In this study, I improved the production efficiency of Fe3O4 nanoparticles by using a novel microwave synthesizer. Also, by modifying a poly-dopamine layer on the surface of nanoparticles to increase the surface area of nanoparticles, I succeeded in enlarging the Hg2+ ions adsorption capacity and releasing efficiency under hydrochloric acid of poly-dopamine-modified Fe3O4 nanoparticles and maintaining the high magnetic property of the material. The magnetic property of poly-dopamine-modified Fe3O4 nanoparticles makes it eco-friendly to reuse the material.