We investigate the electronic and structural properties of two activated carbon models interacting with a Ni2+ ion via theoretical calculations. Interaction distances, charge transfer, adsorption energy, electrostatic potential maps, and density of states are applied in this work to support which activated carbon model is more favorable for nickel removal from the environment. We show that the existence of a defect in the activated carbon molecular structure causes a considerable change in the properties of the system, including its nickel ion removal feature via the adsorption process. Display omitted •The adsorption mechanism between Ni2+ and activated carbon is investigated by the DFT method.•Calculations show Ni2+ adsorption is a spontaneous, endothermic reaction.•Defect taking an essential point regarding the reactivity of the activated carbon system.