Desalination of seawater ion complexes by MFI-type zeolite membranes: Temperature and long term stability

Abstract

Ceramic membranes made from zeolites possess the nanoporous structure required for desalination of saline water including seawater. In this research, an α-Al2O3 supported MFI-type silicalite membrane was synthesised by the direct in-situ crystallisation method via a single hydrothermal treatment in an autoclave under autogenous pressure. Desalination performance of the prepared silicalite membrane was carried out with a seawater solution (0.3wt% TDS (total dissolved solids)) over a long period of around 180 days at a constant pressure of 700kPa at various temperatures. The prepared silicalite membrane achieved a high rejection (>93%) for all major seawater ions including Na+ (except for K+, 83%) at an applied pressure of 700kPa and room temperature (22°C), but showed a continuous decrease in ion rejection when increasing the temperature from 22°C and 90°C. Permeation flux of the zeolite membrane significantly increased with increasing in temperature. Upon closer observation of the major cations, size selective diffusion in the zeolite membrane was observed over the temperatures tested. Larger ions Ca2+ and Mg2+ were less responsive to temperature than smaller ions Na+ and K+. No changes in membrane structure were observed by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) after 180 days seawater exposure. However, energy-dispersive X-ray spectroscopy (EDS) mapping on the surface of the membrane revealed a small quantity of tightly bound divalent cations present in the structure, which appear to have penetrated the zeolite, accelerated by temperature. They were suspected to have altered the permstructure, explaining why original high rejections at room temperature were not reversed after heat exposure. The work has shown that zeolite membranes can desalinate seawater, but other unusual effects such as ion selective diffusion as a function of temperature indicate a unique property for desalination membrane materials.