Surface effects on cation transport across porous alumina membranes

Transport behavior of monovalent and divalent solutes across mesoporous Anopore gamma-alumina membranes was investigated as a function of pore diameter, pH, ionic strength, and nature of the salt or complexing species in solution. Radiotracers Cs-137, Sr-85, Na-22, and Ca-45 were present in the feed solutions at very low concentrations, ranging from 10(-9) to 10(-12) M and total salt concentrations from 0.1 to 10(-4) M. The divalent cations Ca2+ and Sr2+ exhibit lower diffusion rates (3-7 times slower) than the monovalent cations Cs+ and Na+ for membranes with 20 nm diameter pores. Differences between monovalent and divalent cation diffusion rates for the membranes can be explained in terms of a Donnan exclusion effect from the positively charged alumina surface. The rate of Sr2+ transport across the 20 nm alumina membranes was greatly increased by raising the pH (reducing the membrane surface charge) from 5 to 8 for both the feed and receive sides. Increased ionic strengths and the addition of complexing agents or specific salt solutions also facilitated divalent ion transport. Diffusion coefficients for divalent cations increased 3-fold for the 100 nm pore diameter membranes.