Structural effects on SAPO-34 and ZIF-8 materials exposed to seawater solutions, and their potential as desalination membranes

Abstract

The effects of saline water on the material properties of two promising salt rejecting structures, SAPO-34 and ZIF-8, were explored for desalination. Immersing SAPO-34 in seawater revealed a release of Al which involved some replacement by Ca, K, Mg and Na. X-ray powder diffraction revealed a robust structure in deionised and seawater solutions. ZIF-8 on the other hand, lost 1% of its mass due to the release of Zn into both deionised water and seawater solutions, yet the structure remained intact. N2 and positron annihilation lifetime spectroscopy porosimetry techniques revealed loss of surface area and mesopores on both materials, explained by capillary forces acting to close them. Membrane reverse osmosis revealed no liquid water flux for SAPO-34 due to the intactness of the membrane and very small pore size which contributed to prohibitive resistance to liquid water flow. ZIF-8 membranes permeated water, but did not demonstrate significant salt rejection indicating that either the slight mass loss impacted membrane integrity to reject salt, or that liquid water diffusion in ZIF-8 does not follow a salt rejecting path. SAPO-34 and ZIF-8 structures are stable structures for aqueous applications including sea salts, but further work is needed to develop them for reverse osmosis desalination.