Surface effects on cation transport across porous alumina membranes

Transport behavior of monovalent and divalent solutes across mesoporous Ano pore gamma-alumina membranes was investigated as a function of pore diamete r, pH, ionic strength, and nature of the salt or complexing species in solu tion. 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 Ca2and Sr2+ exhibit lower diffusion rates (3-7 times slower) than the monovale nt cations Cs+ and Na+ for membranes with 20 nm diameter pores. Differences between monovalent and divalent cation diffusion rates for the membranes c an be explained in terms of a Donnan exclusion effect from the positively c harged alumina surface. The rate of Sr2+ transport across the 20 nm alumina membranes was greatly increased by raising the pH (reducing the membrane s urface charge) from 5 to 8 for both the feed and receive sides. Increased i onic strengths and the addition of complexing agents or specific salt solut ions also facilitated divalent ion transport. Diffusion coefficients for di valent cations increased 3-fold for the 100 nm pore diameter membranes.