Selenium (Se(IV) and Se(VI)) in wastewater is of particular concern because of its high mobility, ready bioaccumulation, and toxicities to organisms. In the present study, three types of MFe2O4 (M = Mn, Cu, Co) spinel ferrite nanoparticles (NPs) were prepared through a hydrothermal method. The removal of Se(IV) and Se(VI) by MFe2O4 NPs were investigated, and the adsorption mechanisms of Se(IV) and Se(VI) were discussed. The results showed that the adsorption capacity of Se(IV) and Se(VI) followed the order of CuFe2O4 > CoFe2O4 >> MnFe2O4. The adsorption capacity of Se(IV) was primarily determined by the hydroxyl group contents on the surface of MFe2O4, while the adsorption capacity of Se(VI) mainly depended on the surface charges of MFe2O4. Increasing pH led to a decrease of Se(IV) and Se(VI) adsorption. After Se(IV) adsorption, the pH(PZC) of MFe2O4 NPs shifted toward lower pH, indicating the formation of inner-sphere complexes. After Se(VI) adsorption, the surface charges of MFe2O4 NPs also decreased especially at pH lower than pH(PZC) but the pH(PZC) did not altered, suggesting the formation of outer-sphere Se(VI) complexes. Attenuated total reflection flourier transformed infrared spectroscopy also revealed that inner-sphere and outer-sphere complexes formed for Se(IV) and Se(VI) adsorption by MFe2O4, respectively. The results indicate that CuFe2O4 and CoFe2O4, compared with MnFe2O4, could be promising adsorbents for Se(IV) and Se(VI) removal because of their simple synthesis process, high saturation magnetization, and excellent performance. (C) 2015 Elsevier B.V. All rights reserved.