Nanocrystalline iowaite, a Mg/Fe-based layered double hydroxide (LDH) intercalated with chloride, was synthesized to evaluate its performance for arsenic removal from water and to investigate the contributing dearsenication mechanisms. It is characterized by fast arsenic sorption rates and has a much higher arsenic uptake capacity than other LDHs that are commonly used for water dearsenication. The surface adsorption of the solution arsenic onto the iowaite samples and the anion exchange of the arsenic in solution with chloride, which is originally in the iowaite interlayers, are the primary mechanisms for the uptake of arsenic by iowaite. In addition to the Coulombic attraction between arsenate/arsenite and positively charged layers of iowaite, the inner-sphere complexation of arsenic with Fe (instead of Mg) in the iowaite layers is responsible for the formation of more stable and stronger arsenic bonds, as indicated by both XPS and EXAFS analyses. Specifically, bidentate-binuclear and monodentate-mononuclear As-Fe complexes were detected in the arsenate removal experiments, whereas bidentate-mononuclear, bidentate-binuclear, and monodentate-mononuclear As-Fe complexes were present for the arsenite-treated iowaite samples. This study indicates that Nano-iowaite is a promising material for the treatment of both industrial wastewaters with arsenic concentrations over several hundreds of mg/L and contaminated natural waters with a comparatively lower range of arsenic concentrations (typically from several hundreds of µg/L to several mg/L). For industrial-scale applications, Nano-iowaite must be synthesized as microspheres with a macro porous structure or as a homogeneous mixture with other chemically stable materials, such as quartz sand, so that the material has sufficient diffusion pathways for arsenic-bearing waters. In terms of water dearsenication, one limit of Nano-iowaite is that the final solution is usually alkaline (pH 10 - 11). However, this may be advantageous when Nano-iowaite is used to treat acidic arsenic-rich waters from, for example, acid geothermal discharge or acid mine drainage. By carefully designing the treatment conditions, the final solution pH could be within the neutral range, and arsenic could be efficiently removed from water. One problem possibly occurring during the treatment of acid waters by Nano-iowaite is that it may be not as stable as in alkaline solutions. A systematic test on the stability of Nano-iowaite at wider ranges of pH and arsenic concentration needs to be performed before it will be put into practice in acid water treatment.