How to Purify Drinking Water Using Sunlight

Scientists have found a novel way to purify drinking water quickly and efficiently using graphene oxide and sunlight, an advance that may be a game changer for countries such as India.

"We hope that for countries where there is ample sunlight, such as India, you'll be able to take some dirty water, evaporate it using our material, and collect fresh water," said Mr. Srikanth Singamaneni, associate professor at Washington University.

The new approach combines bacteria-produced cellulose and graphene oxide - hailed as a wonder material - to form a bi-layered biofoam.

"The nanoscale cellulose fibre network produced by bacteria has excellent ability to move the water from the bulk to the evaporative surface while minimising the heat coming down, and the entire thing is produced in one shot," said Mr. Singamaneni.

"You have a bi-layered structure with light-absorbing graphene oxide filled nanocellulose at the top and pristine nanocellulose at the bottom," he said.

"When you suspend this entire thing on water, the water is actually able to reach the top surface where evaporation happens," he added.

"Light radiates on top of it, and it converts into heat because of the graphene oxide - but the heat dissipation to the bulk water underneath is minimised by the pristine nanocellulose layer," said Mr. Singamaneni.

"You don't want to waste the heat; you want to confine the heat to the top layer where the evaporation is actually happening," he said.

The cellulose at the bottom of the bi-layered biofoam acts as a sponge, drawing water up to the graphene oxide where rapid evaporation occurs.

The resulting fresh water can easily be collected from the top of the sheet.

The process in which the bi-layered biofoam is actually formed is also novel.

In the same way an oyster makes a pearl, the bacteria forms layers of nanocellulose fibers in which the graphene oxide flakes get embedded.

"While we are culturing the bacteria for the cellulose, we added the graphene oxide flakes into the medium itself," said Qisheng Jiang, a graduate student in the Singamaneni lab.

"The graphene oxide becomes embedded as the bacteria produces the cellulose. At a certain point along the process, we stop, remove the medium with the graphene oxide and reintroduce fresh medium," said Jiang.

"That produces the next layer of our foam. The interface is very strong; mechanically, it is quite robust," he said.

The new biofoam is also extremely light and inexpensive to make, making it a viable tool for water purification and desalination.

"Cellulose can be produced on a massive scale, and graphene oxide is extremely cheap - people can produce tonnes of it," Mr. Singamaneni said.

"The properties of this foam material that we synthesised has characteristics that enhances solar energy harvesting," said Mr. Pratim Biswas, professor at Washington Univsersity.

The research appears in the journal Advanced Materials.

Did you know that solar radiation from the sun can purify (disinfect) water and make it safe for drinking from harmful bacteria?

While so often we hear or read about the bad things that the sun is capable of doing to us, we should also know that some of the same energy from the sun can be harnessed for doing good. A few good examples include solar electric panels, solar hot water panels, and solar ovens.

Well here is one more good example. Harnessing the solar energy from the sun to purify (disinfect) water from a lake or stream into safe drinking water. It is much simpler than you may think. Here’s how…

UV-A rays from the sun, (Ultraviolet-A, longwave, 315-400 nm), will terminate harmful bacteria, parasites, and viruses in water, given enough sun exposure.

A clear plastic bottle filled with water, exposed to the sun for 6 hours will make the water safe to drink (see the caveat list). In fact, the effectiveness of terminating  harmful bacteria is an amazing 5-Nines, that is, 99.999 percent!

List of germs that are terminated from UV-A sunlight exposure at 6 hours

Bacteria – Escherichia coli

Bacteria – Vibrio cholera

Bacteria – Salmonella

Bacteria – Shigella flexneri

Bacteria – Campylobacter jejuni

Bacteria – Yersinia enterocolitica

Virus – Rotavirus

Parasites – Giardia

Parasites – Cryptosporidium (needs 10 hours exposure)

List of caveats to UV-A sunlight water purification

This method kills germs. If the water is already contaminated with chemicals from pollution, the chemicals will remain.

If the water is cloudy and dirty, it should be filtered first to allow the UV-A rays to effectively penetrate into the water.

The plastic water bottle should be no bigger than 2 liters. In moderately cloudy water, UV-A will lose 50 percent effectiveness at a depth of 10 mm (about 0.5 inch), whereas UV-A will only lose 25 percent effectiveness at a depth of 10 mm in clear water. Just use a typical size soda bottle or water bottle.

How to purify water with sunlight

The recommended bottle to use is a “PET” bottle. It is very common and is typically used for soda or other soft drinks.