A novel cell separation and immobilization method for Cr (VI)-reduction under alkaline conditions was developed by using superparamagnetic Fe3O4 nanoparticles (NPs). The Fe3O4 NPs were synthesized by coprecipitation followed by modification with sodium citrate and polyethyleneimine (PEI). The surface-modified NPs were monodispersed and the particle size was about 15 nm with a saturation magnetization of 62.3 emu/g and an isoelectric point (pl) of 11.5 at room temperature. PEI-modified Fe3O4 NPs possess positive zeta potential at pH below 11.5. presumable because of the high density of amine groups in the long chains of PEI molecules on the surface. At initial pH 9.0, Pannonibacter phragmitetus LSSE-09 cells were immobilized by PEI-modified NPs via electrostatic attraction and then separated with an external magnetic field. Compared to free cells, the coated cells not only had the same Cr (VI)-reduction activity but could also be easily separated from reaction mixtures by magnetic force. In addition, the magnetically immobilized cells retained high specific Cr (VI)-reduction activity over six batch cycles. he results suggest that the magnetic cell separation technology has potential application for Cr (VI) detoxification in alkaline wastewater. (C) 2011 Elsevier B.V. All rights reserved.