Drinking water from pipe

Salt and lithium could be filtered from seawater with next-generation material

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Researchers from institutions in Australia and the USA have developed a method for efficiently filtering ions from water using metal-organic frameworks (MOFs) to capture compounds such as salt and useful ions

MOFs are a ‘next-generation material’, made up of metal ions coordinated to organic molecules to form complex structures in one, two or three dimensions. They have the greatest internal surface area of any known substance. These spongey crystals have been used to capture, store and release various compounds.

Now, researchers from Monash University, the Commonwealth Scientific and industrial Research Organisation (CSIRO) and the University of Texas at Austin have demonstrated in a Sciences Advances paper that it is possible to use MOFs to filter water, mirroring the ion selectivity of cell membranes in their own membranes.

At present, the majority of the world’s desalination and the final stage of most water treatment processes use reverse-osmosis membranes. During reverse osmosis, a pressure is applied to remove ions and molecules from water.

Despite this approach being widespread, current reverse osmosis-membranes are not without their limitations, and could have their energy efficiency doubled or tripled. These membranes could be used to both remove salt from seawater and separate metal ions. This ability would prove useful not just in providing water security, but also in mining, for reduction of water pollution and for the recovery of valuable materials from salt water.

Thanks to the work of the international team, which uses an approach operating on the principle of dehydration of ions – a process which occurs naturally in biology – this type of filtration is possible.

“We can use our findings to address the challenges of water desalination,” said Professor Huanting Wang of 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.”

“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 seawater, so this has implications for the mining industry [that] currently use inefficient chemical treatments to extract lithium from rocks and brines. Global demand for lithium, required for electronics and batteries, is very high. The membranes offer the potential for a very effective way to extract lithium ions from […] seawater.”

“The prospect of using MOFs for sustainable water filtration is incredibly exciting from a public good perspective, while delivering a better way of extracting lithium ions to meet global demand could create new industries for Australia,” said Dr Anita Hill, executive director of future industries at CSIRO.

In April 2017, it was reported that researchers at the University of Manchester had used a “sieve” made of graphene to filter salt from seawater to produce safe drinking water for the first time.

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