Abdulaziz Alharbi

تخصص استدامة المياه · دفعة 2026

Mixed-Salt Effects on Water Uptake and Ion Selectivity in Commercial Anion Exchange Membranes

Anion exchange membranes are usually characterised in solutions containing a single salt, yet the water they treat contains several competing anions at once. This study measures water uptake and Donnan selectivity for three commercial membranes, Fumasep, PiperION and Sustainion, in solutions containing five competing anion pairs at three concentrations, giving 45 measurements. Membrane chemistry was the dominant factor: Sustainion absorbed approximately eight times more water than Fumasep, and this ordering held in every measurement without exception. Contrary to the expectation from hydration energy, sulfate-containing pairs did not give the highest water uptake. Ion chromatography showed that all three membranes preferentially absorb chloride over sulfate, with selectivity coefficients between 2.2 and 6.6, giving the sorption order NO₃⁻ > Cl⁻ > SO₄²⁻. Wet mass was independent of external concentration over a fifty-fold range, varying by less than 1% for Sustainion. Selectivity showed an inverse trend with hydration for the chloride–sulfate pair but not for the nitrate–chloride pair, consistent with charge dilution affecting discrimination between ions of different valence more strongly than between ions of the same valence. The results indicate that single-salt characterisation does not transfer directly to mixed-ion conditions.

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