A stable magnetic nanoparticle with a shell core structure of gamma-Fe(2)O(3)@Fe(3)O(4) was developed. Water soluble Polyethylenimine (PEI) was grafted to the nanoparticle to prepare a positive charged adsorbent, which was characterized by XRD, FTIR and SEM. The adsorbent was able to effectively remove anionic Cr(VI) in the pH range of 2-3 due to the large amount of protonated imine groups on its surface, and could be magnetically separated from liquid quickly. Adsorption equilibrium was reached within 30 min and independent of initial Cr(VI) concentration. The Cr(VI) maximum sorption capacity at a temperature range of 35-15 degrees C was obtained using the Langmuir adsorption isotherm. The calculated thermodynamic parameters (Delta G, Delta H, and Delta S) indicated that adsorption of Cr(VI) was spontaneous and exothermic in nature. Competition from coexisting ions (K(+), Na(+), Ca(2+), Cu(2+), Cl(-), and NO(3)(-)) was found insignificant. The adsorbent had satisfying acid-alkali stability and could be regenerated by 0.02 mol L(-1) NaOH solution. The results suggested the potential application of the PEI-modified magnetic nanoparticles in selective removal of Cr(VI) from wastewater. (C) 2011 Elsevier B.V. All rights reserved.