Startseite Microwave-assisted extraction of acetosolv lignin from sugarcane bagasse and electrospinning of lignin/PEO nanofibres for carbon fibre production
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Microwave-assisted extraction of acetosolv lignin from sugarcane bagasse and electrospinning of lignin/PEO nanofibres for carbon fibre production

  • Emmanuel Mkumbuzi , Bishop Bruce Sithole und Werner Ewald van Zyl EMAIL logo
Veröffentlicht/Copyright: 26. Juli 2024
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Green Processing and Synthesis
Aus der Zeitschrift Green Processing and Synthesis Band 13 Heft 1

Abstract

Sugarcane bagasse (SB) is an agricultural waste with massive potential as a source of lignin for the production of renewable materials. In this study, acetosolv lignin from SB was extracted efficiently and sustainably via microwave (MW)-assisted extraction within 10 min. Subsequently, acetosolv lignin was subjected to electrospinning into lignin nanofibres (LNFs) for carbon fibre production using an 85/15% w/w lignin/poly(ethylene oxide) blend ratio in a DMF solution after optimisation of suitable electrospinning parameters. The structural characterisation of lignin was accomplished via pyrolysis-gas chromatography with mass spectrometry, heteronuclear single quantum coherence, Fourier-transform infrared spectroscopy, size-exclusion chromatography, and thermogravimetric analysis while the electrospun LNFs were characterised by transmission electron microscope as randomly arranged fibres with diameters ∼15 nm. SB is a readily available and valuable source of lignin for facile MW extraction using acetic acid, while electrospinning was a fast and efficient method for the fabrication of LNFs.

Graphical abstract

Keywords: microwave-assisted; delignification; electrospinning; lignin nanofibres; sugarcane bagasse

1 Introduction

Carbon fibres (CF) are highly versatile materials widely used in several sectors of the economy, including aerospace, construction, automotive, and energy [1]. However, at least 50% of the cost of CF production is due to the expensive petroleum-derived polyacrylonitrile (PAN) precursor. Cheaper and renewable PAN alternatives are required to enable the cheaper production of CFs [2,3]. In particular, sustainable lignin carbon fibre (LCF) production using lignin derived from agricultural lignocellulosic wastes such as corn stover, wheat husks, corn cobs, and sugarcane bagasse (SB) are advantageous since they do not compete with food security [4,5,6]. SB from sugar processing is a good biomass precursor for LCF production; as a material, it has a reasonably steady supply (annual global production is about 500 million tonnes) [5,7], has a centralised source, can easily be collected and transported relatively cheaply, and it has a generally homogenous material composition [8,9].

Although biomass-derived LCFs have been successfully fabricated [10], they tend to be unsuitable for structural applications due to their numerous defects [11]. The voids resulting from the volatilisation of side chains in lignin, carbohydrate impurities, and plasticisers used in lignin blends during the carbonisation process [12]. Nevertheless, LCFs have found important applications in energy storage devices, including supercapacitors [13,14,15], sodium-ion batteries [15,16], and lithium-ion batteries [17,18,19], where their voids are beneficial as they enable a greater intercalation volume for ions used in energy storage [20,21]. LCFs have also been successfully used as adsorbents for water purification and remediation [22,23] and as catalyst support in heterogeneous catalysis [24,25].

The most common techniques used in the fractionation/extraction of the lignin, the LCF precursor, from biomass, such as the Kraft, soda, and sulphite processes, generally use environmentally harmful chemicals while also being energy-intensive and time-consuming [26,27]. In addition, impure lignin by-product contaminated by inorganic contaminants, with limited application downstream, is often produced, relegating copious amounts of lignin by-product to underutilisation as boiler fuel in the biorefinery [21,28,29,30,31]. Efficient and environmentally benign lignin extraction processes producing lignin with fewer inorganic contaminants are critical for lignin valorisation [4,32,33,34]. In this regard, organic solvents such as formic acid, acetic acid (AA), methanol, and ethanol have been touted as crucial solvents in lignin extraction [35]; in addition, these solvents can be produced on-site by the bio-refineries from the holocellulose components (hemicellulose and cellulose) of the same biomass, encouraging a circular bio-economy overall [35,36,37,38,39,40].

By following sound circular economy principles, including biomass fractionation and subsequent LCF production, several techniques have been developed. To enable efficient lignin extraction, organic solvents have been coupled with external non-ionising radiation such as microwaves (MW), dramatically reducing extraction time and efficiently extracting the lignin [41,42]. MW-assisted heating has higher heating rates compared to conventional heating, whose vessels are slower to warm up. In MW-assisted heating, the energy is transferred more efficiently to the material electromagnetically, rather than via a thermal heat flux down a thermal gradient, as with traditional heating methods [43,44,45,46,47,48,49].

The efficiency of MW irradiation derives from the rapid volumetric heating [43,44,45,46,47,48,49], which is generated by the swift migration of ions and rotation of dipoles in molecules, resulting in molecular friction and heat generation. Successful MW lignin extractions from biomass using organic solvents have been achieved using bamboo in a formic acid/AA/water solution with 6% HCl [50], birch in formic acid [31], and SB in 90% (v/v) AA and 2% HCl [42].

After extracting the lignin precursor, efficient and economical lignin nanofibre (LNF) production is also vital, and can be achieved using electrospinning [21,51,52]. Electrospinning is a well-established technique that uses electrostatic forces to fabricate non-woven fibre mats with typically randomly oriented fibres spanning the submicron to micron range in diameter [19,53,54].

Electrospinning is achieved by applying a high voltage to a fluid polymer, inducing an electric charge in the fluid. Once a critical charge is reached, a fluid jet will be formed from the droplet at the needle’s tip and travel towards the region of lower potential (grounded collector) where the fibres will be collected [55]. The electrospinning setup can be horizontally or vertically oriented, or even be a rotating drum to allow for the formation of aligned fibres (Figure 1) [56,57,58]. The overall fibre morphology produced via electrospinning depends on several variables, such as the lignin type, solvent, binder polymer, viscosity of lignin solution, the concentration of lignin solution, electrospinning voltage, electrospinner configuration, feed rate, and the distance between nozzle and collector [20,56,57,58,59,60].

Figure 1 Common electrospinning configurations. (a) Horizontal, (b) top-down, (c) bottom-up, and (d) rotating drum.
Figure 1

Common electrospinning configurations. (a) Horizontal, (b) top-down, (c) bottom-up, and (d) rotating drum.

This study aimed to extract lignin sustainably and efficiently from SB and subsequently fabricate LNFs for the fabrication of LCFs. This was achieved via acetosolv MW-assisted lignin extraction from SB. The LNFs were then fabricated in a facile and efficient manner using electrospinning. To the best of our knowledge, this is the first time lignin from SB has been extracted using a fully acetosolv MW-assisted heating method. Avelino et al. extracted lignin from SB using 90% (v/v) AA and 2% HCl in 30 min using an open configuration [42]. In this study, a closed vessel setup was utilised and we managed to achieve extraction within 10 min without using HCl as a catalyst. In addition, we utilised the extracted lignin to make LNFs via an electrospinning process, which is rare for lignin derived from SB. As part of the study, being aware of structural differences between lignin sources (as a result of lignin source and extraction methods used), we sought to determine the virtually unique optimum parameters that would allow the extracted acetosolv lignin from SB to be electrospun into LNF without causing electrosprays. In this regard, we were able to determine the flow rate, voltage, distance between poles and lignin to plasticiser ratio required to successfully achieve the electrospinning of acetosolv lignin from SB into LNFs.

2 Materials and methods

All chemicals were purchased from commercial sources and were used as received without further purification at analytical grade. SB was procured from the Council for Scientific and Industrial Research in Durban, South Africa.

2.1 MW-assisted acetosolv extraction of lignin from SB

Sugarcane bagasse (SB) was dried overnight at 105°C in a convection oven and sieved with a 1 mm sieve. The powder was collected into a zip-lock bag and stored in a desiccator. The MW-assisted extraction of lignin from SB was carried out in triplicate.

About 1.0 g of SB was weighed into a 35 mL glass MW vial and 20 mL of pure AA (99.5% v/v) was added; after that, a stirrer bar was dropped in. The vial was placed in an MW synthesiser (CEM discover SP model number 909155) set at 120°C for 10 min with a ramp time of 3 min.

Upon completion, a dark brown solution with a residue at the bottom was observed. The residue was filtered using a Buchner funnel. After that, the residue in the Buchner funnel was washed three times using 10 mL of AA. The dark brown filtrate was transferred to a 100 mL round bottom flask and placed in a rotary evaporator to remove the AA solvent to approximately 5 mL. About 40 mL of distilled water was added dropwise while stirring, resulting in the precipitation of dark brown lignin solid. The mixture was centrifuged at 4,000 rpm for 10 min. The brown SB-derived lignin (SBL) was then filtered on a Buchner funnel, washed five times with 10 mL of water, and subsequently dried under vacuum overnight, weighed, and stored in a glass vial.

2.2 Conventional acetosolv extraction of lignin from SB

The conventional extraction of lignin from SB was conducted in an autoclave. The extraction was carried out in triplicate. The reaction conditions were set at a temperature of 120°C for 10 min, with a ramp time of 10 min. After the reaction, the vessel was left to cool down naturally to reach room temperature.

Upon completion, a dark brown solution with some residue was observed. The mixture was separated using a Buchner funnel. The residue in the funnel was washed three times using 10 mL of AA. The dark brown filtrate was transferred to a 100 mL round bottom flask and placed in a rotary evaporator to reduce the AA solvent to approximately 5 mL. After that, 40 mL of distilled water was added dropwise while stirring, resulting in the precipitation of a dark brown solid (lignin). The mixture was centrifuged at 4,000 rpm for 10 min. The brown SBL was then filtered on a Buchner funnel, washed five times with 10 mL of water, dried under vacuum overnight, weighed, and stored in a glass vial.

2.3 Electrospinning of LNFs

A 10% (w/w) solution of an 85/15% w/w lignin/poly(ethylene oxide) (PEO) blend was made by dissolving 170 mg of SBL and 30 mg of PEO (200,000 g·mol−1) in 1.8 g of DMF. After dissolution, the mixture was magnetically stirred for 2 h and then left to stand for another 2 h. Electrospinning was conducted using a Nanospinner-1 (Basic System) Single Nozzle Electrospinning Machine (Turkey) with a voltage range of 0–40 kV. A 1 mL syringe of 4 mm diameter was used at a flow rate of 0.3 mL·h−1, 20 kV voltage, and a distance of 10 cm from the negative plate. Nanofibres were collected on an aluminium foil placed on the negative plate. The nanofibres were placed in a vial, sealed, and subjected to transmission electron microscopy (TEM) analysis.

2.4 Characterisation

For pyrolysis gas chromatography with mass spectrometry (Py-GC/MS), approximately 100–150 μg of the sample were pyrolysed at 550°C for 20 s and the interface temperature to the analytical column was set at 350°C. The samples were analysed by Py-GC/MS detection using a multi-shot pyroliser, EGA/PY-3030 D (Frontier Lab, Japan) attached to a Shimadzu gas chromatograph/mass spectrometer (QP2010 SE). The chromatographic separation of the pyrolysis products was performed using an ultra-alloy capillary column (Frontier Lab, Japan) (30 m × 0.25 mm, 0.25 μm). The injection temperature was set to 280°C and the column flow rate was set to 1.0 mL·min−1 with helium used as a carrier gas. The GC temperature programme used was as follows: (i) held at 50°C for 2 min, (ii) ramped from 50°C to 200°C at a rate of 5°C·min−1, and (iii) then held for a further 18 min. The ion source and interface temperatures in the mass spectrometer were set to 200°C and 300°C, respectively. The scan range used for the mass selective detector was from m/z 40 to 650. The pyrolysis products were identified by comparing their mass spectra with the mass spectra in NIST 14 and Wiley 10 databases.

For thermogravimetric analysis (TGA), about 12.6 mg of the respective lignin sample was weighed in the pan for the analysis. The oven was programmed from 30°C to 900°C at a constant heating rate of 10°C·min−1, then held at 900°C for 5 min. Nitrogen (N2) at a 20 mL·min−1 flow was used to prevent any probable air combustion of the lignin sample. TGA data were collected on a Perkin Elmer STA 6000 machine (USA).

Heteronuclear single quantum coherence nuclear magnetic resonance HSQC NMR data were collected on a Bruker Avance 400 MHz spectrometer (Germany). HSQC NMR spectra were obtained under ambient conditions in DMSO-d 6 deuterated solvent. NMR data are expressed in parts per million (ppm).

A JEOL 1010 (Japan) transmission electron microscope (TEM) was used to characterise the morphology and size of the produced LNFs.

Infrared spectra were obtained at ambient temperature on a Fourier transform infrared (FTIR) spectroscope (Perkin Elmer Spectrometer, Model 100, USA) equipped with a universal ATR sampling accessory. All characteristic peaks are reported in wavenumbers (cm−1) in the range of 4,000–400 cm−1.

For size exclusion chromatography (SEC), the DMAc system consisted of a Shimadzu LC-10AD pump, a Waters in-line degasser AF, and a Waters 717plus autosampler (Japan). A Waters 2487 dual wavelength UV and a Waters 410 differential refractive index were connected in series as detection systems. Dimethyl acetamide (DMAc, HPLC grade, stabilised with 0.05% BHT and 0.03% LiCl) was used as mobile phase at 40°C and a flow rate of 1 mL·min−1, and the system was calibrated using narrow poly(methylmethacrylate) standards ranging from 634 to 1.944 × 106 g·mol−1. The column set included a 50 mm × 8 mm guard column in series with three 300 mm × 8 mm, 10 μm particle size GRAM columns (two 3,000 Å and a 100 Å) obtained from Polymer Standards Service. Concentration in DMAc was set at 1,000 g·L−1.

The polydispersity index (PDI) was calculated using the following formula:

(1) PDI = M w M n

3 Results and discussion

3.1 Extraction

The MW extraction of lignin from SB required no special preparation. In this study, lignin was extracted using 100% (v/v) AA at 120°C in a sealed MW vial after 10 min, without the need for a mineral acid catalyst. This setup was more efficient compared to a similar extraction of lignin by others from SB, which used 90% v/v AA and 2% v/v of HCl at 110°C in 30 min in an open reflux system [42]. The sealed vial used here encouraged the build-up of pressure in the vessel, which aided in the rupture of cell walls to release the lignin [61]. To improve sustainability, the AA used in SBL extractions was recycled and used in other extractions once removed using a rotary evaporator. The hydrophobic nature of the acetosolv SBL produced enabled easy separation from the residual AA solution, as precipitation was readily achieved by simply adding water. This ensured a green extraction of lignin from SB without the use of toxic chemicals or needless wastage of solvents.

AA was chosen due to several factors: it has a high solubility of lignin, which allowed for lignin’s easy extraction and separation from the AA insoluble cellulose in the SB [62]. In addition, AA is an MW-responsive solvent due to its polar nature, ensuring adequate MW absorption during MW extraction of lignin from SB. AA is also a relatively cheap organic acid that is environmentally benign, especially compared to mineral acids such as sulphuric acid [63]. Additionally, AA can be produced from the oxidation of ethanol derived from biomass [37], which can, in turn, encourage a circular economy at potential bio-refineries.

Several factors were optimised during lignin extraction (Figure 2), such as temperature, reaction duration, and AA concentration. These were optimised to an AA concentration of 100% v/v, at a temperature of 120°C for 10 min. The optimised MW conditions were then used in a conventional heating method using an autoclave for comparison. The MW-assisted method took approximately 20 min in total for the ramp-up time, the actual reaction, and the cooling period combined. However, the autoclave required approximately 1 h in total due to its slower heating rate and extended cooling period. The MW-assisted method had an average lignin yield of 205 mg, while the conventional method had an average yield of 183 mg.

Figure 2 Effect of different parameters on the extraction yield of lignin from sugarcane bagasse using MW-assisted heating. (a) MW at 160°C, 12 min and AA concentration 0%, 25%, 50%, 75%, and 100% (v/v). (b) MW at 160°C, AA concentration 100% (v/v), 2–14 min. (c) MW at AA concentration 100% (v/v), 10 min, 80–160°C. (d) MW versus conventional heating, 120°C, 10 min, 100% (v/v).
Figure 2

Effect of different parameters on the extraction yield of lignin from sugarcane bagasse using MW-assisted heating. (a) MW at 160°C, 12 min and AA concentration 0%, 25%, 50%, 75%, and 100% (v/v). (b) MW at 160°C, AA concentration 100% (v/v), 2–14 min. (c) MW at AA concentration 100% (v/v), 10 min, 80–160°C. (d) MW versus conventional heating, 120°C, 10 min, 100% (v/v).

MW-assisted heating is generally known to be more time, energy, and process efficient compared to conventional heating due to its volumetric heating [64,65,66,67]. Because fossil fuels that are often used for energy production have adverse environmental effects [68], MW-assisted heating is, therefore, inherently more environmentally friendly by being energy efficient [69]. MW-assisted heating can also be turned on and off instantaneously, allowing for better process control. The shorter reaction time due to increased processing efficiency allows for a smaller vessel that takes less space to be used to produce the same amount of products as a larger conventional heated vessel would [70].

While MW-assisted heating proved to be an energy and time-efficient technology useful in process intensification because of its rapid volumetric heating when compared to conventional heating, it has not been implemented in the chemical industry on a larger scale. The capital-intensive nature of retrofitting MW reactors onto existing bio-refineries can perhaps not be economically justified by the improvement in efficiency due to MW. In addition to health and safety, concerns regarding MW radiation exposure and also explosion risks are associated with the fast heating rates and the rapid build-up of pressure in vessels.

The proper techno-economic analysis of the economic viability of the extraction of MW lignin requires quantitative scale-up models, which can be used to predict the behaviour of MW heating at a larger scale. A computational study of general MW-assisted heating by Goyal et al. predicted an eventual decrease in the energy absorbed as the vessel size increased [71]. Penetration limitations of MW radiation could produce coldspots and hotspots within larger vessels, and multiple MW generators (magnetrons) could be fitted to strategic positions to ensure even heating within the vessel if technically feasible. It would be interesting to investigate using a computational study how multiple magnetrons would improve the challenges identified.

3.2 Electrospinning of lignin fibres

Attempts to replicate the experimental electrospinning conditions set by Wang and co-workers [72] using organosolv (Alcell) lignin were unsuccessful, likely due to structural differences between the different lignin sources used, a result of different lignin extraction methods used that inadvertently altered the lignin structure [27,73] and differences in monolignol composition (H/G/S ratio) [74]. These differences, in turn, altered the rheological properties of the spinning solution used, even though both lignins used were organosolv lignins. New parameters had to be determined to enable the SBL to be electrospun without sputtering and producing electrosprays [75] by optimising the lignin/PEO ratio, the distance from the collecting plate and the voltage, among other parameters. This highlights the need to investigate the optimal electrospinning parameters for each unique lignin source to enable various lignin sources to be efficiently electrospun into LNFs.

The starting parameters used for this electrospinning were replicated from the work done by Wang and co-workers [72] using organosolv (Alcell) lignin. Initially, a 90/10% w/w lignin/PEO in DMF solution at a flow rate of 0.3 mL·h−1 at 6.5−7.0 kV and a needle tip-to-plate substrate distance of 10 cm was used. The voltage was increased in 1 kV increments until 20 kV, which induced a Taylor cone, a polymer jet, and the resultant electrospray of the polymer solution onto the collecting plate. This formation of droplets instead of fibres implied that the solution lacked enough plasticity to be spun into fibres. The concentration of the PEO plasticiser was therefore increased from 10% w/w to 15% w/w before jets of the polymer solution began to form electrospun fibres on the collecting plate. The conditions that enabled electrospinning to occur were determined as an 85%/15% w/w lignin/PEO solution in DMF at 20 kV, 10 cm distance, and flow rate of 0.3 mL·h−1.

Electrospinning LNFs from neat lignin alone is challenging, attributable to molecular weight distribution, crosslinking and intermolecular interaction within the lignin framework. Ideally, narrower molar mass distributions to obtain viscous flow and fibre formation when dissolved in solvents are desirable [19]. Small amounts of a second high MW polymer acting as a plasticiser are often required for electrospinning to occur, as pure lignin solutions typically produce electrosprays rather than be drawn into fibres. Common plasticisers include PAN [76], poly(vinyl alcohol), or PEO [72]. The inclusion of these synthetic polymers is often necessary to improve the lignin solution’s rheological properties and promote fibre formation. The addition of a plasticiser is believed to cause the formation of hydrogen bonds with the hydroxyl groups of the lignin [77], which provides the necessary chain entanglement (Figure 3) [78,79]. Incorporation of renewable natural biopolymers such as poly(lactic acid) [80,81], cellulose [82], and poly(hydroxybutyrate [80] for greener fibre formation is ultimately imperative for more sustainable production of electrospun LNFs.

Figure 3 The formation of hydrogen bonds between PEO and lignin molecules to improve plasticity and promote fibre formation during electrospinning.
Figure 3

The formation of hydrogen bonds between PEO and lignin molecules to improve plasticity and promote fibre formation during electrospinning.

Good reproducibility in electrospun fibre characteristics across batches could be affected by variations in relative humidity and ambient temperature in response to prevailing weather conditions [83], necessitating the need for air-conditioned rooms. Slight variations in lignin structure can also be anticipated when using lignin from different sugarcane varieties, and different breeds and hybrids are grown depending on the region. These differences may alter the electrospinning conditions that are optimal for particular LNF characteristics [84].

3.3 Characterisation of lignin and LNFs

3.3.1 SEC

SBL was characterised using SEC to determine the average molecular weight (M w) and the number average molecular weight (M n) of the lignin and its fragments to determine its molecular size using the average molecular weight and evaluate the distribution of its fragments [85]. An M w of 12,124 g·mol−1 and M n of 5,655 g·mol−1 were determined and a PDI of 2.14 was calculated. Figure 4 shows the molar mass distribution curve of SBL.

Figure 4 Molar mass distribution curves of SBL.
Figure 4

Molar mass distribution curves of SBL.

3.3.2 FTIR spectroscopy

Lignin is a complex amorphous polymer, and the theoretical data assignment is inherently challenging because of the overlap of multiple vibration modes from numerous different functional groups within lignin, particularly below 1,400 cm−1. The band assignments in lignin were accomplished based on the characteristic absorption bands of moieties in the lignin skeleton using data from various lignin model compounds and lignins published in previous reports [27,86,87,88,89,90,91,92]. Figure 5 shows the FTIR spectrum for SBL, and Table 1 shows selected absorption bands. Absorptions attributed to the lignin’s three common monolignol units were observed and anticipated, given the herbaceous nature of SB [88]. The bands at 1,604, 1,510, and 1,427 cm−1 were assigned to the aromatic ring stretching mode [87,88,90,91,92], suggesting that the aromatic structures of the selected lignins remained reasonably unaffected by the MW-assisted AA extraction process. In particular, the bands at 1,322 and 1,113 cm−1 were assigned to characteristic syringyl (S) units [87,91,92], while the band at 1,026 cm−1 was attributed to the presence of guaiacyl (G) units [91], and the C–H out of a plane in the p-hydroxyphenyl propane units (H) were at 836 cm−1. The band at 3,396 cm−1 was ascribed to the O–H stretching signal of hydrogen-bonded hydroxyl groups in the aliphatic and aromatic sections of lignin’s structure, respectively [92]. The C–H stretch in methyl and methylene groups was at 2,934 cm−1 [91,92]. The bands at 2,854 cm−1 also corresponded to the C–H stretching in methylene, methyl, and methoxy groups.

Figure 5 FTIR spectrum of SBL.
Figure 5

FTIR spectrum of SBL.

Table 1

Selected FTIR signals and band assignments

Peak (cm−1) Band assignment
3,396 Phenolic and aliphatic O–H stretching
2,934 C–H stretching in methyl, methylene, or methane group
2,854 C–H stretching in methyl, methylene, or methoxy group
1,706 C═O stretch in unconjugated carbonyl groups, carbonyl and ester groups, or LCC
1,634 C–O stretch in conjugated ketone
1,604 C–C stretching (aromatic skeleton)
1,510 C═C stretching (aromatic skeleton)
1,462 C–H deformation (asymmetric in CH3 and CH2)
1,427 C–C stretching (aromatic skeleton) with C–H in plane deformation
1,373 Aliphatic C–H stretch in CH3
1,322 Syringyl ring breathing with C–O stretching
1,221 Aromatic C–O stretching
1,160 C–O stretch in ester groups
1,113 Aromatic C–H in-plane deformation for syringyl type
1,026 Aromatic C–H in-plane deformation for guaiacyl type
836 C–H out of plane in the p-hydroxyphenyl propane units

3.3.3 HSQC NMR

The macromolecular structure of the SBL was studied further using HSQC; this important tool was used to establish the structure of lignin by providing detailed information on the various types of hydrogen and carbon atoms, confirming the presence of functional groups and types of linkages in the lignin sample used for analysis [92]. Notable chemical functionalities were assigned based on the various lignin model compounds from a variety of reports and databases [27,42,92,93,94,95]. Figure 6 shows the HSQC spectrum for SBL.

Figure 6 HSQC spectrum of SBL displaying chemical shifts and suggested assignments.
Figure 6

HSQC spectrum of SBL displaying chemical shifts and suggested assignments.

In lignin studies using HSQC, particular moieties are found in certain regions. The aliphatic side chain is at chemical shift (δ) = 0.5–2.8 ppm, aliphatic oxygenated side chain δ = 2.5–5.8 ppm, the aromatic groups δ = 5–8.5 ppm, and the aldehyde functional group δ = 9–10 ppm. The signals at 0.83, 1.22, 1.45, and 1.94 ppm were assigned to aliphatic groups of lignin. The DMSO-d 6 solvent residual signal was at 2.5 ppm [95]. The significant peak overlap from an intense broad peak from 2.80 to 3.85 ppm was attributed to the presence of lignin carbohydrate complexes (LCCs) in the form of cellulose and hemicellulose derivatives in the SBL sample [96]. The presence of signals associated with the 3 main monolignols (H/G/S) corroborated the FT-IR data. The signals at δ C/δ H 103.2/6.65, 116.3/6.27, and 116.3/6.77 were attributed to G and/or S units of lignin, and the presence of either G or S units was also substantiated by the cross-peak at δ C/δ H 55.8/3.7 associated with the methoxy group (–OCH3), which can be found on 3/5 positions on G or S units. The correlations at δ C/δ H 145.3/7.42, 145.8/7.34, and 131.78/7.50 were associated with the p-hydroxyphenyl (H) unit, which was the furthest downfield signal observed in the HSQC spectrum.

3.3.4 Py-GC-MS

Py-GC-MS was used to elucidate the structure of lignin in more detail, and the phenolic compounds were determined based on their retention times and compared to a database. This analysis confirmed lignin markers derived from H, S, and G phenylpropanoid units in SB and gave an insight into SBL’s monolignol composition, as suggested by FT-IR and NMR data, although we did not go into greater detail regarding the H:G:S ratio. The py-GC-MS spectrum is shown in Figure S1. From the compounds observed in the spectrum, they maintained their ring substitution patterns from the lignin polymer and could be identified and grouped as being derived from the p-hydroxyphenyl (H), guaiacyl (G), or syringyl (S) units [88].

Table 2 shows the ten most abundant aromatics assigned to their perceived monolignol origin in descending order of their abundance. Most of the compounds were predominantly H type, followed by G-type compounds, while there was only one S-type compound. The data can be interpreted as a measure of the dominance of H-type monolignols in the lignin, as inferred by Klein et al. [88]. Alternatively, the thermally induced fragmentation of S-type fragments could have been demethoxylated into G and H monolignols, while G types were converted into H types, resulting in H-type monolignol units’ perceived dominance, as depicted in Figure 7.

Table 2

Primary aromatic compounds based on pyr-GC-MS analysis

Percentage area Compound
6.64* Benzaldehyde, 2-methyl-
2.97* 2-Propenoic acid, 3-(4-hydroxyphenyl)-, methyl ester
2.33# 1,2-Benzenediol, 4-methyl-
1.9* Phenol, 4-ethyl-
1.65# Vanillin
1.57# 2-Methoxy-4-vinylphenol
1.4* Phenol
1.24* Benzaldehyde, 4-hydroxy-
1.11& Benzaldehyde, 4-hydroxy-3,5-dimethoxy-
0.99# Phenol, 2-methoxy-

Key: *H-type compounds, #G-type compounds, &S-type compounds.

Figure 7 Demethoxylation of S-unit to G-unit and then to H unit.
Figure 7

Demethoxylation of S-unit to G-unit and then to H unit.

3.3.5 TGA

The thermal properties of the extracted lignin were investigated using TGA, and they provided useful information regarding suitable temperature parameters when applying lignin in the carbonisation of lignin for the fabrication of CFs. The thermal stability of lignin is greatly dependent on its macromolecular structure, molecular weight, inherent chemical linkages, crosslinking degree, and carbohydrate impurity content (as a result of LCCs) [75,97], among other variables, mainly due to the source of lignin and/or its extraction process [98,99,100].

The thermograms of SBL are shown in Figure 8. The thermogravimetric (TG) thermogram shows the temperature ranges as degradation ensued, the corresponding percentage weight loss, maximum thermal decomposition temperature (DTGmax), and percentage residual carbon of the SBL, while the DTG curve signified the rate of weight loss with temperature. The thermograms displayed three significant thermal events of weight loss as the decomposition of various bonds proceeded, at 60°C, 350°C, and ∼600°C, as observed from the decrease in the DTG.

Figure 8 TG, derivative TG, and DSC thermograms of SBL.
Figure 8

TG, derivative TG, and DSC thermograms of SBL.

The first stage from ∼60°C involves the elimination of solvent/moisture and low molecular weight volatiles in the SBL sample [87,101,102,103]. Stage 2 occurs around 180–350°C. This is the greatest weight loss, as shown by the DTG curve, which shows the greatest decrease in this region. Decomposition in this stage involves the fragmentation of inter-unit monolignol ether linkages, aliphatic chains, and decarboxylation reactions, transferring monomers and other aromatic derivatives into the vapour phase, partly depending on the differences in the content of C–C [104,105]. The major products of this stage are gaseous products, organic and phenolic products, and coke [87]. The degradation of carbohydrate impurities in lignin samples also occurs, converting them into volatile gases such as CO, CO2, and CH4 [27,106]. The third significant degradation occurs after ∼600°C, and the process is associated with the decomposition of the stable aromatic rings [107]. The lignin samples are pyrolysed into a highly condensed carbonaceous residue of non-volatilised char [27], with a resultant residual weight of ∼10%.

3.3.6 TEM

After the fabrication of LNFs via electrospinning, they were analysed using TEM to determine their general pattern and size (Figure 9). Electrospun LNFs were observed as haphazardly arranged fibres of diameters ∼15 nm, forming a mat-like material. The LNFs generated in this study had significantly smaller diameters of about 20 nm compared to ∼500 nm collected by Wang et al. using a similar method [72]. The differences in the structure of the lignin (and monolignol proportion) could play a role in this disparity, as well as the differences in the lignin/PEO ratio and the voltage.

Figure 9 TEM images of the synthesised lignin-PEO blend nanofibres.
Figure 9

TEM images of the synthesised lignin-PEO blend nanofibres.

4 Conclusions

The MW-assisted extraction of lignin from SB, an underutilised agrowaste, was sustainably and efficiently achieved using AA, within 10 min, without the addition of a mineral acid catalyst. The lignin was subsequently utilised to fabricate LNFs using an electrospinning method, an easy, cheap, and fast production method of fibres, with the ultimate goal of carbonising the LNFs to produce non-structural CFs from SB-derived lignin for application in lithium ion or sodium ion batteries. This displayed the immense potential of SB in the production of more valuable sustainable materials, especially when process intensification methods such as MW-assisted heating are used for process efficiency, which can contribute positively to the circular economy and the process economics of typical sugarcane processing biorefineries

During electrospinning, neat acetosolv lignin alone was not electrospinnable; instead, it produced electrosprays. This necessitated the addition of a PEO plasticiser to encourage spinning. An 85/15% w/w lignin/PEO solution in DMF was sufficient to induce electrospinning of the lignin/PEO solution to form a mat of randomly woven nanofibres. We also observed differences in conducive electrospinning parameters to those used by researchers previously in terms of voltage, which necessitated the adoption of new electrospinning parameters after trial and error. The structural differences between the lignin sources were likely responsible for the parameter differences that were conducive for electrospinning.

The application of MW-assisted heating was important in this study as an efficient process intensification technique within industrial, educational, and research spaces. Scaling up of MW processing has been slow despite considerable research output over the years. This could be a result of high equipment cost, which outweigh potential financial gains, after considering the long-term effects on operations and the cost of ownership; an inadequate understanding of the processing limitations involved in MW penetration, health and safety aspects regarding the exposure to large amounts of MW radiation, explosion risks associated with the rapid temperature increase and pressure build-up. The development of safe, robust, and scalable MW-assisted reaction vessels calls for an intimate alliance among multidisciplinary researchers and equipment manufacturers.

tel: +27 (0)31 260 3188

Acknowledgements

We thank the School of Chemistry and Physics, University of KwaZulu-Natal (UKZN), South Africa, for financial support and facilities, and the research supported by the Eskom TESP Program (P677), and in part by the National Research Foundation of South Africa (Grant Number: 132014).

  1. Funding information: The research was supported by the Eskom TESP Program (P677), and in part by the National Research Foundation of South Africa (Grant Number: 132014).

  2. Author contributions: Emmanuel Mkumbuzi: conceptualisation, visualisation, writing – original draft, writing – review and editing. Bishop Bruce Sithole: review and editing. Werner Ewald van Zyl: conceptualisation, writing – review and editing, supervision and finances.

  3. Conflict of interest: Authors state no conflict of interest.

  4. Data availability statement: Data sharing is not applicable to this article as no datasets were generated or analysed during the current study.

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Received: 2023-12-18
Accepted: 2024-05-22
Published Online: 2024-07-26

© 2024 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

Artikel in diesem Heft

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  125. Retraction
  126. Retraction of “Biosynthesis and characterization of silver nanoparticles from Cedrela toona leaf extracts: An exploration into their antibacterial, anticancer, and antioxidant potential”
  127. Retraction of “Photocatalytic degradation of organic dyes and biological potentials of biogenic zinc oxide nanoparticles synthesized using the polar extract of Cyperus scariosus R.Br. (Cyperaceae)”
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https://doi.org/10.1515/gps-2023-0258
Schlagwörter für diesen Artikel
microwave-assisted; delignification; electrospinning; lignin nanofibres; sugarcane bagasse
Creative Commons
BY 4.0

Artikel in diesem Heft

  1. Research Articles
  2. Green polymer electrolyte and activated charcoal-based supercapacitor for energy harvesting application: Electrochemical characteristics
  3. Research on the adsorption of Co2+ ions using halloysite clay and the ability to recover them by electrodeposition method
  4. Simultaneous estimation of ibuprofen, caffeine, and paracetamol in commercial products using a green reverse-phase HPTLC method
  5. Isolation, screening and optimization of alkaliphilic cellulolytic fungi for production of cellulase
  6. Functionalized gold nanoparticles coated with bacterial alginate and their antibacterial and anticancer activities
  7. Comparative analysis of bio-based amino acid surfactants obtained via Diels–Alder reaction of cyclic anhydrides
  8. Biosynthesis of silver nanoparticles on yellow phosphorus slag and its application in organic coatings
  9. Exploring antioxidant potential and phenolic compound extraction from Vitis vinifera L. using ultrasound-assisted extraction
  10. Manganese and copper-coated nickel oxide nanoparticles synthesized from Carica papaya leaf extract induce antimicrobial activity and breast cancer cell death by triggering mitochondrial caspases and p53
  11. Insight into heating method and Mozafari method as green processing techniques for the synthesis of micro- and nano-drug carriers
  12. Silicotungstic acid supported on Bi-based MOF-derived metal oxide for photodegradation of organic dyes
  13. Synthesis and characterization of capsaicin nanoparticles: An attempt to enhance its bioavailability and pharmacological actions
  14. Synthesis of Lawsonia inermis-encased silver–copper bimetallic nanoparticles with antioxidant, antibacterial, and cytotoxic activity
  15. Facile, polyherbal drug-mediated green synthesis of CuO nanoparticles and their potent biological applications
  16. Zinc oxide-manganese oxide/carboxymethyl cellulose-folic acid-sesamol hybrid nanomaterials: A molecularly targeted strategy for advanced triple-negative breast cancer therapy
  17. Exploring the antimicrobial potential of biogenically synthesized graphene oxide nanoparticles against targeted bacterial and fungal pathogens
  18. Biofabrication of silver nanoparticles using Uncaria tomentosa L.: Insight into characterization, antibacterial activities combined with antibiotics, and effect on Triticum aestivum germination
  19. Membrane distillation of synthetic urine for use in space structural habitat systems
  20. Investigation on mechanical properties of the green synthesis bamboo fiber/eggshell/coconut shell powder-based hybrid biocomposites under NaOH conditions
  21. Green synthesis of magnesium oxide nanoparticles using endophytic fungal strain to improve the growth, metabolic activities, yield traits, and phenolic compounds content of Nigella sativa L.
  22. Estimation of greenhouse gas emissions from rice and annual upland crops in Red River Delta of Vietnam using the denitrification–decomposition model
  23. Synthesis of humic acid with the obtaining of potassium humate based on coal waste from the Lenger deposit, Kazakhstan
  24. Ascorbic acid-mediated selenium nanoparticles as potential antihyperuricemic, antioxidant, anticoagulant, and thrombolytic agents
  25. Green synthesis of silver nanoparticles using Illicium verum extract: Optimization and characterization for biomedical applications
  26. Antibacterial and dynamical behaviour of silicon nanoparticles influenced sustainable waste flax fibre-reinforced epoxy composite for biomedical application
  27. Optimising coagulation/flocculation using response surface methodology and application of floc in biofertilisation
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  29. Potent antibacterial nanocomposites from okra mucilage/chitosan/silver nanoparticles for multidrug-resistant Salmonella Typhimurium eradication
  30. Trachyspermum copticum aqueous seed extract-derived silver nanoparticles: Exploration of their structural characterization and comparative antibacterial performance against gram-positive and gram-negative bacteria
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  33. Green quantitative methods for linagliptin and empagliflozin in dosage forms
  34. Enhancement efficacy of omeprazole by conjugation with silver nanoparticles as a urease inhibitor
  35. Residual, sequential extraction, and ecological risk assessment of some metals in ash from municipal solid waste incineration, Vietnam
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  42. Preparation and characterization of composite-modified PA6 fiber for spectral heating and heat storage applications
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  45. Green synthesis and studies on citrus medica leaf extract-mediated Au–ZnO nanocomposites: A sustainable approach for efficient photocatalytic degradation of rhodamine B dye in aqueous media
  46. Cellulosic materials for the removal of ciprofloxacin from aqueous environments
  47. The analytical assessment of metal contamination in industrial soils of Saudi Arabia using the inductively coupled plasma technology
  48. The effect of modified oily sludge on the slurry ability and combustion performance of coal water slurry
  49. Eggshell waste transformation to calcium chloride anhydride as food-grade additive and eggshell membranes as enzyme immobilization carrier
  50. Synthesis of EPAN and applications in the encapsulation of potassium humate
  51. Biosynthesis and characterization of silver nanoparticles from Cedrela toona leaf extracts: An exploration into their antibacterial, anticancer, and antioxidant potential
  52. Enhancing mechanical and rheological properties of HDPE films through annealing for eco-friendly agricultural applications
  53. Immobilisation of catalase purified from mushroom (Hydnum repandum) onto glutaraldehyde-activated chitosan and characterisation: Its application for the removal of hydrogen peroxide from artificial wastewater
  54. Sodium titanium oxide/zinc oxide (STO/ZnO) photocomposites for efficient dye degradation applications
  55. Effect of ex situ, eco-friendly ZnONPs incorporating green synthesised Moringa oleifera leaf extract in enhancing biochemical and molecular aspects of Vicia faba L. under salt stress
  56. Biosynthesis and characterization of selenium and silver nanoparticles using Trichoderma viride filtrate and their impact on Culex pipiens
  57. Photocatalytic degradation of organic dyes and biological potentials of biogenic zinc oxide nanoparticles synthesized using the polar extract of Cyperus scariosus R.Br. (Cyperaceae)
  58. Assessment of antiproliferative activity of green-synthesized nickel oxide nanoparticles against glioblastoma cells using Terminalia chebula
  59. Chlorine-free synthesis of phosphinic derivatives by change in the P-function
  60. Anticancer, antioxidant, and antimicrobial activities of nanoemulsions based on water-in-olive oil and loaded on biogenic silver nanoparticles
  61. Study and mechanism of formation of phosphorus production waste in Kazakhstan
  62. Synthesis and stabilization of anatase form of biomimetic TiO2 nanoparticles for enhancing anti-tumor potential
  63. Microwave-supported one-pot reaction for the synthesis of 5-alkyl/arylidene-2-(morpholin/thiomorpholin-4-yl)-1,3-thiazol-4(5H)-one derivatives over MgO solid base
  64. Screening the phytochemicals in Perilla leaves and phytosynthesis of bioactive silver nanoparticles for potential antioxidant and wound-healing application
  65. Graphene oxide/chitosan/manganese/folic acid-brucine functionalized nanocomposites show anticancer activity against liver cancer cells
  66. Nature of serpentinite interactions with low-concentration sulfuric acid solutions
  67. Multi-objective statistical optimisation utilising response surface methodology to predict engine performance using biofuels from waste plastic oil in CRDi engines
  68. Microwave-assisted extraction of acetosolv lignin from sugarcane bagasse and electrospinning of lignin/PEO nanofibres for carbon fibre production
  69. Biosynthesis, characterization, and investigation of cytotoxic activities of selenium nanoparticles utilizing Limosilactobacillus fermentum
  70. Highly photocatalytic materials based on the decoration of poly(O-chloroaniline) with molybdenum trichalcogenide oxide for green hydrogen generation from Red Sea water
  71. Highly efficient oil–water separation using superhydrophobic cellulose aerogels derived from corn straw
  72. Beta-cyclodextrin–Phyllanthus emblica emulsion for zinc oxide nanoparticles: Characteristics and photocatalysis
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  74. Influential eradication of resistant Salmonella Typhimurium using bioactive nanocomposites from chitosan and radish seed-synthesized nanoselenium
  75. Antimicrobial activities and neuroprotective potential for Alzheimer’s disease of pure, Mn, Co, and Al-doped ZnO ultra-small nanoparticles
  76. Green synthesis of silver nanoparticles from Bauhinia variegata and their biological applications
  77. Synthesis and optimization of long-chain fatty acids via the oxidation of long-chain fatty alcohols
  78. Eminent Red Sea water hydrogen generation via a Pb(ii)-iodide/poly(1H-pyrrole) nanocomposite photocathode
  79. Green synthesis and effective genistein production by fungal β-glucosidase immobilized on Al2O3 nanocrystals synthesized in Cajanus cajan L. (Millsp.) leaf extracts
  80. Green stability-indicating RP-HPTLC technique for determining croconazole hydrochloride
  81. Green synthesis of La2O3–LaPO4 nanocomposites using Charybdis natator for DNA binding, cytotoxic, catalytic, and luminescence applications
  82. Eco-friendly drugs induce cellular changes in colistin-resistant bacteria
  83. Tangerine fruit peel extract mediated biogenic synthesized silver nanoparticles and their potential antimicrobial, antioxidant, and cytotoxic assessments
  84. Green synthesis on performance characteristics of a direct injection diesel engine using sandbox seed oil
  85. A highly sensitive β-AKBA-Ag-based fluorescent “turn off” chemosensor for rapid detection of abamectin in tomatoes
  86. Green synthesis and physical characterization of zinc oxide nanoparticles (ZnO NPs) derived from the methanol extract of Euphorbia dracunculoides Lam. (Euphorbiaceae) with enhanced biosafe applications
  87. Detection of morphine and data processing using surface plasmon resonance imaging sensor
  88. Effects of nanoparticles on the anaerobic digestion properties of sulfamethoxazole-containing chicken manure and analysis of bio-enzymes
  89. Bromic acid-thiourea synergistic leaching of sulfide gold ore
  90. Green chemistry approach to synthesize titanium dioxide nanoparticles using Fagonia Cretica extract, novel strategy for developing antimicrobial and antidiabetic therapies
  91. Green synthesis and effective utilization of biogenic Al2O3-nanocoupled fungal lipase in the resolution of active homochiral 2-octanol and its immobilization via aluminium oxide nanoparticles
  92. Eco-friendly RP-HPLC approach for simultaneously estimating the promising combination of pentoxifylline and simvastatin in therapeutic potential for breast cancer: Appraisal of greenness, whiteness, and Box–Behnken design
  93. Use of a humidity adsorbent derived from cockleshell waste in Thai fried fish crackers (Keropok)
  94. One-pot green synthesis, biological evaluation, and in silico study of pyrazole derivatives obtained from chalcones
  95. Bio-sorption of methylene blue and production of biofuel by brown alga Cystoseira sp. collected from Neom region, Kingdom of Saudi Arabia
  96. Synthesis of motexafin gadolinium: A promising radiosensitizer and imaging agent for cancer therapy
  97. The impact of varying sizes of silver nanoparticles on the induction of cellular damage in Klebsiella pneumoniae involving diverse mechanisms
  98. Microwave-assisted green synthesis, characterization, and in vitro antibacterial activity of NiO nanoparticles obtained from lemon peel extract
  99. Rhus microphylla-mediated biosynthesis of copper oxide nanoparticles for enhanced antibacterial and antibiofilm efficacy
  100. Harnessing trichalcogenide–molybdenum(vi) sulfide and molybdenum(vi) oxide within poly(1-amino-2-mercaptobenzene) frameworks as a photocathode for sustainable green hydrogen production from seawater without sacrificial agents
  101. Magnetically recyclable Fe3O4@SiO2 supported phosphonium ionic liquids for efficient and sustainable transformation of CO2 into oxazolidinones
  102. A comparative study of Fagonia arabica fabricated silver sulfide nanoparticles (Ag2S) and silver nanoparticles (AgNPs) with distinct antimicrobial, anticancer, and antioxidant properties
  103. Visible light photocatalytic degradation and biological activities of Aegle marmelos-mediated cerium oxide nanoparticles
  104. Physical intrinsic characteristics of spheroidal particles in coal gasification fine slag
  105. Exploring the effect of tea dust magnetic biochar on agricultural crops grown in polycyclic aromatic hydrocarbon contaminated soil
  106. Crosslinked chitosan-modified ultrafiltration membranes for efficient surface water treatment and enhanced anti-fouling performances
  107. Study on adsorption characteristics of biochars and their modified biochars for removal of organic dyes from aqueous solution
  108. Zein polymer nanocarrier for Ocimum basilicum var. purpurascens extract: Potential biomedical use
  109. Green synthesis, characterization, and in vitro and in vivo biological screening of iron oxide nanoparticles (Fe3O4) generated with hydroalcoholic extract of aerial parts of Euphorbia milii
  110. Novel microwave-based green approach for the synthesis of dual-loaded cyclodextrin nanosponges: Characterization, pharmacodynamics, and pharmacokinetics evaluation
  111. Bi2O3–BiOCl/poly-m-methyl aniline nanocomposite thin film for broad-spectrum light-sensing
  112. Green synthesis and characterization of CuO/ZnO nanocomposite using Musa acuminata leaf extract for cytotoxic studies on colorectal cancer cells (HCC2998)
  113. Review Articles
  114. Materials-based drug delivery approaches: Recent advances and future perspectives
  115. A review of thermal treatment for bamboo and its composites
  116. An overview of the role of nanoherbicides in tackling challenges of weed management in wheat: A novel approach
  117. An updated review on carbon nanomaterials: Types, synthesis, functionalization and applications, degradation and toxicity
  118. Special Issue: Emerging green nanomaterials for sustainable waste management and biomedical applications
  119. Green synthesis of silver nanoparticles using mature-pseudostem extracts of Alpinia nigra and their bioactivities
  120. Special Issue: New insights into nanopythotechnology: current trends and future prospects
  121. Green synthesis of FeO nanoparticles from coffee and its application for antibacterial, antifungal, and anti-oxidation activity
  122. Dye degradation activity of biogenically synthesized Cu/Fe/Ag trimetallic nanoparticles
  123. Special Issue: Composites and green composites
  124. Recent trends and advancements in the utilization of green composites and polymeric nanocarriers for enhancing food quality and sustainable processing
  125. Retraction
  126. Retraction of “Biosynthesis and characterization of silver nanoparticles from Cedrela toona leaf extracts: An exploration into their antibacterial, anticancer, and antioxidant potential”
  127. Retraction of “Photocatalytic degradation of organic dyes and biological potentials of biogenic zinc oxide nanoparticles synthesized using the polar extract of Cyperus scariosus R.Br. (Cyperaceae)”
  128. Retraction to “Green synthesis on performance characteristics of a direct injection diesel engine using sandbox seed oil”
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Heruntergeladen am 20.10.2025 von https://www.degruyterbrill.com/document/doi/10.1515/gps-2023-0258/html
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