Passive membranes have been used for separations ranging from seawater desalination via reverse osmosis to the separation of particles with microfiltration membranes. However, active membranes, achieved by immobilization of macromolecules containing multiple functional sites to microfiltration membranes, allow for more selective separations. We have designed a novel membrane system consisting of cellulose-based microfiltration membranes functionalized with poly(amino acid)s (2500-10 000 MW). Because of the high carboxyl content of the poly(amino acid)s, these membranes have been shown to be extremely useful in the separation of heavy metals from aqueous solutions. The advantages of the membrane system, including high capacity and rapid sorption, have been demonstrated in this paper. A model has also been presented relating the effect of the pore size, poly(amino acid) attachment density, pH, and metal type to the initial metal sorption rate. St should be noted that, in contrast to homogeneous systems, the molar sorption capacities of the functional carboxyl sites are significantly enhanced in the membrane pores because of counterion condensation that results partly from the extremely high charge densities in the membrane pores. This phenomenon must also be incorporated in a kinetic model for the prediction of sorption behavior.