Working with colleagues in Stuttgart and Singapore, Samuel Sánchez’s group used graphene oxide to make their microscale motors, which are able to adsorb lead from industrial wastewater from a level of 1000 parts-per-billion to down to below 50 parts-per-billion in just an hour. The lead can later be removed for recycling, and the micromotors can be used over and over again.
“The outer shell of the microbot, which is graphene, captures the lead,” says Samuel, an ICREA professor and group leader at IBEC and the Max-Planck Institute for Intelligent Systems in Stuttgart. “The inner layer of platinum works as the engine, decomposing hydrogen peroxide as fuel so that the bot can self-propel.” When hydrogen peroxide is added to the wastewater, the platinum decomposes it into harmless water and oxygen bubbles, which are ejected from the back of the microbot to propel it forward. “It’s important to use a system of pollutant removal that doesn’t produce any additional contamination,” explains Samuel.
Between the graphene oxide and platinum layers is a layer of nickel that allows researchers to control the movement and direction of the microbot magnetically from outside. “A magnetic field can be used to collect them all from the water when they’ve finished,” says Samuel. “In the future, our microbot swarm could be controlled by an automated system that magnetically guides them to carry out various tasks.”
Heavy metal contamination in water – by lead, arsenic, mercury and other metals – stems from industrial activities and poses a serious risk to public health and wildlife. These new microbots – each one smaller than the width of a human hair – offer a solution that is potentially faster and cheaper than current methods of water cleaning, as well as being environmentally friendly: they enable the gathered pollutants to be dealt with responsibly by relinquishing the lead afterwards for recycling, as well as being reusable themselves.
“We now plan to develop our microbots to be able to collect other contaminants, as well as reducing the cost of making them and being able to mass-produce them,” says Samuel, who also works on self-propelling micro- and nanorobots for applications in areas such as drug delivery.
Source paper: Diana Vilela, et al. “Graphene-Based Microbots for Toxic Heavy Metal Removal and Recovery from Water.” Nano Letters, 10.1021/acs.nanolett.6b00768
Samuel Sánchez: Q&A
- Samuel, how are your microbots an improvement on the current methods that are used to decontaminate water polluted with heavy metals?
“Current methods are reverse osmosis, ion exchange, membrane filtration and chemical precipitation of heavy metal ions, which are either expensive or produce secondary waste. The main advantage of our microbots is that they are more effective, and offer the possibility to recover the wasted heavy metal for recycling without creating any secondary waste or sludge. Additionally, our microbots are easy and simple to use, with very low installation costs.
“Graphene itself is already a very good material for removing heavy metals, and our results shows that we can increase its effectiveness by using active microbot design – meaning that it is the microbot that moves around in the water, and not the water that is flushed through membranes, etc., using external sources such as high pressure. That decreases the energy consumption and thus the cost.
“However, the platinum used for the catalytic material is rather expensive. It’s possible to use an alternative catalyst material such as manganese dioxide to reduce the cost for possible commercial applications; and re-usability decreases the cost further, of course. Also, we demonstrated in our paper that the use of peroxide can be avoided by using a magnetic system to make them swim actively, which is still effective but not as effective as the self-propelled method in the low concentration of hydrogen peroxide. Hydrogen peroxide is a green chemistry reagent and leaves nothing but water and oxygen after the decontamination process.”
- How big a problem is heavy metal pollution of water, in financial terms?
Heavy metal pollution costs around $5 billion per year in the USA and even more in China.
- Where do you envisage your microbots being used? Can they be put into a river that might have been contaminated by industrial processes, for example?
“We envision the use of microbots for the pre-treatment of industrial wastewater before dumping into a river. The microbots will swim in a controlled environment, such as small reservoirs, pipes, containers, but not yet in open waters. This technique is scalable, so we envision that small companies could use microbots to decontaminate their water in case they cannot afford to bring their waste water to decontamination plants. Instead of bringing the water to the plant, we bring the microbots to the company.
“A different mechanism could be used for self-propulsion without using hydrogen peroxide but the lifetime of the microbot would be very short. We believe at this point they are more practical to use in the controlled environment, like in a heavy metal-contaminated wastewater tank in the industrial plant.”
- Can your microbots be used to clean up other types of pollutants from water, such as oil?
“Microbots are very versatile in terms of their environmental applications, depending on their chemical functionalization for different targets. Our group first reported efficient degradation of organic pollutants by self-propelled systems with an iron surface (Soler et al., ACS Nano 2013 7 (11), 9611-9620), and our recent results show that they can be used multiple times over several months without much decrease in activity (Parmar et al., Adv. Funct. Mat. 2016 DOI: 10.1002/adfm.201600381). Oil capture was reported by Dr. Joe Wang’s group by using a different hydrophobic surface functionalization but, as I explained before, it is challenging to use microbots in the wild, due some of the limitations of the current system (Wang et al., ACSNano 2012).”
- When do you think your microbots will be available for commercial use?
“We’re currently funded by an ERC Proof of Concept grant with the aim of commercializing or creating a spin-off from this project. Additionally, we are in contact with several companies interested in our technology. Nonetheless, some aspects need to be taken care of, such as mass production and cost. At the moment it’s hard to say, but in my opinion we should see first commercial use of self-propelled microbots in next couple of years.”