Controlling the transport of cations through permselective mesoporous alumina layers by manipulation of electric field and ionic strength

The electric field-driven transport of ions through supported mesoporous gamma-alumina membranes was investigated. The influence of ion concentration, ion valency, pH, ionic strength, and electrolyte composition on transport behavior was determined. The permselectivity of the membrane was found to be highly dependent on the ionic strength. When the ionic strength was sufficiently low for electrical double-layer overlap to occur inside the pores, the membrane was found to be cation-permselective and the transport rate of cations could be tuned by variation of the potential difference over the membrane. The cation permselectivity is thought to be due to the adsorption of anions onto the pore walls, causing a net negative immobile surface charge density, and consequently, a positively charged mobile double layer. The transport mechanism of cations was interpreted in terms of a combination of Fick diffusion and ion migration. By variation of the potential difference over the membrane the transport of double-charged cations, Cu2+, could be controlled accurately, effectively resulting in on/off tunable transport. In the absence of double-layer overlap at high ionic strength, the membrane was found to be nonselective.