A new way to filter salt and metal ions from water

Australian and US researchers have discovered a way to utilise metal-organic frameworks (MOFs) — a next-generation material that has the largest internal surface area of any known substance — to make sea water suitable for drinking. The sponge-like crystals can be used to capture, store and release chemical compounds — in this case, the salt and ions in sea water.

The discovery was made by researchers at Monash University, CSIRO and The University of Texas at Austin, who found that MOF membranes can mimic the filtering function, or ‘ion selectivity’, of organic cell membranes. With further development, these membranes have the potential to perform the dual functions of removing salts from sea water and separating metal ions in a highly efficient and cost-effective manner, offering a revolutionary new technological approach for the water and mining industries.

Reverse osmosis membranes are currently responsible for more than half of the world’s desalination capacity, and the last stage of most water treatment processes, yet these membranes have room for improvement by a factor of 2 to 3 in energy consumption. Furthermore, they do not operate on the principles of dehydration of ions, or selective ion transport in biological channels, and therefore have significant limitations.

In the mining industry, membrane processes are being developed to reduce water pollution, as well as for recovering valuable metals such as lithium. However, at current rates of consumption, there is rising demand likely to require lithium production from non-traditional sources, such as recovery from salt water and waste process streams.

If economically and technologically feasible, direct extraction and purification of lithium from such a complex liquid system would have profound economic impacts. These innovations are now possible thanks to the new research, published in the journal Science Advances.

“We can use our findings to address the challenges of water desalination,” said Professor Huanting Wang from Monash University. “Instead of relying on the current costly and energy-intensive processes, this research opens up the potential for removing salt ions from water in a far more energy-efficient and environmentally sustainable way.

“Also, this is just the start of the potential for this phenomenon. We’ll continue researching how the lithium-ion selectivity of these membranes can be further applied. Lithium ions are abundant in sea water, so this has implications for the mining industry who current use inefficient chemical treatments to extract lithium from rocks and brines.

“Global demand for lithium required for electronics and batteries is very high. These membranes offer the potential for a very effective way to extract lithium ions from sea water, a plentiful and easily accessible resource.”

Image credit: ©Dmytro Tolokonov/Dollar Photo Club