Imagine being a lost and thirsty hiker in an arid region…
You finally discover water! And it’s not a mirage! But now you have a life-or-death decision to make…
Do I drink the water or not?
Is it clean or will I succumb to sickness?
Stanford Engineering has some interesting new developments on this survival situation. They report about an innovative solar purifier that creates its own powerful and safe disinfectant using water and sunlight:
The experimental water purifier, developed in the lab of Xiaolin Zheng, associate professor of mechanical engineering, 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. But, as the team describes in the journal Advanced Energy Materials, 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, she says. 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 Stanford system easily reached well over 400 parts per million of H2O2 in five hours.
Zheng says 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. But 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, Zheng and Xinjian Shi, the graduate student leading the project, 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.
For all the engineering, hacker-spacers and DIY-ers out there, here are the basics of how the solar purifier is made and how it cleans water:
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.
It is a new take on what is known in engineering circles as a photoelectrochemical (PEC) system. PEC systems have been much studied since the 1970s for their ability to convert sunlight to fuel and other useful chemicals, like hydrogen and oxygen. Prior PEC experiments have produced hydrogen peroxide but none of these previous experiments has been as successful as the present research.
Best yet, this solar purifier is an “unassisted system,” says project leader, Shi.
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.
The left over power from using the system could one day power an LED light for itself and/or some other type of wireless indicator for the lightweight purifier.
Reader more about their work at Stanford Engineering!
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Image: Suncat Stanford