Fig. 1. Coconutfiber was ground in a Willye knife mill (STAR FT-80; Fortinox,Piracicaba/SP, Brazil) with a 1-mm-large sieve. Delignification wasperformed as described by Nascimento et al. (2014) with slightmodifications.

Briefly, after grinding, the fiber was added to acetosolv solutioncontaining 93% (w/w) acetic acid and 0.3% (w/w) HCl in the ratio of1:20 (w/v), heated under continuous reflux and stirring for 3 h. Afterpulping, the fibrous material was filtered through a Whatman no2 paper filter and rinsed with fresh acetosolv solution at 80 ◦C. Theblack liquor, this effluent from this step, was stored for later ligninrecovery. The delignified cellulose pulp was rinsed with water untilthe pH was neutral.

The fiber was bleached in the proportion of 1:20 (w/v). First, thepulp was stirred for 90 min at 50 ◦C with 5% (w/w) H2O2 and 3.8(w/w) NaOH. The bleached pulp was filtered through a Whatman no2 paper filter and rinsed with distilled water. This bleaching processwas repeated once more. Finally, the fibers were stirred with 5.7%(w/w) KOH for 120 min at 90 ◦C, filtered through a Whatman no 2paper filter, and rinsed with distilled water to obtain the cellulosebleached pulp (Fahma et al., 2011).

The pulping black liquor was concentrated in a rotary evapo-rator (R-210/215; Buchi, Flawil, Switzerland), diluted 10× in hotwater (∼80 ◦C), filtered through Whatman no 2 paper filter, andrinsed with distilled water to recover lignin. The recovered ligninwas stored in a silica-gel desiccator until characterization analyses(Morandium-Gianetti et al., 2012). The yield of recovered lignin,the percentage of Klason lignin in the fiber that was successfullyrecovered, was calculated using Eq. (1):

%Yrecovered lignin = mrecovered lignin

mfiber × %LKfiber× 100 (1)

where mrecovered lignin is the weight of the recovered lignin, mfiberis the weight of the coconut fiber and%LKfiber is the lignin Klasoncontents of the samples.

The pulping effluent of the lignin recovery (effluent A) (172 mLof effluent/1 g of coconut fiber) and the mixed effluent of bleach-ing and lignin recovery (effluent AB) were collected (397 mLof effluent/1 g of coconut fiber) and analyzed to determine thebiodegradability and biochemical methane potential (BMP) assaysvia anaerobic digestion.

ensive approach for obtaining cellulose nanocrystal from coconut016), http://dx.doi.org/10.1016/j.indcrop.2015.12.078

2.3. Cellulose nanocrystal extraction

Cellulose nanocrystal extraction was performed using fourdifferent methods: two acidic hydrolysis-based extractions, one

onut fiber fractionation.

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mmonium persulfate (APS) oxidation-based extraction, and oneigh-power ultrasound-based extraction (Fig. 1). The yield of thextraction was estimated as the ratio of the cellulose nanocrystalsass and the cellulose mass in the fiber used in the extraction step,

sing Eq. (2):

Ynanocrystals = mnanocrystals

mpulp × %C˛−cellulose100

× 100 (2)

here mnanocrystals is the weight of the extracted nanocrystals,pulp is the weight of the cellulose pulp (bleached pulp for CNH1,

NH2 and CNU methods, and delignified pulp for CNO method)nd%C�-cellulose is the �-cellulose contents of the samples.

.3.1. Acidic hydrolysisAcidic hydrolysis was performed in the proportion of 1:20

w/v) under stirring in two different conditions of H2SO4: (i)NH1, with low H2SO4 concentration (44% w/w), long reactionime (360 min), and high temperature (60 ◦C); (ii) CNH2, with high2SO4 concentration (60% w/w), short reaction time (45 min), and

ow temperature (45 ◦C) (Nascimento et al., 2014; Pereira et al.,014).

Cellulose nanocrystal cellulose extracted by both processesCNH1 and CNH2) were diluted by 10-fold and centrifuged (CR2GIII; Hitachi, Tokyo, Japan) with deionized water for 15 min at3 krpm (26.4 kg) until the supernatant became turbid (three cen-rifugation steps). Thereafter, the suspension was dialyzed in stillistilled water for 72 h, ultra-sonicated (UP400S; Hielscher, Teltow,ermany) for 5 min, and freeze-dried. The final yield was calculateds the percentage of cellulose nanocrystal mass compared to thenitial bleached pulp mass (Eq. (2)).

.3.2. APS oxidationDelignified pulp was added to a 18.5% (w/w) APS solution

(NH4)2S2O8] at 60 ◦C at a ratio of 1:100 (w/v) under vigoroustirring for 16 h (Leung et al., 2011). The carboxylated cellu-ose nanocrystal (CNO) colloidal suspension was centrifuged witheionized water at a pH close to neutral, and the yield was calcu-

ated as the percentage of cellulose nanocrystal mass compared tohe initial delignified pulp mass (Eq. (2)).

.3.3. High-power ultrasoundBleached pulp was mixed with deionized water at a ratio of

:200 (w/v) and kept for 24 h. The mixture was wet-milled (T50,KA Works, Inc., Wilmington, NC, USA) for 10 min and transferredo a jacketed stainless-steel reactor with cold-water circulation.he mixture was treated with a high-power ultrasound processorUIP1500hd; Hielscher) at 20 kHz frequency and 2.5-cm-diameteritanium sonotrode, which was immersed 2/3 of the length in the

iddle of the mixture for 20 min at 1200 W (Li et al., 2013). The col-oidal suspension of cellulose nanocrystals (CNU) was centrifugednd freeze-dried. The final yield was calculated as the percentagef cellulose nanocrystal mass compared to the initial bleached pulpass (Eq. (2)).

.4. Fiber chemical composition

The hemicellulose and �-cellulose contents of the samplesere determined in accordance with TAPPI T203cm-99 (2009) andokoyama (2002). Klason lignin was measured as described by theAPPI T 222 om-22 (2002) standards with minor modifications.

Please cite this article in press as: Nascimento, D.M.d., et al., A comprehfiber. Part I: Proposition of technological pathways. Ind. Crops Prod. (2

.5. Biodegradability and MPP

Anaerobic biodegradability evaluation was based on specificethanogenic activity (Soto et al., 1993). The initial inoculum was

PRESSs and Products xxx (2016) xxx–xxx 3

home sewer sludge from CAGECE (Water and Sewerage Companyof Ceará). Methane production was recorded using an anaerobicrespirometer (Micro-Oximax; Columbus Instruments, Columbus,OH, USA). Biodegradability and BMP were calculated as describedby Costa et al. (2014).

2.6. FTIR

Cellulose nanocrystal samples were ground and pelleted usingKBr. Spectra were recorded from 4000 cm−1 to 400 cm−1 (Cary640;Agilent Technologies, Santa Clara, CA, USA) using Fourier transforminfrared spectroscopy.

2.7. XRD

Cellulose nanocrystal crystallinity indexes (CI) were analyzedusing an X-ray diffractometer (Xpert Pro MPD; PANalytical, Almelo,Netherlands), with the Co tube at 40 kV and 40 mA in the 2� rangefrom 3◦ to 50◦. CI was calculated using two different methods: (i)by determining the ratio between the integrated area of all crys-talline peaks and the integrated total area using PeakFit® softwarefrom Systat Software, Inc. (San Jose, CA, USA) (Garvey et al., 2005),and (ii) by determining the ratio between the peak intensity ofthe total crystallinity and the total peak intensity, considering theamorphous (2� = 18,5◦) and crystalline (2� = 22,5◦) contents (Segalet al., 1959).

2.8. Electronic microscopy

Fibers were gold sputtered in a metalizer (K550; Emitech, FallRiver, MA, USA) and analyzed by scanning electron microscopy(DSM 940A; Zeiss, Jena, Germany) at an acceleration voltage of15 kV.

Cellulose nanostructures were dropped in a 300-mesh coppercarbon grid, draining the excess of water during sample prepara-tion. The cellulose nanocrystals were contrasted with a 2% (w/v)uranyl acetate solution and analyzed by transmission electronmicroscopy (JEM-1011; Jeol, Tokyo, Japan). Nanocrystal sizes andaspect ratios (length/width) were calculated using GNU ImageManipulation Program (GIMP 2.8) software.

2.9. Thermal analyses

Approximately 15–20 mg of samples in an alumina sam-ple holder were evaluated in a thermal analyzer (STA 6000;PerkinElmer, Waltham, MA, USA) under a nitrogen atmospherewith 40 mL/min of gas flux, heating rate of 10 ◦C/min, and tem-perature ranging from 50 to 800 ◦C.

3. Results and discussion

3.1. Fiber chemical composition

The chemical composition of the unripe coconut coir fiber, delig-nified cellulose pulp, bleached cellulose pulp, and recovered ligninare presented in Table 1. The major component of coconut fiberwas lignin (35.1%), with a similar content (35%) as that reported byvan Dam et al. (2006). Both values are between the lignin contentsreported by Brígida et al. (2010) (43.1%) and Bledzki et al. (2010)(27%); these values are all related to coconut fiber.

Cellulose was the second major component (31.6%) and thevalue was similar to that reported by Rosa et al. (2010) (32%) and

ensive approach for obtaining cellulose nanocrystal from coconut016), http://dx.doi.org/10.1016/j.indcrop.2015.12.078

Muensri et al. (2011), but below the values recorded by Brígidaet al. (2010) (45.9%) and Bledzki et al. (2010) (34%). Hemicellulosecontent (25%) was similar to that reported by Muensri et al. (2011)(23%) and Bledzki et al. (2010) (21%).

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Table 1Chemical composition of coconut fiber, delignified pulp, bleached pulp, and recov-ered lignin.

Sample Chemical composition (%)

�-Cellulose Hemicelluloses Klason lignin

Coconut fiber 31.6 ± 0.4 25.5 ± 0.4 35.1 ± 2.2Delignified pulp 41.8 ± 4.9 42.2 ± 5.3 19.1 ± 2.1

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Table 2Anaerobic biodegradability and biochemical methane potential (BMP) of effluentsA and AB.

Sample Biodegradability (%)Total PPM (Lmethane/kgefluente)

removal of lignin and hemi-celluloses, which generally hold cel-

Bleached pulp 70.0 ± 0.6 28.0 ± 0.5 0Recovered lignin – – 94.5 ± 0.8

The chemical composition of plant fibers varies as a functionf several factors, including genotypes, environments, physiologi-al conditions, and harvest techniques, among others (Kruse et al.,008; Pettigrew, 2001). Differences among the chemical compo-ition were similar to expected values and support the previousharacterization of each fiber batch.

High lignin and hemicellulose contents decrease the celluloseer mass of fiber and block the accessibility of reagents to cellulose.oconut fiber is a complex and interesting plant fiber, and it is not

easible to use it for cellulose nanocrystal extraction. Approacheso add value to this fiber must consider adding value to lignin andemicellulose, since more than half of the components are non-ellulosic biomaterials. Thus, the three macrocomponents muste simultaneously used on an industrial scale, and the developedethod may be useful as a model for other low-cellulose agroin-

ustrial biomasses.After acetosolv delignification and alkaline peroxide bleaching,

he cellulose content increased to 70% and lignin content decreasedo below the detection limit. Brígida et al. (2010) reported 41.9% cel-ulose and 47.3% lignin after coconut fiber bleaching with sodiumhlorite, 62.8% cellulose and 45.1% lignin with sodium chlorite andaOH, and 43.9% cellulose and 42.7% lignin using hydrogen perox-

de.

.2. Lignin fractionation, anaerobic biodegradability, andiochemical methane potential

The yield of recovered lignin was ∼30.1% and the purity was94.5%. The yield may be a consequence of the long reaction timend high solubilization of the recovered lignin from the black liquor.his result is related to the reaction conditions and how such con-itions affect the macromolecule depolymerization. The higher theleavage of ether bonds in the lignin subunits, the higher the dis-olution of lignin fragments. Despite the relatively small amountf recovered lignin, it was important to separate the lignin ratherhan destroying it during the pulping process. This makes thehole fractionation process more cost-competitive and improves

he manufacture of products from this biomass.Residual lignin from acetosolv pulping usually presents lower

olecular mass and higher reactivity than residual lignin fromther pulping processes. Such characteristics are responsible for theesidual lignin in the coconut fiber to be easily removed, reducinghe amount of chemicals in the bleaching process (Xu et al., 2007).esidual lignin in the bleaching effluents usually is so degraded andodified that it can not be recovered as an useful chemical (Pandey

nd Kim, 2011). Further researches aiming to increase the overallield of the lignin recovery in the pulping step and to find innova-ive uses for the residual lignin in the bleaching effluents shall beerformed.

The effluents did not show significant differences in biodegrad-bility and BMP after 30 days of treatment (Table 2). However,

Please cite this article in press as: Nascimento, D.M.d., et al., A comprehfiber. Part I: Proposition of technological pathways. Ind. Crops Prod. (2

ffluent AB required a longer time for bacterial digestion, indicatinghe presence of possible bacterial growth inhibitory chemicals, suchs furfural, 5-hydroximethylfurfural, phenolic monomers from

Effluent A 45.21 ± 3.3 2.71 ± 0.20Effluent AB 43.4 ± 0.7 2.00 ± 0.02

lignin and reactive oxygen species, including H2O2, HO−, and O21−

(Monlau et al., 2014, 2013).The BMP values for both effluents were inferior to the results

of Costa et al. (2014) for autohydrolyzed sugarcane bagasse(197.5 Lmethane/kgeffluent). This difference is due to the no use of thehydrolyzed with the fiber, presence of inhibitory chemicals, lowcontent of hemicelluloses removed from the coconut fiber (Table 1),and to the use of no concentrated effluents. In spite of the lowmethane production, effluent AB can be used as a methane source,and this prevents it to be dispose in water bodies, preventing pol-lution in rivers, lakes, and groundwater.

3.3. Cellulose nanocrystal extraction

The yields of the cellulose nanocrystal extraction methods(CNH1, CNH2, CNO, and CNU) ranged from 33% to 88% as describedin Eq. (2). The low yield of CNH2 (32.8 ± 0.2%) relative to CNH1(59.8 ± 2.1%) (both of which involved H2SO4) could be explainedby the acid concentration. Higher acid content in the reaction isassociated with harsher hydrolysis conditions, and even crystallinedomains in the cellulose structure are broken in oligo and monosac-charides, decreasing cellulose nanocrystal levels and overall yield(Chenampulli et al., 2013).

The yield of CNO (49.6 ± 0.3%) can be attributed to APS reactionwith cellulose, promoting concomitantly hydrolysis and degrada-tion due formation SO4

2− and HSO41− species. Hydrolysis yield

with APS depends strongly on the reaction time and tempera-ture. Harsh conditions (high time and temperature) result in higherdegradation and low yield (Cheng et al., 2014). Pulp after acetosolvpulping still presents a high lignin content, and this biomacro-molecule is a hinder for the ionic attack of SO4

2− and HSO41−. APS

procedure presents an advantage in the challenge of nanocellu-lose production from lignin-rich fibers, as agroindustrial biomass,because APS procedure can bleach and hydrolyze such fibers in onestep.

The yield of cellulose nanocrystal from CNU (88.1 ± 1.5%)(20 min and 1200 W) was higher than that CNH1, CNH2 and CNOsamples. The use of mechanical methods lead to the productionof cellulose nanocrystals with higher yield (Li et al., 2012b, 2013;Oksman et al., 2011). However, this method can produce heteroge-neous and lower colloidal stability. These results indicate that CNUhad the best yield among the three different nanocrystal extrac-tion approaches. Moreover, this yield was higher than that obtainedusing H2SO4, CNH2, and CNH1, the standard procedures for cellu-lose nanocrystal extraction.

3.4. Electron microscopy

Fiber morphology was affected by pretreatment, whichremoved some macromolecular components (Fig. 2) (Johar et al.,2012). Raw coconut fiber had an irregular and compact surfacecompared to the delignified and bleached pulps, which had a roughsurface and loosely bundled macrofibrils.

Decompression of the whole fiber occurred because of the

ensive approach for obtaining cellulose nanocrystal from coconut016), http://dx.doi.org/10.1016/j.indcrop.2015.12.078

lulose fibers together. After disassembly of the lignocellulosicmatrix, cellulose macrofibrils were more accessible for the cel-lulose nanocrystal extraction process because of the removal of

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lp; an

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Fig. 2. Scanning electron microscopy of (a) coconut fiber; (b) delignified pu

on-cellulosic components as well as the increased surface areaRosa et al., 2010).

Cellulose nanocrystal micrographies (Fig. 3) and the calculatedize parameters length (L), width (W), and aspect ratio (L/W)Table 3) indicated that CNH1 and CNH2 were needle-like and well-ispersed, with average (L) values of 128 nm and 208 nm, averageW) values of 6.6 nm and 4.9 nm, and aspect ratio of 19 and 42.ellulose nanocrystals produced by acidic hydrolysis showed anspect ratio close to that of nanocrystals from banana pseudostem21) (Pereira et al., 2014), Agave tequilana bagasse (28) (Espinot al., 2014), brewer’s spent grains (36) (Martínez-Sanz et al., 2014),otton linter (24) (Morais et al., 2013), and bleached white coir, pre-ared using other pulping methods (22) (Nascimento et al., 2014)r (39) (Rosa et al., 2010).

CNO had a spherical shape and was well dispersed, with an aver-ge (W) of 116 nm. Liocel cellulose nanocrystals had an averageidth of 96 nm, while microcrystalline cellulose nanocrystal hadidths ranging from 100 to 270 nm. The aspect ratio for spheri-

al nanoparticles is 1, which means a smaller specific surface area

Please cite this article in press as: Nascimento, D.M.d., et al., A comprehfiber. Part I: Proposition of technological pathways. Ind. Crops Prod. (2

ompared to other morphologies nanoparticles and therefore, moreusceptibility of surface modifying reactions (Hsieh, 2013; Zhao and

inter, 2014).

able 3ength (L), width (W), and aspect ratio of cellulose nanocrystals extracted usingifferent methods.

Sample L (nm) W (nm) L/W

CNH1 128 ± 52 6.6 ± 1.5 19CNH2 208 ± 34 4.9 ± 0.5 42CNO 116 ± 27 116 ± 27 1CNU 307 ± 165 14.6 ± 2.5 21

d (c–d) bleached pulp. Scale bar = 50 �m to (a), (b) e (c), and 20 �m to (d).

The CNU structure shows needle-like and cluster formationbecause of the absence of surface-charged groups produced in othernanostructures, with average (L) of 307 nm, average (W) of 14.6 nm,and an aspect ratio of 21. A minimum aspect ratio of 10 is requiredfor efficient tension transfer between the load and the matrix,enabling the nanostructure to be used as a reinforcement (Azeredoet al., 2009).

CNH2 showed the highest aspect ratio because it had the short-est reaction time. Although high-power ultrasound did not producenanocrystals with the highest aspect ratio, the CNU aspect ratio wasstill well above the minimum threshold, showing a similar size toCNH1 and CNH2.

3.5. FTIR

FTIR spectra of raw coconut fiber, delignified cellulose pulp,bleached cellulose pulp, and cellulose nanocrystal are shown inFig. 4 and the summary of the main absorption peaks are shownin Table 4.

The absence of the peaks around 1730, 1612, 1516, and1450 cm−1, as well as the significant reduction around 1639 cm−1,in the delignified and bleached pulps indicate lignin removal. Thepersistence of peaks around 1163 and 899 cm−1 in the bleachedpulp indicates the presence of hemicelluloses resulting from par-tial removal of this set of macromolecules, which was in accordancewith the chemical composition analyses (Section 3.1).

Cellulose nanocrystal samples showed peaks at 1431, 1319,

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1034, and 897 cm−1, which typical cellulose absorption peaks. InCNO, a peak was observed at around 1730 cm−1, indicating the pres-ence of C O groups after oxidation with APS (Habibi et al., 2010).Cellulose nanocrystal carboxylation preferentially occurs at the C6

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F sulfurh

ooess

ig. 3. Transmission electron microscopy for cellulose nanocrystals extracted by

igh-power ultrasound (CNU). Scale bar = 500 nm.

f the crystal surface and allows more reactive nanoparticles to be

Please cite this article in press as: Nascimento, D.M.d., et al., A comprehfiber. Part I: Proposition of technological pathways. Ind. Crops Prod. (2

btained. Cellulose oxidation degree (OD) can be calculated by thequation OD = 0.01 + 0.7(I1735/I1060), where I1735 is the peak inten-ity of carbonyl group, and I1060 is the peak intensity of cellulosekeleton vibration (Habibi et al., 2006). The calculated OD for CNO

Fig. 4. FTIR spectra of coconut fiber, delignified pulp, bleache

ic acid hydrolysis (CNH1 and CNH2), ammonium persulfate oxidation (CNO), and

was 0.197. Their surfaces can be easily modified to improve the

ensive approach for obtaining cellulose nanocrystal from coconut016), http://dx.doi.org/10.1016/j.indcrop.2015.12.078

versatility of the nanofiller and result in nanocomposites that canbe more easily processed (Cheng et al., 2014; Leung et al., 2011). Inboth CNH1 and CNH2, a band at 810 cm−1 was observed, which isrelated to sulfate esters in the nanocrystals (Bellamy, 1980; Felício

d pulp, recovered lignin, CNH1, CNH2, CNO, and CNU.

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Table 4Main absorption wavelengths (cm−1) and relation to chemical groups and lignocellulosic molecules (Gonc alves et al., 2014; Rehman et al., 2013; Rosa et al., 2010 Rosa et al.,2010).

Wavenumber (cm−1) Chemical significance

3411 Axial deformation of O H2993 Axial deformation of methyl and methylene C H2920 Axial deformation of C H1735 Axial deformation of C O in non-conjugated ketones and esters, usually carbohydrates1718 Axial deformation of C O at � position of carboxyl groups1639 Angular deformation of H2O; Axial deformation of lignin C O1612 Vibration C C of lignin aromatic skeleton1516 Vibration C C of lignin aromatic skeleton1450 Vibration C C of lignin aromatic skeleton; Asymmetric angular deformation of CH3 e CH2

1431 Angular deformation of cellulose and lignin C H1377 Angular deformation of cellulose, hemicellulose, and lignin C H1319 Wagging vibration of cellulose CH2

1265 Vibration of guaiacyl (G) aromatic ring1250 Axial deformation of hemicelluloses and guaiacyl C O; and Angular deformation of cellulose C H1225 Axial deformation of C O e C C on the guaiacyl (G) aromatic ring1163 Asymmetric axial deformation C O C of hemicelluloses1119 Plane angular deformation of C H in the syringuyl and guaiacyl (GS) aromatic ring1107 Glucosidic ring vibration in cellulose; Plane deformation of lignin C H1060 Glucosidic ring vibration of C O1034 Vibration of p-hidrofenilpropano (H) units958 Out of plane angular deformation of aromatic C H

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t al., 2008). None of the nanocrystals showed absorption peaks at612, 1516, or 1450 cm−1, which are related to lignin. Thus, thisacromolecule contains no lignin.

.6. XRD

The crystallographic profile of all samples is typical of cellulose, with diffraction peaks around 15◦ (plan 101), 17◦ (plan10ı), 22◦

plan 021), 23◦ (plan 002), and 35◦ (plan 040) (Garvey et al., 2005)Fig. 5). The chemical reactions affected the crystallinity index, butid not modify the cellulose allomorph structure in any sample.

The increment of the crystallinity index was observed from theoconut fiber to the delignified fiber, to the bleached fiber, and washe highest in the nanocrystals (Table 5). This increment is a con-equence of the removal of amorphous components, mainly ligninnd hemicelluloses.

CNO showed a high crystallinity index, indicating the removalf non-cellulosic components, such as lignin, by oxidation. Lignin

s destroyed by oxidative processes, and the APS reaction pro-uces cellulose nanocrystals and oxidizes residual lignin, acting as

bleaching reaction.CNH2 showed the highest crystallinity index, regardless of the

ethod used to calculate the index (Segal or deconvolution). Thisas expected because H2SO4 nanocrystal extractions are harsh

nd generate nanocrystals with high crystallinity indexes (Marianot al., 2014).

The crystallinity index calculated for CNU and CNO differed

Please cite this article in press as: Nascimento, D.M.d., et al., A comprehfiber. Part I: Proposition of technological pathways. Ind. Crops Prod. (2

epending on the method used. While the deconvolution approachhowed values of 82% for CNU and 81% for CNO, the Segal approachhowed a smaller crystallinity index (75%) for CNU and a higherndex (85%) for CNO. Different methodological approaches may

able 5rystallinity indexes of coconut fiber, delignified pulp, bleached pulp, CNH1, CNH2,NO, and CNU.

Sample Coconutfiber

Delignifiedpulp

Bleachedpulp

CNH1 CNH2 CNO CNU

Crystallinity index (%)

Deconvolution 52 59 61 68 71 81 82Segal 57 64 77 79 80 85 79

of C H in the guaiacyl (G) aromatic ring C-2, C-5, and C-6ous domains

generate different values, suggesting that it is important to usemethods that best approximate the actual values. Segal’s method-ology (Segal et al., 1959) is simpler and is used more frequently,but it is based only on the height of the recorded peaks (Terinteet al., 2011). Deconvolution methodology is not typically used forcrystallinity index calculations, but presents values closer to thecrystallinity index measured by RMN 13C, which is considered tobe the gold standard and reveal true values (Park et al., 2010).

CNO had a higher crystallinity index because hydrolysis of amor-phous regions occurred. Cheng et al. (2014) obtained similar resultsfor the hydrolysis and oxidation of liocel fiber with 18.5% (w/w) APSafter a 20-h reaction, with 94% of crystallinity as measured usingSegal’s method.

The increase in the crystallinity index in the CNU validates thatsome cellulose amorphous regions were removed after high-powerultrasound treatment. The results are corroborated by the results ofother studies, in which ultrasound was suggested to create cavityeffects that preferentially fragment cellulose amorphous regions(Li et al., 2012a,b).

Segal’s crystallinity indexes for all cellulose crystals were higherthan those reported by Rosa et al. (2010) (66%) and Fahma et al.(2011) (57%) for coconut fiber from cellulose pulps prepared usingdifferent pretreatments. Acetosolv delignification and bleaching aswell as the cellulose nanocrystal extraction methodologies werequite effective for increasing the hydrolyzing reagents to the cel-lulose content and reducing the overall amorphous content in thecrystals.

3.7. Thermal analyses

Loss weight values and initial degradation temperatures (Ton-set) are presented in Table 6.

The thermogravimetric curve for the raw coconut fiber was sim-ilar to the curves reported by Rosa et al. (2010) and Nascimentoet al. (2014). The thermal stability of the coconut fiber increased

ensive approach for obtaining cellulose nanocrystal from coconut016), http://dx.doi.org/10.1016/j.indcrop.2015.12.078

after the pulping treatments. All analyzed samples showed a smallloss in weight at temperatures below and around 100 ◦C, whichwas related to water evaporation. The bleached pulp showed a sin-gle loss mass event at approximately 328 ◦C, which was related to

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Fig. 5. X-ray diffractograms of coconut fiber, delignified pulp, bleached pulp, CNH1, CNH2, CNO, and CNU.

Table 6Thermal events of TG/dTG thermograms under inert atmosphere.

Samples 1st event 2nd event 3rd event

m (%) Tonset (◦C) m (%) Tonset (◦C) m (%) Tonset (◦C)

Coconut fiber 23.9 260.2 52.6 335.3 – –Delignified pulp 81.9 328.1Bleached pulp 82.2 314.5 – – – –CNH1 80.7 300.6 – – – –

1.1

3.5

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CNH2 8.5 227.8 3CNO 33.3 210.3 3CNU 86.1 324.3 –

he removal of hemicellulose and lignin and increase in celluloseontent, which has a higher thermal stability.

Cellulose nanocrystal samples showed a small loss in water atemperatures below and around 100 ◦C. CNH1 presented a sole

ass loss event at approximately 300 ◦C, while CNH2 showed threeass loss events at 227 ◦C, 279 ◦C, and 343 ◦C. The small thermal

tability of both samples in comparison to the bleached pulp waselated to the presence of sulfate groups on the surface of the cel-ulose chains. This reduced the activation energy and eased theepolymerization, dehydration, and thermal decomposition reac-ions (Teodoro et al., 2011). The lower stability and higher numberf thermal events of the CNH2 are related to the greater number ofulfate esters in the cellulose chains.

Rosa et al. (2010) reported that residual lignin in the nanocel-ulose suspensions prepared by acidic hydrolysis may contributeo increasing the thermal stability and improving the compatibil-ty of the filler with hydrophobic matrices. A further approach foreaching this aim is to incorporate small amounts of lignin in theellulose pulp, reducing the severity of the bleaching treatment,aving reagents and power.

CNO involved two main weight reduction events, includingt approximately 210 and 325 ◦C. Bamboo cellulose nanocrystalsroduced by APS oxidation showed higher thermal stability thanhe nanocrystals produced by acidic hydrolysis (Hu, 2014). CNO

Please cite this article in press as: Nascimento, D.M.d., et al., A comprehfiber. Part I: Proposition of technological pathways. Ind. Crops Prod. (2

oconut cellulose nanocrystal showed thermal stability similar tohat observed for TEMPO-oxidized cellulose nanofibrils (Carlssont al., 2014). Further studies are necessary to optimize the reac-

279.7 31.6 343.7324.8– – –

tion times and reagent concentrations for coconut delignified pulpoxidation.

CNU showed a single weight reduction event at around 324 ◦C,which was 86.12% of the original mass. The presence of sul-fate groups in the surface of the sulfuric acid-hydrolyzed CNCsdecreases the thermal stability in comparison to the CNU, whichwere produced in aqueous medium, with no sulfur esters (Oksmanet al., 2011). This behavior is determined because thermal degrada-tion reactions begin in the cellulose amorphous domains (Ciolacuet al., 2011). The higher thermal stability of the ultrasound-produced nanocellulose was a consequence of the absence ofsulfate groups on the crystal surface, combined with a high crys-tallinity index. Since thermal degradation begins in the amorphousdomains, fewer amorphous regions would result in the degradationof fewer regions (Ciolacu et al., 2011).

Since CNU showed the highest thermal stability of all producednanocrystals, this nanocellulose can be incorporated in polymersover a wide temperature range. For example, in further studies, thiscellulose nanocrystal can be used in polymer extruders becausemost of the typically used thermoplastic matrixes have meltingtemperatures of around 200 ◦C (Chen et al., 2011; Roman andWinter, 2004).

4. Conclusions

ensive approach for obtaining cellulose nanocrystal from coconut016), http://dx.doi.org/10.1016/j.indcrop.2015.12.078

Coconut fiber contains a low cellulose content compared toother lignocellulosic biomasses, but high lignin content and fairhemicellulose content. Thus, all main macromolecules in this

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ndustrial fiber must be used as an economic source of added-valuehemicals or power, increasing its technical competitiveness andconomic viability to overcome issues related to coir disposal. Inhis study, we developed an approach that complies with principlesf green chemistry and integral use of biomass.

Coconut fiber was successfully fractionated into carbohydratend lignin materials without using chlorine processes. Hemicel-uloses were successfully used as a power source for methaneroduction.

This study described several methods for transforming rawber in cellulose nanocrystals with high yield, purity, aspect ratio,rystallinity index, and thermal stability above 200 ◦C. Among thevaluated methods, high-power ultrasound showed results thatere superior to the standard procedure based on sulfuric acid

ydrolysis, and may be scaled up for white coconut fiber nanocel-ulose manufacture.

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