Md.Sakinul Islam ,Nhol Kao ,*,Sati N.Bhattacharya ,Rahul Gupta ,Hyoung Jin Choi

1 Department of Chemical Engineering,School of Engineering,RMIT University,Melbourne,VIC 3001,Australia

2 Department of Polymer Science and Engineering,Inha University,Incheon 22212,South Korea

1.Introduction

Nanocrystalline cellulose(NCC)is a bio-nano-material that has gained the attention of many researchers and scientists at the international level due to its suitability for use in pharmaceutical and biomedical applications.The development of bio-nanomaterials holds great features to mitigate many of the sustainability problems,offering the potential of renew ability and biodegradability,and a path away from harmful additives.The research presented here involves with focusing the potential aspect of a gro-waste biomass for converting them to highly value-added products such as NCC.While NCC has successfully been produced in the lab scale from various lignocellulosic biomasses[1,2],the mass production of NCC from low cost raw materials with developing efficient technology is still scarce at this moment.

Agro-waste biomass is a suitable candidate for the production of NCC by considering the economic point of view and abundant supply all over the world.The annual global primary production of biomass is about 220 billion tonnes on dry mass basis,and it is possible to harvest 10–13 t of dry biomass per hectare per annum.The top four major agricultural crops grown in the world are rice,maize,wheat,and sugarcane,respectively in terms of total cultivated area and production.The above estimate of annual biomass production from rice,maize,wheat and sugarcane crops yields about 5358.54 million tonnes of dry biomass[3].Though its major part is utilized as feed for animals,but a huge amount of this biomass remains unutilized and/or is burnt in open environment.Rice husk is among the common agricultural residue.Rice husk amounts to 20%–25%of the harvested rice grain on dry mass basis,which is usually separated from rice grain at the processing centres[4].

An emission estimate of direct combustion of biomass yields is about 1599 kg carbon dioxide,111.3 kg carbon monoxide,9.2 kg methane,5.6 kg hydrocarbon and 4.8 kg particulate matter per tonne of dry biomass[5].Though,the direct burning of biomass in open environment theoretically does not produce carbon dioxide,as biomass is carbon neutral and does not contribute to the greenhouse effect.However,it is nota good and recommendable practice for sustainable development.Therefore,this biomass must be utilized properly for the production of value-added products.

From the last two decades,researchers have been focusing on the use of lignocellulosic biomass for miscellaneous applications like biooil,bio-fuel,bacterial cellulose,bioethanol and biomolecule production due to its low cost and abundance in supply around the world.Agricultural waste biomass such as coconut husk[6],cassava bagasse[7],banana fibre[8],mulberry bark[9,10],soybean pods[11],wheat straw,and soy hulls[1],cornstalks[12],and sisal fibres[13]have been studied as a resource in the production of NCC.Significant numbers of research have already been published on NCC production in small scale by microbial[14,15],mechanical and chemical processes[14,16,17].Nevertheless,extensive research is still required for large scale production.Even a variety of natural fibres have been investigated and reported in literature,the use of rice husk as a natural source for the production ofNCC has not widely been explored yet.Limited work has been reported for the production of cellulose[6,18–20]and microcrystalline cellulose(MCC)[21,22]and NCC[19,23]from rice husk biomass by a chemical extraction process.In these published works,NCCs obtained from rice husk are still bottle-necked,and the process is not efficient.Even production scale is not well discussed in the conversion of biomass to NCC[18–20].Rice straw has also been reported as a source of raw materials for the production ofNCC[24,25]which is reserved in the harvesting season for cattle food.Rice husk,which is the waste produced during the rice harvesting season,has been used for NCC production in this study.

The rice milling industry generates an enormous amount of rice husks in Australia during the paddy milling process from the fields.There are approximately 2000 farms eligible to grow 1 million tonnes of rice each year in Australia.There are about 412 rice farms under the SunRice Corporation,producing 244787.0 t of rice every year from 22446.0 hectare harvested land with average yield 10.9 × 10?4t·m?2(SunRice,Australia).Apart from this rice production statistics,Australia is a very prospective country by considering agro-waste resources because it can be utilized to produce value-added product.To the best of the authors' knowledge,this research was performed first in Australia for the production of NCC from rice husk biomass in order to explore its greatpotential.Simultaneously,this study also further enhances the fundamental understanding of the production of NCC from rice husk by a chemical extraction process as well.

Rice husk comprises of 33%cellulose,26% hemicellulose,7%lignin as reported by Jackson et al.and Kirubakaran et al.[26,27],but it varies from one place to other due to the weather and geographical positions.Instead of treating this biomass as a waste,a better option is to extract the cellulose content for value-added products like NCC,nanofibrillated cellulose(NFC),CNC,etc.[28,29].Therefore by considering huge supply all over the world including Australia,the use of rice husk as the primary source of raw material for producing NCC is a promising approach.Furthermore,the agro-waste biomass can only be considered sustainable if it is economically sufficient and profitable,socially viable—providing a netbenefitin improving the environmental performance and rural development.By considering viability,annual production,and all those required features,there is a great potential for converting rice husk to NCC as a value-added product.

In this research,NCC was produced as a potential value-added product from R-RHB to evaluate the sustainability of this agricultural resource.The major concerns of NCC production are the mass production,purity,time duration,process development and overall processing costs[30–33].Therefore,the aim of this study was to produce high purity NCC from low-cost agro waste biomass within short duration by delignification,bleaching and hydrolysis processes.R-RHB was chosen for its low cost,availability and significant amount of cellulose content.Apart from this study,a fundamental knowledge of NCC extraction from biomass willbe developed for industrial production.Agro-waste biomass(rice husk)is not to be neglected as waste;it has great potential for producing value-added products as explored in this research by the production and characterization of NCC from R-RHB.

2.Materials and Methods

2.1.Materials

The lignocellulosic materials,raw rice husk biomass(R-RHB),provided by Downes Rice Hulls Pty.Ltd.,Australia,were used as a main raw material for the production of NCC.Rice hull/husk is the hard protective covering on grains of rice which is yellow-brown in colour with a very coarse surface.After processing,R-RHB was stored in the plastic bags in order to protect from moisture attack.

2.2.Chemicals

Sodium hydroxide(99%purity)was used for alkaline treatment in the delignification process,and strong mineral acids were used for acid hydrolysis in order to degrade cellulose to produce the final product.Sodium hypochlorite(10.5%–15%)solution was used for the bleaching process.Other chemicals such as acetic acid,sodium acetate,phosphoric acid,ammonium oxalate,benzene,ethanol and potassium hydroxide were used for the determination of chemical constituents of R-RHB by TAPPI(Technical Association of Pulp and Paper Industry)and ASTM(American Society for Testing and Materials)standard methods.All chemicals,manufactured by Sigma-Aldrich,in reagent grades,were purchased from Science Supply Australia.

2.3.Production of nanocrystalline cellulose(NCC)

2.3.1.Overview of production

For the production ofNCC from agro waste biomass,R-RHB was used as a raw material,which was processed to remove dust and unwanted weeds.The processed R-RHB was then grinded and characterized by FT-IR,EDX and XRD analyses.The chemical compositions of R-RHB,delignified rice husk pulp(D-RHP),and bleached rice husk pulp(BRHP)were estimated,followed by TAPPI and ASTM standards to investigate the separation performance and quality of NCC.Dilute suspension ofNCC was purified by further processes such as dialysis,sonication,and filtration as shown in the following schematic diagram(Fig.1).Finally,purified NCC was analysed by SEM,TEM,AFM,and XRD.Some NCC suspension was dried and kept for further investigations,while simultaneously a small scale investigation was carried out to estimate the ultimate yield of NCC from raw rice husk.

2.3.2.Delignification process

Raw rice husk biomass(R-RHB)was crushed by Rockwell ring mill(Rocklabs,New Zealand)and converted into powder form of particle size 75–710 μm prior to the delignification process.The chemical constituents,crystallinity and FT-IR analysis were performed for R-RHB in orderto compare with the extracted pulp and NCC suspension at various stages of extraction processes.The delignification of R-RHB was performed by 4 mol·L?1NaOH solution under predetermined experimental conditions using a 2.5 L jacketed glass reactor at 80°C for 12 h with constant stirring of 400 r·min?1.In this process,250 g of RRHB powder was placed in the reactor,followed by the addition of 2000 ml NaOH solution.The reaction was carried out to remove lignin with other impurities such as hemicelluloses,pectin,and waxy materials.These are dissolved in alkali solution during delignification and separated by washing and filtration using deionized distilled water(DDW).After this process,delignified rice husk pulp(D-RHP)was bleached to remove remaining impurities.Apart from the filtration process,the waste alkali reagent(black solution)was neutralized by strong acid and discarded after dilution with sufficient water.The delignification set-up in this research was used for the first time,but the chemistry is similar with some reported works[18–21].A typical view of delignification processes is given in Fig.2.

Fig.1.Schematic diagram of NCC production from R-RHB.

2.3.3.Bleaching process

Approximately 500 ml sodium hypochlorite solution(15%)was added to 200 g of wet D-RHP sample in a 1000 ml reaction vessel.The reaction was performed at 60 °C with constant stirring of 800 r·min?1for 60 min.In this process,the extracted cellulose was converted into white pulp(or paste)due to removal of remaining lignin and hemicelluloses.The bleaching treatment breaks down the phenolic compounds and chromophoric groups present in the lignin molecule,thus whitening the D-RHP sample.The bleached rice husk pulp is coded as B-RHP.At the end of this process,B-RHP was washed five times using DDW and neutralized by acetic acid and sodium acetate buffer solution for further modification processes.This final product B-RHP comprises of rod-like highly crystalline cellulose fibril,which is used for the production of NCC by the acid hydrolysis process.However,the chemistry of a bleaching process is well established and in this step,hypochlorite treatment was performed for B-RHP production from D-RHP.This same chemical approach was found to bleach other biomasses,but the experimental aspects are different with process parameters[23–25].

Fig.2.Delignification process of raw rice husk biomass(R-RHB).

2.3.4.Acid hydrolysis

Approximately 200 g of wet B-RHP sample was mixed with 4 mol·L?1sulphuric acid in order to degrade defibrillated cellulose to NCC suspension.The hydrolysis was performed in a 1000 ml reaction vessel for 60 min at 60 °C with continuous stirring of 800 r·min?1.Later on the hydrolyzed rice husk pulp(H-RHP),which is called NCC suspension,was diluted ten-fold with DDW and neutralized with buffer solution.NCC suspension was separated from aqueous solution followed by centrifugation(10000 r·min?1,10 °C,10 min).The aqueous acid solution apart from centrifugation could be reused but finally it was neutralized by strong alkali solution before safely discarded.The raw suspension of NCC,which is white fluorescent residue,was dialyzed against ultra-pure water followed by the method as reported by Lu and Hsieh[24].However,in this step low acid concentration was used(4 mol·L?1)for B-RHP because it is prepared from nonwoody biomass.Finally,dialyzed sample was sonicated for 30 min using an ultrasonicator in the dark chamber applying～10800 kJenergy.The resultant suspension,considered as NCC,was then transferred into a plastic container with a tightly closed stopper and finally preserved in the refrigerator under 5°C for further analysis.Before analysis,a certain amount of NCC suspension was dried in a freeze dryer.The yield of NCC production(YNCC)was estimated using the following Eq.(1),where WR-RHBis the mass of R-RHB and WNCCis the mass of dry NCC which is approximately 80 g.

2.4.Characterization

2.4.1.Chemical composition analysis

The chemical composition of R-RHB,D-RHP,B-RHP,and NCC was determined in accordance to ASTM and TAPPI standard methods for different components of R-RHB and resultant products—namely T222 OS-83 for lignin,T203 OS-61 for cellulose,ASTMD 1104-56 for hemicelluloses,T245 OM-94 for silica and ASTM D 4442-92 for moisture content.These standards are commonly used by many researchers for determining biomass constituents and to investigate the purity[23,34].

2.4.2.Fourier transform infrared spectroscopy(FT-IR)

FT-IR spectra were measured using a Spectrum 100 FT-IR spectrometer(Perkin Elmer,USA)at ambient conditions.Samples were finely ground with KBr(1:100 w/w)and compressed into pellet form.The spectra were collected in the transmittance mode from an accumulation of 128 scans at a 4 cm?1resolution over 4000–600 cm?1range.

2.4.3.X-ray diffraction(XRD)

The overall crystalline phases of samples were determined by the Bruker AXS D8 ADVANCE wide angle X-ray diffraction instrument using the following model Eq.(2)proposed by Segal et al.[27,35].

where I002is the maximum intensity of the(002)lattice diffraction peak,and Iamis the intensity scattered by the amorphous part of the sample.The diffraction peak for plane(002)is located at a diffraction angle around 2θ=22 °C,and the intensity scattered by the amorphous part is measured as the lowest intensity at a diffraction angle around 2θ =18 °C.

2.4.4.Scanning electron microscopy(SEM)

The microstructures and surface morphologies were examined by a scanning electron microscope(Philips XL30,FEI/Philips,USA)in the RMMF laboratory at RMIT.To conduct the analysis,the sample was mounted on aluminium stubs with conductive carbon tape and sputtered with gold to improve conductivity.The EDS-system was coupled with SEMusing mixed BSE(back scatterelectron)+LSE(lateral secondary electron)signal detectors.

2.4.5.Transmission electron microscopy(TEM)

A JEOL 1010 Transmission Electron Microscope,operated ata 100 kV accelerating voltage,was used to obtain high magnification images of NCC.A drop of 10 μl diluted NCC suspension after dialysis and filtration was deposited onto Holey-carbon coated TEM grids(GYCu200)and the excess liquid was removed by blotting with a filter paper after 2 min.Finally,it was processed by drying in a vacuum oven at 40°C for 12 h.The length and diameter distributions of NCCs were measured on several hundred representative of NCCs crystal using digital image analysis software(ImageJ).

2.4.6.Atomic force microscopy(AFM)

AFM imaging of NCC was performed using the D3100 Atomic Force Microscope(Santa Barbara,CA).A few drops of the diluted suspension were deposited onto silicon wafer and allowed to dry in the vacuum at 40°C for 12 h.Samples were scanned in air at ambient relative humidity and temperature in tapping mode with OMCL-AC160TS standard silicon probes(tip radius＜10 nm,spring constant=28.98 N·m?1,resonant frequency= ～310 kHz)(Olympus Corp.)under a 1 Hz scan rate and 512 pixels×512 pixels image resolution.The average height of nanocrystals layer was determined from height profile measurement by image analysis software(Gwyddion).Several hundreds of nanocrystals were randomly selected,and two measurements for each nanocrystal were used to determine the average thickness.

3.Results and Discussion

3.1.Chemical composition

Table 1 shows the chemical composition of R-RHB,D-RHP,B-RHP,and NCC at different experimental conditions on dry basis.It is clear that the chemical modifications of R-RHB significantly increase the cellulose content by removing amorphous components such as lignin,hemicelluloses,pectin,waxes and silica.This confirms that during the delignification process substantial breakdown of lignocellulogic structure took place—resulting in improved defibrillation.The partial hydrolysis of hemicelluloses and depolymerisation of a lignin unit occurred during the delignification process as well,which gave rise to sugars and phenolic compounds that are soluble in water as have been reported by other authors[36,37].When the lignocellulogic biomass is subjected to chemical modification in the alkaline medium,the hemicelluloses and lignin components present in the R-RHB are dissolved[38].In the chemical modification,some loose substance in the R-RHB can be easily removed except for some hard components which are difficult to remove from the cell wall[36].In this way the percentage of cellulose content of R-RHB increases when it undergoes delignification by the alkali treatment.On the contrary,when the D-RHP undergoes a bleaching process,there is a further decrease inthe percentage of hemicelluloses and lignin and thus increasing the percentage of cellulose component present in the sample.

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Table 1 Chemical composition of R-RHB,D-RHP,B-RHP and NCC sample

The physical properties such as colour, fibre diameter and volume also changed in every stage of chemical modification as illustrated in Fig.3.The effectiveness of chemical modification,as could be seen from these pictures,is apparent.Similar observations have been reported in the published work as well[18,23].

The final sample obtained from the bleaching process is coded as B-RHP,which contains 93%cellulose.The B-RHP has trace amounts of hemicellulose and lignin,as shown in Table 1,which were later removed by acid hydrolysis to produce 95%NCC.In NCC the remaining 5%most probably contained chemical impurities like sodium,sulphate,hydroxide,and trace amount minerals.Based on the wet basis analysis of R-RHB,D-RHP,B-RHP and NCC at different experimental conditions it has been reported that the moisture content of NCC is greater than R-RHB,D-RHP and B-RHP due to the increase of a--OH functional group.The chemical modification significantly increases the cellulose content in each subsequent stage,which contains three--OH radical in each unit at C2,C3and C6[38,39].This--OH is hydrophilic in nature;hence the moisture content has increased with chemical modification during delignification,bleaching and hydrolysis processes.The removal of this amorphous region by the acid hydrolysis increases the crystallinity index of nanocellulose suspension[40].The ultimate yield of NCC by considering input raw biomass(R-RHB 250 g),as estimated from Eq.(1),was found to be～30.8%.Raw biomass contains 35%cellulose,so 250 g RH has 87.5 g total cellulose.From 250 g RH,we produced～81 g bleached pulp which is approximately 90%recovery since total cellulose 87.5 g.From 81 g B-RHP,we found 77 g NCC on dry mass basis(total cellulose lose 12%).Further research has to be conducted to investigate possible routes and prevention of cellulose loss.The production rate of NCC from B-RHP is～95%.The overall conversion of R-RHB to NCC was found to be feasible and great potential for large scale production.R-RHB is a suitable candidate to extract cellulose by a chemical extraction process.The cost analysis of NCC production from R-RHB is still under investigation.However,R-RHB is good raw materials for NCC production by considering its rich cellulose content and which is extractable by general acid-alkali treatments.

3.2.X-ray diffraction(XRD)

XRD studies of the raw(R-RHB),delignified(D-RHP),bleached(B-RHP)and hydrolyzed(H-RHP i.e.NCC)samples were conducted in order to investigate the crystalline behaviour of the resultant products at various stages of modification.From the XRD results(Fig.4),it is clear that NCC suspension shows high crystalline structure as compared to others.Due to the acid hydrolysis,NCC exhibits higher crystallinity because of more efficient removal of non-cellulosic polysaccharides and dissolution of amorphous zones.Accordingly,the above results demonstrate that hydrolysis took place preferentially in the amorphous region.This increase of crystallinity after acid treatment has been reported by several authors in the open literature[2,41].Cellulose has crystalline structure contrary to hemicellulose and lignin,which are amorphous in nature.According to Zhang and Lynd[42],cellulose has a crystalline structure due to hydrogen bonding interactions and van der Waals forces between adjacent molecules.It has also been reported that dilute acid has no effecton the crystalline domains,but destroys the amorphous region of the fibre[43].On the other hand,Mwaikambo and Ansell reported that the alkali treatment can be performed on the plant fibres to increase the stiffness of fibre as the impurities present in the fibres can be removed during delignification[44].The crystallinity index(CrI)was determined for all samples,and the results are summarized in Table 2.A continuous increase of the CrI value was observed upon the successive chemical treatments,and the highest value(65%)corresponds to NCC.The subsequent increase of the CrI value upon acid hydrolysis of purified cellulose rice husk fibre is indicative of the dissolution of amorphous cellulosic domains.During the hydrolysis process,hydronium ions can penetrate the more accessible amorphous regions of cellulose and allow the hydrolytic cleavage of glycosidic bonds,which eventually releases individual crystallites[45].In addition,during the preparation of cellulose nanocrystals,the growth and realignment of monocrystals may occur in parallel and thus can improve the cellulose crystallinity[9,10].This increase in the cellulose fibre crystallinity was also expected to increase their stiffness and rigidity.

Fig.3.Photographs of(a)R-RHB,(b)D-RHP,(c)B-RHP and(d)NCC at different stages.

Fig.4.X-ray diffraction of NCC,B-RHP,D-RHP and R-RHB samples.

Table 2 Crystallinity index(CrI)of raw,delignified,bleached,and hydrolyzed samples

However,from this XRD analysis,it could be concluded that product quality is obviously better than some relevant works as reported in the literature.The crystallinity of NCC was found to be～65%in this work which is better than Johar et al.,Fan et al.,and Battegazzore et al.[18,20,23].The cellulose contentin NCCsuspension is about95%,which is good for better quality.In NCC suspension,5%impurities were found from composition analysis.These impurities might be apart from SO42?,Cl?and degradation products due to chemical treatments.

3.3.Fourier transform infrared spectroscopy(FT-IR)

Chemical modification of R-RHB was performed for the production of NCC by the separation of impurities such as lignin,hemicelluloses,and waxy materials.FT-IR analysis was performed with the aim of verifying that lignin and hemicelluloses were removed during a chemical modification process.In the NCC production,cellulose fibrils were produced at the beginning by delignification.And then micro fibrils were produced from cellulose fibrils by bleaching and acid hydrolysis processes.The acid hydrolysis breaks the micro fibril bundle and finally converted them to nano fiber.Nano fibers comprise of cellulose chains,and each chain is made up of a cellulose unit.There are three hydroxyl groups present in a cellulose anhydroglucose unit.One is a primary hydroxyl group at C6,and the other two are secondary hydroxyl groups present at C2and C3[46].Although the primary hydroxyl group is more reactive than the secondary groups,the chemical treatment breaks the--OH group of carbon 6 and carbon 2 positions during the reaction[35,38].Due to the steric effect,chemical reaction cannot form a bond with an O atom at C3position.A typical view of various stages of RH biomass micro and nano fibrillation during NCC production by chemical treatment is shown in Fig.5.

The FT-IR spectra of R-RHB,D-RHP,B-RHP and NCC samples upon various modifications are shown in Fig.6 whereas Table 3 presents the band position and possible stretching.The absorption bands at 800–950 cm?1region were observed for all samples due to the presence of C--O and C--H stretching vibrations[21].The spectrum of R-RHB clearly shows the presence of the characteristic band of a C--O group in the region of 1300–1000 cm?1.In this spectrum,the absorption band at the region near 1700 cm?1may be due to the carboxyl group of acetyl ester in cellulose and the carboxyl aldehyde in lignin[35,47].This band is not apparent in the case of D-RHP,B-RHP and NCC samples.This means that the chemical modification significantly removed lignin from D-RHP,B-RHP and NCC samples.The broad absorption band with peaks located from 3300 to 3400 cm?1is due to the stretching of the--OH groups and the other one near 2900 cm?1is related to the C--H stretching vibrations.The band at 1650 cm?1could be assigned to the C=C stretching of aromatic rings for lignin observed in the spectrum of R-RHB which is absent in the spectra of D-RHP,B-RHP and NCC.According to various authors[8,40,48,49],this band relates to the bending mode of adsorbed water.As pointed out by Sun et al.,a strong peak at 1000–1100 cm?1arises from C--O--C pyranose ring skeletal vibration[48].

3.4.Morphological analysis

The morphological structure of R-RHB and D-RHP is given in Fig.7.1 foruntreated and delignified samples,respectively.The white waxy surface ofR-RHB is shown in Fig.7.1(A-1 and A-2).Panels A-1 and A-2 were captured at 50 and 20 μm spatial resolutions respectively,to investigate the surface morphology of R-RHB.The R-RHB surface is fully covered by white waxy materials as clearly appeared in theses picture as marked by red circles.These white waxy materials are amorphous in nature and comprise of lignin,hemicelluloses,pectin and other non-cellulosic components that act as a protective surface.Lignocellulosic biomass was treated to remove these impurities in order to increase the crystallinity and defibrillation.The images of delignified sample(D-RHP)are shown in Fig.7.1(B-1 and B-2)at 50 and 20 magnifications.From these pictures,it is clearly apparent that,after the delignification process,the white waxy materials were significantly removed except for few impurities.Approximately 80%of white waxy materials were removed,and cellulose fibre was separated out.This phenomenon increases the crystallinity as confirmed in the chemical composition analysis.In Fig.7.1(B-2),defibrillation of RH fibre is indicated by arrow and rectangular box whereas in Fig.7.1(A-1 and A-2),R-RHB is entirely covered by white materials.The D-RHP contains few impurities as found in the TAPPI analysis but this could be removed by bleaching treatment.

Fig.5.Fibrillation stages of raw rice husk biomass(R-RHB)during the production of NCC.

Fig.6.FT-IR spectra of R-RHB(A),D-RHP(B),B-RHP(C)and NCC(D)samples.

Table 3 FT-IR spectral peak assignments for R-RHB,D-RHP,B-RHP and NCC

Fig.7.2(C-1 and C-2)focuses on the surface morphology of bleached sample(B-RHP)that was captured at 50 and 20 μm spatial resolutions respectively.From these images,it is clear that no impurities remain on the surface and fibre bundles are separated.This result confirms that B-RHP has more cellulosic contents and high crystallinity than the R-RHB and D-RHP samples.The XRD and chemical composition analyses fully support this result.Finally,the bleached sample(B-RHP)was hydrolyzed in order to produce NCC.The surface morphology of NCC was investigated by AFM and TEM and is discussed in the subsequent sections.From this SEM results,it can be concluded that chemical modification significantly removes non-cellulosic components and increases the crystallinity as confirmed in XRD and chemical composition analyses.The fibre diameter was found to be decreasing with successive modifications.Furthermore,through the chemical modification,a lot of value-added products such as bioethanol,bio-oil and crystalline cellulose pulp are also produced from biomass[50,51].A similar aspect of raw and treated surfaces has been reported in the open literature during the production of NCC from lignocellulosic biomass[52,53,54].

3.4.2.Atomic force microscopy(AFM)

AFM images of NCC(Figs.8.1&8.2)confirm that the acid hydrolysis causes defibrillation leading to individualization of the cellulose nano fibres from the cellulose pulp without degrading them to soluble mono-and/or oligosaccharides.The existence of individual fibril in both Figs.8.1(A)and 8.2(B)is clearly visible.The height profiles and height profile distributions of NCC particles in these figures are discussed below.In Fig.8.1,height profile response(A-1)was found from 3 μm scan area apart from two scans in the image(A).The height profiles of NCC domains for two scans in Fig.8.1(A)are shown in Fig.8.1(A-1).In addition,the height profile distribution of NCC apart from Fig.8.1(A-1)is also shown in Fig.8.1(A-2)which ranges from 1 to 50 nm.This AFM height profile and distribution showed NCC particles to be(25±15.14)nm thick.

On the other hand,in the case of Fig.8.2,the height profile response and height profile distributions are shown in B-1 and B-2 respectively apart from image B.In Fig.8.2(B-1)both height profiles range from approximately 5–25 nm as observed from a profile pick.The height profile distribution ranges from 1 to 50 nm.AFM height profile and distribution showed NCC particles to be(27±15.14)nm thick which is almost similar with Fig.8.1(A-2).The information obtained from AFM height profile and distribution confirmed that the NCC particle thickness is approximately(25±15.14)nm or(27±15.14)nm,which is comparable to the dimensions obtained from TEM results.In addition,during the AFM analysis,nanoparticles of NCC were measured in tapping mode,and it was found that most of the nano- fibres have diameter in the range of 10–50 nm.Various dimensions of nano- fibres have been reported in published work for coconut husk[6],cassava bagasse[7],rice husk[23]and pineapple leaf[28]which supports this AFM result.

Fig.7.1.SEM images of R-RHB(A-1,A-2),D-RHP(B-1,B-2)samples.

3.4.3.Transmission electron microscopy(TEM)

Fig.7.2.SEM images of B-RHP sample(C-1,C-2).

Fig.8.1.AFM images of NCC is A,height profile images of A is A-1,height distribution histograms of A is A-2.

Fig.8.2.AFM image of NCC is B,height profile image of B is B-1,height distribution histograms of B is B-2.

Fig.9(Aand B)shows the transmission electron micrographs obtained for NCC resulting from sulphuric acid hydrolysis of B-RHP,which supports the evidence for the production of individual nanoparticles from R-RHB.During the hydrolysis of B-RHP,cellulose initially breaks down to produce MCC,and then further hydrolysis of MCC produces NCC.The chemical treatments were performed in the NCC production which should eventually reduce the size of the fibres from micron to nanometre as well[10,55].In the TEM images,nano- fibres clearly appear as indicated by red colour boxes.Thick layers of NCC are indicated by red colour boxes in the image A.From this image observation;it can be said that cellulose micro fibrils were significantly degraded at the time of hydrolysis,so the technique used in this work is effective.In the AFMimage,crystalline domains of NCC were investigated like fibrillar shape,and these are also found similar in the TEM image as well.Rods like crystalline domains of NCC are shown by the rectangular boxes in both TEM images.

The width and length distributions of NCC are shown in Fig.9(A-1)and(A-2)for image A.The width and length distributions of NCC are shown in Fig.9(B-1)and(B-2)for image B,respectively.The average width and length of NCC in A-1 and A-2 are(50±29.68)nm and(550±302.75)nm,respectively.On the other hand,average width and length of NCC in panels B-1 and B-2 are(35±17.38)nm and(275±151.38)nm respectively.However,from TEM analysis the aspect ratio has also been confirmed,that is～11:1(length/diameter;550/50)for TEM image Fig.9(A).On the other hand,for TEM image Fig.9(B),the aspect ratio has been confirmed to be 8:1(275/35).The aspect ratios for both images Fig.9(A)and(B)are almost the same,and particles' dimensions are found in nano-meter scale.The individual crystalline domains(P-1 and P-2)of NCC suspension are shown in image Fig.9(C)with their dimensions.The crystalline domain P-1 has 10 nm width and 100 nm length approximately.So the aspect ratio of crystalline domain P-1 is～10:1.Crystalline domain P-2 has width of 20 nm,but its length can't be determined due to incomplete view.The aspect ratio found in TEM image Fig.9(C)for nano-particles strongly supports the aspect ratios as obtained for TEM image Fig.9(A)and(B).The aspect ratio of nano-cellulose produced from rice straw is smaller than this study which is reported by Lu et al.and Jiang et al.[2,17,24].Nevertheless,the similar kind of work has been reported in the literature for banana fibres,sugarcane bagasse,and pineapple leaf fibres as well,which support this work[23,28,48,56].The results obtained from AFM and TEM analyses in this investigation are in good agreement with each other;the NCC particles are in nanometer range.

Fig.9.TEM images of NCC are A,B,and C,width and length distributions of A are A-1 and A-2 respectively,width and length distributions of B are B-1 and B-2 respectively.

4.Conclusions

NCC has been successfully produced from R-RHB by a chemical extraction process as confirmed by TEM and AFM analyses.The yield of NCC from B-RHP was found to be approximately 95%,and the recovery of cellulose from R-RHB is about 90%.The chemical treatments induced an increase of the cellulose content of R-RHB from 35%to 95%purity and the crystallinity index from 34.0%to 65.0%.

Overall,the pathways followed in this research for the conversion of R-RHB to NCC are found to be efficient in comparison to published literature.The utilization of agro-waste biomass could bring great economic and environmental benefits for Australia by considering its great supply of raw material containing high cellulose content such as rice husk.In addition,the process developed in this research is universal,which can be adopted for all types of rice husk available in various geographical locations.

Acknowledgements

The research work reported in this paper was funded by RMIT University,Melbourne,VIC 3001,Australia.The authors would like to acknowledge the assistance received from Cameron Crombie,Dr.Sandro Longano,Peg Gee Chang,Mike Allan and Dr.Muthu Panniselvam from the School of Engineering for their kind help during the lab work and sample analyses.The authors would also like to thank Phil Francis,Dr.Matthew Field and other technical staff from the RMIT Microscopy and Microanalysis Lab for their assistance with the SEM,AFM,and TEM analyses as well.The rice husk samples received from Downes Rice Hulls Pty.Ltd.,Australia are greatly appreciated.The statistical information provided by Tom Howard on behalf of SunRice Corporation and the Rice Growers Association(RGA)of Australia is gratefully acknowledged as well.

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