The Future for Cleaning Water?

Permaculture magazine
Tuesday, 13th September 2016

A tiny device has been created to clean dirty water from bacteria in just 20 minutes. The device uses all visible parts of the solar spectrum, rather than the more common water cleaning method which uses UV rays.

Clean water is not easily accessible for many people across the world.

To clean contaminated water, many people either boil it, or leave the water in plastic bottles in the sun, so that UV rays will kill microbes. However, UV rays only carry 4% of the sun's energy, which means this method can take up to 48-hours.

The Department of Energy’s SLAC National Accelerator Laboratory and Stanford University have recently created what could be the future for cleaning water. They have manufactured a nanostructured device, which is about half the size of a postage stamp, that disinfects water much faster than the UV method. The device makes use of the visible part of the solar spectrum, which contains 50 percent of the sun’s energy.

According to reports from Stanford University, recent experiments showed the device triggered the formation of hydrogen peroxide and other disinfecting chemicals, when it was hit by sunlight. These chemicals killed more than 99.999% of bacteria within 20 minutes. Once their work was done, the chemicals dissipated, leaving behind pure water.

The Science

The surface of the tiny device is made up of closely spaced lines, making it look like a fingerprint. These lines are very thin films (called nanoflakes) of molybdenum disulfide, stacked on edge on top of a rectangle piece of glass.

Usually molybdenum disulfide is used as a lubricant, but in this case it becomes a photocatalyst - when hit by light, the electrons leave their usual place, and they and the 'holes' they leave behind are eager to take part in chemical reactions. Scientists have been able to make these walls just the right thickness to absorb the full range of visible sunlight, and by topping them with a thin layer of copper, which also acts as a catalyst, they were able to use the sunlight to trigger reactions they wanted. In this case, that was the production of hydrogen peroxide (commonly used as a disinfectant) to kill bacteria in the surrounding water.

Molybdenum disulfide is cheap and easy to make, which is necessary of this device is going to be spread across the developing world. It also absorbs a much broader range of solar wavelengths than traditional photocatalysts.

water_disinfection_v01.jpg

Solving Pollution

Although this is a positive step towards cleaner water, the device cannot remove chemical pollutants. At this stage it has been tested on only three strains of bacteria, but scientists see no reason why it would not kill other bacteria strains, such as viruses.

The work was funded by the Department of Energy Office of Science through SIMES, and carried out in collaboration with Professor Alexandria Boehm’s group in the Stanford department of civil and environmental engineering.

For full details on the device and the team behind it, visit: HERE

Further resources

Using floating ecosystems to clean waterways

How mushrooms can clean up radioactive contamination

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