New water purifier produces disinfectant using sunlight

Stanford scientists have developed a portable purifier that uses sunlight and water to produce small amounts of hydrogen peroxide, a powerful and common antiseptic.

The experimental water purifier is a variant of the better-known process of using solar energy to split water into hydrogen, a clean-burning fuel, and oxygen, a life-sustaining element.

However, instead of fully splitting oxygen and hydrogen, the new process reduces oxygen and oxidizes water to produce hydrogen peroxide, or H2O2.

Even just a small amount will purify the water, said Xiaolin Zheng, an associate professor at Stanford University in the US.

Hydrogen peroxide disinfects water at a level of tens of parts per million. That's about two tablespoons in 25 gallons of water. In tests using tap water, the system easily reached well over 400 parts per million of H2O2 in five hours.

The team will have to change some of the materials in the process to make its blend of ordinary water and hydrogen peroxide safe to drink.

However, they think that one day, a person in desperate thirst could pull out their lightweight solar purifier, pour in some suspect H2O and, given enough time, produce enough H2O2 through the sun-activated process to turn any fresh water into a veritable oasis.

In addition to future drinking water applications, researchers also imagine that their system might be adapted into self-sustaining swimming pools purified with solar-created hydrogen peroxide rather than chlorine, or solar-powered water purification stations for use in developing regions where fresh water is a precious commodity.

The prototype consisted of two electrodes, an anode and a cathode, thrust into water. The anode was made of bismuth vanadate (BiVO4), a photosensitive semiconductor. Simple carbon served as the cathode.

When exposed to sunlight, the bismuth vanadate semiconductor sent negatively charged electrons flowing toward the cathode, while positively charged carriers - or "holes" as they are known in physics - flowed back toward the anode.

The flow of electrons turned oxygen into hydrogen peroxide while the holes acted to transform water into hydrogen peroxide, forming the purifying compound at both electrodes.

"Ours is an unassisted system. It requires zero energy input and only light, water and oxygen to work. Water is the 'fuel' of our system. In fact, it works with tap water," Xinjian Shi, the graduate student at Stanford.

The system produces hydrogen peroxide on both sides of the reaction, at the anode and the cathode. At the end of it all, there's even a small amount of electricity remaining, due to the efficiency of the chemical reactions.

While not a great amount, that additional energy might be used to light an LED bulb as an indicator that the system is working properly, researchers said, letting the thirsty owner drink with confidence.