Reusable microbots make meal of toxic metals

Artist depiction of graphene-coated microbot cleaning lead from wastewater.

Researchers have developed a tube-shaped microbot that offers a cheaper and more effective way of removing heavy metals than previous methods. The self-propelled microbots use an outer layer of graphene that binds to lead ions it comes in contact with. The scientists found that they can remove 95 percent of lead from polluted water in one hour, and once they have a full payload, they can be cleaned and reused multiple times.

Heavy metal pollutants, such as lead, cadmium, mercury and arsenic, are a persistent problem in industrial processes and their subsequent runoff. Besides battery and electronics manufacturing, mining and electroplating are especially guilty of generating hazardous levels of these toxic byproducts. The researchers have focused on lead removal initially as a proof-of-concept, since lead is a major wastewater pollutant from the battery industry and others.

But the microbots are capable of removing other metals as well. "Each metal of contaminant has different affinities with this nanomaterial,"says lead author Samuel Sanchez. "These microbots could remove all contaminants that graphene can adsorb."

The microbots have three functional layers, including the outer layer of graphene oxide. The middle layer is made up of nickel, which gives the microbot ferromagnetic properties, allowing them to be controlled by an external magnetic field. The third and inner layer is platinum. When hydrogen peroxide is added to the water, it gets decomposed by the platinum into water and oxygen microbubbles, which are ejected from the back of the microbot to propel it forward.

The water is continually analyzed using external analytical devices. Once the contamination level stops decreasing, the researchers know the microbots are full, and a magnetic field is used to collect them from the water. To remove the lead (which can be reused), the microbots are treated with an acidic solution. "Different methods have been proven in this work without using organic compounds, such as decreasing temperatures," said Sanchez. "The best option was acid media that later could be neutralized."

According to Sanchez, the graphene surface is not really affected by the decontamination process and the recovery of lead, so the tubes can be reused until they start to look damaged, potentially lasting for months.

For the study, the researchers used approximately 400,000 microbots/mL for levels of 1000 parts per billion of lead in 3 mL of water to achieve the more than 95 percent lead removal. "The contamination determines the quantity of microbots," said Sanchez. "If the water has lower contamination, we could use less quantity."

The microbots could eventually be controlled by a smart device that automatically guides them through the water magnetically to achieve their cleanup tasks. As of now, using the microbots "in the wild" presents too much of a challenge, and work best in a contained area, such as wastewater tanks.

The study appeared in a recent issue of Nano Letters.

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