Hydrous cerium oxide (HCO) nanoparticles were synthesized by a simple precipitation process, and their arsenic adsorption performances were investigated. Due to their high specific surface area (198 m(2)/g) and the presence of high affinity surface hydroxyl groups, HCO nanoparticles demonstrated exceptional adsorption properties in terms of adsorption capacity and kinetics on both As(III) and As(V). At neutral pH, the arsenic adsorption capacity of HCO reached over 170 mg/g for As(III) and 107 mg/g for As(V). Even at very low equilibrium arsenic concentrations, the amount of As(III) and As(V) adsorbed by HCO nanoparticles was still over 13 mg/g at 10 mu g/L and over 40 mg/g at 50 mu g/L, which are higher than the arsenic adsorption capacity for most commercial adsorbents. Over a wide pH range from 3 to 11, HCO nanoparticles could readily remove As(III) by adsorption, which was not observed previously on other arsenic adsorbents. Such exceptional arsenic adsorption properties by HCO nanoparticles were shown to derive from the strong inner-sphere complexion. (c) 2012 Published by Elsevier B.V.