Efficient salt recovery from aqueous streams using multistage liquid-liquid extraction with thermo-responsive polymers

Liquid-liquid extraction (LLX) using thermo-responsive polymers is a promising low-energy alternative for concentrating hypersaline brines from reverse osmosis (RO). This study presents a comprehensive evaluation of a multistage crosscurrent LLX process, comparing poly(propylene glycol) 400 (PPG 400) and the nonionic surfactant Dehypon® LS54. The methodology includes determining liquid-liquid equilibrium (LLE) data, a theoretical graphical multistage countercurrent LLX analysis, four-stage crosscurrent batch experiments, solvent regeneration, and product purification via thermally induced phase separation (TIPS). LLE results revealed that PPG 400 possesses higher water over salt selectivity. Using ternary diagrams from LLE results, it was found that PPG 400 and Dehypon® LS54 require four and five theoretical stages, respectively, to increase the NaCl concentration from 7 to 20 wt%. In comparative four-stage crosscurrent experiments, PPG 400 achieved a higher final raffinate NaCl concentration (17.8 wt%) than Dehypon® LS54 (13.8 wt%). Using TIPS, for both polymers high recovery (>99.5%) was achieved at 80 wt% to 97 wt% polymer purity. TIPS was also proven to be an effective separation for the raffinate streams, reducing polymer contamination to as low as 0.3 wt% for PPG 400 and 0.04 wt% for Dehypon® LS54. The co-extracted salt in the extract yielded a secondary brine stream (1.3 wt% to 10.4 wt% NaCl) rather than clean water, showing that more sophisticated processing and/or downstream purification is necessary to obtain high quality potable water. This work elucidates the fundamental trade-offs between thermodynamic selectivity, solvent loss, and product purity that govern the design of LLX systems for brine concentration and valorization.