Everyone Was Wrong About Reverse Osmosis--Until Now
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science
May 2023
Menachem Elimelech never made peace with reverse osmosis. Elimelech, who founded Yale's environmental engineering program, is something of a rock star among those who develop filtration systems that turn seawater or wastewater into clean drinking water. And reverse osmosis is a rock star among filter technologies: It has dominated how the world desalinates seawater for about a quarter of a century. Yet nobody really knew how it worked. And Elimelech hated that.
Still, he had to teach the technology to his students. For many years, he showed them how to estimate the high pressures that push the water molecules in seawater across a plastic polyamide membrane, creating pure water on one side of the film and leaving an extra-salty brine on the other. But these calculations relied on an assumption that nagged Elimelech and other engineers: that water molecules diffuse through the membrane individually. "This always bothered me. It does not make any sense," he says.
This might seem like an arcane engineering question, but Elimelech's beef with reverse osmosis is based on a real-world problem. Over 3 billion people live in areas where water is scarce. By the year 2030, demand is set to outstrip supply by 40 percent.
And transforming water from salty seas into something potable has always been energy intensive. Older thermal desalination plants in the Gulf States--where energy is plentiful--distill seawater by boiling it and capturing the vapor. A newer generation of reverse osmosis desalination plants, which run the water through an array of plastic membranes, have cut the energy demand a little, but it's not enough. It still takes a lot of power to push water through dense filters, so even minor improvements in membrane design go a long way.
In a study published in April, Elimelech's team proved that the once-frustrating assumption about how water moves through a membrane is, indeed, wrong. They replace it with a "solution-friction" theory that water molecules travel in clusters through tiny, transient pores within the polymer, which exert friction on them as they pass through. The physics of that friction matter, because understanding it could help people design membrane materials or structures that make desalination more efficient or better at screening out undesirable chemicals, Elimelech says.
More effective membranes could also improve municipal water systems and expand the reach of desalination. "This is one of those major breakthroughs," says Steve Duranceau, an environmental engineer at the University of Central Florida, who spent 15 years designing desalination plants before becoming a professor. "This will change the way that people start modeling, and interpreting how to design these systems."
"They've nailed it," agrees Eric Hoek, an environmental engineer at UCLA who trained under Elimelech 20 years ago but was not involved in the study. "Finally, somebody has put the nail in the coffin."
The roots of the new solution-friction idea are actually old. The molecular math behind it dates to the 1950s and 1960s, when Israeli researchers Ora Kedem and Aharon Katzir-Kachalsky, and UC Berkeley researcher Kurt Samuel Spiegler, derived desalination equations that considered friction--meaning how water, salt, and pores in the plastic membrane interact with each other.
Friction is resistance. In this case, it tells you how hard it is for something to get across the membrane. If you engineer a membrane that has less resistance to water, and more resistance to salt or whatever else you want to remove, you get a cleaner product with potentially less work.
But that model got shelved in 1965, when another group introduced a simpler model. This one assumed that the plastic polymer of the membrane was dense and had no pores through which water could run. It also didn't hold that friction played a role. Instead, it presumed that water molecules in a saltwater solution would dissolve into the plastic and diffuse out of the other side. For that reason, this is called the "solution-diffusion" model.
Diffusion is the flow of a chemical from where it's more concentrated to where it's less concentrated. Think of a drop of dye spreading throughout a glass of water, or the smell of garlic wafting out of a kitchen. It keeps moving toward equilibrium until its concentration is the same everywhere, and it doesn't rely on a pressure difference, like the suction that pulls water through a straw.
The model stuck, but Elimelech always suspected it was wrong. To him, accepting that water diffuses through the membrane implied something strange: that the water scattered into individual molecules as it passed through. "How can it be?" Elimelech asks. Breaking up clusters of water molecules requires a ton of energy. "You almost need to evaporate the water to get it into the membrane."
Still, Hoek says, "20 years ago it was anathema to suggest that it was incorrect." Hoek didn't even dare to use the word "pores" when talking about reverse osmosis membranes, since the dominant model didn't acknowledge them. "For many, many years," he says wryly, "I've been calling them 'interconnected free volume elements.'"
Over the past 20 years, images taken using advanced microscopes have reinforced Hoek and Elimelech's doubts. Researchers discovered that the plastic polymers used in desalination membranes aren't so dense and poreless after all. They actually contain interconnected tunnels--although they are absolutely minuscule, peaking at around 5 angstroms in diameter, or half a nanometer. Still, one water molecule is about 1.5 angstroms long, so that's enough room for small clusters of water molecules to squeeze through these cavities, instead of having to go one at a time.
About two years ago, Elimelech felt the time was right to take down the solution-diffusion model. He worked with a team: Li Wang, a postdoc in Elimelech's lab, examined fluid flow through small membranes to take real measurements. Jinlong He, at the University of Wisconsin-Madison, tinkered with a computer model simulating what happens at the molecular scale as pressure pushes salt water through a membrane.
Predictions based on a solution-diffusion model would say that water pressure should be the same on both sides of the membrane. But in this experiment, the team found that the pressure at the entrance and exit of the membrane differed. This suggested that pressure drives water flow through the membrane, rather than simple diffusion.
They also found that water travels in clusters through the interconnected pores, which, though tiny, are large enough that the water doesn't have to scatter into single molecules to squeeze through. Those pores seemed to appear and disappear across the membrane over time, thanks to the applied pressure and natural molecular motion.
Depending on the membrane material, these pores interact differently with water, salt, or other compounds. Elimelech thinks engineers could design membranes to better reject salt (by maximizing how much the pores interact with them) or reduce friction with water (by making the pores less attracted to it, so it slips on by). Making it easier to separate the two means you could use less pressure and reduce energy cost.
Or, he thinks, engineers could tailor membranes to filter out environmental nasties, like boron and chlorides. Roughly 20 percent of boron from seawater slips through membranes as boric acid. That quantity is safe for people but potentially toxic for crops that are irrigated with wastewater. In Israel, water purification plants have to take extra detoxifying steps just to cut out the boron and chlorides in water used for agriculture. If you can filter these out on the initial pass, Elimelech says, "You can save on capital costs and energy."
Hoek thinks the idea is plausible--but not quite there yet. (His colleagues recently explored designing membranes for boron rejection.) Engineers might tinker with channel size, local pH, or electrical charges on the membrane pores, he suggests.
And this may go beyond boron, chloride, or even desalination. Municipal utility plants use reverse osmosis to remove hazardous PFAS "forever chemicals" from drinking water. Current membranes are still regarded as the best approach, but many researchers are determined to design better ones to capture the toxic compounds.
Duranceau dreams of membranes that are as flexible and customizable as clothing--which can be selected based on whatever the user needs. After all, membranes are plastics, the paragon of customizability. Maybe, the engineers think, this knowledge will lead to membranes made of materials other than polyamide that would be better at screening out PFAS or lead. Or perhaps the membrane one chooses will depend on how salty the water is--from brackish to brine.
That may take a while--Elimelech even wonders if it would be best to use an algorithm to search for a membrane material that can beat polyamide, the way biotech companies have turned to machine learning to screen for new drugs. "But it's very challenging," he points out, because in the last 40-odd years, no one has found anything better. At least now, though, the science of water flow is running clear.
