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Article

Corn Straw Fibers, an Agro-Industrial Residue, Used as Reinforcement in Polyurethane Foams in Dye Removal in Wastewater

by
Andressa dos Santos
1,*,
Gabriel Vinicius Alvez Silva
2,
Gabriel Fornazaro
2,
Sílvia Luciana Fávaro
2 and
Fauze Jacó Anaissi
1
1
Laboratório de Materiais e Compostos Inorgânicos, Chemistry Department, Universidade Estadual do Centro-Oeste, Guarapuava 85040-167, PR, Brazil
2
Grupo de Estudos de Polímeros Reciclados Eletrofiados e Compósitos, Engineering Mechanical Department, Universidade Estadual de Maringá, Maringá 87020-900, PR, Brazil
*
Author to whom correspondence should be addressed.
Processes 2025, 13(4), 939; https://doi.org/10.3390/pr13040939
Submission received: 26 February 2025 / Revised: 17 March 2025 / Accepted: 19 March 2025 / Published: 21 March 2025
(This article belongs to the Section Environmental and Green Processes)
Browse Figures
Versions Notes

Abstract

:
Alternative adsorbent materials containing natural fibers are a research topic that has garnered increasing attention, with greater relevance when they come from agro-waste. The removal of contaminants, such as dyes, by adsorption methods has been a low-cost alternative to these materials that impedes the adverse effects of water pollution. This study proposes the preparation of an environmentally sustainable material with an excellent reduction in production costs, based on the manufacturing of polyurethane foam composites containing natural fibers from agro-industrial waste. The foam was synthesized by partially replacing the conventional polyol used in polyurethane foams with corn straw fibers, to form a composite material. The composites were prepared according to a statistical design, and the responses were evaluated using Version 13 of Design Expert® software. The composite samples were characterized by mechanical tests which were performed to determine the resilience, Young’s modulus, and permanent compression, and the morphological properties were analyzed using scanning electron microscopy techniques. To solve environmental problems in the textile and paper industries, such as water pollution, the composite material was evaluated physico-chemically for its application as an adsorbent for dyes, the main cause of ecological imbalance—specifically methylene blue cationic (MB), and Congo red anionic (CR). Owing to their high efficiency in dye removal, the corn straw fibers inserted in the composite proved to be a good sustainable adsorbent with improved mechanical characteristics, making this a project that directly contributes to the sustainable development goal (SDG) #6—drinking water and sanitation; it is a low-cost, high-quality synthesized material from post-harvest waste, and it can be reused after use in wastewater treatment in construction, scientifically contributing to SDGs 12, 14, and 15.

1. Introduction

Corn is the most widely produced multi-purpose grain crop worldwide due to its importance as a food crop for the growing human population, it being used in consumer items, industrial products, biofuel, and processed foods, and it being the main ingredient in corn oil, starch, and syrup; is also an important component in livestock feed. Notably, for the 2023–2024 season, in the United States Department of Agriculture’s (USDA) “World Agricultural Supply and Demand Estimates” report, it was estimated that 1222.77 million metric tons would be produced, surpassing the current season’s production of 1150.68 million metric tons [1]. According to the USDA, Brazil is the world’s third largest corn producer, having produced 135 million metric tons in the 2022–2023 harvest, 9.52% of the global supply [2].
In Brazil, the use of no-tillage farming systems has increased significantly, as it involves depositing plant residues on the soil surface, generating profit and protecting the environment [3]. After grain harvesting, a large amount of dry straw (straw, stalks, foliage) is generated, which can vary from 2 to 15 tons per hectare [4,5,6,7]. Excess straw coverage, poorly distributed straw, or uneven micro-relief in the soil contribute to an increase in pest infestations and greenhouse gas emissions, as well as a reduction in planting density, which causes uneven emergence and delayed initial development [8,9].
Given the abundance of crops and cereals grown in Brazil, agricultural waste has been evaluated as an alternative biomass source for innovative materials with low-cost, good absorption capacity, and wide availability [10,11]. Straw has fibers composed of a cellulosic structure comprising crystalline and amorphous phases, which are characterized by the diversity of functional groups present in the cell wall, which contains hemicellulose, lignin, and cellulose macromolecules [10,12].
In recent years, researchers have investigated the application of natural fibers as reinforcements in composite materials; the major challenge has been the hydrophilic nature of vegetable fibers, the use of which results in the failure of interfacial adhesion with a given polymer [13,14,15,16]. However, studies in this field are advantageous due to the specific properties of these fibers, as they are associated with reductions in mass and low cost, and can be obtained from renewable and biodegradable sources [17]. Natural fibers can be added to polyurethane matrices to modify both the mechanical strength and reduce the cost and biodegradation of the material [18,19]. Natural organic fibers are of plant origin, with cellulose compositions and a degree of polymerization close to 10,000 [20]. Cellulose has a microcrystalline structure, which explains its high mechanical strength. Therefore, the efficiency of natural fiber reinforcement is related to the nature and crystallinity of cellulose [21]. The use of such materials in composites has increased because of their relatively low price compared to synthetic fibers, such as glass and carbon fibers [22], their biodegradability, which makes them easy to recycle, and the fact that they compete with other synthetic reinforcements in terms of increased mechanical strength per mass of material added to the composite.
Treating wastewater, mainly from the textile, paint, paper, leather and printing industries, requires greater focus. Recent information indicates that ≥100,000 companies are the biggest contributors to water pollution, releasing products and dyes into the aquatic environment and damaging ecosystems of aquatic life and, consequently, human health [23,24]. Dyes and their effluents absorb and reflect sunlight that falls on them, causing an ecological imbalance [25]. Techniques such as coagulation, precipitation, flocculation, ion exchange, chemical oxidation, ozonation, adsorption, and membrane filtration have been used to address water pollution. However, among the strategies studied, adsorption is the best and cheapest method for removing various dye effluents, as the adsorption effect of natural fibers is mainly favored by large surface areas per unit volume and increases in pore volume [26,27,28,29]. Nevertheless, when the excess dosage of adsorbent is used, adsorption sites become inaccessible to the dye molecules, or, in other words, the unsaturated surface. This makes it possible to use a small amount of polyol substituted with natural fibers, which is sufficient for the adsorption of dyes to be effective [29]. The insertion of natural fibers into the polyurethane foam could aggregate and increase the distribution of available active sites, favoring greater adsorption and easy pollutant removal.
Therefore, the focus of this study was to develop a hybrid composite of flexible polyurethane foams containing agro-industrial waste, in the form of corn straw fibers (CSs), with the main aim of solving environmental problems, and then to obtain a quality and efficient composite with a greater substitution of synthetic materials with natural straw fibers, and to achieve a better formulation of the composite for the removal of dyes (cationic and anionic) with improvements in mechanical properties.

2. Materials and Methods

2.1. Materials

Fresh CS samples were collected several hours after harvest (summer/2023), in Pinhão City, Paraná, Brazil. Polyurethane foams were manufactured using 4,40-diphenylmethane diisocyanate (MDI) (CH2(C6H4NCO)2, 250.25 g mol−1, from Sigma-Aldrich, St. Louis, MO, USA); tin(II)octanoate (initiator) ([CH3(CH2)3CH(C2H5)CO2]2Sn, 405.12 g mol−1, ≥95%, Sigma-Aldrich, St. Louis, MO, USA); poly(methylhydrosiloxane) (PMHS) ((CH3)3SiO[(CH3)HSiO]nSi(CH3)3, ~1900 g mol−1, Sigma-Aldrich, St. Louis, MO, USA); glycerol; and polyethylene glycol (PEG) (H(OCH2CH2)nOH, ~1500 g mol−1, Sigma-Aldrich, St. Louis, MO, USA). The methylene blue CI 52015 (MB) in powder form (C16H18ClN3S.3H2O 319.85 g mol−1, ≥95%, Nuclear, Diadema, Brazil) and Congo red CI 22120 (CR) in powder form (C32H22N6Na2O6S2, 696.66 g/mol, P.A., NEON, Suzano, Brazil) dyes were purchased and used without prior treatment.

2.2. Manufacture of Polyurethane Foam with CS

The foams were prepared in accordance with dos Santos et al., 2022, used in polypropylene containers 52 mm in height and 43.5 mm in diameter [30]. Two containers were used to synthesize foam without fibers (in this case, a white foam). In container 1, the polyol was added, having been previously heated to 70 °C for 1 h, and then the PMHS and glycerol were added. In container 2, MDI was added, heated to 70 °C for 1 h, and then the initiator was added. Once both containers were heated, the contents of container 2 were poured into container 1. After manual stirring, a foam was formed. To obtain a foam with fibers, the fibers were added together with PMHS and glycerol in container 1. The following quantities used were: 6 g PEG; 3 g of MDI; 0.6 mL of PMHS (0.636 g); 1 drop of glycerol (0.064 g); and 3 drops of initiator (0.19 g).
For the polyurethane foam composites containing natural corn straw fibers (PUCS), corn straw fibers were prepared according to dos Santos et al. 2024 [28]. A 22 statistical factorial design in duplicate, using Version 13 of Design Expert® software, was used to determine the composition of each foam to be produced with untreated CS, as shown in Table 1.

2.3. Mechanical Characterization

Resilience tests were performed using a resiliometer that met ASTM D3574 [31] standards. The procedure consists of releasing a steel ball (with a diameter of 16.0 mm ± 0.1 mm and a mass of 16.7 g ± 0.6 g) from the top of the graduated tube; this ball must fall, touch the body in the test, and rise again without making contact with the graduated tube. The maximum value reached by the sphere was the resilience values. Three accurate measurements were obtained for each specimen. Young’s modulus tests were performed according to ASTM D3574 [31] using a texturometer, model TA.XT Plus, with a load cell of 50 kgf. The permanent deformation in compression test (PDCT) was performed according to ASTM D395 [32]. The test consists of two metal plates that compress the specimen to 50% of its initial size, at 70 °C, for 22 h. Foam morphology was analyzed using scanning electron microscopy (SEM), and images were obtained using a SEM-Vega3 LMU from Tescan (Prague, Czech Republic).

2.4. Dye Removal

The dye removal tests were conducted with 1.0 g of PUCS composite in a 100 mL Erlenmeyer flask containing 50 mL of dye solution which underwent stirring at 40 rpm at room temperature, around 25 °C, for 1 h. The MB and CR dye solutions, of 15 mg g−1 (pH = 6.0) and 40 mg g−1 (pH = 7.0), respectively, were used. Concentrations were determined using ultraviolet spectroscopy, using a Shimadzu UV-1800 spectrophotometer (Kyoto, Japan) at 664 nm (MB) and 500 nm (CR). The amount of adsorbed dye (qe) in mg g−1 was calculated between the initial concentration (considering the addition of initial water), C0, and the final concentration, Cf, in g L−1, using Equation (1),
q e = C 0 − C f V w
where V is the volume of the final aqueous solution (L), and w is the mass of the adsorbent (g). The dye removal percentage (%) can be calculated using Equation (2):
%   d y e   r e m o v a l = C 0 − C f C 0 × 100

3. Results and Discussion

3.1. Statistical Analysis of Composite

A factorial experimental design was used as a statistical methodology to analyze the impact of the number of fibers on the mechanical properties and amount adsorbed, considering the influence of the amount of fibers replaced by PEG in the polyurethane foams. The experimental factors, their respective levels, and the observed responses are listed in Table 2.
Statistical analysis was performed using Design Expert® 13 software, using regression analysis to calculate the effects of the various terms in the model, with a significance level of 0.05. For the resilience response, p < 0.05 indicated that all the factors and their interactions had a significant effect, as shown in the mathematical model in Equation (3).
R e s i l i e n c e = 30.13 + 1.12 A − 1.88 B + 0.1250 A B R 2 = 0.9871
For the other responses, the mathematical models are presented below, as Equations (4)–(9):
Y o u n g ’ s   M o d u l u s = 0.6524 − 0.0226 A + 0.1614 B + 0.1544 A B R 2 = 0.9202
P D C T = 17.66 + 5.55 A + 6.51 B + 10.98 A B R 2 = 0.9352
%   M B   R e m o v a l = 86.35 + 2.83 A + 9.55 B + 0.2227 A B R 2 = 0.9845
M B   q e = 0.7190 + 0.0249 A + 0.0729 B R 2 = 0.9873
%   C R   R e m o v a l = 74.24 − 2.07 A + 8.04 B − 9.92 A B R 2 = 0.9550
C R   q e = 1.45 − 0.0275 A + 0.1510 B − 0.2039 A B R 2 = 0.9555
An analysis of variance (ANOVA) confirmed the quality of fit of the mathematical models, with R-squared values < 0.9200 and F-test values (p-values) < 0.05, indicating that the null hypothesis could be rejected with a probability of <5%. Therefore, the models obtained were considered significant because the p-values were substantially lower than 0.05 and the R-squared values were close to 1.0, demonstrating a good fit to the experimental data, as shown in the response ANOVA Tables (Table S1).
Statistically significant interactions were identified using ANOVA. The number of fibers (Factor B) had a significant influence on the resilience, Young’s modulus, % MB removal, and MB qe. For resilience, this effect was negative, with a lower number of inserted fibers tending to increase the response values, an opposite trend to the other responses. The size of the fibers (Factor A) had no significant influence; however, when there was an interaction (A:B) between the factors, PDCT, % CR removal, and CR qe stood out, with more substantial positive effects on PDCT. According to the ANOVA of the MB qe responses, the main effects were statistically significant, except for the AB interaction, which showed p-values > 0.05, making them statistically insignificant.
Different properties were evaluated for each material, which showed different interactions between the polymer matrix and the fiber under study (Figure 1). The resilience results (Figure 1a) showed that increasing the number of fibers resulted in a reduction in resilience, which was even greater than that of the control. This decrease indicates that the composite absorbs a greater amount of impact energy, which can be explained by the agglomeration of fibers in the foam microstructures, limiting their elastic properties [30,33]. The size of the fibers is the second most significant factor in the resilience response, because increasing the size of the fibers generates greater resistance, producing less energy conservation during the impact [34]. Large fibers stiffen the composite, which increases the resilience of the samples [34,35].
Resilience values are related to the absorption of impact energy, whereas the Young’s modulus characterizes softness, because they are intrinsically related properties [30,36]. The lower the resilience, the greater the rigid phase, that is, the higher the Young’s modulus. The addition of CS fibers leads to a large increase in the Young’s modulus, as shown in Figure 1b. This is due to the good matrix–fiber adhesion, because the agglomeration of fibers, when interacting with the polymer phase of the composite, limits the elastic properties of the foam [18]. Based on Prasad et al. (2015), the insertion of natural fibers creates fragility, which hinders the mobility of the polymer chains in the matrix [37]. The foams became more rigid as the proportion of the elastic matrix phases was replaced by an increase in the amount of CS fibers.
In the PDCTs, foams with low values are considered to be wear-resistant [38]. Figure 1c shows that a low amount of inserted fibers has a low level of permanent compression; however, when associated with larger fiber sizes, the compressive strength decreases, thereby increasing the PDCT values. The combination of these two factors is directly related to the pore arrangement of the matrix and the dimensional stability of the fibers [39]. The formation of fiber agglomerates with larger diameters led to a greater number of gaps or pores inside the sample, resulting in greater permanent deformation during compression. However, smaller fibers tend to disperse better in the polymer matrix, reducing the availability of pores in the polymers, and leading to improved wear resistance.
Removal tests of the dyes MB and CR, of different natures, cationic and anionic, respectively, were performed, as shown in the response surface graphs (Figure 2). Regarding the response to the MB dye, the removal percentage (Figure 2a) and the amount adsorbed (Figure 2b) were higher for composites with a greater number of fibers with larger diameters. Furthermore, the main factor is the amount of fiber replaced and present in the foam, which is due to the fact that a fiber and cationic dye interaction is evident, as reported by dos Santos et al. (2024) [28]. Interactions occur between the active coordination sites of the adsorbent (-COOH) and the aromatic rings of the MB through hydrophobic interactions of the π-π bond type [40]. However, the amounts adsorbed were lower than those for the natural fiber alone, as reported previously [28]. This factor may be associated with the weak interaction between the surface of the polyurethane foam, which has active urethane groups, and the cationic dye MB, which has an acidic character, thereby reducing the adsorption of MB by the composite.
A slightly different behavior was observed in the response to the anionic dye Congo Red, as shown in Figure 2c,d, with respect to the percentage of removal and the amount adsorbed by the composite. The greater the insertion of fibers in the foams, the higher the percentage of removal and the amount adsorbed; in this case, smaller fiber sizes were relevant to the removal process. Another main factor is the amount of fibers inserted in the foam, with higher qe values for CR than for MB of 1.80 mg g−1 and 0.82 mg g−1, respectively. The increase in the amount adsorbed with the fibers inserted in the foams showed an interaction between CR and part of the polyurethane, because when CS fibers alone were studied for the removal of the dye, they showed a lower affinity for CR, as reported by dos Santos et al. (2024) [28]. This increase may be associated with strong interactions between the active coordination groups of the functional group (-COO-) and the primary amine of the CR dye, which form effective H-bonds [41].

3.2. Morphology Analysis

The surfaces of the foams containing CSMB of different sizes and quantities were analyzed using SEM images. As illustrated in Figure 3a, the SEM image of the control sample reveals the fiber structure embedded within the internal foam, suggesting the presence of some points of interaction that potentially enable the availability of active sites in the fiber structure [42]. This configuration of the composite exhibits more points of failure and reduced mechanical properties relative to the post-dye-removal composite, which may be directly attributable to enhanced interactions within the composite parts [42,43]. The presence of fibers in all images is indicated by arrows, which show existing evidence of interactions with the polymer. For samples containing smaller fibers (Figure 3a,b), it was possible to observe that the amount of fiber favored agglomeration at one point on the surface, which corroborates the resilience results. When the size of the fibers increases (Figure 3c,d), there is greater dispersion of the fibers in the polymer, dissipating the conservation of energy and stiffening in making the composite [34,35].

4. Conclusions

In this study, we obtained new flexible polyurethane foams with partial substitutions of 5–30% (m/m) of polyethylene glycol with natural CS and applied statistical analysis to verify the best combination of the granulometric size of the fibers in the composite. The amount of fibers was the main factor that was significant in the mechanical analysis and dye removal. Mechanical analysis showed that the obtained foam had excellent Young’s modulus values, resistance to permanent deformation in the compression test, and low resilience values, making them suitable for application as impact absorbing materials. In the dye removal test, the composite proved to be more effective at removing the anionic dye CR, owing to the strong interaction of the H bond between the -COO- functional group and the primary amine of the CR structure. The morphology study proved the presence of interactions at the interfaces between the polymer and CS fiber with absorbed dye.

Supplementary Materials

The following supporting information can be downloaded at www.mdpi.com/article/10.3390/pr13040939/s1, Table S1. Response Variance Analysis values.

Author Contributions

Conceptualization, A.d.S., S.L.F. and F.J.A.; methodology, A.d.S. and S.L.F.; formal analysis, A.d.S., G.V.A.S. and G.F.; investigation, A.d.S., G.V.A.S. and G.F.; writing—original draft preparation, A.d.S.; writing—review and editing, A.d.S., G.V.A.S., G.F., S.L.F. and F.J.A.; supervision, S.L.F. and F.J.A.; project administration, A.d.S. and F.J.A.; funding acquisition, A.d.S., S.L.F. and F.J.A. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—Brasil (CAPES)—Finance Code 001 and CNPq. A. dos S. thanks CNPq for the postdoctoral scholarship (152453/2022-9). S.L.F thanks CNPq (406233/2021-7). F.J.A. is grateful for the CNPq Productivity grant (310815/2022-3).

Data Availability Statement

The data used to support the findings of this study are available from the corresponding author upon request.

Acknowledgments

We thank COMCAP-UEM for the SEM analysis. We thank Sobrado Campeiro farm for supplying the corn straw used in this study.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
USDAUnited States Department of Agriculture
CScorn straw
MDI4,40-diphenylmethane diisocyanate
PMHSpolymethylhydrosiloxane
PEGpolyethylene glycol 1500
PUCSpolyurethane foam composites containing natural corn straw fibers
SEMscanning electron microscopy
MBmethylene blue
CRCongo red
PDCTpermanent deformation in compression test

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Processes 13 00939 g001
Figure 1. Three-dimensional response surface graphs of the mechanical properties. (a) Resilience, (b) Young’s modulus, and (c) PDCT.
Figure 1. Three-dimensional response surface graphs of the mechanical properties. (a) Resilience, (b) Young’s modulus, and (c) PDCT.
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Processes 13 00939 g002
Figure 2. Three-dimensional response surface graph of the dye removal test; (a) % MB removal; (b) MB qe; (c) % CR removal; and (d) CR qe.
Figure 2. Three-dimensional response surface graph of the dye removal test; (a) % MB removal; (b) MB qe; (c) % CR removal; and (d) CR qe.
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Processes 13 00939 g003
Figure 3. SEM images of the (a) PMCS and PMCSMB with (b) 5% of fiber of the 250 µm, (c) 30% of fiber of the 600 µm, (d) 30% of fiber of the 250 µm, and (e) 5% of the fiber of the 600 µm. Red arrows indicate polymer and fiber interaction.
Figure 3. SEM images of the (a) PMCS and PMCSMB with (b) 5% of fiber of the 250 µm, (c) 30% of fiber of the 600 µm, (d) 30% of fiber of the 250 µm, and (e) 5% of the fiber of the 600 µm. Red arrows indicate polymer and fiber interaction.
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Table 1. The 22 statistical factorial design, in duplicate, in the manufacturing of PUCS.
Table 1. The 22 statistical factorial design, in duplicate, in the manufacturing of PUCS.
RunFactor A:
Granulometric (µm)		Factor B:
Amount of Fiber (%)
1250 (−1)5 (−1)
2250 (−1)5 (−1)
3600 (+1)30 (+1)
4600 (+1)5 (−1)
5250 (−1)30 (+1)
6600 (+1)5 (−1)
7600 (+1)30 (+1)
8250 (−1)30 (+1)
Table 2. Results of mechanical characterization and dye removal of PUCS foams obtained in the 22 factorial design.
Table 2. Results of mechanical characterization and dye removal of PUCS foams obtained in the 22 factorial design.
FactorMechanical ResponseDye Removal Response
RunA 1B 2Resilience
(%)		Young’s Modulus
(Pa)		PDCT
(%)		MB
Removal
(%)		MB qe
(mg g−1)		CR
Removal
(%)		CR qe
(mg g−1)
12505310.6718.8973.390.6162.531.21
22505310.6714.2875.000.6354.191.04
360030290.8847.6199.230.8273.731.43
46005330.235.5981.740.6874.051.48
525030270.768.4892.600.7792.741.80
66005330.405.8577.060.6674.051.47
760030301.0133.8098.670.8166.861.31
825030270.606.8093.100.7695.801.80
Control 30055 ± 50.60 ± 0.1420.90 ± 4.0484.47 ± 2.500.70 ± 0.0288.94 ± 4.611.78 ± 0.07
1 Granulometric (µm), 2 amount of fiber (%), 3 polyurethane foam without CS.
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MDPI and ACS Style

dos Santos, A.; Silva, G.V.A.; Fornazaro, G.; Fávaro, S.L.; Anaissi, F.J. Corn Straw Fibers, an Agro-Industrial Residue, Used as Reinforcement in Polyurethane Foams in Dye Removal in Wastewater. Processes 2025, 13, 939. https://doi.org/10.3390/pr13040939

AMA Style

dos Santos A, Silva GVA, Fornazaro G, Fávaro SL, Anaissi FJ. Corn Straw Fibers, an Agro-Industrial Residue, Used as Reinforcement in Polyurethane Foams in Dye Removal in Wastewater. Processes. 2025; 13(4):939. https://doi.org/10.3390/pr13040939

Chicago/Turabian Style

dos Santos, Andressa, Gabriel Vinicius Alvez Silva, Gabriel Fornazaro, Sílvia Luciana Fávaro, and Fauze Jacó Anaissi. 2025. "Corn Straw Fibers, an Agro-Industrial Residue, Used as Reinforcement in Polyurethane Foams in Dye Removal in Wastewater" Processes 13, no. 4: 939. https://doi.org/10.3390/pr13040939

APA Style

dos Santos, A., Silva, G. V. A., Fornazaro, G., Fávaro, S. L., & Anaissi, F. J. (2025). Corn Straw Fibers, an Agro-Industrial Residue, Used as Reinforcement in Polyurethane Foams in Dye Removal in Wastewater. Processes, 13(4), 939. https://doi.org/10.3390/pr13040939

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