Desalinating (and drinking) seawater: Towards novel desalination membranes with enhanced performance

Desalinate seawater at reduced costs: this is the ambitious goal of the study of a team of engineers from the Department of Energy at the Politecnico di Torino (Torino, Italy), in collaboration with the Massachusetts Institute of Technology-MIT (Cambridge, USA) and the University of Minnesota (Minneapolis, USA). The journal Nature Communications has recently published the results of this research, thus opening up a new path in the development of technologies for membrane-based desalination processes, which could provide innovative solutions for the current water scarcity issue in several countries.

Seawater can be desalinated and made drinkable by means of a membrane, namely a "sieve" capable of separating water molecules from dissolved salt ions. The energy required in this separation process can be provided by heat sources, electromagnetic field or hydraulic pressure. In particular, the research presented by the Italian and American institutions has focused on the desalination process by reverse osmosis, which is based on the capacity of some porous materials -- under pressures larger than the osmotic one -- to be permeated by only water molecules, while rejecting salt ions.

This process can be better pictured as a series of vehicles queueing at the tollbooths for entering the highway. "Suppose that motorcycles are water molecules while cars are dissolved salt ions, and that both are patiently in line at the tollbooth," researchers of Politecnico illustrate. "Now, let's imagine that the opening of the tollbooth is only one meter 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."

Researchers at the Politecnico di Torino, MIT and the University of Minnesota, however, have taken a step further, being 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. In fact, the research laboratory at MIT has experimentally measured the diffusion coefficient of the permeated water, namely the mobility of water molecules while crossing the membrane. These membranes are made of zeolite, which is a material characterized by a dense (and ordered) network of pores with subnanometer diameter (less than a billionth of meter). However, the experimental diffusion coefficient of water appears to be almost a million times lower than the one expected by simulations and theoretical analyses, as measured by the researchers at Politecnico di Torino. A puzzle that required more than two years of activities between Torino and Boston, thanks to the MITOR collaborative research programme funded by Compagnia di San Paolo.

Researchers then explain that: "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." In fact, the water transport through the membrane is governed by a series of two phenomena: first, water molecules have to find an open pore (surface resistance to transport); then, they can enter and diffuse within 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 tollbooth is available, in order to avoid traffic jams at the entrance (and exit) of the highway" researchers say.

The scientists have thus shown that the orders-of-magnitude difference between theoretical and experimental values of membrane permeability is due to the resistance to water transport shown by the surface of membranes. This resistance stems from the current manufacturing techniques of zeolite membranes, which cause the closure of more than 99.9% of the available pore mouths. In other words, water molecules can permeate through a minimal fraction (one per thousand) of surface pore openings: 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, Matteo Fasano, Alessio Bevilacqua, Eliodoro Chiavazzo, Pietro Asinari (Multi-Scale Modelling Lab, Department of Energy at Politecnico di Torino), Thomas Humplik, Evelyn Wang (Device Research Laboratory, MIT) and Michael Tsapatsis (Tsapatsis Research Group, University of Minnesota) 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 can be achieved by manufacturing techniques allowing to reduce surface resistances to transport, namely to open a larger fraction of surface pores. The researchers estimate that membranes manufactured following these criteria have the potential to achieve permeability 10 times larger than current ones, thus reducing the operating costs in desalination processes. This new understanding of surface and volumetric transport phenomena also breaks new ground in other applications in which nanoporous materials are used: from technologies for sustainable energy (for example, thermal storage) to removal of pollutants from water (for example, molecular sieves), up to nanomedicine (for example, drug delivery).