*3.7. BET Characterization of the Soil Samples and Biomass Slag*

In order to investigate the surface characteristics of soil and OPBS, the BET surface area measurements were performed to provide information such as adsorption isotherms, specific surface area, pore volumes, and pore diameter. The collected gas adsorption/desorption isotherms of the samples are shown in Figure 7. The shape of isotherms corresponds to type IV which represents a mesoporous surface in which capillary condensation occurs. A hysteresis is generally observed between the adsorption and desorption curves. According to the gas adsorption isotherms, the OPBS has an adsorption volume of 33.05 (cm3/g STP) at the relative pressure (P/P<sup>0</sup> = 0.99), whereas soil samples have maximum adsorption capacities of 7.70, 7.34, and 6.80 (cm3/g STP) for S1, S2, and S3, respectively. This result demonstrates the high adsorption capacity of OPBS when compared with soil samples. The specific surface area represents the total area divided by the mass unit (g) and refers to the gas adsorption rate into the available pores in lowtemperature conditions. The average pore diameter shows that OPBS has a higher pore

diameter of 20.73 (nm) compared to soil samples. It is common for particles with smaller pores to have a higher specific surface area, but the surface area of a given particle is also determined by the number of pores in that particle, i.e., its porosity. Consequently, a particle can have very small pores, but only in a limited number, resulting in a small specific surface area as demonstrated for Soils 1, 2, and 3 (Table 5). All the samples were dominated by mesopores as shown in the pore diameter distribution (Figure 7). However, the OPBS adsorption in the function of pore volume was greater than in soil samples indicating the higher number and volume of mesopores in OPBS compared to soil samples. (nm) compared to soil samples. It is common for particles with smaller pores to have a higher specific surface area, but the surface area of a given particle is also determined by the number of pores in that particle, i.e., its porosity. Consequently, a particle can have very small pores, but only in a limited number, resulting in a small specific surface area as demonstrated for Soils 1, 2, and 3 (Table 5). All the samples were dominated by mesopores as shown in the pore diameter distribution (Figure 7). However, the OPBS adsorption in the function of pore volume was greater than in soil samples indicating the higher number and volume of mesopores in OPBS compared to soil samples.

In order to investigate the surface characteristics of soil and OPBS, the BET surface area measurements were performed to provide information such as adsorption isotherms, specific surface area, pore volumes, and pore diameter. The collected gas adsorption/desorption isotherms of the samples are shown in Figure 7. The shape of isotherms corresponds to type IV which represents a mesoporous surface in which capillary condensation occurs. A hysteresis is generally observed between the adsorption and desorption curves. According to the gas adsorption isotherms, the OPBS has an adsorption volume of 33.05 (cm3/g STP) at the relative pressure (P/P0 = 0.99), whereas soil samples have maximum adsorption capacities of 7.70, 7.34, and 6.80 (cm3/g STP) for S1, S2, and S3, respectively. This result demonstrates the high adsorption capacity of OPBS when compared with soil samples. The specific surface area represents the total area divided by the mass unit (g) and refers to the gas adsorption rate into the available pores in low-temperature conditions. The average pore diameter shows that OPBS has a higher pore diameter of 20.73

*C* **2023**, *9*, x FOR PEER REVIEW 16 of 22

*3.7. BET Characterization of the Soil Samples and Biomass Slag* 

**Figure 7.** N2 adsorption–desorption isotherms and pore size distribution diagrams of (**a**) Soil 1, (**b**) Soil 2, (**c**) Soil 3, and (**d**) OPBS. **Figure 7.** N<sup>2</sup> adsorption–desorption isotherms and pore size distribution diagrams of (**a**) Soil 1, (**b**) Soil 2, (**c**) Soil 3, and (**d**) OPBS.

**Table 5.** BET specific surface area (SSA) and pore volumes of soil samples.

