CO2 removal (or separation) is the key step for biogas upgrading. This research aims to investigate aqueous solutions of amino acid ionic liquids to achieve effective CO2 separation. In this work, three cholinium-based amino acid ionic liquids ([Cho][AA]s) (i.e. cholinium glycinate ([Cho][Gly]), cholinium alaninate aChol[Alal) and cholinium prolinate aChollProp) were synthesized and characterized. The effect of water on the viscosity, CO2 absorption loading (m and alpha) and apparent absorption rate constant was systematically studied. The CO2 absorption mechanism in the aqueous solution of [Cho][Gly] was explored by C-13 Nuclear Magnetic Resonance (NMR). The results demonstrate that the absorption loading (m) and viscosity increase with increasing IL concentration, while the apparent absorption rate constant decreases. The absorption loading decreased with increasing temperature. The CO2 absorption mechanism in the aqueous [Cho][Gly] solution started with the chemical reaction to form carbamate at low absorption loading (alpha), and followed by the hydrolysis of carbamate and CO2 hydration reaction at high absorption loading (alpha). Moreover, the aqueous solution with 5 wt % [Cho][Gly] showed the highest regeneration efficiency, and the absorption and regeneration performance of the aqueous solution of [Cho][Gly] was compared with commercial CO2 absorbents with promising results. (C) 2017 Published by Elsevier B.V.