Remediation of Water and Wastewater Using Metal Organic Framework based Membranes

Introduction

There is an increasingly global demand for safe drinking water. The world population and anthropogenic activities are growing, and the supplies of water are easily polluted. 2.1 billion people have no access to clean drinking water (water free of pollution) reported by the World Health Organization (WHO) (WHO and UNICEF 2017). This equivalent to 29% of the global population. In water and wastewater systems, membrane processes such as forward osmosis (FO), reverse osmosis (RO), nanofiltration (NF), microfiltration (MF) and ultrafiltration (UF), membrane distillation (MD) are commonly used (Ezugbe and Rathilal 2020; Madhura et al. 2018). Membranes have taken on an ever more critical role in the separation and purification technology in recent decades. Polymer membrane production and operation technologies are relatively advanced due to the advantages such as cost-effectiveness, high processability and low power consumption. However, there is usually a trade-off between selectivity and permeability polymer membranes. To overcome this obstacle and due to their tunable and distinct pore size and porous nature, Metal-organic frameworks (MOFs) have been identified as perfect candidates for membrane purification applications. MOFs were first discovered in 1965 by Tomic (Tomic 1965). Earlier it was called supramolecules. The term “MOF” was introduced by by Yaghi et al. in 1995 (O M Yaghi, Li, and Li 1995). MOFs are organic and inorganic, hybrid crystalline materials that consist of collection of metal ions with a positive charge and are characterized by transparent, porous structures. In comparison to other porous materials, MOFs provide various advantages such as unusual structural complexity, including uniform pores, structural uniformity at atomic level, tuneable porosity, network topology, structure, proportions and chemical functionality versatility (Khan, Hasan, and Jhung 2013; Kitagawa, Kitaura, and Noro 2004; X. Li et al. 2017; Omar M Yaghi et al. 2003).

In the past, researchers have developed several hundred various MOFs have been studied for different applications (e.g. adsorption, membrane preparation, catalysis) including gas separation, gas storage, energy storage, and environmental applications (Rui et al. 2018; Seo et al. 2016; M. Zhang et al. 2018). MOFs can be synthesized using chemical or physical techniques by combining metallic and organic linker. MOFs, in the form of adsorbents, have also been used for the removal of water contaminants, such as lead (Pb) (Luo, Ding, and Luo 2015), arsenic (As) (C. Wang et al. 2015), E coli (X. ping Wang et al. 2020), nuclear waste (J. Cheng et al. 2018) etc. from aqueous phases. Different MOFs such as ZIFs, HKUST, UiO-66, MIL-101, MIL-100, MIL-125, etc. have been used for water purification in the past (Cook, Zheng, and Stang 2013; Elrasheedy et al. 2019; J. R. Li, Sculley, and Zhou 2012; Stock and Biswas 2012).

This chapter will focus on the purification of the aqueous stream through MOFs based membranes. The objectives of this chapter are: (i) to introduce types of contaminants and existing treatment technologies, (ii) familiarise with different types of MOFs based membranes and their synthesis techniques, (iii) outline applications of the aforementioned MOFs based membranes in water purification. The chapter has been organized into seven sections. At first, various contaminants and their treatment technologies are introduced. Different MOFs based membranes are discussed such as bare MOFs membranes, porous matrix membranes (PMM), mixed matrix membranes (MMM), and thin-film nanocomposite (TFN) membranes. Then synthesis methods of MOFs based membranes are outlined with various examples. Application of MOFs based membranes in FO, RO, NF, UF and MF with recent studies is provided. At last, an outlook from our point of view is given.