Wood Sci Technol DOI 10.1007/s00226-015-0764-2 ORIGINAL
Fr actionation of or ganic substances fr om the South Afr ican Eucalyptus grandis biomass by a combination of hot water and mild alkaline tr eatments Jonas K. Johakimu 1 • Andr ew J erome1 Br uce B. Sithole1,2 • Lekha Prabashni1
•
Received: 11 April 2015 Ó Springer-Verlag Berlin Heidelberg 2015
Abstr act An alternative way of fractionating lignocellulose biomass into its individual components, hemicelluloses, lignin and cellulose, was investigated. South African Eucalyptus grandis wood chips were fractionated using a combination of hot water and alkaline treatments with or without AQ. Initially, the biomass samples were treated in hot water to remove hemicelluloses. At optimum prefraction conditions, the data acquired revealed that almost 12 % of the E. grandis wood biomass could be recovered as hemicelluloses. When the hemicelluloses preextracted biomass was further treated using sodium hydroxide with or without AQ, the data indicated that the amount of lignin and cellulose that could be recovered was 22 and 36 %, respectively (as % of the wood mass). The substrate was characterised by a higher amount of a -cellulose (91–93 %), lower kappa no (12–13), viscosity (327–450 g mg/L) and DP (1078–1536). It was then presumed that such pulp could meet end-user requirement of the dissolving pulps. Industrial acceptance of this biomass fractionation concept, however, will further require careful assessments of various options for treating and purifying the hemicelluloses and lignin in their respect streams.
&
Jonas K. Johakimu
[email protected]
1
Forestry and Forest Products Research Centre, Council of Scientific and Industrial Research of South Africa, P.O. Box 17001, Congella, Durban, South Africa
2
Discipline of Chemical Engineering, University of Kwazulu-Natal, Mazisi Kunene Rd, Glenwood, Durban 4041, South Africa
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Intr oduction Wood biomass is abundant not only in South Africa but also in many other countries of the world. This renewable resource represents an alternative and cheap feedstock for production of value-added products such as chemicals and biofuels (Hamelinck ¨ hman 2006). Typical wood biomass contains valuable organic et al. 2005; O substances, notably cellulose, hemicelluloses and lignin (Christensen 1998; Girio et al. 2008). These organic substances, with appropriate fractionation technologies, can be separated and each individual component can be converted to valuable products similar to those obtained from fossil oil in the petroleum industry ¨ hman 2006). This implies that if appropriate wood (Hamelinck et al. 2005; O biomass fractionation strategies are implemented, these organic substances may make an important contribution in helping relieve the world’s dependency on fossil oils. In current practices, only cellulose is exploited for value-added applications, e.g. wood-based pulp production in pulp and paper industry. Consequently, there are limited economical down streams for processing hemicelluloses or lignin to value¨ hman 2006; Girio et al. 2008). Furthermore, implementation of added products (O the biorefinery concept based on the existing pulp and paper industry seems to be dependent on specific mill operating conditions and also may be limited to a few mills. This has necessitated the need for new strategies that will enable optimal utilisation of the organic substances present in the lignocellulosic biomass (Zheng et al. 2009; Europe 2020 strategy, AFORE programme 2014). It is therefore envisaged that the success of such initiatives will open new economic streams as the wood biomass will be accessible not only in the pulp and paper sector, but also in other economic sectors such as in the chemical industries. For example, the cellulose rich stream can be used for specialty pulp applications, nano-crystalline cellulose, lactic acid, ethanol etc., whereas the hemicelluloses can be used in production of specialty chemicals (e.g. oxygen barriers for food packaging, xylitol and furfural or furfural derivatives), and/or biofuel (butanol or ethanol). Lignin has potential applications in carbon fibre, dust suppressants, phenolic resins technologies, etc. Wood biomass, however, has complicated structural and compositional features which negatively affect the yields and the purities of these organic substances upon separation (Hamelinck et al. 2005; Girio et al. 2008; Xiao et al. 2011). For instance in the cell wall, lignin forms a shield around the hemicelluloses and cellulose. In addition, part of the lignin is covalently linked to the carbohydrates forming lignincarbohydrates complexes (LCC). Since the transformation of these organic substances into value-added products such as biofuel or chemicals are largely dependent on their purity (Mosier et al. 2005; Zheng et al. 2009; Yoon and Van Heiningen 2010), it is necessary that a high level of selectivity is achieved during biomass fractionation, which in turn will guarantee acceptable yields and purities in each fractionated stream. The main challenge has been developing technologies that are technically effective and economically viable on an industrial scale (NNFCC Project No 10/003
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2009). Typically, in existing pulping technologies such as acid bisulphite and kraft processes, sulphur compounds are incorporated in chemicals used to break up the wood materials, resulting in the production of spent liquor that is contaminated with sulphur. Spent liquor contaminated with sulphur is an environmental nuisance. Moreover, organic substances recovered from such system have limitation in conversion to value-added products. For example, lignin cannot be post-processed in systems that involve catalysts (NNFCC Project No 10/003 2009). Several studies were conducted to find alternative sulphur-free cooking liquor. Amongst others these alternative technologies include organosolv and alkaline cooking liquors that are fortified with pulping aids (Christensen 1998; Testova et al. 2014). Such pulping aids include anthraquinone (AQ), NABH4, surfactants, urea. However, at industrial scale, only soda ? AQ pulping is widely applied. Its application, however, is currently limited to cellulose pulp production (Christensen 1998; Testova et al. 2014). It is therefore important that the potential of the soda ? AQ process alkaline is explored in the context of biomass fractionation to recover all organic substance. This biomass fractionation strategy could be easily implemented using capital equipment currently used in pulp and paper mills. This would be an advantage over the other competing technologies, e.g. organosolv technologies. However, biomass fractionation strategies involving soda ? AQ process would need to differ from that used in cellulose pulp production. This is because unit operations involving recovering and purifying the organic substances in their respective streams need to be integrated with the traditional soda ? AQ pulping process. Furthermore, the potential application of the products delivered from such process needs to be defined. Previous studies have reported on different biomass pre-treatment strategies prior to processing wood biomass for either ethanol or pulp production. When hot water is used for pre-treatment, the cell walls in the wood biomass are disrupted and consequently the pre-treated biomass responds favourably in the subsequent downstream processes (Cheng et al. 2010; Johakimu and Andrew 2013). More importantly, during hot water pre-treatments a high proportion of the hemicelluloses can also be solubilised, this is because hemicelluloses are easily degraded into oligomeric and monomeric sugars, whereas lignin and cellulose remain insoluble (Christensen 1998; Sixta and Schild 2009). Thus, hot water pre-treatment can also be used to remove and recover hemicelluloses which can be used as a separate economic stream. Nabarlatz et al. (2007) and Boussarsar et al. (2009) studied the influence of hot water treatment processes on treating biomass destined for ethanol production and found that substantial amounts of hemicelluloses could be removed from the biomass. Similarly, Tunc and Van Heiningen (2008) studied the extraction of hemicelluloses from wood biomass prior to pulp production; however, this strategy was not beneficial due to excessive pulp yield loss in the hemicelluloses extracted pulps. This is due to the fact that conditions known to promote extraction of hemicelluloses from wood also facilitate the degradation of cellulose during posttreatments in alkaline liquor (Tunc and Van Heiningen 2008; Garcı´a et al. 2011). To avoid this drawback, it is imperative that the alkaline cooking liquors used in the post-hemicellulose extraction are fortified with cellulose protector.
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Thus, instead of processing the remaining solid residue into either ethanol or pulp as done in previous studies, the lignin and cellulose rich stream can further be fractionated using soda ? AQ process. Soda ? AQ process is one of the alkaline sulphur-free technologies in which the sodium hydroxide is used as the source of the cooking liquor. The role of AQ is to protect cellulose against the peeling reactions (Christensen 1998; Gullichsen and Fogelholm 1999). During soda ? AQ treatments, lignins are solubilised and most of cellulose can be retained (Christensen 1998). The dissolution of lignin in alkaline solvent involves cleavage of the ester bonds (Christensen 1998; Gullichsen and Fogelholm 1999). At the same time, cellulose swells and the degree of polymerisation and crystallinity are reduced. It is also worthwhile to note that soda ? AQ process requires high alkaline concentrations and as a result cause cellulose cleavage reactions that are detrimental to the cellulose yields. It is therefore envisaged that a process that avoids high alkaline concentrations whilst maintaining effective removal of lignin would yield substrate that is richer in cellulose. Previous studies on soda ? AQ pulping have shown that limiting the alkali concentration to about 20 g/L as Na2O during the cook offers good retention of cellulose (Christensen 1998; Gullichsen and Fogelholm 1999; Sturgeoff and Pitl 1993). Alternatively, mild treatments, e.g. using lower cooking temperatures can also be used to minimise the negative effect on the cellulose yield (Christensen 1998; Sturgeoff and Pitl 1993). In the present work, fractionation of the lignocellulose biomass into its individual components hemicelluloses, lignin and cellulose using a combination of hot water and mild alkaline treatment was investigated. South African Eucalyptus grandis wood chips were used as source of raw materials. Initially, the biomass samples were treated in hot water to recover the hemicelluloses, and this process was called a pre-fractionation process. In the subsequent process ‘‘post pre-fractionation’’ stage, separation of lignin and cellulose from the hemicelluloses pre-extracted biomass was accomplished by performing mild alkaline treatments using sodium hydroxide with or without AQ. Mild alkaline treatment was preferred as a means of minimising the negative effects of the soda ? AQ process. As it will be seen later, the alkaline solution and the cooking temperature were kept in the lower range. The proposed concept is based on the existing pulping technologies. However, it is a completely new biomass processing strategy with the objective of recovering all organic substances and beneficiate into various valuable products.
Mater ials and methods Mater ials Eucalyptus grandis woodchips obtained from a kraft pulp mill in South Africa were used in this study. The woodchip samples were screened using a vibrating screen to remove under- and over-sized chips, knots, and bark. Air-dried chips with an average thickness of 3–8 mm were collected and stored in plastic bags for the subsequent experiments. Chip moisture contents were determined according to TAPPI method T258 om-94.
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In order to determine the chemical composition of the wood chips, they were ground into sawdust in the range of 40–60 meshes using a Wiley mill. Klason lignin was determined according to TAPPI method T222 om-88. The sugars were analysed using HPLC as reported in previous studies (Cheng et al. 2010; Johakimu and Andrew 2013). The data were used for evaluating the mass balance and/or the recovery rate of the organic substances in the various fractionated streams. Pr e-fractionation exper iments Hot water treatments In these experiments, water was used as the solvent. All the experiments were performed in a 7-L rotating digester. Wood chips were charged in digester at a rate of 600 g (oven dried equivalent mass) per each run. The water to wood ratio was maintained at 4.5:1. Thereafter, the digester was electrically heated up to the maximum treatment temperature of 170 °C at a constant ramping rate of 1.6 °C/ min. The reaction times at maximum temperature were varied between 15 and 90 min. At the end of each pre-fractionation experiment, free spent liquor was drained out and collected from each experiment. A portion of the spent liquor was collected and stored at 4 °C until required. The determination of sugars in the extract was performed using HPLC. Prior to analysis, the pH of the extract was first adjusted to between 5 and 6 using 6 mol/L HCl, and then the sugars were hydrolysed via heating with 4 % H2SO4 at 121 °C for 1 h. For sugar yield calculations, the HPLC results for arabinose, galactose, glucose, xylose, and mannose were corrected for arabinan, galactan, glucan, xylan, and mannan (Yoon and Van Heiningen 2010; Janson 1974). The measured extract volume was used to estimate the amount of hemicelluloses extracted. This was based on the monosugars contents (mg/L) which were determined by HPLC analysis: C ¼ Glu
162 180
Man 162 b 180
ð1Þ
where b = 1.6 (as an average value for number of mannose units per glucose in hemicelluloses of hardwood), Glu = concentration of glucose (mg/L) and Man = concentration of mannose (mg/L) as determined by HPLC analysis. H ¼ ðArab þ xylÞ
132 162 þ ðGal þ Glu þ ManÞ 150 180
ð2Þ
C
Arab ? xyl = sum of the concentration of arabinose and xylose (mg/L), and Gal ? Glu ? Man = sum of the concentration of galactose, glucose and mannose (mg/L) as determined by HPLC analysis. All the remaining solid residues (pre-treated woodchips) were defiberised using a disc refiner equipped with defibration plates. The defiberised pulp samples were spin-dried to remove excess water, weighed, and stored in plastic bags at 4 °C until further use. The wood loss was evaluated gravimetrically. The solid residue
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obtained after performing the pre-fractionation was expressed as a percentage of the wood mass charged into the digester (on an oven dry basis). Sugars and Klason lignin in the remaining solid residue were analysed using similar procedures as discussed previously. Post pre-fractionation experiments Mild alkaline treatments Selected pre-hemicelluloses extracted samples that gave a relatively higher yield of hemicelluloses were chosen for the post pre-fractionation studies. All post prefractionation experiments were performed in the same rotating digester that was used for the pre-fractionation stage. The only exception was that the free spent liquor from each pre-fractionation experiment was drained out prior to performing the post pre-fractionation experiment. A sodium hydroxide solution of 110 g/L (as Na2O) was prepared and used as the source of alkaline liquor. These samples were then treated at 150 °C using various sodium hydroxide dosages (12–20 %) with or without AQ. The AQ was added to the alkaline liquor and the dosage was maintained at 0.08 % (on oven dried woodchips weight). The AQ in a dispersed form was supplied by Buckman laboratory SA. The liquor to wood ratio was maintained at 4.5:1, whereas a constant reaction time of 2 h was used for all experiments. At the end of each post pre-fractionation experiment, a portion of the spent liquor was collected and stored at 4 °C until further use. Thereafter, the pulp slurry was washed using deionized water, spin-dried and also stored at 4 °C until required. The amount of sugars and lignin in the spent liquor and in the remaining solid residue was quantified using the same protocols as discussed earlier during the prefractionation experiments. However, the carbohydrate compositions were not corrected with the Janson formula. In addition, substrate properties; kappa, viscosity and solubility were characterised using TAPPI standard methods. In particular, the solubility data in terms of degraded cellulose were used to calculate the content of the cellulose retained (a -cellulose). The degree of polymerisation (DP) was calculated as follows (Sun et al. 2005): DP0.9 = 1.65X [g] mg/L, where g refers to CED viscosity CED values. Lignin was recovered from the extract by acidic precipitation. The extract was first filtered to remove suspended solids. Thereafter, a 50 mL portion of the filtrate from each sample was transferred into a 150-mL beaker. The sample was stirred at a constant rate using a magnetic stirrer plate. A few drops of 6 M sulphuric acid were added until the desired pH was reached. The reaction time was set for 2 h. The solution was then filtered and washed with deionized water. The weight of precipitated lignin was determined gravimetrically, by calculating the difference between the mass of lignin obtained after precipitation and mass of dried lignin. The resulting mass was thereafter used to determine the concentration of lignin in the extract (i.e. lignin concentration (g/L) = (X0 - Xd)/VL), where X0 and Xd are the mass of lignin (in g) before and after being dried, respectively, and VL is the volume of filtrate used, in L.
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Str uctural changes in substr ate biomass and lignin Structural changes in the substrate biomass and lignin were characterised using field emission gun scanning electron microscopy (FEG SEM) and FT-IR Spectrometer. FEG SEM tests were limited to untreated, pre-treated and post-treated samples. The oven dried samples were mounted onto aluminium stubs using carbon tape. The samples were sputter coated with gold using a Polaron SC500 sputter coater. Thereafter, the gold-coated samples were viewed at 5 kV using a Carl Zeiss FEG SEM. FT-IR tests were also conducted on the same samples as in FEG SEM, the only exception was that recovered lignin was also analysed. The FT-IR spectra were recorded from a Nicolet NETXUS 670 FTIR spectrometer. Samples were pretreated by tabletting the mixture of sample and KBr into a very thin film with a diameter of 5 mm. All the spectra were recorded in the absorbance mode from 4000 to 400 cm- 1 at room temperature.
Results and discussion Pr e-fractionation of hemicelluloses Effects of pre-fractionation conditions on wood loss and pH of the resulting extracts The wood loss and the pH of the resulting extracts are given in Table 1. The results showed that an increase in the severity of the pre-fractionation conditions by increasing the treatment time resulted in progressively increasing wood losses. Wood loss was in the range between 16 and 27 % for the pre-fractionation conditions examined in this study. Longer pre-fractionation time resulted in higher wood losses, indicating that more carbohydrates and lignins were solubilised. As reported in previous studies (Nabarlatz et al. 2007; Tunc et al. 2010), the resulting extracts were acidic (i.e. pH 3.0–3.2). This is because during hot water treatment, hydrolysis of the acetyl groups in the hemicelluloses occurs (Tunc and Van Heiningen 2008; Tunc et al. 2010). Hydrolysis of acetyl groups leads to formation of acetic acid which causes the pH of the resulting extract to drop to acidic levels. It has been shown in previous studies that acidic conditions lead to the degradation of carbohydrates and can result in excessive wood losses (Xiao et al. 2011). However, when pH buffering reagents are added to the water during hot water treatment, acid neutralisation occurs. As a result, using the appropriate buffering dosage, the pH can be maintained at a near neutral pH level (Mosier et al. Table 1 Wood loss and pH of the resulting extract
Treatment time (min)
Wood loss (%)
pH
15
15.7 ± 0.0
3.2 ± 0.2
45
24.0 ± 1.0
3.1 ± 0.0
60
25.2 ± 0.2
3.0 ± 0.4
90
27.1 ± 0.9
3.0 ± 0.0
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2005). However, the hemicelluloses yield is significantly lower when compared to hot water treatment (Mosier et al. 2005; Sixta and Schild 2009). Effects of pre-fractionation conditions on dissolution of the wood organic substances To ascertain the effects of the pre-fractionation conditions on these organic substances, chemical composition of raw wood and pre-fractionated wood samples were performed and the results are shown in Table 2, expressed as a percentage of the original wood weight. As anticipated, most of the short-chain carbohydrates (hemicelluloses) were solubilised during pre-fractionation; an indication that hemicelluloses depolymerize much faster than lignin and cellulose in hot water. As discussed earlier, similar findings have been reported in previous studies (Tunc and Van Heiningen 2008). However, in those studies, hot water treatment processes were used to treat biomass destined for either ethanol production or extraction of hemicelluloses from wood biomass prior to pulp production. Therefore, despite the fact that different approaches were used, hot water treatment shows uniqueness in removing hemicelluloses. This finding appears to suggest that biomass fractionation strategies could be designed to use this advantage by removing the hemicelluloses in earlier stages of the biomass fractionation process. Indeed, this practice has been used in production of ‘‘hemicelluloses-free’’ dissolving pulps where a hot water process step ‘‘steam pre-hydrolysis’’ is employed specifically for removing hemicelluloses (Christensen 1998). It is also worthwhile to note that the removal of hemicelluloses from wood requires that covalent (ester and ether) bonds which link the hemicelluloses mostly to lignin are broken out. In contrast, it is only the ester bonds that are easily cleaved, whilst the ether linkages remain stable (Christensen 1998). This implies that it is almost impossible to remove all the hemicelluloses in pure form.
Table 2 Chemical composition of the wood residue, % of original wood mass Treatment time (min)
Sugars Arabinan (%)
Wood
Total sugars Galactan (%)
Glucan (%)
Xylan (%)
Mann (%)
Klason lignin (%)
0.17 ± 0.1
1.05 ± 0.0
51.4 ± 0.3
11.9 ± 0.1
2.2 ± 0.0
66.7 ± 0.1
15
\ DL
\ DL
47.9 ± 0.4
7.1 ± 0.0
1.1 ± 0.2
56.1 ± 0.7
22.5 ± 0.0
45
\ DL
\ DL
49.5 ± 0.7
3.3 ± 0.3
0.7 ± 0.1
53.5 ± 0.3
20.8 ± 0,2
60a
\ DL
\ DL
50.1 ± 0.2
2.6 ± 0.1
0.2 ± 0.0
53.0 ± 0.1
20.6 ± 0.3
90
\ DL
\ DL
49.8 ± 0.3
2.3 ± 0.5
0.4 ± 0.0
52.5 ± 0.2
20.2 ± 0.5
\ DL denotes less than detection limits a
Lignin
This sample was selected for the subsequent ‘‘post pre-fractionation studies’’
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26.0 ± 0.4
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In particular, the arabinan and the galactan in pre-fractionated wood samples were not detected by HPLC, indicating that these hemicelluloses where completely removed from the wood and dissolved in extract. It can also be seen that the removal of wood sugars was more pronounced for the hemicelluloses (xylan and mannan) than for cellulose (glucan). The hemicelluloses solubilisation reached a maximum when a pre-fractionation time of 60 min was applied, i.e. approximately 82 % of the original hemicelluloses content was solubilised. At this pre-fractionation condition, the cellulose retained in the pre-fractionated wood biomass was almost 98 % of the original cellulose content, whereas the amounts of lignin removed were approximately 21 % of the original content or 6 % of the original wood mass. Presumably, some of the lignin was depolymerized and gave rise to phenolic compounds that are soluble in water (Cheng et al. 2010). Therefore, although the pre-fractionation process stage may only be intended to remove the hemicelluloses up front from the wood biomass, in practice, it may be necessary to recover the lignin. The amount of lignin removed, however, is relatively small when compared to the amounts that remained in the pre-fractionated wood. This could be an advantage, as it could simplify the downstream operation required for purification/separation of the lignin from the hemicelluloses stream. It can also be observed that solubilisation of the hemicelluloses was progressively as the severity of the pre-treatment conditions increased and reached the maximum level at 60 min. Thereafter, at longer treatment time (90 min) the concentration of hemicelluloses started to drop. It is presumed that longer treatment time leads to degradation of some of the hemicelluloses solubilised in the extract. This is especially true if one takes into consideration the acidic nature of the extract. Extract under acidic condition can lead to degradation of hemicelluloses and cellulose (Hamelinck et al. 2005; Girio et al. 2008). According to previous studies (Sixta and Schild 2009), hot water treatments produce extracts that are characterised by hemicelluloses which are in oligomer and monomeric form. However, the type of sugar present in extract was not investigated in this study. During pre-fractionation, most of the hemicelluloses are solubilised and may be further degraded and, as a result cannot be recovered. Therefore, it was necessary to determine the actual hemicelluloses that could be recovered from the extract. The method proposed by Janson (1974) was used to establish this data. The results for Table 3 Chemical composition in the extract, as % of original wood mass Treatment time (min)
Carbohydrates Cellulose (%)
Hemicelluloses (%)
Total carbohydrates (%)
15
0.64
6.30
45
0.60
9.10
9.70
60a
0.50
11.70
12.20
90
0.40
10.20
10.60
a
6.90
This sample was selected for the subsequent ‘‘post pre-fractionation studies’’
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Hemicellulose recovery rate (%)
the chemical composition in the extracts after the various pre-fractionation treatments are summarised in Table 3. The sugars dissolved in the extracts were mostly dominated by the hemicelluloses. This trend supported the sugar yield data observed in the hemicelluloses pre-extracted wood samples, in which most of the cellulose were retained (Table 2). The highest hemicelluloses yield obtained was 11.7 % of the wood mass. This is equivalent to a recovery rate of 76 % (based on original hemicelluloses content in the wood). Furthermore, the purity, defined as the ratio between hemicelluloses recovered, was determined (Table 3) and the total amount of the hemicelluloses solubilised in the extract (Table 2). The purity was approximately 93 %. Impurities content referred to the solubilised chemical compounds other than hemicelluloses which may include degraded hemicelluloses products (Hamelinck et al. 2005; Girio et al. 2008), such as furfural, hydroxymethyl furfural (5-HMF). The results for the hemicelluloses recovered were plotted against the prefractionation time (Fig. 1). It can be seen that the hemicelluloses recovery rate increased when pre-fractionation time was increased from 15 to 60 min and thereafter started to decrease. This may be attributed to the acidic nature of the extract that favours the degradation of the solubilised hemicelluloses (Hamelinck et al. 2005; Girio et al. 2008). As a result fewer amounts of hemicelluloses were available for recovering. For this reason, the pre-fractionation time of 60 min was selected as the optimal fractionation time at 170 °C and was used to prepare samples for the post pre-fraction studies. It is worthwhile to mention that practically, the amount of hemicelluloses extracted can be separated from the spent liquor (extract) by using a process such as membrane filtration. Sixta and Schild (2009) demonstrated that the ultrafiltration membrane with a pore size of 10 kDa, operating at 40 °C and a pressure of 200–800 kPa can be used to isolate and recover the hemicelluloses. In the current scale-up trials, this method was adopted.
15 13 11 9 7 5 3 1 10
25
40
55
70
85
100
Pre-fractionation time (Min) Fig. 1 Relationship between hemicelluloses recovery rate and pre-fractionation time
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Post pre-fractionation Because substantial amounts of hemicelluloses were removed from the wood biomass during the pre-fractionation treatments, in the subsequent experiments the strategy was to achieve effective separation of lignin and cellulose. Effects on substrates properties after post pre-fractionation The substrate yield was defined as the ratio between wood solid residues recovered after post pre-fractionation and the total amount of the wood originally charged into the digester. The biomass yield preserved after post pre-fractionation (hot water treatment followed by mild alkaline treatment [with or without AQ)] was in the range between 35 and 39 %, whereas the kappa no was in the range of 36–9, respectively. As shown in Table 4, an increase in the sodium hydroxide dosage resulted in a decrease in substrate yield as well as the kappa no, an indication that more lignin, residual hemicelluloses and some cellulose were degraded from the hemicelluloses pre-extracted wood biomass. Surprisingly, the kappa no. attained appears to be slightly higher than expected at a yield range of 35–40 %. Presumably, hot water treatments resulted in formation of condensed residual lignins. Condensed lignin could be formed as a result of the induced effects of the auto-hydrolysis, as the pH dropped to extremely lower values, i.e. all the pH were \ 10 (Table 1) required preventing the lignin condensation reactions (Christensen 1998; Gullichsen and Fogelholm 1999). Condensed lignins are known to be more resistant to delignification during alkaline treatments (Christensen 1998).
Table 4 Substrate yield obtained after post-treatments of the pre-hemicelluloses extracted wood samples, expressed as % of original wood mass Sample
Yield (%)
Kappa no
Viscosity g (mL/g)
DP
a -Cellulose (%)
Wood
100
NA
NA
NA
NA
HT60
75 ± 1.8
NA
NA
NA
NA
HT-MA12
37.7 ± 0.7
36 ± 0.9
NA
NA
NA
HT-MA12 ? AQ
38.3 ± 0.2
23 ± 0.4
NA
NA
NA
HT-MA16
35.9 ± 0.1
15 ± 0.2
327 ± 2.2
1078
91 ± 0.5
HT-MA16 ? AQ
36.3 ± 0.2
12 ± 0.5
450 ± 1.5
1536
93 ± 0.2
HT-MA20
35.1 ± 0.4
13 ± 0.1
NA
NA
NA
HT-MA20 ? AQ
35.1 ± 0.2
9 ± 0.5
NA
NA
NA
HT60 denotes hot water pre-fractionated sample at 60 min which was subsequently post-fractionated. HT-MA denotes hot water treatment followed by mild alkaline treatment, whereas the numbers 12, 16 and 20 refer to sodium hydroxide dosage used with or without AQ. Samples produced using 8 % NaOH with or without AQ were excluded because a substantial amount of the material remained uncooked. Presumably, this dosage was insufficient and as a result the delignification reaction was limited. NA denotes that no tests were done. DP denotes degree of depolymerisation
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The effect of AQ on protecting the cellulose against degradation during alkaline treatments was also observed when the sodium hydroxide dosage applied was limited to the range between 12 and 16 %. At these levels all samples produced using sodium hydroxide liquor ? AQ, exhibited an increase in substrate yield to almost 1 %. Considering that the residual lignins in the AQ samples were not at the same level as in the pulp samples produced without AQ (Table 5), the amount of hemicelluloses in substrate was almost negligible (0.3 %). Furthermore, the amount of hemicelluloses in substrate was almost negligible (0.3 %). This result tended to suggest that although AQ samples retained more cellulose, but at the same time retained relatively less amount of lignin. This is justifiable by the fact that although alkaline treatment with AQ showed the amount of cellulose retained was in the range of 1–2 %, the substrate yields were only increased by 1 %. It is therefore clear that the discrepancy in mass balance yield data was attributed to the amount of lignin that also was retained in the substrates. To reveal the impact on carbohydrates, the viscosity and a-cellulose properties were further investigated; however, these tests were limited only to HT-MA16 and HT-MA16 ? AQ samples. As it will be seen later, these samples were identified to be produced at the optimum post-fractionation conditions (Table 5). It can also be seen that when AQ was applied, pulp samples produced were characterised by a reasonable value of viscosity and retained a higher amount of a -cellulose with relatively high purity, i.e. contained less than 0.27 % of xylan (Table 5). The degree of polymerisation was also relatively reasonable (1536). These data also support the substrate yield and cellulose data in Tables 4 and 5, respectively. It was then presumed that such pulp could meet the end-user requirements of the dissolving pulps (Sixta and Schild 2009).
Table 5 Chemical composition after post-treatment of the pre-hemicelluloses extracted wood, as % wood Sample
Sugars Arabinan
Lignin Galactan
Glucan
Xylan
Mannan
Wood
0.17
1.05
51.4
11.9
2.2
26
HT60
\ DL
\ DL
50.1
2.6
0.2
20.6
HT-MA12
\ DL
\ DL
33.0
0.6
\ DL
3.0
HT-MA 12 ? AQ
\ DL
\ DL
35.0
0.3
\ DL
1.8
HT-MA16
\ DL
\ DL
32.4
0.57
\ DL
1.7
HT-MA16 1 AQ
\ DL
\ DL
34.4
0.27
\ DL
0.6
HT-MA20
\ DL
\ DL
32.0
0.5
\ DL
1.3
HT-MA20 ? AQ
\ DL
\ DL
33.2
0.5
\ DL
0.3
HT-MA denotes hot water treatment followed by mild alkaline treatment, whereas the numbers; 12, 16 and 20 refer to sodium hydroxide dosage used with or without AQ. Bold samples were used for lignin precipitation studies \ DL indicates less than detection limits
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Effects on chemical composition after post pre-fractionation Post pre-fractionated solid residue samples contained predominantly cellulose with a small percentage of lignin remaining. This can clearly be seen from analysis of the chemical composition data in Table 5. The cellulose retained in the biomass was in the range of 62–70 % of the original cellulose content. As anticipated, alkaline treatment conditions with AQ exhibited a slight increase in the amount of cellulose retained (1–2 % higher than in alkaline treatment performed without AQ). At the same time more lignin was solubilised when AQ was applied (40–65 % higher than in the alkaline treatment performed without AQ). The lignin solubilised in the extract was in the range between 90 and 99 % of the original content, indicating that most of the a -benzyl ether linkages between lignin and hemicellulose were cleaved during the post-alkaline treatments (Pandey 1999; Yang et al. 2014). It appears that a substantial amount of cellulose can be retained, whilst at the same time more lignin can be solubilised when the sodium hydroxide dosage is maintained at 16 %. It is also very clear that lignin removal was also associated with cellulose degradation. The cellulose yield loss during alkaline treatment might also contribute to the induced effect of the auto-hydrolysis treatments (Tunc et al. 2010; Mosier et al. 2005). Auto-hydrolysis has been reported to create a whole flock of new end reducing groups (Tunc et al. 2010). Subsequently, during the alkaline treatments severe peeling reaction occurs. It was therefore presumed that these reactions attributed to the cellulose loss observed. Generally, the result suggests that alkaline treatment fortified with AQ may be an interesting technique for improving the selectivity during the separation of lignin and cellulose. However, the application of AQ as pulping aids may be restricted in the near future (Testova et al. 2014). Therefore, it may be of interest to use alternative pulping aids such as urea and surfactants as a substitute of AQ in the proposed biomass fractionation concept. Changes in the structural substrate biomass To reveal the structural changes, unfractionated, pre-fractionated (HT60) and alkaline post-fractionated samples (HT-MA16 and HT-MA16 ? AQ) were subjected to FEG SEM and FT-IR tests. FEG SEM and FT-IR have been shown to be reliable and versatile tools to investigate the structural changes in biomass, i.e. during biomass fractionation (Corrales et al. 2012; Sun et al. 2005; Yang et al. 2014). The FEG SEM micrographs in Fig. 2a–h show the structural changes in E. grandis wood fibres for unfractionated, pre-fractionated and post-fractionated substrate samples. Unfractionated fibres showed a rigid and compact fibre wall structure (Fig. 2a, b). Upon pre-fractionation with hot water pre-treatment, debris and residues were observed on the surface of the fibre (Fig. 2c, d). This debris could be described as molten lignin and some remaining hemicelluloses as discussed by Xiao et al. (2011). After the post-alkaline fractionation and irrespective of whether AQ was used or not, no residues or debris were observed on the fibre surfaces (Fig. 2e–h). The fibre surfaces of the post-treated samples had uneven folds and appeared more porous
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Fig. 2 Micrographs of E. grandis fibres a, b of unfractionated wood samples; c, d after pre-fractionation with hot water; f, e after post pre-fractionation without AQ and g, h after post pre-fractionation with AQ
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when compared with pre-fractionated and unfractionated wood fibres (Fig. 2e–h cf. Fig. 2a–d). Similar findings have been reported by Corrales et al. (2012) who studied biomass fractionation based on steam treatment in the presence of SO2 and CO2. They found that pre-treated bagasse biomass had a more porous structure than untreated ones. Pre-treatment disrupts the cell wall and thus opens up the biomass matrix. As a result, porosity or permeability increases. This phenomenon is not only critical for dissolution of hemicelluloses, but is also important in enhancing the removal of lignin in the subsequent processing steps (Hamelinck et al. 2005; Nabarlatz et al. 2007). The porous appearance of the fibre surfaces could be attributed to the removal of the hemicelluloses and lignin from the cell wall matrix (Fig. 2f, h). It can also be seen that the use of AQ resulted in more defibration of the fibre surfaces, exposing the micro fibrils on the fibre surfaces (Fig. 2h). Fibrillation of the fibre wall or opening of the cell wall matrix by the use of AQ resulted in a more efficient removal of lignin. This is also supported by the chemical composition data (Table 5). The FT-IR spectra of the biomass samples are shown in Fig. 3. The spectrum of the samples was quite similar, which indicates that the basic biomass structure was not changed during the biomass fractionation process. However, by comparing the intensity of the signals between raw wood and pre-fractionated (HT60), the O–H stretch band (at 3500 cm- 1) appears to disappear in the pre-fractionated samples. The O–H stretch band corresponds to the aliphatic moieties in lignin and polysaccharides (Yang et al. 2014). On the other hand, the shoulder at 1742 cm- 1 is attributed to the acetyl and uronic ester groups of the hemicelluloses and appeared to disappear after the hot water pre-treatment, indicating that most of these bonds were cleaved (Pandey 1999). The C–O band stretch band (1000 cm- 1)
100
Control (Sawdust) HT60
Transmittance / a.u.
95
1742
HT-MA16 HT-MA16 + AQ
90 85 80 75 1106
70 65 1054
60 4000
3500
3000
2500
2000
Wavenumber / cm
1500
1000
500
-1
Fig. 3 Impact on structural changes; raw wood, HT60-pre-fractionated sample, HT-MA16 and HTMA16 ? AQ post-fractionated without and with AQ, respectively
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shows deformation in cellulose and lignin present in the raw wood (saw dust) and in the hot water pre-treated samples (Pandey 1999; Yang et al. 2014). The noted partial structural changes upon hot water treatment were attributed to the induced effects of the hot water pre-treatments which resulted in opening up the cell structure (Cheng et al. 2010), as a result partial dissolution or oxidation of lignin and polysaccharides occurred. When the substrate samples collected after hot water treatments were further treated (post-fractionation), the O–H and C–O stretch bands were more evident, indicating deformation occurred in cellulose and lignin that remained in the substrate. A stretch band from the C–H (2890 cm- 1) indicating stretching vibration of methyl and methylene units can also be seen (Pandey 1999). It may be this signal that indicates breakdown of the lignin and it was more pronounced in the HT-MA16 samples than in HT-MA16 ? AQ samples, indicating that lignin in HTMA16 ? AQ sample was more fragmented and subsequently some lignin fraction was dissolved. The bands in the region between 1500 and 1400 cm- 1 show the aromatic C=C stretch from the aromatic ring of lignin (Pandey 1999), which indicated that the basic lignin structure was not much changed. It can also be seen that the intensity at C–O stretch band (1000 cm- 1) decreased for the HTMA16 ? AQ and HT-MA16 samples, an indication that most of the H-type lignin was dissolved during the post-treatments. The absorbance in the region of 1500–900 cm - 1 is associated with the typical cellulose values (Pandey 1999; Yang et al. 2014). Thus, it was concluded that partial structural changes occurred in the biomass and the effect was more pronounced during the post-alkaline treatments. Furthermore, by comparing sample produced with and without AQ, again the result confirmed that AQ enhanced the dissolution of lignin whilst protecting cellulose. Recover y of lignin During post pre-fractionation most of the lignin was solubilised, but some may have been degraded to the extent that they cannot be recovered. Therefore, an attempt was also made to quantify the amount of lignin that could possibly be recovered. In ¨ hman 2006), it was shown that effective lignin precipitation may previous studies (O be achieved when the pH is maintained at 10. However, these studies were based on the precipitation of lignin from kraft pulp mill black liquor. Therefore, it was critical to determine the optimum pH for lignin precipitation based on the approach used in this study. Preliminary trials were performed to determine the effect of pH on lignin precipitation. It can be seen in Fig. 4 that pH in the range of 2–4 favoured a higher lignin recovery. Therefore, a higher acid consumption in the precipitation process may be necessary as compared to acid precipitation carried out at pH 9, e.g. ¨ hman 2006). The lignin precipitation of lignin from kraft black liquor (O precipitation conditions were further optimised and a pH of 3.5 was selected for the subsequent studies, in which the selected samples HT-MA16 and HTMA16 ? AQ were further tested. The data presented in Table 6 indicate that approximately 76 % of the original lignin content could be recovered when a mild alkaline treatment with AQ is applied. The higher lignin recovery from the spent
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Concentration of the lignin (mg/L)
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2
3
4
5
6
7
8
9
10
pH of the extract Fig. 4 Effects of pH during lignin precipitation. The initial pH of the extract was 13.5
Table 6 Recovery of lignin from the post pre-fractionation extracts Sample
Lignin concentration (mg/L) in the extract
Lignin as % of wood
Lignin as % of the original content in the wood
Wood
n/a
29
100
HT-MA16
211
16
55
HT-MA16 ? AQ
290
22
76
liquor when AQ (HTMA16-AQ) was applied is due to the fact that liquor had a relatively higher amount of lignin than that which was found in the spent liquor of the treatment performed without AQ (HTMA16). Contrarily, pulp produced without AQ (HTMA16) has a relatively higher amount of lignin retained in pulp samples. This is clearly supported by the data in Table 5; for example, less lignin was retained in HTMA16-AQ pulp samples than in HTMA16 pulp samples (0.6 vs 1.7 %). In practice, the amount of lignin in the spent liquor can be isolated using filtration techniques such as applied in LignoBoost process. This method has been reported ¨ hman 2006). In the current scale-up trials, a lignin recovery process elsewhere (O using acidification followed by filtration process using a centrifuge instead of a filter press has been adopted. To reveal the structural changes, lignin recovered corresponding to the production of HT-MA16 and HT-MA16 ? AQ substrate samples were studied. The FT-IR spectra of the lignin samples are shown in Fig. 5. The bands in the region of 1700, 1595, 1511 and 1455 cm- 1 correspond to aromatic ring vibrations (Yang et al. 2014). The signal at 1700 cm - 1 indicates non-conjugated carbonyl group, either from carboxyl or the ester linkage of the lignin side chain (Yang et al. 2014). However,
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100 HT-MA16
90
Transmittance / a.u.
HT-MA16 + AQ
1700
80
1153 1322 1422 1511 1595 1455
70
1028
1212
60
1109
50 3500
3000
2500
2000
Wavenumber / cm
1500
1000
500
-1
Fig. 5 Impact on recovered lignin structural changes: HT-MA16 and HT-MA16 ? AQ post-fractionated without and with AQ, respectively
the lignin recovered from HT-MA16 exhibited lower intensities than HTMA16 ? AQ samples. It may indicate that most of these bonds in the lignin recovered from HT-MA16 ? AQ samples were cleaved and subsequently were broken down into lignin monomer units (Pandey 1999; Yang et al. 2014). An examination of converting hemicelluloses, lignin and cellulose into valueadded products for the wood biomass fractionated by using a combination of hot water and mild alkaline treatment was beyond the scope of this study and will be reported elsewhere [the paper entitled production of carbon fibres, polylactic acid and cellulose nano-crystals (CNC) is still in preparation].
Conclusion The overall objective of this study was to develop a better understanding of the feasibility of fractionating the lignocellulose biomass into its individual components, namely hemicelluloses, lignin and cellulose using a combination of hot water and mild alkaline treatments process steps. The approach adopted was to quantify the recovery rate of the organic substances in their respective streams. Furthermore, the impact on structural changes was studied. The results acquired have revealed that the amount of hemicelluloses, lignin and cellulose that could be recovered are 12, 22 and 36 %, respectively (as % of the
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wood mass), indicating that almost 70 % of the wood biomass could be recovered and used economically. The substrate was characterised by a higher amount of acellulose, lower kappa no and the viscosity and DP values obtained were also in a very reasonable range. It was presumed that such pulp could meet end-user requirement of the dissolving pulps. Industrial acceptance of this biomass fractionation concept, however, will further require careful assessments of various options for treating and purifying hemicelluloses and lignin in their respect streams. Such method may include application of the membrane filtration for recovering hemicelluloses and lignin recovery process that is similar to the LignoBoost process, which was beyond the scope of this study. It is also worthwhile to highlight that application of AQ as pulping aids may be restricted in the near future (Testova et al. 2014). Therefore it would be interesting to explore other sulphur-free reagents such as urea and surfactants as a substitute of AQ in the proposed biomass fractionation concept. Acknowledgments The authors wish to acknowledge CSIR for providing financial support which enabled the accomplishment of this research work (Project EIEB002). Special thanks are due to the technical staff at FFP laboratory for their assistance in performing the experiments.
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