*2.6. Soil and Olive Pomace Biomass Slag (OPBS) Characterization*

Various characterization techniques were used in this study in order to compare the physical and chemical properties of soil samples and OPBS. The Brunauer–Emmett– Teller (BET) analysis was used to determine the adsorption characteristics such as N adsorption–desorption curves, specific surface area, and porosity of the soils and (OPBS) under N2 adsorption at 77 K using the Micromeritics Tristar II 3020 Surface Area Analyzer (Micromeritics Instr. Corps., Norcross, GA, USA). Scanning electron microscopy (SEM) was employed to determine the morphology of the soil samples and OPBS using a (SEM, Hitachi, Tokyo, Japan, S-4800). At the same time, an x-ray fluorescence spectrometer (XRF, PANalytical Axios FAST simultaneous WDXRF, Malvern PANalytical Ltd., Almelo, The Netherlands) was used to determine the mineral composition of the fine-soil fraction (<63 µm fraction) and OPBS. The crystalline phases of the soil's complete fraction and OPBS were determined by X-ray diffraction (XRD) using an X-ray diffractometer PANalytical

X'Pert Pro (Malvern PANalytical Ltd., Almelo, The Netherlands). Thermo-gravimetric and differential calorimetric scanning TGA/DCS analysis of the soil's fine and complete fractions was carried out using an SDT Q600 V20.9 Build 20 (TA Instruments, Newcastle, DE, USA). *C* **2023**, *9*, x FOR PEER REVIEW 6 of 22

**Figure 1.** Slag residue from olive pomace combustion (OPBS)*.*  **Figure 1.** Slag residue from olive pomace combustion (OPBS).

### *2.6. Soil and Olive Pomace Biomass Slag (OPBS) Characterization 2.7. Column Study*

Various characterization techniques were used in this study in order to compare the physical and chemical properties of soil samples and OPBS. The Brunauer–Emmett–Teller (BET) analysis was used to determine the adsorption characteristics such as N adsorption– desorption curves, specific surface area, and porosity of the soils and (OPBS) under N2 adsorption at 77 K using the Micromeritics Tristar II 3020 Surface Area Analyzer (Micromeritics Instr. Corps., Norcross, GA, USA). Scanning electron microscopy (SEM) was employed to determine the morphology of the soil samples and OPBS using a (SEM, Hitachi, Tokyo, Japan, S-4800). At the same time, an x-ray fluorescence spectrometer (XRF, PANalytical Axios FAST simultaneous WDXRF, Malvern PANalytical Ltd., Almelo, The Netherlands) was used to determine the mineral composition of the fine-soil fraction (<63 µm fraction) and OPBS. The crystalline phases of the soil's complete fraction and OPBS were determined by X-ray diffraction (XRD) using an X-ray diffractometer PANalytical X'Pert Pro (Malvern PANalytical Ltd., Almelo, The Netherlands). Thermo-gravimetric and differential calorimetric scanning TGA/DCS analysis of the soil's fine and complete fractions was carried out using an SDT Q600 V20.9 Build 20 (TA Instruments, Newcastle, DE, USA). *2.7. Column Study*  In order to assess the soil retention capacity of NO<sup>3</sup> − in the R'mel area, a study of nitrate leaching via vertical columns was carried out. A total of six columns were used in this study. The glass columns used for this study had a length of 35 cm and a diameter of 5 cm. The soil profiles were brought to the field conditions by controlling the apparent density of the soil by adding the necessary amounts of water to promote the vertical movement of solute. Generally, sandy soils are characterized by their high permeability allowing the effluent to infiltrate by gravity and do not necessarily need a force to be drained. All the experiments consisted of the addition of 100 mL constant daily volume of N-NO<sup>3</sup> −. In the first experiment, soils 1, 2, and 3 were filled in columns, namely C1, C2, and C3, conditioned with ultrapure water (T0), and an initial concentration of 51.60 mg/L N-NO<sup>3</sup> − was added on a daily basis for 17 days. In the second experiment, the columns labeled C4, C5, and C6 were filled with soils 1, 2, and 3, respectively. Before initiating the experiment, the three columns were flushed several times with ultrapure water in order to remove the excess nitrogen already contained in the samples. Afterward, the three columns were loaded with an increasing concentration of N-NO<sup>3</sup> − ranging from 0 to 102.83 mg/L. Figure 2 shows the initially loaded concentrations for experiments 1 and 2. The collected solutions were analyzed every day for TN and expressed as N-NO<sup>3</sup> −. Figure 3 shows the column setup for the nitrate-leaching experiments of the R'mel soils.

In order to assess the soil retention capacity of NO3− in the R'mel area, a study of

5 cm. The soil profiles were brought to the field conditions by controlling the apparent

umn setup for the nitrate-leaching experiments of the R'mel soils.

umn setup for the nitrate-leaching experiments of the R'mel soils.

*C* **2023**, *9*, x FOR PEER REVIEW 7 of 22

**Figure 2.** Daily applied concentration of nitrate during experiment 1 (C1, C2, and C3) and experiment 2 (C4, C5, and C6). **Figure 2.** Daily applied concentration of nitrate during experiment 1 (C1, C2, and C3) and experiment 2 (C4, C5, and C6). **Figure 2.** Daily applied concentration of nitrate during experiment 1 (C1, C2, and C3) and experiment 2 (C4, C5, and C6).

density of the soil by adding the necessary amounts of water to promote the vertical movement of solute. Generally, sandy soils are characterized by their high permeability allowing the effluent to infiltrate by gravity and do not necessarily need a force to be drained. All the experiments consisted of the addition of 100 mL constant daily volume of N-NO3−. In the first experiment, soils 1, 2, and 3 were filled in columns, namely C1, C2, and C3, conditioned with ultrapure water (T0), and an initial concentration of 51.60 mg/L N-NO3<sup>−</sup> was added on a daily basis for 17 days. In the second experiment, the columns labeled C4, C5, and C6 were filled with soils 1, 2, and 3, respectively. Before initiating the experiment, the three columns were flushed several times with ultrapure water in order to remove the excess nitrogen already contained in the samples. Afterward, the three columns were loaded with an increasing concentration of N-NO3<sup>−</sup> ranging from 0 to 102.83 mg/L. Figure 2 shows the initially loaded concentrations for experiments 1 and 2. The collected solutions were analyzed every day for TN and expressed as N-NO3−. Figure 3 shows the col-

density of the soil by adding the necessary amounts of water to promote the vertical movement of solute. Generally, sandy soils are characterized by their high permeability allowing the effluent to infiltrate by gravity and do not necessarily need a force to be drained. All the experiments consisted of the addition of 100 mL constant daily volume of N-NO3−. In the first experiment, soils 1, 2, and 3 were filled in columns, namely C1, C2, and C3, conditioned with ultrapure water (T0), and an initial concentration of 51.60 mg/L N-NO3<sup>−</sup> was added on a daily basis for 17 days. In the second experiment, the columns labeled C4, C5, and C6 were filled with soils 1, 2, and 3, respectively. Before initiating the experiment, the three columns were flushed several times with ultrapure water in order to remove the excess nitrogen already contained in the samples. Afterward, the three columns were loaded with an increasing concentration of N-NO3<sup>−</sup> ranging from 0 to 102.83 mg/L. Figure 2 shows the initially loaded concentrations for experiments 1 and 2. The collected solutions were analyzed every day for TN and expressed as N-NO3−. Figure 3 shows the col-

**Figure 3.** Column setup for the nitrate-leaching experiments of the R'mel soils. **Figure 3.** Column setup for the nitrate-leaching experiments of the R'mel soils. **Figure 3.** Column setup for the nitrate-leaching experiments of the R'mel soils.
