Chemical Characterization of Alkali Lignins Isolated from Rapeseed Stalks

TL;DR

This study compares two methods for extracting lignin from rapeseed stalks to determine which is more effective and environmentally friendly.

Contribution

The study provides a detailed chemical comparison of lignin from two pulping methods, highlighting the Soda-AQ process as a cleaner alternative.

Findings

Lignin samples from Soda-AQ pulping showed structural similarities suitable for high-value applications.

The Soda-AQ process avoids sulfur emissions, making it an environmentally cleaner alternative to the Kraft method.

Advanced analytical techniques confirmed structural differences and performance indicators of the isolated lignins.

Abstract

Rapeseed stalks (Brassica napus), an abundant agricultural residue, represent a promising non-woody raw material for the pulp and paper industry. This study focuses on the chemical and structural characterization of alkali lignins isolated from black liquors generated by two common delignification methods: Kraft and Soda-Anthraquinone Pulping of rapeseed stalks. The objective is to understand how the chemical environment of each process influences the final structure, fragmentation degree, and reactivity of the isolated lignin. In practice, lignin samples are recovered from black liquors produced under varying conditions (alkali charge, time, and temperature) to achieve defined levels of delignification. Detailed characterization was performed using advanced analytical techniques, including Gel Permeation Chromatography, Solid-State Cross-Polarization/Magic-Angle-Spinning Nuclear Magnetic Resonance, and FT-IR and UV-Vis Spectroscopy. The findings provide essential data on the structural differences, confirming the suitability of the resulting materials for potential high-value applications. Furthermore, the structural similarities and performance indicators suggest that the Soda-AQ process enables successful delignification of rapeseed stalks without the sulfur emission issues associated with the Kraft method, thus validating the former as an environmentally cleaner alternative for non-wood biomass utilization supporting the complete valorization of rapeseed agricultural waste.

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1. Introduction

Rapeseed (Brassica napus) is a significant agricultural crop that is extensively cultivated for its oilseeds, which are a vital source of edible oil and biofuel [1]. The stalks left in the field after harvesting represent an agricultural byproduct that has been underappreciated and underutilized, despite its significant potential [2,3]. They constitute a significant, annually renewable source of biomass (50–72% of total crop biomass), the utilization of which aligns seamlessly with circular bioeconomy principles and the biorefinery concept [4,5,6,7,8]. The objective of processing this nonwood raw material in the context of a biorefinery is to obtain maximum yields for all three basic components—cellulose (resulting as fibers) [9,10], hemicelluloses [11], and lignin [12]—as shown in Figure 1.

Chemical alkaline delignification processes, such as the Sulfate (also referred to in the literature as the Kraft process) and Soda-Anthraquinone (also found in the literature as Soda-AQ, Natron-Anthraquinone, or Natron-AQ) methods, are essential for the separation of cellulose fibers, simultaneously generating black liquor [13]. This residual solution contains a rich organic fraction (approximately 40–60% of the initial material) composed of dissolved lignin, hemicellulose-derived sugars, extractables, and solubles [14].

Despite the predominant industrial practice involving the combustion of black liquor for energy and chemical recovery [15,16], achieving complete biomass recovery necessitates a shift from a combustion-based approach to the recovery of sustainable biochemicals with added value [17]. Sugars, for instance, can be utilized in the production of biofuels such as ethanol. Lignin, in both modified and unmodified forms, can be converted into polymers, adhesives, or aromatic chemical precursors, including vanillin and phenolic derivatives [18]. Furthermore, due to its complex chemical structure, lignin and its derivatives are of great interest for environmental applications. These compounds have been utilized as dispersants, stabilizers, coagulants, and biosorbents for the adsorption of heavy metals and other pollutants [19].

Lignin, the most prevalent natural aromatic biopolymer, exhibits a structure and reactivity that are profoundly influenced by the chemical environment during delignification [20]. Alkaline lignins, particularly those derived from annual plants, exhibit active functional groups, including phenolic and enolic hydroxyl, carboxyl, and carbonyl groups [21]. These functional groups contribute to increased reactivity, making them suitable for applications such as the production of polymers, adhesives, dispersants, and other novel bio-based products [22]. Therefore, a comprehensive understanding of how alkaline processes affect the structure of lignin is imperative to unlock its potential as a raw material in green chemistry value chains.

Although pulping was initially carried out using the simple Natron process [23], the results (due to low yield and inferior fiber characteristics) were not encouraging, making the Natron-Anthraquinone variant preferable for comparative study with the Sulfate process [24], considering the improved properties of the cellulose and the milder working conditions achieved.

In this context, the present study aims to provide a detailed characterization of alkaline lignins, as the nature of the process (Sulfate vs. Natron-Anthraquinone) and the chemical pulping environment determine structural transformations and changes in the degree of fragmentation of the dissolved lignin. The Natron-AQ process is of particular importance as a sulfur-free pulping process, which accelerates delignification, reduces lignin condensation and, most importantly, eliminates the inherent pollution of the sulfate process with sulfur compounds [13,23,24]. This process thus represents a cleaner and more efficient alternative for the valorization of rapeseed biomass, directly contributing to the objectives of the circular bioeconomy.

Starting from the need to fully utilize rapeseed stalks and to study the resulting by-products in detail, the main objectives of this research are:

• Detailed chemical and structural characterization of alkaline lignins (sulfate lignin and sodium lignin-AQ) isolated from black liquors, using advanced techniques (Gel Permeation Chromatography (GPC), Nuclear Magnetic Resonance Spectroscopy (NMR), FT-IR and UV-Vis Spectroscopy), in order to establish their quality as a raw material for sustainable biochemicals;
• Evaluation of the influence of delignification parameters (type of process and alkali addition) on the degree of lignin fragmentation (molecular mass distribution) and on the structure of functional groups, providing critical information for efficient conversion into value-added products;
• Comparison of the performances of the Sulfate and Natron-AQ processes to justify the use of the Natron-AQ method as a superior, ecologically clean technological alternative that supports the successful delignification and complete valorization of rapeseed stalks, in line with circular bioeconomy principles and the reduction in the environmental impact associated with the production of biofuels and chemicals.

This research addresses the structural and molecular characterization of lignins obtained from rapeseed stalks, providing essential data for the development of efficient and superior valorization processes.

2. Materials and Methods

2.1. Raw Materials: Rapeseed Stalks Preparation and Properties

The plant material utilized in the laboratory experiments (as shown in Figure 2) was represented by rapeseed stalks, harvested in June 2009 from Neamț county, Romania, which remained in the field after the seeds were collected, with a relative moisture content of 15%.

The physical characteristics of the analyzed stalks exhibited a height ranging from 1 to 1.5 m. The diameters exhibited a decreasing variation along the length of the stalk, with measurements ranging from 20 to 30 mm at the base, 15 to 25 mm at 0.5 m in height, 10 to 20 mm at 1 m in height, and 5 to 10 mm at the tip. The stalks, being the only component with qualities suitable for obtaining cellulose, were transformed by manual cutting into chips. The dimensional analysis of this chopped material revealed that 97% of them fell within the normal range of 5–10 cm in length, with the remaining 3% constituting small fragments and particulate matter, defined as chaff. From a compositional perspective, the initial stalk was separated into free bark, representing 84% of the total absolute dry matter, and core, which amounted to 13% of the total biomass.

An essential physical property that was the focus of this study was density, given its direct influence on the logistics of delignification. Due to the unique characteristics of annual plants, the volumetric weight of rapeseed chips is significantly lower than that of wood chips. A variation was identified depending on the fractions, with the volumetric weight measuring approximately 42 kg/m^3^ for whole stalks and 75 kg/m^3^ for chips measuring 5–7 cm long. It is noteworthy that, at equivalent levels of humidity, the chips exhibited a volumetric weight that was nearly double that of the standard stalks. This phenomenon has been attributed to the increased degree of arrangement present in the chips. The chemical characteristics of the stalks and chips were evaluated in order to ascertain their potential as a cellulosic raw material.

2.2. Alkaline Pulping

The delignification of rapeseed stalks was carried out by two major alkaline processes, in order to obtain cellulose with high yield and strength characteristics: Sulfate Pulping (Kraft) and Anthraquinone Soda (Soda-AQ) [25,26]. These represent heterogeneous solid–liquid processes that aim at the selective dissolution of lignin from the plant material at high temperatures and pressures. The chemical composition of the rapeseed stalks utilized in this study was 40–44% cellulose, 19–21% lignin, 21–23% pentosans, and 2–3% inorganics, consistent with values reported in the literature [27,28,29]. This relatively low lignin content, combined with the low density of the material compared to woody species, allowed for the use of milder working conditions and significantly shorter pulping times to achieve efficient delignification.

The pulping conditions were varied to study the influence of the main parameters, as follows:

• Sulfate Process: Active alkali at 18, 20, and 22% (expressed as NaOH) was added based on the absolutely dry material, at process temperatures of 150, 160, and 170 °C. The compositions of the pulping solutions, designated as A6, A9, and A12, are reported in Table 1.
• Natron-AQ process: The conditions were similar to those for the sulfate process, using the same levels of active alkalis (18, 20, and 22% expressed as NaOH) and the same working temperatures (150, 160 and 170 °C). Furthermore, 0.5% Anthraquinone (AQ) was added based on the absolutely dry material. The concentration of the chemical solution used is described in Table 1.

Delignification were carried out in a batch system, in a laboratory installation (schematic shown in Figure 3) whose main element was a rotary digester (1) with a volume of 10 L, equipped with electric heating and automatic temperature control. After loading the chopped material and the reagent solution, the temperature was raised according to a standard pulping diagram (Figure 4), with a heating time of 60 min to the regime temperature, followed by maintaining it at this temperature for 60 min. The ratio of cooking liquor to rapeseed stalk (i.e., the liquid:plant material ratio) was maintained at 5:1.

At the end of the delignification stage, black liquor samples were taken by degassing. Immediately after the sample was taken, the digester was depressurized in a controlled manner, evacuating the gases and a portion of the hot black liquor, and then it was cooled to a temperature of approximately 60 °C to allow for opening and removal of the defibered pulp.

The brown stock was washed in a washing and thickening tank (2), followed by mechanical defibrillation in a cylindrical disintegrator (3) for 15 min for complete fiber individuation. Finally, the pulp was sorted on a plane-vibrating sorter (4) with 0.3 mm × 35 mm slots, in order to separate the fibrous material (accept) from the non-defibered fraction (reject). The yield of sorted cellulose fibers (η) and the total yield of defibered material (ηt_otal_) were calculated based on the absolute dry masses of the obtained material and the initial raw materials. Detailed studies on the yield and physical–mechanical characteristics of the obtained cellulose have been described in previous publications [13,23,24].

2.3. Black Liquor Initial Characterization

The black liquor obtained from the pulping processes was analyzed to determine its physicochemical characteristics, including density, pH, silicate content, active and total alkali, total solids, and organic and inorganic substance contents, according to the standardized TAPPI Methods: T625—Analysis of Soda and Sulfate Black Liquor and T650—Solids content of Black Liquor.

The experimental procedure for analyzing the black liquor is schematized in Figure 5. A sample of the black liquor was initially filtered to remove impurities, then used directly for Gel Permeation Chromatography (GPC) analyses and, after appropriate dilution, for UV–Vis Spectroscopy measurements [30]. The experimental protocol combined size-exclusion chromatography with complementary spectroscopic techniques (FT-IR, UV–Vis, and ^13^C NMR) in order to achieve comprehensive structural characterization of the dissolved lignin fractions [31].

For further morphological and structural characterization, alkaline lignin was isolated from the black liquor samples, following a well-known protocol. The process involved a controlled acid precipitation method [32]. The initial phase consisted of heating the black liquor in a water bath at 90–95 °C under continuous stirring. Subsequently, the hot solution was acidified in a controlled manner with a 20% (w/w) sulfuric acid solution until a final pH of 2 was reached. After acidification, the mixture was left to settle to allow the decantation of lignin, which was then separated by centrifugation. The lignin precipitate was washed several times with acidified water at pH = 2 to remove impurities, and finally air-dried. The recovered lignin was presented in the form of a brown, non-agglomerated powder, with a characteristic, unmistakable odor.

The lignin isolation procedure in this study was designed to obtain representative samples for structural analysis rather than to maximize the total recovery yield. Lignin was precipitated from the black liquor collected immediately following the delignification stage. It is important to note that the lignin fraction remaining in the subsequent fiber washing steps was not recovered, as the concentration and processing of these dilute filtrates were not feasible under laboratory-scale conditions. Therefore, the lignin reported herein represents the specifically isolated alkali-lignin fraction from the black liquor.

In a commercial-scale operation, however, the implementation of advanced multi-stage washing and concentration systems would allow for substantially higher recovery rates, effectively capturing the vast majority of the lignin dissolved during the alkaline pulping process

2.4. Structural and Molecular Characterization Techniques

Scanning Electron Microscopy (SEM) is an efficient analysis method for visualizing the surface morphology of organic and inorganic materials at the nanometer to micrometer scale. The SEM analysis was performed using a Quanta 200 Environmental SEM (FEI Company, Hillsboro, OR, USA), capable of operating in high vacuum, variable pressure, and environmental modes. The specimens were analyzed uncoated and untreated at 5 kV, utilizing the X-ray spectrometer for element detection and spectral imaging.

Fourier Transform Infrared Spectroscopy (FT–IR) Spectroscopy was performed using a Bruker Vertex 70 spectrometer (Bruker Optics, Billerica, MA, USA) on potassium bromide (KBr) pellets with a 2 cm^−1^ resolution. The concentration of the samples was maintained at a constant ratio of 2 mg of lignin per 200 mg of KBr. Spectral data were subsequently imagined using the KnowItAll Academic Edition Software (Version 9.5, 2012) and analyzed using the Spectragryph—Optical Spectroscopy Software (Version 1.2.16.1, 2025). This technique is highly efficient and non-destructive, providing crucial molecular-level information regarding the functionality and structural transformation of the isolated lignins [33,34].

Ultraviolet-Visible Spectroscopy (UV–Vis) is the most convenient and useful method for the quantitative and qualitative analysis of lignin in solution [35]. Due to its aromatic structure, lignin strongly absorbs UV light, exhibiting characteristic maxima whose locations depend on the lignin type, chemical modifications, and the solvent used, allowing for concentration and structural inference in black liquor. The spectral characterization was performed by tracing and interpreting the spectra of the black liquor extracts using a Jasco V–550 (JASCO Corporation, Tokyo, Japan) UV–Vis spectrophotometer. The spectra were registered in the 200–800 nm region, utilizing a 1:50 dilution of the prepared extracts.

Gel Permeation Chromatography (GPC) was employed as the analytical method for determining the molecular weight distribution of the isolated lignin [36,37]. It was performed using a Shimadzu LC 20AT (Shimadzu Corporation, Kyoto, Japan) liquid chromatograph with a SPD M20A ultraviolet diode array (UV) detector set at 280 nm. In the GPC analysis, non-acetylated lignin samples were analyzed using TSK-gel GMPWxl GPC columns (300 × 7, 8 mm, G3000PW, 500–800 kDa). Elutions were performed with a pH 11 phosphate-buffered solution, at a flow rate of 0.5 mL/min. A calibration curve was obtained using standards of poly(styrene sulfonic acid) sodium salt, provided by Fluka (Buchs, Switzerland). The reported molecular weights are relative to this standard and should not be considered as absolute lignin molecular weights.

Solid-State ^13^C Cross-Polarization Magic-Angle Spinning Nuclear Magnetic Resonance (^13^C CP/MAS NMR) Spectroscopy experiments were conducted for detailed structural analysis of the lignin samples [38,39]. The measurements were carried out on a Bruker Avance 400 WB spectrometer (Bruker Biospin, Billerica, MA, USA), operating at 100.613 MHz for ^13^C. The lignin samples were tightly packed into 4 mm diameter zirconia rotors. The experimental conditions for ^13^C CP/MAS were set as follows: a 2.7 μs pulse for the 90° pulse, a recycle delay of 5 s, a contact time of 2 ms, and a rotating speed of 10 kHz. Tetramethylsilane (TMS) was utilized as the primary chemical shift scale reference for all ^13^C analyses [40].

3. Results and Discussion

3.1. Characteristics of Residual Black Liquors

The delignification of rapeseed stalks was performed using two alkaline pulping processes—Sulfate (kraft) and Soda–Anthraquinone (Soda-AQ)—under comparatively mild conditions, reflecting the low lignin content and low bulk density of this non-woody raw material. The chemical compositions of the cooking liquors and the resulting black liquors provide insight into the delignification efficiency, alkali consumption, and dissolution behavior of lignin and accompanying components.

The initial cooking liquors differed significantly in their chemical composition (see Table 1). The Sulfate process was characterized by the presence of sodium sulfide and a sulfidity of 22.5%, resulting in higher total, active, and effective alkali contents compared to the Soda-AQ system, which relied solely on sodium hydroxide with anthraquinone as a redox catalyst. These differences fundamentally influenced the delignification reactions and the extent of carbohydrate stabilization during pulping [41,42,43].

Analysis of the black liquors obtained after pulping revealed systematic trends as a function of alkali charge and temperature for both processes (see Table 2). In the Sulfate pulping series (A6, A9, A12), the residual active alkali decreased progressively with increasing cooking severity, indicating more extensive alkali consumption associated with enhanced lignin dissolution and neutralization of acidic degradation products. This tendency was accompanied by a corresponding increase in residual sodium carbonate, reflecting the conversion of hydroxide during pulping reactions. The presence of residual sodium sulfide further confirms the Kraft-type chemistry and its role in promoting selective lignin cleavage through nucleophilic reactions at ether linkages.

In contrast, the Soda-AQ black liquors (B6, B9, B12) exhibited lower overall residual alkali concentrations, consistent with the absence of sulfide species and the lower initial alkali charge. Nevertheless, the addition of anthraquinone enabled efficient delignification, as evidenced by the comparable levels of dissolved organic matter relative to the sulfate process, which correlate with the fiber properties [40,44,45]. This highlights the catalytic role of anthraquinone in stabilizing carbohydrates and enhancing lignin fragmentation through reversible redox cycling.

The solids content of the black liquors, determined at 105 °C, remained relatively constant across both pulping systems, with values around 180 g/L. However, the distribution between organic and inorganic fractions differed. The sulfate black liquors showed a higher inorganic content, attributable to the presence of sulfide- and carbonate-based sodium salts, whereas the Soda-AQ liquors contained a higher proportion of organic matter. This suggests that, under the applied conditions, the Soda-AQ process favored the dissolution of lignin-derived fragments relative to inorganic salt formation.

The pH values of all black liquors remained strongly alkaline (approximately 11.7–12.0), confirming that the pulping reactions proceeded under sufficiently alkaline conditions to maintain lignin solubility and prevent premature precipitation.

Silicate concentrations were comparable across all samples, indicating limited variation in silica dissolution from the rapeseed stalks and suggesting that inorganic extractives played a secondary role under the applied conditions.

Comparison between the initial alkali charge of the cooking liquors and the residual alkali concentrations measured in the corresponding black liquors indicates a substantial consumption of alkali during delignification. This decrease reflects the participation of alkaline species in lignin dissolution reactions, neutralization of acidic degradation products, and the formation of carbonate species during pulping.

3.2. Scanning Electron Microscopy Analysis

Scanning electron microscopy analysis of the lignin samples at two different magnifications (1000× and 2000×) provides a consistent and complementary view of the morphology of lignin precipitated from alkaline black liquor (please see Figure 6). At the lower magnification (30 µm scale), the lignin powder appears as a highly agglomerated material composed of irregularly shaped clusters distributed across the field of view. These clusters form a continuous, heterogeneous network with poorly defined particle boundaries, indicating extensive inter-particle association. The agglomerates typically extend over several micrometers and exhibit a broad size distribution, reflecting the intrinsic heterogeneity of technical lignin and the non-uniform nature of the precipitation process.

Higher-magnification imaging (10 µm scale) reveals additional structural details within these larger aggregates. The agglomerates are shown to consist of fused submicron primary entities that have coalesced into dense secondary particles. Individual primary particles are not clearly distinguishable, suggesting that precipitation involved rapid macromolecular association and collapse rather than stepwise particle growth. The surfaces of the lignin aggregates are markedly rough and uneven, characterized by protrusions, depressions, and fine-scale texturing, which further supports a kinetically controlled precipitation mechanism.

The absence of well-defined spherical particles or smooth surfaces indicates that lignin precipitation occurred through rapid acidification of the alkaline medium, leading to protonation of phenolic and carboxylic groups and a sudden loss of solubility. As a consequence, lignin macromolecules underwent strong intermolecular interactions, including hydrophobic association and hydrogen bonding, resulting in the formation of compact, amorphous aggregates. This behavior is typical of lignins recovered from black liquor, which are known to possess broad molecular weight distributions and chemically heterogeneous structures due to alkaline pulping reactions.

Furthermore, the dense packing of the aggregates and the limited presence of large inter-particle voids suggest partial structural densification during post-precipitation handling and drying. Capillary forces acting during solvent removal, combined with continued intermolecular association, likely contributed to the consolidation of the lignin particles into compact clusters. The overall amorphous appearance and lack of crystalline features are consistent with the non-crystalline nature of lignin and its tendency to form disordered solids upon recovery from solution.

Taken together, the SEM observations at both magnifications confirm that the lignin precipitated from alkaline black liquor exhibits a highly aggregated, irregular morphology with rough surfaces and micrometer-scale secondary particles formed through rapid, kinetically driven precipitation. This structural organization is characteristic of technical lignins and has direct implications for their physicochemical behavior, including solubility, reactivity, and performance in subsequent material or chemical valorization processes.

3.3. Structural Analysis by FTIR

FT-IR spectroscopy was used to investigate and compare the structural features of lignins precipitated via Kraft and Soda-AQ delignification processes carried out at an active alkali charge of 20% and a cooking temperature of 170 °C. The resulting spectra, presented in Figure 7a,b, exhibit the characteristic absorption bands of technical lignins, confirming the preservation of the main lignin structural moieties after alkaline pulping and precipitation.

Both lignin samples display a broad absorption band in the region around 3388–3379 cm^−1^, attributed to O–H stretching vibrations. This band reflects the presence of phenolic and aliphatic hydroxyl groups and indicates extensive hydrogen bonding, which is typical for lignins subjected to alkaline treatment and subsequent precipitation. Slight differences in band position and intensity suggest variations in hydroxyl group content and intermolecular interactions between the Kraft and Soda-AQ lignins.

The C–H stretching vibrations of aliphatic methyl and methylene groups are observed at approximately 2937 cm^−1^ for the sulfate lignin and at 2920 and 2850 cm^−1^ for the Soda-AQ lignin. These bands are associated with alkyl side chains linked to the aromatic backbone and indicate that both lignins retain significant aliphatic character. The more pronounced splitting observed in the Soda-AQ lignin spectrum may be related to differences in side-chain integrity and a lower extent of condensation reactions compared to the sulfate lignin.

Absorption bands at 1732 and 1655 cm^−1^ correspond to C=O stretching vibrations, which are generally attributed to unconjugated carbonyl groups such as ester, aldehyde, or carboxylic functionalities. The presence of these bands suggests partial oxidation and side-chain modification during alkaline pulping, with their relative intensity potentially reflecting differences in carbohydrate residues or oxidation reactions between the two processes.

The aromatic skeletal vibrations characteristic of lignin are clearly visible in both spectra, with bands at approximately 1608 cm^−1^ and 1512–1508 cm^−1^, confirming the presence of substituted aromatic rings. These bands indicate that the fundamental aromatic structure of lignin is preserved in both pulping systems, despite the severe alkaline conditions applied.

Deformation vibrations of CH_2_ groups appear around 1460 cm^−1^, while combined CH and CH_2_ bending vibrations are observed at 1423–1421 cm^−1^. These features are typical of lignin macromolecules and further confirm the aliphatic–aromatic hybrid nature of the recovered lignins. The band near 1329 cm^−1^ is associated with O–H deformation vibrations, commonly linked to syringyl units and condensed phenolic structures.

Bands observed in the region 1120–1117 cm^−1^ are assigned to C–O–C stretching vibrations of ether linkages, whereas absorptions at 1043–1036 cm^−1^ correspond to C–OH stretching vibrations in aliphatic alcohols. The absorption region between 1100 and 1033 cm^−1^ is characteristic of C–O vibrations in primary and secondary aliphatic alcohols, indicating the presence of residual side-chain hydroxyl groups. The absorption in the range 1240–1210 cm^−1^ is attributed to ether-type C–O–C linkages, which are fundamental structural elements of lignin.

Distinct absorption bands associated with syringyl and guaiacyl units are also evident. The band near 1330 cm^−1^ is characteristic of syringyl units, while the absorption around 1270 cm^−1^ is attributed to guaiacyl structures. The presence of both bands confirms that rapeseed stalk lignin contains a mixed S/G character, typical of non-woody biomass. Differences in the relative intensities of these bands between the sulfate and Soda-AQ lignins suggest variations in the S/G ratio and in the extent of ether bond cleavage and condensation reactions induced by the respective pulping chemistries.

3.4. UV-Vis Spectroscopic Analysis

The recorded spectra are characteristic of lignins dissolved in alkaline media, exhibiting a pronounced absorption maximum around 205 nm, a shoulder in the 260–280 nm region, and a plateau extending over the 320–350 nm range (please see Figure 8). As observed, noticeable differences in lignin absorbance values occur as a function of alkali charge and sample origin. A clear correlation is evident between absorbance intensity and the concentration of dissolved lignin, which in turn reflects the degree of delignification. The UV spectra of lignins obtained under different alkali additions clearly demonstrate the crucial role of the alkaline medium in promoting lignin solubilization.

UV–Vis spectroscopy was used to comparatively evaluate the structural features of lignins precipitated via Kraft and Soda-AQ pulping processes under different cooking severities, defined by active alkali charge (18, 20, and 22%) and cooking temperature (150, 160, and 170 °C). The recorded spectra exhibit absorption patterns characteristic of technical lignins, dominated by strong absorption in the ultraviolet region, associated with aromatic chromophores and conjugated structures.

All lignin samples display an intense absorption band in the range of approximately 200–220 nm, which is attributed to π–π* electronic transitions of the aromatic rings present in guaiacyl and syringyl units. The intensity of this band increases systematically with both increasing temperature and alkali charge, indicating a higher concentration of dissolved and precipitated lignin fragments with preserved aromatic structures. This trend suggests that more severe pulping conditions promote enhanced lignin fragmentation and solubilization, leading to increased recovery of UV-active aromatic moieties.

A secondary, less intense absorption shoulder is observed in the region of 270–290 nm, which is commonly associated with n–π* transitions of non-conjugated carbonyl groups and conjugated phenolic structures within the lignin macromolecule. The gradual intensification of this shoulder with increasing cooking severity indicates a higher degree of structural modification, including oxidation and cleavage of β–O–4 ether linkages, resulting in an increased number of phenolic hydroxyl and carbonyl-containing groups.

Comparing the two pulping systems, the Soda-AQ lignins generally exhibit higher absorbance values across the entire UV–Vis range, particularly at higher temperatures and alkali charges. This behavior suggests that the presence of anthraquinone enhances lignin depolymerization while limiting excessive condensation reactions, leading to lignin fractions with a higher content of UV-active chromophores and a more open, less condensed structure. In contrast, Kraft lignins tend to show comparatively lower absorbance intensities, which may be attributed to increased condensation reactions facilitated by sulfide species during Kraft pulping, resulting in more compact aromatic structures with reduced chromophore accessibility.

The effect of temperature is particularly pronounced at an alkali charge of 22%, where the absorbance maxima reach their highest values at 170 °C for both pulping processes. This observation confirms that temperature plays a dominant role in promoting lignin dissolution and structural transformation. However, the relative differences between the Kraft and Soda-AQ lignins remain evident, with Soda-AQ samples consistently displaying slightly higher absorbance, reflecting differences in lignin chemistry induced by the two delignification mechanisms.

3.5. Molecular Weight Distribution by GPC

Gel permeation chromatography (GPC) was employed to characterize the molecular weight distribution of lignins dissolved in the residual liquors obtained from the delignification of rapeseed stalks. The molecular weight distribution curves derived from the chromatograms, as seen in Figure 9a, reveal that all lignin samples exhibit broad and asymmetric distributions, which is typical of technical lignins generated under alkaline pulping conditions. This behavior reflects the intrinsic structural heterogeneity of lignin and the simultaneous occurrence of depolymerization and condensation reactions during delignification [46]. The corresponding chromatograms in Figure 9b show well-defined elution profiles, with absorbance maxima located in the low to intermediate elution volume range, indicating the predominance of oligomeric lignin fragments.

A clear correlation can be observed between the active alkali charge applied during delignification and the amount and molecular characteristics of lignin dissolved in the residual liquors. For both pulping processes, increasing the alkali charge leads to higher chromatographic peak intensities, indicating an increased concentration of dissolved lignin. Simultaneously, the elution maxima shift toward lower elution volumes, accompanied by a broadening of the molecular weight distribution toward lower molecular masses. This behavior suggests that higher alkali additions enhance not only lignin solubilization, but also the extent of lignin fragmentation.

Quantitative analysis of the molecular weight parameters (as given in Table 3) supports these observations. In the Sulfate (Kraft) lignin series (A6, A9, A12), the weight-average molecular weight (Mw) decreases slightly with increasing alkali charge, while the number-average molecular weight (Mn) shows a similar decreasing trend. The polydispersity index (IP) increases marginally from 2.63 to 2.68, indicating a broader molecular weight distribution at higher pulping severity. This suggests that, although Kraft pulping promotes lignin dissolution, it also favors secondary condensation reactions that maintain a relatively high fraction of high-molecular-weight species.

In contrast, the Soda-AQ lignins (B6, B9, B12) exhibit systematically lower Mw and Mp values compared to the sulfate lignins, along with narrower molecular weight distributions, as reflected by lower polydispersity indices (2.25–2.31). The progressive decrease in Mw from 3941 to 3621 Da with increasing alkali charge indicates a more pronounced depolymerization of lignin macromolecules in the presence of anthraquinone. This effect can be attributed to the catalytic redox role of anthraquinone, which promotes selective cleavage of ether linkages while limiting excessive condensation reactions.

The shift in the chromatograms toward lower molecular weight regions and the increased contribution of low-molecular-weight (LMW) fractions indicate that lignin fragmentation intensifies with increasing alkali charge for both pulping systems. This fragmentation results from the combined action of the alkaline medium and reactive species formed in the cooking liquor, leading to the degradation of high-molecular-weight (HMW) lignin fragments into smaller oligomeric units. The effect is more pronounced in the Soda-AQ process, where the stabilization of carbohydrate end groups and suppression of lignin recombination reactions favor the accumulation of LMW lignin fractions.

3.6. Solid-State 13C CP/MAS NMR Analysis

The structural features of lignins dissolved during the delignification of rapeseed stalks were further investigated by Solid-State ^13^C NMR spectroscopy [47]. Figure 10a,b present the ^13^C NMR spectra of sulfate lignin (A-9) and Soda-AQ lignin (B-9), respectively, while the corresponding signal assignments are summarized in Table 4. The spectra provide detailed information on the aliphatic and aromatic carbon environments and enable a comparative evaluation of the structural modifications induced by the two alkaline pulping processes.

Both spectra exhibit the characteristic resonance regions typical of technical lignins obtained under alkaline conditions [48]. Signals in the aliphatic region (0–50 ppm) are associated with saturated carbon atoms in lignin side chains and residual aliphatic structures. The resonances at approximately 29.45 ppm (sulfate) and 28.20 ppm (Soda-AQ) correspond to aliphatic carbons (C–C, C–H) in the lignin side chain, while the signals at ~32 ppm in both samples are attributed to methylene (CH_2_) groups. The close similarity of these signals suggests that the basic aliphatic backbone of lignin is largely preserved in both delignification processes [49].

A prominent signal observed at 55.04 ppm for sulfate lignin and 54.64 ppm for Soda-AQ lignin is assigned to methoxyl (–OCH_3_) groups in syringyl (S) and guaiacyl (G) units. The comparable intensity and chemical shift of this signal indicate that demethoxylation reactions are limited under the applied cooking conditions [50]. However, the slightly lower chemical shift observed for the Soda-AQ lignin may reflect a more uniform chemical environment for methoxyl groups, consistent with reduced condensation reactions in the presence of anthraquinone [51].

In the region between 60 and 90 ppm, signals associated with oxygenated aliphatic carbons in lignin side chains are clearly visible. The resonances at ~64 ppm correspond to Cγ carbons in β–5 substructures, while those at ~74 ppm are attributed to Cβ carbons in β–O–4 ether linkages. The presence of these signals in both spectra confirms that β–O–4 structures, although partially cleaved during alkaline pulping, remain a significant structural motif in the dissolved lignins. Notably, the Soda-AQ lignin exhibits slightly better-defined signals in this region, suggesting a higher preservation of ether linkages compared to the sulfate lignin.

The aromatic region (100–160 ppm) provides critical insight into the distribution and substitution pattern of syringyl and guaiacyl units [52]. Signals at approximately 104.58 ppm (sulfate) and 103.63 ppm (Soda-AQ) are assigned to C2/C6 carbons in syringyl units, confirming the presence of S-type lignin, which is characteristic of non-woody biomass such as rapeseed stalks. The resonances at ~132.8 ppm correspond to C1 carbons in non-etherified guaiacyl units, while those at ~147–148 ppm are attributed to C3 carbons in non-etherified G units. These signals are slightly shifted to higher ppm values in the Soda-AQ lignin, which may indicate a higher degree of phenolic character resulting from enhanced ether bond cleavage.

Signals in the range of 150–155 ppm are associated with etherified aromatic carbons, particularly C4 in guaiacyl units linked through ether bonds or conjugated with carbonyl groups (α-C=O). The signal at 152.23 ppm in sulfate lignin and 153.50 ppm in Soda-AQ lignin suggests that etherified aromatic structures are present in both samples, although their chemical environment appears more oxidized in the Soda-AQ lignin.

The carbonyl region (170–190 ppm) shows notable differences between the two lignins. The sulfate lignin displays signals at 172.95 and 174.95 ppm, attributed to aliphatic ester carbonyls and aromatic aldehyde (Ar–CHO) groups, respectively. In contrast, the Soda-AQ lignin exhibits carbonyl signals shifted toward higher chemical shifts (177.99 and 180.87 ppm), indicating a higher degree of oxidation. This observation suggests that Soda-AQ pulping promotes the formation of carboxylic and conjugated carbonyl functionalities, likely due to the redox activity of anthraquinone and enhanced oxidative cleavage of lignin side chains.

These findings are in good agreement with the GPC results, which indicate lower molecular weights and narrower polydispersity for Soda-AQ lignins, and confirm that the Soda-AQ process produces structurally less condensed and more chemically functionalized lignin fractions compared to the conventional Sulfate process.

4. Conclusions and Future Perspectives

The black liquor analyses demonstrated that both pulping processes were effective in dissolving lignin from rapeseed stalks under mild operating conditions. The sulfate process exhibited higher alkali consumption and greater inorganic residue formation, consistent with its more aggressive chemical environment. In contrast, the Soda-AQ process achieved comparable levels of organic matter dissolution at lower residual alkali concentrations, underscoring the efficiency of anthraquinone-assisted delignification for non-woody biomass. These results confirm that rapeseed stalks are well suited for alkaline pulping and that Soda-AQ represents a viable alternative to Kraft pulping when reduced sulfur content and lower chemical loads are desired.

The FT-IR analysis demonstrated that both sulfate and Soda-AQ lignins exhibited the typical functional groups and structural motifs of technical lignins. However, subtle differences in band intensities and positions indicate that the Soda-AQ lignin retains a less condensed structure with a higher preservation of ether linkages and hydroxyl functionalities, whereas the Sulfate lignin shows signs of increased condensation and structural modification due to the presence of sulfide species and the more aggressive Kraft pulping environment.

The UV–Vis spectral analysis indicated that increasing pulping severity enhances the aromatic character and chromophore concentration of the recovered lignins. The comparative results further demonstrate that Soda-AQ pulping yields lignins with higher UV–Vis absorbance and potentially lower degrees of condensation compared to sulfate lignins. These findings are consistent with the catalytic role of anthraquinone in promoting selective lignin fragmentation and stabilizing lignin structures during alkaline delignification, and they correlate well with the trends observed in FT-IR and SEM analyses.

The GPC results demonstrated that both the alkali charge and the type of delignification process have a significant influence on the molecular weight characteristics of dissolved lignin. While Sulfate pulping produces lignins with higher molecular weights and broader polydispersity due to concurrent depolymerization and condensation, the Soda-AQ process yields lignins with lower average molecular weights and narrower distributions, indicative of more efficient and selective lignin depolymerization. These findings are consistent with the trends observed in the UV–Vis and FT-IR analyses and confirm the suitability of Soda-AQ pulping for producing less condensed lignin fractions from non-woody biomass such as rapeseed stalks.

The comparative ^13^C NMR analysis demonstrated that both pulping processes yield lignins with similar fundamental structural motifs, including methoxylated aromatic units and residual β–O–4 linkages. However, significant differences arise in terms of oxidation state and condensation behavior. Sulfate lignin is characterized by a slightly higher degree of structural heterogeneity and the presence of less oxidized carbonyl groups, consistent with concurrent depolymerization and condensation reactions typical of Kraft pulping. In contrast, Soda-AQ lignin exhibits a higher degree of oxidation, better preservation of ether linkages, and a more uniform aromatic structure, supporting the conclusion that anthraquinone enhances selective lignin depolymerization while limiting secondary condensation reactions.

The combined spectroscopic, chromatographic, and morphological analyses suggest that the structural characteristics of lignin recovered from rapeseed stalks can be effectively tailored through controlled adjustment of the pulping severity and alkali charge. In this context, the Soda-AQ process emerges not only as an environmentally favorable alternative to Kraft pulping, but also as a promising route for producing lignin fractions with enhanced structural homogeneity and functional group accessibility. These features are particularly relevant for downstream valorization pathways, where the reactivity, solubility, and molecular weight distribution of lignins are critical parameters.

From an application-oriented perspective, the lower degree of condensation and narrower molecular weight distributions observed for Soda-AQ lignins indicate a higher potential suitability for chemical modification, polymer blending, and the production of lignin-based functional materials. Conversely, the more condensed sulfate lignins may be advantageous in applications requiring higher thermal stability or char-forming capacity, such as carbon materials or binders. This highlights the possibility of selecting or designing pulping processes based on targeted lignin end-uses rather than solely on delignification efficiency.

Future work should focus on correlating the structural features identified in this study with lignin performance in specific valorization routes, such as phenolic resin synthesis, polyurethane formulations, antioxidant additives, or bio-based carbon materials. In addition, a more detailed investigation of lignin reactivity, including quantitative analysis of phenolic hydroxyl groups and interunit linkage distribution, would further clarify the structure–property relationships governing lignin functionality. The integration of Soda-AQ pulping with lignin-first or biorefinery-oriented processing strategies could therefore represent a viable pathway toward the high-value utilization of agricultural residues such as rapeseed stalks.

From a techno-economic perspective, the valorization of rapeseed stalks via alkaline pulping processes demonstrates significant potential for industrial commercialization. As an abundant agricultural residue, this feedstock offers a substantial cost advantage over traditional wood-based resources, significantly reducing initial feedstock expenditures. The feasibility of the process is further supported by its compatibility with existing Kraft mill infrastructure, requiring minimal capital investment for implementation of the sulfur-free Soda-AQ route. The ability to achieve efficient delignification under milder thermal and chemical regimes suggests a reduction in operational expenditures (OPEX) related to energy and reagent consumption.

At the same time, while the chemical and structural properties of rapeseed stalk lignin confirm its potential for high-value applications, its commercial feasibility is governed by external logistical and socioeconomic factors. The primary challenge lies in the low bulk density of the stalks, which necessitates a localized processing model to minimize transport expenditures in a specific area close to the mill. Additionally, the industrialization of such a process is highly sensitive to feedstock pricing stability; the perceived value of agricultural residues often shifts once industrial potential is established, leading to price volatility. Consequently, the commercial success of a rapeseed-based biorefinery depends not only on the technical efficiency of the Soda-AQ or Kraft processes but also on optimization of the collection radius and the establishment of sustainable, long-term partnerships with agricultural producers to mitigate logistical and market-driven risks.

Ultimately, by shifting the paradigm from lignin combustion to the isolation of structurally preserved, high-value aromatic precursors for the biopolymer and resin industries, this approach enhances the overall economic sustainability of integrated biorefinery systems and aligns with the strategic goals of a circular bioeconomy.

Overall, the study shows that annual plants represent an increasingly attractive feedstock for integrated biorefinery concepts, due to their fast growth rates, wide availability, and favorable chemical composition, enabling the simultaneous production of cellulose-rich fractions for materials and pulp applications, fermentable sugars for biofuels, and structurally diverse lignins. In particular, the Soda-AQ lignin, characterized by its lower molecular weight and enhanced functional group density, represents a superior precursor compared to traditional Kraft lignin for sustainable biochemicals [53], polymeric resins [54], and advanced composite materials [55,56,57,58]. This transition from traditional energy recovery through combustion to the production of lignin-based derivatives [59] and biofuels [60] underscores the potential of rapeseed stalks to support a zero-waste, circular bioeconomy.