3.3. Physicochemical Characterizations
- a.
Scanning electron microscopy coupled to EDX (SEM/EDX)
SEM observations showed a heterogeneous fibrous structure of fresh leaf surfaces, with water pockets of 44.65 µm diameter (
Figure 4A), which disappeared after drying them at 50 °C (
Figure 4B). The observation of the stem in a cross section indicated the presence of long channels with diameters varying between 22.88 to 33.29 µm (
Figure 5A). The longitudinal section of these channels showed a stratified fibrous structure, very homogeneous in thickness (
Figure 5B,C). Similar observations were previously reported, the images show the presence of homogeneous parenchyma cells and fibro-vascular bundles in Cardoon leaves [
6].
The results of the qualitative analysis by surface electron scattering (EDX) (
Table 4), revealed that the elements carbon and oxygen were dominant, which is consistent with the organic nature of these materials (i.e., carboxylic, lactonic, and phenolic compounds). The macro-elements (i.e., Magnesium, Potassium, Chlorine, Sodium, and Calcium) were present in relatively small quantities in both types of samples. Silica and Sulphur were not identified on the surface of stems. These results are similar to those obtained by previous studies, which showed that the average content of carbon was 41% and that of oxygen 49% [
6,
7,
8,
9,
10,
11,
12,
13,
14,
15].
- b.
X-ray diffraction (XRD)
The obtained diffractograms for both organs (leaves and stems) were similar (
Figure 6). They showed three diffraction peaks of slightly different intensities, which are conventionally encountered in cellulosic compounds. The peaks positioned around 2θ = 15.4°, 2θ = 21.5° and 2θ = 32°, correspond respectively to the diffractions of the (101), (002), and (040) planes of crystalline cellulose I.
The crystallinity index (CI) was calculated according to the method of Segal, ref. [
18] using Equation (5):
where:
The calculated crystallinity index is 10.24% for Leaves while it is about 13.22% for Stems (
Table 5).
- c.
Fourier Transform Infrared Spectroscopy (FTIR)
The infrared absorption spectra had similar appearances, with practically the same absorption bands with only slight difference in intensity (
Figure 7). In fact, a band was observed 3300 cm
−1, attributed to the hydroxyl group -OH of cellulosic and hemicellulosic molecules (
Table 6). The absorption bands at 2922 cm
−1, 2852 cm
−1, 1407 cm
−1, and 1367 cm
−1 indicated the presence of C-H bond of cellulose, whereas another band around 1730 cm
−1 was attributed to the carbonyl group C=O related to esters and/or carboxylic acids in hemicellulose and lignin. The band at 1600 cm
−1 confirmed the cellulosic nature of samples.
The absorption bands around 1250 and 1251 cm
−1 were characteristic for the vibration of C-O bonds. Glycose ring stretching was recognized through the visible bands around 1020 and 1024 cm
−1 and the low intensity bands at 815 and 817 cm
−1 were attributed to the C-H vibrations of the glycosidic ring of β-glycosidic bonds [
19]. The bands at 830 and 700 cm
−1 were explained by the out-of-plane deformation mode of the C-H bond within the aromatic rings.
The same absorption bands were detected in the study of Ouldmoumna, A. et al. on Cardoon leaves [
17].
Table 6.
Absorption band assignments of infrared spectra of Cardoon leaves and stems.
Table 6.
Absorption band assignments of infrared spectra of Cardoon leaves and stems.
Wavenumber (cm−1) | Assignment [19,20] |
---|
Leaves | Stems |
---|
3300 | 3300 | H-O (cellulose) |
2922 | 2922 | C-H (CH2) (cellulose/hemicellulose) |
2852 | 2852 | C-H (cellulose) |
1730 | 1730 | C=O (carboxylic acid /ester) hemicellulose |
1600 | 1600 | O-H (cellulose) |
1407 | 1407 | C-H (cellulose) |
1367 | 1367 | C-H (cellulose) |
1320 | 1315 | O-H (cellulose) |
1251 | 1250 | C-O (the ester) |
1020 | 1024 | C-O (cellulose/hemicellulose) |
815 | 817 | C-H (glycosidic cycle) |
770 | 775 | C-H (aromatic ring) |
- d.
Inductively coupled plasma atomic emission spectroscopy (ICP-AES)
The results of the elemental analysis (
Table 7) revealed that the leaves contained various elements, such as Sodium (Na), Calcium (Ca), Boron (B), Magnesium (Mg), Phosphorus (P), and Potassium (K), which dominated the vegetal mass with concentrations of 3.72 mg/g, 1.7 mg/g, 1.64 mg/g, 1.59 mg/g, 1.54 mg/g, and 1. 32 mg/g, respectively. The dominant elements in stems were Sodium (Na), Potassium (K), Calcium (Ca), Magnesium (Mg), Boron (B) and Phosphorus (P), with contents respectively of 2.7 mg/g, 2.03 mg/g, 1.07 mg/g, 0.91 mg/g, 0.64 mg/g, and 0.59 mg/g, along with other trace elements (i.e., Al, Fe, Sr, Zn, Mn, and Ti).
These results agree with those found by surface electron scattering (EDX). The study of Angelova, V. et al. showed that the mineral composition of Cardoon also contained Calcium (1.6%), Magnesium (1%), Potassium (0.68%), and Nitrogen (0.13%) [
21].
- e.
Thermogravimetric analysis (TGA/DTA)
The thermogram of leaves showed a mass loss of 86.49% in the temperature range 100–700 °C divided into three main steps (
Figure 8):
- -
The first step: of 45.3% occurs around 230 °C and was attributed to the departure of volatile matter.
- -
The second step: of 9.06% at 280 °C, was due to the degradation of the hemicelluloses [
22,
23].
- -
The third step: of 30.41% at 350 °C, is usually attributed to the decomposition of cellulose and lignin [
22,
23].
For stems, the total mass loss was 87.91% within similar temperatures range. Equally, there were observed three stages, the first at 240 °C with a loss of 47.33%, the second at 280 °C with a mass loss of 16.43%, and the third at 410 °C with a mass loss of 23.83% (
Figure 8).Damartzis et al. studied the thermal analysis (DTG curves) of stalks and leaves of Cardoon and noted that the pyrolysis occurred in the temperature range of 200 and 500 °C with two peaks, the former due the decomposition of hemicelluloses and the latter due to cellulose decomposition [
16]. The study of Ouldmoumna, A. et al. confirmed our findings, by showing that the first stage of carbonization occurs in the temperature range of 200–500 °C (70% loss) and is due to the groups lignin, hemicellulose and cellulose [
17].
The results showed that Cardoon stems and leaves consisted mainly in organic matter and had a basic character (pHpzc = 8.39) for stems and neutral for leaves (pHpzc = 7.35). The ash contents were 6.35% for leaves and 8.1% for stems. This type of vegetal waste contained 7% minerals, including Sodium (Na), Calcium (Ca), Boron (B), Magnesium (Mg), Phosphorus (P), and Potassium (K). The analysis by XRD and FTIR showed that Cardoon consists mainly of lignocellulosic compounds (i.e., cellulose, lignin, and hemicellulose), with an index of crystallinity for cellulose not exceeding 13%, indicating that this plant is rich in amorphous phase. The morphological analyses of Cardoon revealed a homogeneous fibrous and porous structure, both on the surface and in profile.
The results showed that Cardoon wastes have very interesting characteristics, making it a very valuable raw material in various economic domains with practical applications.
For example, the high content of ash indicates that Cardoon wastes could be used in the preparation of activated carbon. The findings of XRD and FTIR analyses encourage the extraction of cellulose, hemicellulose, and lignin from these wastes and their energetic capitalisation.