Journal of Polymers and the Environment https://doi.org/10.1007/s10924-018-1249-9

ORIGINAL PAPER

Comparative Study of Xylan Extracted by Sodium and Potassium Hydroxides (NaOH and KOH) from Bagasse Pulp: Characterization and Morphological Properties Parizad Sheikhi1 · Seyed Rahman Djafari Petroudy2

© Springer Science+Business Media, LLC, part of Springer Nature 2018

Abstract Xylan is the second most abundant polysaccharide and the predominant hemicellulose component of soda bagasse pulp. The present endeavor focuses on increasing the value addition to underutilized agro-industrial residue such as bagasse. For this purpose, xylan was isolated by two conventional alkali extraction methods i.e. NaOH and KOH. The recovery rate and sugar composition of different reaction times and alkali consumptions were monitored with advanced method such as High Performance Liquid Chromatography (HPLC). The Fourier Transform Infrared Spectroscopy (FTIR) and Wide Angle X-ray spectroscopy (WAXS) were respectively employed to characterize the functional groups and Crystallinity Index (CrI) changes during the extraction process. It was explored that highest xylan recovery rates were obtained with 6% of NaOH at 120 min and 6% KOH at 45 min. The xylan morphology via WAXS was found that its structure to be amorphous. HPLC results also showed KOH had higher effectiveness than NaOH in terms of extracted xylan purity. Highest XGRs (Xylose to Glucose Ratios) were also achieved by KOH processes. Hence, this study contributes to the adequate utilization of agricultural residues, with promising potential for applications in the production of certain novel materials and chemical conversion industries. Keywords  Xylan · Soda bagasse pulp · Sodium hydroxide · Potassium hydroxide

Introduction The development of new technologies for the production of more valued materials such as advanced chemicals and fuels from agro-waste resources can be defined as a biorefinery concept and it has substantially increased during the last decades. Recently, the production rate of agricultural residues has been increased in Iran [1], and a sustainable agricultural residues management is needed. Annually there * Parizad Sheikhi [email protected] Seyed Rahman Djafari Petroudy [email protected] 1



Department of Mechanical engineering, Dezful Branch, Islamic Azad University, P.O. Box 313, Daneshgah BLVd, Azadegan Quarter, Dezful, Khuzestan, Iran



Cellulose Nanotechnology and Carbohydrate Chemistry Laboratory, Department of Biorefinery, Faculty of New Technologies Engineering, Shahid Beheshti University (SBU), Zirab, Savadkoh, Mazandaran, Iran

2

are approximately 4.3 million tons of bagasse in Iran [2]. This large number of lignocellulosic biomasses, both from the main culture and its residues, opens up new possibilities for the biofuel and the advanced materials industries. It should be said that a part of these materials are using in different applications such as pulp and paper, Medium Density Fiber (MDF), biofuel, burned and landfilled [3, 4]. Nowadays about 70% of generated sugarcane bagasse in Iran is used to generate heat and power to run the sugar mills and ethanol plants. The remaining portion is usually stockpiled. However, because the heating value of carbohydrates is approximately half of that of lignin [5], it would be beneficial to develop a more economical use of carbohydrates. Cellulose, hemicelluloses and lignin, which are all cell wall components, are the main components of bagasse and other agricultural residues. Sugarcane bagasse typically contains 5–7% extractives, 1–3% ash, 39–45% cellulose, 23–27% hemicelluloses and 19–32% lignin [6–11]. The main xylan found in bagasse is the arabinoxylan (AX), with substitutions of arabinose and slightly acetylation in the backbone of xylose [12]. The xylan can be extracted based on chemical

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(acidic or alkaline treatment), hot water extraction, steam explosion processes, DMSO extraction [13] and enzymatic processes [14], which depends on the final application (mono, oligo or polysaccharides). In this line, there are a lot of research works related to alkaline extraction of xylan from other agro-waste resources [15, 16]. The direct alkaline extraction of xylan with KOH, NaOH from less lignified annual plants such as bagasse pulp at milder temperatures appears to be favorable in order to obtain polymeric xylan [17, 18].Compared to KOH and NaOH, the xylan yield of the DMSO extraction is lower [14, 19, 20]. Xylan extracted by DMSO has the advantage of preserving the acetyl ester compounds and glycosidic linkages, which is desirable for structural studies [21]. Evtuguin et al. reported that into the acetylated xylan extracted from eucalyptus no significant difference in the position of acetyl groups occurred during the delignification process [22]. Nitren is also investigated for the production of dissolving pulp from paper-grade pulp and the additional recovery of polymeric xylans [17, 18]. It was found nitren extraction has the advantage of a much lower chemical charge (5–6%) compared to NaOH (10%) or KOH (14%) and the nitren extractions tend to result into xylan with higher ­DPw values compared to alkaline hydroxide ones [18] but the necessity of an effective removal of the residual nickel was disadvantage of aforementioned extraction [18]. Xylan-based compounds are valuable products used in a wide variety of applications, ranging from films to foams and food ingredients [23, 24].The potential application of pure xylan and its compounds can mainly be determined by physico-chemical properties such as composition (i.e. heterogeneity), degree of substitution, and molar mass [25]. In one hand, extraction of xylan from raw bagasse is challenging due to presence of lignin and some extractives and the other hand, this study contributes to the further utilization of bagasse pulp. It is necessary to note that Iran has an annual bagasse pulp production of 27,000 thousand tons for one existing company. For this reason, bleached chemical bagasse pulp was used as a pristine material for xylan extraction. Two extraction processes by sodium hydroxide and potassium hydroxide were employed. This report provides information on the extracted yield, composition, functional groups, Crystallinity Index ­(CrI) and Xylose/Glucose Ratios (XGRs) by two different alkaline agents.

Journal of Polymers and the Environment

by the following operational conditions: Soda pulping under cooking conditions of 15% Active Alkali (AA), digester pressure at 7  bar and cooking time of 15  min at maximum temperature 170 °C. Produced unbleached bagasse chemical pulp was afterward bleached by an ECF bleaching process using one step with sodium hypochlorite, NaClO (H sequence). Consequently, this pulp was then used as a pristine fiber for xylan extraction. Prior to the extraction process the starting pulp had the following carbohydrate compositions: 73.3% cellulose, 25.3% xylose, 0.8% arabinose and 0.1% galactose. The residual lignin and ash content of pristine fiber were also measured to be 0.11 and 0.39%, respectively.

Xylan Extraction Process The pristine bagasse fiber was disintegrated in distilled water and was centrifuged to approximately 75% dry content. Xylan extraction was carried out following to the different reaction conditions described into the Table 1 both sodium (NaOH) and potassium hydroxide (KOH) alkaline extraction. Each batch containing 20 g BSBB was used to xylan extraction in polyethylene bottles for different times at 30 °C on a temperature-controlled roller mixer. Extraction was conducted with three levels of alkali (6, 8 and 10% based on the oven dry pulp). The liquor-topulp ratio (L:P ratio) was selected as 20:1. The extract was separated from the pulps by vacuum filtration over sintered glass crucibles (G1). Afterwards, the pulp was washed with 2% sodium hydroxide, distilled water, acetic acid and hot water. The alkali extract was then precipitated in 1200 ml ethanol and the pH was adjusted to 6 with 10% acetic acid. The extracted xylan was purified through 5 times with 100 ml ethanol and also 3 times with 100 ml with ethyl ether.

Table 1  The xylan extraction conditions applied for alkaline treatments and their corresponding nomenclatures Alkaline concentration (%)

Extraction time (min)

NaOH

KOH

6

45 90 120 45 90 120 45 90 120

A1 A2 A3 B1 B2 B3 C1 C2 C3

D1 D2 D3 E1 E2 E3 F1 F2 F3

Materials and Methods Starting Pulp Characteristics The Bleached Soda Sugarcane Bagasse (BSBB) pulp was kindly provided as never dried by Pars Paper factory in HaftTapeh, Ahvaz, IRAN. According to information which obtained from the supplier, this kind of pulp was produced

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Fourier Transform Infrared Spectroscopy (FTIR)

Results and Discussion

The functional groups of extracted xylan were determined via IR spectrum of FT-IR spectrometer (Bruker Tensor 27, Bruker, Germany). The specimens were ground with KBr powder and pressed into pellets for aforementioned measurement in the wave-number range of 4000–400 cm− 1 at a resolution of 4 cm− 1. The output data was based on transmission.

Recovery Rate of Extracted Xylan

X‑ray Diffraction (XRD) X-ray Diffraction (XRD) profiles of extracted xylan with the different condition were taken using a STOE powder diffraction system (Darmstadt, Germany). The samples in powdered form (sub-micrometer particles) were irradiated by Cu (λ = 0.15060 nm) at 40 kV and 40 mA with a symmetric reflection geometry in the range of 2θ = 10°–40° with a step of 0.06°. The Crystallinity Index ­(CrI) of the specimens was evaluated by the method of Segal et al. [26] based on the Eq. (1): [ ] I − Iam Cr I(%) = 100 × 002 (1) I002

The recovered extract described in the Table 2 denotes the actual amount of gained extraction consisting mainly of dissolved cellulose and xylan because of other hemicelluloses such as low Degree of Polymerization (DP) ones i.e. arabinose and galactose were dissolved during the initial steps of alkaline extraction i.e. pulping process [7]. Figure 1 shows the NaOH consumption and extraction time variables on the recovery rate of xylan. As can be seen from this figure, the recovery rate displayed different behavior with reaction variables. In this regard, highest recovery rate was obtained with 6% of NaOH at 90 min, which could recover 55% of the initial pulp material. The recovery rate was decreased along with the highest the level of NaOH consumption (10%). 8 and 10% NaOH resulted into a recovered extract of 50 and 39% at reduced reaction time. Previously, Xu et al. reported that three extraction yields of 55.5%, 59.1 and 62.1% were acquired for 1M NaOH during 18 h at 20, 30 and 40 °C on

where ­I002 is the intensity of [002] reflection (2θ = 21°–23°) and ­Iam is the intensity of amorphous part of the samples (2θ = 18°).

The sugar content of extracted xylan was analyzed by High Performance Liquid Chromatography (HPLC, Smartline, Knauer, Germany). Eurokat H column (Knauer, Germany) was used as chromatography column. 0.01 N H ­ 2SO4 at the flow rate of 0.8 ml/min was used as eluent and oven temperature was set at 75 °C. Refractive Index (RI) detector was used for sugar detection.

Xylan recovery rate (g)

High Performance Liquid Chromatography (HPLC)

6% 0.8 0.75 0.7 0.65 0.6 0.55 0.5 0.45 0.4 0.35 0.3

45

8%

10%

90 Reaction Time (min.)

120

Fig. 1  The recovery rate of NaOH extractions in different conditions Table 2  The Recovery rate of extracted xylan at different conditions in terms of alkaline concentration and reaction times

Sample code

Recovery rate (g/g fiber)

Sample code

Recovery rate (g/g fiber)

A1 A2 A3 B1 B2 B3 C1 C2 C3

0.54 (0.015)* 0.55 (0.018) 0.53 (0.020) 0.52 (0.017) 0.46 (0.019) 0.51 (0.021) 0.43 (0.014) 0.43 (0.016) 0.39 (0.018)

D1 D2 D3 E1 E2 E3 F1 F2 F3

0.76 (0.025) 0.63 (0.028) 0.74(0.024) 0.59 (0.021) 0.64 (0.022) 0.65 (0.026) 0.69 (0.027) 0.68 (0.025) 0.73 (0.029)

*Values in the brackets are the corresponding standard deviation

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Journal of Polymers and the Environment

findings with NaOH [18]. This result was so interesting since Sixta et al. concluded that combined sodium alkaline pre and post-extractions leads to the isolation of polymeric xylan from eucalyptus [28]. Low recovery rate of some samples such as C ­ 3 and ­E1 can be ascribed to inaccessible xylan, which is strongly bound on the inner surface of cellulose fibril and could not be leached out of cell wall by diluted alkaline extraction [29].

Fourier Transform Infrared Spectroscopy (FTIR) Samples were characterized by Infrared (IR) spectroscopy in the 400–4000 cm− 1 frequency range. Regarding the characterization of bagasse xylan by FTIR spectroscopy some spectra is shown in Figs. 3, 4 and 5. In all spectra, some of typical bands for the native hemicelluloses the absorbance at 1638, 1580, 1414, 1268, 1170, 1045, and 898 cm− 1 present in the spectrums [30].The prominent band in 1044 cm− 1 attributed to the C–O–H bending in hemicelluloses. Also a sharp band at 897 cm− 1 was detected, which is typical of β-glycosidic linkages between the sugar units in hemicelluloses [31, 32]. The band 1164 cm− 1 is attributed to C–C and C–O–C stretching in hemicelluloses [33]. The peak of O–H

A3

D3

120 Transmittance (%)

bagasse pulp, respectively [15]. The best results in terms of xylan extraction was obtained with 8% NaOH compared to other level of NaOH consumption. Increasing the reaction time affected the recovered extract significantly specially at 10% of NaOH. The lowest recovery rate was reaped with 6% NaOH at 45 min of reaction time. It can be concluded that 8% of NaOH consumption at lessened reaction time was presumably able to extract more xylan from cellulosic surface and even between the cellulose fibrils. This is in accordance with results more recently reported by Djafari Petroudy et al. [7]. For the KOH alkaline extractions, the recovery rates were different from minimum amount of extracted xylan (59%) for nethermost level of KOH consumption to maximum ones (76%) by applying uttermost level of KOH. Figure 2 shows the influence of KOH dosage and extraction time on the recovery rate. It can be seen that, ascending trend was also detected by extraction with KOH unlike NaOH namely the recovery rate increased with increasing the KOH consumption and also increment of reaction time. Also, increasing the extraction time of KOH from 90 to 120 min increased the recovery rate to uppermost level i.e. 73%. Overall, both aforementioned KOH variables increased the recovery rate. The highest recovery rate was achieved with 6% KOH at 45 min and there were significantly differences between varying levels of KOH at this reaction time. This can be ascribed to this fact which KOH is not capable to dissolve xylan at low reaction time and its ions ­(K+, ­OH−) needs adequate time to penetrate the cellulosic structures [27]. An egregious difference of recovery rate at prolonged reaction time with KOH could clearly be seen from Fig. 2. In summary, KOH extraction process was found to be more effective than NaOH in terms of extracted xylan. As shown in Table 2 in order to obtain higher recovery rate, 6% KOH at 120 min of reaction time can be recommended. It is worth to mention that Janzon et al. also reported the similar

100 80 60 40 20 0

400

1400

2400 Wavenumber (Cm-1)

3400

Fig. 3  IR spectra of extracted xylan with 6% NaOH and KOH at 120 min 0.8 0.75 0.7 0.65 0.6 0.55 0.5 0.45 0.4 0.35 0.3

8%

10% B2

100 80 60 40 20 0

45

90 Reaction Time (min.)

120

Fig. 2  The recovery rate of KOH extractions in different conditions

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E2

120 Transmittance (%)

Xylan recovery rate (g)

6%

400

1400

2400 Wavenumber (Cm-1)

3400

Fig. 4  IR spectra of extracted xylan with 8% NaOH and KOH at 90 min

Journal of Polymers and the Environment C1

A1

120

900

100

700

80 60 40

600 500 400 300 200

20 0

D1

800 Intensity(a.u.)

Transmiance(%)

F1

100 0

400

900

1400

1900 2400 2900 Wavenumber (Cm-1)

3400

3900

10

15

20

25 2ϴ(degree)

30

35

40

Fig. 5  IR spectra of extracted xylan with 10% NaOH and KOH at 45 min

Fig. 6  X-ray diffraction pattern of extracted xylan with 6% NaOH and KOH at 45 min

stretching as a broad band at 3400 cm− 1 (3000–3600 cm− 1) can be monitored to determine the cellulose and hemicellulose [31]. A main adsorption band at about 3400 cm− 1 can be attributed to glycosidic groups in extracted sugars. Briefly, the main bands of extracted xylan were summarized in the Table 3 according to Da Costa Lopes et al. [34].

Similarly, the boarder peak of NaOH extracted xylan took place at B ­ 1 (See Table 4, for more information and Fig. 8) condition with 19.8% crystallinity. Although better xylan extraction in terms of crystallinity can be obtained at 6% NaOH consumption. Low level of NaOH consumption was yielded amorphous structures whereas high level of KOH consumption (10%) was resulted less crystalline structure. Also, there are no explicit correlation between increasing of extraction time and crystallinity both NaOH and KOH processes. Since calculated ­CrI for all extracted xylan were less than 30% (Table 4) so as expected, this polymer is well known as an amorphous polymer [35, 37–41] due to the heterogeneity of their chemical constituent. In this connection, Ahvenainen et al. have categorized xylan as an amorphous polymer along with lignin and amorphous cellulose [41]. Although Gabrielii et al. reported that xylan film extracted from type of aspen was crystalline and they have addressed this result to be a consequence of the lack of O-acetyl groups [42]. In this regard, Park et al. indicated that increased amorphous section will be the main contributor to XRD peak broadening [43]. However, in addition amorphous content, there are other intrinsic factors that influence peak broadening, such as crystallite size and non-uniform strain within the crystal.

X‑ray Diffraction The most important nonwood hemicellulose is xylan to about 80–90% of the nonwood hemicelluloses [7, 35]. Figures 6 and 7 present the X-ray diffraction pattern of extracted xylan from bleached soda bagasse pulp. The broad peak at 20° indicates some short-range order in the amorphous polymeric structure of the hemicelluloses [36] and also the peak broadness from 20 to 22° can clearly be seen. This peak indicates most of crystalline cellulose was removed during the extraction process. Here, there is no indication for cellulose Iβ peaks i.e. 2θ = 15°–16°, 22°–23°, and 34°–35° [6, 37]. Figure 6 belongs to extracted xylan XRD profile at minimum level of alkaline consumption i.e. (6%) of NaOH and KOH. Interestingly, The KOH alkaline extraction created broader peak at the 20 to 22° compared to NaOH and can be concluded that this peak represents amorphous peak of xylan [38].

Table 3  Characteristic of FTIR absorption bands for cellulose and hemicelluloses Absorption ­(Cm− 1)

Description

Cellulose

Hemicellulose

1061–1066

C–O–C ether linkage of the skeletal vibration of both pentose and hexose unit contribution Contribution of C–O strtching and C–O–C glycosidic linkage in xylan Arabinosyl side chain

*

*

– –

* *

1043–1049 993–998

*Indicates the presence of the FTIR absorption band in the polymer

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Journal of Polymers and the Environment F1

%Glucose

C1

Intensity (a.u.)

1200 1000 800 600 400 200 0

10

15

20

25 2ϴ(degree)

30

35

40

Fig. 7  X-ray diffraction pattern of extracted xylan with 10% NaOH and KOH at 45 min

Extract compositions (%)

1400

70 60 50 40 30 20 10 0

A1

A2

Sample code

CrI%

A1 A2 A3 B1 B2 B3 C1 C2 C3

24.34 23.10 24 19.78 26.76 27.26 36.03 30.43 32.31

D1 D2 D3 E1 E2 E3 F1 F2 F3

23.99 30.07 25.04 25.42 27.37 26.10 25.72 23.44 30.69

F2

Intensity (a.u.)

1000 800 600 400 200 10

15

20

25 2ϴ (degree)

30

35

40

Fig. 8  X-ray diffraction pattern of extracted xylan with 8% NaOH at 45 min and 10% KOH at 90 min

HPLC Results Standard HPLC method was used to quantify the relative xylose content in the extracted sugars. Based on the monosaccharide composition of the extracts (can be shown in Figs. 9, 10) two mono-saccharides i.e. xylose and glucose, were determined. Thus, cellulose degradation was also

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C2

C3

90 80 70 60 50 40 30 20 10 0

D1

D2

D3

%Xylose

E1 E2 E3 KOH Extraction

F1

F2

F3

Fig. 10  Extracted carbohydrate compositions percent with KOH

B1

1200

0

B1 B2 B3 C1 NaOH Extraction

%Glucose

Extract compositions (%)

CrI%

A3

Fig. 9  Extracted carbohydrate compositions percent with NaOH

Table 4  Calculated ­CrI at varying condition of alkaline extraction by NaOH and KOH Sample code

%Xylose

taking place during both alkaline treatment. It was observed that as the concentration of alkali increased, there was a decrease in xylose content. The highest xylose content was explored with lowest NaOH consumption (6% at 90 min) and applying higher concentration of NaOH may be yielded more degradation of cellulose and resulted higher content of glucose with HPLC. From Fig. 10, it can be concluded that, there are no significantly differences between the xylose content of 6% NaOH at prolonged time (from 45 to 120 min). The lowest xylose content was measured to be 39% in the sample code of ­C3. Figure 10 also shows that a decrement trend of xylose content in extracted with an increment of NaOH concentration. This result implies that the residual xylan is not easily accessible to hydrolysis by NaOH and it (inaccessible xylan) binds tightly with cellulose fibrils. Teleman et al. concluded that the xylan of the bleached eucalyptus pulp was recognized in two forms: (1) xylan easily accessible to NaOH, which appears either in a free form or on the surface of cellulose microfibril; and (2) xylan inaccessible to base solution, which is strongly bound on the inner surface of cellulose fibril [29].

Journal of Polymers and the Environment NaOH

KOH

3.5 3 XGR

2.5 2 1.5 1 0.5 0

A1-D1 A2-D2 A3-D3 B1-E1 B2-E2 B3-E3 C1-F1 C2-F2 C3-F3 Alkaline Extractions

Fig. 11  Calculated Xylose-Glucose ratios (XGRs) during the bagasse xylan extraction

While with KOH, the xylose contribution into the dissolved substance was higher than glucose (higher XGR) so that, the maximum percentage of produced xylose was 75.8 (XGR = 3.1) when 6% KOH used for 45 min. of reaction time (sample code of D1 in Fig. 10) although similar results were yielded in higher level of KOH consumption i.e. sample codes of D3, F1 and F3. It would be interesting to mention that 8 and 10% KOH eventuated lower concentration of xylose in the extract and these concentrations need extended reaction time to dissolve the xylose. Eventually, lowest xylose concentration with KOH was found to be 59% which is more than that of NaOH. On the other hand, Timell (1967) addressed that the KOH is often preferred over NaOH because it has shown to give maximum xylan yield with minimum contamination from glucomannan and hardwood xylan could be isolated by use of the KOH xylan extraction method to a maximum yield of up to 80% [44]. Based on the aforementioned HPLC results, the XyloseGlucose ratio (XGR) has been calculated (Fig.  11). The dissolved xylose decreased with level of NaOH consumption increased and the amount of degraded cellulose significantly increased at different time of reaction. For example, the minimum XGR was 0.64 which obtained at maximum level of NaOH consumption and reaction time (See sample code C3 in Fig. 11) however, the best xylose extraction result was evaluated with XGR of 3.1 during the KOH extraction process. As a result, the current study was demonstrated that xylan extraction with KOH had higher effectiveness rather than NaOH.

Conclusion The present study concentrates on increasing the value addition to underutilized agro-industrial waste such as bagasse. For this purpose, we demonstrated the effect of different variables of two conventional alkali extraction process i.e. on the extracted xylan properties. Bleached soda bagasse pulp was reacted with NaOH and KOH at different time

and alkali concentrations. Advanced methods such as High Performance Liquid Chromatography (HPLC), Fourier Transform Infrared Spectroscopy (FTIR) and Wide Angle X-ray Spectroscopy (WAXS) were employed to characterize the carbohydrate compositions, functional groups and crystallinity index of extracted materials. We conclude that the extracted xylan with both of two reaction processes displayed an amorphous structure. KOH had higher effectiveness than NaOH in terms of extracted xylan purity. Highest XGRs (Xylose to Glucose Ratios) were also achieved by KOH process. Thereby, the current attempt to value addition of sugarcane bagasse has immense importance for production of certain novel materials and chemical conversion industries under biorefinery concept. Acknowledgements  The authors wish to thank the Dezful Branch, Islamic Azad University for the financial support and also Shahid Beheshti University (SBU) technicians for their skillful laboratory work. The authors would like to thank all the editor and reviewers for their comments in the development and improvement of this paper.

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