No Filter: Microfluidics Makes Desalination Membrane-Free

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Seven billion people share one percent of Earth’s water. What’s more worrisome: our demand for freshwater increases every day. Advances in desalination have promised access to the oceans’ vast supply, but the majority of desalination facilities burn fossil fuels and require expensive membrane materials. 

Nevertheless, a new breakthrough in desalination technology may make desalination more viable in the future. Hayder Abdulbari and Esmail Basheer, chemists from the University of Malaysia Pahang, recently developed a membrane-free filtration technique that may be a first step toward better desalination technology. They investigated a well-known filtration process called directional solvent extraction (DSE). Their contribution, however, lay in shrinking the lab down to the size of a fingertip.

Abdulbari and Basheer turned to the emerging technology of microfluidics to shrink down DSE. Microfluidic devices called chips are small blocks of a resin called PDMS. The chips are etched with tubes less than a millimeter wide that enable researchers to constrain fluids in a way that changes their behavior entirely. At such a small scale, cohesive, adhesive, and electrical forces overpower gravitational forces. Like the capillary action that draws liquid up tree trunks and coffee stirrers, fluid is pulled up as it adheres to the surface inside a tube, defying gravity. This behavior allows researchers to manipulate diffusion, dissolution, and mixing much more precisely than at larger scales.

Normally, DSE exploits differences in density to separate solvent and solute, relying on gravity, but this process is inefficient and doesn’t remove much salt. However, at the microfluidics scale, the researchers were able to approach extraction in a new way: instead of removing the salt from the water, they removed the water from the salt.

This may seem impossible, but microfluidics makes it a reality. A saline solution passes into a microfluidic chip, where it mixes with octanoic acid—a key step in desalination. Octanoic acid is a polar fatty acid, so it can bind to the polar hydrogen atoms in water molecules while repelling the Na+ and Cl ions from salt. Thus, as the solution moves through a long network of channels on the chip, more and more water molecules bind to octanoic acid, lowering salinity even further. The salt remains behind while the water continues through the capillary. Finally, the octanoic acid is separated from the water in a decanting process.

The microscale version of this experiment is almost eighty percent more efficient than the macroscale version. However, microscale DSE still has a long way to go. Yale graduate student Xuechen Zhou noted that microfluidics is still expensive and applicable only at the research scale: “It’s complex. Right now, it’s only at the lab scale for fundamental study. In the future, it depends. It’s hard to say.” The study of microfluidics is still in its infancy, and microfluidic DSE even more so. Nonetheless, as microfluidics continues to grow and intersect with more fields, innovations that bring costs down are likely to be on the horizon. Solutions in science are incremental, and each first step is a step in the right direction.