Lignin and sodium lignosulfonate production from the black liquor generated during the production of bioethanol from rice straw

Rice straw is considered as an abundant resource to recover hemicellulose, cellulose, and lignin. Lignin, considered as waste from the biomass pretreatment process for bioethanol production, has numerous applications in value-added chemical products such as lignosulfonate, vanillin, guaiacol, quinones, cresol, etc. Lignosulfonate was used in various industrial processes such as concrete admixtures, oil well dispersants, dyestuff, coal water slurry dispersants, agricultural chemicals, and other industrial binders. However, lignosulfonate production from rice straw is less attractive due to the difficulty of lignin recovery. Therefore, the objective of this study is to recover lignin and optimize the sulfomethylation of lignin originated from rice straw by sodium sulfite for the highest reaction efficiency to produce lignosulfonate. The effects of reactive conditions, including reaction time, temperature, and sodium sulfite/lignin ratio on the sulfomethylation are investigated by experiments and analyzed by response surface methodology (RSM). The results demonstrate that second-order quadratic modeling is compatible with experimental data (R-squared value of 99.9 %), and all three factors have a great impact on reaction efficiency (p < 0.05). The optimal reaction condition predicted from the empirical modeling is the reaction time of 150 minutes, the temperature of 80 oC, and the sodium sulfite/lignin ratio of 1:2 g/g. The verified experiment at the optimal condition produces sodium lignosulfonate with a high reaction efficiency of 96.2 % and low surface tension of 48 N/m.

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Vietnam Journal of Science and Technology 58 (6A) (2020) 63-72 doi:10.15625/2525-2518/58/6A/15465 LIGNIN AND SODIUM LIGNOSULFONATE PRODUCTION FROM THE BLACK LIQUOR GENERATED DURING THE PRODUCTION OF BIOETHANOL FROM RICE STRAW Do Huu Nghi 1, * , Le Minh Tan 2, 3 , Duong Hoang Phi Yen 2, 3 , Do Nguyen Hoang Nga 2, 3 , Doan Ly Xuan Huong 2, 3 , Tran Tan Viet 2, 3 , Le Thi Kim Phung 2, 3, * 1 Institute of Natural Products Chemistry, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Ha Noi, Viet Nam 2 Refinery and Petrochemicals Technology Research Center (RPTC), Ho Chi Minh City University of Technology (HCMUT), 268 Ly Thuong Kiet Street, Ho Chi Minh City, Viet Nam 3 Vietnam National University Ho Chi Minh City (VNU-HCM), Linh Trung , Thu Duc District, Ho Chi Minh City, Viet Nam * Email: phungle@hcmut.edu.vn, nghi@inpc.vast.vn Received: 5 September 2020; Accepted for publication: 29 December 2020 Abstract. Rice straw is considered as an abundant resource to recover hemicellulose, cellulose, and lignin. Lignin, considered as waste from the biomass pretreatment process for bioethanol production, has numerous applications in value-added chemical products such as lignosulfonate, vanillin, guaiacol, quinones, cresol, etc. Lignosulfonate was used in various industrial processes such as concrete admixtures, oil well dispersants, dyestuff, coal water slurry dispersants, agricultural chemicals, and other industrial binders. However, lignosulfonate production from rice straw is less attractive due to the difficulty of lignin recovery. Therefore, the objective of this study is to recover lignin and optimize the sulfomethylation of lignin originated from rice straw by sodium sulfite for the highest reaction efficiency to produce lignosulfonate. The effects of reactive conditions, including reaction time, temperature, and sodium sulfite/lignin ratio on the sulfomethylation are investigated by experiments and analyzed by response surface methodology (RSM). The results demonstrate that second-order quadratic modeling is compatible with experimental data (R-squared value of 99.9 %), and all three factors have a great impact on reaction efficiency (p < 0.05). The optimal reaction condition predicted from the empirical modeling is the reaction time of 150 minutes, the temperature of 80 o C, and the sodium sulfite/lignin ratio of 1:2 g/g. The verified experiment at the optimal condition produces sodium lignosulfonate with a high reaction efficiency of 96.2 % and low surface tension of 48 N/m. Keywords: lignosulfonate, lignin, rice straw, sulfomethylation. Classification numbers: 1.1.2, 1.3.2, 3.7.3. 1. INTRODUCTION Do Huu Nghi, Le Thi Kim Phung, et al. 64 Lignin is considered as one of the abundant and renewable natural resources. In the plant, lignin plays a role as an adhesive of hemicellulose and cellulose. It makes the plant cell wall strongly hydrophilic, and anti-microorganism [1]. Additionally, due to the polyaromatic structure of lignin, lignin can be applied to produce the aromatic chemicals likewise benzene, phenol methoxyl phenol, cyclohexane, etc. Moreover, lignin also can be used as a desperation, stabilization, or admixture chemical in rubber production, construction, or plastic industry [2]. However, the application of lignin faced many challenges such as the difficulties of recovery lignin, the solubility of lignin, the impurities of lignin, etc. Therefore, improvement of lignin structure is one of the keys to solving that problem, and lignosulfonate with high water solubility is a solution [3]. Lignosulfonate can be used as a dispersant, binder in concrete admixtures, de- foamer, etc. Thus, the study of production of lignosulfonate is necessary for expanding its application in high-value products. Rice straw accounts for 75 % of agricultural waste in Viet Nam. Lignin is one of the main components of rice straw, accounts for 20 % of the weight of rice straw, which indicated the potential of rice straw for recovery lignin [4]. The recovery and application of rice straw lignin are hindered because of the intricate linkage of lignin and other components in rice straw such as cellulose, hemicellulose, or silica. There are many researches about developing the method, recovering lignin from rice straw effectively. Therein, pretreatment is an outstanding method to solve this situation [5]. Moreover, black liquor, waste from the stage of rice straw pretreatment for bioethanol production, is absolutely intricate to treat and cause negative influences on the environment. However, with high lignin content, this liquor is known as a potential source for lignin production. Therefore, in this study, the rice straw was pretreated with sodium hydroxide as the step of rice straw pretreatment in bioethanol production, and then lignin was recovered by acidification method. The obtained lignin was characterized by Fourier-transform infrared (FT-IR) and X-ray powder diffraction (XRD) to confirm the chemical structure, inorganic matter. To produce sodium lignosulfonate (NaLS), the sulfomethylation was conducted between lignin, formalin, and sodium sulfite. This process was optimized of reaction condition for the best of reaction yield and mass of sodium lignosulfonate by response surface methodology (RSM). The obtained sodium lignosulfonate from the optimal condition was determined by chemical structure and surface tension. 2. MATERIALS AND METHODS 2.1. Materials 2.1.1. Rice straw Rice straw was collected from Thai My village, Cu Chi district, Ho Chi Minh City, Viet Nam. The paddy straw was thoroughly rinsed with water and dried under sunlight until their moisture is 15 % or under. After that, the dried rice straw was cut into pieces of 0.5 - 1 cm and packaged in a closed bag for further use. 2.1.2. Lignin Lignin and sodium lignosulfonate production from the black liquor generated during 65 Lignin was recovered by the alkaline method, which has been developed at the pilot scale for bioethanol production [6]. Rice straw was soaked with sodium hydroxide (NaOH) 1M at 90 o C, the ratio of rice straw to NaOH of 1/15 (g/mL), for 2 h. After that, the obtained liquid was filtrated for rice straw residue and then, adjusted to a pH value of 5.5 by hydrochloric acid (HCl) and added with ethanol 95 % following the ethanol/liquid ratio of 3/1. The liquid was then stabilized for 6 hours before precipitation was removed. Next, the filtrate was evaporated and acidified to a pH of 1.5 with HCl to recover the lignin. The obtained lignin was dried at 105 °C for 8 h [7]. The following reagents were purchased from commercial suppliers in pure grade: sodium hydroxide (NaOH), sodium sulfite (Na2SO3), hydrochloric acid (HCl), and formalin (HCHO). 2.2. Synthesis of sodium lignosulfonate from lignin method A volume of 0.5 mL of formalin 37 w/v% (HCHO) and 0.5 g sodium sulfite (Na2SO3) was heated to reaction temperature before 1 g lignin was added. The mixture was simultaneously mixing and heating in controlled condition as reaction time, temperature, and Na2SO3-Lignin ratio (Table 1). When the reaction finished, the mixture was cooled down to reach room temperature; then the precipitate was removed by vacuum filtration. The filtration was adjusted to a pH value of 7 by HCl 0.5 M before incubating at 0 - 5 o C to separate inorganic salts by vacuum filtration. The obtained liquid was evaporated and dried at 105 o C to gain NaLS [8]. The reaction yield was calculated by the formula: H = where: A is the mass of lignin before reaction, and B is the mass of excess lignin after the reaction. 2.3. Analysis method The rice straw composition was analyzed by the NREL/TP-510-42618 method [9]. The ash content of lignin was found by treating samples at 900 ± 25 °C for 6 hours in Nabertherm muffle furnace, model LT3/11. The chemical structure of lignin and lignosulfonate was determined by FT-IR spectra, ranging from 400 to 4000 cm -1 with a 4 cm -1 resolution, acquired on KBr pellets using a PerkinElmer Frontier IR instrument. The XRD analysis was conducted by using Bruker-D8 Model equipment to record the diffraction patterns. The operating program was from 10 to 80º (2) with an angle step of 0.019º and a time step of 43.00 s at ambient condition. A CuKα Ni-filtered radiation (λ = 1.5406 Å) was applied with a working voltage of 40 kV. The surface tension was determined by the droplet volume method and calculated by the formula [10]: Therein: V: Volume of a droplet (m 3 ); D: Density of liquid (kg/m 3 ); g: Gravitational acceleration (m/s 2 ); r: The capillary radius (m). Do Huu Nghi, Le Thi Kim Phung, et al. 66 2.4. Optimization method The experiments were followed by Box-Behnken design to find out the most suitable regression equation [11]. In this study, the independent variables including temperature (X1: 60 C - 100 C), time (X2: 30 - 240 min), Na2SO3/lignin ratio (X3: 0.1 - 0.5 g/g) were investigated for their influence on 2 response of yield of sulfomethylation (Y1 - %) and mass of NaLS (Y2 - g). Therefore, the design for three-variable optimization with its 15 experimental points, including 3 replicates, at the center points. The center points defined the experimental error and reproducibility of the data. The codification of the levels of the independent variable consists of transforming each real value into coordinates inside a scale with dimensionless values, which must be proportional to its localization in the experimental space [12]. The code levels of the independent variables correspondence with real value are described in Table 1. The values of the independent variables are expressed in codes, as −1, 0, and +1 interval, corresponding to the lower, center, and upper levels of each variable, respectively. Each experiment was performed three times to obtain the average Y1 and Y2 values. Table 1. The code levels of the independent variables correspondence with real value. Coded Time (X1) Temperature (X2) Na2SO3/lignin ratio (X3) -1 30 60 0.1 0 135 80 0.3 +1 240 100 0.5 3. RESULTS AND DISCUSSION 3.1. The characterization of rice straw recovered lignin The composition of rice straw as shown in Table 2, illustrates the complicated linkages of lignin and other components, especially the bonding of lignin and silica (ash); therein lignin associates with polysaccharides, especially hemicellulose, via covalent bonds to form lignin- carbohydrate complexes, whereas silica interacted with cellulose and lignin [13, 14]. According to Kargbo et al., the silica content accounts for approximately 75 % weight of rice straw’s ash [15]. Moreover, when rice straw was soaked with NaOH in the pretreatment step of bioethanol production, the linkages of lignin and the other is released and the amount of lignin and silica is dissolved. The waste liquid from this step, known as black liquor, contains lignin and silica and is difficult to treat. Therefore, based on previous studies, the black liquor was acidified by HCl to pH 1.5 as a description in section 2.1.2 to recovery lignin for lignosulfonate synthesis [7]. Table 2. The component of rice straw. Cellulose (%) Hemicellulose (%) Lignin (%) Ash (%) Moisture (%) Rice straw 45.7 22.45 19.6 12.25 15.27 Lignin and sodium lignosulfonate production from the black liquor generated during 67 The chemical structure of obtained lignin from acidification black liquor was confirmed by FT-IR spectra and XRD patterns, shown in Figure 1. Therein, the domination band from 3000 to 3500 cm -1 assigned to OH stretching vibration. The aromatic skeletal vibration (C=C) was found around 1510 cm -1 , 1605 cm -1 , illustrated the aromatic structure of lignin. Additionally, the specific peak of lignin appeared at 1260 and 1031 cm -1 (guaiacyl) and 1126 cm -1 (syringyl). Moreover, the results revealed that the spectrum of obtained lignin is similar to spectra reported by other authors [16–18]. Thus, it can be absolutely verified that lignin was recovered from rice straw. The XRD spectra consist of peaks at 31.5 o , 45.5 o, and 56 o , which reveals existing of inorganic matter like sodium chloride (NaCl), silica, etc. [19, 20]. The ash content of lignin only is 9.31 %, and NaCl contaminated in the lignin can be removed by rinsing with distilled water [7, 21]. 3.2. The optimal condition of sulfomethylation The optimization of sulfomethylation is conducted with 2 responses of yield and mass of NaLS, and the result in Table 1 shows that the yield of reaction and the mass of NaLS ranges from 57 % to 97.2 % and from 0.57 g to 1.28 g, respectively. Therein, the highest yield of sulfomethylation (97.2 %) is obtained at the temperature of 80 o C, the time of reaction of 240 minutes, and the Na2SO3/lignin ratio of 0.5 or at the temperature of 100 o C, the time of reaction of 135 minutes, and the Na2SO3/lignin ratio of 0.5. The corresponding reaction conditions for the highest NaLS content is at the temperature of 100 o C, the time of reaction of 135 minutes, and the Na2SO3/lignin ratio of 0.5. The regression analysis in Table 2, exported from Design Expert 11.0.4 software, indicated that all of the variables significantly impacted on the responses because of p-value < 0.05 [12]. X3 variable has a strong influence on 2 responses as similarly as the effect of X3 2 variable on the Y1 response due to p-value < 0.0001. The F-values of Lack of Fit of Y1 and Y2 responses were 5.37 and 6.54, respectively, implying that the lack of fit was insignificant. The coefficient of determination (R 2 ) was 0.9990 (Y1) and 0.9892 (Y2), mean that more than 99.90 % and 98.92 % of the response variability were explained, and the ability of the established model. The Model F-value is very high, and the model p-value is very low on 2 responses, so there is only a 0.01 % chance that a Model F-value so large could occur due to noise. b a Figure 1. The FT-IR (a) and XRD (b) spectra of lignin. Do Huu Nghi, Le Thi Kim Phung, et al. 68 Table 3. Box-Behnken design with three factors and response of NaLS mass. No. Time X1 (Min) Temperature X2 ( o C) Na2SO3/lignin ratio X3 (g/1g) Yield Y1 (%) Mass of NaLS Y2 (g) 1 30 60 0.3 80 0.82 2 240 60 0.3 83.7 0.75 3 30 100 0.3 84.5 0.87 4 240 100 0.3 85 0.82 5 30 80 0.5 95.6 1.15 6 240 80 0.5 97.2 1.06 7 30 80 0.1 56.8 0.62 8 240 80 0.1 58.2 0.57 9 135 60 0.5 93 1.2 10 135 100 0.5 97.2 1.28 11 135 60 0.1 57 0.62 12 135 100 0.1 60 0.65 13 135 80 0.3 83.7 1.05 14 135 80 0.3 83.5 1.06 15 135 80 0.3 83.3 1.03 This indicated that the accuracy of the polynomial model was adequate. Therefore, in this case, the regression equations of yield and mass of NaLS response are a quadratic function as described in Eq 1 and Eq 1, respectively: Y1 = 83.5 + 0.89X1 + 1.63X2 + 18.88X3 -6.52X3 2 (1) 2 = 1.05 – 0.033X1 + 0.28X3 – 0.16X1 2 – 0.072X2 2 (2) The visualization of the predicted model equation can be obtained by the surface response plot (Fig. 2). This graphical representation is a two-dimensional surface in the three-dimensional space because one variable is set to a constant value to visualize the plot as Fig. 2. It can be observed that the yields of reaction increased with the increase of Na2SO3/Lignin ratio whilst the interaction of independent variables is less effective on the reaction yield (Figs. 2a,b,c). Mass of NaLS increases linearly with the decreases of Na2SO3/Lignin ratio and the increase of reaction time inside the experimental region (Figs. 2d,e,f). The optimal condition of the reaction was determined by solving the regression equations and exported from Design Expert software. The experiment was conducted to follow the predicted conditions (148.78 min, 80 o C, 0.5 g/g) to compare the obtained values as yield and mass of NaLS. The results are demonstrating that the actual values obtained from the experiment gave the reaction yield of 96.2 % and the mass of NaLS of 1.27 g and it was close to the predicted values (Yield = 95.97 %, the mass of NaLS = 1.28 g). Additionally, the coefficient of variation of yield and mass of the NaLS model is very low (< 5 %). Therefore, this model can be used to simulate the interaction of time, temperature, and Na2SO3/lignin ratio to the sulfomethylation. Lignin and sodium lignosulfonate production from the black liquor generated during 69 Figure 2. The surface response plot of Yield response (a,b,c) and mass of NaLS response (d,e,f). 3.3. Characterization of NaLS and surface tension property Figure 3. The FT-IR (a) and UV-Vis (b) spectrum of NaLS. The spectra of FT-IR and UV-Vis used to analyze the characteristics of NaLS, that obtained at the optimal condition, were shown in Fig. 3. In general, the FT-IR spectrum of NaLS (Fig. 3a) is similar to that of lignin (Fig. 1a) in some functional groups. However, the FT-IR spectrum of NaLS occurred more peaks at 974, 770, 630, 608, 494 cm -1 than the ones of lignin. Therein, the peak of 630 and 608 cm -1 was typically attributed to the S-O stretching vibrations of the sulfonic B a b Do Huu Nghi, Le Thi Kim Phung, et al. 70 group. The peaks at 770 cm -1 and 974 cm -1 are correspondent to C-S vibrations [17]. Moreover, the UV-Vis spectra of NaLS (Fig. 3B) also indicated that the maximal absorption band of NaLS is 195 - 210 nm, and there is a peak at 280 nm. This result is very consistent with previous researches [22, 23]. Thus, it can be concluded that the sulfonic group (-SO3) is crosslinked with the monophenolis of lignin. Additionally, the surface tension of NaLS was determined by the droplet volume method, and their surface tension of NaLS is 48.0409 mN/m, equivalent with commercial lignosulfonate (48 mN/m) and lignosulfonate produced (47.4 mN/m) by Ouyang et al. [24]. With this value, NaLS can be applied as a de-foaming agent, adhesives, preservation, etc. in industry. 4. CONCLUSIONS In this study, lignin was successfully recovered from black liquor generated from the bioethanol plant. Sodium lignosulfonate from obtained lignin was produced by sulfomethylation with a reaction yield of 96.2 %. The significant effects of time, temperature, Na2SO3/lignin ratio on the yield of sulfomethylation, and the mass of NaLS were determined by mathematical modeling. Additionally, the optimal condition of sulfomethylation for the highest NaLS content of 1.27 g was confirmed at the temperature of 80 o C, the reaction time of 148.78 min, and Na2SO3/lignin ratio of 0.5. Moreover, the obtained NaLS have surface tension of 48.0409 N/m and potential for use in the industry. This technique got rid of the waste of bioethanol manufacturing plant to create high-value products, contributing to the product diversification from rice straw. Acknowledgements. This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number FWO.104.2017.03, and Ministry of Science and Technology (NĐT.45.GER/18). We acknowledge the support of time and facilities from Ho Chi Minh City University of Technology (HCMUT), VNU-HCM for this study. CRediT authorship contribution statement. LMT: Formal analysis, Investigation, Manuscript preparation. DHPY: Formal analysis, Supervision, Manuscript preparation. DNHN: Formal analysis, Supervision, Manuscript revision. DLXH: Experimental design, Supervision, Manuscript revision. TTV: Anesthesia domain knowledge, Experimental design, Supervision, Manuscript rev
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