Researchers in the Czech Republic have used tiny robots to remove contaminants from water. These included the toxic substance arsenic. An international team of nano-robot researchers described in the lab experiment this week Nature- Communications† It is highly questionable whether the technology can also be used outside the lab.
Arsenic belongs to the group of heavy metals, just like lead and mercury, and is ‘highly toxic’ according to the World Health Organization. It occurs naturally in the earth’s crust. In some places, arsenic ends up in drinking water. For example in Bangladesh and Turkey where arsenic is in the soil.
For example, a traditional way to remove arsenic from drinking water is by adding certain molecules to the water to which arsenic binds. The compound can then be filtered out of the water. But that is an expensive technique.
An alternative method, not yet widely applied, is the temporary addition of cleaning nano-robots to the drinking water that can be used repeatedly. These usually have a size of hundreds to tens of thousands of nanometers (a nanometer is one millionth of a millimeter). These nano-robots move through the water and are controlled by small internal motors or magnets. Most of those robots use metal as a ‘cleaning agent’; on the surface of the robots, this reacts with contaminants in the water. The only problem was that those robots don’t last long. If metal is in the water for a long time, it will rust.
The nanorobots will clump when the water gets warmer
That’s why the researchers built nano-robots that don’t use metal, but that capture the contaminants by surrounding them together. This encirclement occurs through an increase in water temperature. At 5 degrees Celsius, the robots move independently of each other through the water. When the temperature of the water is raised to 25°C, the robots begin to clump and form a kind of gel. Contaminants are trapped between the clots.
The clumping takes place thanks to a temperature-sensitive layer around the robots, Pluronic Triblock Copolymer (PTBC). PTBC can have molecular chains of different lengths, and depending on this, they may or may not clump at a certain temperature. The length of the robots has been adjusted in such a way that they clump at 25, but not at 5°C.
Once the robots have clumped together and have trapped contaminants, the researchers can use magnets to remove the clumps of robots from the water. When they cool them down again to 5°C, the robots release each other and the contaminants and they can go back into the water.
Bowl of water in the lab
The researchers tested the nano-robots in several experiments in a basin of water in the lab. In the experiments where many robots went into the water (18.6 mg per liter), they succeeded in removing the majority of both toxic substances (the other substance was the pesticide atrazine).
“Beautiful complex chemistry, but not applicable in practice,” responds Mark van Loosdrecht, professor of biotechnology at TU Delft and not involved in the study. “Purifying water has to be very cheap, a few cents per cubic meter. For this method, heating the water alone is too expensive due to the amount of energy required. The researchers first devised a nice technology, and only then linked it to a relevant social problem. That happens often in the academic world.”
Bert Meijer, professor of chemistry at Eindhoven University of Technology, was also not involved in the study: “Another problem is that the researchers can capture hundreds of types of substances in the water with this method; they do not specifically capture one contaminant. Things went well in the lab, because then there was only arsenic or atrazine in the water, but there are all kinds of molecules in drinking water. There are substances that only bind with a specific contaminant. Only when the robots, with the help of such substances, only capture specific contaminants, do I see a future in this.”
A version of this article also appeared in NRC Handelsblad of 4 March 2022
A version of this article also appeared in NRC in the morning of March 4, 2022