New membranes could help eliminate brine waste from desalination

Membranes packed with charge help overcome the current salinity limit, making it easier to harvest valuable minerals from desalination waste.
Jovan Kamcev, an assistant professor of chemical engineering and macromolecular science and engineering, demonstrates how he tests the performance of filters that remove salts and other chemicals from water. Each glass cell contains two compartments separated by a membrane — one A concentrated brine of table salt, the other pure water. Kamcev measures how much salt passes through the membrane by sticking conductivity probes into the compartments that hold the pure water. Because the salt exists as electrically charged particles in water, the conductivity of the pure water will increases as salt diffuses in. January 13, 2024. Photo by Marcin Szczepanski/Lead Multimedia Storyteller, Michigan Engineering

Membranes packed with charge help overcome the current salinity limit, making it easier to crystallise ocean salts and harvest valuable minerals from desalination waste.

Desalination plants, a major and growing source of freshwater in dry regions, could produce less harmful waste using electricity and new membranes made at the University of Michigan.

The membranes could help desalination plants minimise or eliminate brine waste produced as a byproduct of turning seawater into drinking water. Today, liquid brine waste is stored in ponds until the water evaporates, leaving behind solid salt or a concentrated brine that can be further processed. But brine needs time to evaporate, providing ample opportunities to contaminate groundwater.

Space is also an issue. For every litre of drinking water produced at the typical desalination plant, 1.5 litres of brine are produced. According to a UN study, over 37 billion gallons of brine waste are produced globally every day. When space for evaporation ponds is lacking, desalination plants inject the brine underground or dump it into the ocean. Rising salt levels near desalination plants can harm marine ecosystems.

“There’s a big push in the desalination industry for a better solution,” said Jovan Kamcev, U-M assistant professor of chemical engineering and the corresponding author of the study published today in Nature Chemical Engineering. “Our technology could help desalination plants be more sustainable by reducing waste while using less energy.”

To eliminate brine waste, desalination engineers would like to concentrate the salt so it can be easily crystallised in industrial vats rather than ponds occupying over a hundred acres. The separated water could be used for drinking or agriculture, while the solid salt could be harvested for valuable products. Seawater contains sodium chloride—or table salt—and valuable metals such as lithium for batteries, magnesium for lightweight alloys and potassium for fertiliser.

Desalination plants can concentrate brines by heating and evaporating the water, which is very energy-intensive, or with reverse osmosis, which only works at relatively low salinity. Electrodialysis is a promising alternative because it works at high salt concentrations and requires relatively little energy. The process uses electricity to concentrate salt, which exists in water as charged atoms and molecules called ions.

Here’s how the process works. Water flows into many channels separated by membranes, and each membrane has the opposite electrical charge to its neighbours. A pair of electrodes flank the entire stream. The positive salt ions move toward the negatively charged electrode and are stopped by a positively charged membrane. Negative ions move toward the positive electrode, stopped by a negative membrane. This creates two types of channels—one that both positive and negative ions leave and another that the ions enter, resulting in streams of purified water and concentrated brine.

But, electrodialysis has its salinity limits. As the salt concentrations rise, ions start to leak through electrodialysis membranes. While leak-resistant membranes exist on the market, they transport ions too slowly, making the power requirements impractical for brines more than six times saltier than average seawater.

The researchers overcome this limit by packing a record number of charged molecules into the membrane, increasing its ion-repelling power and conductivity. This means it can move more salt with less power. With their chemistry, the researchers can produce membranes that are ten times more conductive than relatively leak-proof membranes on the market today.

The dense charge attracts many water molecules, limiting the amount of charge that can fit in conventional electrodialysis membranes. The membranes swell as they absorb water, and the charge is diluted. In the new membranes, carbon connectors prevent swelling by locking the charged molecules together.

The level of restriction can be changed to control the leakiness and the conductivity of the membranes. Allowing some leakiness can push the conductivity beyond today’s commercially available membranes. The researchers hope the membrane’s customizability will help it take off.

“Each membrane isn’t fit for every purpose, but our study demonstrates a broad range of choices,” said David Kitto, a postdoctoral fellow in chemical engineering and the study’s first author. “Water is such an important resource, so it would be amazing to help make desalination a sustainable solution to our global water crisis.”

Study: Fast and Selective Ion Transport in Ultrahigh Charge Density Membranes (DOI: 10.1038/s44286-025-00205-x)

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