Diffusion behaviour of multivalent ions at low pH through a MFI-type zeolite membrane

Zeolite membranes have been widely examined for desalination. Work to date has shown effective performance in monovalent rich solutions, but the understanding of the unique ion-zeolite interactions for multivalent ion rich solutions, such as those found in acidic mining wastewaters, has not yet been studied. Filtration performance of MFI-type zeolite membrane was evaluated on a model multivalent ion, Fe3+, Al3+, Ca2+ and Mg2+, solution with total dissolved solids (TDS) of 97,000 mg L-1 and pH 2.03 at between 3 MPa and 7 MPa and 21 degrees C to 70 degrees C. At 7 MPa and 21 degrees C, rejection for Fe3+ was 97%, 80% for Al3+ and Mg2+, and 50% for Ca2+. This behaviour followed the rejection of ions with larger hydrated diameter, except for Al3+ which was attributed to its unique strong interaction with zeolites. However, an unusual trend of increasing rejection with increasing temperature was observed. Instead of activated transport which occurs with monovalent or dilute solutions, temperature accelerated the infiltration of multivalent ions into the zeolite structure to further block ions and synergistically increased rejection. Zeolite membranes exhibited unique effects in multivalent ion rich solution that could be further utilised in niche desalination applications or benefit other applications such as sensors.