Water purification through membrane separation has become a crucial process, especially in communities where water scarcity is common. Lawrence Livermore National Laboratory (LLNL) researchers designed carbon nanotube (CNT) pores to efficiently separate particles from water in a desalination process.
The findings have recently been published in the journal Science Advances describing an efficient desalination process. The tiny pores were designed to be 0.8 nanometers in diameter compared to human hair that is nearly 80,000 to 100,000 nanometers wide.
Traditional methods of desalination to produce potable water involve seawater or brackish water sources. Typically, seawater would be pressurized in a high-pressure pump before beginning multiple stages of purification.
The water would then go through reverse osmosis via thin-film composite membranes to filter out the salt ions. However, not all ions and micropollutants in the water are removed, such as chemical compounds or biological species like bacteria, requiring additional processes that increase costs and energy.
Depending on the type of water, additional steps include multiple activated carbon filters with smaller pores to trap ions. The water may also be treated with ultraviolet light to sterilize microbes that may have gone through the numerous reverse osmosis membrane filters.
In rural areas without desalination plants, some people use portable reverse osmosis water processors. These mobile devices are also commonly used by travelers going on island camping or fishing trips. However, some bacteria may still pass through the filter of these systems.
Designing Efficient Filters
Biological water channels, also known as aquaporins, can be used as a blueprint to develop structures that can improve water desalination processes, according to the study. Since aquaporins have "an extremely narrow inner pore that squeezes water down to a single-file configuration," it can efficiently block solutes such as ions and protons.
However, this may be a costly alternative and may have issues with long-term stability. Instead, the team designed an artificial water channel in carbon nanotubes that mimic the structure and functions of aquaporins.
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Desalination Via Carbon Nanotubes
Alex Noy, a chemist at LLNL, said, "Carbon nanotubes represent some of the most promising scaffold structures for artificial water channels because of the low friction of water on their smooth inner surfaces, which mimic the biological water channels." Its tiny structure enables strong selectivity, efficiently separating salt ions and micropollutants from water.
Using computer simulations, the team measured how water and chloride ions were transported through the carbon nanotube porins, short segments of CNTs. Their results showed that the activation energy was significantly lower than previous methods. When compared to controls, the vesicles (lipid bilayer of cells) "were extremely stable with no detectable content leaking," unlike traditional methods that need multiple filters.
"This process allowed us to determine the accurate value of water-salt permselectivity in narrow CNT pores," said materials scientist Tuan Anh Pham. "Atomistic simulations provide a detailed molecular-scale view of water entering the CNTP channels and support the activation energy values."
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