Biomass and waste materials as potential sources of

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Dec 5, 2018 - biomass and waste cellulosic materials as renewable sources for NCC extraction. Newer and/or .... process temperatures and lower production rate compromise the ... effect on yield and the relative quality, i.e., properties, of the resulting product. ...... wastes. Waste and Biomass Valorization, 8(1), 41–56.
Carbohydrate Polymers 207 (2019) 418–427

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Biomass and waste materials as potential sources of nanocrystalline cellulose: Comparative review of preparation methods (2016 – Till date) Shweta Mishra, Prashant S. Kharkar, Anil M. Pethe


Shobhaben Pratapbhai Patel School of Pharmacy and Technology Management, SVKM’s NMIMS (Deemed to be University), Vile Parle (W), Mumbai, 400 056, India



Keywords: Nanocrystalline cellulose NCC preparation Ultrasonication Ionic liquids Cellulases

Nanocrystalline cellulose (NCC) has gained much popularity over the last decade as a preferred nanomaterial in varied applications, despite its laborious industrial production and higher cost. Its production methods have undergone a great deal of metamorphosis lately. The main emphasis has been on the environment-friendly and green processes, in addition to the sustainable and renewable feedstock. Globally, the researchers have explored biomass and waste cellulosic materials as renewable sources for NCC extraction. Newer and/or improved process alternatives, e.g., ultrasonication, enzymatic hydrolysis and mechanical treatments have been applied successfully for producing high-quality material. Detailed investigations on optimizing the overall yield from cheaper feedstock have yielded obvious benefits. This is still work in progress. The present review majorly focuses on the advances made in the NCC preparation field from biomass and waste cellulosic materials in last three years (2016 - till date). Collaborative efforts between chemical engineers and research scientists are crucial for the success of this really amazing nanomaterial.

1. Introduction Biopolymers intrigued mankind since the dawn of civilization. Of the three major types of biopolymers, polysaccharides are the most abundant; cellulose being the most common of all the biopolymers on Earth, with annual production in excess of 75 billion tonnes (Habibi, Lucia, & Rojas, 2010). The ‘PubMed’ search using term ‘Cellulose’ yielded 89430 hits (conducted on November 15, 2018). Such a large number of hits only emphasizes the utility of cellulose and its derivatives. Countless number of research and review articles have been published describing the utility of cellulose-based materials in electronics (Agate, Joyce, Lucia, & Pal, 2018; Sethi, Farooq, Österberg, Illikainen, & Sirviö, 2018; Sun et al., 2018; Wu, Zhou, & Huang, 2018; Zhang, Yang, Yan, & Ding, 2018), bioethanol production (Aguilar et al., 2018; Bolivar-Telleria et al., 2018; Robak & Balcerek, 2018; Qiu et al., 2018), specialty chemicals production (Delbecq & Len, 2018; Ehsanipour, Suko, & Bura, 2016; Zhang, Xi, Zhang, Yu, & Wang, 2017), biomedical and allied fields (Fu et al., 2018; Kabir et al., 2018; Meng, Wang, Ma, & Zhu, 2017; Teng et al., 2018; Ullah, Wahid, Santos, & Khan, 2016), and many others. In recent years, cellulose and its

derivatives have attracted significant attention, mostly due to a) increased awareness of the availability of its resources, mainly from the agricultural waste or biomass (Arevalo-Gallegos, Ahmad, Asgher, ParraSaldivar, & Iqbal, 2017; Kaur, Kumar, Sachdeva, & Puri, 2018), b) improved methods of their production (Hutterer, Kliba, Punz, Fackler, & Potthast, 2017; Kasirajan, Hoang, Furtado, Botha, & Henry, 2018; Mazarei et al., 2018; Yang, Berthold, & Berglund, 2018) and c) recent developments in the techniques for cellulose solubilization and selective chemical modifications (Jiang, Zhao, & Hu, 2018; Kostag, Jedvert, Achtel, Heinze, & El Seoud, 2018; Li, Wang, Liu, & Zhang, 2018). Microcrystalline cellulose (MCC), one of the most widely used forms of cellulose in food, cosmetic, pharmaceutical and allied industries including material sciences, has been the subject of intensive research with respect to functionalization for a specific property, e.g., adsorption of dyes (Wei et al., 2018). Huang, Xie, and Xiong (2018) carried out surface modification of MCC using urea under microwave irradiation and further prepared composite films with chitosan exhibiting superior mechanical properties. In yet another interesting study, MCC was used for the delivery of a recombinant protein-based antigen in mice (Jeon et al., 2018). This was a pioneering study where MCC served as a

Abbreviations: PCL, polycaprolactone; DLS, dynamic light scattering; XPS, X-ray photoelectron spectroscopy; CS, crystallite size; DP, degree of polymerization; odp, oven-dried pulp; RSM, response-surface methodology; BBD, Box-Behnken design; SCB, sugarcane bagasse; DCF, discarded cigarette filters; PRD, powder X-ray diffraction; BET, Brunauer–Emmett–Teller; EDS, energy dispersive spectroscopy; EFBP, oil palm empty fruit bunch pulp; FESEM, field-emission scanning electron microscopy; OPEFB, oil palm empty fruit bunch ⁎ Corresponding author. E-mail address: [email protected] (A.M. Pethe). Received 16 October 2018; Received in revised form 19 November 2018; Accepted 4 December 2018 Available online 05 December 2018 0144-8617/ © 2018 Elsevier Ltd. All rights reserved.

Carbohydrate Polymers 207 (2019) 418–427 Mechanical reinforcing agent in - 3D scaffolds (Shaheen, Montaser, & Li, 2018), film packaging (Sukyai et al., 2018); heavy metal adsorbent (Khoo et al., 2018); antibacterial material (Tavakolian, Okshevsky, van de Ven, & Tufenkji, 2018) Substitute for plastics (Su et al., 2018); barrier films/coatings (Lu, Guo, Xu, & Wu, 2018); food products (Parés et al., 2018); bone regeneration (Safwat et al., 2018); papermaking (Boufi et al., 2016); viscosity modifier (Paukkonen, Ukkonen, Szilvay, Yliperttula, & Laaksonen, 2017) Tissue engineering (Osorio et al., 2018; Ullah et al., 2016); microbial control (Li, Cha et al., 2018); ultrafiltration membrane (Xu et al., 2018); biomimicry (Reis, Berti, Colla, & Porto, 2018); controlled extended drug delivery (Alkhatib et al., 2017)

vaccine delivery vehicle. Alemán-Domínguez et al. (2018) used MCC as an additive in polycaprolactone (PCL) matrices for 3D-printed scaffolds with superior mechanical and biological properties. Such novel applications of MCC or any other forms or derivatives of cellulose inspire the researchers from various fields to further explore their utility. Pethe, Kharkar, and Desai (2017a) and Pethe, Kharkar, and Desai (2017b) prepared MCC from agricultural waste and explored its application as an excipient in pharmaceutical formulations. The discussion on countless number of cellulose derivatives and their applications is beyond the scope of this review. Being so abundant in the plant kingdom, cellulose polysaccharide units exist mainly in three hierarchal (nano)structural forms, namely, nanocrystalline cellulose (NCC), nanofibrillated cellulose (NFC) and bacterial nanocellulose (BNC). Xue, Mou, and Xiao (2017) recently reviewed the fine structural aspects, properties, present status and the future prospects of nanocellulose in general, for biomedical applications. Table 1 lists the comparative properties of the three types of nanocellulose, including preparation techniques and applications. The nanocellulose field has undergone a major revamp in the last few years with reference to its preparation, functionalization and interesting applications in the biomedical, material and other allied sciences (Brinchi, Cotana, Fortunati, & Kenny, 2013; Habibi, 2014; Jarvis, 2018; Klemm et al., 2011; Lam, Male, Chong, Leung, & Luong, 2012; Li, Cha et al., 2018; Xue et al., 2017). Of the three types, bacterial nanocellulose is comparatively different from NCC and NFC owing to its preparation methods, properties and applications (Table 1). Detailed information on BNC can be found somewhere else (Gama, Dourado, & Bielecki, 2016). Despite the uniqueness amongst the nanocelluloses and varied applications such as heavy metal adsorbent, mechanical reinforcing agent in 3D scaffolds, polymer composites, films, etc. (Table 1) of NCC, its production cost is a major hurdle. The problems are manifold. Extraction processes such as acid hydrolysis with strong mineral acid (62–64 wt% H2SO4) (Mondal, 2017), physical, mechanical or microbial methods are inefficient. In addition to the longer reaction times, higher process temperatures and lower production rate compromise the overall yield and the quality of the resulting product. Many of these methods work excellently on lab-scale, but fail miserably in the production plants (Islam, Kao, Bhattacharya, & Gupta, 2017). This has hindered easy commercial availability of the NCC on mass-scale. Even though there is a huge gap between the production capacity and market demand, the situation will improve only in near future with reduction in prices. The current nanocellulose market mostly focuses on NFCs as a substitute for plastics, resins, synthetic thickeners and strengtheners (The Nanocellulose Investment & Pricing Guide, 2018). Products based on NFCs have made it to the market for packaging and composites sector. The main drivers for the nanocellulose industry are newer, sustainable, renewable sources of raw materials with underlying supply and price stability and the resulting unexplored applications. The present review article mainly focuses on the NCC preparation methods, reported from 2016 till date, from such renewable and cheaper materials, the comparative analyses of the process parameters (temperature, time, concentration of the reagents and/or catalysts, etc.) and their effect on yield and the relative quality, i.e., properties, of the resulting product. The main emphasis is on the biomass and waste cellulosic materials for the obvious reasons such as cost and sustainability. Effective management of the biomass and the agricultural waste for the production of value-added products such as NCC is likely to reduce our carbon footprint with a step closer to much sought-after ‘circular biobased economy’ from the present-day ‘oil-based economy’. The applications of NCC obtained from such sources have been discussed wherever the information was available

Bacterial synthesis Low-molecular weight sugars and alcohol Microbial cellulose, bacterial cellulose, bio cellulose BNC 3

Delamination by mechanical, chemical or enzymatic treatments Microfibrillated cellulose, microfibril aggregates, nanofibril, nanofibre, nanofibrillar cellulose, microfibril NFC 2

Wood, sugar beet, potato tuber, hem and flex

Length: 100 – 250 nm Diameter: 5 – 70 nm Length: Several μM Diameter: 5 – 60 nm Diameter: 20 – 100 nm Acid hydrolysis Cellulose nanocrystal (CNC), cellulose whisker, nanowhisker, nanowire, nanorod NCC 1

Cotton, mulberry bark, wheat straw, Avicel, sugar beet, wood

Preparation technique Sources Synonyms Type Sr. No.

Table 1 Comparison and applications of nanocelluloses – NCC, NFC and BNC (Klemm et al., 2011; Xue et al., 2017).

Particle size

Major applications

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2. NCC preparation from biomass: state-of-the-art (Pre-2016) and recent updates Every year billions of tonnes of biomass are generated all over the 419

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Fig. 1. Typical process flow diagram for NCC extraction from suitable source using conventional acid hydrolysis method.

process flow have been attempted (Brinchi et al., 2013; Yang, Chen, & van de Ven, 2015). This included acid hydrolysis using other mineral acids such as HCl, H3PO4, HBr, etc. (Habibi et al., 2010). But 64% w/w H2SO4 is the reagent of choice due to the resultant stable NCC suspensions. To complete the hydrolysis process, at times prolonged reaction times are required. This, in turn, leads to loss of cellulosic material due to formation water-soluble impurities (e.g., xylose, glucose), compromising the overall yield. The disposal of the effluent generated during the process is another issue. To reduce the time required for hydrolysis, sonication has been proposed (Wang, Ding, & Cheng, 2007). Additionally, ionic liquids have been used as catalysts for cellulose hydrolysis (Man et al., 2011). If MCC is used as the starting material (which usually is the case), then the alkali treatment and bleaching may be optional. In brief, the technology transfer, i.e., scale-up, from lab- to bulk-scale is one of the major issues. Overall, several routes and detours have been taken to reach the NCC extraction destination. Some routes are shorter, while others are longer. The journey so far has not been so very lucrative. Nevertheless, researchers continued walking on the path with the hope that some day they would succeed in making NCC commercially viable, surmounting all the present-day obstacles. Recent literature specifically tried to fill the technical gap between lab- and bulk-scale extraction of NCC from cheaper feedstock. Table 2 summarizes newer NCC preparation methods, thoroughly discussed below, along with the respective biomass or waste source. Islam, Kao, Bhattacharya, Gupta et al. (2017) and Islam, Kao, Bhattacharya, Gupta, and Bhattacharjee (2017) studied the effect of low- and high-pressure processes coupled with chemical treatment for preparation of NCC from raw rice husk biomass. Conventional work-flow, i.e., delignification and bleaching followed by hydrolysis was used except that the two variants of the first step (delignification) were tried. The high-pressure process used – pressure: 5 bar, temperature: 80 °C, aqueous NaOH (4 M), time: 12 h. The low-pressure process parameters were similar except that it was carried out at the atmospheric pressure. The delignified material was bleached (20% NaOCl) and hydrolysed (4 M H2SO4) to produce high-quality NCC, which was characterized thoroughly with standard techniques. Both processes yielded NCC (diameter: 10–20 nm, length: 1–2 μM) with high-pressure process being more efficient (yield: 62%) than its low-pressure counterpart (yield: 51%). Overall, the readily available, cheaper and renewable biomass, i. e., raw rice husk, was efficiently used for NCC production. In their recent work on developing a novel method for producing NCC, Surov et al. (2018) used oxidation-hydrolysis strategy. The authors utilized solution plasma-chemical processing of cellulosic materials (MCC and filter paper) as a highly efficient and advanced oxidation technique for NCC production, owing, particularly to, production of highly-active hydroxyl radical (•OH) (Fig. 2). The use of glow discharge

world. Developed nations have systems in place for managing biomass systematically. Biomass is used for power generation, biofuels or other bio-based products. Developing or underdeveloped nations use biomass, mostly for burning and for production of specialty chemicals. To a larger extent, utilization of biomass is an unorganized activity in such countries. With global warming and other environmental issues such as pollution (Smog in Delhi, 2018), concerted efforts are being taken to recycle the biomass appropriately (instead of burning) so that the emissions are controlled and the environmental damage minimized. This problem gives birth to an opportunity in the sense that the biomass can potentially be used for cellulose production. With reference to NCC, coconut husk, cassava bagasse, banana fibre, mulberry bark, soybean pods, corn stalks and soy hulls have been used for its extraction on labscale (Islam, Kao, Bhattacharya, Gupta et al., 2017). Khoo, Chow, and Ismail (2018) recently reviewed the usefulness of sugarcane bagasse NCC for polymer reinforcement and heavy metal adsorption. Fig. 1 outlines the schematic of the complete process flow in the conventional industrial-scale production of NCC from a suitable source using acid hydrolysis. Treatment with a strong mineral acid (62–64 wt% H2SO4) hydrolyzes the glycosidic linkages in the polysaccharide chains, simultaneously leading to surface modifications such as sulfation and oxidation. Many of the NCC characteristics such as negative zeta potential, are due to the surface functionalization. There is always a tradeoff between the process parameters (e.g., reaction temperature and duration) and the final product characteristics such as crystallinity, thermal stability, etc., which finally dictate the utility of such material in several applications (Beck-Candanedo, Roman, & Gray, 2005; Yousefi et al., 2015; Yousefi, Tanaka, Bagheri, Mahmood, & Ikeda, 2016). Brinchi et al. (2013) reviewed the methods of NCC extraction from lignocellulosic biomass, along with its applications in polymer composites as a mechanical reinforcing agent. Typically, it is a two-step process namely, pretreatment of a suitable source, followed by acid hydrolysis. Usually, finely-ground source material is subjected to alkali treatment to remove hemicelluloses, followed by bleaching to get rid of lignin and related impurities. Apart from the chemical pretreatment, steam explosion technique is in practice for quite some time (Cherian et al., 2010). The milled material is heated (200–270 °C) under pressure (14–16 bar) for relatively short duration (20 s – 20 min). Rapid drop in pressure leads to explosion breaking down the material. The resulting mass is known to ease the enzymatic hydrolysis further down the process flow. The dried cellulosic mass is then subjected to strong mineral acid (usually H2SO4) hydrolysis, addition of water to stop the reaction and further processing to remove acid, typically using dialysis. Additional mechanical treatment such as sonication yields stable NCC suspension. The last step involves drying of the suspension, e.g., by spraydrying, to obtain solid material. Several variants of the complete 420

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Table 2 Summary of newer NCC preparation methods from the corresponding source material reported in the literature (2016-till date). Sr. No.

Source Material

1 2

7 8 9 10 11 12

Alkali treatment, bleaching and acid hydrolysis using H2SO4 (62-64 wt%) (conventional) Raw rice husk Conventional with modified delignification (high- and atmosphericpressure) process MCC and filter paper (Grade 1) Solution plasma-chemical processing of cellulosic materials Pruning waste of Zizyphus spina christi Conventional with bleaching using H2O2:glacial HOAc (1:1) Roselle (Hibiscus sabdariffa) fibreConventional using 50% H2SO4 derived MCC Cassava (Manihot esculenta) bagasse Enzymatic removal of residual starch, followed by conventional process with multiple bleaching steps Sugar palm (Arenga pinnata) fibres Conventional coupled with mechanical process Cotton linters Enzymatic pretreatment using cellulase followed by conventional process Empty fruit bunch (EFB) Conventional Sugarcane bagasse Steam explosion, bleaching and acid hydrolysis Discarded cigarette filters Conventional with alkaline deactivation step to hydrolyse cellulose acetate Rice husk Conventional with low-pressure delignification process


Tetra Pak waste


Pine wood and corn cobs


MCC or wood



Oil palm empty fruit bunch pulp (EFBP) Oil palm empty fruit bunch (OPEFB)MCC Jute dried stalks


Wheat MCC

3 4 5 6




Conventional with hydrapulping to separate the cellulosic mass from polyethylene and aluminium Conventional with chemical pretreatment (acetosolv) and bleaching with H2O2:NaOH mixture Acid recycling using charcoal treatment to the acid solution from production batch i) Treatment with alkali, followed by DMSO ii) Acid hydrolysis with ultrasonication TEMPO-mediated oxidation coupled with mechanical treatment, i.e., sonication Ultrasonication combined with microwave-assisted pretreatment, followed by acid hydrolysis Enzymatic hydrolysis coupled with ultrasonication

Islam, Kao, Bhattacharya, Gupta (2017) Surov et al. (2018) Hindi (2017) Kian et al. (2018) Travalini et al. (2018) Ilyas et al. (2018) Beltramino et al. (2018) Song et al. (2018) Sukyai et al. (2018) Ogundare et al. (2017) Islam, Kao, Bhattacharya, Gupta, Bhattacharjee et al. (2017) Diop and Lavoie (2017) Ditzel et al. (2017) Sarma et al. (2017) Zianor Azrina et al. (2017) Rohaizu and Wanrosli (2017) Chowdhury and Abd Hamid (2016) Cui et al. (2016)

analyse NCC surface modifications, exhibited two peaks – the first corresponding to C–O hydroxyl and ether groups and the other corresponding to the C–OeC hemiacetal moieties. All three modes produced materials of comparable composition with reference to the relative ‘O’ and ‘C’ content and ‘C]O’ and ‘COOH’ groups. Overall, mode (ii) resulted in the highest NCC yield from MCC. The authors claimed that this was the first report of NCC production using the plasma-chemical oxidation-hydrolysis strategy. It can be potentially applied to MCC obtained from any type of biomass, known or unknown in the literature. The utility of this technique for bulk-scale production of NCC remains to be tested. Nonetheless, the oxidative-hydrolytic process overcomes several limitations of the conventional and most widely used hydrolytic process. Hindi (2017) extracted NCC from pruning waste of Zizyphus spinachristi (Christ’s thorn jujube), a shrub or tree native to most of the Africa and southern and western Asia. The wood is mostly used for burning and yields excellent quality charcoal, while the fruits are used for alcohol preparation (by fermentation) and as food (Ziziphus spina-christi, 2018). In the reported study, NCC was extracted from the wood, after removing bark and pith, in a systematic process involving – i) Soxhlet extraction of the chipped wood with benzene-EtOH mixture; ii) digestion of the material from stage (i) with 1:1 mixture of H2O2 and glacial HOAc at 60 °C till complete whiteness, followed by drying in oven and, iii) extraction of NCC from the material from stage (ii) with 64% w/w H2SO4 at 70 °C. The extracted NCC was then thoroughly characterized. The CI was 86.75% and the average crystallite size (CS) was 2.78 nm. The XRD studies confirmed the presence of cellulose I. Crystal growth studies demonstrated the ability of NCC to self-assemble. Overall, these studies, including thermal analyses, confirmed the suitability of the biomass from Zizyphus spina-christi. In an interesting study, Kian, Jawaid, Ariffin, and Karim (2018) described preparation of NCC from roselle fibre-derived MCC. Roselle (Hibiscus sabdariffa, Family: Malvaceae) is a perennial herb or sub-shrub used mostly for the production of bast fibre. In India, fleshy calyx part of roselle plant is used for preparing a popular dish, while in Nigeria, it is used for preparing zobo, a popular refreshing drink. The stem fibre of

Fig. 2. Schematic diagram of solution plasma processing of MCC/filter paper as NCC Source (adapted from Surov et al., 2018).

between the graphite anode in the gas phase and electrolyte solution surface as cathode (discharge gap length: 3 mm) led to redox reactions in the solution surface layer under ion bombardment. The bulk solution (∼50 mL), due to the presence of secondary active moiety – H2O2 – underwent oxidation too. Other process parameters were – processing time: 5–80 min; glow-discharge voltage: 305–1680 V; solution temperature: 22–68 °C; discharge current: 40 mA. Three different modes of cellulose treatment such as - i) plasma-chemical treatment of MCC and Whatman filter paper (Grade 1) in H2SO4 solutions; ii) initial plasmachemical treatment of MCC and filter paper in distilled water followed by chemical hydrolysis using H2SO4 solutions and, iii) plasma-chemical treatment of aqueous suspension of NCC (prepared from MCC and H2SO4). All the grades of NCC were thoroughly characterized by advanced instrumental techniques [dynamic light scattering (DLS), infrared spectroscopy (IR), transmission electron microscopy (TEM), Xray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS)]. The results indicated that the processing mode (ii) was better in producing NCC (up to 56% yield) from MCC while both the other modes (i) and (ii) utilizing filter paper as NCC source were similar (approximately 30% yield). The NCC films exhibited high crystallinity index (CI) (81–90%) and the XRD analyses confirmed the peaks arising due to cellulose Iβ. The XPS analysis (C1s XPS spectra), due to its ability to 421

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precisely indicated that the cellulase treatment drastically reduced the average fibre length, increased cellulose crystallinity, enhanced NCC yield post-hydrolysis up to 90% and reduced NCC surface charge, albeit slightly. There was a trade-off between the enzyme quantity and the reaction time for optimal NCC yield. Prolonged treatment with the cellulase led to loss of NCC (overall yield) due to its conversion to sugars, owing to the β-galactosidase activity of the cellulase. The effluent could possibly be used as a feedstock for bioethanol production. The statistical analysis of the data generated using the experimental design proposed the optimal conditions for enzymatic pretreatment as – enzyme dose: 20 U/g odp and duration: 24 h with overall yield of 79%. The study successfully proved the utility of a ‘green step’ in NCC extraction from biomass (cotton linters) in enhancing overall industrial scalability of the process. Song, Chew, Choong, Tan, and Tan (2018) investigated the usefulness of empty fruit bunch (EFB), a fibrous biomass available abundantly from oil mills, for NCC extraction. It is generally considered as waste with little or no commercial value. The authors used response-surface methodology (RSM) - Box-Behnken Design (BBD) - for optimizing three input variables, namely, NaOH concentration during alkaline treatment, H2SO4 concentration during acid hydrolysis and the hydrolytic reaction time. A typical process for NCC extraction (10–30% NaOH treatment at 80 °C for 6 h, bleaching with H2O2−HOAc at 70 °C for 7 h, acid hydrolysis using 56–64 wt% H2SO4 at 45 °C for 1–3 h) was used. The particle size of the prepared NCC was below 300 nm. All three input variables played critical role in affecting the particle size distribution (dependent variable). Overall, the study identified EFB as a renewable and sustainable biomass, useful for the extraction of NCC. Further exploration of EFB-derived NCC, i.e., its mechanical, thermal and related properties, could strengthen the above proposition. Sukyai et al. (2018) used sugarcane bagasse (SCB) for extraction of NCC due to its abundant availability and relatively higher cellulose content (40–50%). The biomass is also used for the production of ethanol and pulp. The process involved extraction of cellulose from biomass using steam explosion (195 °C, 15 min), followed by bleaching (1.4% w/v NaOCl2 X7). The extracted cellulose was further subjected to acid hydrolysis (60% v/v H2SO4, 45 °C, 75 min). Further processing involved acid removal by centrifugation and dialysis. The prepared NCC was subjected to thorough characterization by standard techniques which demonstrated successful removal of hemicelluloses and lignin following steam explosion and bleaching. The purified cellulose (αcellulose content: 87.68 ± 0.95%) yielded short and needle-shaped NCC (length: 200–300 nm, diameter: 20–40 nm) with CI of 68.28%, which degraded at 220 °C. The isolated NCC was further used as a mechanical reinforcing agent in a whey protein isolate-based film for food packaging. The results confirmed the superior mechanical properties of the composite films containing NCC.

roselle plant is mostly disposed off as an agricultural waste. Kian et al. used acid hydrolysis (50 wt% H2SO4) of roselle MCC for varying time lengths for producing NCC. A thorough characterization of the prepared NCC was carried out using standard analytical techniques. Overall yield of 21.92% was obtained with NCC-III (1 h reaction time). The needleshaped NCC crystal widths ranged from 4.67 to 13.06 nm. All the grades of NCC exhibited good stability in aqueous suspensions. The XRD analysis demonstrated superior crystallinity (79.5%) for NCC derived after longer reaction time (1 h). The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) clearly demarcated the thermal stability of resulting NCC as a function of shorter reaction time. In conclusion, the studies precisely proved the utility of roselle fibre as a sustainable source of NCC. However, direct utility of the roselle plant (or its parts) for NCC preparation is yet to be proved. Cassava starch, extracted from tuberous roots of Manihot esculenta (Family: Euphorbiacea) (cassava) is an important commercial product. The bagasse from cassava represents a sustainable biomass for NCC production due to its higher cellulose (up to 50 wt%) and lower lignin content (3 wt%) (Travalini, Prestes, Pinheiro, & Demiate, 2018). Travalini et al. started off with the aim of improving the crystallinity of NCC extracted from cassava bagasse, to make it more amenable as a mechanical reinforcing agent. Initial stages of the process flow included enzymatic removal of residual starch (50–80 wt%) to obtain cellulosic fibres, followed by bleaching (0.7% w/v aqueous NaClO2 treatment) and delignification using aqueous NaOH (17.5% w/v). The bleached, delignified and micronized cassava bagasse fibres were then subjected to hydrolysis using 62.4 wt% H2SO4 at 45 °C for 2 h. Further processing (spray-drying) yielded NCC (15% overall yield) with promising CI of 84.1%. The repeated bleaching steps were proposed to be responsible for the higher CI. There was a slight decrease in thermal stability, as seen from TGA curves, which was attributed to the sulfate groups on the NCC surface following acid treatment. The study, beyond doubt, demonstrated the utility of cassava bagasse as a promising, sustainable and renewable source of NCC with improved mechanical properties in applications involving polymer composites. Recently sugar palm fibres were reported as a renewable source of NCC by Ilyas, Sapuan, and Ishak (2018) for the first time. Sugar palm tree (Arenga pinnata, Family: Palmae) is a very useful tree found mainly in the tropical region. Intrigued by the multidimensional uses of the sugar palm fibres, the authors initiated the extraction of NCC using chemical and mechanical methods, followed by detailed characterization. Initially, the fibres were subjected to delignification (NaClO2, 70 °C) followed by hemicellulose removal (5% w/v aqueous NaOH). Acid hydrolysis (60 wt%, 45 °C, 45 min) of the dried sugar palm cellulose yielded pure cellulose with negligible lignin and hemicellulose content. The final yield of NCC was 29% (average diameter: 9 ± 1.96 nm; length: 130 ± 30 nm). The degree of polymerization (DP) and the molecular weight were 142.86 and the 23 kD, respectively. The lower DP was thought to be due to extensive delignification and hydrolytic processes. Following the characterization using XRD, the CI of the NCC was observed at 85.9%. Thermal analyses precisely confirmed the reduction in thermal stability of the sugar palm fibres after bleaching and further increase after alkali treatment. Overall, the thermal properties improved following acid treatment as compared to the untreated fibres. The study unequivocally demonstrated the usefulness of sugar palm fibres as a sustainable source of NCC. In line with the general theme of using biomass for producing NCC, Beltramino, Blanca, Vidal, and Valls (2018) assessed the optimal conditions for enzymatic pretreatment as an initial step during NCC extraction from cotton linters. Commercially available cellulase preparation was used for the pretreatment with enzyme dose (2–20 U/g ovendried pulp, odp) and reaction time (2–24 hrs) as the study variables and 22 factorial design (total 7 experiments). The enzyme-pretreated and control fibres were subjected to acid hydrolysis using 62% (w/w) H2SO4 with time: 25 min and temperature: 47 °C. Following the complete process, NCC samples were thoroughly characterized. The results

3. NCC extraction from discarded materials/waste In addition to biomass, researchers have explored the possibility of extracting NCC or other cellulosic materials from discarded materials or waste. Such an interesting study reported the use of discarded cigarette filters (DCF) for NCC extraction (Ogundare, Moodley, & van Zyl, 2017). The DCFs present a substantial threat to both terrestrial and aquatic life due to the humongous quantity generated and its persistence in the environment for two to ten years. It mainly consists of plasticized cellulose acetate, which can be broken down to cellulose, followed by acid hydrolysis to yield NCC. The report by Ogundare et al. was the first ever in utilizing DCFs directly for the production of NCC. The process flow is shown in Fig. 3. The prepared NCC samples were characterized by 1Hand 13C-NMR, FT-IR, TEM, SEM, powder X-ray diffraction (PRD), TGA, specific surface area and porosity measurements. The overall yield of NCC from DCF was 29.4%. The NMR spectra generated data on the degree of substitution on DCF-cellulose (2.34) and % distribution of acetyl moiety, mostly on C-6 hydroxyl group 422

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from paddy mills (125 million tonnes, annually). It contains ∼33% cellulose, 26% hemicellulose and 7% lignin (Islam, Kao, Bhattacharya, & Gupta, 2017; Islam, Kao, Bhattacharya, Gupta, & Bhattacharjee, 2017). To overcome the issues associated with previously reported extraction of NCC from rice husk, Islam, Kao, Bhattacharya, and Gupta (2017) and Islam, Kao, Bhattacharya, Gupta, and Bhattacharjee (2017) used low-pressure alkaline delignification process for the first time to improve pulping efficiency with direct effect on overall yield of NCC. Aqueous solution of NaOH (4 N, 80 °C, 24 h) was used to remove lignin and other impurities such as pectin, hemicelluloses and waxy materials. This was followed by bleaching (15% NaOCl, 60 °C, 60 min) and finally acid hydrolysis (4 M H2SO4, 60 °C, 60 min). The overall yield of NCC was 32%. A thorough analysis using various techniques confirmed the formation of NCC with CI: 61%, particle dimensions as diameter: 15–50 nm, thickness: 10–50 nm and length: 275–550 nm. The utilization of engineering fundamentals to test the feasibility of large-scale production of NCC yielded fruitful outcome with improvements in overall yield and product quality. Such studies are needed to overcome the inherent scale-up issues from lab- to production-scale. Collaborative efforts of the lab researchers with chemical engineers in particular, will fill the technology gap haunting the industrial production of NCC. Tetra Pak® are so commonplace in our lives. The convenience comes at a price – generation of large quantities of waste! Diop and Lavoie (2017) investigated the use of Tetra Pak®-derived tertiary forest residues for extraction of NCC. This approach addresses several issues with one solution, namely, waste management, recycling of waste, generation of value-added products, and of course, the environmental damage. The process flow is schematically represented in Fig. 4. In brief, the cellulosic mass (63% cardboard) was separated from plastic (30% recovery of polyethylene) and aluminium (7% recovery) with hydrapulper, followed by treatment with NaOH (2% and 5%) twice, both at 90 °C for 2 h each, bleaching (75 ± 5 °C for 3 h using HOAc and NaClO2) and finally acid hydrolysis (64 wt% H2SO4 at 90, 120 and 180 min). The first alkali treatment removed several impurities (silica ash, pectin, salts, proteins, waxes and others) while the second treatment specifically reduced the lignin content significantly. The overall yield of NCC from raw unpurified recycled Tetra Pak cellulose and bleached purified recycled Tetra Pak cellulose were 36% and 41%, respectively, after 90 min of hydrolysis at 45 °C. Structural and chemical analyses using energy dispersive spectroscopy (EDS), TEM, FT-IR, XRD and TGA provided the vital information. The chemical elemental analysis of NCC surface (EDS) detected negligible amounts of Al and S,

Fig. 3. Process flow for extraction of NCC from discarded cigarette filters (Ogundare et al., 2017).

(43%). The efficiency of deacetylation was confirmed by FT-IR on monitoring the disappearance of C]O stretching of the acetate group. The TEM precisely gave the mean length of 143.5 ± 37.43 nm and the width of 8.28 ± 3.26 nm. The higher aspect ratio (above 10) of the prepared NCC indicated its usefulness in reinforcing applications. The CI values of the NCC film and the freeze-dried form were 96.77% and 94.47%, respectively, as measured from PRD data. In thermal analyses, freeze-dried material exhibited higher resistance to thermal degradation over its film counterpart. The mean Brunauer–Emmett–Teller (BET) specific surface area of the freeze-dried sample was found to be 7.78 ± 0.11 m2/g, almost seven-fold higher than the film. In conclusion, this interesting study provided a solid foundation for such environment-friendly moves which not only reduced the waste (i.e., DCF) but also produced useful commercial-grade material (Best from the Waste!). Rice husk represents yet another waste material obtained mostly

Fig. 4. Schematic representation of NCC extraction process from waste Tetra Pak containers. (adapted from Diop & Lavoie, 2017) 423

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step forward in the direction of ‘green’ synthesis of NCC from biomass. In line with the above studies, researchers have used oxidation using TEMPO (2,2,6,6-tetramethylpiperidine-1-oxy) radical, followed by mechanical treatment for NCC synthesis (crossref. Rohaizu & Wanrosli, 2017). Treatment of the pulp with TEMPO led to the oxidation of 1 °C-6 hydroxyl groups of the fibres, making their dispersion in water easy. In their seminal studies with oil palm empty fruit bunch (OPEFB)-MCC, Rohaizu et al. extracted NCC using sono-assist TEMPO-mediated oxidation followed by mechanical treatment, i.e., sonication. The overall effect was increased reaction rates, and the efficiency of the process. There was quantitative increase in the carboxylate content and ∼39% in the NCC yield from the OPEFB-MCC. The resulting NCC was – i) 6 nm in width and 122 nm in length; ii) CI: 72% with prominently cellulose I structure and, iii) thermally less stable than its MCC counterpart. Despite these observations, the mechano-chemical method could offer substantial advantages by avoiding the acid hydrolysis step. Further optimization of such methods for the other sources would be highly desirable with respect to industrial-scale production of NCC. Chowdhury and Abd Hamid (2016) investigated the use of ultrasonication combined with microwave-assisted pretreatment as an adjunct to acid hydrolysis in their study – production of NCC using jute dried stalks. The process modifications included the use of microwaves during alkali treatment to speed up the delignification process (due to swelling of the amorphous domains of cellulose) and ultrasonication during acid hydrolysis using ‘green solvent’ such as ionic liquid or conventional H2SO4. Bleaching process, prior to hydrolysis, was carried out with the help of 30% H2O2. This resulted in rod-like NCC crystals with length 62–105 nm and diameter 10–15 nm. The overall yield with the ionic liquid was significantly higher than the conventional acid treatment (48% versus 42%). The CI of NCC from acid treatment was higher than that from ionic liquid (88% versus 83%). The thermal stability of the NCC was lower due to smaller size. In conclusion, a green and sustainable approach for extracting NCC was developed with high potential for industrial-scale production. Cui, Zhang, Ge, Xiong, and Sun (2016) used wheat MCC to extract NCC using an environment-friendly enzymatic hydrolysis coupled with ultrasonication with an overall yield of 22.57% (120 h). The ultrasonic treatment had significant impact on the yield, particle dimensions and the crystallinity. Cellulase enzyme (Celluclast® 1.5 L, 700 endoglucanase unit/g) was used at varying time durations (72, 96 and 120 h) under ultrasonic treatment for speeding up the enzymatic catalysis. For comparison, same source material was subjected to conventional acid hydrolysis treatment. The NCC dimensions were – length: 50–80 nm and width: < 10 nm. The CI decreased from 87.46% (30 min ultrasonic treatment) to 82.26% (60 min ultrasonic treatment). Surprisingly, the thermal stability of NCC from enzymatic process was better than its acid-treated counterpart. The study was relevant from several aspects such as ‘green process’, better quality of the product, scalability and commercial feasibility. Further optimization with respect to enzymatic treatment duration and other process parameters such as sonication intensity as well as duration are likely to yield obvious benefits.

in few batches. This was possibly due to the extended reaction time (180 min). The average particle length was 150 nm, and width 14 nm. The XRD analysis confirmed the presence of cellulose I crystalline structure with CI 94%. There was a decrease in thermal stability of the NCC after extraction. Nonetheless, the study beautifully demonstrated the ‘Best from the Waste!’ concept. Ditzel, Prestes, Carvalho, Demiate, and Pinheiro (2017) extracted NCC from pine wood and corncobs by using environment-friendly pulping process, organosolv. The acetosolv (oraganosolv using EtOH, organic acids or ketones) pulping was carried out with the finelyground (#80) pine wood and corncobs. The main advantages offered by the acetosolv process are - a) avoidance of chlorinated reagents (e.g., NaOCl, NaClO2) for bleaching; b) use of H2O2 as an oxidizing agent for completion of delignification, and c) improved quality of delignified cellulose, which can be directly taken up for subsequent acid hydrolysis for NCC extraction. The chemical pretreatment (acetosolv) involved use of 92.9% w/w acetic acid, 0.3% w/w HCl (as a catalyst) and deionized water (q.s.), at reflux temperature (115 °C for 3 h). The bleaching process (50 °C, 2.2–3.5 h) utilized 1:1 mixture of 4% NaOH and 24% H2O2. Finally, the acid hydrolysis was carried out with H2SO4 (62% w/w, 44 °C, and 1.5 h). The generated NCC suspensions were evaluated extensively using zetasizer, gravimetric analysis, XRD and SEM/TEM. The CIs for the NCC from pine wood and corncobs were 67.8% and 70.9%, respectively. The corresponding yields of NCC were 9% from pine wood and 23.5% from corncobs. The authors concluded that further optimization in the process parameters, especially hydrolytic conditions, was necessary to make it feasible commercially. 4. Commercial NCC preparation: marching towards sustainability Despite several issues, commercially NCC is produced following acid hydrolysis of a suitable source, such as wood or MCC. Generation of dilute stream of H2SO4 containing sugars produced during hydrolysis presents challenges for recycling and/or disposal. Membrane technologies are inefficient, to some extent, in removing the sugars completely. This is likely to complicate the acid recycling for the next batch. Sarma, Ayadi, Brar, and Berry (2017) employed a simple, yet innovative and efficient approach for removing sugars and other impurities for acid recycling to be feasible. Activated charcoal (1 to 100 g/L, 15 to 180 min, 4–45 °C) was used for removing carbohydrates (mainly glucose and xylose) from the acid liquor sample from the commercial NCC production batch. A large number of parameters were evaluated for optimizing the conditions of activated charcoal treatment using statistical experimental design (13 runs). The results indicated precisely the dominant role of activated charcoal concentration in effectively removing sugars. The recommended dose was 50 g/L to avoid loss of acid solution and reduce the cost involved. The adsorbent could be recycled at least five times, further adding to the overall process efficiency. Any innovative step in reducing the hazardous waste such as acid by recycling, is beneficial in bringing down the NCC production cost, which in turn, will have a positive impact on its commercial utility. The NCC synthesis either by acidic method coupled with mechanical or enzymatic treatment or by non-acidic process will possibly offer unique advantages for various reasons cited at the beginning. Zianor Azrina et al. (2017) isolated NCC from oil palm empty fruit bunch pulp (EFBP) using acid hydrolysis coupled with ultrasound treatment. This was the first attempt where EFPB was used as a source of NCC. The process involved typical alkali treatment followed incubation with DMSO at 80 °C. The material, after filtering, washing with deionised water and drying, was subjected to acid hydrolysis (64% w/v H2SO4, 45 °C, 2 h) in ultrasonic bath. Further processing led to dried NCC, which was characterized for/by chemical composition, FT-IR, fieldemission scanning electron microscopy (FESEM), XRD, and TGA. Surprisingly, the NCC was spherical (diameter: 30–40 nm) in shape and exhibited CI of 80% with significantly higher thermal stability from the source and intermediate materials. The results were promising and a

5. Recent patents on NCC preparation (2015-till date) A closure look at the patents published/granted in last three years (January 1, 2015 – November 18, 2018) revealed few interesting methods of NCC preparation. Table 3 provides a brief summary of these methods. Use of these (to be) patented methods are likely to be beneficial for the large-scale production of NCC. The number of patents exploring applications of NCC or NCC composites with large variety of materials are humongous and are not discussed here since such subject matter is out of scope of the present review. The authors’ lab is instrumental in extracting MCC and NCC from unexplored biomass or waste materials. Two Indian Patent Applications have recently been published (Pethe et al. (2017b, 2017a)). There is a great scope in 424

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Table 3 List of recent patents featuring NCC preparation from biomass/waste materials. Sr. No.

Patent Title




Method for production of nanocrystalline cellulose Nanocrystalline cellulose, its preparation and uses of such nanocrystalline cellulose Production of nanocrystalline cellulose

Flash lyophilized-acidic hydrolysis method

Hindi and Abohassan(2017)

Use of metal oxide or hydroxide for neutralizing NCC prepared from acid hydrolysis

Mueller and Briesen (2016)

Use of solvent mixture (organic solvent and acid catalyst) followed by second solvent mixture (second organic solvent with little or no water) for NCC extraction from cellulosic starting material

Kunaver, Kos, Anzlovar, Zagar, and Huskic (2015)

2 3

thankful to Dr. Bala Prabhakar, Dean, SPPSPTM, SVKM’s NMIMS, for her constant support and motivation during preparation of this manuscript.

generating intellectual property and reaping commercial benefits by being more creative and rational so that the quality NCC production becomes cheaper and sustainable. The need of the hour, given the growing popularity of NCC and the unheard and really cool applications of the materials such as composites, make this area even more interesting. The increasing number of publications and patents/patent applications only endorse its popularity.

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6. Conclusions & perspectives Nanocrystalline cellulose, a well-known and popular nanomaterial, finds a large number of potential applications in materials, life and biomedical sciences. Despite in existence for quite some time, the commercial preparation of NCC is troublesome due to longer reaction times, lower yield and environmental damage. Over the last few decades, the discipline has come a long way due to ground-breaking research addressing the present issues. Researchers are finding innovative alternatives to the conventional alkali-bleaching-acid hydrolysis process. Alternative methods such as ultrasonication, use of green oxidants, enzymes, organosolv process, flash-lyophilized acid hydrolysis, etc. are making headways towards sustainable and commercially viable largescale NCC production. The objective is not only to develop cost-effective method but also to improve the quality of the generated NCC. The innovative chemical and physical (i.e., mechanical, microwaves) methods or any combination thereof, in the coming years, will ensure that the NCC is popular as a preferred nanomaterial for varied applications. The use of sustainable, green and cheaper feedstock, e.g., biomass and waste materials, as potential sources of NCC will substantially bring down its production cost. We need a three-prong approach – sustainable and cheaper feedstock, high-yielding processes and least or no damage to the environment. Most of these so-called innovative processes work excellently in lab but present obvious problems on bulk scale. Logically speaking, researchers are expected to work proactively in tandem with the chemical engineers to covert labscale research into industrial-scale production and commercialization. This will only expand the applicability domain of a popular nanomaterial like NCC and fascinating products derived from it, e.g., nanocomposites, films, etc. On the similar lines, one process alternative tried and tested for one source, ideally, should be tried and tested for the other sources. This will ensure the availability of the optimized process for the possibly best source to yield the best quality NCC at reasonably lower cost. Concerted efforts and contributions towards converting ‘waste into best’ should be the 21st century mantra. Conflict of interest Authors declare no conflict of interest. Acknowledgements The authors thank Mr. Sumit Nerkar, 2nd Year M. Pharm. (Pharmaceutical Quality Assurance) student of Shobhaben Pratapbhai Patel School of Pharmacy and Technology Management (SPPSPTM), SVKM’s NMIMS, for his help with the figures. The authors are also 425

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