Abstract--Citrus bio-residues play an important role in obtaining different materials through the sol-gel method. This method is a friendly technique. These waste consists of organic compounds with a wide variety and chemical reactivity. In this way, it has been recognized that this kind of waste, generated mostly by the food industry, has an important potential to obtain silica to be used as a catalyst support. Provided that the principles of green chemistry and circular economy are present. This work aims to use the peel and juice of orange bio-residues, in acidic hydrolysis to obtain sol-gel silica. FT-IR spectra confirm silica formation according to the centered band at 1070 cm-1. In addition, SEM and TEM images of the synthesized materials were compared to a reference sample to contrast their morphology. Strictly speaking, orange bio-residues promote silica polymerization, due to the availability of complex organic compounds and their abundant acidic contribution.
Keywords-- citrus waste; SEM; TEM; FT-IR; circular economy
I. INTRODUCTION
The waste generated from citrus fruits have received close attention in recent years due to the large amount generated thereof, causing different problems for the soil, air and accumulation of them in different places. Internationally, these bio-residues are mostly produced by the juice industry generating millions of tons of said waste per year. However, it is currently destined for livestock feeding, recovery of essential oils, pectin extraction or to obtain bioethanol. But its high degree of acidity hinders adequate use demanding expensive neutralization processes (Boluda-Aguilar et al., 2010).
On the other hand, the use of these citrus fruits as bio-residues for the synthesis of new materials has not been studied enough. This work presents a series of criteria regarding the selection of this type of waste for the formulation of a siliceous material. It will be done through the sol-gel technique, using peels and orange juice. Here you have the advantage of using its metabolites of an acidic nature, of bio-residues as natural catalysts, without the need to depend on complex processes of optimization of the raw material.
In recent investigations (Venkata et al.,
2016), it has been shown that from orange waste, chemicals and other high
-importance products can be isolated. This faces a recent challenge so that
such exploitation can be low
Figure 1: Circular economy applied to sustainable reuse of orange waste (Clark, 2019).
cost, generate profits and increase the competitiveness of said activity. Which is part of the context of the circular economy (EC) (Fig. 1). Here all the mitigation and exploitation processes of biomass are incorporated into the gear of productive and industrial processes (Zuin, 2016). The objective of this work is based on the re-use of the shells and the orange juice to obtain silica, using those bio-residues as acid catalysts in the acid hydrolysis of the sol-gel technique to obtain silica.
II. METHODS
A. Material synthesis
From the orange waste (peels and juice), silica were synthesized using the silicate alkoxide polymerization by sol-gel technique (Cordero Castaño, 2020). Fresh orange shells were cut into squares and a certain time were ground. Subsequently, under a controlled N2 atmosphere different solutions were prepared using a 100 ml beaker containing 6 g of ground peel (NC) and orange juice (NZ), separately. 15 ml of tetraethyl orthosilicate (TEOS) were dissolved in a mixture of 37 ml of anhydrous ethyl alcohol with 5 ml of distilled water, maintaining a constant molar ratio, this mixture was added to the peels and juice orange previously prepared. The solutions obtained were stirred at 300 rpm and ambient temperature (20°C), by 45 min, using a magnetic agitator, until observing the formation of a defined gel.Once the gelified materials were obtained, they were cool using an agate mortar in order to turn them into a fine powder. Using a muffle,at 100 ºC, for 3h, the powders were calcined. A similar procedure was performed with glacial acetic acid, as a catalyst, to obtain a reference silica sample (SAA), thus replacing the use of bio-residues already described to make a comparison with the gel obtained from the peel and juice orange.
B. Characterization of solids
The textural properties of the solids, such as the specific surface area (SBET), the pore volume and pore size, were determined by adsorption/desorption in Micromeritics Accusorb 2100 equipment (USA), using N2 as absorbablegas. Before the measurement, each sample was degassed at 100°C for 12 h. The evaluation of the acidic properties of solids was carried out by potentiometric titration with n-butylamine, in a Metrohm 794 Basic Titrino titrator (Switzerland), with a double-junction electrode. 0.025 ml/min of an n-butylamine solution in acetonitrile (0.025 N) was added to 0.025 g of sample, previously suspended in 45 ml of acetonitrile, keeping the stirring time, before to add the first drop, constant (540 seg) and a waiting time of the drop of 60 seg, while stirring constantly. FT-IR (Fourier transform infrared spectroscopy) spectra were obtained using Bruker IFS 66 equipment (Germany) and pellets of the sample in KBr, at room temperature, measured in 400 and 4000 cm-1range. Scanning electron microscopy (SEM) was carried out to obtain different micrographs of the solids, in JEOL equipment, JSM-6390LV (Japan), using a voltage of 20 kV. Samples were supported on graphite and metallized with a sputtered gold film. TEM (Transmission electron microscopy) measurements were performed on a JEOL (model JEM 2011) instrument, operated at an accelerating voltage of 120 kV. Samples were prepared by their suspension in milli-Q water and placing 5 µl over carbon-coated copper grids, allowing the samples to dry in a desiccator for 16 h at room temperature.
III. RESULTS
The FT-IR spectra observed in the literature, for the determination of the functional groups of the sol-gel silica, indicate that thesynthesized xerogels have a wide variety of vibrations. This helps to describe the possible generated molecular structure model (Innocenzi, 2003).
Figure 2 observed the complete FT-IR spectra of the non-calcined and calcined SAA samples obtained by synthesis with TEOS as precursor and acetic acid as catalyst. When we analyze the spectra at lower values of wavenumbers, four well defined bands: at 460, 800, and 1080 and around 1200 cm-1 are observed (dotted lines) corresponding to vibrations of silicon-oxygen bonds. And can be classified by the type of movement of the oxygen atom respect to the silicon atoms in balancing, bending and stretching (Kirk, 1988). Those bands are typical of SiO2 and systematically appear in these kind of materials (Table 1).
When analyzing the SEM and TEM images, the SAA
microstructure can be seen with staggered and irregular shapes typical of those
materials obtained by acid catalysis (Fig. 3a and 3b). Regarding the
characterizations of
Figure 2. FT-IR of silica obtained with acetic acid
Table 1: FT-IR of silica with acetic acid (SAA).
|
Band |
Wavenumber (cm-1) |
Band Assignment |
Structural Unit |
|
1 |
3778 |
Si-OH |
Si-OH |
|
2 |
3443 |
ν OH |
O-H |
|
3 |
2926 |
νa (CH2) |
-CH2 |
|
4 |
1883 |
νβ Si-O |
Si-OH |
|
5 |
1640 |
δ H-O-H |
H-O-H |
|
6 |
1182 |
νS (Si-O-Si) |
Si-O-Si |
|
7 |
1079 |
νa (Si-O-Si) |
Si-O-Si |
|
8 |
945 |
νβ (Si-O) |
Si-OH |
|
9 |
797 |
νs (Si-O) |
Si-O-Si |
|
10 |
555 |
ν (Si-O) |
SiO2 |
|
11 |
452 |
δ (CCO) |
CH3-CH2-O |
δ: Deformation vibration, s: Symetric deformation vibration, ν: Stretching vibration, νas: antisimetric stretching vibration,
νs: symmetric stretching vibration, νβ: stretching vibration in the plane
the silica obtained using orange peels and orange juice, the potentiometric titration with n-butylamine is showed. The non-calcined NZ sample showed an Ei of 210 mV and the 130 mv calcined, while the non-calcined NC sample showed a 95 mv and calcined a value of 130 mv of Ei. It can be deduced that water or organic solutions of the mixture in wet gelation in calcination to reach these values are lost.
Figure 4 shows the titration curves for all samples synthesized with orange bio-residues. The curves of the SAA (Fig. 5), comparatively have an Ei of 130 mV, both un-calcined and calcined.
In Fig. 6 and Fig. 7, the SEM and NC and NZ micrographs are presented with the morphological similarities of SAA (Figs. 3a and b).
If the specific SBET surfaces of the SAA samples are compared with the respective NC, it can be seen that the un-calcined SAA sample has a value of 1043.6 m2/g that decreases to 68.4 m2/g for the NC and the calcined from the SAA low to 516.7 m2/g, while the NC increases to 75.6 m2/g. This change may be due to the aggregate of the fresh ground orange peel to the synthesis for obtaining silica is a mixture of more solid consistency, which could be interpreted as a loss percentage of the liquid phase of the mixture by reactions when incorporating the peel.
Figure 3a. SEM un-calcined of SAA (Mag. x1000)
Figure 3b. TEM un-calcined of SAA (Bar: 50 nm)
Figure 4. Potentiometric titration curves of silica samples: NZ and NC
The isotherms
obtained by adsorption-desorption of nitrogen disorders of the calcined and
non-calcined SAA sample is presented in Fig 8 and those obtained for the NZ and
NC samples in Fig 9 and Fig 10. In reference to the values obtained for SBET
and the isotherms of calcined
Figure 5. Potentiometric titration curves of silica SAA
(a)
Figure 6 a): SEM of un-calcined NC (Mag x500);
(b)
Figure 6 b): SEM of un-calcined NZ (Mag. x2500)
NZ (315.8 m2/g),
it presents a type IV isotherm with a behavior that could be assimilated to a
greater loss of liquid (orange juice and other organic soluble), which implies
an increase in the surface area with respect to NC. For its part, the
non-calcined NZ has an area of 274.7 m2/g.It is observed that the
SAA has a Type I isotherm, reversible type reversible Langmuir, a
characteristic of microporous solids with a long plateau, while those
corresponding to the NC sample is type III, convex with respect to the relative
pressure axis (p/po) throughout the pressure range, this indicative
of weak interactions between the adsorbate and adsorbent. Figure 11 shows the
spectra of the samples NZ (a) and NC (b) both uncalcined and calcined. The main
absorption peaks that are
Figure 7a). TEM of un-calcined NC (Mag.0.2 nm)
Figura 7b).TEM un-calcined NZ (Mag.0.2 nm)
(a)
(b)
Figure 8.N2 adsorption-desorption isotherms obtained for the samples: a)SAA un-calcined, b) calcined SAA
observed are
associated with the vibration modes of the Si-O-Si links: 1182 cm-1
(SAA) and 1185 cm-1 (signal 6) for NC. For the NZ spectrum, the main
absorption peaks that are observed are associated with the vibration modes of
the Si-O-Si links: at 1071 cm-1 (signal 7) and at
(a)
(b)
Figure 9. N2 adsorption-desorption isotherm obtained for the samples: a) non-calcined NZ; b) NZ calcined.
(a)
(b)
Figure 10. N2 adsorption-desorption isotherm obtained for the samples: a) non-calcined NC and b) calcined NC
1186 cm-1
(signal 6). It can be seen that together with the vibration mode of the syndic
Si-O-Si links of the silica,
(a)
(b)
Figure 11. FT-IRspectra of silica samples with orange juice(a) and peel (b)
other associated vibration modes appear that displace and widen the main band. These associated vibration modes may correspond to the impurities that are adsorbed to the surface of the sample.
FT-IR spectrum allows to observe a broad band around 3400 cm-1 assigned to the symmetric stretching of OH bonds corresponding to physically adsorbed water on the solid. The signal at 1640 cm-1 is attributed to bending OH vibrations of water molecules in the solid retained by hydrogen bonds (Wong et al., 2017). The presence of these two bands indicates that the xerogel silica contains a high amount of molecular water and hydroxyls. Finally, the band centered around 950 cm-1 is assigned to the vibration of Si-OH bonds (silanols) (Luna-López et al., 2009; Oh, 2010).
It is important to highlight that the time that takes silica formation with acetic acid is 18 days (SAA), while with the use of waste the results are: with orange peel 15 days and with orange juice 12 days.
In the case of orange juice, there is an improvement in the speed of acid hydrolysis for silica formation through the sol-gel technique.
V. CONCLUSIONS
The broader purpose of this study was to develop a work based on sustainability to solve current problems, proposing new directions to deal with the large volumes of organic waste produced by the food industry and, at the same time, obtain new materials of interest for different sectors of the industry. For this, new materials were synthesized using the sol-gel method, a simple and fast technique that allowed the inclusion of bio-waste in silica matrices.This work meets several points that describes the principles of green chemistry. It can be concluded, in general terms, that the proposed objectives have been achieved
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Received: August 2, 2022
Sent to Subject Editor: August 29, 2022
Accepted: September 19, 2022
Recommended by Subject Editor Laura Briand