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Article

Morphological Differences between Virgin and Secondary Fibers

by
Edyta Małachowska
1,2,*,
Marcin Dubowik
2 and
Piotr Przybysz
1,2
1
Institute of Wood Sciences and Furniture, Warsaw University of Life Sciences—SGGW, 159 Nowoursynowska Str., 02-787 Warsaw, Poland
2
Natural Fibers Advanced Technologies, 42A Blekitna Str., 93-322 Lodz, Poland
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(10), 8334; https://doi.org/10.3390/su15108334
Submission received: 21 March 2023 / Revised: 24 April 2023 / Accepted: 19 May 2023 / Published: 20 May 2023
(This article belongs to the Special Issue Recent Advances in Sustainable Bio-Based Materials: Preparation, Modification and Utilization)
Browse Figures
Versions Notes

Abstract

:
The properties of the fibers determine the quality of the pulp and, thus, the quality of the paper made from it. Recognition of properties, which fiber and paper pulp should be characterized by, in order to achieve required paper properties, is, therefore, a subject of research and interest of many papermaking research experts and scientists. Fibers are subject to deformation and possible weakening under the influence of chemical and mechanical factors, and therefore the quality of the fibers decreases each time they are used in production when it comes to recycled pulps. Then again, the key factor determining the quality of the primary fiber is the degree of pulp delignification. In the article, an attempt was made to define the impact of delignification of virgin pulp on morphological properties of fibers, and compare them with the properties of recycled paper pulp, in order to find correlations. The current economic and raw material situations in the wood market force one to seek new solutions to limit the use of virgin fibers, which is extremely important for the economy of the paper mill, environmental protection and raw material management.

1. Introduction

Properties of fibers determine, to a high degree, the usable properties of final paper materials and the cost of their production [1,2]. They have a particular impact on structural and strength properties of paper products [3]. Hence, the ability to control fiber properties during the initial stage is a determining factor in effective quality control and also the cost of paper production.
Although fiber properties in virgin pulps can be modified by a delignification and refining process, this is more difficult in pulp from recovered fiber. Although secondary pulps, for both ecological and economic reasons, are good raw materials for the production of paper or cardboard, the properties of fibers in wastepaper deteriorate each time during later production cycles. One of the main problems with recycled pulp fiber sheets is their lower tensile strength compared to that of virgin pulp fiber sheets. However, many paper products do not require cellulose pulp with high strength properties; paper pulps with low delignification degree or recycled pulps can be used for such products.
Degree of delignification is expressed as the Kappa number, which describes in papermaking the content of residual lignin in the pulp. Depending on conditions of delignification, one can obtain pulps of different properties and different morphological characteristics of fibers. Therefore, the degree of delignification of cellulose pulp defines its papermaking ability [1,2]. By modification of the degree of delignification pulps, and thus paper, an increased papermaking ability can be obtained, with desirable usable properties.
Due to the fact that both wood and electrical energy prices are constantly rising [4,5], increasing the use of various fibrous pulps with high Kappa number is desirable. The relatively good quality and increased efficiency of these pulps make them an attractive paper semi-product. In particular, they could alleviate the severe shortage of paper semi-products. However, while methods to increase the efficiency of wood pulp above a Kappa certain value (45–46) provide better efficiency, they decrease the quality of the obtained cellulose pulp [6,7,8]. Moreover, an increased Kappa number of a cellulose pulp corresponds to poorer pulp beatability; that is, greater specific energy consumption is necessary to refine the wood pulp [9,10,11,12]. Therefore, through aiming to optimize the delignification process, in fact, the economy of paper production may be remarkably improved.
The degree of delignification of a pulp also has an impact on its beatability due to the fact that, depending on Kappa number, i.e., the amount of a lignin in the pulp, the rigidity of fibers changes, causing a different tendency to be shortened during the refining process. Therefore, even in the case of the same pulp, refined in the same conditions, any previous changes in the conditions of delignification have an impact on subsequent papermaking abilities of that pulp.
The proper choice of pulp for the production of paper with specified properties is, therefore, very problematic, because there is no clearly stated set of properties of pulps determining their papermaking ability [13,14,15]. Most often it is made intuitively and based on experience, which may be a repeated cause of big economic losses due to a large-scale paper production. Due to the complexity of a production of cellulose pulps, until now, relevant and constant correlations between delignification of a pulp and morphological properties of fibers were not specified either. This inspired the authors to conduct research into this topic. Going further, the authors decided to verify whether there is such a degree of pulp delignification at which the fiber properties correspond to the properties of recycled fibers.
Indeed, this work was a continuation of our studies on pulps with different delignification degrees. In a previous publication [16], technological and economic evaluations of the manufacture and application of cellulose pulps with different Kappa number were made. This work focused on the characteristics of the fiber itself after various conditions of chemical treatment and the reference of the results to the recycled paper pulps used on an increasingly larger scale.

2. Materials and Methods

2.1. Materials

The following materials were selected for research:
  • Pine wood (Pinus sylvestris L.)—industrial woodchips containing 7–8% moisture;
  • White wastepaper, including products of bleached pulps, scraps of wood-free paper, little printed, no glue, no waterproof paper and no colored paper (ranked 3.04 according to the EN643 ‘List of European standard types of wastepaper’ [17]);
  • Mixed wastepaper composed of unsorted wastepaper, formally classified as the entire spectrum of the second type of paper, that is medium-sized varieties. These included newspapers and printed office wastepaper, among others (ranked 3.19 according to the EN643 ‘List of European standard types wastepaper’).
Three samples of white (marked as 1.1, 1.2 and 2.2) and mixed (marked as 2.2, 2.4 and 2.5) wastepaper from different batches delivered by the paper mill were taken for the research. Wastepaper was crushed manually (pieces of approximately 2–5 cm) and mixed to ensure that the sample was mixed homogeneously. The wastepaper prepared in this way was placed in the described PP foil bags, which were subsequently stored in barrels with tight covers to protect the samples from moisture and contamination. After mechanical shredding, the wastepaper samples were packed in tight containers and stored at a constant temperature of approximately 15 °C.

2.2. Delignification Process of Virgin Pulps

Cellulosic pine pulps were prepared using the sulphate method described by Modrzejewski et al. [18]. Briefly, 20–38% active alkali was added (per batch) and the water to wood ratio (v:w) was 4. The dry weight (DW) of all materials was determined before pulping. The delignification processes were conducted in 15 dm3 PD-114 stainless steel reactors (Danex, Katowice, Poland) with regulated temperature (using a water jacket) and agitation (three swings per minute, 60° swing angle). Suspensions of the disintegrated materials were heated for 120 min to achieve a temperature of 172 °C and incubated at this temperature for a further 120 min. The temperature was then decreased to 25 ± 5 °C using a jacket with cold tap water. After delignification, the material was washed several times (in the amount determined on the basis of previous experimental trials) with demineralized water and incubated overnight in demineralized water to remove the residual alkali-soluble fractions. The solids were disintegrated for 3 min in a laboratory JAC SHPD28D propeller pulp disintegrator (Danex, Katowice, Poland), and the fibers were screened using a PS-114 membrane screener (Danex, Katowice, Poland) equipped with a 0.2 mm gap screen. After screening, the pulps were dried at room temperature (20 ± 2 °C) for 48 h. The dry pulps were stored in hermetically sealed vials until being used in further experiments. Pulping process advances and method descriptions that are presented in this section were described in our earlier published research [19,20,21].

2.3. Pulp Refining

Before processing, the pulp was soaked in water for 24 h. Then, virgin and secondary pulps were treated in the Danex JAC SHPD28D propeller pulp disintegrator (Danex, Katowice, Poland) according to PN EN ISO 5263-1 (2006) with 23,000 revolutions. The refining process was performed in the Danex JAC PFID12X PFI mill (Danex, Katowice, Poland), with a single batch of dried pulp (22.5 g), according to PN-EN ISO 5264-2 (2011). Pulps were refined to 30 ± 1° SR. The Schopper-Riegler freeness was measured using Schopper-Riegler apparatus (Danex, Katowice, Poland), according to PN-EN ISO 5267-1 (2002).

2.4. Fiber and Pulp Properties

Pulps were characterized in terms of Kappa number, fiber dimension, fine content, water retention value (WRV) and freeness. The residual lignin content, expressed as the Kappa number, was determined according to ISO 302:2015 (the more lignin residue in the pulp, the higher Kappa value is). The dimensions of fibers were measured according to ISO 16065-2:2014 using the Morfi Compact Black Edition apparatus (Techpap, Grenoble, France). WRV was determined according to ISO 23714:2014. All analyses were performed on both unbeaten and refined pulps. Recycled pulps, which at the very beginning were characterized by freeness higher than 30° SR, were excluded from further analyses.

3. Results and Discussion

3.1. Pulp Properties

The parameter describing the changes undergoing in the refined pulp, and, in the meantime, correlating to the strength properties of the paper, is a degree of WRV (water retention value) fibers swelling. The degree of paper pulp swelling is physically the amount that is so-called ‘trapped’ in the pulp water, meaning the part of the whole water, which under industrial conditions cannot be mechanically removed (both in forming and press section). Thus, using the WRV indicator enables defining the border amount of water, which can be removed from paper web after pressing.
In Table 1, changes in WRV indicator for tested pulps are presented. It was determined that, with the increase in Kappa number up to a value of around 42, a significant increase in WRV indicator of virgin refined pulps was noticed. Further growth of Kappa number does not cause relevant changes in investigated indicators. Most probably it is caused by the fact that, for virgin pulps with the lowest delignification degree, the residual lignin causes hemicellulose swelling, whereas for pulps with the lowest Kappa number in those pulps hemicellulose, which is susceptible for swelling, it was practically fully removed.
In the case of a secondary pulps, there was no correlation between the lignin content and the degree of water retention, which shows how heterogeneous the raw material is in terms of properties. Refined recycled pulps showed a slower WRV index (an average of about 30% regardless of the type of wastepaper) compared to virgin pulps. This is probably due to the hornification of pulp fiber that occurs when recycled pulp fibers are dried many times. Fiber swelling affects fiber conformability and flexibility which, in turn, affect the strength properties of the paper [22]. Reduced water retention value of the recycled fibers leads to lower bond strength between the pulp fibers, leading to lower tensile strength of wastepaper compared to that of virgin pulp fiber sheet, which is highly undesirable for many types of papers.
On the other hand, no association has been shown between the considered indices in the case of unbeaten pulps. However, the water retention values are much higher for beaten pulps. Beating increases fiber swelling, which is attributed to internal fibrillation of the fiber wall. According to some researchers, the increased swelling after beating straightens the fiber segments, thus enhancing their activation [23,24], which is reported to be beneficial to the tensile index and elastic modulus of the fiber network, desired for many sanitary papers.
Fine fraction is one of the basic factors shaping the papermaking ability of cellulosic pulps. It is responsible, to a high degree, for many interactions during the technological process of papermaking, e.g., the ability to dewater formed and pressed paper web and the efficiency of papermaking machine, as well as directly influencing the properties of the final product [25,26]. Secondary fine fraction produced during the process of refining of a fibrous pulp positively influences the paper properties [27,28]. Due to the evolved outer surface, very weak dewatering and high swelling rate, it exerts a negative impact on the ability of dewatering of paper pulp [29,30].
The course of changes in the content of fine fraction, depending on a degree of delignification of not-refined and refined to freeness of 30° SR pulps, is presented in Table 1. Regardless of a degree of delignification, the beginning amount of fine fraction (% in length) for all investigated virgin pulps was around 4–7% (and 0.5–1.5% in area). The increase in Kappa numbers within the range of 19–30 did not cause significant changes in the amount of fine fraction in the refined pulp. Further decrease in delignification had a proportional impact on the increase in the amount of fine fraction within the range of 34–46% in length (4–6% in area) (a result of from the longer time of water treatment of fibrous cellulose pulp). However, these are still much lower than those obtained for secondary pulps (Table 1, Figure 1). This growth is explained by the fact that fibers characterized by high lignin content during the refining process easily become destroyed and detached fiber elements create significant amounts of fine fraction. On the other hand, fibers well solubilized during the refining process easier become elastic (swell), and are, therefore, less prone to damage leading to creation of fine fraction.
Microscopic images of the virgin and secondary pulps, recorded using a Morfi Compact Black Edition camera, are shown in Figure 1. The images provided clearly show the differences in the fine content as well as a fiber length between the pulps.
The fine fraction content in unbeaten pulps is not dependent on the delignification degree and is incomparably higher in the case of secondary pulps. In practice, this translates into an inferior dewatering capacity of these pulps and the efficiency decrease in the paper machine [31]. This fact was confirmed via the analysis conducted, wherein a positive correlation was found between fine fraction content and pulp freeness (Table 1).
Aside from its impact on the technological process of paper production, the fine fraction has a direct influence on the properties of the final product. Secondary fine fraction, emitted during pulp refining, has a positive effect on the static properties of paper. However, the increase in fine fraction causes a decrease in the dynamic strength properties of paper, which causes at least the lower tear resistance of paper, which is highly undesirable especially for sanitary papers.

3.2. Fiber Dimensions

The dimensions of fibers are one of the most basic properties of pulps, having a great impact on the paper structure (bulk, smoothness of surface, opacity, etc.) and dynamic strength properties of paper products. An in-depth overview of various fiber properties for virgin and recycled pulps are presented in Table 2 and Table 3.
One of the most important fibers properties, affecting the strength properties of paper [32,33,34], is their arithmetic length. It is a ratio of the total length of measured fibers and their amount; however, calculated in this way, results may carry significant mistakes due to the count of fine fraction, i.e., particles of dimensions under 0.2 mm. To eliminate the influence of fine fragments of cellulose fibers on the results and error of measurements, the progress of shortening is usually determined based on such parameters as the average length-weighted length or average weight-weighted length of fibers [35,36,37].
The change in the fiber length, depending on the delignification degree for the investigated virgin pulps and recycled pulps, is presented in Table 2. Regardless of the rate of delignification degree, the initial weighted length of virgin fibers for all unrefined pulps was around 2300 μm. Tiny differences might have been caused by nonuniformity of the raw material. For refined pulp, however, the increase in Kappa number within the range 19–35 causes the growth of fiber resistance on shortening. Further decrease in delignification does not cause relevant changes in mean length of fibers in refined pulps.
For recycled pulps (whether white or mixed) no simple relationships were found. Regardless of the type of wastepaper and its chemical composition, the length of the fibers remained at a similar level, which was about two times lower than the virgin pulp (Table 2, Figure 1).
The diameter of fibers was increased proportionally to the growth of the Kappa number, and this parameter may also show the papermaking potential of the virgin pulp. In the case of secondary pulps, as in the case of the previously discussed properties, no such dependencies were observed. The mean fiber width of the secondary fiber is about 29–37% (depending on the type of wastepaper) smaller than that of virgin fiber.
Another analyzed property of fibers in investigated pulps was a coarseness indicator. It indirectly characterizes the thickness of fibers and it also is used as a direct indicator of their strength and pliability [38]. Based on the analysis of the received data for the virgin pulps, it was stated that the coarseness indicator increases proportionally (from the value of 0.18 to 0.26 mg/m) to the growth of Kappa number included in the range of 19–90 (Table 2). The coarseness index for recycled pulps was, on average, 54% lower. The degree of refining of the pulp does not influence the change in coarseness indicator. The change in coarseness indicator is caused by removal of parts of hemicelluloses and lignin from the inner structure of fibers, which causes the fiber to practically maintain its diameter, whereas its pulp decreases.
During mechanical processing of the pulp, especially with high concentration, curling of the fibers takes place. It is the same with kinks in the fibers. Research shows that both of these fiber deformations significantly affect the strength properties of the paper and the light scattering coefficient [39], which is crucial for hygiene papers. More scattering of reflected light results in a slightly higher brightness and opacity and matte appearance [40]. Curling of the fibers exerts an important impact on the tensile strength or tear resistance [13]. The increase in the degree of curl index of the fibers causes the structure of a paper to loosen, which means that a sheet formed with curled fibers has low tensile index but can have high tear strength [41]. This is explained by uneven distribution of stress along the length of a curled fiber in a fracture zone [42]. The pulp sheets containing straight fibers have low extensibility both as wet web and in dry state [43,44]. Indeed, the more kinked the fibers are, the higher the wet rupture energy. Thus, fiber kinks affect the wet strength of the pulp [45]. The fiber curl, in absence of other effects, also raises bulk and porosity of the pulp sheet, especially desirable in sanitary papers.
It should be noted that during refining, especially in the laboratory PFI mill, the main effect is the combed and straightening of the fibers. Therefore, most of fiber deformations, curl and kinks vanish during beating of the pulp [39], which is confirmed by tests performed. This phenomenon may explain the lower amount of fiber damage and deformation in the secondary pulps (Table 3). Straightening of the fibers reduces the degree of curling of the pulp, so that the strength properties return to the level of unrefined pulp. Fiber straightening during refining process improves the load carrying ability and improves the stress distribution in the fiber network and, therefore, both the elastic modulus and the tensile strength increase in papers made from refined pulps [46]. During the refining process in high concentration, in turn, pulp curling increases. However, industrial refining mills struggle to remove any fiber deformation [46]. In industrial pulps, fiber deformations caused during the production process have a significant impact on the quality of paper, which cannot be seen during tests in laboratory conditions. It is worth mentioning here the irreversible damage of fibers caused by the chemical degradation during pulping and mechanical damage contribute.
When considering the data in a degree delignification context, regardless of the Kappa number, the curl index for all virgin refined pulps was around 9% and small differences were caused probably by nonuniformity of a raw material. The same dependencies were observed for an indicator of kinked fiber—amounting to approximately 30%. Moreover, for a macrofibryllation index and for the broken fibers, no significant changes nor correlations to the degree of delignification of the pulp were observed (Table 3). It was also argued that the some of the fiber deformations, curl and kinks vanish after refining of the pulp. Some of them remain on a similar level.
Like the previously described indicators, the properties of recycled fibers are significantly different from virgin ones and none of the degrees of virgin pulp delignification are reflected in the recycled paper pulps.
Surprisingly, in none of the cases under consideration do the white and mixed wastepaper show any differences with each other, which proves the heterogeneity of the raw material. The secondary raw material used in the research was characterized by high variability. Some bales of white wastepaper subjected to analysis exhibited a raw material that should be described with a different variety. In addition to the 3.04 variety, there are also 3.02 varieties, i.e., mixed shreds of printing and writing papers, slightly colored in mass, containing a minimum of 90% of wood-free paper; 3.03, i.e., bookbinding scraps made of wood-free paper, slightly printed, with glue, without paper dyed in mass. However, the range of variation in the quality of wastepaper has been determined in other studies [47] and this was not the purpose of this publication. However, the lack of precise requirements in paper mills as to the quality of secondary raw material could cause difficulties in obtaining unambiguous correlations and dependencies and contribute to the fact that no significant differences were obtained in the characteristics of white and mixed wastepaper fibers, which clearly indicates the need to control the quality of recycled raw materials in paper mills before will go into the production process.

4. Conclusions

The obtained results clearly indicate no significant similarities in the morphological properties of recycled fibers with virgin fibers, regardless of the degree of their delignification. The quality of the fibers decreases each time they are used in production, and even very weak delignificated virgin pulp has better fiber characteristics than recycled pulp, which is reflected in the quality of the final product. However, that doesn’t change the fact that the dimensions of fibers and properties of fibrous pulp are strongly correlated to the degree of delignification, especially for refined pulp and the influence of the refining on the fiber deformations and damages of the pulp fibers significant.

Author Contributions

Conceptualization, E.M.; methodology, E.M. and M.D.; data processing, M.D. and E.M.; literature review, E.M.; writing—original draft preparation, E.M.; writing—review and editing, E.M.; supervision, P.P.; funding acquisition, P.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Center of Research and Development in Poland, grant number POIR.01.01.01-00-0084/17.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

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Sustainability 15 08334 g001a 550Sustainability 15 08334 g001b 550
Figure 1. Microscopic images of the tested pulps recorded using a Morfi Compact Black Edition camera.
Figure 1. Microscopic images of the tested pulps recorded using a Morfi Compact Black Edition camera.
Sustainability 15 08334 g001aSustainability 15 08334 g001b
Table
Table 1. Characteristics of virgin and secondary pulps.
Table 1. Characteristics of virgin and secondary pulps.
SampleKappa NumberSchopper–Riegler FreenessFine ContentFine ContentWRV
[-][°SR][% in Area][% in Length][%]
UnrefinedUnrefinedRefinedUnrefinedRefinedUnrefinedRefined
Virgin
pulps		89.7121.4766.3176.5746.08112.4189.6
76.5111.3215.7075.8141.35118.9191.3
63.8121.2165.6695.8437.77115.4186.4
46.6110.9914.7724.7432.61118.0188.4
41.9110.9964.2174.9233.92119.8184.6
35.2111.0254.4465.1534.82117.6179.6
29.6121.0814.1405.0927.22121.5181.4
26.9120.8693.9855.2229.33122.6178.9
23.7120.6783.8965.1226.40121.9172.6
19.1120.5353.5024.0727.41125.2160.8
White 1.149.73621.05–62.24–130.9–
White 1.249.65623.63–65.94–136.3–
White 2.219.42013.3216.4547.3152.88101.3128.2
Mixed 2.339.62216.6318.7851.0855.3891.1125.7
Mixed 2.439.62213.4215.1844.9848.9897.6128.8
Mixed 2.540.23526.78–66.60–107.2–
Table
Table 2. Morphological characteristics of tested pulps fibers (part 1).
Table 2. Morphological characteristics of tested pulps fibers (part 1).
SampleKappa NumberMean Artithmetic Fiber LengthMean Weighted Fiber LengthMean Fiber WidthMean Fiber Coarseness
[-][μm][μm][μm][mg·m−1]
UnrefinedRefinedUnrefinedRefinedUnrefinedRefinedUnrefinedRefined
Virgin
pulps		89.7182711352337177038.035.50.260.26
76.5182611472302180937.135.00.240.24
63.8173511712254180936.034.50.240.23
46.6170211952307184234.934.00.220.23
41.9171211892301184034.133.80.210.21
35.2170411812292182933.933.60.200.21
29.6171111622322176733.733.20.200.21
26.9169911342284179933.333.00.200.21
23.7169111162267173633.033.00.200.21
19.116469922288156732.432.00.190.18
White 1.149.7652–994–25.4–0.16–
White 1.249.6600–925–24.0–0.15–
White 2.219.470969396092021.822.40.110.10
Mixed 2.339.664362688484621.323.80.110.10
Mixed 2.439.662360682579721.524.20.100.09
Mixed 2.540.2648–985–23.1–0.150.11
Table
Table 3. Morphological characteristics of tested pulps fibers (part 2).
Table 3. Morphological characteristics of tested pulps fibers (part 2).
SampleKappa NumberKinked Fiber ContentMean Fiber Curl IndexMacrofibrillation IndexBroken Fiber Content
[-][%][%][%][%]
UnrefinedRefinedUnrefinedRefinedUnrefinedRefinedUnrefinedRefined
Virgin
pulps		89.732.9928.658.169.040.1961.44640.4545.83
76.530.6428.508.179.390.1941.41940.1845.45
63.835.5629.238.908.800.1651.27540.7743.45
46.634.7731.488.949.250.1971.06437.7543.54
41.935.2130.568.569.020.1961.05436.9842.91
35.234.8730.028.928.960.1991.00337.0943.27
29.637.5732.199.659.280.2040.92435.9242.95
26.936.3231.079.419.310.2020.95436.2643.59
23.739.6234.529.949.480.2140.90236.4743.20
19.137.1228.759.659.420.2050.95935.7944.62
White 1.149.721.01–6.80–1.11–32.26–
White 1.249.622.52–7.01–1.24–33.38–
White 2.219.426.1225.357.356.820.820.7526.9126.39
Mixed 2.339.623.1422.096.726.040.861.0327.0528.48
Mixed 2.439.624.1022.676.636.100.881.1025.6226.88
Mixed 2.540.225.51–7.27–1.06–30.81–
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Małachowska, E.; Dubowik, M.; Przybysz, P. Morphological Differences between Virgin and Secondary Fibers. Sustainability 2023, 15, 8334. https://doi.org/10.3390/su15108334

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Małachowska E, Dubowik M, Przybysz P. Morphological Differences between Virgin and Secondary Fibers. Sustainability. 2023; 15(10):8334. https://doi.org/10.3390/su15108334

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Małachowska, Edyta, Marcin Dubowik, and Piotr Przybysz. 2023. "Morphological Differences between Virgin and Secondary Fibers" Sustainability 15, no. 10: 8334. https://doi.org/10.3390/su15108334

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