PEO flocculation process to improve the lignin removal from the pre-hydrolysis liquor of kraft-based dissolving pulp production process

Bioresource Technology 102 (2011) 5177–5182

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A combined acidification/PEO flocculation process to improve the lignin removal from the pre-hydrolysis liquor of kraft-based dissolving pulp production process Haiqiang Shi a,b,⇑, Pedram Fatehi b,⇑, Huining Xiao b, Yonghao Ni b a b

Liaoning Key Laboratory of Pulp and Paper Engineering, Dalian Polytechnic University, Dalian 116034, China Limerick Pulp and Paper Centre, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3

a r t i c l e

i n f o

Article history: Received 18 December 2010 Received in revised form 21 January 2011 Accepted 24 January 2011 Available online 1 February 2011 Keywords: Pre-hydrolysis liquor Acidification PEO Biorefinery Lignin

a b s t r a c t The presence of lignin impairs the utilization of the hemicelluloses dissolved in the pre-hydrolysis liquor (PHL) of the kraft-based dissolving pulp production process. In this paper, a novel process was developed by combining the acidification and poly ethylene oxide (PEO) flocculation concepts to improve the lignin removal. The results showed that the lignin removal was improved by the addition of PEO to the acidified PHL, particularly at a low pH of 1.5. The main mechanisms involved are the lignin/PEO complex formation and the bridging of the formed complexes. This hypothesis was supported by the turbidity, FTIR and particle size measurements. Interestingly, the hemicelluloses removal from the acidification/PEO flocculation was marginal, which would be beneficial for the down-stream ethanol production from the PHL. Additionally, a process flow diagram was proposed that incorporates this new concept into the existing configuration of kraft-based dissolving pulp production process. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Today, the substitution of oil-based products with bio-based products is of great importance due to environmental concerns, lack and high cost of oil supply, and the limited availability of oil-based resources. The production of bio-based material/polymers from natural resources is another incentive for pursuing the biorefinery strategy. In the kraft-based dissolving pulp production process, hemicelluloses are removed from the wood chips in the pre-hydrolysis step (Li et al., 2010; Saeed et al., 2010). The pre-hydrolysis liquor (PHL) can be utilized in the production of various value-added products, e.g., ethanol and xylitol (Zhuang et al., 2009; Liu et al., 2011). However, the presence of other dissolved materials in PHL, e.g., lignin, hampers the utilization of the dissolved hemicelluloses for the production of ethanol or xylitol, for instance. Lignin is one of the main inhibitors of the ethanol production from hydrolyzed carbohydrates (Martinez et al., 2000, 2001; Hahn-Hagerdal et al., 2007; Zhu et al., 2010). It is partly converted to phenolic compounds during the hydrolysis stage and dissolved ⇑ Corresponding authors at: Liaoning Key Laboratory of Pulp and Paper Engineering, Dalian Polytechnic University, Dalian 116034, China. Tel.: +86 41186322799 (H. Shi), +1 506 452 6084 (P. Fatehi). E-mail addresses: [email protected] (H. Shi), [email protected] (P. Fatehi). 0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.01.073

in the PHL. The functional group (e. g., the phenolic group), molecular weight and hydrophobicity of lignin, are important parameters on the inhibition effect of lignin compounds in the PHL (Larsson et al., 1999; Palmqvist and Hahn-Hagerdal, 2000a,b; Klinke et al., 2004). It was reported that the low molecular weight phenolic compound is the most toxic element in the PHL for the ethanol production via fermentation (Palmqvist and HahnHagerdal, 2000b). Additionally, the functional groups and hydrophobicity of phenolic compounds can interact with enzymes (Palmqvist and Hahn-Hagerdal, 2000b), which hinders the enzymatic activity. Lignin can be used as a fuel source (van Heiningen 2006; Leschinsky et al., 2008), dispersing agent, emulsion stabilizer, or rheology control (Jonsson and Wallberg, 2009). It can also be used as a raw material for the production of value added products, e.g., phenols, carbon fibers, binder (van Heiningen, 2006). Therefore, the removal of dissolved lignin in the PHL is of great importance. We previously reported that acidification could remove the dissolved lignin from the PHL (Liu et al., 2011). However, the hydrolysis process conditions influence the molecular weight and structure of lignin dissolved in PHL, which in turn affects the lignin removal efficiency via acidification (Leschinsky et al., 2008). Thus, it is desirable to develop improved processes for the lignin removal. The interaction of PEO with lignin has been comprehensively studied in the past (Xiao et al., 1995; Takase and van de Ven,

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1996; van de Ven and Alince, 1996; Alince and van de Ven, 1997; Wu et al., 2007). It was claimed that the phenolic groups of lignin interacted with PEO and induced complexes (van de Ven and Alince, 1996; Gaudreault et al., 2005; Negro et al., 2005; Wu et al., 2007). These complexes promoted the deposition of clay, latex and fiber fines on papers (van de Ven and Alince, 1996; Takase and van de Ven, 1996; Wu et al., 2007; Wu and van de Ven, 2009). It was also evident that the presence of salt (particularly calcium ions) facilitated the interaction of PEO with lignin (Takase and van de Ven, 1996; Gaudreault et al., 2005). The PHL commercially produced from the kraft-based dissolving pulp production process contains lignin and salt (Liu et al., 2011; Saeed et al., 2010). Therefore, the application of PEO to the PHL may enhance the lignin removal through the formation and bridging of lignin/PEO complexes. Based on the above hypothesis, we proposed a novel process by combining acidification with PEO flocculation treatment for removing lignin of the PHL. The concept may be implemented in practice as one process unit or two discrete units. Additionally, the adoptability and integration of this process in the existing kraft-based dissolving pulp production process was described in this work. Alternatively, the utilization of acidification and PEO treatment was experimentally evaluated on industrially produced PHL. In the experimental evaluation, the effects of the process variables and PEO properties on the efficiency of lignin removal were systematically taken into account. Finally, the influence of acidification and PEO treatment on hemicelluloses removal of the PHL was also investigated.

2. Methods 2.1. Materials Poly ethylene oxide (PEO) with two different molecular weights, i.e., 100 k and 600 k, were purchased from Aldrich co. and dissolved in water (0.5 g/l) prior to using. KBr powder (analytical grade) was also received from the same company. The prehydrolysis liquor (PHL) was received from a mill located in Eastern Canada. According to the VisCBC technology for the kraft-based dissolving pulp production process, the pre-hydrolysis is performed using steam at 170 °C. Two displacement steps, first by the strong black liquor and then by the white liquor, are subsequently followed after the hydrolysis for extracting, degrading and removing the hemicelluloses from the wood chips. The feed stock of the mill is a mixture of 70% maple, 20% poplar, and 10% birch. The total cooking cycle is about 300 min, including chip filling, pre-hydrolysis, displacement, pulping and discharging.

2.2. Acidification and PEO treatment of PHL Approximately, 100 ml of the PHL sample was acidified using concentrated (98%) sulfuric acid to a pH of 3.5, 3.0, 2.5, 2.0 and 1.5, respectively, and kept for 30 min under stirring. Then, the samples were centrifuged at 2500 rpm using a laboratory centrifuge for 10 min. The filtrates were then collected for analysis and PEO treatment. In one set of experiments, the concentration of PEO with the MW of 600 k was maintained at 350 mg/l in PHL solution at various pHs, and kept for 10 min at 120 rpm. The samples were then filtered and the lignin and COD of the filtrates were assessed. In another set of experiments, the concentration of PEO with a MW of 600 k or 100 k was maintained at 275 mg/l in the PHL at various pHs and the turbidity of the PHL samples was analyzed afterwards. Subsequently, the lignin and hemicelluloses removals from the PHL were assessed.

2.3. Turbidity, particle size and FTIR analyses The turbidity of the PHL samples before and after the PEO treatment was determined using a HACH 2100AN Turbiditmeter (Colo, USA) at room temperature. Similarly, the particle size of the formed lignin/PEO complexes in the PHL was measured by using a Brookhaven ZetaPlus Particle Size analyzer (Holtsville, NY, USA) operating with the software of 90plus/BI-MASS. The scattering angle and operating wavelength were 90° and 658 nm, respectively. The analysis was conducted automatically to yield the mean diffusion coefficient. Then, the apparent hydrodynamic sizes of the polymers and formed complex were assessed from the Stokes– Einstein equation (Buchhammer et al., 2003; Fatehi et al., 2010). An average of three testing results was reported. The PHL and PEO solutions were filtered using a 0.45 lm Nylon syringe filter prior to the PEO addition and the particle size analysis. Additionally, the acidified lignin, PEO powder and the complexes formed at 275 mg/l PEO concentration in the PHL (at pH 2) were collected for FTIR analysis via employing a Fourier Transform Infrared Spectroscopy (FTIR) (Perkin Elmer Spectrum 100 FTIR Spectrometer, USA). The analysis was conducted via embedding the samples in KBr pellets in a mixture of about 1% (wt.). The spectra were recorded in a transmittance mode in the range 800–3800 cm 1. 2.4. Lignin and COD analyses The lignin content of the samples was determined based on the UV/Vis spectrometric method at 205 nm by following Tappi UM250 (Liu et al., 2011; Saeed et al., 2010). The chemical oxygen demand (COD) of the PHL samples before and after the treatments was assessed by using an incubator, Thermoreaktor CR2200, Germany, according to PAPTAC standard procedure. A calibration curve of UV absorbency at 620 nm was measured and the amount of COD in the PHL samples was determined using the calibration curve (He et al., 2004; Sharma et al., 2007). 2.5. Hemicellulose analysis The concentration of hemicelluloses in the PHL before and after the acidification and PEO treatment was determined by using an ion chromatography unit equipped with CarboPacTM PA1 column (Dionex-300, Dionex corporation, Canada) and a pulsed amperometric detector (PAD). To convert oligosaccharide to monosaccharide, an additional acidic hydrolysis was carried out on the samples under the conditions of 4% sulfuric acid at 121 °C for 1 h in an oil bath (Neslab instruments Inc., Portsmouth, NH, USA) by following the same procedure as reported earlier (Liu et al., 2011; Saeed et al., 2010). The PAD settings were E1 = 0.1 V, E2 = 0.6 V and E3 = 0.8 V. Deionized water was used as eluant with a flow rate of 1 ml/min. A solution of 0.2 M NaOH was used as the supporting electrolyte with 1 ml/min flow rate. The samples were filtered and diluted prior to analysis. The hemicelluloses content of the samples were reported as monosugars in this work. 3. Results and discussion 3.1. Proposed process for removing lignin of PHL from the combined acidification and PEO flocculation system In the kraft-based dissolving pulp production process, the hemicelluloses are removed from the hardwood chips in the prehydrolysis stage prior to kraft pulping. However, a part of lignin is also removed from the wood chips and dissolved in pre-hydrolysis liquor (PHL) during this process (Leschinsky et al., 2009; Liu et al., 2011; Saeed et al., 2010). Presently, the PHL is concentrated and

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sent to the recovery boiler of the mill for burning. However, as the dissolved organic content of the PHL is very low (3–4%), this concentrating process is very energy intensive (Saeed et al., 2010). On the other hand, since PHL contains hemicelluloses, it can potentially be used for the production of other value-added products, e.g., ethanol or xylitol. As described earlier, lignin is a typical inhibitor for the fermentation process of hemicelluloses to produce ethanol or xylitol, and hence it should be removed (Hahn-Hagerdal et al., 2007). The acidification can remove the dissolved lignin (Liu et al., 2011); however, the lignin removal depends on the molecular weight and structure of lignin and other organic materials dissolved in the PHL. Fig. 1 shows a process configuration for improving the removal of dissolved lignin of the PHL. As can be seen, the lignin removal can be conducted in two pathways. In stream 1, the PHL is first acidified and the insoluble lignin is separated via filtering and considered a by-product. Subsequently, the acidified PHL is treated with PEO, which leads to the formation of lignin/PEO complexes. These lignin complexes can potentially be by-products of the process and used as retention aids in papermaking (Takase and van de Ven, 1996; van de Ven and Alince, 1996; Wu et al., 2007) or as fuel. In the latter, due to a much higher solid content of the complexes in comparison with that of the original PHL, the utilization of lignin complexes as fuel sources may be more economical. In stream 2, the filtration step between the acidification and PEO treatment is skipped. Thus, the acidification and PEO flocculation treatment are combined into one step. Stream 2 is simpler than stream 1, but at the expense of limited process flexibility and number of by-products. Subsequently, the treated PHL is further detoxified for removing other inhibitors, e.g., acetic acid, furfural, and subsequently fermented to produce ethanol or xylitol. 3.2. Experimental results The experimentation in Section 3.2 was conducted by following the procedure of stream 1 in Fig. 1.

3.2.1. Effect of pH on complex formation The lignin content and sugar compositions of the PHL are listed in Table 1. As can be seen, the total solid content of the PHL was about 5.3% (wt.), in which lignin and sugars (sum of the monomeric and oligomeric sugars) account for 1.0% (wt.) and 2.3% (wt.), respectively. Acetic acid, furfural and inorganics were also present in the PHL (not shown), as noted in an earlier study (Saeed et al., 2010). The characteristics of the PHL at various pHs before and after PEO treatment (at 350 mg/l PEO concentration and room temperature) are listed in Table 2. As shown in Table 2, acidifying the PHL led to a decrease in the lignin concentration; however, the reduction was limited. This is in contrast to the results in an earlier study (Liu et al., 2011). It has been well documented in the literature that the lignin removal/precipitation from the acidification process is significantly affected by the structure/functional groups and molecular weight of the dissolved lignin. For example, the black liquor of kraft pulping process contains a significant amount of acidinsoluble lignin, which can be easily removed via acidification (García et al., 2009; Toledano et al., 2010), while the lignin of bi-sulfite spent liquor is difficult to be removed/separated via acidification (Fredheim and Braaten, 2003; Ringena et al., 2005). The hydrolysis of wood chips under different conditions produces the dissolved lignin with various molecular weights (MW) and structures in the PHL (Leschinsky et al., 2008). The PHL sample used in the earlier work had significantly different properties (e.g., lignin and sugar contents) compared with the present sample, which indirectly implies that the properties of the PHL samples must have been influenced by the two displacement steps, as described in Section 2.1, leading to altered behaviors in the acidification treatment. However, further study is necessary to confirm this hypothesis. The results in Table 2 further showed that the addition of PEO to the PHL was effective in removing lignin, and its efficiency was increased as the pH decreased.

Steam

Wood chips

Pulping

Bleaching

Bleached pulp

1 PHL 2 Acid Lignin

Acid

Acidification/PEO treatment

PEO Lignin/PEO complex

PEO

Acidification Lignin/PEO complex

PEO treatment

Detoxification

Fermentation

Ethanol or xylitol Fig. 1. Proposed process for removing the dissolved lignin of the PHL prior to fermentation.

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Table 1 Lignin content and sugar compositions of the industrially produced pre-hydrolysis liquor (PHL). Solid content, %

Lignin, g/l

Xylan, g/l

Mannose, g/l

Glucose, g/l

Arabinonse, g/l

Rhamanose, g/l

Galactose, g/l

Total sugar, g/l

5.34

10.3

9.8

3.1

7.8

1.7

3.7

1.7

23

Table 2 Lignin and COD concentrations of the PHL after acidification and PEO (600 k) treatment (PEO concentration = 350 mg/l) conducted at room temperature. Sample ID

1(Original) 2 3 4 5 6

pH

3.7 3.5 3.0 2.5 2.0 1.5

Lignin, g/l

COD, g/l

After acidification

After PEO treatment

After acidification

After PEO treatment

– 10.28 10.24 10.09 10.01 9.91

10.18 10.07 9.84 8.91 7.96 7.74

89.4 88.7 86.9 86.2 84.8 83.8

88.7 86.7 82.9 78.8 71.4 69.2

As can be seen, three strategies were followed in Table 2: discrete acidification, PEO treatment of original PHL, and acidification together with the PEO treatment. The results in Table 2 showed that the acidification to pH 1.5 insignificantly reduced the lignin (from 10.3 to 9.91 g/l). The PEO treatment of original PHL also showed a marginal change in lignin (from 10.3 to 10.18 g/l). However, the lignin concentration was reduced to 7.74 g/l (i.e., 22% lignin removal) by the subsequent acidification and PEO treatment via having 350 mg/l PEO in the PHL at pH 1.5. Under these conditions, the COD removal of 17.3% was obtained. Thus, the acidification/PEO treatment was very effective in removing lignin of the PHL. Consequently, the acidification/PEO treatment is favored compared with the application of PEO on original PHL or discrete acidification treatment for lignin removal. Fig. 2 shows the turbidity of the PHL as a function of pH via adding PEO with the MW of 600 k. Evidently, at the PEO concentration of 275 mg/l in PHL, the turbidity of the PHL increased significantly as the pH decreased, indicating that the PEO and lignin interaction and the formation of lignin/PEO complexes would be strongly dependent on the pH. A similar set of experiments was conducted on the same PHL via adding PEO having a MW of 100 k; however, the changes in the lignin and COD concentrations were limited, thus not presented in this work. The increase in the turbidity, thus the lignin removal, via decreasing pH may be due to two reasons: (1) the interaction of lignin and PEO were enhanced at a lower pH (Takase and van de Ven, 1996). It was claimed that the interaction of lignin and PEO occurred via the oxygen atoms of the phenolic, hydroxyl and

carboxyl groups in lignin and the ether oxygen of PEO (van de Ven and Alince, 1996; Gaudreault et al., 2005). (2) The acidic environment screens out a part of anionic charges of lignin, and hence decreases the debye length of lignin macromolecules, which induces a stronger interaction between lignin and PEO, and between those of the lignin/PEO complexes in solutions.

Fig. 2. Turbidity of the PHL as a function of pH at the PEO (MW of 600 k) concentration of 275 mg/l in the PHL (after conducting at room temperature and 120 rpm for 10 min).

Fig. 3. Turbidity and hydrodynamic size of the lignin/PEO complexes formed at different concentrations of PEO (MW of 600 k) in the PHL at pH 2 (after conducting at room temperature and 120 rpm for 10 min).

3.2.2. Effect of PEO dosage on forming lignin complexes Fig. 3 shows the turbidity and hydrodynamic sizes of the complexes formed at different concentrations of PEO in the PHL (pH 2). Evidently, by increasing the concentration of PEO to 350 mg/l, the turbidity of the PHL solution increased to the maximum of 2500 NTU. In the same vein, the hydrodynamic size of the complexes increased to the maximum of 800 nm. In the literature, the changes in the turbidity and hydrodynamic size of particles were employed

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H. Shi et al. / Bioresource Technology 102 (2011) 5177–5182 Table 3 Summary of the IR band observed for the acidified lignin of the PHL, PEO (MW of 600 k) and the formed complexes obtained from the FTIR analysis.

a b c

Component

Acidified lignin

PEO

Lignin/PEO

No.

Band position, PS

Band position (Kubo and Kadla, 2005)

Band position, PS

Band position (Stachurek and Pielichowski, 2007)

Band position, PS

Assignment

1 2 3 4 5

3450 2850–2950 2850 1700 1600

3421 2840–2937 2840 1682 1603

3500

3513

2850 1710

2890 1700

3450 2850 2850 1700

6 7

1520 1460

1514 1462

1500b

1470b

1520 1500

8

1430

1425

9 10 11 12

1340 1240 1050 895

1327 1269 1044a 903a

OAH stretching CAH stretching CH stretching in CH2 C@O stretching (unconjucated) Aromatic skeletal vibration + C@O stretching Aromatic skeletal vibration CAH deformation (methyl and methylene) CAH in plane deformation with aromatic ring stretching CAO of the syringyl ring CAO of guaiacyl ring CAO stretching of CAOAC b-glycosidic

1420 1290c 1130c 1100 980

1346c 1148c 1112 960c

1280c 1120c 1050–1100 960c

Ren et al. (2009). CAH vibration in CH2. CAH bonds in CH2.

for confirming the formation of complexes (Buchhammer et al., 2003; Fatehi et al., 2010). An increase in the turbidity of the PHL is due to the formation of lignin/PEO complexes. The increase in the hydrodynamic size and turbidity implied that by adding more PEO, larger size complexes were formed, which may be due to the bridging effect of formed complexes as a consequence of the higher concentration of PEO in the PHL. The IR band of the FTIR analysis and their assignments for the lignin, PEO and lignin/PEO complexes are listed in Table 3. It can be seen that the peaks at 1700–1750, 1460, and 1430 cm 1, which are assigned to C@O stretching (unconjucated), CAH deformation (methyl and methylene), and CAH in plane deformation with aromatic ring stretching, respectively, are observed for the acidified lignin and the hardwood lignin studied in the literature (Kubo and Kadla, 2005). Additionally, the IR bands of PEO investigated in this study was similar to those reported in other investigations (Popelka et al., 2007; Stachurek and Pielichowski, 2009), but some peaks shifted slightly (Table 3). The complexes also possessed peaks of both lignin and PEO (Table 3). 3.2.3. Effect of PEO dosage on mass removal The effects of PEO dosage on the lignin and COD removals of the acidified PHL are presented in Fig. 4. Evidently, the lignin was removed by 15% (wt.) via increasing the PEO concentration up to 75 mg/l. With further increase in the PEO concentration, the removal was gradually increased to the maximum of 22% (wt.). Similarly, the COD removal was increased (15%) by increasing the PEO concentration to 75 mg/l in the PHL.

Fig. 4. Removals of lignin and COD from the PHL (pH 2) at different PEO (MW of 600 k) concentrations (after conducting at room temperature and 120 rpm for 10 min).

A further increase in the PEO from 75 to 350 mg/l increased the COD removal to about 20%. Interestingly, the results in Figs. 3 and 4 showed that there were two steps in lignin removal. By increasing the PEO concentration up to 75 mg/l, the lignin removal occurred significantly. By further increase in the PEO concentration, the lignin removal slightly increased. The first step might be due to the formation of complexes between the lignin and PEO, while the second step might be due to the bridging effect of complexes as a result of adding more PEO to the PHL. 3.2.4. Effect of time on complex formation Fig. 5 shows the removals of lignin and COD at different shaking times. Evidently, the removals were dominantly occurred during the first 5 min. This result implies that the complex formation, which caused the lignin removal from the PHL, was a fast process. This is particularly important from industrial point of view. It was documented in the literature that the interaction of PEO and lignin took place within 10 min under different conditions conducted (van de Ven and Alince, 1996; Gaudreault et al., 2005). 3.2.5. Effect of PEO on hemicelluloses removal Since the PEO-treated PHL would presumably be used for the ethanol production, it is important to minimize the removal of hemicelluloses in the stage. For this reason, the hemicelluloses removal was determined during the acidification/PEO flocculation process. We determined the hemicelluloses concentration of the PHL before and after the acidification/PEO treatment. The results are listed in Table 4. It is evident that the hemicelluloses removal

Fig. 5. Removals of lignin and COD from the PHL (pH 2) at PEO (MW of 600 k) concentration of 350 mg/l (after conducting at room temperature at 120 rpm).

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Table 4 Effect of acidification and PEO (MW of 600 k) treatment on the hemicelluloses concentration of the PHL at pH 2.

Xylose, g/l Glucose, g/l Galactose, g/l Arabinose, g/l Rhamnose, g/l Mannose, g/l Total sugar, g/l

Acidified to pH 2

PEO concentration, 100 mg/l

PEO concentration, 200 mg/l

PEO concentration, 300 mg/l

9.7 7.8 1.7 1.7 3.5 3.0 22.7

9.7 7.7 1.7 1.7 3.2 3.0 22.7

9.5 7.7 1.7 1.6 3.1 3.0 22.4

9.1 7.4 1.7 1.5 2.8 3.0 21.6

was very limited under the conditions studied. For instance, the xylose removal was about 7% at the PEO concentration of 300 mg/l in the PHL. These results indicated that the acidification/PEO treatment was a selective process for removing lignin from the PHL. However, it is evident in Figs. 4 and 5 that the total lignin removal was rather low (<25%). One possible explanation is due to the presence of lignin/hemicelluloses complexes in the PHL. This is supported by the experimental evidence that the dissolved hemicelluloses were not removed from the system (Table 4). 4. Conclusions The combined acidification/PEO flocculation process can improve the lignin removal from the pre-hydrolysis liquor (PHL) of the kraft-based dissolving pulp production process. The formation of lignin/PEO complexes was confirmed by means of turbidity, particle size and FTIR analyses. The combined acidification/PEO flocculation increased the lignin and COD removals from 3.8% and 6.2% (acidification to pH 2 only) to 22 % and 20 % (pH 2 and 350 mg/l PEO), respectively. Our results further demonstrated that the proposed acidification/PEO flocculation process was selective in the lignin removal compared with the hemicelluloses removal from the PHL. Acknowledgements This project was funded by an NSERC CRD grant, Canada Research Chairs programs of the Government of Canada, and National Natural Science Foundation of China (Grant #20906006). References Alince, B., van de Ven, T., 1997. Effect of polyethylene oxide and kraft lignin on the stability of clay and its deposition on fibers. Tappi J. 80 (8), 181–186. Buchhammer, H.M., Mende, M., Oelmann, M., 2003. Formation of mono-sized polyelectrolyte complex dispersions: effects of polymer structure, concentration and mixing conditions. Colloids Surf. A 218, 151–159. Fredheim, G.E., Braaten, S.M., 2003. Christensen BE. Comparison of molecular weight and molecular weight distribution of softwood and hardwood lignosulfonates. J. Wood Chem. Technol. 23 (2), 197–215. Fatehi, P., Kititerakun, R., Ni, Y., Xiao, H., 2010. Synergy of CMC and modified chitosan on strength properties of cellulosic fiber network. Carbohydr. Polym. 80, 208–214. García, A., Toledano, A., Serrano, L., Egüés, I., González, M., Marín, F., Labidi, J., 2009. Characterization of lignins obtained by selective precipitation. Sep. Purif. Technol. 68, 193–198. Gaudreault, R., van de Ven, T.G.M., Whitehead, M.A., 2005. Mechanisms of flocculation with poly (ethylene oxide) and novel cofactors. Colloids. Surf. A 268, 131–146. Hahn-Hagerdal, B., Karhumaa, K., Fonseca, C., Spencer-Martins, I., Gorwa-Grauslund, M.F., 2007. Towards industrial pentose-fermenting yeast strains. Appl. Microbiol. Biotechnol. 74, 937–953.

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