Graphene filters to purify water

In a two-step process, engineers have successfully sealed leaks in graphene. First, the team fabricated graphene on a copper surface (top left) — a process that can create intrinsic defects in graphene, shown as cracks on the surface. After lifting the graphene and depositing it on a porous surface (top right), the transfer creates further holes and tears. In a first step (bottom left), the team used atomic layer deposition to deposit hafnium (in gray) to seal intrinsic cracks, then plugged the remaining holes (bottom left) with nylon (in red), via interfacial polymerization. Credit: MIT

We have plenty of water on our Planet and yet water, the blue gold, is still a dream for many people and many areas.

Oceans are full of water but it is salty and useless. Rain pours in tropical area but mix with soil and bacteria making it unsafe for drinking.

Actually, pure safe water has been the last invention of mankind (read the nice book, A history of the world in 6 glasses, where it shows that drinkable water was the last to come after beer, wine, spirit, coffee, tea, cola).

A lot of work has been done, and it is being done, in finding way to purify water, a process that today is quite expensive. 

One way to desalinate and purify water uses membranes that have tiny pores blocking undesired compounds, like metallic ions, salt and so on. These membranes are some 200 nm thick. Using membrane of graphene has been a goal for engineers because of they are much thinner, six hundred times thinner as a matter of fact, and that will provide higher filtering capacity.

The problem is to create a graphene membrane that has no holes throughout its surface, something almost impossible with todays manufacturing technology.

Now, researchers at MIT have found a way to seal the holes on a graphene layer by using chemical deposition and polymerisation to stop the leaks. 

They first create a membrane of the desired size (a few square centimetres, that may not seem big but when you look at it from the atomic scale of graphene it is about creating a surface with 10,000,000,000,000,000 - ten million billion- beads -atoms- with no faults) and then go on fixing imperfections that would let molecules go through by plugging all holes using hafnium atoms and then by polymerising the surface. The hope is to bring water purification within affordability limits to all those people that are still water starved, and that is 1.1 billion people today.

Author - Roberto Saracco

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