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Novel composites of iron oxide encapsulated in macroporous silica with excellent arsenic adsorption performance have been successfully developed. Macroporous silica foams with large pore sizes of ≈100 nm and a high pore volume of 1.6 cm3 g−1 are chosen as the porous matrix. Electron tomography technique confirms that γ-Fe2O3 nanoparticles with an average particle size of ≈6 nm are spatially well-dispersed and anchored on the pore walls at even a high γ-Fe2O3 content of 34.8 wt%, rather than forming aggregates inside the pores or on the external surface. The open large-pore structure, high loading amount, and the non-aggregated nature of γ-Fe2O3 nanoparticles lead to increased adsorption sites and thus high adsorption capacities of both As (V) and As (III) without pre-treatment (248 and 320 mg g−1, respectively). Moreover, the composites can reduce the concentration of both As (V) and As (III) from 100 to 2 μg L−1. It is also demonstrated that the composites can be applied in a household drinking water treatment device, which can continuously treat 20 L of wastewater containing As (V) with the effluent concentration lower than the World Health Organization standard. [ABSTRACT FROM AUTHOR]