Biofoam Sheets Use Sunlight for Simple Water Purification

A bilayer biofoam sheet placed on water efficiently uses sunlight to create purified steam. (Image courtesy of Washington University.)

A team of engineers has recently developed a technique for using sheets of graphene oxide to obtain clean, drinkable water. The technique involves an efficient method of using sunlight to heat dirty water to a boil, creating purified steam that can be collected and safely consumed.

A Bilayered Biofoam

The idea behind the technique is simple. When you boil water, you kill any microbes that can cause intestinal problems in humans. So, by collecting the steam and allowing it to condense, you get water that’s safe to drink.

But there’s a problem with most solar-steam techniques: they’re inefficient, as a lot of energy is wasted by heating the entire volume of water at once. The solution to this problem is to confine heat energy to the boundary between air and water, so that you only heat the relevant evaporation surface.

The engineers have devised a method of heat localization using what’s called a bilayered biofoam. This material is composed of bacterial nanocellulose (BNC) and reduced graphene oxide (RGO). The bilayer structure was created by growing Gluconacetobacter hansenii bacteria in the presence of graphene oxide flakes.

"The process is extremely simple," said engineer Srikanth Singamaneni. "The beauty is that the nanoscale cellulose fiber network produced by bacteria has an excellent ability to move the water from the bulk to the evaporative surface while minimizing the heat coming down and the entire thing is produced in one shot. You have a bi-layered structure with light-absorbing graphene oxide filled nanocellulose at the top and pristine nanocellulose at the bottom”

An illustration of the process used to obtain the bilayered structure composed of BNC and RGO. See the full paper for details. (Image courtesy of Advanced Materials.)

Using the bilayer biofoam is also extremely simple. All you have to do is suspend the material on the surface of the dirty water and the sun does the rest. The top layer of RGO/BNC captures the heat from sunlight and the bottom layer of pure BNC draws water from underneath while shielding it from the heat above.

“When you suspend this entire thing on water, the water is actually able to reach the top surface where evaporation happens,” said Singamaneni. “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 minimized by the pristine nanocellulose layer.”

Since the biofoam is inexpensive to make and highly scalable, the technique is quite promising as a means of large-scale water purification. "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 Singamaneni.

You can read the team’s paper in Advanced Materials.

To learn about another method of using graphene oxide to obtain clean water, check out Graphene-Based Nanoscrolls Could Improve Water Purification. You can also learn about another technique for using sunlight to purify water in Novel Device Purifies Water with Sunlight.