Desalination systems play an important role in improving access to fresh water, but some types of the natural resource are too salty for current solutions to handle. Such hypersaline water can have salt content as much as 10 times greater than seawater, and now scientists have developed a solar-powered desalination system that relies on a special coating to take the fight to this super-salty foe.
Hypersaline water can be produced through a number of industrial processes, including oil and gas production, along with current desalination approaches that separate out fresh water and create concentrated brine at the same time. This can’t be treated using common desalination techniques as it requires pressures that are too great, and is therefore expensive and energy-intensive. Disposing of hypersaline water, meanwhile, is typically a costly affair.
So scientists are working on next-generation desalination technologies that can take on this demanding task, including one group at Columbia University who last year demonstrated how a solvent could be added to the hypersaline brine to extract the salt content in a low-cost and efficient manner. The new breakthrough out of Rice University is also claimed to be low-cost and efficient, but takes a different approach entirely by leveraging a two-dimensional form of boron nitride.
Boron nitride comes in various shapes and sizes, but when fashioned into 2D hexagonal patterns just a single atom thick, it takes on some very useful properties. Sometimes called “white graphene,” this material has shown promise in the development of ultra-thin electronics and advanced solar cells, and now the Rice University team has found it could play a crucial role when it comes to treating salty water.
Its value in this type of application, the team found, lies in how it can facilitate greater temperature differentials on either side of the membrane in a desalination device. Generally speaking, as brine flows over one side of these porous membranes, it is heated by a special coating, which creates a difference in temperature with cold fresh water on the opposite side. This in turn generates a pressure gradient that forces water vapor through the membrane to filter out salts and other contaminants.
The trouble is hypersaline water is more corrosive when heated, which promptly destroys the heating elements in the device and renders it useless. And this is where the 2D boron nitride comes in, with the team growing the hexagonal layers on top of a stainless steel mesh wire, which acts as the heating element and was incorporated into a commercial membrane.
This formed part of a solar-powered desalination device, where light-activated nanoparticles fixed to the membrane gather all the required energy from the sun. In testing, the researchers found that this design was able to desalinate hypersaline water with “exceptionally” high outputs of fresh water, at 42 kg (92 lb) per square meter (11 sq ft) of membrane, which they claim is more than 10 times greater than what current ambient solar membrane technologies are capable of.
This was with the voltage of the heating element running at the same 50-Hz frequency of a household AC supply and power densities of up to 50 kW per sq m. The material reportedly maintained excellent stability and demonstrated high thermal conductivity, vapor permeability and resistance to corrosion, thanks to the special coating. Notably, the team was able to incorporate the technology into a coiled membrane system, which bodes well for a compact solution that could be transported and deployed as needed. The researchers are now looking for industry partners to help develop a larger-scale version for field tests.
“We’re ready to pursue some commercial applications,” says Qilin Li, professor of civil and environmental engineering at Rice University.“ Scaling up from the lab-scale process to a large 2D CVD sheet will require external support.”
The research was published in the journal Nature Nanotechnology.
Source: Rice University
