Universidade Federal de Santa Maria

Ci. e Nat., Santa Maria, v. 41, e53, 2019.

DOI: http://dx.doi.org/10.5902/2179460X39877

Received: 05/09/2019 Accepted: 17/10/2019

 

by-nc-sa

 


Section Environment

 

Residual lignin from biomass pretreatment for 2G ethanol production as an adsorbent in dye removal

 

Anita Ribas AvanciniI

Juliana Silva LemõesII

Ester Schiavon MatosoIII

Nidria Dias CruzIV

Sérgio Delmar dos Anjos SilvaV

Cláudia Fernanda Lemons SilvaVI

 

I     Universidade Federal de Pelotas, RS, Brasil – anita.avancini@hotmail.com

II   Universidade Federal do Rio Grande do Sul, RS, Brasil - julianalemoes@yahoo.com.br

III   Universidade Federal de Pelotas, RS, Brasil - ester_schiavon@hotmail.com

IV  Universidade Federal de Pelotas, RS, Brasil - cruz_nidria@gmail.com

V   Empresa Brasileira de Pesquisa Agropecuária RS, Brasil - sergio.anjos@embrapa.br

VI   Universidade Federal de Pelotas, RS, Brasil - lemonsclau@gmail.com

 

Abstract

Liquid waste from some industrial activities generates a serious environmental impact due to the presence of dyes. However, during the production process of second-generation ethanol, a lignin-rich residue is generated, which may be able to absorb the dyes in the waste. The goal of this work was to test the potential of lignin as a dye-removal adsorbent. Two quantities of precipitated lignin (200 and 500 mg) were tested as adsorbents by contact with Methylene Blue, Eriochrome Black T, and Bromocresol Green dyes. Kinetics curves for the adsorption capacity (Q) of lignin quantities were constructed and the percentage of removal of each dye was calculated. The Q of the three dyes increased with contact time and was higher in the 200-mg treatment; however, the strongest dye removal was presented by the 500-mg treatment. The maximum removal achieved was 84.77% with 500 mg lignin in the methylene blue solution and the minimum was 21.32% with 200 mg in the Bromocresol green solution. Therefore, it is concluded that lignin has potential as an adsorbent, as increased quantities are more efficient in removing dyes.

Keywords: Lignin; Adsorption; Dyes

 

1 Introduction

Industries that process materials such as textiles, leather, dyes and paper are the main effluent generators due to the high water consumption of their different operations (SUTEU et al., 2010). Dyes are present in most of the effluents and are difficult to remove because of their recalcitrant nature (HOLKAR et al., 2016). Thus, liquid waste from industrial activities becomes extremely polluted and generates a serious environmental impact (SU et al., 2016). In addition, dyes discharged into watercourses as effluents are toxic, and can cause conditions such as anemia to high fever and cyanosis in humans (MICLESCU; WIKLUND, 2010).

In Brazil, CONAMA Resolution No. 430/2011 establishes effluent discharge standards specifying that dyes from anthropogenic sources should be visually absent. In addition, Resolution CONSEMA No. 355/2017 provides standards for liquid effluents discharged into water in the state of Rio Grande do Sul and establishes that polluting sources must discharge their liquid effluents directly or indirectly without coloring the recipient body. In this sense, it is necessary to search for alternatives that reduce the amount of dyes contained in the effluents, and adsorption is a technique that has been effectively used (IBANEZ et al., 2012; MÜNCHENET et al., 2015; SILVA et al., 2018).

Adsorption is defined as a physicochemical phenomenon in which molecules in a fluid accumulate spontaneously on the surface of a solid material (NASCIMENTO et al., 2014). Lignin is a natural macromolecule available in large quantities in residues such as black liquor, which may be used as a dye adsorbent owing to its structural chemical properties, such as the presence of carboxylic groups (WATKINS et al., 2014; WANGA, et al., 2019). Black liquor is generated at the end of the pretreatment of lignocellulosic biomass for the production of second-generation (2G) ethanol, which is essential for enzymatic hydrolysis of cellulose and hemicellulose (CASTRO; PEREIRA JR., 2010; SIVAGURUNATHAN et al., 2017).

2G ethanol is a renewable and environmentally recommended biofuel as it is produced from tailings or inedible products (SHARMA et al., 2017). However, black liquor has high toxicity and can cause damage to the health of living beings, as well as providing pollution to the environment (OLIVEIRA et al., 2017), and it is necessary to find a destination for this residue. In this sense, the present work aimed to test the potential of lignin generated in the biomass pretreatment for the production of 2G ethanol as adsorbent in dye removal.

 

2 Methodology

The present study was developed at Embrapa Temperate Climate in Pelotas, Rio Grande do Sul. The residue from the alkaline pretreatment step (0.5 M NaOH, 127° C, 30 min) that occurred during the production of 2G ethanol was used. The biomass submitted to the process was rice straw from the cultivar Puitá Inta CL.

To obtain lignin, 500 mL of black liquor was used, which was acidified with 50% v/v sulfuric acid (H2SO4) until a pH of 2 was reached (BES et al., 2019). After precipitation, the lignin was separated by the vacuum-filtration method. The liquid generated as waste was discarded for proper treatment while lignin, the solid fraction, was oven dried at 40° C until constant mass was obtained (adapted from SCIBAN et al., 2011). The procedure was repeated 10 times. From the five liters of black liquor precipitated it was yield a total of 105 g of lignin.

The obtained lignin was macerated and subjected to a washing process. For each 1.5 g of the sample, 80 mL of deionized water was added. The solution remained under magnetic stirring for 15 h and then, was vacuum filtered again. The precipitate was collected and dried in a drying oven at 40° C. The process was repeated twice (adapted from GUO et al., 2008). At the end of the two washing steps, 26 g of lignin was obtained, which was macerated and sieved through a 0.25 mm mesh to provide uniformity and increase the contact area of the sample (SCIBAN et al., 2011). Lignin adsorption capacity was tested by contact with methylene blue, Eriochrome Black T, and Bromocresol green. Because of the chemical differences between the dyes, the measured pH of each solution was 5.17, 2.99, and 3.55 for methylene blue, Eriochrome Black T, and Bromocresol green, respectively.

Two amounts of lignin, 200 and 500 mg, were used in 100 mL of a 20 mg L-1 solution of each dye. The dye and lignin solution was kept under magnetic stirring for 30 min and every 2 min, a 2 mL aliquot was removed and centrifuged to separate the lignin from the solution. The aliquot was measured by UV/vis spectrophotometry, according to the corresponding wavelength for each dye: 660 nm for methylene blue, 530 nm for Eriochrome Black T, and 455 nm for Bromocresol green, to determine the dye concentration.

Three repetitions were performed for each treatment and for the six contact times in each dye. The initial concentrations and those after dye adsorption by lignin were calculated from calibration curves constructed for each dye, and with these values, kinetics curves were constructed to determine the adsorption capacity (Q) following the equation Q = {[(Co – Ce).V]/m} described by Nascimento et al. (2014) where: Co: initial concentration of adsorbate; Ce: adsorbate concentration in equilibrium; V: volume of solution; m: mass of adsorbent. The percentage of removal of each dye by the two amounts of lignin was also calculated. The results were subjected to analysis of variance and, in case of statistical significance, were compared by a t-test at 5% probability.

 

3 Results and discussion

 The adsorption capacity of 200 mg of lignin when in contact with the methylene blue dye ranged from 5.59 to 8.50 mg g-1 when the contact time increased from 5 to 30 min. When the amount of adsorbent was 500 mg, the adsorption capacity was 3.39 mg g-1 at 5 min and did not exceed 3.79 mg g-1 at 30 min of contact (Figure 1A). From the figure, it is clear that the minimum time required for the system to reach equilibrium was 25 min.

When lignin was kept in contact with the Eriochrome Black T dye, the adsorption capacity was lower for both amounts tested. With 500 mg, the maximum adsorption capacity was 2.21 at 30 min and at 200 mg 3.67 mg g-1 (Figure 1B). At 5 min of contact of the lignin with the dye in question, the solution had already reached equilibrium, that is, at this time, the adsorbent had already adsorbed the maximum capacity, and there was no need for the solution to stay in contact any longer.

The lowest adsorption capacity when using 500 mg of adsorbent was in contact with the green Bromocresol dye, reaching only 1.9 mg g-1. Figure 1C shows the range of 2 to 3.66 mg·g-1  adsorption capacity of 200 mg of lignin with the dye in question. As the graph shows, the behavior was similar to that of the methylene blue dye, and at 25 min, the solution reached equilibrium.

For all dyes, the lignin adsorption capacity increased with longer contact time, but was lower with higher lignin content. This is because there was a loss in performance of the solution (MARCO, 2015). It is important to find the best ratio between the amount of adsorbent used and the amount of dye adsorbed.

Dyes have distinct characteristics according to their chemical structure: methylene blue is a basic (cationic) dye, whereas Eriochrome Black T and Bromocresol green are acidic (anionic), and Eriochrome Black T is further distinguished by being azoic (presence of N = N), which raises concerns about its disposal in the environment (MURMU et al., 2018). Due those differences the adsorption of each dye was distinct.

With Pinus elliottii lignin mass and Zn2+ ions as adsorbents, Bortoluz (2019) performed tests with amounts of lignin ranging from 25 to 250 mg of adsorbent with the solution subjected to mechanical stirring at 150 rpm, pH ~ 4.75, and 25 °C for 4 h. It was found by Bortoluz that the Qt values decreased as lignin mass increased when performing the kinetics of adsorption capacity.  The results found in this study corroborate those of the author.  

 

Figure 1 - Kinetic adsorption curve of two amounts of lignin as a function of contact time for (a) methylene blue, (b) Eriochrome Black T, and (c) Bromocresol green dyes.

 

The kinetic adsorption curve presents an inverse behavior when compared with the percentage of removal. The decrease of adsorption capacity as increased the mass of adsorbent is due to sites that were not occupied during the adsorption process. In other words, due to saturation of adsorption sites corresponding to a decreased harnessing of the adsorbent that can also occur because of the aggregation of lignin resulting in a decrease of the effective surface area for the adsorption of adsorbent (Deniz and Karabulut 2017).

The values found for dye removal by lignin were increasing when the amount of adsorbent increased (Table 1). This behavior is due to the greater number of active sites available when there is a higher amount of adsorbent, allowing a greater interaction between the lignin and dye (TODORCIUC et al., 2015). Methylene blue dye achieved 94.85% removal at 30 min with 500 mg·L-1 lignin. In the solution containing 200 mg L-1, the maximum dye removal was 85.08%. Eriochrome Black T dye reached a maximum removal of 40.96 and 60.74% with 200 mg L-1 and 500 mg L-1, respectively, and Bromocresol green dye ranged from 21.32 to 37.75% when 200 mg of the adsorbent was used, and from 34.13 to 60.11% with 500 mg.

 

Table 1 - Percentage of removal of methylene blue, Eriochrome Black T, and Bromocresol green dyes as a function of two amounts of lignin (QL), in grams, used as adsorbents with six contact times.

Dyes

QL (mg)

Contact Time (min)

5

10

15

20

25

30

Methylene Blue

200

60.30 b

71.50 b

78.35 b

81.25 b

83.10 b

85.08 b

500

84.77 a

90.47 a

92.54 a

93.48 a

94.79 a

94.85 a

Eriochrome Black T

200

31.10 b

37.14 b

38.43 b

40.91 b

40.91 b

40.96 b

500

51.34 a

58.71 a

59.31 a

60.71 a

60.74 a

60.74 a

Bromocresol

Green

200

21.32 a1

27.14 b

31.07 b

34.58 b

37.79 b

37.75 b

500

34.13 a

43.39 a

49.33 a

51.39 a

54.45 a

60.11 a

1Averages followed by the same letter in the columns do not differ from each other by the t-test (p ≤ 0.05), comparing the effect of lignin amounts (200 and 500 mg) on dye removal within each contact time.

 

It is important to note that even reaching high removal values, the methylene blue dye did not completely lose its visible coloration, as required by legislation; perhaps for this dye, other amounts of lignin per liter or contact time are necessary. In contrast, the Eriochrome Black T and Bromocresol green dyes lost much of their coloration, even reaching median removal values, as can be observed in Figure 2.

 

Figure 2 - Comparison between dye solution 20 mg, L-1 (left) and dye after 30 min of contact with 500 mg lignin (right).

 

The methylene blue dye was more susceptible to lignin adsorption. The values found corroborate the study by Budnyak et al. (2018), who used a lignin and silica composite to absorb the same dye and achieved a maximum removal of 99%. The authors indicate that the hydroxyl, carboxyl, and lignin aromatic ring functional groups can form hydrogen bonds, electrostatic interactions, and π (between aromatic) interactions with the cationic nitrogen atoms and aromatic rings present in the dye. Manna et al. (2017) observed that the percentage of removal increased as the adsorbent mass increased, using lignocellulosic materials, up to an equilibrium of 0.01 g mL-1, a mass greater than that used in this study, which reached a maximum of 0.005 g mL-1. The untreated lignin they used was able to remove ~93% of the dye. Eriochrome Black T is resistant to photodecomposition and some chemical reagents, which makes its removal and color reduction difficult (BARKA et al., 2011). The work of Losada et al. (2017) using photocatalytic degradation showed a maximum of 54.75% removal. Almeida et al. (2017) achieved 100% removal using modified perlite as adsorbent at pH = 3; however, the initial dye concentration was lower than that used in this study which may influence the percentage of removal as observed by Murmu et al. (2017).

The most studied method for Bromocresol green removal is photocatalytic degradation, Ying et al. (2017) and Osuntokun et al. (2018) obtained an efficiency of 64.34% and 60%, respectively; although their method are different, the efficiency was close to that obtained in this study.

The behavior of Bromocresol green dye under the influence of reaction pH may differ depending on the removal method applied. Bai et al. (2016) obtained the highest removal tax (85%) at pH 7 using electrocatalysis, whereas Murmu et al. (2017) observed the best result (99.9%) at pH 0.5 using Phragmites karka adsorption. At a pH of 3, close to that used in this study, the percentage of removal obtained by them was ~60%, corroborating the data obtained in this work.

The rate of stirring of the reaction may also influence the removal. Murmu at al. (2017) observed an increase from 60.61 to 99.99% when they varied the stirring rate from 50 to 250 rpm. Thus, the removal results for Bromocresol green in the present study agree with the results of the other authors and may be improved with variations in other factors that were not studied.

 

4 Conclusions

It was concluded that lignin recovered from the black liquor generated in the biomass pretreatment for the production of 2G ethanol has great potential as an adsorbent in dye removal. In terms of quantity, 500 mg of lignin was more efficient and more advantageous than 200 mg because the removal percentage was higher for all dyes tested.

 

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