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A thin coating of the 2D nanomaterial hexagonal boron nitride is the key ingredient in a cost-effective technology developed by Rice University engineers for desalinating industrial-strength brine.
Boron Nitride Coating - Key Ingredient in Desalination Technology
Hypersaline water, which can contain 10 times more salt than seawater, is an increasingly important challenge for many industries. Industrial processes, in general, produce salty wastewater because the trend is to reuse water. Many industries are trying to have ‘closed loop’ water systems. Each time you recover freshwater, the salt in it becomes more concentrated. Eventually the wastewater becomes hypersaline and you either have to desalinate it or pay to dispose of it.
Conventional technology to desalinate hypersaline water has high capital costs and requires extensive infrastructure. NEWT, a National Science Foundation (NSF) Engineering Research Center (ERC) headquartered at Rice’s Brown School of Engineering, is using the latest advances in nanotechnology and materials science to create decentralized, fit-for-purpose technologies for treating drinking water and industrial wastewater more efficiently.
Membrane Desalination Process
One of NEWT’s technologies is an off-grid desalination system that uses solar energy and a process called membrane distillation. When the brine is flowed across one side of a porous membrane, it is heated up at the membrane surface by a photothermal coating that absorbs sunlight and generates heat. When cold freshwater is flowed across the other side of the membrane, the difference in temperature creates a pressure gradient that drives water vapor through the membrane from the hot to the cold side, leaving salts and other nonvolatile contaminants behind.
A large difference in temperature on each side of the membrane is the key to membrane desalination efficiency. In NEWT’s solar-powered version of the technology, light-activated nanoparticles attached to the membrane capture all the necessary energy from the sun, resulting in high energy efficiency. Li is working with a NEWT industrial partner to develop a version of the technology that can be deployed for humanitarian purposes. But unconcentrated solar power alone isn’t sufficient for high-rate desalination of hypersaline brine, she said.
2D Nanomaterial Hexagonal BN-coating Protects Heating Element
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The energy intensity is limited with ambient solar energy,” said Li, a professor of civil and environmental engineering. “
The energy input is only one kilowatt per meter square, and the production rate of water is slow for large-scale systems.”
Adding heat to the membrane surface can produce exponential improvements in the volume of freshwater that each square foot of membrane can produce each minute, a measure known as flux. But saltwater is highly corrosive, and it becomes more corrosive when heated. Traditional metallic heating elements get destroyed quickly, and many nonmetallic alternatives fare little better or have insufficient conductivity.
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We were really looking for a material that would be highly electrically conductive and also support large current density without being corroded in this highly salty water,” Li said.
Boron nitride’s combination of chemical resistance and thermal conductivity has made its ceramic form a prized asset in high-temperature equipment, but hBN, the atom-thick 2D form of the material, is typically grown on flat surfaces.
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This is the first time this beautiful hBN coating has been grown on an irregular, porous surface,” Li said.
“It’s a challenge, because anywhere you have a defect in the hBN coating, you will start to have corrosion.”
The researchers showed that the wire mesh coating, which was only about one 10-millionth of a meter thick, was sufficient to encase the interwoven wires and protect them from the corrosive forces of hypersaline water. The coated wire mesh heating element was attached to a commercially available polyvinylidene difluoride membrane that was rolled into a spiral-wound module, a space-saving form used in many commercial filters.
Source: Rice University