Description of the preferential transport of monovalent salts through Na–mordenite membrane: Physico-chemical aspects

A Na–mordenite membrane was prepared by using sol–gel synthesis protocol, and spin-coating was chosen as the impregnation methodology. The gel was impregnated onto commercial ceramic tubular support (α-alumina) and surface characterizations were carried out on Na–mordenite powder prepared with the same gel. The Na–mordenite surface charge is not appreciably modified by a contact with monovalent or divalent salts. The average pore diameter was estimated in the range of 8 nm. Previous results showed that monovalent salts were not retained by the stabilized Na–mordenite membrane while after an alkaline treatment, rejections were observed according to the order of hydration energies (NaF > NaCl > NaBr > NaI). After an acid cleaning, the membrane recovered its initial properties. A new estimation of average pore radius proved that steric effects were not responsible for rejection themselves. A new filtration test of mixed monovalent salt solution was carried out. Fluoride ions were retained by a majority while the others were almost not retained. Infrared characterizations indicate that, after an alkaline treatment, surface chemistry modifications may be at the origin of these interactions. Especially, the apparition of an ultrapolar layer all over the Na–mordenite surface greatly modifies Van der Waals interactions between the hydrated anions and the Na–mordenite surface.

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► Na–mordenite membranes are produced by seeding hydrothermal synthesis.
► Na–mordenite surface treatment induces a preferential transport of Cl−, Br− and I− ions in regard with fluoride transport.
► Brönsted surface sites |H|[Al–Si–O] are transformed into |Na+|[Al–O−] sites after silicon extraction.
► Alkaline treatment of Na–mordenite leads to an increase of surface.