Activated carbon (AC) was prepared from waste tea (WT) and the obtained product, WTAC, was modified with ethylenediamine (MWTAC). Unmodified and modified WTAC samples were characterized in terms of structural, morphological and chemical properties. Carbon dioxide (CO2) adsorptions from pure CO2 and swine farm biogas of WT, WTAC and MWTAC were investigated to determine the effect of activation and modification as well as the effect of methane in real swine farm biogas on the CO2 adsorption performances. The adsorption capacities of pure CO2 on the WT, WTAC and MWTAC were 15.39, 87.42 and 108.97 mg/g, respectively, while the adsorption capacities of 40% CO2 from swine farm biogas were 4.22, 60.64 and 78.98 mg/g, respectively, indicating that modified AC leads to a higher CO2 adsorption capacity. In addition, density functional theory (DFT) calculations revealed that the adsorption energies for graphene + CO2, unmodified AC + CO2 and modified AC + CO2 models were -7.53, -50.74 and -65.21 kJ/mol, respectively, indicating the modified AC model led to a higher adsorption than that of the unmodified AC and graphene models, owning to hydrogen bond and dispersion interactions between the CO2 molecule and more active atoms on the surface of the modified AC model. (C) 2019 Elsevier Ltd. All rights reserved.