*3.3. Soil Thermal Characterization*

The adoption of a technique such as TGA-DSC could alleviate the problem of soil decomposition and provide an accurate description of soil composition by comparing the different temperature intervals of the soil. The simultaneous (TGA-DSC) measurements for both the fine and complete fractions of the sampled soils are presented in Figure 5. The observed effects of TGA variation include three intervals; the first ranges from 0 to 105 ◦C and represents the loss of interstitial water from the samples intra-pores, the second interval refers to the pyrolyze/oxidation of OM under a maximum temperature of 550 ◦C, whereas the third interval represents the decomposition of CaCO<sup>3</sup> at a temperature exceeding 550 ◦C. Soil 2 was characterized by higher derivative peaks (600 ◦C and 700 ◦C) in its FF and CF; these results confirm the presence of CaCO<sup>3</sup> in this soil. This will be also confirmed by both XRD and XRF results (CaO and CaCO3), in addition to the soil's higher calcium content and pH. It is noteworthy that the higher mass losses are accompanied by the higher peak of derivative mass loss, thus highlighting the significant changes in weight (inflection points) and demonstrating the areas corresponding to the decomposition of the soil during the heating process. Moreover, the points corresponding to changes in heat flow were highlighted by higher derivative peaks. The TGA/DSC of the soil FF displays a clear peak when compared with the CF. The mass loss of the FF was clearly higher than the CF which is mainly due to higher water content, organic matter, and carbonates in the clayey fraction. This difference between the TGA/DSC curves provided information on the parameters influencing the soil retention capacity, such as the presence of fine particles, water-holding capacity, and porosity.

**Figure 5.** Thermal decomposition profiles of the R'mel soils: (**a–c**) represent the TGA/DSC curves for the complete fraction of Soils 1, 2, and 3, respectively. Images (**d–f**) represent the TGA/DSC curves for the fine fraction (<63 μm) of Soils 1, 2, and 3 respectively. **Figure 5.** Thermal decomposition profiles of the R'mel soils: (**a**–**c**) represent the TGA/DSC curves for the complete fraction of Soils 1, 2, and 3, respectively. Images (**d**–**f**) represent the TGA/DSC curves for the fine fraction (<63 µm) of Soils 1, 2, and 3 respectively.

### *3.4. Olive Pomace Biomass Slag (OPBS) Analysis 3.4. Olive Pomace Biomass Slag (OPBS) Analysis*

The composition of OPBS was characterized as major (Table 2). The OPBS has a very alkaline pH of 12.1. This higher pH is related to the presence of dissolved metals as basic metal salts, oxides, and carbonates formed during the combustion of biomass. Accordingly, OPBS could be used to increase the pH of acidic soils in the R'mel region. The OPBS represents a moisture content of 7.18% calculated as dry mass. This moisture content was much higher when compared to soil samples which did not exceed 1% of the total mass based on the generated TGA curves. These findings demonstrate that the pore The composition of OPBS was characterized as major (Table 2). The OPBS has a very alkaline pH of 12.1. This higher pH is related to the presence of dissolved metals as basic metal salts, oxides, and carbonates formed during the combustion of biomass. Accordingly, OPBS could be used to increase the pH of acidic soils in the R'mel region. The OPBS represents a moisture content of 7.18% calculated as dry mass. This moisture content was much higher when compared to soil samples which did not exceed 1% of the total mass based on the generated TGA curves. These findings demonstrate that the

pore spaces between OPBS particles could hold more interstitial water than the R'mel soils, implying that the application of OPBS to soil may boost soil water-retention capacity. During biomass combustion, the organic carbon present in the slag corresponds to the unburned fraction of the biomass. The OPBS contains 19.97% of total organic carbon (TOC) that has not been burnt. Batra et al. [55] investigated the presence of unburned carbon in bagasse fly ash sampled from bagasse cogeneration power plants in India and found more than 25% of unburned carbon. The same authors reported that unburned carbon resulted in disposal issues, provided challenges when employed in cement formulations, and would thus be better suited for alternative uses. In this context, there is a distinct possibility to apply the OPBS as a carbon-containing amendment to R'mel sandy soils recognized by their lower organic carbon as demonstrated by thermogravimetric analysis. In addition to the high carbon content, the major element compositions of OPBS showed significant amounts of macronutrients which decrease in the sequence of Ca > K > Mg > Fe > Na. The leaching of essential macronutrients is common in coarse-textured soils and reduces nutrient availability to plants. By comparing the macronutrient level in OPBS with soil, it is convincible to exploit the fertilizing capacity of this material to amend R'mel soils. In addition, many farmers in the R'mel region use lime in order to control soil acidity mainly resulting from irrigation and excessive nitrogen use. The cationic exchange capacity (CEC) plays an important role in adsorbing and releasing nutrients needed by plants, as well as assessing the potential harm of certain contaminants. As a result, the R'mel soils have shown quite low CEC. The higher level of macronutrients contained in OPBS such as calcium, potassium, and magnesium could be introduced to increase the soil CEC, and thus macronutrient availability to the plants. OPBS, on the other hand, has the potential to limit the excessive use of N fertilizers by not only giving essential nutrients but also preserving their availability in the soil. Motesharezadeh and collaborators [56] concluded that the use of lime in sandy soils increases the CEC and reduces the leaching of NO<sup>3</sup> − and K.

**Table 2.** pH, moisture content, unburned carbon, and major elements in OPBS.


The combustion of biomass could result in the accumulation of trace elements in ashes and slag residues. For example, Wang et al. 2014 [57] reported that woody biomass blended in the fuels could generate large amounts of As, Cd, Cu, Cr, Pb, and Zn in fly ash and slag. In this study, OPBS was analyzed to assess trace-element toxicity in order to insure a safe utilization of this residue in soil (Table 3). The heavy metal content in OPBS decreases in the order of Mn > Zn > Cr > Cu > Ni > Co > Mo > As > Pb > Cd. It can be seen that the levels of heavy metals such as, Cd, Co, Pb, and Mo were quite low in the OPBS samples, whereas Cu, Ni, Cr, Mn, and Zn occurred in background levels, and no element present any potential risk of contamination. Moreover, the toxicity of the slags is not intrinsically linked to their trace-element levels, but rather to their leaching. A previous similar study of four biomass slags from a fired power plant showing approximately the same composition of OPBS has demonstrated too low leaching amounts of trace elements [34]. The same authors have concluded that the biomass slags did not represent any risk of contamination related to their utilization. In another study, the fertilizer value of fly ash derived from burning bark and wood chip was investigated by Numesniemi et al. [58], and the findings revealed that the levels of hazardous elements (As, Cd, Cr, Cu, Hg, Pb, Ni, and Zn) were low. In addition, the comparison between the level of trace elements in the clayey fraction of the soil and OPBS revealed that the fine fraction contained higher trace elements than the OPBS; these results indicate that the use of OPBS will not affect the levels of trace elements in the soil. From another point of view, the OPBS, which has a similar composition to fly ashes, could represent a great solution for the immobilization of heavy metals in the soil. Indeed, several studies reported the efficiency of fly-ash addition to soil on trace-element immobilization [57–65].

**Table 3.** Trace element composition in OPBS.

