This study involved preparation of positively charged nanofiltration membranes by chemical modification of P84 copolyimide asymmetric membranes using branched polyethylenimine (PEI). Optimized membrane performance was shown to be 50.9 +/- 5.1% salt rejection to a 2.0 g/L NaCl solution with a flux of 1.2 +/- 0.1 m(3) m(-2) day(-1) at 13.8 bar and room temperature. The rejection sequence of CaCl2 > NaCl > Na2SO4 indicates a positive charge at the membrane surface, which may be attributed to amine groups remaining after the reaction between PEI and polyimide. Upon modification, the pores of the membranes were filled with PEI molecules so as to cause the membranes to exhibit nanofiltration properties. The PEI-modified polyimide membranes likely have a highly cross-linked structure which makes these membranes stable in various operating environments including high temperature (100 degrees C), organic solvents, and mild acid and base (2 <= pH <= 10). These membranes showed efficient removal of multivalent heavy metal ions (> 95%) and may potentially be used for treatment of industrial wastewater. (c) 2008 Elsevier B.V. All rights reserved.