Natural Zeolites for Seawater Desalination

Abstract

Freshwater shortage is one of the major challenges confronting the world today mainly due to climate change, population growth and urbanisation. Seawater desalination appears to be an attractive means for freshwater production. Utilisation of natural zeolites in seawater desalination is currently one of the understudied pathways probably due to other competing technologies, variable properties which makes designing and controlling the zeolite-based process difficult. In some cases, zeolites do not produce water suitable for human consumption. However, zeolites abundancy, low cost of production, high water fluxes, high ion rejections, high thermal and chemical stability as well as compatibility with soil when disposed by landfilling renders them an attractive and sustainable option for intergration into future seawater desalination applications. Research into the use natural zeolites in desalination is therefore ongoing. In this chapter, we review the recent research that focuses on seawater desalination using natural zeolites. The chapter starts with a brief overview of natural zeolites properties and relative abundancy. We then talk about current desalination technologies that exist commercially in industry before delving into the research on seawater desalination using natural zeolites. Generally, not much work has been done on the use of zeolites to desalinate seawater when compared to its use in wastewater treatment. The available literature suggests that researchers have applied natural zeolites in adsorptive desalination, membrane and ion exchange technology taking advantage of zeolites’ molecular sieve and cation exchange properties. On adsorptive desalination zeolites have demonstrated potential to replace or work in combination with silica gel as adsorbents. In the membrane technology the thermally driven pervaporation is preferred over reverse osmosis for both natural and synthetic zeolites. When natural zeolites sectioned from host rocks are used directly without any modification they tend to deliver low water fluxes. On the other hand, synthetic zeolites are costly to produce, although they offer opportunities to tailor make the required membrane properties. Research into the use of natural zeolites for seawater desalination to produce water for agricultural processes by exploiting the cationic exchange properties of this material seems to be gaining traction especially in Japan. The research so far indicates high rejection of Na+ from seawater and when the cationic exchanger is coupled to an anion exchange material, water suitable for agricultural purposes can be produced. This use of natural zeolites is attractively sustainable in that it can remove only the unwanted salts leaving those that are beneficial to plants in the main product. This ensures that the exchanger can treat more volumes before getting saturated. The use of seawater for agricultural purposes alleviates the dependence on freshwater sources making it available for other needful purposes like drinking, sanitation and industrial manufacturing uses.