1. Introduction
Based on the main notions of the circular economy and the bioeconomy [
1,
2], the concept of using agricultural waste is discussed. The common objective is to minimize the generation of waste from economic or urban activities related to agriculture. The circular economy turns out to be attractive for understanding the sustainable challenges we face in terms of social, economic and environmental aspects. However, in the case of bio-waste, taking advantage of the immense amount of agricultural waste generated, the processes could become a complex and difficult operation [
3]. This is especially true for the development of new materials from citrus fruit bio-waste.
Citrus fruits are believed to have originated from the warm southern slopes of the Himalayas in north-eastern India and northern Myanmar [
4]. At present, citrus crops are among the most cultivated fruits in the tropical and subtropical regions of the world. Regarding the bio-waste issue, approximately one-third of citrus fruits are utilized for processing, which produces around 50–60% of organic waste [
5].
With the growing importance that organic farming has acquired, both in developed and developing countries, there are no studies of these socio-economic realities [
6]. Organic agriculture [
7,
8] has remained a haven against the invasion of agrochemicals and industrialization in the food supply chain. It represents less than 1% of global agriculture, specifically reported as 0.98% by Willer and Lernoud [
9]. On the contrary, agriculture is the fastest-growing food sector in the world [
10]. South America has a strong presence in the agriculture sector: Argentina (3,191,255 ha), Uruguay (930,965 ha) and Brazil (705,233 ha) [
9,
10,
11]. Approximately 40% of the oranges produced globally are used in the production of different commercial products [
12]. Due to processing, large amounts of waste products (bio-waste), such as peels from the juice industry, are generated and accumulated [
13].
Considering this waste accumulation, different investigations propose the use of waste and bio-waste in order to obtain new materials, especially in sol-gel synthesis to obtain silica-based materials [
13,
14,
15,
16,
17,
18,
19]. The sol-gel process can be described as the formation of an oxidic network through the polycondensation reaction of a molecular precursor in a liquid. The term sol refers to a dispersion of colloidal particles in a liquid medium where the stability of the colloids is due to both Brownian motion and small particle size. The term gel refers to a three-dimensional network of a solid phase interwoven with an entrapped and immobilized continuous liquid phase [
18,
19,
20]. In our previous research, the sol-gel method was used as a versatile synthesis route characterized by the low temperatures at which inorganic materials and organic/inorganic hybrids were synthesized [
21,
22].
In this research, two main issues were addressed at the same time: the use of citric bio-waste and the obtention of materials using the sol-gel technique—looking for their use in heterogeneous catalysis as a bi-functional support system. Silica has been identified as an ideal support material for its strong hydrophilicity, acknowledged biocompatibility, shape and chemistry surface [
23]. Two different strategies have been explored to combine bio-waste with the synthesis of silica-based materials. The first step was to obtain silica through acidic or alkaline hydrolysis by using acetic, citric, hydrochloric acids and an alkali (ammonium hydroxide). In the second step, lemon or orange peels were added to the obtained mixtures. The objective of this mixed synthesis was to find the influence of organic bio-waste on acids/alkalis, to partially replace them with organic orange or lemon peels, which could presumable provide citric acid, among other compounds. For this, different amounts of bio-waste were used in the various syntheses [
24,
25].
As a precursor of silica, tetraethyl orthosilicate (TEOS) was used, and its hydrolysis and condensation were studied, using absolute ethanol as a green solvent due to its renewability [
26,
27]. This solvent plays an important role in the synthesis of silica-based materials, both in the case of the reaction with TEOS, once they act as a homogenizer agent for substances with different solubilities that participate in the condensation reaction, and with lemon and orange peels, extracting compounds of interest from them [
28,
29,
30]. All synthesized solids were characterized in-depth by integrating different physical-chemical techniques, such as scanning electron microscopy (SEM) and x-ray diffraction (XRD). The chemical compositions of the solids were investigated through Fourier transform infrared spectroscopy (FT₋IR) and the acidic properties by
n-butylamine potentiometric titration.
With the physicochemical properties discussed in this research, silica-based materials synthesized with bio-residues could have the potential to be used as supports for heterogeneous reactions (whose analysis is reserved for future research).
2. Materials and Methods
The starting point to specifying the proposed objectives includes having the necessary material and carrying out an adequate design of the experiments. For the design of these steps, two levels of complexity were selected, when required. This means that some of the operating variables remain fixed. Thus, reliable results can be obtained with the fewest possible experiences [
21,
22].
Before starting with the sol-gel synthesis, the oranges and lemons were peeled (20 of each of them), and the peels were cut into small pieces and put on the stove for 3 days, at 70 °C. Once calcined, the peels become brittle, and it is easier to grind them to a small size, so they were ground in a mortar (
Scheme 1). This size allows uniformity between the liquid gel and ground peels mixture. It should be noted that in 2020 a doctoral thesis was carried out with citrus peels without calcining (lemon, orange and mandarin) to obtain silica-based materials [
31].
The sol-gel synthesis was carried out in a chamber with a nitrogen-controlled atmosphere at room temperature. First, a portion of the solvent, absolute ethanol (CH3CH2OH, Carlo Erba), and the corresponding catalyst for acid hydrolysis were placed into a beaker: acetic acid glacial (CH₃COOH, HPLC grade, J. T. Baker), citric acid (C6H8O7 anhydrous for synthesis, Merck) or hydrochloric acid (HCl, 37%, Merck). The same was done in the alkaline hydrolysis using ammonium hydroxide (NH4OH, Anedra analytical reagent). Then, the TEOS (Si(OC2H5)4, Aldrich 98%) was added, followed by the last portion of solvent and distilled water, respectively.
Each mixture, outside the nitrogen-controlled atmosphere, was placed on a magnetic stirrer at 500 rpm; the respective amounts of orange or lemon peels were added, maintaining stirring for 2 h. The mixture was left for 3 weeks at room temperature; then, the gel formed was dried at 100 °C for 1 h. The details of each synthesis are shown in
Table 1,
Table 2,
Table 3 and
Table 4 (considering the total volume of ethanol added). Each table details the ratios of the compounds and the amounts used of lemon and orange peels (25 and 50%) in relation to the pure silica mass—considering as 100% the 5 g of pure silica obtained with fixed volumes of 17 mL of TEOS and 21.8 mL and, as appropriate, 5 mL of CH₃COOH; 0.43 mL of C
6H
8O
7; 7.5 mL of HCl (so proton concentration is the same in all cases) or 1.8 mL of NH
4OH. The amounts of all the reagents were varied in order to maintain the relationship between the TEOS precursor, the acid and alkaline catalysts and the solvent, with which the same silica-based solids should be generated, but adding citrus peels.
The evaluation of the acidic properties was carried out by potentiometric titration with n-butylamine using a Metrohm 794 Basic Titrino titrator (Switzerland) with a double-junction electrode. For that, 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 and keeping the stirring time constant (540 s) before adding the first drop and a waiting time of the drop of 60 s, while stirring constantly. FT₋IR spectrum was obtained using Bruker IFS 66 equipment (Germany). Pellets of the sample in KBr, at room temperature, were measured in a range between 500 and 4000 cm−1. SEM was applied to obtain different micrographs of the solids using the JEOL equipment, JSM-6390LV (Japan), with a voltage of 20 kV. Samples were supported on graphite and metalized with a sputtered gold film. Finally, XRD patterns were recorded by means of a PANalytical X’Pert Pro 3373/00 device with the following conditions: Cu Kα radiation (λ = 1.5417 Ǻ); Ni filter; 20 mA and 40 kV in the high voltage source; scanning angle (2θ) from 5° to 40°, scanning rate 2° (2θ)/min.
4. Conclusions
In this study, we synthesized new materials using the sol-gel method, a simple and fast technique that allowed the inclusion of bio-waste into silica matrices, proposing a first stage for the development of new silica-based supports that can be used in heterogeneous catalysis. The analyses of the influence of the orange and lemon peel percentages in the acidity and morphology of siliceous solids revealed promising results, suggesting that the bio-waste used can provide acidity to partially replace the acid catalysts investigated in acid hydrolysis—since the values obtained using 25 and 50% of bio-waste are similar to those of pure silica—but it is not possible to replace them completely. In addition, the mixed materials also showed the ability to maintain the siliceous structure. This is an important step for future studies in order to replace, for example, strong inorganic acids with a renewable raw material (bio-waste), which has a lower cost and suitable morphology. Research based on sustainability aspects is increasingly important and necessary in the current model of production and consumption in which we live. The chemistry required for a circular model will materialize only through a new attitude toward research, chemical engineering and the design of new processes and products.