We successfully and simultaneously prepared paramagnetic Fe(OH)(3) nanoplates and ferrimagnetic Fe3O4 nanoparticles with specific surface areas of 6.6 and 30.1 m(2)/g, respectively, via a one-pot hydrothermal process. This work represents the first reported synthesis of Fe(OH)(3) nanoplates using a facile hydrothermal process. The Fe(OH)(3) nanoplates are approximately several hundred to thousands of nanometres in width, and the Fe3O4 anoparticles are approximately 50 nm in size. These Fe(OH)(3) nanoplates and Fe3O4 nanoparticles were both used to adsorb arsenic ions. The arsenic adsorption capacity of the Fe(OH)(3) nanoplates (approximately 45 mg/g) was superior to that of other Fe(OH)(3) and metal hydroxide materials, and the arsenic adsorption capacity of the Fe3O4 nanoparticles (approximately 93 mg/g) was greater than that of other arsenic-ion adsorbents. Hence, the Fe(OH)(3) nanoplates and Fe3O4 nanoparticles developed in this work have significant potential for treating wastewater via the removal of arsenic ions. (C) 2014 Elsevier B.V. All rights reserved.