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

Highlights

• One hundred and eighty days stability of MFI-type zeolite membrane for seawater desalination was confirmed.

• The membrane achieved a high rejection (>93%) for the major seawater ions including Na⁺.

• Increasing the temperature decreased the ion rejection and increased the permeation flux.

• Larger ions Ca²⁺ and Mg²⁺ were less responsive to temperature than smaller ions Na⁺ and K⁺.

• Ions infiltrate the zeolite material and alter its size selective property.

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 (0.3 wt% TDS (total dissolved solids)) over a long period of around 180 days at a constant pressure of 700 kPa 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 700 kPa 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.

Introduction

Desalination is now commonly performed using membrane technology in reverse osmosis (RO) mode. However, the membrane types for desalination are limited to a handful of materials which imposes strict pretreatment requirements, such as chlorine/oxidant removal, abrasive particle removal and reduced operation temperature prior to feeding water to the polymer membrane. Also, the polymer material is subject to fouling which damages the membrane, either by the foulant itself, or the cleaning chemicals used to reverse the fouling [1]. Research and development in alternative membrane materials could enable more desalination opportunities where current membranes are limited. Harnessing inorganic materials for developing desalination membranes could be a more robust alternative relieving the costly pretreatment requirements required to conform to these material limitations.

The fundamental requirement of the membrane to carry out desalination by RO is an inherent ability of the membrane to repel ions, but pass water. High ion rejection properties are essential in tandem with high water diffusion [2]. Nanoporous inorganic membranes have been studied both theoretically and experimentally to reject ions by filtration, utilising single layers [3] and a novel bilayer concept [4], [5]. Single layers are a more simplistic approach but the material must possess the required pore size between ions and the water. There have been some studies to date applying different membrane materials such as zeolites [2], [3], [6], [7], [8], [9], [10], and hybrid organically bridged silica [5] for membrane desalination. Zeolite materials are highly configurable through their chemistry and offer unique frameworks for a wide variety of applications including chemical sensing, water treatment and chemical reaction [2], [3], [6], [7], [8], [9], [10], [11], [12], [13]. Their tuneable pore size, typically in the range of 0.3–1 nm, makes them highly suitable for molecular sieving (i.e. ion rejecting) applications. Over the last decade, significant progress in the preparation and characterisation of zeolite membranes has stimulated research in their application for various molecular level separations including gas phase and liquid phase mixtures. Zeolites have also been shown to be outstanding candidate materials for desalination membranes as they possess the required small pore properties to reject ions [6], [10] as well as the thermal, chemical, and mechanical stability of ceramics [14].

A molecular dynamic simulation study conducted by Lin and Murad [10] showed that 100% rejection of Na⁺ could be achieved using a perfect (single crystal), pure-silica ZK-4 zeolite membrane by RO. They found that zeolite pore structure is ideally suited to reject ions. The size exclusion of hydrated ions is the separation mechanism of the perfect ZK-4 zeolite membrane [15]. The aperture of the ZK-4 zeolite (diameter 0.42 nm) is significantly smaller than the kinetic sizes of hydrated ions (e.g. Na⁺ 0.716 nm, K⁺ 0.662 nm, Ca²⁺ 0.824 nm, Table 1) [16].

Following this computational simulation study, several research groups have explored the possibility of using MFI-type zeolite membranes for desalination [2], [6], [7], [8], [9]. The MFI-type zeolite has orthorhombic crystal symmetry with nearly cylindrical, 10-member ring channels. The aperture size of the MFI-type zeolite is around 0.56 nm [8], which is smaller than the sizes of hydrated ions [17] but larger than the kinetic diameter of water. Performance testing of MFI-type zeolite membranes working in RO demonstrated that high rejections of even the smallest ions, including Na⁺ abundantly found in saline waters, are achievable [2], [6].

In general, permeation in an ideal molecular sieve zeolite membrane should occur only through the regular intracrystalline pores of the zeolite selective layer. In reality, however, the permeation properties will often be modified due to the existence of intercrystalline defect porosity caused by insufficient intergrowth of crystals, thermal removal of the template (e.g. tetra-propyl ammonium hydroxide (TPAOH)) [18], [19], or the complete de-watering of the membrane layer [20], [21]. Several researchers have reported changes in the unit cell dimensions of MFI-type zeolite crystals during heat treatment [21], [22], [23], [24]. Our recent studies [25], [26] also showed that the interaction between MFI-type zeolites and the major cations in seawater causes changes not only in structure but also porosity, which is expected to affect diffusion properties of these materials when used as membranes for desalination. An easily modified feature of MFI-type zeolites is the Si/Al ratio which allows structures to be tailored to optimise the sorption uptake and species selectivity [27]. For example, increasing the content of alumina can alter properties such as surface hydrophobicity and surface charge which can have a significant impact on diffusion of electrolytes [27], [28]. Al-rich MFI-type zeolite (ZSM-5) membranes were recently reported to deliver higher water fluxes when compared with pure silica (silicalite-1) membranes using a pervaporation setup for desalination of NaCl solutions, but the silicalite-1 membrane exhibited relatively high robustness during a long term (560 h) stability testing [29]. Little work however exists on longer term (e.g. 180 days) performance of MFI-type zeolite membranes for reverse osmosis desalination of seawater ion complexes, or the performance as a function of temperature which may reveal unique diffusion effects through the dynamic zeolite cage and grain boundaries.

In this work, a MFI-type silicalite membrane was developed by a direct in-situ hydrothermal synthesis method. The as-synthesised zeolite membrane underwent long term (180 days) desalination of seawater ion complexes in the RO mode at different temperatures. The structure and morphology of the zeolite membrane was also investigated by XRD and FESEM techniques.