Concentration-polarization in nanofiltration of low concentration Cr(VI) aqueous solutions. Effect of operative conditions on retention

Abstract Nanofiltration of low concentration Cr(VI) aqueous solutions by using an AFC80 membrane from PCI has been shown to be feasible with high retentions that increase for higher pressures, temperatures and recirculation speeds. This makes nanofiltration a suitable tool to decrease Cr(VI) levels below those recommended by the world health organization. In some cases this would require a few consecutive NF steps. Nanofiltration of Cr(VI) has been modelled by using the Spiegler-Kedem theory modified to include concentration-polarization. This allowed us the simultaneous evaluation of the three characteristic parameters of the model: reflection coefficient σ , solute permeability P and mass transfer coefficient K m from rejection experiments. Both σ and P are shown to decrease with concentration and to reach a relatively high concentration plateau. Other methods habitually used to evaluate the mass transfer coefficient have also been compared showing, when applicable, similar results for K m .