Optimizing the Poly Ethylene Oxide Flocculation Process for Isolating

(4, 5) The prehydrolysis liquor (PHL), which contains the dissolved hemicelluloses and lignin, is presently mixed with black liquor of kraft process a...
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Optimizing the Poly Ethylene Oxide Flocculation Process for Isolating Lignin of Prehydrolysis Liquor of a Kraft-Based Dissolving Pulp Production Process Haiqiang Shi,*,†,‡ Pedram Fatehi,*,§ Huining Xiao,‡ and Yonghao Ni‡ †

Liaoning Key Laboratory of Pulp and Paper Engineering, Dalian Polytechnic University, Dalian, China, 116034 Limerick Pulp and Paper Centre, University of New Brunswick, Fredericton, NB, Canada E3B 5A3 § Chemical Engineering Department, Lakehead University, Thunder Bay, ON, Canada, P7B 5E1 ‡

ABSTRACT: In this work, poly ethylene oxide (PEO) with various molecular weights (MW)s was employed to separate the lignin of prehydrolysis liquor (PHL) of a kraft-based dissolving pulp process. The results revealed that, the higher the MW and concentration of PEO in the PHL at pH 2, the higher are the removals of lignin and chemical oxygen demand (COD) but with a marginal removal of sugars. Alternatively, poly aluminum chloride (PAC) (200 mg/L) and PEO (8 MDa MW) (200 mg/L) were employed in a dual polymer system to extract the lignin, and the PAC/PEO system (MWs of 2 kDa/8 MDa) was more effective than the PEO/PEO system (MWs of 0.1 MDa/8 MDa) in removing lignin and COD of PHL. The maximum lignin, sugar, and COD removals were 46%, 18.8%, and 32%, respectively, under the conditions of 400 mg/g PEO (with a MW of 8 MDa) at pH 2 and room temperature in a singular PEO system.

1. INTRODUCTION Today, there is a strong incentive for producing value-added chemicals in addition to the traditional products in the pulp and paper industry.1−3 This concept fits well into the integrated forest biorefinery (IFBR) model.1 The kraft-based dissolving pulp production process can potentially be converted to an integrated forest biorefinery unit. In the current technology practiced, wood chips are pretreated with steam that dissolves the majority of hemicelluloses and a part of lignin of the wood chips.4,5 The prehydrolysis liquor (PHL), which contains the dissolved hemicelluloses and lignin, is presently mixed with black liquor of kraft process and burned in the recovery boiler of kraft mills.4,5 However, these lignocelluloses can be isolated/extracted from PHL and then used in the production of value-added chemicals, e.g., ethanol or xylitol.6,7 The lignin of PHL can also be used for producing various phenolic compounds and biofuel.8,9 Additionally, the extraction of lignin from PHL will facilitate the production of ethanol/xylitol from hemicelluloses dissolved in PHL.10−12 The extraction of lignin from PHL may also promote the handling of treated PHL in wastewater systems of kraft mills, as the extraction decreases the organic content of PHL and hence decreases the load of PHL treatment in the wastewater system of mills. Efforts have been made on removing the lignin of various hydrolysates in the literature.13−15 In our previous work, we attempted to isolate the lignocelluloses of PHL employing adsorption and flocculation processes.16−18 We also developed different processes that could be adapted in a kraftbased dissolving pulp process to produce value-added chemicals.7,12 The interaction of poly ethylene oxide (PEO) and lignin was comprehensively studied in the past.19−23 It was reported that the phenolic groups of lignin interacted with the ether oxygen of PEO and induced complexes.21,23−25 These complexes © 2012 American Chemical Society

promoted the deposition of clay, latex, and fiber fines on papers.20,21,23,26 In one report, lignin/PEO complexes were used as retention aids for papers made of unbleached pulp.27 On the basis of the complexation tendency of PEO and lignin, we developed a process via adding PEO to PHL in order to separate the lignin of PHL in our previous work.12 One objective of the current work was to optimize the process conditions, under which PEO could be most effective and selective in removing the lignin of PHL. A dual polymer system has been recognized as an efficient flocculation process for removing the dissolved materials of various spent liquors and for retaining dissolved materials on pulp fibers in papermaking processes.19,23,27−31 In a dual polymer system, a polymer with a small molecular weight (MW) interacts with the dissolved materials and forms small flocs/complexes via charge interaction and patching phenomena. The addition of large polymers helps bridge the formed flocs/complexes, which will promote the removal of complexes from the system (i.e., settling of complexes). Another objective of this work was to investigate the efficiency of a dual polymer system, which consists of PEO and poly aluminum chloride (PAC), i.e., a dominant flocculant for wastewater systems, in removing lignin from PHL. In the work presented herein, PEO with various MWs and/ or PAC will be added to an industrially produced PHL. The effects of MW, concentration, and temperature of PEO treatment in removing lignin from PHL were studied, and the process conditions were optimized for a singular PEO polymer system. Alternatively, PEO and PAC were used in dual Received: Revised: Accepted: Published: 5330

January 23, 2012 March 17, 2012 March 19, 2012 March 19, 2012 dx.doi.org/10.1021/ie300141k | Ind. Eng. Chem. Res. 2012, 51, 5330−5335

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Table 1. Lignin, Sugars, and COD of Original and Acidified PHL

original acidified

pH

lignin, g/L

xylan, g/L

mannose, g/L

glucose, g/L

arabinose, g/L

xylose, g/L

rhamanose, g/L

galactose, g/L

total sugars, g/L

COD, g/L

4 2

9.2 8.8

31.1 26.1

3.2 3.0

7.8 7.6

1.7 1.7

12 10.5

2.5 2.4

1.8 1.7

60.1 53.0

148.7 131.3

Alternatively, the concentration of PEO with a MW of 8 MDa was maintained at 400 mg/L in untreated (original) PHL (pH 4) at room temperature and 120 rpm for 20 min under the conditions demonstrated above in order to investigate the effect of PEO in removing the lignin of original PHL. The reason for conducting these experiments for 20 min was to possess the same treating conditions as the dual polymer systems that will be described below. This analysis was repeated using PAC (400 mg/L concentration) at room temperature, pH 4, and 120 rpm for 20 min. The lignin, hemicellulose, and COD were determined before and after the PEO or PAC treatment in order to determine the efficiency of PEO or PAC treatment. 2.4. Dual Polymer System. In one set of experiments, the concentration of PEO with the MW of 100 kDa was controlled at 200 mg/L in the untreated PHL (pH 4) for 10 min at room temperature and 120 rpm, and subsequently, PEO with a MW of 8 MDa was added (concentration 200 mg/L) to the system under the same treatment conditions so that the total concentration of PEO in the PHL was kept at 400 mg/L for another 10 min. Then, the lignin, hemicelluloses, and COD of PHL were determined after centrifuging the formed complexes. Alternatively, this analysis was repeated using PAC (200 mg/L) in the PHL for 10 min, which was followed by PEO (with a MW of 8 MDa) treatment (200 mg/L) for another 10 min at 120 rpm under the above-mentioned conditions in order to evaluate the efficiency of a PAC/PEO dual polymer system in removing the lignocelluloses of PHL. Additionally, the PEO/ PEO and PAC/PEO dual polymer systems were conducted on acidified PHL (pH 2) to evaluate the efficiency of the systems under acidic conditions described above. Finally, the PAC/PEO dual polymer system was conducted on acidified PHL (pH 2) at different temperatures, under otherwise the same conditions, in order to determine the efficiency of PAC/PEO system at different temperatures. The lignin, hemicelluloses, and COD analyses were conducted after centrifuging the PAC/PEO complexes from the PHL. 2.5. Turbidity and Particle Size Analyses. The turbidity of PHL samples was determined using a HACH 2100AN Turbidity meter (Colo, USA) at room temperature. Similarly, the particle size of the formed lignin/PEO complexes in the PHL was measured using a Brookhaven ZetaPlus Particle Size analyzer (Holtsville, N.Y. 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.32,33 An average of three testing results was reported. The PHL, PAC, and PEO solutions were filtered using a 0.45 μm Nylon syringe filter prior to adding PEO or PAC to PHL. 2.6. Lignin and COD Analyses. The lignin content of the samples was determined on the basis of the UV/vis spectrometric method at 205 nm by following Tappi UM250.5,7 The chemical oxygen demand (COD) of PHL samples was assessed using an incubator, Thermoreaktor CR2200, Germany, according to PAPTAC standard procedure.

PEO/PEO or PAC/PEO polymer systems, and the results were compared with a singular PEO system.

2. MATERIALS AND METHODS 2.1. Materials. Poly ethylene oxide (PEO) with various molecular weights (MW), i.e., 100 kDa, 600 kDa, 2MDa, 4MDa, and 8MDa, and poly ethylene oxide (PAC) with a MW of 2 kDa were purchased from Aldrich Co. and dissolved in water (0.5 g/L) prior to use. Sulphuric acid (98%) was purchased from Fisher Scientific Co. The prehydrolysis liquor (PHL) was received from a mill located in Eastern Canada. As explained in our previous work, the PHL was produced according to VisCBC technology via steaming wood chips (70% maple, 20% poplar, and 10% birch) at 170 °C for 30 min.12 Two displacement steps, first by the strong black liquor and then by the white liquor, were subsequently followed after the hydrolysis for extracting, degrading, and removing hemicelluloses from the wood chips. The prehydrolysis liquor (PHL) was collected after these steps and used as received in this research. Meanwhile, at least three repetitions were conducted for each analysis and treatment, and the average value was reported in this work. 2.2. Acidification of PHL. Approximately, 200 mL of PHL sample was acidified using concentrated sulfuric acid (98%) to pH 2.0 and kept for 30 min under stirring at room temperature.7 Then, the sample was centrifuged at 2500 rpm using a laboratory centrifuge for 10 min. The filtrate was then collected for analysis and PEO treatment. The lignin, hemicelluloses (sugar), and chemical oxygen demand (COD) analyses of PHL before and after the acidification reflected the efficiency of acidification in removing lignocelluloses of PHL. 2.3. Singular PEO Treatment System. In our previous work, we reported that 10 min of treating of PHL with PEO was sufficient to form complexes; thus, most of the mixing analysis of PHL with PEO was performed for 10 min throughout the current research. In one set of experiments, the concentration of PEO having various MWs in the acidified PHL (pH 2) was controlled at 300 mg/L and room temperature, while stirring at 120 rpm for 10 min. Afterward, the turbidity of PHL and hydrodynamic size of complexes in the PHL were analyzed. The samples were then centrifuged at 2500 rpm for 10 min and the lignin, hemicelluloses (sugar), and COD of the filtrates were assessed. In another set of experiments, the concentration of the most effective PEO (the one with 8 MDa MW) was varied in the acidified PHL (pH 2) under the aforementioned conditions, and then, the turbidity and hydrodynamic size of complexes in the PHL were determined. Subsequently, the samples were centrifuged under the conditions mentioned above, and the remaining amounts of lignin, hemicelluloses, and COD in the PHL samples were assessed. Alternatively, the analyses were repeated using the PEO with 8 MDa MW at a 400 mg/L concentration at different temperatures, i.e., room, 30, 40, 50, 60, and 80 °C, and then, the turbidity, hydrodynamic size of complexes in the PHL, and lignin, hemicelluloses, and COD of filtrates were measured. 5331

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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.34,35 2.7. Hemicellulose Analysis. The concentration of hemicelluloses in the PHL was determined using an ion chromatography unit equipped with CarboPac PA1 column (Dionex-300, Dionex corporation, Canada) and a pulsed amperometric detector (PAD). In this work, all oligomers of PHL were converted to monomers. The concentrations of xylan and monosugars of original PHL and acidified PHL were reported in Table 1. However, hemicelluloses were reported as monosugars in other sections of this study. To convert oligomers (xylan) to monomers, 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.5,7 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 a supporting electrolyte with a 1 mL/min flow rate. The samples were filtered and diluted prior to sugar analysis using an ion chromatography.

Figure 1. Turbidity of PHL and hydrodynamic size of the complexes in the PHL as a function of PEO molecular weight (300 mg/L PEO concentration, pH 2, room temperature, 120 rpm, and 10 min).

increasing the MW of PEO to 8 MDa. Similarly, the hydrodynamic size of the complexes in the PHL was 450 nm via adding PEO with the MW of 0.1 MDa and increased to 550 nm via increasing the MW of PEO to 8 MDa. These results imply that, by increasing the molecular weight (size) of the PEO, larger complexes were formed. It is well-known that large polymers have a greater tendency than small polymers do to bridge the dissolved/suspended complexes in solutions.24,36 In one study on the PEO/polystyrene latex system, larger PEO (with a MW of 4 MDa) was more efficient than smaller PEO (MW of 1 MDa) in complex formation and hence in removing polystyrene latex from the system.37 Figure 2 shows the effect of PEO molecular weight on the lignin, sugars, and COD removals of PHL at pH 2. It is evident

3. RESULTS AND DISCUSSION 3.1. Acidification of PHL. The properties of original and acidified PHL are listed in Table 1. As can be seen, the PHL contained 9.2 g/L lignin, 43 g/L xylan, and some monosugars (total sugars 60.1 g/L). Although a similar lignin content was reported for an industrially produced PHL in the literature, the sugar content of the PHL in the current study was significantly higher than that reported earlier.12 In the VisCBC hydrolysis process, the PHL is mixed with black/white liquor after hydrolysis, which results in sugar degradations due to the high pH of black/white liquor.5,12 The high sugar content of PHL in Table 1 suggests that the PHL of this study was minimally mixed with a black/white liquor in the process, as the PHL had a high sugar content (i.e., minor sugar degradations). The high COD of PHL is also attributed to the high sugar (organic) content of PHL (Table 1). The results also showed that acidification of the current PHL caused 4.3% lignin, 11.8% total sugars, and thereby 11.7% COD removals, respectively. These results imply that acidification was not effective for lignin removal of this PHL. In the literature, contradictory results were reported on the efficiency of acidification for lignin removal of PHL: in one report, 50% of lignin was isolated via acidification,7 while in another report less than 10% of lignin was isolated.12 As described earlier, the mixing conditions of the present PHL with black/white liquor were different from those of the previous PHL (i.e., the one studied in our earlier work) with black/white liquor.12 It was stated in the literature that the operating conditions of the hydrolysis process significantly affected the properties of dissolved lignin in PHL.6 Therefore, different acidification efficiencies (in lignin removal) imply that the properties of lignin dissolved in the current and previous PHLs were different. 3.2. Impact of PEO Molecular Weight. In this set of experiments, the concentration of PEO was maintained at 300 mg/L via adding PEO with different MWs to the PHL at pH 2. Figure 1 shows the effect of PEO molecular weight on the turbidity of PHL and on the hydrodynamic size of the complexes formed as a result of adding PEO to the PHL. Evidently, the turbidity of PHL was 950 NTU via adding PEO with the MW of 0.1 MDa and increased to 1220 NTU via

Figure 2. Lignin, sugar, and COD removals of PHL as a function of PEO molecular weight (room temperature, 300 mg/L PEO concentration, pH 2, and 120 rpm for 10 min).

that the lignin and COD removals were increased from 24% to 35% and from 7% to 18% via increasing the molecular weight of PEO from 0.1 to 8 MDa, respectively. It seems that the sugar removal was not significantly affected by the PEO molecular weight. Considering the results presented in Table 1 and Figure 2, the total removal of lignin, sugar, and COD were 40%, 18%, and 28%, respectively, as a result of acidifying and adding PEO (300 mg/L) to PHL. As more promising results were obtained via treating PHL with PEO having a MW of 8 MDa, the remaining experiments were conducted using this PEO. 3.3. Impact of PEO Concentration. Figure 3 shows the effect of PEO (with the MW of 8 MDa) concentration on the turbidity and hydrodynamic size of complexes formed in the PHL. It is evident that, by increasing the concentration of the 5332

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Figure 3. Turbidity of PHL and hydrodynamic size of complexes in PHL as a function of PEO (with a MW of 8 MDa) concentration in PHL (pH 2, room temperature, and 120 rpm for 10 min).

Figure 5. Lignin, sugar, and COD removals of PHL as a function of the temperature of the PEO/PHL system (400 mg/L PEO with 8 MDa, pH 2, and 120 rpm for 10 min).

PEO to 400 mg/L, the turbidity of PHL was increased to 1250 NTU, and the size of complexes reached to 600 nm. Figure 4 shows the lignin, sugar, and COD removals of PHL as function of PEO (MW of 8 MDa) concentration in the PHL.

hinder the removals of lignin and COD of PHL after treatment with PEO at an elevated temperature, e.g., 60 °C. It was also noticed in Figure 5 that the sugar removal was not affected by a change in the temperature range studied. 3.5. Dual vs Singular Polymer System. In a dual polymer system, the addition of low MW polymers induces the complexes with a relatively small size. Depending on the size of the formed complexes, a part of the complexes may precipitate and be removed from the system. The subsequent addition of high MW polymers helps bridge the formed complexes in solutions and facilitates the precipitation of complexes. As PAC is commonly used in wastewater systems of pulp/paper mills, we employed PAC in a dual polymer system of PAC/PEO to investigate its lignin removal efficiency. In this case, our intention was to make best use of the chemicals already employed in mills for various purposes/scenarios. In this set of experiments, dual polymer systems consisting of PAC/PEO and PEO/PEO (two different MWs) are compared with the singular system of PAC or PEO, while the total concentration of polymers was fixed at 400 mg/L in the PHL. The lignin, sugar, and COD removals of original PHL (pH 4) as a result of PEO and PAC treatments were listed in Table 2. It is evident that the lignin removal was the most, and the sugars removal was the least in the singular polymer system of PEO (with the MW of 8MDa) at pH 4. However, these results were inferior to those reported in Figure 4. As the experiments in Figure 4 and Table 2 were conducted at different pHs, but under otherwise the same conditions, it is implied that the PEO was more effective under more acidic conditions (pH 2).12 In contrast, the lignin removal was low and the sugar removal was relatively high in the singular system of PAC at pH 4 (original PHL). The removals of lignin, sugar, and COD from the PHL in the PAC/PEO and PEO/PEO systems were also listed in Table 2. The lignin, sugar, and COD removals of the PAC/PEO system were similar to those of the PEO (with MW of 8 MDa) singular polymer system at pH 4. Interestingly, the efficiency of the PAC/PEO system was higher on lignin, sugar, and COD removals at pH 2 than 4 but was inferior to those of PEO singular system at pH 2. In one study, it was reported that the efficiency of PAC was higher at pH 2 than pH 7 in removing the COD of effluent of pulp and paper mills in a dual system of PAC/fly ash.29 The results also showed that the PEO/PEO system was not as effective as the PAC/PEO system for lignin, sugar, and COD removals, regardless of the pH of PHL.

Figure 4. Lignin, sugar, and COD removals of PHL as a function of PEO (with a MW of 8 MDa) concentration in the PHL (pH 2, room temperature, and 120 rpm for 10 min).

Interestingly, by increasing the PEO dosage to 400 mg/L, the lignin, sugar, and COD removals increased to 43%, 8%, and 23%, respectively. The total lignin, sugar, and COD removals were 46%, 18.8%, and 32%, respectively, considering the results of Table 1 and Figure 4 (acidification and PEO treatment). In this case, with increasing the concentration of PEO in solutions, more lignin (i.e., phenolic compounds) could interact with PEO via patch and bridging mechanisms in a PEO/lignin system.24 3.4. Impact of Temperature of Singular PEO System. Figure 5 shows the lignin, sugar, and COD removals of PHL at a PEO (with a MW of 8 MDa) concentration of 400 mg/L as a function of temperature. Evidently, the removals of lignin and COD were insignificantly affected by the change in temperature when it was in the range of 25 to 40 °C. Further increase in the temperature caused a significant decrease in their removals and hence deteriorated the performance of PEO. The reason for this trend might be due to the entropy increase of the system: as the entropy of the PEO and lignin segments were increased in the PHL at a higher temperature, e.g., 60 °C, their tendency to interact with each other via developing hydrogen bonding was reduced, which decreased the lignin and COD removals. Additionally, the higher temperature may also increase the solubility of lignin/PEO complexes,24 which might directly 5333

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Table 2. Effect of PAC/PEO, PEO/PEO, PEO, and PAC Systems on Lignin, Sugar, and COD Removals of PHL under the Conditions Elaborated in the Experimental Sectiona

a

polymer system

dosage, mg/L

MW

pH

lignin removal, %

COD removal, %

sugars removal, %

PAC PEO PEO PEO PAC/PEO PEO/PEO PAC/PEO PEO/PEO

400 400 400 400 200/200 200/200 200/200 200/200

2 kDa 0.1 MDa 8 MDa 8 MDa 2 kDa/8MDa 0.1 MDa/8 MDa 2 kDa/8MDa 0.1 MDa/8 MDa

4 4 4 2 4 4 2 2

21.8 16.0 25.3 44.0 24.7 23.5 37.5 28.5

18.2 12.1 21.2 24.0 20.4 18.3 28.5 21.5

10.8 5.1 7.5 8.0 6.9 5.9 10.1 8.8

Excluding those presented in Table 1.

3.6. Impact of Temperature of Dual Polymer System. As the PAC/PEO system induced more promising results than the PEO/PEO system did in a dual polymer system, the PAC/ PEO system was selected for further analysis. Figure 6 shows

4. CONCLUSIONS The results revealed that, the higher the MW and concentration of PEO in the PHL at pH 2, the higher are the lignin and COD removals but with insignificant change in the sugar removal. The maximum hydrodynamic size of complexes in the PHL and the turbidity of PHL were approximately 600 nm and 1250 NTU, respectively, via adding PEO to PHL. The dual polymer system of PAC/PEO (MWs of 2 kDa/8 MDa) was more effective than PEO/PEO (MWs of 0.1 MDa/8 MDa) in removing lignin and COD of PHL at pH 2 and 4. However, the dual PAC/PEO system was less effective than the singular PEO (with MW of 8 MDa) system in lignin and COD removals. Generally, by increasing the temperature of the system, the efficiency of the PEO or PAC/PEO system in removing lignin and COD was reduced. The maximum lignin, sugar, and COD removals were 46%, 18.8%, and 32%, respectively, via having PEO (with a MW of 8 MDa) concentration of 400 mg/L in the PHL at pH 2 and room temperature in a singular polymer system.

Figure 6. Lignin, sugar, and COD removals from the PHL as a function of the temperature of the system (200/200 mg/L PAC/PEO concentration (MWs of 2 kDa/8 MDa), 10 min/10 min, pH 2, 120 rpm).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.S.); [email protected] (P.F.). Tel: +86-411-86323327 (H.S.); 807-343-8697 (P.F.). Fax: +86-411-86323736 (H.S.); 807-346-7943 (P.F.).

the impact of temperature on the removals of lignin, sugar, and COD of PHL in the PAC/PEO system at pH 2. As can be seen, similar to the singular PEO system (Figure 5), the increase in the temperature hampered the lignin, sugar, and COD removals. As demonstrated earlier, the higher entropy of the system might have prohibited the formation of complexes and eventually their precipitation from the PHL. In the literature, the temperature increase was claimed to increase the solubility of complexes formed via adding PAC to different river water samples.30 Additionally, a comparison between the results of Figures 5 and 6 revealed that the lignin and COD removals were adversely affected by increasing temperature, regardless of singular or dual polymer system studied. However, the temperature increase affected the lignin and COD removals in the dual polymer system (Figure 6) more pronouncedly than in the singular PEO system (Figure 5). The general higher COD removal in the dual PAC/PEO system (Figure 6) than singular PEO system (Figure 5) was attributed to a higher sugar removal in the dual polymer system. All in all, in addition to the PEO singular system, analysis showed that PAC/PEO system could be employed for lignin removal of PHL, particularly if PAC was already used in the mill. The PAC/PEO system may be cheaper than the PEO singular system, but less removal will occur using the dual polymer system than singular PEO system.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was funded by an NSERC CRD grant, Canada Research Chairs programs of the Government of Canada, National Natural Science Foundation of China (Grant #20906006), and Program for Liaoning Excellent Talents in University.

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REFERENCES

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dx.doi.org/10.1021/ie300141k | Ind. Eng. Chem. Res. 2012, 51, 5330−5335