Ferromagnetic carbon-coated Fe nanoparticles (core size of 15 nm, saturated magnetization of M_s = 218 emu g⁻¹ and coercivity of H_c = 62 Oe), fabricated at a mild temperature, demonstrate a strong ability to effectively remove more than 95 wt% of Cr(VI) in waste water via carbon shell physical adsorption, which is much higher than the commercially available Fe NPs.