Ionic transport across tailored nanoporous anodic alumina membranes

Highly ordered Nanoporous Alumina Membranes (NPAMs) with a precise control of the pore size and different porosity but thickness around 63 μm were fabricated by the two-step anodization process using different acidic aqueous solutions, oxalic (Al-Ox), sulfuric (Al-Sf), and phosphoric (Al-Ph) acids, respectively. The pore size was controlled by properly changing the anodization voltage and the electrochemical bath conditions, obtaining the following average pores diameter values, d(p), as determined by scanning electron micrographic analysis: Al-Ox (d(p)=46±2 nm), Al-Sf (d(p)=27±2 nm), and Al-Ph (d(p)=240±20 nm). A pore increasing of around 5% for samples Al-Ox and Al-Sf was obtained after membranes immersion in 5 wt.% phosphoric acid for a certain etching time. Electrochemical characterization of NPAMs was performed with the samples in contact with NaCl solutions at different electrolyte concentrations. Ionic transport numbers and effective membrane fixed charge were determined from membrane potential measurements, which clearly show the significant influence of the pore diameter on ions transport. Moreover, frictional and electrical effects on mass transfer parameters (salt and ions diffusion coefficients) into the pores of alumina membranes were also evaluated from these results.