, Volume 25, Issue 4, pp 2629–2645 | Cite as

Ultrasound-assisted conversion of cellulose into hydrogel and functional carbon material

  • Teck Wei Ching
  • Victoria Haritos
  • Akshat Tanksale
Original Paper


Microcrystalline cellulose (MCC) was fibrillated using an ultrasound probe to produce a hydrogel, which after freeze-drying and carbonisation under N2 atmosphere at elevated temperatures produced highly porous carbon. Ultrasound treatment in the absence of acid resulted in high aspect ratio, nanocrystalline cellulose due to fibrillation of the outer layers of the MCC fibre bundles, whereas in the presence of acid, cleavage of glycosidic bonds resulted in smaller aspect ratio fibres. Carbonisation of the acid-generated nanocrystalline cellulose samples at 800 °C provided the highest BET surface area of 917.0 m2/g, with over 18% pore volume in mesopores. The resulting high surface area carbon was able to absorb 100% of methylene blue in a solution having an initial concentration of 10 mg/L in 20 min which is comparable with many commercially available activated carbon products.

Graphical Abstract

Ultrasonication of microcrystalline cellulose resulted in nanocrystalline cellulose hydrogel which after freeze drying and carbonisation provided high surface area mesoporous carbon.


Microcrystalline cellulose Ultrasound Nanocellulose hydrogel Carbonisation Dye adsorption 


Cellulose is the most abundant natural polymer on earth with an estimated annual production of over 7.5 × 1010 tons (Habibi et al. 2010), and due to its availability, biodegradability and mechanical attributes, it is widely used for industrial applications. Nanocellulose is a valuable form of cellulose and is characterised by large surface area, high aspect ratio and highly reactive surface properties due to extensive hydroxyl side groups (Brinchi et al. 2013). It has various applications across the biomedical field (Gatenholm and Klemm 2010), nanocomposite films(Cho and Park 2011; Li et al. 2012b) and paper industry (González et al. 2012; Hu et al. 2013). Nanocellulose can be suspended in water to prepare hydrogels which can be applied in various fields, in particular biotechnology and biomedical applications (Korhonen et al. 2011; McKee et al. 2014; Zander et al. 2014). While high pressure homogenisation is one of the most commonly used methods to obtain nanocellulose, (Chakraborty et al. 2005; Nakagaito and Yano 2004) it requires high pressures in the range of 20,000 psi (~ 1400 bars) and multiple passes (20–50) to produce homogenised nanocellulose (Lee et al. 2009; Li et al. 2012a). High pressure homogenisers consumed the highest energy (78.8 MJ/kg) to produce nanocellulose among the methods studied by Spence et al. (2011). Grinding methods may be used for the nanocellulose production, in which pulp slurry undergo super-grinding by being allowed to pass through a rotating grind stone and a static grind stone (Iwamoto et al. 2005). However, nanocellulose produced in this way were found to have large distribution of fibre length. Aqueous counter collision method, in which cellulosic material is reduced into nano-size using a pair of high pressure water jets also results in high energy requirements (Kondo et al. 2014). A single pass using this method subjects the substrate to 6.7–18.1 kJ/mol of kinetic energy, which exceeds the typical dipole–dipole, London dispersion forces and weak hydrogen bonds (Kose and Kondo 2011). Various chemical methods, such as high concentration acid hydrolysis (Siró and Plackett 2010) and enzyme approach (Henriksson et al. 2007) can be used to obtain nanocellulose from amorphous fractions of cellulose. TEMPO-mediated oxidation of cellulose was able to obtain fine nanocellulose with diameters in the range of 3–4 nm (Saito et al. 2006). Ionic liquids such as N,N-dimethylacetamide with lithium chloride (McCormick et al. 1985) and 1-butyl-3-methylimidazolium (Kim and Jang 2013; Mahadeva et al. 2011) was used to promote swelling and dissolution of cellulose, leading to easier separation of the substrate into nanofibers. Most of the methods commonly used for nanocellulose are energy intensive and/or require high concentration of corrosive and hazardous chemicals chemicals. Therefore, ultrasound treatment is gaining popularity as an alternative means for the production of nanocellulose (Chen et al. 2011; Cheng et al. 2009, 2010). Ultrasound treatment utilizes cavitation, which is the rapid formation and collapse of microbubbles in water. The collapse of the bubbles are able to generate a temperature of up to 5000 °C and pressure of 1000 atm with heating and cooling rates beyond 1010 K/s (Suslick and Price 1999). The energy provided by cavitation is approximated to be in the range of 10–100 kJ/mol, which is greater than the hydrogen bond free energy content (Tischer et al. 2010). Therefore, ultrasound treatment can gradually decrease the size of cellulose and defibrillate the fibres (Chen et al. 2011).

Recently, Budarin et al. (2007) reported the synthesis of mesoporous carbon materials from starch hydrogels. High-amylose corn-starch was gelatinised by heating in water to produce mesoporous starch, which was then dried via solvent exchange and subsequently carbonised under vacuum to produce a porous material (Budarin et al. 2006). It would seem that cellulose, as a relatively cheap, stable, highly abundant and non-food product would be an excellent candidate for production of a mesoporous material but to date this has not been reported. Porous carbon materials have application in catalysis, gas purification, water treatment, energy storage, as well as decolouration (Rodriguez Correa et al. 2017; White et al. 2009).

Dyes are extensively used in the fabrics, food, paper, plastics and cosmetics industry (Robinson et al. 2002) and due to the complicated chemical nature of textile effluents, there is no single effective method for textile wastewater treatment (Cooper 1993). It is estimated that about 40,000 tonnes of dye are discharged as wastewater (O’Neill et al. 1999) and aside from the visual impact of coloured water in rivers or lakes, it presence reduces light penetration into the water, leading to an adverse effect on photosynthetic activity and also potential toxicity to fishes and underwater organisms (Mckay et al. 1980). Therefore, it is vital to remove dyes before discharging wastewater. Existing processes to achieve this reduction involve solution oxidation (Hsu et al. 2001; Perkins et al. 1996), UV radiation (Fung et al. 2000; Uygur and Kök 1999), chemical precipitation (Kadam and Lee 2015), biological (Punzi et al. 2015), membrane separation (Ciardelli et al. 2001) or adsorbent contact (Malik 2004; Namasivayam and Kavitha 2002). However, many of these processes are costly to implement and do not remove all dyes from wastewater (Valix et al. 2004), whereas adsorbents are suitable for this purpose due to their simplicity, effectiveness and reusability (Chen et al. 2016; Nguyen-Le and Lee 2015).

In this paper, we report the synthesis of a highly porous carbon from microcrystalline cellulose via a scalable process. Microcrystalline cellulose was treated using an ultrasound probe to create hydrogels followed by carbonisation under N2 atmosphere. Varying the conditions of the ultrasound treatment affected the morphology, particle size, crystallinity and aspect ratio of the nanocellulose product and the porosity and surface area of carbonised cellulose was highly dependent on carbonisation temperature. Carbonisation at 800 °C yielded material comparable to commercially-available activated carbon in terms of porosity and available surface area, and its suitability for applications in dye removal were demonstrated.

Materials and methods

Ultrasound treatment of cellulose

Microcrystalline cellulose (2 g, Sigmacell cellulose, Type 20, abbreviated as MCC) was added to 75 ml of MilliQ water in a 100 ml beaker (2.6 wt% loading). The MCC suspension was ultrasound-treated using Qsonica Q700 Sonicator, with or without acid loading (1 mmol-H2SO4/g-MCC), at 100% amplitude for 2 h total treatment time delivered as 15 s pulses with 15 s breaks, while being cooled by ambient temperature water bath to avoid excessive heating of samples. After ultrasound treatment, the thick stable suspension with the appearance of a hydrogel were placed at − 20 °C for 12 h. The frozen samples were freeze dried using Labconco FreeZone Freeze Dry System. Same procedure was conducted with microfibrillated cellulose (MFC, Celish, Daicel) and fibre cellulose (FC, SigmaAldrich).

Carbonisation of ultrasound pretreated cellulose

Freeze dried nanocellulose samples were carbonised under 120 ml/min of N2 flow by heating from room temperature to 120 °C at 2 °C/min ramp, then held for 10 h, then heated at 2 °C/min to target temperatures (300–800 °C), and held for 2 h.

Electron microscopy

SEM images were captured using Phenom XL Benchtop SEM at accelerating voltage of 10 kV and working distance of 4 mm. Samples were secured on a carbon tape and sputter coated using Quorum SC7620 Sputter Coater prior to imaging.

Ultrasound treated samples were viewed under FEI TECNAI G2 T20 TEM with a working voltage of 200 kV. TEM grids (carbon coated copper grids) were plasma treated for 1 min prior. To image individual nanocellulose fibrils, the samples were diluted to 0.01 wt% and 3 µl droplet was deposited on the TEM grid, followed by drying with a mild stream of N2. Samples were then negatively stained with 2 wt% uranyl acetate substitute (samarium and gadolinium triacetate) for 30 s, followed by gentle washing with water. Excess liquid was removed using filter paper and finally drying under N2 again prior to viewing under TEM. The distributions of length and diameter of fibres were analysed using ImageJ software (NIH).

Gel point measurement

Gel point of cellulose suspension produced after ultrasound treatment was estimated using the sedimentation method (Varanasi et al. 2013). Briefly, cellulose suspensions ranging in concentration from 0.01 to 1.00% were prepared in 20 ml glass vials and agitated to ensure contents were fully suspended, the initial height of the suspension was recorded (Hi). Samples were allowed to settle for at least 24 h, the final height (Hf) of the sediment layer was recorded. The ratio of final sediment height (Hf) to initial height (Hi) was plotted against cellulose concentration. Data was then fitted with a quadratic equation, with the coefficient of the linear term representing the gel point concentration of the suspension (C).

Effective medium theory (EMT) (Celzard et al. 2000) and crowding number theory (CN) (Kerekes and Schell 1992) were both used to estimate the aspect ratio (A) of the cellulose suspension from the gel point solids mass fraction (C). The aspect ratio were estimated using the equation, A = 3.19C−0.58 (EMT) and A = 6.00C−0.5 (CN) (Varanasi et al. 2013).

X-ray diffraction (XRD)

A Rigaku Miniflex 600 was used to determine XRD patterns of treated and untreated cellulose. Samples were analysed from 2θ ranging from 10° to 30°, with a working voltage of 40 kV, current of 15 mA and Ni-filtered Cu-Kα radiation. Scanning speed used was 1°/min with step size of 0.02°.

The crystallinity index of cellulose samples was determined by measuring the height of the [2 0 0] peak (I200 at 2θ ~ 22.6°) and the minima between the [2 0 0] and [1 1 0] peaks (Iam at 2θ ~ 18.4°), calculated using Segal method as shown in Eq. 1.
$$C_{I} = \left( {\frac{{I_{200} - I_{am} }}{{I_{200} }}} \right) \times 100\%$$
Crystallite size of cellulose was calculated using the Scherrer equation (Eq. 2) where D is the apparent crystallite size, K is the shape factor and a value of 0.94 is used (Revol et al. 1987), λ is the wavelength of X-ray source, β is the width of the diffraction peak measured at half maximum height (FWHM).
$$D = \frac{K\lambda }{\beta cos\theta }$$

Surface area determination via N2 adsorption

Surface area of samples (SBET) were determined using Brunauer–Emmett–Teller (BET) equation. Degassing of the samples were performed at 120 °C for at least 12 h using Micromeritics Smart VacPrep. Then, N2 adsorption was carried out at 77 K using Micromeritics TriStar II. Micropore volume (Vµ) was also calculated using Dubinin–Radushkevich (D–R) equation (Dubinin 1989). Total pore volume (VT) was defined as volume adsorbed at relative pressure of P/P0 = 0.995 (Basta et al. 2009), and mesopore volume (Vm) was determined to be the balance between VT and Vµ. Average pore width was calculated using the equation 4VT/SBET.

Fourier transform infrared spectroscopy

FTIR ATR was carried out using PerkinElmer UATR Two (450–4000 nm wavelength) to study the functional group changes of cellulose materials after ultrasound and carbonisation treatment.

Dye adsorption on carbonised materials

Carbonised nanocellulose (50 mg) was added to 50 ml aqueous solution of Methylene Blue (MB, Basic Blue 9, 100 mg/L) and stirred. Samples of liquid were collected at frequent time intervals and filtered using a 0.2 µm syringe filter and the concentration of MB was determined by UV/Vis spectrophotometer (DR 5000 UV–Vis Spectrophotometer) at 665 nm. Adsorption capacity is defined as the ratio of mass of MB adsorbed to the mass of adsorbent used (Eq. 3). C0 (mg/L) is the initial MB concentration, C (mg/L) is the concentration at any time of interest, V (L) is the volume of the solution and W (g) is the mass of adsorbent used.
$$Adsorption\,capacity, q = \frac{{\left( {C_{0} - C} \right)V}}{W}$$

Results and discussion

XRD analysis of ultrasound-treated cellulose

As expected, untreated MCC showed peaks at 2θ = 22.5° and 16.4° (Fig. 1a), confirming the crystalline structure of Cellulose I type (Chen et al. 2011; Li et al. 2012b; Tang et al. 2011; Tian et al. 2016; Xiong et al. 2012). After ultrasound treatment, however, the peaks of the MCC sample were shifted to the higher 2θ; the [0 0 2] peak shifted from 22.5° to 22.8° and the [1 0 1] peak shifted from 16.0° to 16.4°. Similar shifts in the XRD peaks were observed in earlier reports (Li et al. 2012b) where Avicel MCC was subjected to ultrasound treatment, however this was not explained by the authors. Here, we postulate that MCC particles are subjected to intense physical stresses from cavitation during ultrasound treatment, the breakage of MCC particles generate internal stresses within the crystal lattice, and this in turn caused a minor shift in the XRD peaks of the treated MCC. Ultrasound treatment in the absence of acid decreased the crystallinity of MCC gradually from 77.8 to 69.0% after 2 h of treatment, and crystallite size decreased from 3.5 to 2.4 nm (Fig. 1 a).
Fig. 1

XRD of freeze dried nanocellulose following ultrasound treatment in a water only and b water containing 1 mmol/g H2SO4. The time periods (min) refer to ultrasonic treatment time

In the presence of acid, however, ultrasound treatment showed less significant change in the crystallinity and crystallite size of MCC; both attributes decreased slightly after 15 min of treatment, and then remained almost constant up to 2 h (Fig. 1b). The peak shift due to ultrasound treatment was also more gradual (increasing with treatment duration) than the case in absence of acid. The [0 0 2] peaks in Fig. 1b remained sharper relative to the corresponding peaks in Fig. 1a. This suggests that sulphuric acid attacked only the relatively more amorphous regions in MCC, catalysing partial depolymerisation of β-1,4 glycosidic bonds (Shrotri et al. 2013). Ultrasound treatment of MCC in the absence of acid resulted in lower crystallinity, this is evident from the broader peak in Fig. 1a as compared to 1b and summarised in Table 1. Overall, the reduction in crystallinity of MCC with ultrasonic treatment was minor which suggests that supramolecular ordering of MCC is not significantly affected (Zhang et al. 2013), whereas the morphology and particle sizes of MCC experience more significant change after ultrasound treatment (Table 1 and SEM section).
Table 1

Crystallinity index and crystallite size of ultrasound treated MCC

Treatment Duration (min)

Crystallinity index, CI (%)

Crystallite size, D (nm)

Without acid

With acida

Without acid

With acida


























a1 mmol H2SO4/g-MCC during ultrasound treatment

Gel point measurement

Due to their entanglement, it is difficult to identify the ends of individual cellulose fibres using microscopy methods (Henriksson et al. 2008; Ishii et al. 2011), therefore, a sedimentation method was used to estimate the aspect ratio of suspended fibres (Martinez et al. 2001; Varanasi et al. 2013; Zhang et al. 2012). This method estimates the aspect ratio based on gel point concentration of suspension, a concentration at which fibres form a continuous network in a suspension. The gel point of MCC samples decreased with increasing ultrasound treatment duration (Fig. 2). The gel point of untreated MCC was 16.8 wt% and was reduced to 10.6 wt% with the addition of acid (without ultrasound treatment). However, after 2 h of ultrasound treatment the gel point reduced to 0.0002 wt% (without acid) and 0.5605 wt% (with acid). Ultrasound cavitation causes fibrillation of MCC, leading to nanocrystalline cellulose formation which increases the aspect ratio and results in hydrogel formation at lower weight loadings.
Fig. 2

Gel point measurements of ultrasound treated MCC determined over different times a without acid and b with 1 mmol H2SO4/g-MCC determined by the sedimentation method. The lines of best fit are fitted with a quadratic equation, with the coefficient of the linear term representing the gel point concentration of the suspension

The aspect ratio of nanocellulose fibres increased with increasing ultrasound treatment duration, and the samples treated without acid exhibited a higher aspect ratio compared to those with acid (Table 2). It is hypothesized (Fig. 3) that in the absence of acid, the originally bundled fibres of MCC were fibrillated from the outer layers, producing fibres that are thin and long, therefore having a higher aspect ratio. In the presence of acid, however, the cleavage of glycosidic bonds was promoted by the acid, resulting in shorter fibre bundles and a smaller aspect ratio.
Table 2

Estimated aspect ratio of nanocellulose in hydrogels formed via ultrasound treatment of MCC


Estimated aspect ratio after ultrasound treatment duration (min)






Without Acid














With Acid














Aspect ratio calculated using gel point measurement via sedimentation method (Varanasi et al. 2013)

Fig. 3

Proposed effect of ultrasound treatment on MCC in the absence and presence of acid

Analysis of ultrasound-treated cellulose by electron microscopy

TEM images shown in Fig. 4 provide supporting evidence of the effects acid has during ultrasound treatment of MCC. Increasing the treatment time without H2SO4 is shown to have yielded thinner fibres, from 20 to 26 nm diameter in 30 min treatment, to 10–18 nm diameter and 400–500 nm length after 120 min ultrasound treatment (Fig. 5a, b). From the SEM images, surface modification is observed on the cellulose treated by ultrasound which are produced by shockwaves generated during the treatment that promotes newly exposed surfaces and breakage of particles (Fig. 6 and 7). At longer treatment times, MCC particles appear more fibrillated, consistent with the observation by Mishra et al. (2011). In the presence of H2SO4, SEM images reveal that samples are more aggregated and less fibrillated (Fig. 7) as compared to samples treated without acid (Fig. 6). Formation of these aggregates were similarly observed by Cheng et al. (2009) and Fan and Li (2012). This is most likely due to the formation of hydrogen bonds causing smaller cellulose particles to come in contact with each other (Li et al. 2012b), the presence of H2SO4 was also previously shown to promote repolymerisation behaviour by Shrotri et al. (2013) With the addition of H2SO4 during ultrasound treatment, the diameter of fibres appear to be unchanged with time at 15–26 nm, however the length distribution was observed to cover a wider range and decreased from 300–550 nm to 200–450 nm after 2 h of treatment (Fig. 5c, d). The thinning of MCC fibres in the absence of acid along with the shortening of fibres in the presence of acid both supports the proposed scheme depicted in Fig. 3.
Fig. 4

TEM images of hydrogels formed by MCC treated with ultrasound without acid for a 30 min, b 120 min and with 1 mmol H2SO4/g-MCC for c 30 min and d 120 min

Fig. 5

Diameter (left) and length (right) distributions, as determined by TEM image analysis, of ultrasound treated MCC without acid for a 30 min, b 120 min and with 1 mmol H2SO4/g-MCC for c 30 min and d 120 min

Fig. 6

SEM images of MCC treated with ultrasound for a 0 min, b 15 min, c 30 min, d 60 min and e 120 min

Fig. 7

SEM images of MCC treated with ultrasound with 1 mmol H2SO4/g-MCC for a 0 min, b 15 min, c 30 min, d 60 min, and e 120 min

MFC and FC were also tested as feedstocks for ultrasound treatment as they are cheaper and more readily available substrates. Figure 8 compares the hydrogel suspensions of the cellulosic materials after 2 h ultrasound treatment; each material produced a hydrogel that remained stable for at least 24 h. TEM images of the MFC and FC hydrogels prepared in the presence of H2SO4 are shown in Fig. 9, MFC yielded longer fibres with diameters well within the nanometre range whereas FC yielded thicker fibres compared to MCC.
Fig. 8

Suspensions of (left) MCC, (middle) MFC and (right) FC following 2 h ultrasound treatment with 1 mmol/g of H2SO4 and left at rest for 24 h

Fig. 9

TEM images of a MFC and b FC treated with ultrasound for 2 h in the presence of 1 mmol H2SO4/g-cellulose

Carbonisation of ultrasound treated MCC

Carbonisation of freeze dried hydrogels resulted in increasing BET surface area and pore volume with carbonisation temperature, giving maximum \(S_{BET}\) = 755.7 m2/g and \(V_{T}\) = 0.37 cm3/g at 800 °C with 2.6 wt% MCC initial concentration. Mesopore volume increased with increasing carbonization temperature, which is similar to the trend observed by White et al. (2009), however the majority of the pores formed in our study belong in the micropore size range, which is a property of activated carbons prepared from biomass sources (White et al. 2009). Control sample without ultrasound treatment but with H2SO4 addition showed significantly lower specific surface area and no mesopore volume (Table 3); which demonstrates the positive effect of ultrasound treatment in generating pores in the material. In addition to the carbonization temperature, the degree of entanglement of fibres in the hydrogel had a significant impact on the porosity of carbon materials. Reducing the MCC loading during ultrasound treatment to 1.0 wt% was expected to have produced a hydrogel with lower entanglement and the resulting carbonized material gave the highest \(S_{BET}\) in our study (917.0 m2/g). Furthermore, it was observed that hydrogels prepared in absence of acid, which had higher aspect ratios and hence higher entanglement, resulted in lower \(S_{BET}\) values. For example, the equivalent 1.0 wt% loading hydrogel prepared without acid (1.0MCC800c, Table 3) resulted in \(S_{BET}\) = 382.0 m2/g only. All the carbonized materials generated in this study showed Type II isotherm (Fig. 10). That is, adsorption before the point of inflexion (P/P0 = 0.05) indicated a monolayer adsorption of N2 on the carbonized product, the almost linear section after the point of inflexion indicated multilayer adsorption, hence a Type II isotherm represents the unrestricted monolayer-multilayer adsorption of the material (Sing 1985). The lower loading sample (1.0MCC800) displayed a significantly higher mesopore volume than the high loading sample (2.6MCC800) as evident by the hysteresis loop (Fig. 10, Table 3). The pore size distribution for the 1.0MCC800 sample were indicated to be mesopores in the 5–15 nm range (Fig. 10). Porous carbonized material synthesised in this study exhibited BET surface areas comparable to commercial activated carbons such as DARCO (584 m2/g) (Sabio et al. 2004), Chemviron Carbon F-300 (1000 m2/g), DARCO GCW (1000 m2/g) and Norit RFZ 1 (800 m2/g) (Lücking et al. 1998). When compared to other porous carbon-based material synthesised in the literature (Table 4), BET surface area of the product in this study is comparable, especially when considering the milder conditions used in other works, such as low NaOH/carbon weight ratio.
Table 3

BET surface area, pore volumes, pore width of MCC at various carbonisation temperature

Sample ID

Carbonisation yield (%)

BET Surface area, \(\varvec{S}_{{\varvec{BET}}}\) (m2/g)

Micropore volume, Vµ (cm3/g)

Total pore volume, VT (cm3/g)

Mesopore volume, Vm (cm3/g)

Average pore width, \(\varvec{d}_{\varvec{p}}\) (nm)

As received MCC



7.3 × 10−4

5.3 × 10−3

4.5 × 10−3





2.5 × 10−1

2.5 × 10−1






5.9 × 10−4

2.1 × 10−3

1.5 × 10−3





4.5 × 10−3

1.5 × 10−2

1.0 × 10−2





1.9 × 10−1

1.9 × 10−1

4.7 × 10−3





2.0 × 10−1

2.2 × 10−1

1.4 × 10−2





3.5 × 10−1

3.7 × 10−1

2.0 × 10−2





4.2 × 10−1

5.1 × 10−1

9.5 × 10−2





1.7 × 10−1

1.7 × 10−1

6.6 × 10−3





3.4 × 10−3

2.5 × 10−2

2.1 × 10−2





2.4 × 10−3

1.5 × 10−2

1.3 × 10−2





2.6 × 10−3

2.2 × 10−2

1.9 × 10−2





4.5 × 10−4

2.9 × 10−3

2.4 × 10−3


All the samples were treated for 2 h in the ultrasound except as labelled

aControl = acid impregnated MCC (1 mmol H2SO4/g-MCC) carbonised at 800 °C without ultrasound treatment

bSample ID nomenclature: 2.6MCC300 = 2.6 wt% loading during ultrasound treatment of MCC substrate followed by freeze drying and carbonation at 300 °C

cUltrasound treatment without H2SO4

Fig. 10

a N2 Adsorption (black line) and desorption (red line) isotherm and b pore size distribution of carbonised MCC. Legends for pore size distribution: (i) 2.6MCC300, (ii) 2.6MCC400, (iii) 2.6MCC600, (iv) 2.6MCC700, (v) 2.6MCC800, (vi) 1.0MCC800 and (vii) 1.0MCC800c

Table 4

BET surface area comparison of current study against similar product in literature

Synthesis method

Product BET surface area (m2/g)


Ultrasound treatment of microcrystalline cellulose for 2 h with mild H2SO4, freeze dried for 24 h, carbonised at 800 °C for 2 h


This study

Fir wood dried at 110 °C for 24 h, carbonized at 450 °C for 1.5 h, concentrated NaOH treated, oven dried, washed, neutralised with HCl, washed

380 at (NaOH/Carbon) = 2

1672 at (NaOH/Carbon) = 3

2406 at (NaOH/Carbon) = 4

Wu and Tseng (2008)

Rice straw carbonised at 800 °C for 1 h, treated with concentrated KOH, dried at 110 °C for 12 h, carbonised at 800 °C for 1 h, washed with HCl, dried at 110 °C for 12 h

657 with 1 stage carbonisation

1917 with 2 stage carbonisation

Basta et al. (2009)

Coconut shell dried at 110 °C for 48 h, ground, carbonised at 500 °C for 2 h, concentrated NaOH treated, dried at 130 °C for 4 h, carbonised at 700 °C for 1.5 h, washed with HCl, dried at 110 °C for 24 h

783 at (NaOH/carbon) = 1

1842 at (NaOH/carbon) = 2

2825 at (NaOH/carbon) = 3

Cazetta et al. (2011)

Gelatinisation of starch, retrogradation at 5 °C, ethanol solvent exchange followed by vacuum drying at 50 °C for 24 h, first stage carbonisation at 170 °C for 12 h with acid catalyst, second stage carbonisation at 900 for 3 h


White et al. (2009)

While MFC and FC are lower cost feedstocks compared with MCC, the porosity of ultrasonic-treated carbonized materials generated from them at temperatures up to 900 °C was very low. This indicates that the dense entanglement of hydrogels created by MFC and FC fibres following ultrasound treatment were unable to maintain a pore structure. SEM images of carbonized materials generated from MCC, MFC and FC are compared following similar treatments in Fig. 11; the MCC samples after carbonisation at 800 °C appeared to retain the particle structure, whereas MFC and FC samples appeared to be significantly more defibrillated and formed a flatter and denser sheet. It appears from this comparison that the crystallinity of MCC is essential to generate and maintain pore structure in processed, carbonized cellulose.
Fig. 11

SEM images of carbon samples a 2.6MCC800, b 1.0MCC800, c 2.6MFC800 and d 2.6FC800MCC

The three sources of cellulose investigated here—MCC, MFC and FC—were compared by FTIR at various stages from untreated, freeze dried hydrogel following ultrasonic treatment, heating and carbonization at 800 and 900 °C. Untreated MFC showed highest moisture content as evident by absorbance at 3300 and 1640 cm−1 corresponding to the stretching of OH groups and bending mode of absorbed water (Fig. 12a–c) (Lan et al. 2011; Pappas et al. 2002). After carbonisation at temperatures up to 300–400 °C, all cellulose sources showed a reduction in peaks corresponding to moisture, OH, C–H, C–OH, C–O and CH2 groups (Fig. 12). These groups are present in glycosyl linkages in cellulose (Xie et al. 2009), indicating a partial decomposition of the glycosidic structure. There was also emergence of new peaks at 1600 cm−1 (C=C stretching) and 1720 cm−1 (C=O stretching) became more prominent with heating at these temperatures. When the carbonization temperature was further increased to 700–900 °C, the glycosidic structure appeared to be completely decomposed in all the samples, giving an overall spectra nearly transparent to IR except for the peak at 2800–3000 cm−1 which corresponds to the stretching of C–H groups (Fig. 12a-c).
Fig. 12

FTIR ATR spectra of a MCC, b MFC and c FC materials as untreated, ultrasonic-treated and freeze dried, and following carbonization steps

Dye adsorption on carbonised material

Methylene Blue was selected as the target dye in this study due to its common use in adsorption measurements and its basicity, since the acid groups present on the porous carbon material act as an anchoring site due to electrostatic interactions (Hameed et al. 2007; Pereira et al. 2003). The removal of Methylene Blue (MB) dye from the solution is due to the adsorption of the dye in its cationic form (MB+), onto the surface of substrate (Kannan and Sundaram 2001). Since the adsorbent synthesised in this study were carbonised in the presence of H2SO4, there may be presence of sulphonic group (R-SO3) that promotes the electrostatic interaction of cationic Methylene Blue onto the substrate (Jia et al. 2016). As shown in Fig. 13, the majority of the dye was adsorbed within 10 min—79% adsorption on 2.6MCC800 and 98% adsorption on 1.0MCC800—giving adsorption capacity of 79 mg/g and 98 mg/g, respectively. Higher adsorption capacity of the latter sample can be attributed to the higher specific surface area, and pore volume, specifically the higher mesoporous volume which is favourable for MB adsorption (Altenor et al. 2009). The dye adsorption efficiency of the 1.0MCC800 is comparable to the commercially available DARCO 100 mesh activated carbon. The dye adsorption capacity of the sample is also comparable to that of carbon prepared from mechanically treated rice straw with KOH activation at 800 °C of 62.23 mg/g (Basta et al. 2009). When 1.0MCC800c was tested for dye adsorption, the performance was poor due to the low available surface area of the sample and the lack of surface acidity. Karagoz et al. observed a similarly low MB adsorption when activated carbon was prepared without sulphuric acid during carbonization (Karagöz et al. 2008). The dye adsorption capacity of carbon samples prepared at lower temperatures were significantly lower (data not shown). For example, the 2.6MCC400 sample showed no MB adsorption in the first hour, and even after 24 h only 17% of the dye was adsorbed. The performance improved with 2.6MCC700 where 18% MB was adsorbed in the first hour and 22% after 24 h. Therefore, these results clearly show that the adsorption capacity is directly proportional to the pore volume and surface area, which increases with increasing carbonization temperature. Acid loading during carbon synthesis may also be important for the adsorption of a basic dye such as MB.
Fig. 13

Comparison of carbonised material (black circle) 2.6MCC800, (black triangle) 1.0MCC800, (black square) 1.0MCC800c and (cross) DARCO activated carbon in Methylene Blue adsorption. Data beyond 60 min not shown


Ultrasound treatment of cellulosic materials successfully defibrillated the cellulose and induced hydrogel formation through generation of high aspect ratio fibres. Acid addition during ultrasound treatment reduced the aspect ratio of the fibrils in the hydrogels, which resulted in lower entanglement. Carbonization of freeze dried hydrogels produced from microcrystalline cellulose resulted in the highest BET surface area of 917.0 m2/g and total pore volume of 0.51 cm3/g, which was comparable to commercially available activated carbon. Higher temperature carbonization, acid addition and lower cellulose loading during ultrasound treatment resulted in high porosity carbon materials. Freeze dried hydrogels produced from microfibrillated cellulose or fibre cellulose were highly dense due to the generation of high aspect ratio nanocellulose but these did not form good pore networks and hence were inferior as carbonized materials. However, they may find application in other functional materials such as ultra- or nano- filtration membrane synthesis or biomedical application. The porous carbon materials synthesized in this work demonstrated high adsorption capacity for Methylene Blue (98 mg/g in 10 min) which is comparable to commercially available activated carbon, DARCO 100 mesh.



This work is supported by Monash University Faculty of Engineering International Postgraduate Research Scholarship (FEIPRS) and CSIRO Flagship Collaboration Fund. The Authors acknowledge use of the facilities and assistance of Dr Timothy Williams and Dr Emily Chen at the Monash Centre for Electron Microscopy.

Authors’ contribution

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Authors and Affiliations

  1. 1.Department of Chemical EngineeringMonash UniversityClaytonAustralia

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