## 3 Results and Discussion  
### Variation of Species  
Figure 2 represents the evolution of the ion concentrations over the time when the packed column is operated. Two trends describe the hydroxide, carbonate, and bicarbonate ion concentrations before (Figure 2a) and after (Figure 2b) the gas-liquid contact inside the column. According to reaction R\({}_{5}\), the depletion of NaOH and the production of the carbonates take place. Regarding the consumption of the NaOH, a first look allows us to conclude that the instantaneous reaction between the CO\({}_{2}\) and the hydroxide occurs as soon as both phases are in contact and when pH is still higher than 10. The drastic differences between the inlet and outlet concentration in the first 20 min corroborates the high selectivity of the absorbent to the contaminant. This rapid exhaustion of the initial absorbent will strongly influence the mass transfer coefficient in later calculations. In parallel, the presence of the carbonate ions increases over time as a consequence of the reaction. From 20 to 60 min, NaOH is still present in the sum tank but is not any more at the outlet of the column because all the NaOH contacting the CO\({}_{2}\) is reacting and the alkaline solution is close to the saturation in terms of absorption. In this time frame, carbonate concentration remains almost constant (around 0.1 M, the maximum to be produced according to the stoichiometry 2:1) at the outlet of the column because carbonates are still producing but also being consumed by the reaction with the CO\({}_{2}\) (R\({}_{6}\)). Both reactions take place at the same time when the hydroxide concentration is quite low (10 < pH < 8). The total depletion of the 0.2 M of NaOH arrives in the following moments after 60 min of the process (Figure 2b) has passed, due to the fact that no more hydroxides are available at the outlet at this time and, since that moment, the profile corresponding to the inlet carbonate concentration, which was all the time below the outlet trend, is above up to the end of the process at 80 min. This can be explained because once the NaOH has totally disappeared (pH < 8), carbonates are the only species reacting with CO\({}_{2}\) producing bicarbonateates. This second reaction is seen where both bicarbonate profiles remain in zero for 40 min but slowly increase, first appearing at the outlet of the column (Figure 2b) and, finally, when the presence of bicarbonate is high enough to be detected by titration in the feed tank (Figure 2a). At the end of the experiment, only carbonates and bicarbonateates were in the solvent reservoir. Species concentrations collected in the column followed the same trend when the membrane contactor set up was evaluated. Figure 3 shows the evolution of hydroxides, carbonates and bicarbonates at the inlet and the outlet of the hollow fibers. Similar operating conditions were applied, except in the gas flow rate, being in this case 6.5 L min\({}^{-1}\) (33 L min\({}^{-1}\) in the packed column) due to the smaller dimension of the contactor. Inlet (Figure 3a) and outlet (Figure 3b) concentrations describe linear trends against the drastic absorption that occurs in the column. Here, NaOH depletion is slower due to the fact that less flue gas is coming into the contactor and, therefore, less amount of CO\({}_{2}\) reacts. Thus, the concentration of hydroxide remains higher (because the same initial volume was used in the sum tank) but the production of carbonate is also lower (stoichiometry is respected). Figure 3 shows that no bicarbonates appeared during the experiments because the NaOH concentration was high enough to absorb the CO\({}_{2}\) following the R\({}_{5}\) reaction, which did not allow the reaction with the carbonates (R\({}_{6}\)). Thus, the reservoir only contained sodium hydroxide and carbonates at the end of the experiment. Figure 3: Evolution of inlet (**a**) and outlet (**b**) species concentration as a function of time in the membrane contactor.