Alkali Recycling from Rice Straw Hydrolyzate by Ultrafiltration: Fouling

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Industrial & Engineering Chemistry Research

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Alkali Recycling from Rice Straw Hydrolyzate by Ultrafiltration: Fouling Mechanism and Pretreatment Efficiency Yun Lia,b, Benkun Qia*, Jianquan Luoa, Yinhua Wana* a

State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China b University of Chinese Academy of Sciences, Beijing 100049, PR China Corresponding Author *E-mail: [email protected] (Y. Wan); [email protected] (B. Qi) Tel/Fax: 86-10-62650673

ABSTRACT Alkaline

pretreatment

of

lignocellulosic

biomass

produced

certain

amounts

of

alkaline-soluble lignin and phenolic compounds in the hydrolyzate, which might bring negative effect on the alkali reuse for continuous biomass pretreatment. In the present work, lignin recovery from the alkaline rice straw hydrolyzate by ceramic ultrafiltration was investigated in terms of lignin retention and fouling mechanisms. Results showed that over 75% of lignin was retained using a ceramic membrane with the molecular weight cut-off (MWCO) of 5000 Da. The relative higher cross-flow velocity and lower pressure led to less resistance and membrane fouling, and complete or intermediate pore blocking was the most possible fouling mechanisms. The alkaline solution from ultrafiltration permeate was recycled for the pretreatment of fresh rice straw after simple pH adjustment. By comparing the composition of solid residues and its enzymatic hydrolyzate after pretreatments with fresh and recycled alkaline solutions, it was found that the alkaline hydrolyzate could be polished by ultrafiltration for further reuse (at least four cycles). Moreover, the alkali recycling from hydrolyzate could retard the release of phenolic compounds during the pretreatment. The consumptions of NaOH and water were reduced by 42% and 50% respectively during 1


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pretreatments with reuse of alkali. Alkali recycling benefits lignocellulose biorefinery by decreasing the costs associated with water and alkali supplementation as well as wastewater treatment. 1. INTRODUCTION Nowadays, the fossil fuels shortage and environmental protection boost the biofuels development based on biomass refining. However, due to the recalcitrance of biomass, pretreatment and fractionation are usually required to release the polysaccharides.1,2 Alkali pretreatment is one of the most effective methods, which could decrease the degree of polymerization and crystallinity, disrupt the lignin structure and increase cellulose digestibility, thus promoting the following saccharification process.3 The color of the biomass hydrolyzate after pretreatment by alkaline solution is generally deep black and therefore also called black liquor. The main composition of the alkaline biomass hydrolyzate is alkali and alkali-soluble lignin. In addition, small amount of hemicellulose and many kinds of phenolic compounds are also present in this hydrolyzate. Due to high pH, BOD, COD and color, alkaline hydrolyzate is quite toxic to the environment. 4

Therefore, it is urgent to find an adequate method to clean the alkaline hydrolyzate. In view

of sustainable development, two objectives should be achieved in the treatment of this hydrolyzate: alkali recycling and lignin recovery. On the one hand, alkali pretreatment consumes large amounts of water and alkali, and alkali recycling allows saving alkali and water 5, as well as decreasing wastewater discharge.6 On the other hand, lignin can be used as dispersant in cement and gypsum blends or chelating agent to remove heavy metals or as an adsorbent agent,7-9 implying that lignin recovery from lignocellulosic hydrolyzate is 2



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economically attractive. Many methods have been used to treat alkaline hydrolyzate, such as traditional combustion, electrocoagulation10, bioaugmentation11, electrolysis12, adsorption13, precipitation and ultrafiltration. However, most of these processes are not suitable due to the occurrence of technical and economic problems. For example, because of the deep color and the complexity of alkaline biomass hydrolyzate, adsorption is not effective.13 Regarding precipitation, the colloids formation during acidic precipitation process resulted in a decrease of the lignin purity.14 Ultrafiltration, as an environmental-friendly separation technology, has been applied to recover lignin from alkaline hydrolyzate.9,15-18, Without chemical supplementation, lignin could be retained by suitable ultrafiltration membrane while the impurities passed through, producing less contaminated lignin than that from chemical method.14 In addition, ultrafiltration could fractionate lignin according to molecular weight for different applications. However, membrane fouling is a major obstacle in practical applications, especially for the feed with complex composition. It was reported that concentration polarization, cake layer, membrane pore blocking and adsorption enhanced the fouling situation and led to serious permeate flux decline.19 Moreover, membrane fouling was affected by feed chemistry, membrane characteristics and operation conditions.20 Thus, it is necessary to study the membrane fouling mechanisms during ultrafiltration of alkaline hydrolyzate and to decrease the membrane fouling by optimizing operation conditions. The present work investigated the lignin recovery and alkali recycling from alkaline rice straw hydrolyzate by ultrafiltration, with focus on the membrane fouling mechanism during 3


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ultrafiltration process, as well as the reuse of alkaline solution in successive lignocellulose pretreatments. To our best knowledge, this was the first attempt to recycle alkali from rice straw hydrolyzate by ultrafiltration. The resistance-in-series and membrane blocking models were used to explain the fouling mechanisms during cross-flow ultrafiltration. The compositions of alkaline hydrolyzate, solid residues and subsequent enzymatic hydrolyzate were analyzed and compared for different pretreatment cycles. The outcome of this work not only offers a simple strategy for valorization of rice straw hydrolyzate, but also provides advice for membrane fouling control during ultrafiltration of alkaline hydrolyzate.

2. MATERIALS AND METHODS 2.1. Raw Material. Rice straw was collected from Jingzhou, Hubei Province, China. The naturally dried rice straw was chopped to 1-2 cm size. It was then milled, screened to collect the fraction of 0.5mm-1mm size and dried at 50 °C for 24 h before use. The main compositions of the raw rice straw were as follows: glucan 39.7%, xylan 15.7%, arabinan 3.4%, acid-soluble lignin (ASL) 3.3%, acid-insoluble lignin (AIL) 17.4% and ash 6.9%. The composition analysis of raw rice straw and pretreated rice straw was conducted using NREL21 methods. 2.2. Alkaline Hydrolysis of Rice Straw. Rice straw was pretreated with 0.5 M NaOH solution at a solid: liquid ratio of 1:10 (w/w) in sealed serum bottles, using an autoclave (LDZX-75KB, Shanghai Shen’an Medical Instrument Factory, China) at 121°C for 2 h. At the end of the reaction, the pretreatment slurry was filtered through filter cloth (200 mesh) in the vacuum filter to separate the solid and the liquid. The liquid was collected to analyze lignin 4



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and phenolic compounds content and this liquid was used for the following ultrafiltration process. The initial composition of hydrolyzate was showed in Table 1. Lignin was the main composition in hydrolyzate. In addition, some phenolic compounds also existed in the liquid The solid part was washed with deionized water until permeate became neutral. After that, this solid residue was dried and used for subsequent enzymatic hydrolysis experiments. 2.3.

Experimental

Set-up,

Membrane

and

Ultrafiltration

Procedure.

The

ultrafiltration set-up consists of a feed tank (2L) with a jacket, a feed pump, an ultrafiltration membrane module and a water bath tank quipped with a circulating pump. A temperature sensor was installed in the feed tank and the feed temperature was controlled by the circulating water bath. A 5000 Da molecular weight cut-off ceramic membrane (TiO2-ZrO2, TAMI, France) was employed in the present study. The ceramic membrane had three channels and the length of tubular membrane was 25 cm. The diameter and thickness of this tubular membrane was 1 cm and 0.1 cm, respectively. And the effective membrane surface area was 0.0084 m2. The new ceramic membrane was first washed with 1% NaOH (w/v) solution at high cross-flow velocity (6 m·s-1) at 50 °C for 1 h and then washed with deionized water until permeate became neutral. After that the water permeability was measured. Cross-flow ultrafiltration experiments were carried out using the ceramic membrane. The alkaline rice straw hydrolyzate was poured into the feed tank. The cross-flow velocity was adjusted and measured using an electronic flowmeter. The transmembrane pressure (TMP) was adjusted using a ball valve and the frequency converter of pump. The experiments were performed at different cross-flow velocity (3, 4 and 5 m·s-1) and TMP (0.1 and 0.2 MPa) with 5


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the same hydrolyzate. All the experiments were carried out at a constant temperature (50 °C). The permeate flux of membrane was measured every 4-5 min to determine the fouling situation. Both permeate and retentate were recycled back to the feed tank in order to assure that the hydrolyzate concentration in the tank was kept constant. In addition, samples of feed and permeate were taken to measure the lignin concentration and calculate the membrane retention of lignin. The total operation time was 5 h. In concentration mode, permeate was collected in the bottle for the alkali reuse. After each run, the feed solution was replaced by the pure water and the water permeability was measured. Then the membrane was washed with pure deionized water followed by permeability measurement. At last, the membrane was cleaned at 50 °C with an aqueous NaOH solution of 1% (w/v) for 1h and then washed with deionized water. After the chemical cleaning, the water permeability of membrane was measured again. 2.4. Alkali Recycling. After ultrafiltration process, the main compositions of permeate were alkali and some small phenolic compounds. The permeate was further used as delignification solution in the pretreatment of raw rice straw, under the same conditions described in section 2.2. pH was adjusted to the same value as the initial (nearly 13.90) with NaOH solid supplementation before subsequent pretreatment. This alkaline liquid was recycled for four times and the diagram of whole experimental process is shown in Fig.1.

6



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Fig. 1 Schematic diagram of whole experimental process 2.5. Enzymatic Hydrolysis. In order to evaluate the pretreatment effect of recycled alkali on raw rice straw, the solid residues after alkali pretreatments were used for the enzymatic hydrolysis. Enzymatic hydrolysis was performed in 50 mL of citrate buffer (50 mM, pH 4.8) at 10% (w/v) solid loading with commercial cellulase (Celluclast C2730 derived from Trichoderma reesei, Sigma-Aldrich) at 50°C and 150 rpm for 72 h. The cellulase loading was 20 FPU/g glucan. Samples were taken periodically (6, 12, 24, 48, and 72 h) for glucose analysis after centrifuging at 9000 rpm for 15 min. 2.6. Assessment of Fouling Mechanisms. 2.6.1. Resistance-in-series Model. Resistance-in-series model22-23 is a classical model to describe the fouling process. This model analyzes four types of resistance that could lead to the decline of permeate flux and there is a basic equation to interpret this fouling mechanism: 7


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=

∆

( )

=

∆

(1)

( )

where is the steady filtration flux (m·s-1), ∆P is the transmembrane pressure (Pa), μ is the dynamic viscosity of permeate (Pa·s-1) and Rtot is the total resistance (m-1). Rc, Rg, Ra and Rm represent the resistance due to the concentration polarization layer, gel layer, the internal membrane adsorption fouling and virgin membrane (m-1), respectively. Rm can be calculated from the water flux of clean membrane; Rtot can be obtained from the steady flux with feed solution; the sum of Rm, Rg and Ra can be determined from the water flux when the feed was replaced by deionized water because concentration polarization layer was dispersed; the sum of Rm and Ra can be acquired from the water flux after the gel layer was removed by washing with water.

2.6.2. Membrane Blocking Models. Hermia24 developed four empirical models for dead-end filtration based on constant pressure condition to analyze the membrane fouling mechanism. Vela et al. developed an equation by adapting these models to cross-flow ultrafiltration19:

− = ( − )

(2)

where J is the permeate flux (m·s-1) and is the steady-state permeate flux. The parameter

n depends on the type of fouling: complete blocking (n=2), intermediate blocking (n=1), standard blocking (n=3/2) and cake layer formation (n=0). The details of four blocking models are shown in Table S1. By fitting experimental data using these linear models and comparing their regression coefficients (R2), the most possible fouling mechanism can be found for the membrane processes under different conditions. 2.7. Analytical Methods. Lignin concentration was measured by the absorption value at 8



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280 nm using an UV-vis spectrophotometer (UV757CRT，Lengguang Technology Co., Ltd., Shanghai，China)25. Phenolic compounds were determined by LC (Shimadzu Corp., Kyoto, Japan), equipped with a C18 Column (4.6×250 mm, TC- C18, Agilent, USA) and an UV detector at 320 nm (SPD-20A, Shimadzu Corp., Kyoto, Japan). The mobile phase was acetonitrile/water (20:80) and 1% (v/v) acetic acid at a flow rate of 1 ml/min.26 The injection volume was 20 µL. Sugars concentrations were determined by LC according to previous method27. Dynamic viscosity of permeate was analyzed by Dynamic viscometer (NDJ-1B, Changji Geological Instrument Co., Ltd, Shanghai, China). Five indexes of hydrolyzate and ultrafiltration permeate were carried out by Pony Testing International Group (Beijing, China). Briefly, chromaticity and turbidity were determined by colorimetric method. Titration method was used to measure the total alkalinity. COD was determined by dichromate titration method, which estimated the organics content in alkaline hydrolyzate, mainly comprising lignin, phenolic compounds and hemicellulose. Suspended substance content was analyzed by filtration and weighting methods, which represented the particles in the pretreated rice straw after pre-filtration by 200 mesh filter cloth (liquid-solid separation). Specifically, liquid sample was filtrated by a microfiltration membrane. Then, the membrane was dried at 103-105°C for half hour, cooled and weighed.

3. RESULTS AND DISCUSSION 3.1. Effect of Operating Conditions. Permeate flux and solute retention are two important parameters in membrane separation. As shown in Fig. 2 (a), the permeate flux increased with increasing TMP and cross-flow velocity due to higher filtration driving force 9


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and less fouling resistance, respectively. Fig. 2 (b) shows the lignin retentions under various operating conditions. The observed retention of lignin, !"

= #1 −

% &

!" ,

is defined as

' × 100%

(3)

where + and + are the lignin concentrations in permeate and feed solution, respectively. All the lignin retentions were above 72% under the tested conditions, indicating most lignin was retained by this ultrafiltration membrane. Lignin retention slightly increased with increasing TMP and the highest lignin retention achieved 76.0%. This could be explained as follow: the increasing pressure caused more solvent passing through the membrane pores than solutes due to the stronger interaction of water with the hydrophilic active layer of the membrane and the stronger size exclusion of the solutes, thus resulting in the lower solute concentration in the permeate (i.e. higher solute retentions). In addition, the more compact fouling layer at higher TMP acting as a secondary layer might cause higher rejection of lignin. Similar explanation was also reported by Bhattacharjee et al

28

during ultrafiltration of Kraft

black liquor. The lignin retention was also improved when cross-flow velocity increased from 3 to 5 m·s-1 due to the less concentration polarization (less diffusion gradient) at higher cross-flow velocity. (a)

(b)

10



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Fig. 2 Effect of different operating conditions on permeate flux (a) and lignin retention (b) 3.2.

Fouling

Mechanisms

Analysis.

3.2.1.

Resistance-in-series

Model.

Resistance-in-series model was used to analyze the fouling resistances that lead to flux decline during cross-flow ultrafiltration of alkaline rice straw hydrolyzate. Fouling resistance under different operating conditions is shown in Fig.3. It should be noted that the total resistance decreased with increasing cross-flow velocity at both TMPs but the reasons were different. At 0.1 MPa, the decreasing adsorption fouling at higher cross-flow velocity was responsible for the variation of total resistance, while the concentration polarization was more important to this variation at 0.2 MPa, indicating that higher shear rate on the membrane was prone to alleviate the adsorption fouling at lower TMP and to control the concentration polarization formation at higher TMP, respectively. Thus, a relatively high cross-flow velocity (e.g. 5 m·s-1) should be applied in order to decrease the fouling resistance. (a)

(b)

11


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Fig. 3 Fouling resistances under different operating conditions. The crossflow velocity: 3, 4 and 5 m·s-1; TMP: 0.1 MPa (a) and 0.2 MPa (b). Rtot is total filtration resistance, Rm is the membrane hydraulic resistance, Rc is the resistance of concentration polarization layer, Rg is the gel layer resistance, and Ra is the resistance adsorption fouling Fig. 4 shows the percentage of four resistances in the total resistance under different operating conditions. It can be seen that adsorption fouling and gel formation accounted for more than 60% of the total resistance, which were the main fouling types in this case. Only at a high TMP of 0.2 MPa and a low cross-flow velocity of 3 m·s-1, the concentration polarization resistance became obvious (more than 12%).

Fig. 4 Percentage of four resistances in the total resistance under different operating 12



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conditions. Rtot is total filtration resistance, Rm is the membrane hydraulic resistance, Rc is the resistance of concentration polarization layer, Rg is the gel layer resistance, and Ra is the resistance adsorption fouling

3.2.2. Membrane Blocking Models. Table S2 shows the regression coefficients (R2) of different membrane blocking models by fitting the flux data with time under different conditions. From Table S2, it can be found that complete and intermediate pore blockings were the most possible fouling mechanisms during the filtration of alkaline rice straw hydrolyzate by 5000 Da ultrafiltration, regardless of operating conditions. This indicated that the size of most lignin molecules in the alkaline hydrolyzate was larger than the pore size of tested membrane and the lignin would not adsorb onto the pore wall to form standard blocking. Besides lignin, small amount of hemicellulose was also dissolved in the hydrolyzate. According to Jönsson’s report17, the retention of hemicellulose was higher than that of lignin when ultrafiltration membrane with molecular weight cut-off (MWCO) of 4 -100 kDa was used for treating kraft cooking liquor. Thus, hemicellulose molecules could also contribute to membrane fouling on the surface of membrane. Moreover, considering the analysis of resistance-in-series model in Fig. 4, the resistances of virgin membrane, adsorption fouling and gel layer showed approximately equal contribution to the total resistance. For the gel layer resistance, it might not be dense and compact as a cake layer, implying that the fouling formation was not serious in this case because the alkaline pH of the feed had a self-cleaning effect on the organic fouling. 3.3. Characteristics of Hydrolyzate and Ultrafiltration Permeate. In Table S3, the main characteristics of alkaline hydrolyzate (feed) and ultrafiltration permeate was compared. 13


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As can be seen, the color of virgin alkaline hydrolyzate was deep black and the chromaticity was 1600 times. After ultrafiltration, the chromaticity of permeate decreased by 50%. In addition, the turbidity, suspend substance (SS) and chemical oxygen demand (COD) of permeate decreased by 90.7%, 97.2% and 69.0%, respectively, compared with those of the hydrolyzate. It meant that ultrafiltration removed most of the suspended particles and retained most of organics in the concentrate, making permeate clearer. Since the molecular weight of NaOH was much smaller than the molecular weight cut-off of membrane, theoretically, all the NaOH molecular could pass through the membrane. However, as shown in Table S3, the total alkalinity of the permeate was 70.2% of the alkaline hydrolyzate, indicating that there was an alkali loss during the ultrafiltration process. The possible reason for this was that there was some water residue in the pipes and pump of ultrafiltration system, resulting in a dilution of the permeate. This was inevitable in this case due to the limitation of device. Nevertheless, the NaOH in the ultrafiltration permeate might be reused in the following delignification process. 3.4. Compositions of Hydrolyzate and Ultrafiltration Permeate during Alkali Recycling. During alkali recycling for four times, the compositions of the hydrolyzate and ultrafiltration permeate changed greatly and the results are shown in Table 2. Besides the lignin, the hydrolyzate also contained some small phenolic compounds. The size of these phenolic molecules was much smaller than the pore size of membrane and therefore they could easily pass through membrane and accumulate in the ultrafiltration permeate with increasing cycle times (Table 2). It is also worth mentioning that these phenolic compounds are high value-added products and can be recovered by alkali-resistant nanofiltration for further use, and accordingly, the quality of alkali for reuse can be upgraded by such 14



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nanofiltration treatment.

Fig. 5 Recovery of lignin and lignin yield during four pretreatment cycles Fig. 5 shows the lignin recovery by ultrafiltration during four recycling times, which was calculated according to the following equation: Recovery(%) = #1 −

% ×3% & ×3&

' × 100

(4)

where Cp and CF are the lignin concentrations in permeate and feed, respectively. Vp and VF are the volumes of permeate and feed, respectively. Although ultrafiltration could retain most of lignin, there was still some short-chain lignin passing through the membrane. This small lignin accumulated in the ultrafiltration permeate and its concentration increased with increasing recycling times, resulting in a decline of lignin recovery (Fig.5). Thus, nanofiltration could be used for further removing these small molecular lignin from ultrafiltration permeate to improve the quality of reused alkali. In addition, it should be noted that the lignin yield (i.e. fresh lignin released in each pretreatment cycle) kept almost stable between 0.116 and 0.133 g·g-1 rice straw during four alkali reuse cycles (Fig.5), implying that the recycled alkaline solution was still highly 15


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effective for the delignification. Moreover, the recycled alkaline solution pretreatment also disrupted a little amount of hemicellulose in the hydrolyzate and the hemicellulose yields were between 0.016 and 0.020 g·g-1 rice straw during four alkali reuse cycles (data not shown). 3.5. Properties of Rice Straw Residues Pretreated by Recycled Alkaline Solution. In order to further verify the feasibility of reusing alkaline solution for the delignification, the composition of pretreated rice straw residues was analyzed and compared. The total lignin (ASL and AIL) and glucan contents of raw rice straw were 20.7% and 39.7%, respectively. While as shown in Fig.6 (a) for the pretreated rice straw, the total lignin content decreased to 4.6% and the glucan content increased to 58.9% due to the removal of lignin. When using recycled alkaline solution (ultrafiltration permeate) for the delignification of raw rice straw, the residual lignin content in four cycles was nearly the same as that using fresh alkaline solution, which confirmed that the recycling of alkali from the hydrolyzate for the delignification was feasible. The glucose yield during enzymatic hydrolysis of rice straw residues pretreated by alkaline solution is shown in Fig.6 (b). The enzymatic digestibility of alkali pretreated rice straw was greatly enhanced and a relatively high glucose yield more than 45% was achieved after 72 h enzymatic hydrolysis compared with raw rice straw. For the pretreatments with recycled alkaline solution, the glucose yields during enzymatic hydrolysis of rice straw residues kept around 40-50%, being similar as that using fresh alkaline solution for the pretreatment. Therefore, pretreatment of rice straw with recycled alkaline liquor would not have negative effect on the subsequent enzymatic hydrolysis. 16



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(a)

(b)

Fig. 6 Composition of rice straw residues pretreated by alkaline solution (a) and the glucose yield during enzymatic hydrolysis of rice straw residues pretreated by alkaline solution (b). “0” refers to the rice straw residues pretreated by fresh alkaline solution, “1”-“4” refers to rice straw residues pretreated by recycled alkaline solution. Rocha et al.29 and Cheng et al.

30

investigated the delignification of sugarcane bagasse

with recycled alkali. However, in their work, the lignin was not removed from the rice straw hydrolyzate in each round. Therefore, the lignin accumulation in the hydrolyzate resulted in an increase of hydrolyzate viscosity, decreasing the recycling times and weakening the delignification efficiency. Moreover, without separation process, lignin cannot be recovered and used efficiently. 3.6. Evaluation of Water and Alkali Consumptions. An overall water and alkali consumptions during pretreatment of rice straw with alkali recycling is shown in Table 3. NaOH and water consumptions in the first pretreatment round were 0.200 g·g-1 and 0.010 L·g-1 rice straw, respectively. After recycling alkaline solution for four times, the total consumption of NaOH decreased to 0.116 g·g-1 rice straw, which saved 42% of NaOH 17


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compared with that without alkali recycling. At the same time, the consumption of water decreased to 0.005 L·g-1 rice straw, indicating that 50% of water was saved. Therefore, recycling alkali greatly decreased the consumption of alkali and water, which would have great practical value in biorefinery applications. In addition, alkali recycling also could decrease the wastewater discharge from pretreatment of biomass and benefit the environmental sustainability.

4. CONCLUSIONS A ceramic membrane with 5000 Da MWCO was applied in cross-flow ultrafiltration of alkaline rice straw hydrolyzate. Over 75% of the lignin was retained. The fouling resistance was increased with decreasing cross-flow velocity and increasing TMP, while complete or intermediate pore blocking was the most possible fouling mechanisms. Moreover, the alkaline solution in the ultrafiltration permeate could be reused for the pretreatment of raw rice straw, which achieved a high delignification (75%) and a stable cellulose conversion rate (50%) for at least four recycling times. Alkali recycling could reduce alkali and water consumptions by 42% and 50% during rice straw pretreatment, respectively.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors would like to thank the National Natural Science Foundation of China (Grant No. 18



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21306211) and the National High Technology Research and Development Program of China (863 Program, Grant No.2014AA021902) for the financial support.

SUPPORTING INFORMATION Mathematics Descriptions of Four Empirical Fouling Models in Cross-flow Ultrafiltration (Table S1); Coefficient R2 of Linear Regression of Fitting Experimental Data According to Membrane Blocking Models (Table S2); Main Features of Alkaline Hydrolyzate and Ultrafiltration Permeate (Table S3). The supporting information is available free of charge via the Internet at http: //pubs.acs.org.

REFERENCES (1) Galbe, M.; Zacchi, G. Pretreatment: The Key to Efficient Utilization of Lignocellulosic Materials. Biomass Bioenerg. 2012, 46, 70. (2) Jacquet, N.; Maniet, G.; Vanderghem, C.; Delvigne, F.; Richel, A. Application of Steam Explosion as Pretreatment on Lignocellulosic Material: A Review. Ind. Eng. Chem. Res. 2015,

54, 2593. (3) Singh, R.; Shukla, A.; Tiwari, S.; Srivastava, M. A Review on Delignification of Lignocellulosic Biomass for Enhancement of Ethanol Production Potential. Renew. Sust.

Energ. Rev. 2014, 32, 713. (4) Grover, R.; Marwaha, S.; Kennedy, J. Studies on the Use of An Anaerobic Baffled Reactor for the Continuous Anaerobic Digestion of Pulp and Paper Mill Black Liquors. Process

Biochem. 1999, 34, 653. 19


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(5) Serrano, L.; Ferrer, A.; Labidi, J.; Rodríguez, A. Reuse of Ultrafiltration Permeate as A Preliminary Step In the Pulp and Paper Production. Cellulose Chem. Technol. 2014, 48, 553. (6) Balat, M. Production of Bioethanol From Lignocellulosic Materials via The Biochemical Pathway: A Review. Energ. Convers. Manage. 2011, 52, 858. (7) Buranov, A. U.; Mazza, G. Lignin in Straw of Herbaceous Crops. Ind. Crop. Prod. 2008,

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2014, 59, 63.

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Table 1 Composition

Concentration (g·L-1)

Lignin

18.376

Ferulic acid

0.634

p-coumaric acid

0.166

Vanillic acid

0.042

Vanillin

0.052

Syringaldehyde

0.039

p-hydroxybenzaldehyde

0.117

24



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Table 2 Main Compositions of Hydrolyzate (feed) and Ultrafiltration Permeate after the Pretreatment with Fresh and Recycled Alkaline Solution

Recycle

UF

Lignin

Volume

Ferulic acid p-coumaric acid Vanillic acid

Vanillin Syringaldehyde

p-hydroxybenzaldehyde

(g/L)

(L)

(g·L-1)

(g·L-1)

(g·L-1)

(g·L-1)

(g·L-1)

(g·L-1)

feed

18.376

7.5

0.634

0.166

0.042

0.052

0.039

0.117

permeate

4.962

5.

0.633

0.122

0.032

0.047

0.037

0.113

feed

26.112

3.5

0.745

0.176

0.087

0.089

0.046

0.242

permeate

10.297

2.6

0.486

0.123

0.071

0.053

0.037

0.224

feed

27.860

2.2

0.708

0.212

0.120

0.117

0.042

0.421

permeate

12.722

1.7

0.666

0.211

0.102

0.092

0.036

0.340

feed

33.516

1.2

0.797

0.377

0.133

0.139

0.052

0.493

permeate

17.611

0.8

0.761

0.275

0.114

0.106

0.046

0.352

feed

36.305

0.6

0.983

0.380

0.148

0.140

0.063

0.472

times 0

1

2

3

4

25


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1 2 3 4 5 6 7 Table 3 Evaluation of NaOH and Water Consumptions for the Delignification Process with Alkali Recycling 8 9 10 Rice straw (g) NaOH supplement Deionized Water Ultrafiltration feed Permeate Residue NaOH (g) 11Recycle time 12 13 (g)a supplement (L)b volume (L) volume (L) 14 15 160 1000 200 10 7.5 5.0 80 17 18 1 500 20 0 3.5 2.6 40 19 20 212 260 12 0 2.2 1.7 27 22 23 3 170 7 0 1.2 0.8 11 24 25 264 80 5 0 0.6 27 28 Total consumption 2100 244 29 30 a 31 NaOH was supplemented to the ultrafiltration permeate to achieve the same pH value (13.90) for next pretreatment cycle. Because the 32 33 ultrafiltration permeate contained most of origin alkali, the supplement of NaOH was relatively small for each cycle. 34 35 b 36 There was no deionized water supplement in the pretreatment cycle with reused alkali 37 38 39 40 41 26 42 43 44 45 46 ACS Paragon Plus Environment 47 48


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Figure Captions Figure 1. Schematic diagram of whole experimental process. Figure 2. Effect of different operating conditions on permeate flux (a) and lignin retention (b). Figure 3. Fouling resistances under different operating conditions. The crossflow velocity: 3, 4 and 5 m·s-1; TMP: 0.1 MPa (a) and 0.2 MPa (b). Rtot is total filtration resistance, Rm is the membrane hydraulic resistance, Rc is the resistance of concentration polarization layer, Rg is the gel layer resistance, and Ra is the resistance adsorption fouling. Figure 4. Percentage of four resistances in the total resistance under different operating conditions. Rtot is total filtration resistance, Rm is the membrane hydraulic resistance, Rc is the resistance of concentration polarization layer, Rg is the gel layer resistance, and Ra is the resistance adsorption fouling. Figure 5. Recovery of lignin and lignin yield during four pretreatment cycles. Figure 6. Composition of rice straw residues pretreated by alkaline solution (a) and the glucose yield during enzymatic hydrolysis of rice straw residues pretreated by alkaline solution (b). “0” refers to the rice straw residues pretreated by fresh alkaline solution, “1”-“4” refers to rice straw residues pretreated by recycled alkaline solution.

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Alkali Recycling from Rice Straw Hydrolyzate by Ultrafiltration: Fouling

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