| Passive membranes have been used for separations ranging from seawater desalination via reverse osmosis to the separation of particles with microfiltration membranes. However the attachment of macromolecules, with multiple functional sites, to microfiltration membranes allows for more selective separations. For these reasons, we have designed a novel membrane system, consisting of cellulose-based microfiltration membranes functionalized with polyamino acids (2,500--15,000 MW). Because of the high carboxyl content of the polyamino acids, these membranes have been shown to be extremely useful for the separation of heavy metals from aqueous solutions. The primary objective of this research was to establish the sorption mechanisms of functionalized microfiltration membranes and use these mechanisms to predict the rate behavior of metal transport through these membranes. Both cellulose acetate and pure cellulose were used as membrane support materials. Extensive experiments (pH 3--6) were conducted (under convective flow mode) with the derivatized membranes involving the heavy metals: lead, cadmium, nickel, copper, and selected mixtures with calcium in aqueous solutions. Metal sorption results were found to be a function of derivatization (aldehydes) density of membranes and degree of attachment of the polyfunctional groups, number of functional groups per chain, membrane surface area, and the type of metals to be sorbed. We have obtained metal sorption capacities as high as 1.5 g metal/g membrane. As opposed to homogeneous solution systems, the molar sorption capacities of the functional carboxyl sites are significantly enhanced in the membrane pores because of counterion condensation resulting partly from the extremely high charge densities in the membrane pores. This phenomenon was incorporated in a kinetic model for the prediction of sorption behavior. The model studied the effect of pore size, polyamino acid attachment density, pH, and metal type. Finally, in addition to functionalized membranes, negatively-charged nanofiltration membranes were studied to establish the role of non-aqueous solvents (alcohols) on flux and rejection behavior of ionized organic molecules. |
