## 3 Results and Discussion
### Adsorption isotherms  
In light of the adsorption isotherms (for both CO\({}_{2}\) and N\({}_{2}\) at 25 and 50 \({}^{\circ}\)C) shown in Figure 3, the AC_WS_500 adsorbent appeared to be the best material in terms of CO\({}_{2}\) adsorption capacity at relatively low pressures. For instance, at an absolute pressure of 14 kPa, the highest CO\({}_{2}\) uptake at both temperatures was measured for this adsorbent (1.3 and 0.74 mmol g-1 at 25 and 50 \({}^{\circ}\)C, respectively; as reported in Table 4). This finding is consistent with the highest ultra-micropore volume (\(V_{ultra}\)) reported in Table 3 for this activated biochar (which also exhibited the lowest burn-off degree), since many studies have reported that the CO\({}_{2}\) uptake at absolute pressures of 5-15 kPa is mainly dependent on the availability of ultra-micropores [6; 7; 9; 12].  
Figure 3: Adsorption isotherms of CO\({}_{2}\) and N\({}_{2}\) on the activated biochars at 25 and 50 \({}^{\circ}\)C: AC_VS_600 (**a**), AC_WS_600_P (**b**), AC_WS_500 (**c**), and AC_WS_500_P (**d**).Black filled squares: CO\({}_{2}\) at 25 \({}^{\circ}\)C; black open circles: CO\({}_{2}\) at 50 \({}^{\circ}\)C; red filled squares: N\({}_{2}\) at 25 \({}^{\circ}\)C; red open circles: N\({}_{2}\) at 50 \({}^{\circ}\)C.  
Figure 2: Adsorption isotherms of N\({}_{2}\) at -196 \({}^{\circ}\)C (**a**) and CO\({}_{2}\) at 0 \({}^{\circ}\)C (**b**) on the activated biochars (see Table 1 for sample designation).  
The CO2 adsorption capacities at low pressure (14 kPa) reported in Table 4 for the activated biochars can be considered as more than acceptable considering the relatively simple activation process (without the use of corrosive and/or costly reagents). At the more realistic temperature of 50 \({}^{\circ}\)C, the CO2 uptakes at equilibrium (0.60-0.74 mmol g-1) were within the range of those reported in previous studies for physically activated carbons derived from olive stones and almond shells [13], as well as for KOH-activated lignin-derived brochars [11].  
Besides the CO2 uptakes measured for the activated biochars, Table 4 also lists their apparent selectivities at 25 and 50 \({}^{\circ}\)C, which were estimated according to [PERSON] et al., [11]:  
\[S_{app}=\frac{q^{*}_{CO2}pN2}{q^{*}_{N2}p_{CO2}} \tag{1}\]  
In Equation (1), \(q^{*}\) is the uptake of CO2 or N2 from the respective pure component adsorption isotherms at an absolute pressure equal to the partial pressure of each gas in a binary mixture (\(p_{CO2}\) = 14 kPa and \(p_{N2}\) = 86 kPa). This relatively simple approach, which can provide a reasonable estimate of the binary selectivity from single-component adsorption data [11], represents a good alternative to the most commonly used method, which is based on the ideal adsorption solution theory (IAST). The IAST-based method requires as inputs the fitted adsorption isotherms for a given model (e.g., Toth [11], Sips [13], and dual site Langmuir-Freundlich [23]). Limitations of such models to reproduce perfectly the pure component equilibrium data, in addition to possible deviations from ideality in the adsorbed phase, can result in unrealistic estimates of selectivity [13]. In fact, [PERSON] et al., [11] found that the IAST-predicted selectivities were notably lower than the estimates calculated according to the simplified approach given in Equation (1).  
The apparent CO2-over-N2 selectivities reported in Table 4 are considerably higher than those reported by [PERSON] et al., [11] (about 14 at 50 \({}^{\circ}\)C). It is interesting to note that apparent selectivity decreased with higher adsorption temperature, especially for wheat straw-derived carbons. This negative temperature dependence, which was also observed in previous studies [8; 11; 24; 25], could be related to the higher isosteric heat of adsorption and/or lower diffusion coefficient of CO2 with respect to N2.