(342ar) DFT-Based Solvent Screening for Temperature Swing Solvent Extraction of High Salinity Brines

Recently, temperature swing solvent extraction (TSSE) has been proposed by Boo et al.,^1-2 which is a membrane-less and non-evaporative desalination technology.³ TSSE utilizes solvents with temperature-dependent water solubility for the selective extraction of water over salt from saline feeds with low-grade heat.^{1-2, 4} As reported in previous studies, solvent polarity is a key parameter used to quantify solvent extraction performance. For example, a low-polarity solvent with temperature-dependent water solubility can extract large amounts of water from a hypersaline feed to induce salt precipitation.² However, others found a higher surface polarity of the solvent facilitates more water absorption and contributes to solvent contamination from the salt ions.⁵

The key for the solvents used in TSSE for brine desalination is large water solvation variability within a relatively small temperature window (e.g., ~300 K +/- 25 K). We have performed molecular dynamics (MD) simulations, as well as experimental evaluation of different amine- and imidazole-based solvents in order to determine their TSSE performance. However, due to the large number of potential candidates and time requirements for MD simulations or experimental evaluation, first-principles density functional theory (DFT) calculations can provide initial solvent screening and provide rational design rules. Previously, we use similar approaches to predict CO₂ solubility in ionic liquids, based solely on the ionic polarity characteristics of the individual molecules.⁶ Herein, we are using electrostatic potential properties obtained from the solvent van der Waals surfaces to quantify the polarity of these solvents, along with solvation free energy calculations, in order to understand the TSSE structure-property-performance relationships and provide guidelines for solvent design for brine desalination.

References:

1. Boo, C.; Winton, R. K.; Conway, K. M.; Yip, N. Y., Membrane-Less and Non-Evaporative Desalination of Hypersaline Brines by Temperature Swing Solvent Extraction. Environ. Sci. Technol. Lett. 2019, 6, 359-364.
2. Boo, C.; Billinge, I. H.; Chen, X.; Shah, K. M.; Yip, N. Y., Zero Liquid Discharge of Ultrahigh-Salinity Brines with Temperature Swing Solvent Extraction. Environ. Sci. Technol. 2020, 54, 9124-9131.
3. Davidson, R.; Smith Jr, W.; Hood, D. W., Structure and Amine-Water Solubility in Desalination by Solvent Extraction. J. Chem. Eng. Data 1960, 5, 420-423.
4. Luo, T.; Bajpayee, A.; Chen, G., Directional Solvent for Membrane-Free Water Desalination—A Molecular Level Study. J. Appl. Phys. 2011, 110, 054905.
5. Kim, M.; Choi, O. K.; Cho, Y.; Lee, J. W.; Cho, A. E., Elucidation of the Desalination Mechanism of Solvent Extraction Method through Molecular Modeling Studies. Desalination 2020, 496, 114704.
6. Liu, X.; O’Harra, K. E.; Bara, J. E.; Turner, C. H., Solubility Behavior of CO₂ in Ionic Liquids Based on Ionic Polarity Index Analyses. J. Phys. Chem. B 2021, 10.1021/acs.jpcb.1c01508.