Black liquor used in this research is a by-product of the formacell pulping of OPEFB, as reported by Hidayati et al. [31]. The pulping conditions are presented in detail in the next section. OPEFB was obtained from PTPN Business Unit VII Central Lampung Bekri, Indonesia. Meanwhile, KOH, NaCl, NaOH, HCl, and ethanol were supplied by Merck (Darmstadt, Germany).

The formacell pulping was conducted in the autoclave at a liquor-to-particle ratio of 15:1 for 1 h at 130°C. This ratio of liquor to OPEFB particle ratio was used based on our initial experimental trials in the laboratory, considering the volumetric of OPEFB as a typical example of non-woody biomass. In this ratio, the OPEFB particles can be immersed completely in the chemical liquor. The ratio solvent of acetic acid and formic acid was 85:15 with the addition of HCl of 0.5% as the catalyst. The cooking liquor was collected after filtration and washing the completed softened OPEFB with fresh water at room temperature. This formacell BL was used for the next step.

The formacell BL obtained from the pulping process of 100 mL contained a total solid content of around 25.10%. There are two methods for lignin isolation from formacell BL of OPEFB. The first one, i.e., the isolation of lignin from BL was conducted using KOH with a concentration of 2.5%, 5%, 7.5%, 10%, 12.5%, and 15% (w/v). The second isolation technique used ethanol at concentrations of 5%, 10%, 15%, 20%, 25% and 30% (v/v). As much as 100 mL of the KOH or ethanol solution was added into 100 mL BL, then stirred and measured the pH of the liquor was. The solution was allowed to stand for 10 h, then centrifugated for 20 min at 2500 rpm, and then separated using filter paper. The precipitated lignin was then oven-dried at 50°C–60°C for 24 h. Dried lignin was crushed manually to produce fine lignin powder (Fig. 1). The experiment was conducted in triplicate.

The data were calculated and presented in average and standard deviation tables and graphs. Observations made were pH value, the yield of lignin, lignin methoxyl content, the equivalent weight of lignin, analysis of lignin with Fourier Transform Infra-Red spectroscopy (FTIR) spectrophotometer (Cary 630 FTIR, Agilent, Australia), and lignin morphology using a scanning electron microscope (SEM) (JEOL JSM 6510 LA, Japan).

Lignin yield was calculated based on the method according to Sluiter et al. [32], as presented in Eq. (1).

(1) Lignin Yield (%)=ab×100%

Notation:

a = weight of isolated lignin;

b = weight of lignin in black liquor.

The methoxyl content of lignin was determined by using the ASTM 15120-81 method [33]. As much as 0.5 g of lignin was added to 5 mL of ethanol, following addition to 100 mL of distilled water containing 1 g of NaCl, and made in the form of suspension. Neutralization was performed using 0.1 N NaOH, given 6 drops of phenolphthalein (pp) indicator, and then 25 mL of 0.25 N NaOH was added and homogenized in a closed state for 30 min at room temperature. After that, 25 mL of 0.25 N HCl was added and titrated with 0.1 N NaOH until the end of the lasting color change (at least 30 s). The methoxyl content of lignin was calculated according to Eq. (2) as follows:

(2) Methoxyl (%)=ml NaOH × N NaOH × 3,1Berat sampel (gram)×100%

Isolated lignin equivalent weight was calculated using the method of Brauns et al. [34]. 0.5 g lignin was put into 250 mL Erlenmeyer and moistened with 5 mL ethanol. The mixture is spiked with 1 g of NaCl which is then added with 100 mL of distilled water and 6 drops of phenolphthalein indicator. The solution was then titrated with 0.1 N NaOH until pH 7.5. The equivalent weight of isolated lignin was calculated by using Eq. (3).

(3) Equivalent Weight=1000×gram sample(mLN)NaOH

As much as 1 mg isolated lignin was added with 150 mg KBr, pelletized, and then the samples were analyzed by FT-IR spectrophotometer (Cary 630 FTIR Agilent). The analysis was set at a wavenumber of 4000 to 400 cm⁻¹ (wavelength of 2.5–25 πm).

Morphology of isolated lignin was carried out using Scanning Electron Microscope (SEM) (JEOL JSM 6510 LA) with 1000 × magnification.

The pH of BL from formacell pulping OPEFB is 0.59 ± 0.01, indicating that the liquor is acidic; thus, base or ethanol can be used to precipitate lignin from BL. To speed up the reaction, solvents such as acetic acid and formic acid are used in the formacell pulping process, along with HCl as a catalyst [31]. This process causes the pH of the formacell BL to become very acidic. Zhuang et al. [35] stated that formic acid and acetic acid could delignify lignin because of their acidic nature, so they can degrade lignin well. In another study, a combination of formic acid, acetic acid, and water achieved more than 50% lignin depolymerization in biomass [36]. The pH of the BL will be affected by the addition of KOH solution. According to the research, the pH value obtained from formacell pulping added with KOH solution ranged from 2.43 ± 0.07 to 4.27 ± 0.35 (Table 1), while the addition of ethanol concentrations can slightly increase the pH value from 1.0 ± 0.01 to 1.29 ± 0.03. According to the findings, KOH has a greater effect on increasing pH than ethanol due to the strong base property of KOH.

The pH steadily increased as an up-surging concentration of KOH, where the lowest pH, 2.43 ± 0.07, was obtained from 2.5% KOH, and the highest was a pH of 4.27 ± 0.35 from KOH 15%. Increasing the concentration of KOH solution causes more OH⁻ ions in the BL so that the pH of the solution becomes more alkaline. It can also be explained that KOH is a strong base. The stronger the base, the more OH ions are released, increasing the pH of the solution and making it more alkaline. Ethanol has a higher pKa than KOH, indicating a weaker acid. Unlike KOH, increasing the ethanol concentration slightly increased the pH, with the lowest being 1 and the highest being 1.29 after adding 5% and 30%, as shown in Table 1.

The lignin yield from adding KOH solution at various concentrations in isolation of BL ranged from 3.57 ± 0.38 to 14.95 ± 1.10 (Table 1). In comparison, ethanol isolation resulted in lower lignin yields of about 0.51 ± 0.00 to 1.06 ± 0.00. The highest lignin yield was produced at a concentration of 15% KOH solution (14.95% ± 1.10). Increasing the concentration of KOH solution caused the lignin yield to increase. It can be affected by the increasing concentration of an alkaline solution (KOH) that facilitates easier breaking of lignin bonds with other compounds, and a condensation reaction occurs in the lignin constituent units [37]. It is also supported by Sundin [38] who stated that lignin deposition from BL due to the protonation of phenol groups on lignin molecules caused by the addition of OH⁻ ions from KOH. It reduces the electrostatic force between lignin molecules, resulting in the deposition of lignin and reduced solubility. The more OH⁻ ions, the more lignin deposits will form. The deposit lignin increase occurred because of an increase in ionic strength and protonation. The protonation of the phenolic group in lignin leads the lignin nucleus to form aggregates when the pKa value of lignin is reached [39].

The lignin yield with KOH liquor treatment is higher than that of NaOH, as reported by Hidayati et al. [20]. Lignin was isolated from formacell BL of OPEFB by using NaOH, producing lignin yields ranging from 1.48% to 5.67%. It might be caused by the stronger base of KOH than NaOH. As seen in the periodic system, the metal Na⁺ and K⁺ are included in Group 1A. This is also related to the lower nature of alkali metal bases; the stronger, the lower the metal, the more easily reduced. Besides pH, temperature and ionic strength also affect lignin solubility. The molecular weight distribution in lignins is related to yield [40]. Both molecular weight and the ratio of phenols to aromatic units contribute to the solubility of lignin [40]. The changes in lignin yield in isolation by adding ethanol concentration are very low. The very low electronegativity of ethanol can suggest the presence of carbon compounds that cannot precipitate lignin properly. In the absence of sulfide, delignification was slower in the Alcell process.

As opposed to KOH precipitation, increasing ethanol concentrations leads to almost 0% lignin recovery. This result was in agreement with the precipitation of lignin from Miscanthus x giganteus, where after the addition of 75% of ethanol, there was no lignin precipitated [41]. In the low concentration of ethanol, lignin precipitation is carried out by diluting liquor with water, and the solubility of lignin decreases, resulting in more lignin being recovered. In other words, when the water content in the ethanol is limited, most of the lignin is soluble in the system, and hence the aggregate is not formed.

Methoxyl is one of the chemical groups in lignin for identifying one of the characteristics of lignin. The methoxyl group in each type of lignin has different content because of the different structures of lignin types. Determining methoxyl contents gives information about the average C9 units in lignin, and eventually, the unsaturated side chain and average oxygen content can be calculated [42]. The methoxyl content in KOH lignin ranged from 10.13% ± 1.92 to 13.06% ± 0.9, whereas the use of ethanol concentrations results in methoxyl contents between 11.99 ± 0.02 to 17.03 ± 0.42 (Table 1). The highest methoxyl content in lignin was acquired from isolated lignin by adding a 2.5% KOH solution. Increasing the concentration of KOH and ethanol solution leads to a decrease in the methoxyl content of isolated lignins. This may be due to the use of KOH as a strong base as a precipitating solution, which allows some methoxyl groups to degrade and transform into other compounds. This decrease in methoxyl content can be influenced by the disruption of structural stability in lignin caused by the addition of KOH. Furthermore, according to the pathway proposed by Yin et al. the removal of the methoxy group in lignin created a higher hydroxyl group of lignin by KOH solution. The presence of potassium in lignin isolation decreased the methoxyl content due to cleavage Cα―Cβ through Cβ―O bond. This cleavage leads to the removal of side chains such as methoxyl. The mechanism of this reaction has been proved by Yin et al. [43].

The various methoxyl content in lignin depends on the source of lignin and the delignification process used. Generally, high methoxyl content will inhibit lignin reactivity during use in the resin field. Lignin has phenolic hydroxyl groups that are typically linked to nearby phenyl propane units, allowing lignin to connect with formaldehyde in a way analogous to phenol and formaldehyde [44]. Molecular weight and lignin purity are important factors during the production of biocarbon fibers [44,45].

In the case of lignin application as an adhesive raw material, the low methoxyl content is more beneficial because it will increase the reactivity of aromatic nuclei in lignin [46]. It will facilitate the reaction of methylation or hydroxymethylation with adhesive-forming reactants such as NaOH and formaldehyde, hinting at the ease of gel formation. Recently, Gendron et al. compared four types of lignin for green adhesive, where the lowest methoxyl content of lignin had the highest water solubility and shear resistance as an adhesive [47].

The lignin standard of Indulin AT has a methoxyl content of around 13.5%–14.5% [48], so the methoxyl contents in isolated lignin in this study are still lower. Meanwhile, compared to methoxyl content by Hidayati et al. [20] with a value of 14.61%–20.77% by using NaOH as precipitating solution, the results of this study are also lower. These results are probably influenced by the use of KOH, which is classified as a strong base with a higher reactivity than NaOH, as seen from the periodic system. The influence of harsh chemicals and temperature during isolation causes the structure of lignin to undergo many changes, including the methoxyl group. The methoxyl group is degraded and transformed into other degradation compounds such as aromatic hydrocarbon and phenolic types, resulting in lower methoxyl content [49].

The determination of the equivalent weight of lignin is intended to determine the molecular weight of lignin to identify lignin degradation and condensation reactions. The molecular weight of lignin is an important property that influences biomass recalcitrance and lignin value. Determination of the molecular weight of lignin in the original biomass depends on the source of the raw material used, chemicals used for isolation, and purification methods [50,51]. Based on the results, the equivalent weight of lignin from the KOH solution ranged from 427.03 ± 74.20 to 1372.94 ± 269.73, whereas the addition of ethanol concentration resulted in molecular weights ranging from 449.88 ± 1.9 to 2123.03 ± 27.5 (Table 1).

Increasing the concentration of KOH solution resulted in a decrease in lignin equivalent weight, so the lowest value at 15% KOH solution concentration was 427.03 ± 74.20. This might be attributed to the increasing addition of a KOH solution as a strong base will degrade a portion of lignin into a derivative compound with shorter chains. It can degrade lignin resulting in a low equivalent weight. According to Patil et al., lignin depolymerization has two routes: harsh and moderate, resulting in different lignin structures [52]. The average molecular weight of lignin decreases with increasing precipitation of lignin yield by KOH. It was shown that increasing the amount of lignin with lower molecular weight deposited a high yield of lignin. Lignin is degraded into smaller fragments so that there are more phenolic groups with lower molecular weights [53]. Increasing salt concentration can lower the product’s molecular weight [54,55].

According to Gellerstedt [56], the termination of methyl aryl ether bonds from the aromatic ring of lignin resulted in decreased methoxyl content and low molecular weight of lignin due to the lignin degradation process. Based on several previous studies, lignin molecular weight values were obtained, namely 2500–14,000 [57], and 1327.64–1787.23 [20]. In contrast, the equivalent weight of Indulin AT reached 3357–3366 [58]. This shows that the equivalent weight value of lignin varies greatly.

Lignin that has a high molecular weight can be used as a matrix in polyolefin composites, because a higher molecular mass of lignin has a better affinity with the matrix [59], while lignin with a lower molecular weight can be used for the development of phenol-formaldehyde resins, antioxidants, adhesives, and paints due to their higher reactivity than high molecular weight lignin [59].

Differences in elemental composition, functional group types, proportions, and molecular weight in lignin molecules provide opportunities to use lignin in a variety of value-added applications. Some factors that influence the molecular weight of lignin are the diversity of lignin isolation procedures, macromolecular degradation during isolation, the effect of condensation mainly on acidic conditions, the apparent polydispersity of dissolved lignin, inadequate determination methods used to characterize the isolated lignin, and uncertainties in the properties of lignin. Because lignin is heterogeneous and inconstant, characterizing native lignin is also difficult [60].

The molecular weight of lignin is very important in the development of such polymeric products; similarly, low-molecular-weight lignin isolated from wheat straw by acetic acid pulping is an ideal feedstock for adhesive manufacturing [61]. Doherty et al. [62] stated that lignin from the soda process of wheat plants ranges from 1700 with a methoxyl content of 16. The organosolv pulping process at bagasse produces a molecular weight of around 2000 and a methoxyl content of 15.1. Lignin produced in the isolation process using alcohol or alcell process has a low molecular weight (Mw < 900) with low polydispersity, no sulfur, and low ash content, although the value depends on the analytical method. There is also a molecular weight of 2580 g mol⁻¹ with a polydispersity of 4.3. Sameni et al. compared four types of lignin from different isolation methods, where the highest Mw (13488) was obtained from the steam explosion process with a methoxyl content of 0.69. Meanwhile, with the same methoxyl content, the lowest Mw (2565) was derived from the kraft process [57]. Yun et al. reported that organosolv fractionation from native lignin decreased its molecular weight from 5070 to 1810 [23].

FTIR was performed to analyze differences in the functional groups of lignin in the sample (Figs. 2 and 3). FTIR spectra analysis showed that the peak in the aromatic ring vibrations decreased with the absorption band at 1446 cm⁻¹ (a) and 1408 cm⁻¹ (b), especially in the treatment using the highest concentration of KOH (Fig. 2) and ethanol (Fig. 3). There are similarities in the wavelength of 3500 to 3109 cm⁻¹ which refer to the presence of hydrogen bonds in the hydroxyl group (alcohol and phenolic). A wavelength absorption of 1587 and 1513 cm⁻¹ indicates the aromatic lignin skeletal vibration [60]. Wavenumbers at 1460 to 1420 cm⁻¹ are associated with stretching wavelengths of CH (methyl and methylene) and CH deformation with aromatic ring stretching fields [63]. The peaks observed in the range of 1220 to 1270 cm⁻¹ in all spectra may refer to the C-H deformation vibrations of the guaiacyl [64]. The bands at 1300 cm⁻¹ (syringyl) and 1200 cm⁻¹ (guaiacyl) indicate the presence of both syringyl and guaiacyl in lignin’s chemical structure as the unique peaks of lignin precursor [65]. A similar peak was also found in kraft lignin isolated from Acacia mangium black liquor [27]. In general, peaks in Figs. 2 and 3 show that the treatment using KOH had a sharper peak than that of ethanol treatment. However, no difference in the peaks resulting from these two treatments was determined. Hidayati et al. [20] reported the experiment in isolated lignin from formacell BL of OPEFB using NaOH with the wave number of isolated lignin for guaiacyl lignin at around 1273.02 cm⁻¹.

Scanning Electron Microscope (SEM) was used to observe the size and morphology of lignin. In this study, SEM images were taken from the lowest and the highest concentration of KOH and ethanol as representative lignin morphology. As seen in Fig. 4, increasing the concentration of KOH acid from 2.5% to 15% showed differences in lignin morphology. Lignin isolation using 15% KOH (Fig. 4b) produced a more compact formation and morphology compared to lignin produced by isolation using 2.5% KOH concentration (Fig. 4a). In stark contrast, in treatment using ethanol appears that treatment with a 5% ethanol concentration resulted in a more condensed form and morphology than those using a 30% concentration. This result was in accordance with the findings reported by Solihat et al. [27] and Hamzah et al. [41], where increasing ethanol concentration created more rupture and amorphous lignin surface.

The lignin molecule contains a variety of active functional groups, including aliphatic hydroxyl (Al-OH), phenolic hydroxyl (Ph-OH), carboxyl (-COOH), carbonyl (-C=O), and methoxy groups (-OCH₃), which determine its properties, chemistry, and reactivity of lignin. Lignin’s chemical properties enable it and its derivatives to be used as high-value materials. The lignin molecule can be used in a variety of value-added applications due to differences in molecular weight, elemental composition, and the types and proportions of functional groups. Currently, Saražin et al. investigated the potency of organosolv lignin-based non-isocyanate polyurethane for wood adhesive and the result demonstrated satisfactory mechanical properties [66] and its curing kinetics for interior furniture showed that it was more suitable than urea-formaldehyde (UF) adhesive [67]. Sulfur-free lignin is produced by isolating lignin from the formacell process. Most value-added applications prefer lignin with a lower percentage of sulfur and ash. For example, sulfur-free lignin is preferred for use in aerogel composites. Jõul et al. reacted lignin isolated by organic acid with 5-methylresorcinol-formaldehyde to create aerogel and according to the data, formacell lignin had good reactivity to create a high surface area of aerogel [68]. According to Talabi et al. lignin with rich hydroxyl carbon, low molecular weight, and less in the methoxyl group was suitable for lignin-based-formaldehyde resin with a 73% graphitization level [69], and hence formacell lignin with higher KOH and ethanol concentrations are appropriate for this application. Meanwhile, the high molecular weight of lignin is preferred as a matrix in carbon fiber composites because it improves the mechanical performance of the fiber.