A process of chemical activation with phosphoric acid from coconut shell in air to produce activated carbon was developed. The effects of various preparation parameters on the porous texture and surface characteristics were investigated by means of the nitrogen physisorption, Fourier transform‐infrared spectra (FT‐IR), and X‐ray photoelectron spectroscopy (XPS). The adsorption capacities of activated carbon for chromium, lithium, and cesium were also investigated. The surface modification of activated carbon using nitric acid was performed to enhance the adsorption capacities, and the relationship between the adsorption capacities and the surface functional groups was investigated. The oxygen‐containing functional groups on the surface of an activated carbon played a more important role than its textural structure in adsorption of metal ions, especially cesium ion.