There are two established processes for seawater desalination, and both have disadvantages: “Reverse osmosis”, in which water is forced through a membrane at high pressure, cannot remove all pollutants. And while distillation can produce ultrapure water, it also requires a lot of energy. Researchers at the University of Colorado have now presented a desalination process that is intended to combine the advantages of reverse osmosis and distillation.
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It is based on an aluminum oxide membrane with tiny pores of 60 to 90 nanometers in size. This membrane is coated on one side with fluorinated alkylsilane. This makes one side of the pores water-repellent (“hydrophobic”). Liquid water can no longer penetrate the pores there, and they remain filled with air. However, if the water to be desalinated is put under high pressure of several dozen bars, it evaporates at the contact surface with the hydrophobic pores and penetrates in gaseous form into the air layer there, which is less than 200 nanometers thick. If it comes into contact with the water on the other, uncoated side of the pores, it condenses there again. So it is also a kind of distillation, but it is driven by pressure and not by heat.
The design of ultra-thin air-entrainment membranes for pressure-controlled vapor transport.
(Bild: Nguyen et al., Sci. Adv. 9, eadg6638 (2023) 14 July 2023)
Not only salt was removed
“The feasibility of desalination by pressure distillation is evident from the theory, but the demonstration of the system has not yet been possible due to the lack of suitable membranes,” the researchers write in their paper. According to their calculations, one square meter of membrane can desalinate 88 kilograms of water per hour at a pressure of 12 bar. Not only 99.8 percent of the salt was removed, but also up to 99.1 percent of other pollutants such as boron, urea and dimethylnitrosamine. Classic reverse osmosis can only remove around 45 percent of these substances. Therefore, the desalinated water usually has to be treated in a second step. This step would be omitted with the new method.
The new membrane is also insensitive to chlorine and ozone, which clog conventional osmosis membranes. So far, however, it has been less able to cope with surfactants, which reduce the surface tension of the water, and with biofouling (“fouling”). According to the researchers, suitable membranes can be made not only from aluminum oxide but also from other materials, such as hydrophobic polymers. How the membranes can be produced on a large scale and how they can be made even more robust is the subject of further research, the paper says.
(hrm)
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