A Na+ ion-selective desalination system utilizing a NASICON ceramic membrane
Graphical abstract
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Introduction
Seawater is a massive natural reservoir of minerals and freshwater (Petersen, 1994). A great demand for Na, Ca, Mg, K, and Li exists in the agricultural, industrial, environmental remediation, and medical fields, and they can be profitably extracted from seawater (Loganathan et al., 2017; Shahmansouri et al., 2015). A classic example of this is NaCl (common table salt), which is indispensable for human life and is used in the food, glass, soap, detergent, textile, pulp, and paper industries (Petersen, 1994).
To successfully segregate seawater into water and desirous salt, thermal, electrical, or physical processes are required (Chen et al., 2021; Pistocchi et al., 2020; Pramanik et al., 2020). Among the various processes of seawater separation, electrodialysis (ED) is superior to other technologies for ion-selective desalination (Strathmann, 2010). This process is driven by a difference in electrical potential over a membrane stack, where charged compounds are selectively transferred from a feed solution through a pair of anion and cation exchange membranes (Fig. 1(a)) (Xie et al., 2016). For example, salt manufacturing companies have been economically producing NaCl from seawater using ED since the 1970s at a production rate of 360 000 t/yr during 1970–1980 (Kobuchi et al. 1983).
The ion-selectivity of ED is mainly attributed to the polymeric ion exchange membrane (IEM) (Luo et al., 2018). Ion transport under the driving forces of concentration or electrical potential gradient through IEMs can be explained by the solution-diffusion model (Fig. 1(d)) (Sata, 2000). According to this model, the membrane structure is divided into two phases: the gel and interstitial phase (Kreuer et al., 2004). The gel phase is composed of polymer chains and hydrophilic ion-exchange groups bound to the chains. The interstitial phase is the void between the elements of the gel phase. It is assumed to be a space filled with an electro-neutral solution (water) containing ions, which is responsible for ion transport. The four possible mechanisms for transport in this phase are diffusion, electromigration, convection, and surface site hopping (Saito et al., 2004; Thampan et al., 2000). However, this solution-like ion transfer mechanism may also cause the transport of unwanted ions (other than Na+). Furthermore, water may migrate across the membrane via spontaneous phenomena such as osmosis, electro-osmosis, electro-migration, and diffusion (Lakshminarayanaiah, 1965; Lakshminarayanaiah and Subrahmanyan, 1968). These can have detrimental effects on freshwater production as well as in salt manufacturing (Tanaka, 2011). For example, during salt manufacturing from brine by ED, it is important to control water transfer through the membrane to prevent dilution of the final product (Jiang et al., 2014).
To cope with this problem, previous studies have shown that ion selectivity in polymeric membranes can be improved by modifying the surface of IEMs (Khoiruddin et al., 2017), introducing novel bulk morphology of membranes (Balster et al., 2005), or blending polymers (Zhang et al., 2013). However, no method perfectly prevented the passage of other undesirable ions through a membrane whose transference number was still below unity; 0.89–0.99 (Cwirko and Carbonell, 1992; Długołęcki et al., 2010; Wu et al., 1994).
Previous studies have pointed out the use of ceramic-based membranes with ion-exchange properties as a promising alternative to polymeric IEMs (Dzyazko et al., 2007; Linkov and Belyakov, 2001). Among these, the sodium (Na) super ionic conductor (NASICON)-structured solid electrolyte has attracted considerable attention due to its high Na+ selectivity. This is demonstrated by its integral Na+ transference number of 1 (Song et al., 2016). Owing to its high selectivity, it is applicable in wastewater remediation (Balagopal et al., 1999; Fountain et al., 2008; Girard et al., 1999) as well as in sensors and batteries (Goodenough et al., 1976; Hong, 1976; Ivanov et al., 1994; Leonhard et al., 1994; Zhao et al., 2018).
Herein, the superior Na+-selective and water-impermeable characteristics of NASICON were demonstrated compared to CEM. Subsequently, for the first time, NASICON was introduced in place of CEM in the ED system by constructing the NASICON-applied electrodialysis (N-ED) system, which compensates for the weakness of the conventional ED system (Fig. 1). This approach enabled the selective extraction of Na+ from seawater in the diluate while minimizing water loss. Additionally, the N-ED system was expanded to a seawater battery for desalination (SWB-D) system by substituting NASICON with Na metal as a battery anode interposed between two NASICONs. Thus, the unit cell scale ED, N-ED, and SWB-D systems were constructed to compare their desalination performances. This study collates the above insights and proposes a new NASICON-based ceramic conductive membrane for enhancing electrochemical desalination technologies.
Section snippets
Reagents
Solutions for testing selectivity, osmosis, and electro-osmosis of the membrane were prepared by dissolving NaCl (≥ 99%, Alfa-aesar), Na2SO4 (≥ 99%, Dae-Jung), CaCl2 (≥ 99%, Junsei), MgCl2 (≥ 98%, Sigma Aldrich), and 18.2 MΩ cm−1 Milli-Q water (Merck Millipore). Seawater for the electrode solution of ED and N-ED and catholyte for SWB-D was simulated by blending 38 g of sea salt (Shinan Co. Ltd, Republic of Korea) with DI (deionization) water to prepare 1 L of seawater. This can reduce
NASICON vs CEM
The unique properties of NASICON can be explained by its crystal structure (Fig. S3). The representative composition of NASICON (Na1+xZr2SixP3-xO12 (0 ≤ x ≤ 3, x=2 in this study)) comprises corner-sharing tetrahedra [SiO4], [PO4], and octahedra [ZrO6], which form a 3D network of channels for Na+ transport (Fig. 1(e)). The interstitial Na sites present between the gaps of the framework are simultaneously filled with mobile Na+ and available adjacent vacancies, which promote Na+ diffusion
Conclusions
The unique properties of NASICON were investigated, which is impenetrable by water and ions other than Na+. By utilizing NASICON instead of CEM in the ED system, a NASICON-ED system (N-ED) was suggested for the first time. It outperformed conventional ED in terms of several factors: the productivity of the diluate, the specific energy consumption for NaCl extraction, and the Na+ selective ion removal rate. In this sense, N-ED is expected to be effective for various industries such as salt
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Funding: This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean Ministry of Science and ICT (MSIT) (No. 2020R1A4A1019568), and the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20215610100030).
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