Surface effects on metal ion transport across porous alumina membranes. 2. Trivalent cations: Am, Tb, Eu, and Fe

Transport behavior of trivalent solutes across mesoporous Anopore gamma-alumina membranes was investigated as a function of re diameter, pH, ionic strength, and nature of the salt in solution. Radiotracers, Fe-55, Eu-152, Tb-157, and Am-241, were present in the feed solutions at concentrations ranging from 10(-8) to 10(-10) M. Total salt concentrations in the feed ranged from 1.0 to 10(-4) M. Order of magnitude differences between experimental and theoretical diffusion rates for the membranes can be explained in terms of electrostatic interactions between the cations and the positively charged membrane surface. The rates of Am3+ and Tb3+ transport across the alumina membranes were greatly increased by raising the ionic strength from 10(-3) to 0.1 M in both the feed and permeate solutions; however, EU3+ diffusion rates were only slightly affected in this range of ionic strength. Trivalent cation transport rates at total salt concentrations greater than 0.1 M were slower than divalent cations due to enhanced electrostatic interactions between the alumina surface and the higher valent 3+ cations. Alkali metals present in both the feed and permeate solutions slightly influenced iron transport.