## 4 Discussion
### Application of Biochar Produced by Parabolic Dish Collector  
Biochar is widely used for wastewater treatment and soil amendment strategies by recycling organic waste. The ability of biochar to eliminate pollutants (organic and inorganic) from wastewater depends on its adsorption capacity, which is linked to biochar stability and surface physicochemical properties due to the increased hydrophobic nature with well-organized carbon layers [31]. It is reported that higher pyrolysis temperatures produce biochar with higher aromaticity, hydrophobicity, surface area, pore volume, ash content, and pH. In contrast, biochar produced at lower temperatures are rich in oxygen-containing functional groups (acidic), have higher molar ratios of H/C and O/C (low aromaticity), and lower pH [32]. The biochar with low pH is suitable for soil due to the increased availability of soil nutrients at low pH, i.e., acidic conditions [33]. Applying biochar as a soil amendment has variable effects on soil corresponding to the biochar physicochemical properties and soil type. Depending on the physicochemical parameters of the biochar presented here, biochar produced by the parabolic dish pyrolysis reactor may predict the efficiency of biochar amendment to modify soil properties, especially in alkaline soil.  
The pH levels of the obtained biochar were 5.60 and 6.88 for SB-1 and SB-2, respectively, which could decrease soil alkalinity. Applied low-pH biochar can increase N retention in soil by enhancing NH\({}_{4}\)\({}^{+}\) formation and reducing NH\({}_{3}\) emissions [3]. Biochar application could increase the soil organic matter percentage and subsequently, nitrogen and phosphorus availability [34]. [PERSON] et al. (2019) applied eggplant shoots biochar (BC, pH = 7.6) and low-pH composted biochar with farmyard manure (BC-FYM, pH = 7.4) and sulfur (BC-S, pH = 6.9) in alkaline soil planted with potatoes. They found that the treatment of BC-FYM increased the potato yields by 11.7% and 25.13% under both inorganic and organic fertilizers, respectively, and by 10.53% in the case of BC-S using organic fertilization [35].  
In contrast, original BC reduced the yield in the presence of mineral and organic fertilization by more than 6% and 10%, respectively. This result confirmed that the lower biochar pH reduces the soil pH, which might improve nutrient solubility and availability for plant growth in alkaline soil. The CEC values of 59.59 and 57.61 cmol kg\({}^{-1}\) for SB-1 and SB-2, respectively, were higher than SB-3 due to the presence of carboxyl functional groups primarily induced in acidic groups, confirmed by FTIR data [9]. It was demonstrated that the surface functional group could influence the capability of the biochar water-holding capacity [4]. In addition, it has been reported that biochar as a soil amendment could improve some soil physical properties, especially in coarse-textured and fine-textured soils, by increasing soil porosity, decreasing soil bulk density, and improving soil aggregation [36]. The complicated structure and pore system of the produced biochar could provide a protection arm for the soil microbes, such as fungi, bacteria, actinomycetes, and mycorrhizae [26].