3.1. Chemical Composition of Salty Food-Waste-Derived Biochar
shows the elementary analysis of the salty food waste derived biochar and Na and Cl contents. After flushing, the Cl content decreased to the Cl content in the STD sample, non-salted food waste sample. On the other hand, the Na content significantly increased after flushing compared to the Na content in the STD sample, non-salted food waste sample.
Carbon content increased with the pyrolysis temperature, and there was a difference in the content before and after flushing that resulted from a slight increase in the carbon content relative to the total weight since salt was discharged while flushing.
Hydrogen content decreased as the pyrolysis temperature increased, and there was a difference in the content before and after flushing that resulted from increased relative weight since salt was discharged while flushing, as with carbon.
Nitrogen content did not show any tendency to increase or decrease according to the pyrolysis temperature. However, the nitrogen content of the biochar having 1% salt content decreased after flushing, while the nitrogen contents of the chars having 3% and 5% salt content increased after flushing.
The higher the pyrolysis temperature and the higher the salt content, the lower the O/C ratio, which indicates the stability of biochar [26
]. There was a difference before and after flushing, but this phenomenon may have been caused by changes in salt content before and after flushing. When the O/C ratio is 0.2 or less, the half-life of biochar is estimated to be 1000 years or more. When the O/C ratio is 0.2–0.6, the half-life is estimated to be 100–1000 years [27
]. The H/C ratio decreased as the pyrolysis temperature increased. There was an obvious difference according to pyrolysis temperature, but there was no difference according to the salt content. The Lower values of H/C and O/C at higher pyrolysis temperatures indicate a stable biochar with a low content of O-based functional groups by demethylation and decarboxylation [28
A C/N ratio of 12:1 is the most favored for increasing the nitrogen pool plants can use, and a C/N ratio of 20:1 is used for soil conditioners, fertilizers and compost [29
]. As the pyrolysis temperature increases, the C/N ratio increases [30
]. The experimental result showed that the ratio increased and the ratio was 11–15, which is suitable for use in the soil when the temperature increased from 300 to 500 °C.
H/C, O/C, and C/N values are influenced by pyrolysis temperature, and the content of salt appears to have no noticeable effect on the chemical composition.
The CEC of salty food waste biochars varies depending on raw materials and pyrolysis conditions. Some studies have reported that the CEC value increases as the pyrolysis temperature increase from 300 to 500 °C in relation to the carboxyl groups on the biochar surface [31
]. Table 3
shows that as the pyrolysis temperature increased from 300 to 400 °C, the CEC value increased, peaked at 400 °C, and decreased again at 500 °C. CEC is determined by substitution sites, O-based functional groups such as –OH or –COOH that are made by decarboxylation and deformation as the pyrolysis temperature. Although, further study is needed to specify the required energy for deformation to contain high CEC, it is reliable trend similar to Wu et al.’s study result [32
]. These results may relate to specific surface area. Focusing on O-based functional groups, 300 °C should indicate highest CEC value. But in case of this, complex action between functional group and specific surface area, may be acted on the explanation of this phenomenon. O-based functional groups are decreased gradually as the pyrolysis temperature increases. Specific surface area is increased as the pyrolysis temperature increases [34
]. At 300 °C, there could be more O based functional groups but there is lower specific surface area. Overall, the surface area of pyrolyzed char at 300 °C, including O-based functional groups, may be smaller than at 400 °C. About this assumption, further study is needed.
Compared to the CEC value of the char unaffected by salt, the measured CEC value was higher before flushing and lower after flushing. This may have been the result of the phenomenon in which ionized Na+
is adsorbed on the biochar surface during the flushing. VAN ZWIETEN et al. [35
] showed that the application of biochar in a ferrosol significantly increased CEC of soil. In other words, the pyrolyzed salty food waste biochar is able to hold many cations which are nutritional content used by plants and helps the soil fertility improvement.
The extractable values of Na and K were much higher than those of other elements and significantly decreased after flushing. However, the extractable value of Ca increased after flushing. The value of Mg also increased in the case of 1% and 3% at 300 °C. This phenomenon will be discussed in relation to the result of Fourier transform infrared spectroscopy (FT-IR).
The CEC of soil is an important criterion for determining the ability of plants to retain the cations they use. Sandy soil, with its low organic content, has a CEC value of less than 3 cmolc/kg, but heavy clay soil or soil with a high organic content has a CEC value higher than 20 cmolc/kg [36
]. Every sample except post-flushing samples (300 °C, 1%; 500 °C, 3%) obtained values higher than 20 cmolc/kg; therefore, it is expected that they are able to be used as soil conditioners.
3.2. XRD Analysis of Salty Food-Waste-Derived Biochar
, Figure 3
, Figure 4
and Figure 5
show the XRD results of salty food waste chars before and after flushing according to the pyrolysis temperature, as well as those of NaCl. The crystalline form of NaCl has peaked at 32°, 45.5°, 56.5°, 66°, 75°, and 84° (2θ). When comparing these peaks with the XRD peaks of the salty food waste biochar, the graph before flushing clearly shows the NaCl peaks. On the other hand, the graph after flushing shows that the peaks of NaCl become blurred, and the peaks show a similar tendency to the peaks of standard food waste biochars containing no salt.
In other words, even after carbonization, NaCl remains in the form of crystals. However, after flushing, the remaining NaCl has washed away, and the peaks resemble those of the standard state.
The peak near 23° (2θ) corresponds to the diffuse graphite [37
]. As the pyrolysis temperature increases, the peak shifts from 20° to 25° (2θ). This means that cellulose crystallinity decreases and turbostratic crystallinity increases [39
As the pyrolysis temperature increases from 300 °C to 500 °C, the peak intensity near 28° (2θ) increases. This peak confirms the presence of calcite [40
], and the sharper the peak, the better the calcite crystallization.
3.3. FT-IR Analysis of Salty Food-Waste-Derived Biochar
, Figure 7
and Figure 8
, which show the FT-IR results for salty food waste biochars before and after flushing according to pyrolysis temperature, illustrate that the tendency varies significantly depending on the pyrolysis temperature.
Carboxylic (COOH) bonds have a peak of 1700 cm−1
and are associated with CEC capability [41
]. The peak near 1700 cm−1
that appeared clearly at 300 °C and 400 °C had almost disappeared at 500 °C. Decarboxylation occurred as the pyrolysis temperature increased, thereby decreasing the CEC value.
The peak near 1420–1440 cm−1
increased after flushing. This peak indicates a carboxylate anion [43
]. It seems that NaCl is ionized while flushing and Na ions bind to the carboxylate anion, thereby intensifying the peak. The CEC values of Na and K were lower after flushing because Na was already bound, decreasing the capacity to exchange monovalent cations.
The distinctive peak change at 2800–3000 cm−1
indicates a change in aliphatic C-H, and the peak at 3600–3200 cm−1
reveals O-H hydroxyl groups [41
]. As the pyrolysis temperature increased, the peak of the aliphatic C-H that was clear at 300 °C sharply decreased. The intensity of the peak was insignificant at 500 °C. The peak intensity of hydroxyl groups also gradually weakened. According to the elemental analysis, this demethylation phenomenon may have caused the decreased C/H ratio as the pyrolysis temperature increased.
The peak of aromatic C-H at 700–900 cm−1
] and the aromatic skeletal vibration peak at 1515–1590 cm−1
] became clearly visible and increased in intensity as the temperature increased from 300 °C to 500 °C. This confirms that aromatization occurred as the pyrolysis temperature and stability of biochar increased.
There were differences between the standard peak without any salt and the peak of pyrolyzed carbide containing salt. The peak shift from 550 cm−1
to 570 cm−1
was clearly shown as the pyrolysis temperature increased and salt content was higher. The peak near 550–600 cm−1
indicates C=O deformation of aromatic ketone [46
]. Porchelvi and Muthu [47
] investigated that the 552 cm−1
peak results from SO2
bound to deformed aromatic ketone and found that the 560 cm−1
peak appears due to cyano bound to a deformed aromatic ketone. In other words, a higher salt content induces more bonding of a specific compound to the aromatic ketone deformed in the pyrolysis process. More research is necessary for compounds that show a peak at about 570 cm−1
. In addition, the peak at 700 cm−1
appears in the standard without salt, while the peak is weak or does not appear at all in the presence of salt. This peak indicates the rocking vibration and binding of CH2
with asymmetric deformation of aromatic ketone [47
]. When salt is present, this bond is not induced. Thus, the salt contained in the food wastes affects the aromatic carbon in various biochar structures during the pyrolysis process.
Additionally, the peak at 520 cm−1
, which indicates the bonding of C-Cl [47
], is not shown regardless of the pyrolysis temperature and salt content. This result demonstrates that the NaCl does not affect the biochar structure in the discrete state as Na+
3.4. NMR Analysis of Salty Food Waste Derived Biochar
, Figure 10
and Figure 11
show the solid-state 13
C-NMR spectra. Although there is an apparent difference in the spectra according to the pyrolysis temperature, the peak shift by and influence of salt are negligible.
The peaks at 300 °C are shown at 16.5, 25–35, 64.5, 132, and 174.5 ppm, while broad peaks are seen from 128–132 ppm at 400 °C. At 500 °C, a weak peak is shown from 18–27 ppm and the distinct peak are seen from 110–150 ppm. The peak near 15 ppm represents CH3
carbon, while the peak near 30–65 ppm indicates CH2
]. The aromatic C peak is shown narrowly around 130 ppm and also appears broadly in the range of 90–160 ppm [49
]. The peak at 160–200 ppm means carboxylic or ketone C=O [41
At 300 °C, significant amounts of CH3 and CH2 are present and form the main peak, while aromatic C is insufficient until demethylation occurs. As the pyrolysis temperature increases to 400 °C, sufficient dehydration occurs, and aromatic C is formed. At 500 °C, clearly visible aromatic C indicates the stabilized biochar.