Seawater is a complex cocktail of useful minerals, but it's hard to separate out the specific ones we need. Now, a team of scientists from Australia and the US has developed a new water desalination technique that can not only make seawater fresh enough to drink, but recover lithium ions for use in batteries.
The key to the process is metal-organic frameworks (MOFs), which boast the largest internal surface area of any known material. Unfolded, a single gram of the material could theoretically cover a football field, and it's this intricate internal structure that makes MOFs perfect for capturing, storing and releasing molecules. Recent research into the material could see MOFs put to work as carbon emission sponges, high-precision chemical sensors, and urban water filters.
Currently, reverse osmosis membranes are the most commonly-used technology for water filtration, and they work on a fairly simple principle. The membrane's pores are large enough for water molecules to pass through, but too small for most contaminants. The problem is that to work, these systems require water to be pumped through at relatively high pressure.
MOF membranes, on the other hand, can be more selective and efficient. Researchers at Monash University, the CSIRO and the University of Texas at Austin have now developed just such a membrane. The design was inspired by the "ion selectivity" of biological cell membranes, allowing the MOF material to dehydrate specific ions as they pass through. Better yet, these filters don't require water to be forced through, saving on energy use as well.
"We can use our findings to address the challenges of water desalination," says Huanting Wang, an author of the new study. "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."
But clean drinking water isn't the only end product of the MOF membrane. Lithium is in high demand, thanks to the lithium-ion batteries that power everything from smartphones to electric cars. Those ions are left behind in the spongey structure, ready for the taking.
"Lithium ions are abundant in seawater, so this has implications for the mining industry who currently use inefficient chemical treatments to extract lithium from rocks and brines," says Wang. "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 seawater, a plentiful and easily accessible resource."
The technique could also be put to work filtering waste water from industrial processes like fracking.
"Produced water from shale gas fields in Texas is rich in lithium," says Benny Freeman, co-author of the study. "Advanced separation materials concepts such as ours could potentially turn this waste stream into a resource recovery opportunity."
The researchers say they plan to continue studying how to make MOFs even better at selecting for lithium ions.