06-17 carrying water

[Image above] A boy demonstrates the difference between clean water provided by Oxfam and dirty water from the river Nile. Finding new ways to purify water leads to greater global access to clean water. Credit: Oxfam East Africa, Wikimedia (CC BY 2.0)

Living in the United States near the Great Lakes, the largest freshwater system in the world, can easily lead to taking clean water for granted. However, seeing the lake levels dropping each year should be a clear warning for even the most skeptical among us.

Many of the world’s easily accessible freshwater sources are being drained faster than they are being replenished, prompting researchers to develop new and improved ways of purifying water for consumption.

I discussed research on photocatalytic methods for water cleaning in past CTT posts. Briefly, this technology involves sunlight interacting with various nanoscale ceramic materials to trigger reactions that break down harmful chemicals and cell walls of bacterial contaminants. These methods are promising, and I’ll review some further research in a future CTT.

But today, the title of a recent article in the Journal of the American Ceramic Society caught my attention, mainly because it sounded as if it came right out of science fiction. “Self-propelling nanomotor made from halloysite and catalysis in Fenton-like reaction” describes some very real and very interesting technology which, as it turns out, has potential for advancing water purification.

In this article, researchers led by Shiding Miao from Jilin University in China describe the performance of clay-based materials used as self-propelled catalysts for breakdown of organic contaminants in water.

Clay is widely used in water purification plants because it readily adsorbs contaminants. However, by doping natural tubular halloysite clay tubes with oxides of manganese and iron, the resulting material catalyzes the release of free radicals and oxygen from hydrogen peroxide. The free radicals in turn disrupt the bonds of rhodamine B, an organic compound frequently used to model organic contaminants in natural bodies of water.

Schematic diagram of MnO2–Fe3O4/HNTs nanomotors synthesis for bubble-propelled motion. Credit: Wang et al., Journal of the American Ceramic Society

The shape of the manganese and iron oxide decorations on the surface result in directional release of oxygen bubbles during the reaction. This directional release propels the nanotubes within the water, albeit in various path shapes.

The authors provide many videos of these nanomotors in action, with Video S2 (available here) showing different paths as observed under a microscope. The still images below illustrate the various paths.

Typical trajectories of MnO2–Fe3O4/HNTs nanomotors during the movement. The white streaks are the actual paths; the yellow are explanatory model lines. Scale bar: 100 μm. Credit: Wang et al., Journal of the American Ceramic Society

The randomness of the travel creates a natural stirring action that provides mixing far greater than that achieved by simple Brownian motion of the reactants and catalyst. The result is rhodamine breakdown rates far exceeding those for nonmobile catalysts.

Another very interesting aspect is the potential for removal of the catalysts via magnetic fields. While the catalyst is relatively inert with low chemical toxicity, nanomaterials can have negative environmental effects. With the magnetic susceptibility of the iron oxides, the paths of the nanomotors can be altered to follow external magnetic fields. Thus, the particles can be collected much more readily than with filtration alone.

In addition to low toxicity, these catalytic nanomotors are relatively low cost. Not only are the raw materials low cost, doping the halloysite with the manganese and iron use facile and inexpensive precipitation and hydrothermal methods.

In a follow-up article published this year, the Miao research group reported similar results from untreated pelagic clays extracted from the floor of the Indian Ocean. They found this clay has high iron and manganese content and performed similarly to the treated clays.

They performed further experiments exploring the effects of pH and reusability of the clay. The best performance was found at low pH values, though degradation of the clay decreased the automotive function and thus it was not reusable. Nonetheless, the potential for even lower cost catalysts is demonstrated.

These results are very exciting. I look forward to seeing this research move to the next level. Can they clean larger volumes of water? What happens when they move from model systems to real-life industrial effluent? Can other oxidizers be used to enable safer and more convenient packaging?  What are some other potential uses for this technology?

On the last topic, the authors mention a few possibilities, including drug delivery and nanoscale surgery. Reading about in-vivo applications sparked the science-fiction comment I made early on. It’s not so scary—unless these particles find a way to self-replicate…

The 2021 paper, published in Journal of the American Ceramic Society, is “Self-propelling nanomotor made from halloysite and catalysis in Fenton-like reaction” (DOI: 10.1111/jace.17821).

The 2022 paper, published in Journal of the American Ceramic Society, is “Deep-sea clays using as active Fenton catalysts for self-propelled motors” (DOI: 10.1111/jace.18348).