An adsorption gel was prepared from microalgal waste after extracting biodiesel oil by a simple chemical treatment of crosslinking using concentrated sulfuric acid. The adsorbent exhibited notably high selectivity and adsorption capacity towards Cs+ over Na+ from aqueous solutions, within the pH range of slightly acidic to neutral. The adsorption followed Langmuir isotherm and the maximum adsorption capacity of the gel for Cs+ calculated from Langmuir model was found to be 1.36 mol kg(-1). Trace concentration of Cs+ ions present in aqueous streams was successfully separated from Na+ ions using a column packed with the adsorbent at pH 6.5. The adsorption capacity of the gel towards Cs+ in column operation was 0.13 mol kg-1. Although the adsorbed Cs+ ions were easily eluted using 1 M hydrochloric acid solution, simple incineration is proposed as an alternative for the treatment of adsorbent loaded with radioactive Cs+ ions due to the combustible characteristics of this adsorbent. (C) 2014 Elsevier B.V. All rights reserved.