About the author
Jonathan de Roo
Water, the source of life. We cannot go without, and yet industrial waste water is still poured into rivers. Dead Rivers we call them. They are so heavily polluted that they cannot sustain life anymore. Our drinking water, too, is full of organic dirt such as hormones.
Therefore, Europe is now urging researchers to find sustainable solutions to purify water. Actually we have already found a clever solution: sponges. Researchers have designed a kind of sponge that doesn’t adsorb water but dangerous organic chemicals. How do you do that?
First of all, you have to find a material that likes these dirty organics. Actually, simple and cheap coal already does a very good job. Coal has a very porous structure, just as a sponge and attracts all kinds of pollutants (like bisphenol A). However, the airy structure is unordered and chaotic, and molecules often can’t find their way inside. It’s like a labyrinth and the entrance is crowded and blocked very quickly. Even worse, the holes are very small, less than 2 billionths of a metre (< 2 nanometres). As a result, big molecules – although equally harmful – cannot enter and escape the trap.
So we had to alter the design. We went from disorder to order, from too small to bigger. Actually “bigger” means 10 nanometres, which is still unbelievably small. But anyway, the result looks just like a honeycomb, – see the picture. It’s very organized and contains long, wide tunnels that take the pollutants deep inside the coal where it is trapped.
This material is called ‘mesoporous carbon’.
Now, what if this mesoporous carbon, this chemical sponge, has adsorbed all it can take? Can you squeeze it to remove the dirt like a normal sponge is squeezed to remove the water. No. That’s a pity but the only thing you can do is throw it away, single use only. That is not very sustainable, is it? Well, it’s a start but we aim to do better.
Top researchers are now investigating a new material: boron nitride. It has the same structure as mesoporous carbon but with one additional advantage: after adsorbing dirt, you can throw it in the fire! The boron nitride survives this treatment but all the organic pollutants are burned away. We call this process regeneration, and the material looks brand new and can be reused.
Now that’s a sustainable dirt sponge!
Acknowledgement to Wannes Libbrecht, PhD student at Ghent University