Towards spatiotemporal-resolved spectroscopic measurement of freezing desalination using femtosecond laser filaments

Freezing desalination can produce freshwater from seawater or industrial wastewater by ice crystallization with low energy consumption and less corrosion, and is promising for alleviating current stress on the world's water supply. However, the salt rejection dynamics during the freezing process is less understood due to the lack of real-time, high spatiotemporal-resolved detection methods. Here we present a proof-of-principle, quantitative, and simultaneous multi-salt detection approach for the measurement of salt rejection dynamics in salt-contaminated water at the ppm level using an ultrashort-pulsed femtosecond laser propagating in the filamentation regime. We show that this approach of filament-induced plasma spectroscopy possesses the abilities for both monitoring the change of salt concentration in the remaining unfrozen water during the freezing process and imaging the spatial concentration distributions of metal contaminants on the resultant ice surface. With this approach, we find that the initial salt concentration in water plays an essential role in the efficiency of the freezing desalination process, which shows a metal-species-dependent characteristic. Our results pave the way towards real-time, online, and high spatiotemporal-resolved detection of salt rejection dynamics of freezing desalination by ultrashort-pulsed lasers.