Although the amount of petroleum residues produced from oil refinery processes has increased, there has been limited research for the residues. In this work, we developed petroleum residue-activated carbon (PAC) by chemical modification with citric acid as a novel adsorbent The optimal concentration of citric acid for PAC was chosen as the 6 M (named PAC-CA6) and the adsorption behavior of Co(II) was investigated. The experimental data were analyzed using various isotherm and kinetic models. The maximum adsorption capacity of 12.50 mg/g was determined using the Langmuir model. The pseudo-second-order model was found to be the most suitable for describing the adsorption kinetics. PAC-CA6 was characterized by using various instrumental analyses including FE-SEM, FTIR, and XPS. The Boehm titration method indicated the presence of multiple oxygen groups in PAC-CA6. Conclusively, the mechanism of Co(II) adsorption onto PAC-CA6 involved electrostatic interactions between Co(II) ions and carboxylic groups present on the surface of PAC-CA6. From the result, the PAC-CA6 could be sufficiently used as a potential adsorbent for the removal of cobalt from aqueous solutions.