Globally, two billion people are without clean, safe drinking water. Could a material that can extract drinking water from desert air and seawater make a difference?
Metal-organic frameworks (MOFs) are super-high surface area materials with a lot of potential. They can capture, store and release chemical compounds, such as the salt and ions in seawater, and extract water molecules from the driest air.
Researchers in Australia and overseas have been looking at ways that these properties could be put to use delivering clean drinking water to those most in need.
At MIT, a research team have field tested a water-harvesting device that could fill an unmet need in the world’s most arid deserts.
The researchers say the device can harvest drinkable water from air with relative humidities as low as 10 per cent. This would make it suitable for Australian desert regions, which can have 20% humidity in parts of inland Western Australia, but average around 30% for most arid areas.
This is a big difference from existing methods, such as fog-harvesting and refrigeration systems for dew harvesting. Both of these systems need much higher relative humidity to work (100% and 50% respectively), and refrigeration-based systems are also energy hungry.
The MOF device is also free of moving parts, making it easy to maintain.
The solar powered MOF device was tested at Arizona State University in Tempe, which according to Professor Evelyn Wang, from the Department of Mechanical Engineering at MIT, was representative of the dry areas where the device might be deployed.
“[The test] showed that we can actually harvest the water, even at subzero dewpoints,” she said.
While the solar device can only operate over a single day and night, the researchers said that they could achieve continuous operation using readily available low-grade heat sources including biomass and waste heat.
While there is still a lot of work to do in scaling up this technology to produce more than a few millilitres, Wang said that her team would like to develop units that can be used by households to harvest drinking water from dry air.
“We want to see water pouring out!” she said.
Desalination applications
A collaborative team from CSIRO, Monash University and the University of Texas at Austin have been studying how MOFs can be applied to desalination of seawater.
The team have discovered that MOF membranes can act like organic cell membranes to selectively filter ions.
This has applications in the desalination of seawater, and could also be used to develop economical and efficient ways of separating metal ions for the mining industry.
According to Professor Huanting Wang from Monash University, the team’s research can be used to address the challenges of water desalination. Reverse osmosis membranes, which currently make up more than 50% of global water desalination, are power intensive and have limitations in the ion filtering they can perform.
“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,” Wang said.
