Identification of dielectric effects in nanofiltration of metallic salts

Transport of four metallic salts (CuCl2, ZnCl2, NiCl2 and CaCl2) through a polyamide nanofiltration (NF) membrane has been investigated experimentally from rejection rate and tangential streaming potential measurements. Rejection rates have been further analyzed by means of the steric, electric and dielectric exclusion (SEDE) homogeneous model with the effective dielectric constant of the solution inside pores as the single adjustable parameter. The volume charge density inside pores has been assessed from tangential streaming potential experiments by considering the effective dielectric constant inside pores instead of the bulk one. The conventional NF theory (i.e. disregarding dielectric effects) has been found to be unable to describe the experimental sequence of rejection rates (CuCl2 > ZnCl2 ≈ NiCl2 > CaCl2). It has been shown that the rejection rate sequence can be explained by the combination of the dielectric exclusion via image charges and the Donnan effect in the case of asymmetric salts with divalent counter-ions. The influence of the characteristic size of ions on the magnitude of the Born effect may explain the almost identical rejection rates measured with ZnCl2 and NiCl2 whereas a significant variation of the membrane volume charge density has been put in evidence by electrokinetic measurements carried out in both media.