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. 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 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.

Membrane-based desalination and wastewater treatment are promising strategies that enable water purification from various unconventional resources. However, mineral scaling constrains the performance and efficiency of membrane technologies significantly. Silica (SiO₂) is a major mineral scale that is formed via a polymerization reaction. Due to its amorphous nature and the lack of effective anti-scalants, silica scaling is considered as the Gordian Knot of membrane desalination processes. In this work, we performed a systematic study that investigated the design principle and mechanism of anti-scalants for mitigating silica scaling in membrane desalination. We examined the effectiveness of anti-scalant candidates with different molecular functionalities in hindering silica scale formation in static experiment as well as in dynamic crossflow membrane distillation (MD) and reverse osmosis (RO) experiments. Particularly, I will present three key results that answer the following questions: (1) how the anti-scalant performance behaves as a function of molecular functionality and size; (2) whether the efficiencies of anti-scalants revealed in static experiments predict those in dynamic, crossflow membrane filtration; and (3) whether the anti-scalants possess the same efficiencies in RO and MD. Our findings provide fundamental insights that have the potential to guide successful design of anti-scalants tailored to silica scaling for membrane desalination.

The sustainable management of hypersaline brines (typically ≥70,000 ppm TDS) has become a pressing demand. Although desalination offers an attractive option, energy-efficient hypersaline desalination remains technically challenging. As a mature technology widely applied in brackish water desalination, electrodialysis, ED, has obtained increasing attention regarding hypersaline applications. However, as the key component in ED, current ion-exchange membranes (IEMs) are unsuitable for hypersaline operations due to 1) diminished permselectivity in high ionic strength environment and 2) water transport resulting from transmembrane osmotic pressure difference. In this study, we develop sulfonated polystyrene membranes that can satisfy the two critical needs. Polystyrene-r-sulfonated polystyrene copolymers with carefully controlled sulfonation levels were neutralized with sodium hydroxide and then fabricated into thin films. Wide-angle X-ray scattering analysis and ion exchange capacity and conductivity measurements indicate a critical ion cluster percolation threshold for ions to permeate through the membrane. Near the percolation threshold, fabricated membranes show permselectivity ≥0.82 when characterized by static method with 4.0 and 0.8 M NaCl solutions, superior to commercial IEMs (significantly diminished permselectivity of ≈0.76). Furthermore, when challenged in ED process with 4 M NaCl solution, excellent permselectivity of ≥0.96 is observed (commercial IEM is ≈0.74). Additionally, sulfonated polystyrene membranes exhibit water permeability as low as ≈60 mL μm/(m² h bar), with commercial IEMs typically between 100 to 1000 mL μm/(m² h bar). The study demonstrates a facile approach to fabricating IEMs capable of maintaining high permselectivity but low water permeability under high solution concentrations, thus enabling hypersaline ED desalination and other electromembrane processes.

As an emerging desalination technology for hypersaline wastewater treatment, membrane distillation (MD) faces a critical challenge of membrane wetting. The state-of-the-art wetting mitigation strategy in MD is to use novel membranes that are commercially unavailable and difficult to fabricate. This study proposes an operational mode, negative pressure MD (NPMD), as a novel wetting mitigation strategy in MD operations. Compared with conventional MD, NPMD can substantially enhance the wetting resistance of commercially available hydrophobic MD membranes. Specifically, in a conventional direct contact MD (DCMD) operation, a polyvinylidene fluoride (PVDF) membrane is easily wetted by a 0.1 mM sodium dodecyl sulfate (SDS) feed solution, while in NPMD, the PVDF membrane can remain unwetted with a 0.2 mM feed solution. By determining the liquid entry pressure (LEP) of the PVDF membrane using an impedance-based technique, the working mechanism of NPMD for wetting mitigation is illustrated, and such a mechanism is further confirmed by DCMD experiments using feed solutions containing ethanol. With a negative gauge pressure on the feed stream, the transmembrane hydraulic pressure becomes lower than the LEP of the PVDF membrane, thereby mitigating membrane wetting. As a simple yet effective wetting mitigation strategy, NPMD can be readily implemented in practice with commercially available hydrophobic membranes, showing vast potential to advance MD applications.

Management of concentrate streams in inland applications has an uncertain long-term environmental impact. This study investigates the efficiency of thermal energy management with an intensified air-gap membrane distillation (AGMD)-concentrated solar power (CSP)/photovoltaic (PV) collector. The demonstration-scale system is the first-of-its-kind and has the potential to realize self-sustained zero-waste discharge of concentrate streams in inland and off-grid applications, producing up to 225 kWh of thermal energy directly supplied to the AGMD system and up to 500 L/day of high-quality distillate. Experiments were performed on the hybrid system for a two-year period to evaluate thermal performance for various operating conditions including MD and CSP flow rates, AGMD operating temperature, AGMD vacuum pressure, and thermal storage. Experimental results indicate that doubling the MD flowrate results in 116% increase in thermal energy utilization and nearly doubles distillate production. Compared to the winter months, operating the hybrid system in summer months when direct normal irradiance is at its peak results in a sixfold increase in average distillate production. An upgraded thermal storage configuration increased MD distillate production by approximately 50% compared to the original operational configuration. Furthermore, the relative specific thermal energy consumption decreases by 30% when the thermal storage reservoir is preheated in the winter. A techno-economic assessment was performed, and compared to conventional thermal desalination processes, the AGMD-CSP/PV system has lower specific energy demands and can produce high quality water for <1.50 $/m³. Results from this study highlight important design considerations for integrating thermal desalination with solar energy resources in an operational environment.