Desalination has been increasingly used to address the escalating water shortage. Thermal desalination process and membrane-based techniques are widely used to produce fresh water. However, the thermal technologies are highly energy-intensive, e.g., 14-27 kWh m−3 for multi-stage flash and multiple effect distillation. Membrane-based techniques such as reverse osmosis process also have several drawbacks, including relatively high energy consumption (1.5-6 kWh m−3), and chemical usage in membrane cleaning and dechlorination1,2. Alternatively, salts can be removed efficiently by passing saline water through columns filled with ion-exchangers or adsorbents3. In particular, thermoresponsive ion-absorbent-based desalination process has been implemented in water industry. In this process, the ion adsorption process is energy-efficient, but the thermal energy required for regeneration at elevated temperature such as 80°C is substantial4-6. Is it possible to use the most abundant and renewable source of energy on earth – sunlight – for adsorbent regeneration? And that is how this study start with.
By searching through the photo-reactive molecules, we found that spiropyran (SP) can be converted to zwitterionic state (merocyanine, MC) in dark, and reversed back to SP under visible light irradiation7. Uniquely, merocyanine with positively charged indolium group and negatively charged phenolate group could function as anion and cation binding sites, respectively, to adsorb salts from saline water. Moreover, MC can be accelerated back to SP by visible light (Vis), achieving the regeneration of the absorbent. In this work, polyspiropyran (PSP) molecules are confined into MIL-53 (Al) frameworks to develop a sunlight-regenerable salt adsorbent (PSP-MIL-53) for sustainable desalination.
Figure 1. Schematic illustration and mechanism of salt adsorption by PSP-MIL-53 under dark/UV light and desorption under visible light (Vis) irradiation.
PSP-MIL-53 shows the ability to reversibly capture and release of salts (e.g., NaCl, CaCl2) in water. Under dark conditions or UV light irradiation, the zwitterionic isomer quickly adsorbs multiple cations and anions from water within 30 minutes with high ion adsorption loading, up to 2.88 mmol g−1 of NaCl. With sunlight illumination, the neutral isomer rapidly releases these adsorbed salts within 4 minutes. Importantly, after 10 adsorption-desorption cycles, PSP-MIL-53 demonstrated good water-stability.
PSP-MIL-53 is capable of adsorbing various monovalent and multivalent salts with excellent cycling performance. To demonstrate its potential for practical application, PSP-MIL-53 was further tested to produce fresh water by desalting synthetic salt waters in dark in a single-column setup. With regards to the desalination of 2,233 ppm synthetic brackish water, the ion adsorption loading of PSP-MIL-53 was 1.06 mmol g−1 and 1.33 meq g−1 (milliequivalent g−1). Accordingly, 23.4 mL of fresh water (<600 ppm TDS) per gram of PSP-MIL-53 (mL g−1) could be produced in an adsorption-desorption cycle. In terms of desalting 35,000 ppm synthetic seawater, it exhibited an ion adsorption loading of 2.47 mmol g−1 and 2.66 meq g−1, with a fresh water productivity of 2.3 mL g−1 per cycle due to the high salinity of seawater.
Furthermore, a simple and yet conservative analysis of its water desalination capacity and energy consumption was conducted on a classical two-bed desalination system. A fresh water yield of 139.5 L kg−1 per day and a low energy consumption of 0.11 kWh m−3 would be reached for desalinating 2,233 ppm synthetic brackish water. By utilizing sunlight for adsorbent regeneration, the energy consumption of adsorbent-based desalination process could be highly reduced. This work opens the door for innovative design of stimuli-responsive materials capable of water desalination and purification in a highly sustainable manner.
1 Al-Karaghouli, A. & Kazmerski, L. L. Energy consumption and water production cost of conventional and renewable-energy-powered desalination processes. Renew. Sust. Energ. Rev. 24, 343-356 (2013).
2 Ang, W. S., Yip, N. Y., Tiraferri, A. & Elimelech, M. Chemical cleaning of RO membranes fouled by wastewater effluent: Achieving higher efficiency with dual-step cleaning. J. Membr. Sci. 382, 100-106 (2011).
3 Burn, S. et al. Desalination techniques—A review of the opportunities for desalination in agriculture. Desalination 364, 2-16 (2015).
4 Chandrasekara, N. G. N. & Pashley, R. Study of a new process for the efficient regeneration of ion exchange resins. Desalination 357, 131-139 (2015).
5 Bolto, B. et al. An ion exchange process with thermal regeneration IX. A new type of rapidly reacting ion-exchange resin. Desalination 13, 269-285 (1973).
6 Ou, R. et al. Thermoresponsive Amphoteric Metal–Organic Frameworks for Efficient and Reversible Adsorption of Multiple Salts from Water. Adv. Mater. 30, 1802767 (2018).
7 Klajn, R. Spiropyran-based dynamic materials. Chem. Soc. Rev. 43, 148-184 (2014).