## 1 Introduction
However, the reaction mechanism after CO\({}_{2}\) absorption is similar to that of amines, as follows:  
\[{}^{-}\text{OOC}-\text{R}-\text{NH}_{2}+\text{CO}_{2}(\text{g})\rightleftarrows {}^{-}\text{OOC}-\text{R}-\text{NHCOO}^{-}+\text{H}^{+} \tag{1}\]  
\[\text{CO}_{2}(\text{aq})+\text{OH}^{-}\rightleftarrows\text{HCO}_{3}^{-} \rightleftarrows\text{CO}_{3}^{2-}+\text{H}^{+} \tag{2}\]  
\[{}^{-}\text{OOC}-\text{R}-\text{NH}\text{COO}^{-}+\text{H}_{2}\text{O}_{2} \rightleftarrows\text{HCO}_{3}^{-}+\text{{}^{-}OOC}-\text{R}-\text{NH}_{2} \tag{3}\]  
\[{}^{-}\text{OOC}-\text{R}-\text{NH}_{2}+\text{H}^{+}\rightleftarrows{}^{-} \text{OOC}-\text{R}-\text{NH}_{3}{}^{+} \tag{4}\]  
\[\text{H}_{2}\text{O}\rightleftarrows\text{H}^{+}+\text{OH}^{-} \tag{5}\]  
In Equations (1)-(5), it can be found that the pH of the solution has an effect on the absorption and the distribution of the species. Increasing the pH means decreasing the [H\({}^{+}\)], which favors a forward reaction in Equation (1), and hence favors CO\({}_{2}\) absorption. On the other hand, it favors the formation of HCO\({}_{3}\)\({}^{-}\), \({}^{-}\text{OOC}-\text{R}-\text{NH}_{COO}^{-}\), and \({}^{-}\text{OOC}-\text{R}-\text{NH}_{2}\). Aqueous amino acid salt, which is similar to that of aqueous amino acid zwitterion, will be protonated or deprotonated by changing the pH, as shown in reaction [18], as follows:  
\[{}^{+}\text{NH}_{3}-\text{R}-\text{CO}_{2}\text{H}\overset{\text{PF}_{1}}{ \rightleftarrows}{}^{+}+\text{NH}_{3}-\text{R}-\text{CO}_{2}{}^{-}\overset{ \text{PF}_{2}}{\rightleftarrows}\text{NH}_{2}-\text{R}-\text{CO}_{2}{}^{-} \tag{6}\]  
It can be found that a low pH (pH < pK\({}_{1}\)) amino acid salt represents positive charges, HOOC \(-\text{R}-\text{NH}_{3}^{+}\). In the range of pK\({}_{1}\) and pK\({}_{2}\), the medium pH range becomes the zwitterion form, \({}^{-}\text{OOC}-\text{R}-\text{NH}_{3}^{+}\). At a low pH value, the reaction of amino acid with CO\({}_{2}\) is less. Thus, knowing how to control the pH, temperature, and flow rate in order to obtain the desired absorption rate, efficiency, and mass transfer coefficient becomes significant. This can be achieved by adding an amino acid salt solution into the scrubber, which can maintain the desired pH to a certain value, where CO\({}_{2}\) can be effectively captured. In addition, the selection of the scrubber is also significant, as it affects the performance of the CO\({}_{2}\) absorption. Here, the bubble-column has been used, as it shows a superior performance, such as a high absorption, high mass-transfer coefficient, simple structure, and easy operation, compared with other scrubbers [2]. However, some parameters, such as the gas-flow rate, liquid-flow rate, solution temperature, and the pH of the solution have been found to be key factors affecting the absorption of CO\({}_{2}\). Because of the importance of mass transfer in a bubble-column, some papers have studied the effects of the process parameters on the mass transfer coefficients [31; 32; 33; 34; 35; 36]. The reported values were in the range of 0.01-0.888, depending on the conditions. A comparison of their mass transfer coefficients is listed in Table 1. However, there are no available data in the literature regarding the absorption of CO\({}_{2}\) using a sodium glycinate solution. In addition, the effect of pH on the solution and operating range can be effectively explored using a bubble-column scrubber. Therefore, a study of the absorption and desorption of CO\({}_{2}\) using a sodium glycinate solution becomes significant.