## 3 Results and Discussion  
### Properties of the Used Adsorbents  
The results from proximate and ultimate analyses for the raw biomasses and activated biochars are listed in Table 2. Both raw biomass samples exhibited relatively high carbon contents (47.1 and 44.2 wt. % in dry and ash-free basis for VS and WS, respectively) and low ash contents (below 5 wt. %), indicating their potential to be used as precursors for activated carbons. After pyrolysis and further activation with CO\({}_{2}\), an expected higher carbon content (above 85 wt. % in daf basis), at the expense of both hydrogen and oxygen, was observed. However, the ash content of activated biochars was relatively high (from 9.00 to 16.5 wt. %). This can be explained by the loss of organic matter during the thermal treatments, leading to final mass yields of activated carbons (from the raw feedstock) within the range of 18-20 wt. % (dry basis), and the absence of any acid-leading post-treatment. Despite the fact that the presence of ash can decrease the surface area available for adsorption, biomass-derived ash constituents (especially K) could have positive effect on CO\({}_{2}\) adsorption, as reported by [PERSON] et al., [20]. At this point, it should be noted that the contents of potassium (as K\({}_{2}\)O) in the ashes from the biomass sources used here were found to be considerably high: 18.4 wt. % for VS [18] and 53.2 wt. % for WS [19]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & & & **Sample** & & \\ \hline  & _VS_ & _WS_ & _AC\_VS\_600_ & _AC\_VS\_600\_P_ & _AC\_WS\_500_ & _AC\_WS\_500\_P_ \\  & & & _Proximate (wt. \%)_ & & & \\ Ash & 4.43 & 4.23 & 9.00 & 13.9 & 15.6 & 16.5 \\ Moisture & 7.48 & 7.60 & 0.74 & 0.20 & 3.21 & 3.11 \\ Volatile matter & 75.7 & 74.8 & 13.2 & 10.2 & 12.9 & 12.2 \\ Fixed carbon & 12.4 & 13.4 & 77.1 & 75.5 & 68.3 & 68.2 \\  & & & _Ultimate (wt. \%, daf basis)_ & & & \\ C & 47.1 & 44.2 & 88.8 & 89.7 & 86.5 & 86.9 \\ H & 5.29 & 6.31 & 1.65 & 1.61 & 2.26 & 2.18 \\ N & 0.66 & 0.62 & 3.21 & 3.25 & 2.98 & 3.03 \\ O \({}^{1}\) & 47.0 & 48.9 & 6.33 & 5.44 & 8.26 & 7.89 \\ \hline \hline \end{tabular} \({}^{1}\) Calculated by difference.  
\end{table} 
Table 2: Proximate and ultimate analyses of biomass sources (vine shoots (VS) and wheat straw pellets (WS)) and activated biochars. On the other hand, Table 3 reports the key textural properties (i.e., apparent surface area and pore volumes at different pore size ranges) of activated biochars, which were deduced from the adsorption isotherms of both N\({}_{2}\) and CO\({}_{2}\) at \(-\)196 \({}^{\circ}\)C and 0 \({}^{\circ}\)C, respectively (see Figure 2). As expected, the N\({}_{2}\)-based specific surface areas (\(S_{BET}\)) reported in Table 3 were relatively low due to the high content of ash and the predominantly microporous narrow structure in biomass-derived chars activated with CO\({}_{2}\). From Figure 2a, it can be observed a certain contribution of narrow macropores (isotherms do not reach a plateau at high relative pressures). In addition, a slight increase in the degree of burn-off resulted in a slightly better microporosity development (i.e., higher \(V_{mic}\) values). This trend, which is consistent with previous studies [21; 22], could be explained by a progressive widening of the narrow micropores with the degree of burn-off. However, the mesopore volume did not seem to be correlated with the degree of burn-off, suggesting that the nature of the biomass feedstock and the pressure applied during the pyrolysis play a certain role in the resulting pore size distribution.