Industrial activities, from mining to the manufacturing of batteries, often result in heavily contaminated wastewater. One of the main pollutants is usually heavy metals – such as lead, mercury and arsenic – and in order to remove them from the water, dispersants and other chemicals then have to be added. But what if this clean up could be achieved using just a magnet? 
That is what researchers at the Max-Planck Institute for Intelligent Systems have managed to do, using tiny graphene microbots as their cleaners. In a series of experiments, the team have been able to show that the microbots can remove up to 95 percent of lead in polluted water in just one hour, self-propelling themselves through the water. What’s more, when they're done, the bots can then be easily removed using magnets, treated in an acidic solution, and then reused.
“This work is a step toward the development of smart remediation system where we can target and remove traces of pollutant without producing an additional contamination,” explained Samuel Sánchez, who co-authored the paper published in Nano letters, in a statement. If the researchers can scale up their experiments and reduce the amount of money it costs to make them, then the microbots could potentially offer a more environmental and economical solution to the clean-up of heavy metal pollutants.
Illustration of the graphene microbot removing lead from wastewater. Vilela et al. 2016
The tiny microbots are hollow tubes formed of three separate layers. The outermost is made from graphene oxide that absorbs the lead particles from the wastewater. The middle layer is crafted from nickel, making the microbots ferromagnetic and thus allowing the researchers to control the direction of the little tubes; the inner layer is formed of platinum, which gives the bots propulsion. This is because when hydrogen peroxide is added to the water, the platinum breaks it down into tiny bubbles of water and oxygen, which are then directed out the back of the tube, powering it forward.
When the bots have absorbed as much of the lead in the water as they can, they are then removed using a magnet. They can then be bathed in an acidic solution to remove the lead, which is recovered to potentially be reused, along with the now-clean microbots. “The use of self-powered nanomachines that can capture heavy metals from contaminated solutions, transport them to desired places and even release them for 'closing the loop' – that is a proof-of-concept towards industrial applications,” said Sánchez. The researchers also hope that they could then be applied to environmental issues, helping to clean up heavy metals spills.
Image in text: GIF of the microbots in action being guided by magnets. Viela et al. 2016
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