The effects of counter ion on CO2 capture performance of amino acid salt solutions for direct air capture applications

• K-based AA solutions had remarkably lower viscosity than Na and Li-based ones. • K-based AA solutions had much higher CO 2 absorption rate than Na and Li-based ones. • Proline had higher CO 2 absorption rate than sarcosine, glycine, lysine and alanine. • K-based solutions had higher CO 2 absorption capacity than Na and Li-based ones. • Precipitation occurred only in Li-based solutions when loaded with CO 2 . Direct CO 2 capture from the atmosphere has received growing attention driven by the imperative of achieving global net-zero emission targets. Amino acid salt solutions are promising candidates for liquid-based direct air capture processes. Utilised as a neutralized salt by reacting initially with hydroxides, it has been speculated that their performance may vary with the type of counter ion present in the solution. This work investigates the influence of counter ions, potassium, sodium, and lithium, of amino acid salt solutions on the physical properties, the CO 2 absorption capacities, and the CO 2 absorption kinetics under atmospheric CO 2 concentration levels. The potassium-containing amino acid solutions exhibited significantly lower viscosities and mildly elevated densities compared to sodium- and lithium-containing ones. At 25 °C, the potassium-prolinate solution demonstrated the highest viscosity (8.5 mPa⋅s), whereas K-glycinate exhibited the lowest viscosity (2.5 mPa⋅s). Additionally, potassium-based solutions consistently displayed the highest CO 2 mass transfer coefficients, followed by sodium- and lithium-containing ones. The CO 2 mass transfer coefficient was highest for potassium-prolinate (2.99 mmol m −2 ⋅s −1 ⋅kPa −1 ) with the lowest rate observed for potassium-β-alaninate (1.68 mmol m −2 ⋅s −1 ⋅kPa −1 ). Interestingly, the type of counter ion had minimal impact on the CO 2 absorption capacity, with potassium-based solutions exhibiting only a slightly elevated capacity compared to sodium- and lithium-based solutions. Specifically, potassium-lysinate displayed the highest CO 2 absorption capacity (0.7 mol CO 2 /mol amine), while potassium-β-alaninate showed the lowest (0.65 mol CO 2 /mol amine). It should be noted that all lithium-based solutions of all amino acids formed precipitates during CO 2 absorption. From a practical and operational perspective, these findings suggest that the potassium salt solution of amino acids would be the optimal choice for direct air capture applications due to their enhanced solubility and CO 2 mass transfer rates compared to the corresponding sodium and lithium counter ion salt solutions.