Novel and recyclable superparamagnetic adsorbent Fe3O4-O-CMK-3 was synthesized by in situ growth of nanometer sized magnetite particles (m-NPs) on the surface of mesoporous carbon at low temperature (70 degrees C) using ecologically-benign materials. Structural characterization by transmission electron microscopy (TEM) confirmed the formation of 20 nm thick oxidized mesoporous carbon layers around the magnetic NPs. Thermogravimetric analysis (TGA) results revealed dense carboxylic and phenolic groups on the surface of Fe3O4-O-CMK-3. Because of these abundant polar groups the Fe3O4-O-CMK-3 had stronger adsorption affinity towards Cs than did magnetic mesoporous carbons O-Fe-CMK-3 synthesized by impregnation, and Fe-O-CMK-3 synthesized by co-casting, even in the presence of high concentrations of competing cations (K+,Na+, Ca2+ and Sr2+). The Fe3O4-O-CMK-3 adsorbent reached a steady state quickly (<5 min) with maximum adsorption capacity of 205 mg g(-1) which is sufficiently higher than other magnetic adsorbents (usually lower than 110 mg g(-1)) reported in the literature. The synthesized nanostructure adsorbent could be retrieved in a few seconds using an external magnet and reused at least six times for contaminant removal. (C) 2016 Elsevier B.V. All rights reserved.