## 3 Results and Discussion
### Breakthrough Experiments under Humid Conditions
\begin{table}
\begin{tabular}{c c c} \hline \hline  & \multicolumn{2}{c}{**Activated Carbon**} \\ \cline{2-3}  & \(AC\_VS\_600\) & \(AC\_WS\_500\_P\) \\ \hline Mass of adsorbent (g) & 15.1 & 27.8 \\ Absolute pressure in adsorption steps (kPa) & \(118.8\pm 1.5\) & \(120.0\pm 0.9\) \\ Absolute pressure in desorption steps (kPa) & \(34.6\pm 3.0\) & \(36.9\pm 3.5\) \\ cycle 1 & 0.542 & 0.486 \\ cycle 2 & 0.447 & 0.393 \\ cycle 3 & 0.435 & 0.412 \\ cycle 4 & 0.487 & 0.391 \\ \(q_{\text{CO2}}\) (mmol g\({}^{-1}\)) & cycle 5 & 0.526 & 0.390 \\ cycle 6 & 0.526 & 0.385 \\ cycle 7 & 0.528 & 0.376 \\ cycle 8 & 0.498 & 0.363 \\ cycle 9 & 0.474 & 0.359 \\ cycle 10 & 0.533 & 0.371 \\ Percentage of total CO\({}_{2}\) uptake & & \(28.7\pm 1.6\) & \(42.8\pm 3.5\) \\ at breakthrough time & & \(28.7\pm 1.6\) & \(42.8\pm 3.5\) \\ (excluding first cycle) & & \(-\) \\ cycle 2 & 82.5 & 80.9 \\ cycle 3 & 80.2 & 84.8 \\ cycle 4 & 89.8 & 80.4 \\ cycle 5 & 97.0 & 80.2 \\ cycle 6 & 97.0 & 79.2 \\ cycle 7 & 97.4 & 77.4 \\ cycle 8 & 91.9 & 74.7 \\ cycle 9 & 87.4 & 73.9 \\ cycle 10 & 98.3 & 76.3 \\ Selectivity CO\({}_{2}\)/N\({}_{2}\) & & \(54.0\pm 4.1\) & \(40.9\pm 2.7\) \\ (excluding first cycle) & & \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Summary of results from the breakthrough experiments (humid conditions) conducted at 50 \({}^{\circ}\)C.  
With regard to the CO\({}_{2}\)-over-N\({}_{2}\) selectivity under humid conditions, similar (for AC_WS_500_P) or slightly higher (for AC_VS_600) values than those measured under dry conditions are reported in Table 7. A little impact of water vapor on the CO\({}_{2}\) purity was also observed by [PERSON] et al., [16] for a coconut shell-derived activated carbon using a wet flue gas containing 4.6 vol. % H\({}_{2}\)O. In a more recent  
Figure 6: CO\({}_{2}\) breakthrough curves for ten consecutive cycles (adsorption temperature = 50 \({}^{\circ}\)C) from a wet mixture of N\({}_{2}\) and CO\({}_{2}\) (with a CO\({}_{2}\) composition of 13.75–14.25 vol. %; relative humidity (RH) = 100%) using the AC_VS_600 (**a**) and AC_WS_500_P (**b**) materials. study, focused on simulating a VSA process using activated carbon, [PERSON] and [PERSON] [27] also reported that the CO\({}_{2}\) purity was almost constant regardless of the relative humidity. Although the promising results discussed above, the most negative impact of moisture was on the percentages of used bed, which were significantly lower than those measured under dry conditions, especially for the AC_VS_600 adsorbent (only 28.7% in average, as reported in Table 7). This loss of efficiency could be attributed to the fact that the presence of adsorbed water might substantially reduce the diffusion rate of CO\({}_{2}\) into ultra-micropores during the adsorption step. Better efficiencies were obtained for the AC_WS_500_P adsorbent, showing an average percentage of used bed of 42.8%. This finding suggests that the rate of water adsorption on this material could be lower and a relatively high number of adsorption sites could be available for CO\({}_{2}\) during the initial stage of the adsorption process. As stated by [PERSON] et al., in an excellent review [28], water adsorption on carbonaceous materials is a really complex process, which can be affected by the pore size distribution and the pore connectivity as well as the type of functional groups and their location and distribution on the surface. According to the mechanism firstly suggested by [PERSON] and [PERSON] [29], water adsorption on activated carbon first takes place at the polar and oxygen-containing functional groups (OFGs) on the surface. Then, the water molecules can form clusters, which can grow and coalesce to fill the pores, in a greater or lower extent depending on the pore structure. Several previous studies revealed that the carboxyl group possessed the highest affinity to water molecules [30; 31; 32], due to both the accessibility of the carboxylic group to water molecules and the ability of this functional group to form hydrogen bonds.