(297e) Temperature Swing Solvent Extraction Hypersaline Desalination with Stepwise Release to Advance the Productivity-Selectivity Tradeoff

Hypersaline brines are of growing environmental importance but are presently underserved by conventional treatment methods. Temperature swing solvent extraction (TSSE) is an emerging technology that is able to desalinate hypersaline brines, thereby reducing their volume and facilitating disposal while simultaneously producing fit-for-purpose water to alleviate supply stress. TSSE utilizes a switchable solvent with thermally responsive polarity to extract water from the hypersaline brine while rejecting salts. Low-temperature heat triggers the solvent to switch from a hydrophilic to a more hydrophobic state, causing the product water to disengage from the extract and regenerating the solvent to be recycled into the process. Our previous work revealed a tradeoff between water productivity and water/salt selectivity that constrains the desalination performance of TSSE. In this study, we demonstrate the improved performance of the novel temperature swing solvent extraction stepwise release (TSSE-SR), departing from the nominal productivity-selectivity tradeoff. Thermally switchable solvents of diisopropylamine, triethylamine, and tert-octylamine were evaluated for TSSE-SR desalination of 1.0 M NaCl brines. The introduction of an intermediate temperature step in TSSE-SR allows salt to preferentially disengage over water from the organic solvent extract. TSSE-SR drastically improves salt rejection while minimizing sacrifices in water recovery yield. The potential contribution of the intermediate step to amine loss is assessed and confirmed to be insignificant. Hunter-Nash analysis is conducted on diisopropylamine-water-NaCl ternary phase diagrams to provide a theoretical basis for the enhanced TSSE-SR performance. Finally, TSSE-SR is benchmarked against standard TSSE and the enhancements in productivity and selectivity are quantified. This innovative approach can expand the spectrum of viable solvents for hypersaline desalination to include those that have high water productivity but low selectivity in single-stage TSSE operations.