Engineers at the University of California, Riverside (UCR) (Riverside, California) have developed a new route to recover almost 100% of the water from highly concentrated salt solutions. The system aims to alleviate water shortages in arid regions and reduce concerns surrounding high salinity brine treatment, including corrosion.
The research, which involves the development of a carbon nanotube-based heating system that could vastly improve the recovery of fresh water during membrane distillation processes, was recently published in the journal Nature Nanotechnology. The project was led by David Jassby, an assistant professor of chemical and environmental engineering at UCR.
While reverse osmosis is the most common method of removing salt from seawater, wastewater, and brackish water, it is not capable of treating highly concentrated salt solutions, the researchers explain. Such solutions, called brines, are generated in large amounts as waste products during reverse osmosis and as produced water during hydraulic fracturing. In each process, the brines must be disposed of properly to avoid environmental damage. In the case of hydraulic fracturing, the water is often disposed of underground in injection wells, but some studies suggest this could result in an increase in local earthquakes.
One way to treat brine is membrane distillation, a thermal desalination technology in which heat drives water vapor across a membrane. This technology allows for further water recovery while the salt stays behind.
However, hot brines are highly corrosive—making heat exchangers and other system parts expensive in standard distillation systems. Furthermore, because the process relies on the heat capacity of water, single pass recoveries are less than 10%, leading to stringent heat management requirements.
“In an ideal scenario, thermal desalination would allow the recovery of all the water from brine, leaving behind a tiny amount of a solid, crystalline salt that could be used or disposed of,” Jassby says. “Unfortunately, current membrane distillation processes rely on a constant feed of hot brine over the membrane, which limits water recovery across the membrane to about 6%.”
To improve on this, researchers developed a self-heating, carbon nanotube-based membrane that only heats the brine at the membrane surface. The new system reduced the heat needed in the process and increased the yield of recovered water to close to 100%.
In addition to improved desalination performance, the team also investigated how the application of alternating currents to the membrane heating element could prevent the degradation of carbon nanotubes in the saline environment. Specifically, a threshold frequency was identified where electrochemical oxidation of the nanotubes was prevented, allowing the nanotube films to be operated with no reduction in performance. The insights from the work could also allow carbon nanotube-based heating elements to be used in other applications, where electrochemical stability is a concern.
The research was supported by grants from the U.S. Office of Naval Research (Arlington, Virginia), Department of Energy (Washington, DC), the National Science Foundation (Washington, DC), and the Petroleum Research Fund (Washington, DC).
Source: UCR, ucrtoday.ucr.edu.