Transport behavior of monovalent and divalent solutes across mesoporous Anopore γ-alumina membranes was investigated as a function of pore diameter, pH, ionic strength, and nature of the salt or complexing species in solution. Radiotracers 137 Cs, 85 Sr, 22 Na, and 45 Ca 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 Ca 2+ and Sr 2+ 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 Sr 2+ 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.
Transport behavior of trivalent solutes across mesoporous Anopore γ-alumina membranes was investigated as a function of pore diameter, pH, ionic strength, and nature of the salt in solution. Radiotracers, 55 Fe, 152 Eu, 157 Tb, and 241 Am, 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 Am 3+ and Tb 3+ 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, Eu 3+ 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.
Investigations confirmed the feasibility and potential advantage of using mixed perfluorosulfonate/carboxylate membranes for nitric acid dehydration. Experimentation consisted of in situ generation of pure sulfonate and mixed sulfonate/carboxylate (as high as 71 mol % carboxylate) polymer films from a perfluorosulfonyl fluoride precursor membrane, the characterization of the material, and a study of the transport characteristics of these membranes. The carboxylate concentration was determined using direct Fourier transform infrared and X-ray fluorescence spectroscopy. Nitric acid dehydration transport tests confirmed that bulk fluxes decreased and the water separation factor dramatically increased as the carboxylate side-chain content increased to 53 mol %.
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