Seawater desalination has been hailed as the way of the future for meeting our ever-expanding water needs, but current methods are inefficient and environmentally destructive. In a world first, an international team of researchers has developed a method for extracting more fresh water from the process, and recovering valuable resources from the by-products.
Centre for Technology in Water and Wastewater
Australian Research Council Discovery Project grant
G Naidu, P Loganathan, J Kandasamy, To Vu Hien Phuong, Tanjinna Nur, Youngkwon Choi, Professor Anthony Fane and Professor Rong Wang from Nanyang Technology University Singapore, Professor Hae Moon from Chonnam National University, Korea
Desalination of seawater has been slated as a way to meet Australia’s growing water needs for irrigation, industry and domestic use.
However, the current process of reverse osmosis used by most Australian — and international — desalination plants to turn seawater into potable water produces only 40 to 50 per cent fresh water.
“It also has a detrimental impact on marine ecology,” says Director of UTS’s Centre for Technology in Water and Wastewater, Professor Saravanamuth Vigneswaran, because the process creates significant amounts of concentrated brine as a by-product that is then pumped back into the sea.
Vigneswaran and his team have created a technology to make water desalination more economically viable and more environmentally friendly at the same time by extracting rubidium from the waste brine.
Rubidium is a rare and valuable metal worth about A$10,000 per kilogram and is used in lamps, night vision devices, fibre optics, semiconductor technology and fibre optics.
To access this metal, the UTS researchers are using membrane distillation technology with novel membranes to concentrate the brine and use copper potassium hexacyanoferrate-based (KCuFC) “adsorbent” in a granular form to extract the rubidium.
An adsorbent is a material that extracts certain substances from gases, liquids or solids by attracting the materials to stick to its surface.
“It mops it up,” Professor Vigneswaran says. “It selectively absorbs rubidium, which is then desorbed (released through pores in the material) by using environmentally-friendly chemicals and concentrated using repeated crystallisation.”
The UTS team is collaborating with one of the world’s foremost researchers in membrane distillation technology, Professor Tony Fane, at the Nanyang Technology University in Singapore.
Since the process is more effective and efficient if the brine is more concentrated, the team is reducing the brine’s volume fourfold by membrane distillation. This creates more concentrated rubidium as well as an additional 80 per cent more desalinated water.
“Brine disposal is always a huge issue in desalination plants so this method will solve a lot of the problem for them and they get more value out of it,” says Vigneswaran.
The team’s use of this technology to extract rubidium from seawater brine is a world-first, and they are planning to take it a step further to extract strontium.
Although not as valuable as rubidium, some of the varied uses of strontium combined with other chemicals include creating glass for colour television cathode ray tubes, treatment of osteoporosis, producing the deep red colour in fireworks, ferrite magnets and optics applications.
While other minerals such as lithium, are also found in the brine, they are in much lower traces and not economical to collect.
The only downside of the project is the cost of the thermal energy needed for the membrane distillation process, but Vigneswaran and his team are also looking at ways to use solar and recycled energy with new generation electrospun membranes to address this problem.
Photographer: Jesse Taylor
Photographer: Tim Geers, via Flickr