Optimization of Milling Procedures for Synthesizing Nano-CaCO<sub>3</sub> from <i>Achatina fulica</i> Shell through Mechanochemical Techniques

Gbadeyan, O. J.; Adali, S.; Bright, G.; Sithole, B.; Onwubu, S.

doi:https://doi.org/10.1155/2020/4370172

Journal of Nanomaterials

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Abstract Introduction Discussion Conclusion Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2020 | Article ID 4370172 | https://doi.org/10.1155/2020/4370172

Optimization of Milling Procedures for Synthesizing Nano-CaCO₃ from Achatina fulica Shell through Mechanochemical Techniques

O. J. Gbadeyan,¹S. Adali,¹G. Bright,¹B. Sithole,^2,3and S. Onwubu⁴

Academic Editor: Bhanu P. Singh

Received14 Mar 2020

Revised09 May 2020

Accepted13 May 2020

Published04 Jul 2020

Abstract

The possibility of obtaining calcium carbonate nanoparticles from Achatina fulica shell through mechanochemical synthesis to be used as a modifying filler for polymer materials has been studied. The process of obtaining calcium carbonate nanopowders includes two stages: dry and wet milling processes. At the first stage, the collected shell was dry milled and undergone mechanical sieving to ≤50 μm. The shell particles were wet milled afterward with four different solvents (water, methanol, ethylene glycol, and ethanol) and washed using the decantation method. The particle size and shape were investigated on transmission electron microscopy, and twenty-three particle counts were examined using an iTEM image analyzer. Significantly, nanoparticle sizes ranging from 11.56 to 180.06 nm of calcium carbonate was achieved after the dry and wet milling processes. The size particles collected vary with the different solvents used, and calcium carbonate synthesis with ethanol offered the smallest organic particle size with the average size ranging within 13.48-42.90 nm. The effect of the solvent on the chemical characteristics such as the functional group, elemental composition, and carbonate ion of calcium carbonate nanopowders obtained from Achatina fulica shell was investigated. The chemical characterization was analyzed using Fourier transform infrared (FTIR) and a scanning electron microscope (SEM) equipped with an energy-dispersive spectroscope (EDX). The effect of milling procedures on the mechanical properties such as tensile strength, stiffness, and hardness of prepared nanocomposites was also determined. This technique has shown that calcium carbonate nanoparticles can be produced at low cost, with low agglomeration, uniformity of crystal morphology, and structure from Achatina fulica shell. It also proved that the solvents used for milling have no adverse effect on the chemical properties of the nano-CaCO₃ produced. The loading of calcium carbonate nanoparticles, wet milled with different solvents, exhibited different mechanical properties, and nanocomposites filled with methanol-milled nano-CaCO₃ offered superior mechanical properties.

1. Introduction

Nanosized calcium carbonate (CaCO₃) has received noteworthy consideration for several applications due to its availability, advantageous mechanical strength, and thermal stability [1]. It is commonly used as filler or reinforcement for polymeric materials, papers, and paints. This carbon-based material is obtained from several resources such as rock, humans, and animal waste through different methods. Over many decades, montmorillonite and kaolinite with a high concentration of CaCO₃ commonly referred to as nanoclay are commonly used and soured from rock and synthesized using either gas pressure blasting or explosion method [2–4]. Studies conducted confirmed the effectiveness of nanoclay in improving several composite properties [5, 6]. However, it has been discovered that the end-of-life of the composite produced with nanoclays tends to have negative impacts on human health, and, at times, the composite is more expensive when compared to naturally sourced fillers [7]. These drawbacks consequently have reduced the use of this filler in some countries.

The harmful impact of the abovementioned fillers and factors such as climate change compelled materials scientist to source for alternative filler material. The fillers are sourced from natural resources that not only offer suitable filler material properties but also meet societal needs and support global sustainability. Consequently, in recent decades, calcium carbonate (CaCO₃) of different particle sizes extracted from natural resources such as bones, horns, and animal shells is used as reinforcement to enhance thermal stability, degradation, strength, and physical properties of polymeric materials [8].

Several types of research have investigated the functional group, elemental composition, and minerals present in eggshell, mollusk shell, and animal bone [8–11]. These natural resources were discovered to have a higher content of calcium carbonate (CaCO₃), especially eggshell, which was confirmed to have about 95% of CaCO₃ [12–14]. Furthermore, mollusk shell, oyster shell, and animal bone have been investigated for CaCO₃, and they were found to have adequate filler-appropriate properties for different applications. Consequently, calcium carbonate produced from animal waste has been suggested to be used as an alternative to commercial CaCO₃ for some applications, which include but not limited to the dental and medical applications [8, 9, 15–18]. The mechanism behind the physical properties of shell structure has been studied [19, 20]. Filetin et al. [19] investigated microhardness of Pinna pectinata (Pinnidae), with the Adriatic Sea mollusk shell structure as a function of the indentation load. The result proved that the microhardness of the shell depends on the load for the nacreous (inner layer of the shell) and prismatic (outer layer of the shell) structures. Furthermore, the microhardness value measure for the outer layer was higher than that for the inner layer with a lesser margin. Besides, a notable aragonite platelet structure was seen in between the nacreous and prismatic predominant normal stress, preventing interlamellar sliding/plastic deformation of the shell. This study was consistent with our previous investigation on the physical and morphological study of Achatina fulica shell. The layer of aragonite was referred to as reinforcement and served as the main functional mechanism that prevents plastic deformation which resulted in high resistance to indentation, resulting in relatively high microhardness and tensile strength properties [20]. However, there is limited literature about CaCO₃, synthesized from these natural resources, being used either as a filler material to fabricate composite or as polymer material reinforcement. This output may be because milling a large quantity of these materials to smaller particle sizes comes with many challenges, which include but not limited to particle agglomeration.

Despite the availability of numerous milling techniques used for small organic particle synthesis, the mechanochemical technique has been found to be most effective [4, 21]. This organic synthesis mechanism has been regarded as a significant change towards achieving the sustainable and efficient process of producing small molecular sizes of grains [22]. Numerous researchers confirmed that mechanochemical techniques are a branch of chemistry that covers any chemical transformations induced mechanically or physiochemical changes of the material of any state of combination due to the influence of mechanical energy such as friction, compression, or shear [15, 23–26]. A mechanochemical procedure such as hand grinding or ball milling has been reported to influence the structure and composition of materials. Consequently, it brings about an opportunity for the preparation and fabrication of nanomaterial particles using top-down tactics [27].

Ball mill is a high-energy mill process especially used for an energy-intensive process like mechanical alloying, mechanochemistry, or mechanical activation. Planetary ball mill is used to determine the dependence of process efficiency using milling parameters such as ball size and number, mill geometry, and velocity of the rotating parts. However, the maximum efficiency of the grinding process achieved with high-density balls, and higher rotation speed seems to provide materials with higher impact energy in comparison with small, low-density balls and lower rotation speed [28, 29].

The planetary ball mill theory and its efficient procedure for producing small molecular sizes of grains achieved using the mechanochemical technique made it a promising candidate for solvent-free synthesis [21, 30, 31]. However, solvent-free synthesis may not apply to all materials, especially calcium carbonate-based materials, where the agglomeration of particles is dominant. Research studies have proved that dry milling technique causes a large agglomeration of the particles during synthesis. Most times, this results in a bimodal size distribution, which weakens the bond in nanocomposites [32–35]. Having this in mind, this present study deals with the optimization of milling procedures for synthesizing nano-CaCO₃ from Achatina fulica shell through mechanochemical (wet milling) techniques using different solvents. It further investigates the consequence of the solvent on the reinforcement effect of the nano-CaCO₃ on polymeric material.

2. Experimental Details

2.1. Raw Material Collection, Preparation, and Synthesis

Epoxy resin and hardener supplied with the trade name of LR 30 and LH 30 (medium) was used as a binder.

Achatina fulica snail shells collected from the University of Kwazulu-Natal, Westville campus soccer pitch, were washed and disinfected. Snail shells were soaked in a solution of water and 5% diluted household sodium hypochlorite for seven hours. Afterward, they were rinsed with distilled water and dried in the oven at 150°C for 20 mins to ensure absolute dehydration.

2.1.1. Nano-CaCO₃ Synthesis

The clean shells were kept under room temperature for 24 hours to dry before milling. The milling process was done in two stages. The first stage was dry milling, and the second was wet milling. At the first stage, 30 g of dried Achatina fulica snail shells was measured and dry milled in a planetary ball mill (Retsch® PM 100) to obtain fine particles [12, 36]. The milling setup comprises 50 stainless steel balls of 10 mm diameter and a 500 mL stainless steel jar (inner diameter of 100 mm). The snail shells were milled at 450 rpm for 30 minutes in a clockwise direction. The shell powder after the milling process was sieved using a mechanical sieving shaker (Retsch, AS 200 basics, Germany) to a particle size of ≤50 μm. In the second stage, the collected snail shell powder was wet milled to achieve nanoparticles. Accordingly, 30 g of snail shell particle size of ≤50 μm was measured into the 500 mL stainless steel jar. After that, 100 mL of a different solvent such as water, methanol, ethylene glycol, and ethanol was added differently and wet milled at 450 rpm for 258 mins in a clockwise direction. Subsequently, mixtures of fine particles and solvent were separated by removing the liquid layer that is free of a precipitate using the decantation method. To ensure the total removal of the solvent, settled particles were washed by adding distilled water and separated using the decantation method. This process was repeated five times to ensure the cleanness of fine particles. Then, particles were dried in the oven at 35°C for 72 hours. The fine powders obtained after that were characterized to establish the successful synthesis of the CaCO₃.

2.2. Characterization Approaches

2.2.1. High-Resolution Electron Microscopy

The particle size, shape, and distribution of snail shell particles milled with different solvents were observed under a transmission electron microscope (TEM). The investigation was conducted on JEM Jeol 2100 (Japan). Before this investigation, a small amount of snail shell powder was dispersed in 10 mL of ethanol and sonicated at 10 kV for 10 mins. Afterward, a thin cross section of cryomicrotomed specimens was prepared using a Leica microtome and placed on carbon copper grids. The TEM image was further analyzed on iTEM analyzer software, version 5.0.1 (Japan), to determine the range of particle sizes.

2.2.2. Fourier Transform Infrared Spectroscopy

The Fourier transform infrared (FTIR) spectra were measured to identify the functional group constituents of snail shell particles milled with different solvents. A PerkinElmer Universal ATR spectrometer was used for investigation. A small quantity of each sample was placed in the sample pouch. Subsequently, an initial background check was conducted before scanning within the range of 550-4000 cm⁻¹ at a resolution of 4 cm⁻¹.

2.2.3. Scanning Electron Microscope

The chemical composition of the shell was determined on the Zeiss Ultra FEG-SEM field emission scanning electron microscope (SEM) equipped with an energy-dispersive spectroscope (EDX). Before SEM (field emission, Carl Zeiss) observation, the surface was coated with a thin, electric conductive gold film to prevent a buildup of electrostatic charge.

2.2.4. Nanocomposite Preparation

Nanocomposites were prepared using the conventional resin casting method. To facilitate shell particle dispersion and to reduce matrix viscosity, 100 wt.% of epoxy resin was measured into the beaker using a Snowrex digital electronic scale with 0.1 g and heated up to 70°C. Subsequently, 2 wt.% of CaCO₃ nanoparticles from shell particles was slowly incorporated into the matrix and mixed using a mechanical stirrer at 500 rpm for one hour to ensure homogeneous dispersion of shell particles. Nanocomposites were taken off the stirrer and were allowed to cool down to room temperature. Then, the catalyst was added to nanocomposites at a mixing ratio of 100-30 wt.%. The blend was thereafter poured into an open mold to have a composite panel and allowed to cure for two days. To facilitate the easy removal of the nanocomposite panel, the wax was applied to the inner surface of the plastic mold before pouring. The mechanical strength of the fabricated nanocomposite panel was investigated after 15 days.

2.3. Mechanical Testing

Mechanical tests such as tensile, hardness, and impact were performed on wholly cured composite panel samples obtained after fifteen days of casting.

2.3.1. Tensile

The tensile strength and stiffness of nanocomposite were determined according to the ASTM D3039 test standard. The test was carried out on samples using a Lloyd universal testing machine (Model 43) fitted with a 30 kN load cell. Five samples were tested at ambient temperature, and the constant crosshead speed of testing used was 1.3 mm/min. The mean value of the five samples was reported.

2.3.2. Hardness

The Barcol impressor hardness tester commonly used for composite material was used to determine the hardness property of composite panel samples. The test was performed according to the ASTM D2583 test standard. A standard impressor with a steel truncated cone (6.82 height and a tip diameter of 0.55 mm) was used at an angle of 26°. This intender was positioned on the surface of the composite panel, and a uniform downward press was applied by hand, and readings were collected directly from the dial indicator. Twenty-five indentation readings were randomly collected on the sample, and the mean values were used for graphical illustration and discussion.

3. Result and Discussion

3.1. Characterization

Figure 1 presents the FTIR spectra of the raw snail shell (Achatina fulica) powder and CaCO₃ nanoparticle obtained through ball milling mechanochemical techniques using different solvents. Numerous bands were seen within the range of 550 cm⁻¹-4000 cm⁻¹.

A relatively few FTIR spectra peaks, typically associated with the stretching vibration of the functional group, were observed.

Noticeably, the spectra for the raw shell and synthesized nano-CaCO₃ are quite different within the functional group region. On the contrary, insignificant different FTIR spectra peaks and bends were observed between nano-CaCO₃ wet milled and synthesized with different solvents such as ethylene glycol, ethanol, water, and methanol. Furthermore, the raw shell FTIR spectra display absorption peaks of calcite at about 713 cm⁻¹ and 873 cm⁻¹, while nano-CaCO₃ synthesized with different solvents such as ethylene glycol, ethanol, water, and methanol shows the absorption peaks of aragonite at around 712 cm⁻¹, 854 cm⁻¹, and 1083 cm⁻¹. The peaks of calcite displayed at 713 cm⁻¹ and 873 cm⁻¹ were accredited to the out-of-plane bending and in-of-plane bending vibration modes, asymmetric and symmetric stretching for calcium carbonate (CO₃²⁻) molecules.

Furthermore, a very close prominent absorption peak is observed for both the raw shell powder and the synthesized CaCO₃ nanoparticle (1474 cm⁻¹ and 1448 cm⁻¹) at the functional group region. These were associated with the presence of carbonate ions in both materials. The FTIR spectra observed for the raw shell powder were consistent with the literature for calcite [12, 16, 37, 38]. The absorption peaks of aragonite displayed at around 1083 cm⁻¹ of CaCO₃ nanoparticles synthesized with different solvents such as ethylene glycol, ethanol, water, and methanol could be attributed to symmetric carbonate stretching vibration, and the absorption peaks of aragonite displayed at around 854 cm⁻¹ were accredited to carbonate out-of-plane bending vibrations [37–39]. Additionally, the carbonyl group C=O bending identified around 1788 cm⁻¹ was attributed to solvent (ethylene glycol, ethanol, water, and methanol) used for mechanochemical processes. Similarly, the tiny slope of carboxyl group O-H stretching is observed at around 2533 cm⁻¹. This is associated with traces of water molecules. This suggests that CaCO₃ nanoparticles can be obtained from snail shell Achatina fulica using wet milling with ethylene glycol, ethanol, water, or methanol using mechanochemical procedures.

The elemental composition of the raw snail shell and CaCO₃ nanoparticle synthesized with different solvents such as ethylene glycol, ethanol, water, and methanol is shown in Table 1. An elemental composition such as carbon, oxygen, and calcium dominated different weight percentages in both raw snail shells and the synthesized nano-CaCO₃. It was observed that the raw snail contains a high volume of CaCO₃ of about 99.4 wt.% and other metal oxides of about 6 wt.%. However, this was lesser compared to the synthesized nano-CaCO₃ that has 100 wt.% CaCO₃ irrespective of the solvent used for the wet milling process. This may suggest that the mechanochemical procedure used to synthesize helps in achieving small molecular nanoparticles and the decantation method adopted also helps in purification of the nanoparticle, resulting in clean 100 wt.% CaCO₃.

This performance may be a result of the milling period and solvent used. Among the nano-CaCO₃ synthesized with different solvents and raw snail shells, water-synthesized nano-CaCO₃ contains the highest amount of carbon, which eventually reduces the weight percentage of oxygen and calcium present in the material. The weight percentage of these components suggests that the use of water increases the carbon content in the synthesized CaCO₃, which make it harder than others; this fact can be related to the mechanical property improvement observed in Figures 2–4.

Table 2 presents particle sizes of the synthesized CaCO₃ nanoparticles that were investigated using iTEM Jeol 2100 HR (high resolution). To determine the appropriate solvent for optimizing the wet milling method for producing nano-CaCO₃, TEM images were further analyzed under TEM image analyzer software version 5.0.1 on volume base. Significantly, 28 counts of CaCO₃ particle sizes randomly selected were investigated against each solvent used. Figure 5 shows the TEM image analysis for CaCO₃ powder obtained, and the means of the powder consisted of 25.35 nm–63.68 nm sizes of a particle having semisphere morphology.

The particle size of ethanol-synthesized CaCO₃ ranges from 13.43 nm to 42.56 nm, ethylene glycol-synthesized CaCO₃ ranges from 11.56 nm to 65.78 nm, methanol-synthesized CaCO₃ ranges from 12.57 nm to 98.66 nm, and water-synthesized CaCO₃ ranges from 24.29 nm to 180.06 nm. Notably, nanosized particles within the range of 100 nm were observed for all synthesized CaCO₃. However, water-synthesized CaCO₃ has the biggest particle size of 63.68 nm, considering the mean of the 25 counts, as shown in Table 2. This particle size is consistent with figure observed in Table 1, where water-synthesized CaCO₃ powder has high carbon content.

The presence of water mixtures has been confirmed to affect CaCO₃ polymorphs and morphology [40]. This fact suggests that the inclusion of water before the milling process affected polymorphs and morphology of the shell powder, which eventually prevents the effectiveness of the ball milling process, resulting in larger particle sizes. Furthermore, ethanol-synthesized nano-CaCO₃ has the smallest range of particle sizes. This outcome proved the effect of ethanol on breaking CaCO₃ molecules into small sizes. Additionally, a low standard deviation was observed for 28 counts of CaCO₃ particle sizes randomly selected. This consequence depicts the effectiveness of ball milling in providing uniform particle sizes that are very close to the mean value of the particle sizes.

3.2. Mechanical Properties

The reinforcement effect of synthesized nano-CaCO₃ using different solvents through mechanochemical was investigated. Table 3 shows that the loading of the manufactured nanoparticle enhanced the mechanical properties of the epoxy composite. This trend is consistent with previous findings.

This performance may be attributed to the good dispersion and interconnecting networked structure of the particles incorporated in the matrix that formed a tougher, strengthened, and stronger structure than neat epoxy. This trend is consistent with pieces of literature where filler loading enhances mechanical properties [3, 8, 41]. The mechanical property results shown in Table 3 are higher compared with those reported in some available literature where CaCO₃ synthesized from other shells are used to improved mechanical properties [42–44]. The loading of CaCO₃ synthesized from a shell such as eggshell and mollusk and milled to microparticles improved the mechanical properties; however, loading of Achatina fulica shell offered superior properties [45–47]. This performance may be attributed to excellent dispersion of particles in the polymeric material shown in Figure 6, which was facilitated by the small particle sizes of the synthesized calcium carbonate particles.

(a)

(b)

(c)

(d)

Nano-CaCO₃ produced with ethanol- and methanol-filled composites exhibited almost the same tensile strength and hardness properties, as shown in Figures 2 and 4. The loading of nano-CaCO₃ synthesized with ethanol improved the strength of epoxy by 22.42%, and incorporation of nano-CaCO₃ manufactured by methanol enhanced tensile strength by 21.34%. This improvement may be attributed to the particle size and shape of the synthesized nano-CaCO₃ incorporated.

Tensile strength values for the epoxy composite filled with nano-CaCO₃ synthesized with ethanol glycol and water are on the lower side compared to nano-CaCO₃ produced with the ethanol- and methanol-filled composites. On the contrary, hardness properties for nano-CaCO₃ synthesized with ethanol glycol- and water-filled composites were superior to those of nano-CaCO₃ produced with ethanol- and methanol-filled composites as represented in Figure 4. This confirmed that the loading of nano-CaCO₃ synthesized with ethanol glycol and water increases hardness, which makes it brittles. The brittleness of this material is the reflection of low tensile strength offered by this composite, as shown in Figure 2.

Figure 3 shows that the loading of synthesized nano-CaCO₃ is not only an efficient way to improve the mechanical strength of polymeric material but also enhances stiffness. The incorporation of synthesized nano-CaCO₃ improved the stiffness of epoxy nanocomposite irrespective of the solvent used for synthesizing the nanoparticles. The addition of nano-CaCO₃ synthesized with ethanol through mechanochemical techniques increased epoxy stiffness by 57.7%, and incorporation of nano-CaCO₃ produced with the addition of ethanol glycol enhanced the stiffness of epoxy by 56.8%. The addition of nano-CaCO₃ synthesized with water through mechanochemical techniques increased epoxy stiffness by 10.65%, and the loading of nano-CaCO₃ wet milled with methanol improved stiffness by 84.65%. Although loading of synthesized nano-CaCO₃ improved stiffness, the epoxy composite filled with nano-CaCO₃ wet milled with methanol offered superior stiffness.

TEM images presented in Figure 6 are for the epoxy polymer filled with nano-CaCO₃ milled using different solvents. The dark phase of the image signifies the nano-CaCO₃, and the lighter phase of the image signifies the polymer matrix. The TEM micrograph not only shows the well-dispersed nano-CaCO₃ in the matrix but also shows that the size of the nanoparticles is slightly different. This trend complements different particle sizes of synthesized nano-CaCO₃ shown in Table 2. Furthermore, the homogeneous dispersion of the nanoparticle shown in the TEM image formed an interlocking structure that strengthened the epoxy composite, which eventually led to the improved mechanical properties observed in Table 3.

4. Conclusion

Nano-CaCO₃ was successfully synthesized from Achatina fulica shell. The reinforcement effect of the produced nanocalcium carbonate particles, wet milled with different solvents using the mechanochemical technique, was investigated. High-speed (450 rpm) balling milling machine was used to synthesize nanoparticle sizes. The mixture of 100 mL of different solvents and 30 g of raw snail powder of particle was wet milled at 450 rpm for 258 mins in a clockwise direction to produce nano-CaCO₃. It was discovered that nanoparticle particle sizes (25.35 nm–63.68 nm) of calcium carbonate could be synthesized from Achatina fulica shell using the mechanochemical wet milling technique.

FTIR spectra for the raw shell display absorption peaks of calcite at about 713 cm⁻¹ and 873 cm⁻¹ and were accredited to out-of-plane bending and in-of-plane bending vibration modes, asymmetric and symmetric stretching for calcium carbonate (CO₃²⁻) molecules. On the other hand, nano-CaCO₃ synthesized with different solvents such as ethylene glycol, ethanol, water, and methanol shows the absorption peaks of aragonite at around 712 cm⁻¹, 854 cm⁻¹, and 1083 cm⁻¹. The observed absorption peaks were attributed to carbonate out-of-plane bending vibrations. This bending vibration proves that milling Achatina fulica shell to nanoparticles changed the polymorphs of the shell powder at microsize from calcite to aragonite.

The energy-dispersive spectroscope (EDX) confirmed that the mechanochemical procedure used to synthesize nano-CaCO₃ from Achatina fulica shell helped not only in achieving small molecular nanoparticles but also in purification, resulting in unsoiled 100 wt.% CaCO₃. TEM image analyzer results evidenced that the CaCO₃ powder obtained consisted of a particle size range of 25.35 nm–63.68 nm with semisphere morphology. The mechanical property results show that the loading of nano-CaCO₃ synthesized from Achatina fulica shell through the mechanochemical procedure was an effective way of modifying mechanical strength. Although loading of synthesized nano-CaCO₃ improved strength and stiffness polymetric material however, the epoxy composite filled with nano-CaCO₃ wet milled with methanol offered superior stiffness.

5. Future Work

The completion of this study brought forward some limitations that opened up opportunities for future work. The drawback included but not limited to the long milling period of the shells. Thus, the reduction of the milling process for synthesizing nano-CaCO₃ from Achatina fulica shell shall be a potential area for future work.

Data Availability

The authors have decided to deposit the data in the manuscript with the abovementioned topic in a public repository.

Conflicts of Interest

The authors declare that they have no conflict of interest as all authors are affiliated to the institution as academic staff and students.

Acknowledgments

The author would like to acknowledge the scholarship support towards the remission of school fees from the University of Kwazulu-Natal and financial assistance received from the CSIR and the Department of Science and Innovation (General Business Support Treasury funding).

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Copyright © 2020 O. J. Gbadeyan et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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