Carbon nanotubes (CNTs) functionalized with magnetic nanoparticles are attractive for environmental remediation applications due to their high specific surface area conducive for adsorption of water contaminants and the possibility of recovering these nanohybrids after remediation using an external magnetic field. Most of existing methods for synthesizing magnetic iron oxide/CNTs (MIO-CNTs) composites are carried out in the liquid medium and are tedious, uneconomical, and environmentally unfriendly. Herein, we report a one-pot solid-phase route to synthesize MIO-CNTs composites based on pristine CNTs. MIO-CNTs possess a high specific surface area, good dispersibility, and desirable magnetic properties, making them promising as adsorbents for arsenic removal. The maximum arsenic adsorption capacities are 47.41 and 24.05 mg g(-1) for As(V) and As(III), respectively. These values are among the highest for carbon-based materials. Oxygen-containing groups on the surface of MIO-CNTs play a crucial role in arsenic adsorption. This work is very important for the practical applications of pristine CNTs containing catalyst nanoparticles without the need of purifications. (C) 2014 Elsevier Inc. All rights reserved.