3D composite hydrogels composed of graphene sheets and Fe3O4 nanoparticles (G/Fe3O4) were prepared via a simple hydrothermal method. The composite has an interconnected 3D porous network, and the Fe3O4 nanoparticles with the size of similar to 13 nm are uniformly dispersed onto the graphene sheets. Due to the synergistic effects of the assembled graphene sheets and Fe3O4 nanoparticles, the resultant G/Fe3O4 exhibited efficient adsorption for organic pollutant using Rhodamine B as the adsorbate and electromagnetic wave absorption. The maximum removal ratio of Rhodamine B was 99.6%, and the G/Fe3O4 hydrogel demonstrated wide and strong wave absorption achieved in the frequency range of 2-20 GHz. Present work shows a way to design and prepare lightweight and high performance materials for both electromagnetic wave absorption and wastewater treatment based on 3D graphene and Fe3O4 nanomaterials. (C) Elsevier Ltd. All rights reserved.