A study conducted by researchers at the Politecnico di Torino in Italy, the Massachusetts Institute of Technology (MIT) and the University of Minnesota has demonstrated an innovative way to improve the performance of desalination membranes. The findings of this study, which are published in Nature Communications, could lead to the development of membranes able to desalinate seawater at significantly reduced costs.
One way to desalinate seawater for making it drinkable involves pumping the seawater through a semi-permeable membrane in order to separate water molecules from dissolved salt ions, which are too large to pass through the membrane. The energy required for this process, known as reverse osmosis, can be provided by heat sources, electromagnetic fields or hydraulic pressure.
The process can be pictured as being like vehicles queuing at tollbooths before a highway. "Suppose that motorcycles are water molecules while cars are dissolved salt ions, and that both are patiently in line at the tollbooth,” say the researchers at Politecnico. "Now, let's imagine that the opening of the tollbooth is only 1m wide: motorcycles would be able to easily overcome the barrier and thus enter the highway, while cars would be forced to reverse course. Similarly, membranes for reverse osmosis allow the transport of water molecules, while blocking dissolved salts. Therefore, efficient membranes are characterized by large water transport rates at fixed input energy and effective surface, namely high permeability."
In this study, the researchers have been able for the first time to understand the mechanisms regulating the water transport from one side (salt water) to the other (fresh water) of the membrane. This follows work by the research laboratory at MIT to measure experimentally the diffusion coefficient of the permeated water, namely the mobility of water molecules through the membrane.
These membranes are made of zeolite, characterized by a dense (and ordered) network of pores with sub-nanometer diameter. The researchers discovered that the experimental diffusion coefficient of water appears to be almost a million times lower than the figure predicted by simulations and theoretical analyses. Working out why the experimental and theoretical coefficients are so different has required more than two years of work.
"While previous studies mainly focused on the transport process inside the membrane, we have shifted the attention on what was happening on the surface, where the solution to the puzzle could be actually found," say the researchers.
They discovered that water transport through the membrane is governed by two phenomena. First, water molecules have to find an open pore (surface resistance to transport); then they enter and diffuse through the membrane (volumetric resistance to transport), eventually leaking from the other side of the membrane. "Going back to the previous simile, adding further highway lanes can reveal as an insufficient strategy to speed up the journey of motorcyclists through the highway. In fact, we should also ensure that a sufficient number of open tollbooths are available, in order to avoid traffic jams at the entrance (and exit) of the highway," explain the researchers.
They have now shown that the orders-of-magnitude difference between theoretical and experimental values of membrane permeability is all down to the resistance to water transport at the surface of the membrane. This resistance stems from current techniques for manufacturing zeolite membranes, which result in the closure of more than 99.9% of the available pores.
In other words, water molecules can only permeate through a tiny fraction (one in a thousand) of the pores in the membrane surface: this causes a bottleneck effect, which slows down the overall water transport through the membrane and thus drastically reduces the membrane permeability. After more than two years spent on computer simulations and experiments, the researchers have unveiled this mechanism and proposed an accurate physical model of the overall water permeation process.
These findings clearly indicate that next-generation desalination membranes with enhanced performances could be produced by using manufacturing techniques that ensure a larger proportion of open pores. The researchers estimate that membranes manufactured with such techniques could possess a permeability 10 times larger than current ones, thus reducing the operating costs in desalination processes.
This new understanding of surface and volumetric transport phenomena could also lead to advances in other applications that use nanoporous materials. These applications include sustainable energy (for example, thermal storage), removal of pollutants from water (for example, molecular sieves) and nanomedicine (for example, drug delivery).
This story is adapted from material from Politecnico di Torino, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.