Sugar Cane Bagasse Ash: An Agricultural Residue with Potential Rubber Filler Applications

Abstract

South Africa produces approximately 7 million tons of sugarcane bagasse annually as an agricultural residue, which is treated as waste and its disposal is known to have negative impacts on the environment. To lessen reliance on petroleum and polymers, consideration is given on use of sugarcane bagasse ash as substitute materials for the development of fillers for rubber and other large-scale commodity polymers. This work reports on the mechanical, physiochemical, and structural properties of sugarcane bagasse ash to define the compatibility with the specific polymers that will pave way to the engineering of composites to utilize the potential benefits of these residue-derived fillers. The structural and morphological properties of the untreated and treated sugarcane bagasse ash were performed using XRD, FTIR, and SEM-EDX, respectively. The obtained results confirmed the successful treatment of the sugarcane bagasse ash. The study was successful in showing that sugarcane bagasse ash as potential filler in rubber polymer matrix is a natural resource of silica, which is sustainable and cost-effective, thus should be harnessed for industrial purposes in South Africa.

1. Introduction

Sugarcane commonly known as “saccharum officinarum L” is a commercially grown crop in harvesting season under climate conditions of slightly sunny and colder. And, it is produced annually in most parts worldwide, with 100 million tonnes produced by both Brazil and India. South Africa produces about 19.3 million tonnes of sugarcane annually followed by Nigeria and Uganda with two of largest producers of sugar cane on African soil [1, 2, 3].

Sugar cane in South Africa is grown in the Kwa-Zulu Natal Province and Mpumalanga Province as shown in Figure 1. Sugar cane production in the Kwa-Zulu Natal Province is composed of gross farming located across the two provinces whereby about 22,949 registered sugarcane growers produce annually 20 million tons of sugarcane from approximately 14 million supply sections, extending from southern Kwazulu-Natal to Mpumalanga Lowveld as shown in Figure 1 [4].

Figure 1.

Represents sugarcane plantations regions in the Mpumalanga and KwaZulu-Natal provinces of South Africa.

Sugarcane primarily it is the source of sugar and juice, and can be used as a raw material in various industries for production of sugar, jiggery and syrups. In spite of that these industrial processes generate considerable quantities of residual bagasse, which is often used to produce heat energy via combustion. Interestingly, about 25–30 wt residual lignocellulose bagasse (RSB) is produced per kg of sugarcane processed in sugar mills [5, 6, 7, 8]. The crop accumulates to approximately 380 kg/pa of silicon in a 12-month-old-crop. In addition, silicon in this state is a solid waste as silica (SiO₂). It has been reported that natural silica is safe to handle, cheap and simple work-up extraction from various sources. The fate of bagasse ash poses a challenge to the environment [9].

Nowadays, research has swiftly shifted focus to the industrial agricultural wastes to address the ever-growing concerns for the inadequate disposal of residues produced from agro-wastes. Consequently, led to the development of hybrid composites via reinforcements in polymer science. The use of substitute materials such as sugarcane bagasse as a filler in rubber and cementious application, has become a global concern to look at ways to minimize the disposal of wastes especially those which are non-biodegradable and pose a threat to the eco-system. Therefore, recycling and re-use of these residues released as wastes from industries has been efficient route to minimize the disposal of wastes which are not environmentally friendly and pose a threat to the quality of life (micro-organisms) [10].

Notably the re use of recycled fillers as reinforcements from renewable sources has received an increased attention with the aim of synthesizing alternate materials in addressing shortcomings related to the fairly and low sustainable of conventionally reinforced polymer composites. Carbonaceous nanomaterials from various sources such as wood, jute, cotton and bagasse have been introduced into a polymer matrix as reinforcements which have recently gained momentum in polymeric composites, this is due to their production means being inexpensive, fast and do-able [11].

Additionally, sugarcane bagasse their composites possess good chemical, physical and mechanical properties and is considered a low-cost and low-density material which possesses potential application-specific mechanical strength and stiffness values [12]. Thus, researchers reported on the mechanical properties of the composites (elasticity and elongation at break) whereby they measured using the average of the three properties in Table 1. In relation to the stress–strain, the mechanical properties in terms of tensile strength, the values measured from sugarcane bagasse ash SCBA with optimized silane treatment were found to be the most desired and excellent.

Property	Pure gum	25 per hundred rubber (phr) pristine	25 per hundred rubber (phr) in situ silane treatment	25 per hundred rubber (phr) with optimized silane treatment
Stress (MPa)	4.02	4.23	4.96	11.47
Modulus (E)(MPa)	0.32	0.37	1.03	0.62
Strain (%)	974	625	442	857.4

Table 1.

Mechanical properties of the composites with and with no silane coupling agent.

For modified application of silane coupling agent, Bis[3-(triethoxysilyl)propyl] Tetrasulfide (TESPT), in the SCBA-Natural Rubber interaction, SCBA particles were homogenously dispersed in the NR matrix, from the method reported in [13, 14]. The samples with a significant amount (phr) of SCBA were studied to understand and aim for greater interfacial adhesion within the treatments. Hence, the composites displayed improved mechanical response, the bold in the table imply to the best results observed in the samples. Moreover, the use of SCBA can be a feasible substitute material as filler in Natural Rubber (NR) due to excellent mechanical properties [14].

Interestingly, sugarcane bagasse has been used as reinforcing phase in cementitious and polymer composites. And, it comprises of waste generated from industrial production of sugar, fuel and other beverages derived from sugarcane. Sugarcane bagasse ash is mostly composed of silica in various forms structurally including crystalline, vitreous and amorphous. The amorphous silica from SCBA has been utilized as a filler in polymer composites. Wimolmala and Sombatsompop reported for the first time the utilization of SCBA with a particle size in the range 45–145 μm as a filler in a rubber matrix with a concentration of 15 phr (per hundred rubber). Although, they incorporated high content of N-cyclihexyl-2-benzothiazole sulphonamide (CBS), which, in spite of improving the crosslinks sections and the mechanical resistance of the composites, significantly minimized their life time. Traditionally, the alkaline treatment is utilized followed by a strong acid for neutralization to produce silica gel, as illustrated below [13, 14, 15]:

$${\mathit{SiO}}_2\left(s\right)+2{\mathit{\text{NaOH}}}_{\left(\mathit{aq}\right)}\to {\mathit{Na}}_2\mathit{Si}{O}_{3\left(\mathit{aq}\right)}+H_2{O}_{\left(\mathit{aq}\right)}$$E1

$$N{a}_2\mathit{Si}{O}_{3\left(\mathit{aq}\right)}+2H{\mathit{Cl}}_{\left(\mathit{aq}\right)}\to \mathit{Si}{O}_{2\left(\mathit{gel}\right)}+2\mathit{Na}{\mathit{Cl}}_{\left(\mathit{aq}\right)}+H_{2}O\left(l\right)$$E2

Other studies have reported the use of sugarcane fibers as reinforcing filler, which implies the environmentally-friendly application of this natural resource. In this current study, the structural and morphological properties of sugarcane bagasse ash were studied as well as thermal properties potential for reinforcements in rubber materials. The use of citric acid (an organic acid) during treatment step to remove any inorganic impurities from the ash proved to be useful tool in realizing green methodology for the use of sugarcane bagasse ash [16].

Nanomaterials have allowed researchers to effectively manipulate and exploit materials at the nanometer scale to produce new functionalities. And, to fine-tune their properties from an atomic level, to get desired properties to suit particular applications. In essence, rubber materials are desired to be inexpensive, due to the bio-materials (filler/polymer matrix) and their fabrication methods as well as their resources which is of paramount importance as compared to conventional silica from non-natural resources. In this work surface treatment was performed using citric acid potentially to improve the interfacial adhesion of waste and its higher interactions with elastomeric matrix. The structural, morphological and thermal properties of SCBA potential for filler matrix interaction were analyzed.

2. Materials and methods

2.1 Material used

The chemicals utilized in the preparation were citric acid ≥99.5%, purchased from Sigma Aldrich. Sugarcane was procured from Sugar Illovo South Africa Company. The synthesis was done using deionized water from the Milli-Q water purification system (Millipore, Bedford, MA, USA).

2.2 Preparation of Sugarcane Bagasse Ash (SCBA)

The sugarcane bagasse (a, b) cultivated around the tropical and coastal regions of South Africa is used in this work, shown in Figure 2. Firstly, the incomplete combustion of the bagasse to ash (c, d) was carried out in open space and then heated in an oven at 40°C overnight to obtain uniform particle sizes from dried ash. In a typical procedure the residue of (d) sugarcane bagasse 2.0 g heated at 700°C for 4 hours (residence time) in a muffle furnace at 10°C/min heating rate (gradient time) to burn away volatile organic matter illustrated in Figure 2D.

Figure 2.

Presents images of a typical sugarcane plantation (A), stalks (B), and bagasse (C) from the south African sugar industry, calcination in a muffle furnace (D), and conversion to sugarcane bagasse ash (E).

2.3 Leaching with citric acid

In the first treatment, the bagasse ash was mixed with citric acid in 250 ml beaker and then underwent reflux in a 250 ml volumetric flask stirred at 450 revolution per minute (rpm) at 80°C for 2 hours, shown in Figure 2D,E. The ash was then washed with double deionized water and decant until the pH of the supernatant reached 6.5. The resulting ash was then dried in an oven at 40°C overnight and ground into fine powder using mortar and pestle.

3. Results and discussion

3.1. Structural and morphological characterization.

The crystallinity identification was cross-examined using X-ray diffraction (XRD) on a Bruker AXSD8 Advancement instrument, (Ithemba Labs, South Africa) with Cu-Kα1, λ = 154,050 A. The Bragg angle array was 2Ɵ = 10–90°C with a scanning step 0,035°C. The new surface functionalities were studied using IR by identifying the functional groups and bonding of elements present in the samples. The analysis was undertaken at room temperature with a wavelength range 400–4000 cm⁻¹ and phase composition determined using a PerkinElmer FTIR spectrometer (spectrum two). Scanning electron microscopy analysis was performed using SEM (TESCAN, VEGA) to observe surface morphology of the sugarcane bagasse ash (SCBA). The samples were prepared on an aluminum stub and carbon sputtered on a carbon coater before analysis.

3.1 Characterization of synthesized materials

3.1.1 Powder X-ray diffraction and FTIR analysis

The X-ray diffraction pattern of untreated (i) and treated (ii) Sugarcane bagasse ash (SCBA) is presented in Figure 3a. The x-ray diffraction pattern reveals diffraction peaks associated with the presence of quartz silica @ 2Ɵ = 27 (JCP phase from ICDD: 01–083-0539), and broad peak corresponding to the amorphous nature of silica in the bagasse sample @ 2Ɵ = 12 and between 2Ɵ = 22 and 28. The attributed traces of silica in the form of quartz in the bagasse ash was reported to be as a result of accumulation of minerals from the soil. The obtained bagasse proves to be heavily impure as shown in Figure 1a (i) for untreated bagasse ash. The acid treated bagasse, Figure 1a (ii) ash show crystalline peaks attributed to silica as well as smooth peaks as compared to the untreated bagasse ash [17].

Figure 3.

Presents XRD spectra of (a) untreated bagasse ash (i) and treated bagasse ash with citric acid (ii), and FTIR spectra of (b) untreated bagasse ash (i) and treated bagasse ash with citric acid (ii).

Figure 3b, presents FTIR spectra of untreated (i) and acid-treated (ii) bagasse ash. The key functional groups existing in the bagasse ash were identified and characteristics peaks for SCBA with absorption bands between 1046 and 1221 cm⁻¹ corresponds to asymmetric vibration of the Si-O-Si bonds. The peaks at 464 and 775, correspond to Si-OH present on the surface of the particles. The peaks at 1361 and 1740 are associated with organic matter in the material and C=O, respectively. The peaks at 3014 and 3457 are as a result of organic matter in the ash C-O and OH group, respectively.

SCBA material it is possible to observe the pronounced peaks and intense peaks for the acid leached sample in Figure 3b (ii). It is noteworthy to realize the evolution and disappearance of functional groups and the appearance of key functional groups. This confirms successful incorporation of new functionalities on the acid leached sample (ii) Figure 3b. Moreover, the highly narrow and pronounced band at around 1046 cm⁻¹ which is associated with crystalline silica, is in agreement with the results obtained from XRD diffractograms(s) [17, 18].

3.1.2 SEM–EDX analysis

The surface morphologies for the untreated and treated ash were cross examined using Scanning Electron Microscopy (SEM), presented in Figure 4A untreated bagasse and (c) for acid-treated bagasse ash. The SEM images, it is possible to observe the agglomerates of irregular morphology and sizes as well as spherical-rice like particles present in the samples. Although, organic acid leaching process was effective on the surface morphology as observed in (d) with rice-like shape observed, the morphological differences confirm the surface characteristics analysis (FTIR) discussed earlier, where the pretreatment with citric acid significantly improved the formation of silica for the acid-leached sample [18, 19].

Figure 4.

SEM images of (A) untreated bagasse ash, corresponding EDX spectrum (B) and image of (C) treated bagasse ash with citric acid and corresponding EDX spectrum (D).

The EDX results exhibit that the chemical composition of SCBA untreated and treated with citric acid obtained by SEM–EDX shown in Figure 4B and D, respectively, that acid treatment is an effective tool to mitigate impurities from the increment of Si matter in the samples for acid-leached. In addition, the acid-treated bagasse ash reveal closely packed particles due to the increased formation of hemicellulose removal bonds. The acid leaching treatment results in the degradation of cellulose and hemicellulose-inspired bonds, thus the resulting cellulose chains generate new hydrogen bonds to replace the removed ones. In essence, improves filler polymer interaction [20].

Table 2 shows that citric acid is a useful organic acid for the pretreatment process. The increased morphology of silica in the powders of the ash is as result of the selective removal of the synthesis residues using citric acid, thereby reducing metallic impurities present as compared to the morphology from the untreated ash.

Element	Untreated SCBA (%)	Acid-treated SCBA (%)
C	74.92	69.36
O	22.45	25.37
Si	1.78	5.27
K	0.26	—
Ca	0.59	—

Table 2.

Chemical composition of raw bagasse ash.

The samples show that pretreatment of the ash results in significant reduction in carbon content as shown in Table 1 above, and silicon increased from 1.78% to 5.27%.

It is quite evident that the pretreatment step is a useful tool and sufficient in purifying the bagasse ash by reducing inorganic impurities to achieve high yield of silica implied by the mathematical expressions below [21]

$$100\mathrm{g}\text{of}\mathrm{SBA}\text{introduces approximately}76.34\mathrm{g}\text{of}\mathit{Si}{O}_2$$E3

$$\text{Therefore},\text{unknown}\left(\mathrm{X}\right)\text{amount of SCBA will introduce}73.21\mathrm{g}\text{of}\mathit{Si}{O}_2$$E4

$$\text{The aforementioned implies},=\frac{100x73}{76.34}=\frac{7300}{76.34}=95.625\mathit{\text{gofSCBA}}$$E5

$$\begin{array}{l}\text{For,}A{l}_2{O}_3\text{since}100\mathrm{g}\text{SCBA introduces}6.7\mathrm{g}\text{of}A{l}_2{O}_3,\\ \text{then}:95.625\mathrm{g}\text{of SCBA will introduce}=\frac{95.625x6.7}{100}=\frac{640.688}{100}=6.407\mathit{\text{gofA}}{l}_2{O}_3\end{array}$$E6

$$\begin{array}{l}F{e}_2{O}_3:\text{Since}100\mathrm{g}\text{SCBA introduces}6.3\mathrm{g}\text{of}F{e}_2{O}_3,\\ \text{then}:95.625\mathrm{g}\text{of SCBA will introduce}\frac{95.625x6.3}{100}=\frac{602.438}{100}=6.024\mathit{\text{gofF}}{e}_2{O}_3\end{array}$$E7

$$\begin{array}{l}\mathit{CaO}:\text{Since}100\mathrm{g}\text{SCBA introduces}2.8\mathrm{g}\mathit{CaO},\\ \text{then}:95.625\mathrm{g}\text{of SCBA will introduce}=\frac{95.625x2.8}{100}=\frac{267.75}{100}=2.678\mathit{\text{gofCaO}}\end{array}$$E8

$$\begin{array}{l}\mathit{MgO}:\text{Since}100\mathrm{g}\text{SCBA introduces}3.2\mathrm{g}\mathit{MgO},\\ \text{then}:95.625\mathrm{g}\text{of SCBA will introduce}=\frac{95.625x3.2}{100}=\frac{306}{100}=3.06\mathit{\text{gofMgO}}\end{array}$$E9

$$\begin{array}{l}{P}_2{O}_5:\text{Since}100\mathrm{g}\text{SCBA introduces}4.0\mathrm{g}{P}_2{O}_5,\\ \text{then}:95.625\mathrm{g}\text{of SCBA will introduce}=\frac{95.625x4.0}{100}=\frac{382.5}{100}=3.825\mathit{gof}{P}_2{O}_5\end{array}$$E10

$$\begin{array}{l}N{a}_{2}O:\text{Since}100\mathrm{g}\text{SCBA introduces}1.1\mathrm{g}N{a}_{2}O,\\ \text{then}:95.625\mathrm{g}\text{of SCBA will introduce}=\frac{95.625x1.1}{100}=\frac{105.188}{100}=1.052\mathit{\text{gofN}}{a}_{2}O\end{array}$$E11

$$\begin{array}{l}{K}_{2}O:\text{Since}100\mathrm{g}\text{SCBA introduces}2.4\mathrm{g}{K}_{2}O,\\ \text{then}:95.625\mathrm{g}\text{of SCBA will introduce}=\frac{95.625x2.4}{100}=\frac{229.5}{100}=2.295\mathit{gof}{K}_{2}O\end{array}$$E12

$$\begin{array}{l}\mathrm{LOI}:\text{Since}100\mathrm{g}\text{SCBA introduces}0.9\mathrm{g}\mathit{LOI},\\ \text{then}:95.625\mathrm{g}\text{of SCBA will introduce}=\frac{95.625x0.9}{100}=\frac{86.063}{100}=0.861\mathit{\text{gofLOI}}\end{array}$$E13

3.1.3 Thermal analysis

Thermal properties were investigated using thermogravimetric analysis (TGA) from room temperature to 800°C, reported in [22]. The current study gave insightful information on the thermal characteristics of bagasse ash as shown in Figure 5. The TG curves essentially showed the total mass degradation of Figure 5a 12% and Figure 5b 18%, respectively. It is noteworthy to realize 3 distinct stages of rapid weight loss attributed to moisture drying, volatile organic matter and rapid decomposition of hemicellulose and cellulose. The results are similar to the findings reported in [23, 24]. No further mass loss was observed as a result of thermal stability of nanosilica present in the samples, which account to 84% in mass, as reported in [25].

Figure 5.

TGA curves of (a) untreated sugarcane bagasse ash and treated bagasse ash with citric acid. Reproduced with permission from [22], copyright (2022), MDPI.

4. Conclusions

Based on the findings, we concluded that acid-leaching significantly improved the chemical, physical and thermal properties of bagasse ash. The ash is mostly carbonaceous material. And, the major component of SCBA is silica from the elemental composition studies. The SCBA contains both amorphous and crystalline silica. The reduction of inorganic impurities makes the bagasse residue potential to serve as a filler in polymer composites, and presents a promising and feasible alternative substitute material for the rubber industry. The advantage of this finding is that the use of organic acid proved essentially and equally compatible for the current study as eco-friendly and green method for acid-leaching process.

Furthermore, the filler material will enhance the thermal and mechanical properties of the polymer matrix. These new route to recycle the agricultural residue from the sugarcane industry is pre-conceived as eco-friendly and cost-effective due to the readily available of the plant in tropical countries, and could be used as reinforcing filler in polymer matrix interaction. Therefore, presents a niche application in south Africa as it has never been done before.

Acknowledgments

Funding: This research was funded by National Research Foundation (NRF), South Africa, grant number: 138079 and Tertiary Education Support Program (TESP), Eskom Holdings SOC Limited Reg No 2002/015527/06.

Conflict of interest

The authors declare no conflict of interest.