Characteristic investigations on bio-silica gel prepared from teff (Eragrostis tef) straw: effect of calcination time

A sample of TS was collected from the Debre Zeit Agricultural Research Center (DZARC), situated in Bishoftu, Ethiopia (filagot spp., accession code: Dz-Cr-442 RIL77C). The sample's contaminants, including sand, dirt, and soil particles, were removed using pure tap water. After being sun-dried for fifteen days, the sample was pulverized into smaller particle sizes with the aid of a grinder. The grounded TS particles were activated using a reflux condenser with HCl acid with a 2.5 N concentration for 1 h at 90 ± 5 °C. The acid-activated sample was additionally cooled to room temperature. Once the pH value reached 7, it was repeatedly cleaned with distilled water. An air-forced drying oven was used to dry the pretreatment sample for 12 h at 100 ± 5 °C. The biosilica samples, at 500, 700, and 900 °C with a 2 h calcination time, made from the ash of teff straw, have single-point BET surface areas of 52, 61, and 81 m² g⁻¹, respectively. As more carbon residue breaks down at high temperatures, the surface area of biosilica rises with an increase in ash temperature from 500 to 900 °C. As a result, the maximum surface area is reached after two hours at 900 °C. This increased the number of holes on the surface of the biosilica, increasing its surface area [36]. 'According to the study by E Rafiee, S Shahebrahimi [44]', for 2 h at higher combustion temperatures of 1,000 °C, silica crystallization increased. The section of the pores that were damaged throughout the procedure also caused the specific surface area to decrease as the temperature increased. A programmed muffle furnace (Model: FE-37-FA-013) was used to calcinate a dried sample of TS at various calcination times (2, 3, 4, and 5 h) at 900 °C with a 10 °C min⁻¹ heating rate. The obtained TSA samples were collected after calcination and to prevent absorbing moisture, left to cool in a desiccator.

By following the process, silica gel was produced from TSA using the sol–gel technique reported by S P Ramasamy, D Veeraswamy [21]. As a result, 10 g of TSA samples were combined with 80 ml of 2.5 M NaOH and heated on a hot plate for 1 h at 90 ± 5 °C. Additionally, it was continuously agitated to create a sodium silicate solution by dissolving the silica from the TSA. Filtration via Whatman No. 41 filter paper was used to purge the solutions of any remaining, undissolved ash particles. The obtained filtrate was cooled, titrated with 2.5 M HCl to a pH of 7.0, and continuously stirred on a hot plate at a temperature of 90 ± 5 °C for 1 h. In order to break down the silica in the ash and create the sodium silicate solution, the mixture was continually agitated (reaction can be seen in (equation (1)). The residual, undissolved ash particles were filtered out of the mixture using filter paper (Whatman No. 41). Using reaction equation (2) as a guide, the resulting filtrate was chilled and titrated with HCl (2.5 M) until the pH reached 7.0 while being continuously stirred. As a result, silica particles may have formed [45, 46]. Then, it was fostered for 20 h to encourage gel development. After ageing, the gel was cleaned using Whatman No. 1 filter paper, 110 mm, and boiled double-distilled water. After draining the supernatant, the aqua gels were dried for 15 h at 90 °C in a hot air oven in order to create silica gel. The acquired materials were stored for later analysis in cool, sealed plastic bags, dry environment. The conceptual process for silica gel synthesis using the sol–gel method is shown in figure 1.

$$\begin{eqnarray}&&{{\rm{SiO}}}_{2\left({\rm{solid}}\right)}+2{{\rm{NaOH}}}_{\left({\rm{aquos}}\right)}\mathop{\to }\limits^{{\rm{yields}}}{{\rm{Na}}}_{2}{{\rm{SiO}}}_{3\left({\rm{aquos}}\right)}+{{\rm{H}}}_{2}{{\rm{O}}}_{\left({\rm{liquid}}\right)}\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&{{\rm{Na}}}_{2}{{\rm{SiO}}}_{3\left({\rm{aquos}}\right)}+2{{\rm{HCl}}}_{\left({\rm{aquos}}\right)}\mathop{\to }\limits^{{\rm{yields}}}{{\rm{SiO}}}_{2\left({\rm{solid}}\right)}+2{{\rm{NaCl}}}_{2\left({\rm{aquos}}\right)}+{{\rm{H}}}_{2}{{\rm{O}}}_{\left({\rm{liquid}}\right)}\end{eqnarray} \tag{ 2 }$$

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The TSA samples' surface area was calculated using the Brunauer–Emmett–Teller formula (SA-9600 series, HORIBA, USA). Non-Local Density Functional Theory (NLDFT) techniques of the (Nova 4000e, Quantachrome, USA) analyzer were used to measure the specific surface area, pore volume, and pore diameter using BET and Barrett-Jyner-Halenda (BJH) methods. The sample was degassed for 8 h at 300 °C in a vacuum after making sure all moisture and impurities had been removed. By infusing nitrogen at 77.35 K while adjusting the relative pressure (P/P_o) from 0.057 to 0.305, the Langmuir adsorption isotherm mechanism was taken into consideration. The BJH pore diameter was calculated using the equation (3).

$$\begin{eqnarray}&&{{\rm{d}}}_{p}=4\,\ast \,\displaystyle \frac{{{\rm{V}}}_{p}}{{{\rm{S}}}_{BET}}\end{eqnarray} \tag{ 3 }$$

where SBET is the BET surface area, Vp is the pore volume, and dp is the pore diameter.

To record the spectrum data for TSA, FT-IR spectroscopy (Thermo Scientific FTIR, IS50, USA) was used., which produced silica gels. The scanning range was from 3 to 800 at 2 for a speed of 10.0 degree min⁻¹ using an XRD (RIGAKU-Model: Ultima-IV, USA) with a CuK emission of acceleration voltage of 40 kV and current of 44 mA. A scanning electron microscope (JEOL-Model: JSM-6390, USA) and an EDX at Secondary Electron Image with the same model and a range of 0–15 accelerating voltage were used to analyze the silica gel's surface morphology.

Adsorption is the process by which gas molecules attach to any solid surface. In this way, the exposed active surface area, gas pressure, temperature, and level of solid-gas interaction can all affect how much gas is adsorbed. Due to its great purity and strong interactions with all materials, N₂ gas is often used to assess BET surface area . In the current work, the specific surface area was measured using the BET technique in the relative pressure range (P/P_o) of 0.05 to 0.25. In this case, Po stands for the starting pressure, whereas P stands for the system pressure. The nitrogen adsorption isotherm was utilized to determine the silica nanoparticle's pore volume using a pore size analyzer, which measures the porus size distribution at a relative pressure (P/P_o) of 0.3 [47].

Based on the observations, a straightforward comparison of the pore volume, surface area, and pore diameter of silica gel has been shown in table 1. The silica gel obtained from TSA has a BET surface area that is comparable to earlier findings, particularly bamboo leaf ash, which has a surface area of 667.95 m² g⁻¹ [49], rice husk ash of 653 m² g⁻¹ [1] and rice husk silica gel of 783 m² g⁻¹ [50]. As shown in table 1, the TS silica gel was found to have a larger surface area than the acid-leached TSA. The outcomes show that the silica gel's surface area and pore volume are considerably impacted by the acid leaching products. It is clear that the increase in surface area is mostly due to the effect of acid hydrolysis on cellulose and hemicellulose to transform larger molecules into smaller ones that can be broken down more easily during calcination. As a result, increasing pore volume allows for the achievement and confirmation of high porosity. In the current investigation, acid-leached TS silica gel was shown to have an average pore width of 1.647 nm, which was much smaller than that of other silica materials made from various biomass materials. As a result, mesoporous silica gel was discovered to have been acquired from acid-treated TSA. As shown in table 1, it is also evident that activation with HCl solution greatly increased the pore volume and surface area of the silica gel, which may have led to a reduction in pore diameter [51].

For the purpose of identifying the functional groups present in the sample, Fourier Transform Infrared (FTIR) spectroscopy was executed. The predetermined amount of TSA sample suspension was applied on a copper grid in the current study, and the grid was subsequently darkened to allow ethanol to evaporation. At room temperature, the FTIR spectra of the TSA sample was analyzed between 400 and 4000 cm⁻¹. Figure 2 displays the FT-IR data from the TSA samples that were calculated across calcination times (2, 3, 4, and 5 h). As a result, a band at 436 cm⁻¹ was discovered due to bending vibrations, which was consistent with the current work, and a band at 796 cm⁻¹ was seen due to Si–O–Si bond stretching. The Si–O–Si link's asymmetric vibration was the cause of a strong signal at 1060 cm⁻¹. The band between 3000 and 3500 cm⁻¹ was verified to be caused by the silanol group (Si–OH bonding). At 1600 cm⁻¹, a bending vibration caused by H-O-H was seen. The absence of a peak between 1700 and 2800 cm⁻¹ demonstrated that silica was the only organic compound that was produced [24, 45]. It implied that there was no NaCl present as well. It confirmed that the all alkali, alkaline earth metals, and other pollutants were successfully removed during HCl washing operations.

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The characteristics of the FT-IR absorption band at 1633 and 1645 cm⁻¹ wave numbers were brought about by the vibration of the O–H bond found in Si-OH silanol groups. The wave numbers 3338 and 3475 cm⁻¹ were caused by the O–H bond stretching vibration from the Si–OH silanol groups and the adsorbed water molecules on the SiO₂ surface. The broadband was caused by O–H bond bending vibration at wave numbers of 1633 and 1645 cm⁻¹ [7, 52]. A strong and noticeable absorption peak can be found at wave number 1072 cm⁻¹, which was brought on by the asymmetric stretching of the Si–O–Si (siloxane) bonding. At 795–800 cm⁻¹, the Si–O bending vibrations produced a strong peak. Si-O vibrations that were bent out of plane had a peak at 445–488 cm⁻¹ [24].

The resulting silica's amorphous nature was confirmed by the wide XRD array at approximately 22.5°, which was characteristically amorphous solid [7]. CuK was operated at 40 kV, 44 mA, and 2θ between 5 and 800 in the current investigation to acquire XRD patterns. Figure 3(a) presents XRD patterns of ashes obtained from TS. In this study, the sharp peak position was around 22.1° and 26.6° centered for the sample calcined at 900 °C [45]. Quartz and tridymite (SiO₂) were discovered in the TS at 21.1° and 26.6°, respectively, based on the XRD patterns. Because quartz is a crystalline form of silicon, the ash had a higher crystallinity. The TSA samples, which were proven to be crystalline, showed a strong sharpening band in the current research effort [21, 45].

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The XRD patterns of several silica gel samples obtained from TSA produced at varying calcination times are shown in figure 3(b). As seen in the picture, the extracted silica gel's XRD pattern at 2θ = 22.22° showed a robust and broad pattern, confirming its amorphous nature. These characteristics made them ideal for adsorption, catalysis, and other chemical processes. Sodium chloride, which was created during the precipitation of silica gel from hydrochloric acid, may be the cause of the small, abrupt, and powerful peaks at 2θ = 31.8°, 45.68°, 56.62°, and 75.24° that were seen in the silica gel at 900 °C [1, 36, 45]. The XRD pattern evident from the sol–gel procedure may eliminate practically all other ingredients and change the crystallin silica to amorphous silica gel. 'According to K Wahyuningsih and S Yuliani [53]', the silica produced from rice husks using extraction methods had XRD patterns with broad peaks at 2θ = 15°–30°, demonstrating the amorphous nature of the produced silica particles. For examine the structural properties of amorphous silica gel, which was prepared using sol–gel techniques by XRD analysis at a calcination time of 2, 3, and 5 h at a calcination temperature of 900 °C.

Equation (4) was used to calculate the silica gel yield for the effect of calcination time at a constant temperature. In this investigation, 10 grams of TSA were used to determine the synthesis of silica gel. The yield of silica gel recovered from the calcined biomass was calculated using equation (4) [35, 46].

$$\begin{eqnarray}&&{\rm{Silica}}\,{\rm{gel}}\,{\rm{yield}}=\displaystyle \frac{{m}_{SG}}{{m}_{TSA}}\,\ast \,100 \% \end{eqnarray} \tag{ 4 }$$

where, ${m}_{TSA}$ is the mass of ${\rm{TSA}}$ and ${m}_{SG}$ is mass of silica gel obtained.

By calculating the silica gel production using the previously mentioned equation (4), it is possible to determine the potential of TSA due to the synthesis of silica gel and the effects of the calcination time at a constant calcination temperature. The silica gel yield, which ranges from 78.1 to 90.0%, is shown in figure 4. When compared to earlier research reported using TS gathered from local farmers, the new precursors collected from DZRC have demonstrated a higher silica gel synthesis. Table 2 presents the silica content in various biomass in terms of mass percent. The percentages ranged from 49.5 to 84.5 [36] and 59.38 to 85.85% [35]. The maximum documented silica gel production for other feedstocks, including palm shell, was lower than the present precursor with the best silica gel yield 92% [54], corn cob 47% [19], bamboo Leaf 79.38% [49], sugarcane leaf 80.50% [55], wheat straw (7.00% [21], and rice straw 76.70% [56]. The highest yield from the TS was, however, only slightly higher than that of sugarcane bagasse and rice husk, both of which have yields greater than 95% [57] and 96% [13, 46], respectively. This demonstrates that one of the potential precursors for producing silica gel can be the TS. In this investigation, the average silica gel yield was reported for each calcination time that was performed in triplicate, and the values were 87.3, 78.1, 90.0, and 79.7% for calcination times ranging from 2, 3, 4, and 5 h at 900 °C, respectively. It is clear from the findings that TS has the potential to be a viable and promising bio-precursor for silica gel.

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The SEM images with different magnifications showed the physical morphology of the silica gel's surface [figures 5(a) and (b)]. The figure revealed that the silica gel had a rough and sticky surface due to the presence of clusters of silica particles, each with a diameter of 5 μm. The surface also exhibited variations in size, smoothness, and color, with some regions being larger, smoother, and darker than others. These characteristics were affected by the alkaline treatment and the manual grinding of the silica gel. The alkaline treatment induced the formation of particle aggregates, while the grinding led to an uneven distribution. One of the recent advances in the field of biomass utilization is the production of amorphous silica from wheat straw, which has various potential applications in industry and agriculture. As stated by S P Ramasamy, D Veeraswamy [21] presents a comprehensive study on the method development and characterization of amorphous silica obtained from wheat straw by hydrothermal treatment. They use SEM to analyze the morphology, size, and distribution of the silica particles, as well as to investigate the effect of different parameters such as temperature, time, and acid concentration on the yield and quality of the silica. They also compare the properties of the silica derived from wheat straw with those of commercial silica and other sources of biomass-derived silica. Their results show that wheat straw is a capable raw material for producing high-purity amorphous silica with a spherical shape and a narrow size range, which can be tailored by adjusting the hydrothermal conditions [21].

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The SEM images of the biosilica gel samples calcined at different calcination time and constant calcination temperatures are shown in figures 5(a) and (b). It can be observed that the silica gel samples exhibited more porosity and cavities when they were calcined for 2 and 5 h at 900 °C. This indicates that the organic matter in the teff straw was decomposed and removed during the calcination process, leaving behind a porous structure of silica. The biosilica gel samples remained amorphous in the calcination time range of 2 to 5 h, as confirmed by the SEM analysis, which is consistent with the XRD results shown in figure 3 [58].

The silica gel samples produced from TS were examined using energy dispersive x-rays (EDX). The spectra obtained are shown in figure 6. From the results, it was discovered that the TS biomass burnt fully at 900 °C with calcined times of 2, 3, 4, and 5 h because high-temperature calcination produced ash particles with unique aggregation and agglomeration. It was discovered that the samples contained oxygen, silica, sodium, and chlorine. However, traces of the copper, rubidium, tantalum, hafnium, and titanium in the mineral were also present in the silica gels. The elemental composition from the EDS investigation was examined, and the results are displayed in table 3. There were just two distinct, strong Si and O peaks, and silica has been identified as the primary component. This is followed by 80.57, 72.59, and 56.91%, which together account for approximately 99.77% of the entire element. calcination temperature of 900 °C continuously for 2, 4, and 5 h, respectively, as shown in table 1, and the associated extra pollutants were very minimal. Na and other metals were added to the acid-leached TS after the resultant biosilica gel was confirmed to be 99.77% pure. It proved that the used HCl leaching phase removed the other metals well. During the pretreatment stage, the metallic oxides of TS react with HCl to create soluble chloride salts that were leached by the acid [45]. The results were supported by A B Bageru and V Srivastava [41] who worked with TS stated that the TS had been burned at 900 °C while being treated with HCl, generating biosilica samples with corresponding efficiencies of 91.82%. 'According to S P Ramasamy, D Veeraswamy [21]', In addition, the EDX spectrum revealed a higher silica concentration of 70.1% and a low potassium level of 9.96%.

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The SEM-EDS spectrum revealed the presence of some imperfections in the precipitated silica gel, such as small cracks and pores presented in table 3. The analysis also showed that the precipitated silica gel had a low level of contamination from other elements, such as Cl, Ti, Cu, and Na, which could affect its properties and applications. The teff straw, which was used as the raw material for the synthesis of the precipitated silica gel, contained some elements that were not detected in the final product, such as K and Al. These elements might have been removed during the extraction and purification processes [59, 60]. A previous study by P S Utama, R Yamsaengsung [61] investigated the fly ash generated from the palm oil mill and the precipitation process that occurred. They hypothesized that CO₂ would react with the silica extract and form a carbonate compound containing the carbon element in the carbonate form. They confirmed the presence of these components in the silica precipitate using various analytical methods [61].