Comparative study of pretreated corn stover for sugar production using cotton pulping black liquor (CPBL) instead of sodium hydroxide

Industrial Crops and Products 84 (2016) 97–103

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Comparative study of pretreated corn stover for sugar production using cotton pulping black liquor (CPBL) instead of sodium hydroxide Huan Liu a,b , Bo Pang b , Jinghui Zhou a , Ying Han a , Jie Lu a , Haiming Li a,∗∗ , Haisong Wang a,b,∗ a b

Liaoning Key Laboratory of Pulp and Papermaking Engineering, Dalian Polytechnic University, Dalian 116034, Liaoning, China Key Laboratory of Biofuels, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China

a r t i c l e

i n f o

Article history: Received 18 August 2015 Received in revised form 26 January 2016 Accepted 27 January 2016 Keywords: Corn stover Alkaline pretreatment Cotton pulping black liquor Enzymatic hydrolysis

a b s t r a c t Black liquor is a significant water pollution source, and many investigations related to black liquor have been performed over the years. Recovering cotton pulping black liquor (CPBL) is difficult due to its low lignin content. This study presents corn stover pretreatments using CPBL alone and in combination with other agents to improve the cost-effectiveness of the biomass-to-sugar conversion. The results showed that 78.4%, 74.9% and 82.1% of the lignin was removed after pretreatment with sodium hydroxide (SHP), CPBL (CPBLP) and CPBL combined with sodium sulfite (SS-CPBLP). Accordingly, the glucose and xylose contents in the pretreated corn stover increased to more than 62% and 24% after CPBLP and SS-CPBLP, respectively, which was comparable to SHP. Moreover, the total sugar yield of CPBLP and SS-CPBLP was 66.4% and 75.3%, which was 2.9% and 11.8% higher than that of SHP (63.5%), respectively. The lignin content in the black liquor rose from 6.9 g/L to 17.8 g/L, and the sulfonation degree of the black liquor reached 0.8 mmol/g after SS-CPBLP, which had a positive effect on further lignin utilization. The results indicate that the use of CPBL for corn stover pretreatment not only efficiently produced fermentable sugar products, but also presented a solution to the CPBL pollution issue. Thus, this process represents a method of waste recycling with high industrialization potential. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The soda pulping process is widely used in Chinese paper mills at present. The process produces a waste liquor called “black liquor” (Narapakdeesakul et al., 2013; Zhou et al., 2010). Black liquor is a significant water pollution source, and much work has been focused on overcoming this challenge over the years. Currently, black liquor is concentrated and burned in recovery boilers to generate energy and to recycle inorganic chemicals required for the papermaking process (Gea et al., 2002). Alkaline cotton pulping black liquor has special properties compared to other categories of black liquor, including a deeper color, more organic contaminants (i.e., cellulose, oligosaccharides, and fatty alcohol), a higher pH, and a lower lignin content. The lower lignin content results in a relatively low heating value and makes conventional alkali recycling ineffective for the treatment of cotton pulp black liquor. The discharge of black

∗ Corresponding author at: Liaoning Key Laboratory of Pulp and Papermaking Engineering, Dalian Polytechnic University, Dalian 116034, Liaoning, China. ∗∗ Corresponding author. E-mail addresses: [email protected] (H. Li), [email protected], [email protected] (H. Wang). http://dx.doi.org/10.1016/j.indcrop.2016.01.047 0926-6690/© 2016 Elsevier B.V. All rights reserved.

liquor without any treatment causes severe environmental pollution. Therefore, finding an effective way to utilize cotton pulping black liquor is necessary for cotton pulp mills. With the increase in global energy consumption and the problems associated with CO2 emissions, more attention has been paid to the increased utilization of lignocellulosic biomasses, which are renewable bioresources primarily composed of cellulose, hemicellulose and lignin (Avci et al., 2013; Zhao et al., 2008; Zu et al., 2014). Corn stover is considered one of the best feedstocks for ethanol production due to its high annual biomass yield and high carbohydrate content (Yu et al., 2013a). However, lignocelluloses are resistant to degradation and offer hydrolytic stability and structural robustness primarily due to cross-linking between polysaccharides (cellulose and hemicellulose), the crystalline structure of cellulose, and ester linkages between ester and lignin (Maryana et al., 2014). Therefore, an effective pretreatment method is needed to break the coverage of lignin and disrupt the crystalline structure of cellulose. Many factors can affect the hydrolysis of cellulose, such as the porosity (accessible surface area) of the materials, the cellulose fiber crystallinity, and the lignin and hemicellulose content (McMillan, 1994). Among the pretreatment methods, alkaline pretreatment has received more attention because it is relatively inexpensive, less energy intensive, and effective on many feed-

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stocks, especially forages and agricultural residues (Xu et al., 2010). Compared with acid processes, alkaline processes result in less sugar degradation, can use the existing and mature pulping equipment and the waste water treatment system that is well developed in the pulping industry, and many of the caustic salts can be recovered and/or regenerated (Xu et al., 2015). Xu et al. (2012) used switchgrass-derived black liquor for the pretreatment of corn stover and found that the sugar yield was comparable to that of the biomass pretreated with 1% NaOH primarily due to the high pH and the appreciable amount of carbohydrates in the black liquor. However, CPBL contains more effective alkali and less lignin compared with the black liquor derived from switchgrass but has not been reported as a biomass pretreatment. Our previous work used alkaline sodium sulfite to pretreat corn stover and achieved significant enhancement of the sugar yield due to the higher delignification rate and enzymatic hydrolysis rate (Li et al., 2012; Liu et al., 2015). Therefore, in the current study NaOH was replaced with CPBL containing effective alkali for the pretreatment of corn stover to study its impact on enzymatic hydrolysis. 2. Materials and methods 2.1. Raw materials and chemicals The corn stover used in this study was obtained from Qingdao, Shandong Province, China. Air-dried raw materials without classification were cut into pieces 3–5 cm in length and stored in sealed plastic bags at room temperature. The biomass was ground using a Willey mill, and the particles with sizes between 5 and 40 mesh were collected. The concentrated black liquor was obtained from the soda pulping of cotton and was supplied by the cotton pulping workshop Shandong Henglian Investment Co., Ltd. All chemicals (i.e., sodium hydroxide and sodium sulfite) were analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. The enzymes used for the enzymatic hydrolysis were Celluclast 1.5 L (cellulase, enzymatic activity 43 FPU/mL) and Novozyme 188 (␤-glucosidase, enzymatic activity 741 IU/mL) provided by Sigma–Aldrich China Inc. The enzyme activities were determined according to the method reported by Ghose (Ghose, 1987). 2.2. Pretreatment The pretreatment of corn stover was performed in a cooking reactor (Model PL1-00, Xianyang TEST Equipment Co., Ltd., Xianyang, China) containing 4 bombs with a batch capacity for oven-dried corn stover of 50 g per bomb. The methods for pretreating corn stover were as follows: water pretreatment, sodium hydroxide pretreatment (SHP), cotton pulping black liquor pretreatment (CPBLP), alkaline sodium sulfite pretreatment (ASSP), and sodium sulfite combined with cotton pulping black liquor pretreatment (SS-CPBLP). The pretreatment chemical dosages of each pretreatment process are listed in Table 1. The total alkali charges of the pretreatments (with the exception of the water pretreatment) were 12% calculated based on NaOH. The ratio of cooking liquor to corn stover was 8:1 (v:w). Both the pretreatment solution and corn stover were heated from 30 ◦ C to 140 ◦ C in 30 min. Then, the temperature was maintained at 140 ◦ C for 20 min. At the end of the pretreatment, the bombs were cooled to room temperature with water. The pretreated biomass was recovered and washed with water to remove the excess alkali and dissolved byproducts that might inhibit the enzymes during the subsequent hydrolysis. Approximately 2 g (dry weight) of the pretreated biomass was dried at 45 ◦ C to a constant weight for the composition analysis, and the rest was stored in a sealed plastic bag at 4 ◦ C for enzymatic hydrolysis.

2.3. PFI beating The pulp pretreated with alkali or water was beaten by a PFI refiner (mode PL11-00, Xianyang Test Equipment Co., Ltd., Xianyang, China). The beating conditions were as follows: pulp consistency, 10%; beating gap, 0.3 mm; and PFI refining revolution, 1500 r, 3000 r, or 4500 r. The refined pulp was collected and stored in a refrigerator. The PFI beating not only increases the accessibility of substrates but also reduces the cellulose crystallinity. Continuously increasing the enzymatic saccharification under the relatively mild conditions during the pretreatment process is very helpful. 2.4. Enzymatic hydrolysis Enzymatic hydrolysis of the pretreated corn stover without beating (i.e., the refining revolution was 0 r) or the beaten corn stover were performed at a substrate consistency of 2% (w/v) with 20 mL of sodium acetate buffer (pH 4.8) at 50 ◦ C using a shaking incubator at 95 rpm for 48 h. A mixture of Cellulast 1.5 L with a loading activity of 20 FPU/g substrate and Novozyme 188 with a loading activity of 10 IU/g substrate were used for the enzymatic hydrolysis. After the addition of the enzymes, sodium azide (0.2%, w/v) was added to inhibit microbial growth during the hydrolysis. After the hydrolysis time elapsed, the supernatant was collected for sugar analysis. 2.5. Analytical methods 2.5.1. Analysis of pretreatment and enzymatic hydrolysis The total solid, ash, extractive, carbohydrate, and lignin contents of the raw and pretreated biomasses were determined using the Laboratory Analytical Procedures (LAP) established by the National Renewable Energy Laboratory (NREL) (Sluiter et al., 2008a; Sluiter et al., 2008b; Sluiter et al., 2008c; Sluiter et al., 2005). The major monomeric sugars present in the hydrolysate, which included glucose and xylose, were determined using high performance liquid chromatography (HPLC; model 1200 Agilent, USA). The HPLC system was equipped with a Bio-Rad Aminex HPX-87H column (300 mm × 7.8 mm) and a refractive index detector. The analytical column was operated at 55 ◦ C with 0.005 M H2 SO4 as the mobile phase at a flow rate of 0.5 mL/min, and the quantitative analysis was performed using external standard calibration. Duplicate experiments were conducted, and the mean values were reported. The calculation equations were as follows:



Ysolid(%) =

Mpretendedcornstover Moriginalcornstover

 Eglucose (%) =

 Exylose (%) =



× 100

(1)



Mglucoseinhydroplyzate Mglucoseinpretreatedcornstover Mxyloseinhydrolyzate

Mxyloseinpretreatedcornstover



Rdelignification (%) = 1 −

× 100

(2)

 × 100

Ysolid × Cligninofpretreatedcornstover Cligninofrawcornstover

(3)

 × 100 (4)

where M was the oven dry weight of the corresponding substance (g), C was the percent of the corresponding component content (wt.%), Ysolid was the percentage of the solid yield, Eglucose was the enzymatic hydrolysis rate of glucose, Exylose was the enzymatic hydrolysis rate of xylose, and Rdelignification was the delignification rate. The glucose or xylose yields (%) were expressed as the percentage of glucose or xylose in the enzymatic hydrolysate divided by the corresponding percentages in the original biomass. The total sugar yield (%) was expressed as the combined percentage of glucose

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Table 1 Pretreatment methods with different combinations of chemicals. Pretreatment methods

Chemicals

NaOH (%)

Na2 SO3 (%)

Effective alkali in CPBL (%)

Water SHP CPBLP ASSP SS-CPBLP

– NaOH CPBL + NaOH Na2 SO3 + NaOH Na2 SO3 + CPBL

– 12 6 6 –

– – – 6 6

– – 6 – 6

and xylose in the enzymatic hydrolysate divided by the combined glucose and xylose percentage in the original biomass. 2.5.2. Solid content of black liquor The glass weighing bottle was heated in the oven at 105 ◦ C until a constant weight was obtained. Then, the bottle was removed and cooled to room temperature in a desiccator. A specific volume of black liquor was added to the weighing bottle. The weighing bottle was put in the oven and heated for at least 6 h. Next, the bottle was removed, cooled to room temperature in a desiccator, and then weighed. The solid content of the black liquor was calculated according to the following formula: The solid content of black liquor

g L

=

W2 − W1 × 1000 V

2.5.3. Monomeric sugar content of the black liquor The black liquor was hydrolyzed by 4% sulfuric acid at 121 ◦ C for 30 min. The major monomeric sugars (i.e., glucose) present in the hydrolysate were determined using HPLC. 2.5.4. Total titratable alkali content of the black liquor First, 5 mL of the black liquor was pipetted into a porcelain crucible. Then, methyl orange was added as an indicator. An HCl solution was added to acidify the solution. The porcelain crucible was placed in a water bath until the solution was evaporated to dryness, transferred to a muffle furnace and then burned to ash at 600 ◦ C. The porcelain crucible was removed from the muffle furnace. The ashes was dissolved in deionized water, transferred to a 250-mL volumetric flask and diluted to scale. A total of 25 mL of the diluted liquid was pipetted into a 250-mL Erlenmeyer flask, and 1 mL of potassium chromate indicator solution (50 g/L) was added. Then, the sample was titrated with a 0.1 mol/L AgNO3 standard solution until the color of the solution turned light brick red. The total titratable alkali was calculated by the following formula: Total titratable alkali

L

V × c × 0.031 × 1000 = 5 × 25/100

Effective alkali

g L

=

V × c × 0.040 × 1000 50 × 50/500

(7)

where V is the volume of the standard HCl solution (mL), c is the concentration of the standard HCl solution (mol/L), and 0.040 is the amount of NaOH equivalent to 1 mmol HCl (g). The test methods for the total solid, monomeric sugar, total titratable alkali, and effective alkaline contents in the black liquor were determined according to the methods in the book Analysis and Detection of Pulping and Papermaking (Shi, 2003).

(5)

where W2 is the weight of the glass weighing bottle and sample (g), W1 is the weight of the glass weighing bottle (g), and V is the volume of the black liquor (mL).

g

into a 250-mL Erlenmeyer flask and titrated with a 0.1 mol/L HCl standard solution to pH 8.3. Effective alkali was calculated by the following formula:

2.5.6. Sodium lignosulfonate content in the black liquor Sodium lignosulfonate (reagent grade, Aladdin, China) solutions at different concentrations (20, 40, 100, 200, and 400 mg/L) were prepared, and the absorbency of these samples was measured by a UV Spectrophotometer (752N, Shanghai Jingke, China) at a wavelength of 280 nm. The sodium lignosulfonate content in the black liquor was obtained according to the standard curve of the concentration–absorbency. 2.5.7. Sulfonation degree of the black liquor The sulfonation degree tests of the black liquor were conducted following the previously reported procedure (Yu et al., 2013b). The anion exchange resin (model 717) and cation exchange resin (model 732) were immersed in 2 M NaOH and 2 M HCl, respectively, for 12 h to remove salts and other impurities and then washed to neutral pH with deionized water. First, the diluted black liquor was passed through an anion exchange resin to remove inorganic acid and then passed through a cation exchange resin. As a result, the sodium lignosulfonate was transformed into lignosulfonic acid. A NaOH standard solution of 0.04 mol/L was used as the titrant. The titration end point was determined by a conductivity meter (DDS307, Shanghai Jingke, China). The sulfonation degree of the black liquor was calculated according to the following formula: Sulfonationdegree

(6)

where V is the volume of the standard AgNO3 solution (mL), c is the concentration of the standard AgNO3 solution (mol/L), and 0.031 is the amount of NaCl equivalent to 1 mmol AgNO3 (g). 2.5.5. Determination of the effective alkali in the black liquor A total of 50 mL of black liquor was pipetted into a 500-mL volumetric flask containing approximately 200 mL of water, and then 50 mL of a 100 g/L BaCl2 solution was added and mixed well. When the precipitate had partially settled, a drop of the supernatant liquid was transferred to a test tube containing a few drops of dilute H2 SO4 . If no precipitate formed, 5 mL of the BaCl2 solution was added to the volumetric flask and the test continued. This procedure was repeated until a white precipitate was obtained. Then, the contents of the volumetric flask were diluted to the mark and mixed thoroughly. The supernatant was obtained by filtration with dry filter paper. A total of 50 mL of this clear liquid was pipetted

 mmol  g

=

NNaOH VNaOH n × c/1000 × v/1000

(8)

where NNaOH is the concentration of the standard NaOH solution (mol/L), VNaOH is the volume of the standard NaOH solution (mL), n is the dilution factor of the black liquor, c is the sodium lignosulfonate content in the black liquor (mg/L), and v is the volume of the dilute black liquor (mL). 3. Results and discussion 3.1. Solid yield and chemical composition of the biomass The compositions of the corn stover after different pretreatments are shown in Table 2. Compared to the water pretreatment, the solid yield significantly decreased after the alkali-based pretreatment. The solid yield of the corn stover under different pretreatment conditions was in the range of 53.3–58.8%. During the water pretreatment process, some water-soluble compounds of the plant tissue were removed, such as sugars and some extractives;

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Table 2 The compositions of corn stover after different pretreatments. Pretreatment methods

Glucose (%)

Raw corn stover Water SHP CPBLP ASSP SS-CPBLP

36.7 44.7 65.0 62.9 64.1 62.9

a

± ± ± ± ± ±

0.5 0.3 0.2 0.1 0.3 0.3

Xylose (%) 20.4 19.8 27.8 24.7 25.0 24.1

± ± ± ± ± ±

0.1 0.4 0.3 0.1 0.2 0.4

Klason lignin (%) 18.5 21.2 7.4 8.0 4.2 5.3

± 0.2 ± 0.4 ± 0.5 ± 0.2 ± 0.3 ± 0.3

Acid-solublelignin (%) 0.7 0.7 0.4 0.5 0.5 0.6

± ± ± ± ± ±

0.0 0.0 0.0 0.0 0.0 0.0

Extractivesa (%)

Ash (%)

Solid yield (%)

17.6 ± 0.2 ND ND ND ND ND

5.1 ± 0.1 ND ND ND ND ND

100 85.1 ± 1.1 53.3 ± 1.6 56.9 ± 1.4 56.1 ± 1.2 58.8 ± 1.4

ND: not detected.

Fig. 1. Delignification rate of pretreated corn stover.

lignin rarely dissolved in the water, which lead to the relatively high solid yield compared to the other alkaline pretreatments (Xu et al., 2012). SHP resulted in the lowest solid recovery, whereas the solid recoveries of CPBLP and ASSP were relatively high and comparable because NaOH had the highest pH and the highest dissolution capacity for lignin and carbohydrates. The highest solid yield of 58.8% was achieved by SS-CPBLP. The utilization of cotton pulping black liquor slightly improved the solid yield most likely due to the influence of residues such as lignin, cellulose and hemicellulose in the black liquor on the dissolution of some components. The composition of corn stover is a complex mixture, with cellulose, hemicellulose, and lignin the three major components. Glucose is the major component (36.7%) in corn stover, followed by xylose (20.4%) and acid-insoluble lignin (18.5%). The acid-soluble lignin, extractive and ash contents were 0.7%, 17.6%, and 5.1%, respectively. After pretreatment, the glucose content of the treated corn stover was 44.7% (water), 65.0% (SHP), 62.9% (CPBLP), 64.1% (ASSP) and 62.9% (SS-CPBLP), which was 21.8%, 77.1%, 71.4%, 74.7% and 71.4% higher than the original corn stover, respectively. The xylose content increased to more than 24% for all of the methods except for the water pretreatment. This outcome was not only the result of the dissolution of extractives but also a consequence of the dissolution of a large amount of lignin. This effect will greatly improve the sugar hydrolysis rate of corn stover.

3.2. Delignification rate and enzymatic hydrolysis rate of pretreated corn stover The delignification rate and enzymatic hydrolysis rate are important indices for the evaluation of the effectiveness of the pretreatment. Effective delignification is one of the major results of alkaline pretreatment and causes an improvement in the enzymatic digestibility of the lignocellulosic biomass.

Fig. 2. Enzymatic hydrolysis rate of pretreated corn stover.

The delignification of pretreated corn stover can liberate and expose the internal cellulose, increase the porosity and surface area of the corn stover, and reduce the unproductive binding of cellulase to lignin, which allows the enzymes to penetrate into the fibers and hydrolyzes the cellulose to monomeric sugars (Galbe and Zacchi, 2007; Sun et al., 2000; Yang et al., 2013). Fig. 1 shows the delignification rate of the pretreated corn stover. As shown, approximately 6% of the lignin was removed when the corn stover was pretreated with only water. The delignification rates of SHP, CPBLP and SS-CPBLP were 78.4%, 74.9% and 82.1%, respectively. The ASSP performed excellently on delignification because over 85% of the lignin was removed. This finding is consistent with the results of our previous study (Liu et al., 2015). The excellent delignification rates of ASSP and SS-CPBLP are primarily due to the presence of nucleophilic sulfite (SO3 2− ) in the pretreatment liquor. Nucleophilic sulfite (SO3 2− ) leads to the

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Table 3 Composition and properties of the CPBL and the black liquor collected after SS-CPBLP of corn stover.

CPBL After SS-CPBLP of corn stover

pH

Total solids(g/L)

Glucose(g/L)

Lignin(g/L)

Total titratable alkali (g/L)

Effective alkali (g/L)

Sulfonation degree of black liquor (mmol/g)

13.0 ± 0.1 10.0 ± 0.1

89.0 ± 1.1 119.9 ± 1.5

3.0 ± 0.0 5.3 ± 0.1

6.9 ± 0.1 17.8 ± 0.2

11.5 ± 0.2 9.9 ± 0.2

8.4 ± 0.2 0.6 ± 0.0

– 0.8 ± 0.0

Fig. 3. Yields of (a) total sugars, (b) glucose, and (c) xylose after pretreatment.

sulfonation of lignin as well as the cleavage of ␣-benzyl ether linkages, ␣-alkyl ether linkages, and ␤-benzyl ether linkages on phenolic lignin units (Gierer, 1985). The increased delignification ability of CPBLP and SS-CPBLP might be partially attributed to the appreciable amount of alkali in the CPBL, which improved the dissolution of lignin during corn stover pretreatment. However, some organic compounds (i.e., lignin) in the CPBL influenced the removal of lignin during the corn stover pretreatment process. Therefore, the delignification rates of the pretreatments containing CPBL were consistently lower than the NaOH pretreatment. The enzymatic hydrolysis rate of sugars is displayed in Fig. 2. The enzymatic hydrolysis rates of glucose and xylose without beating were 88.0% and 80.5%, respectively, for the ASSP-treated samples. In contrast, enzymatic hydrolysis rates of only 76.3% and 66.3% were achieved for glucose and xylose, respectively, for the SHPtreated samples under the same conditions because the addition of Na2 SO3 led to the sulfonation of lignin, which made the lignin more hydrophilic and facilitated lignin removal (86.1%). The sulfonation

of lignin also reduces the hydrophobic interactions with enzymes that contribute to the improved enzymatic digestibility of the substrate (Zhu et al., 2009). The enzymatic hydrolysis rate of glucose after SS-CPBLP was slightly lower than ASSP primarily due to the weaker delignification rate compared to ASSP. As shown in Fig. 2, increasing the beating revolutions significantly improved the enzymatic hydrolysis rate of the polysaccharides. However, there was almost no improvement when the beating revolutions increased from 3000 to 4500. Based on the influence of energy consumption, 1500 revolutions were sufficient to improve the hydrolysis rate of both glucose and xylose in SHP and SS-CPBLP, whereas 3000 revolutions were needed to improve the hydrolysis rate of CPBLP and ASSP. 3.3. Sugar yields The total sugar yields under different pretreatment conditions with different beating revolutions are illustrated in Fig. 3(a). The

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total sugar yield increased with the increase in the beating revolutions. Water pretreatment was not sufficient to substantially improve the saccharification of corn stover, and the maximum total sugar yield was 16.3% with PFI refining at 4500 revolutions. The total sugar yields after SHP, CPBLP, ASSP and SS-CPBLP without beating were 63.5%, 66.4%, 75.2% and 75.3%, respectively. The total sugar yield of the CPBL pretreatment was comparable to that of the alkaline pretreatment. The effective alkali in CPBL facilitated the sugar yield increase. As shown in Fig. 3, the total sugar yield after CPBLP with 3000 beating revolutions was 77.3% compared to the 70.3% yield obtained from the SHP-treated samples. With ASSP and SSCPBLP, the total sugar yield increased from 75.2% and 75.3% to 85.1% and 81.8%, respectively, after beating for 3000 revolutions. According to the literature (Xu et al., 2012), the outstanding performance of CPBL in improving the enzymatic hydrolysis of corn stover could be attributed to two factors. The first factor is that high alkalinity of black liquor promotes the removal of lignin. Table 3 shows the composition and properties of CPBL and the black liquor collected after SS-CPBLP. As shown in Table 3, the total titratable alkali and effective alkali of CPBL were 11.5 g/L and 8.4 g/L, respectively, which were sufficient to cause a substantial alteration in the lignocellulose structure. After SS-CPBLP, the total titratable alkali and effective alkali of the black liquor decreased to 9.9 g/L and 0.6 g/L, respectively. The pH of the CPBL was 13.0, whereas the ending pH after SS-CPBLP was 10.0. The second factor is the presence of carbohydrates in the CPBL collected from cotton mill pulping. As shown in Table 3, the glucose content of the black liquor increased by 2.3 g/L after pretreatment. The presence of carbohydrates in the initial cotton pulping black liquor alleviated the degradation of carbohydrates in the corn stover during the SS-CPBLP process. Additionally, the lignin content increased from 6.9 g/L to 17.8 g/L, which facilitated the processing and utilization of the black liquor. Although increasing the revolutions favored the reduction of cellulose crystallinity and improved the sugar yield during enzymatic hydrolysis, it also caused an increase in energy consumption. The glucose and xylose yields of the pretreated corn stover illustrated in Fig. 3(b) and (c) showed regularities similar to the total sugar yield, but the xylose yields were always less than the glucose yields. For instance, the glucose yields after water pretreatment, SHP, CPBLP, ASSP, and SS-CPBLP without beating were 14.9%, 71.9%, 75.4%, 86.1% and 83.9%, whereas the xylose yields were 5.9%, 48.1%, 49.9%, 55.2%, and 59.6%, respectively. Furthermore, the xylose yields of corn stover after CPBLP under the same conditions were significantly higher in most cases compared with the NaOH pretreatment; this effect was an advantage of using CPBL for pretreatment. As shown in Fig. 3(b) and (c), the glucose and xylose yields increased with the increase in the beating revolutions of PFI refining, particularly in the revolutions range of 0–3000. For example, the glucose and xylose yields of SS-CPBLP increased from 83.9% and 59.8% to 92.4% and 62.3%, respectively, as the beating revolutions increased from 0 to 3000. When the beating revolutions of PFI refining was higher than 3000, the changes in the glucose and xylose yields were variable for the different pretreatment methods.

4. Conclusions Black liquor extracted from cotton mill pulping has special features, including a high pH, an appreciable carbohydrate content and a low lignin content, that make it very difficult to handle using conventional methods. However, it can effectively improve the enzymatic hydrolysis of corn stover when it replaces sodium hydroxide for pretreatment. After the cotton pulping black liquor combined with sodium sulfite pretreatment and subsequent beating for 3000 revolutions, the total sugar, glucose, and xylose yields

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