Microfluidics and sunlight combined to purify water
By Ben Coxworth
January 16, 2011
It has been known for some time now that sunlight can be used to purify drinking water. The practice of Solar Water Disinfection (SODIS) basically involves just leaving water sitting in direct sunlight, where a combination of heat and UV rays kill off waterborne pathogens – the process is called photocatalysis, and it’s what’s at work behind both the Solaqua water purification device, and a system recently created by students from the University of Washington. Now, researchers from Hong Kong Polytechnic University have taken things a step further, by combining photocatalysis with microfluidics in a microreactor.
The team from Hong Kong Polytechnic created a planar microfluidic reactor, which consists mainly of a rectangular chamber made up of two facing glass plates, each coated with titanium dioxide (Ti02) – this is the active ingredient in many sunscreens. Tainted water passes between the plates via microchannels, to maximize the surface-to-volume ratio. When exposed to sunlight, the Ti02 releases electrons, which in turn break down contaminants in the water.
"These two technologies have been developed in parallel but there have been few efforts to employ the natural synergy between them," said study author Xuming Zhang. "Our results showed a dramatic improvement in the efficiency of the photocatalyst." When compared to a bulk container with the same amount of water, pollutant (methylene blue), and Ti02 surface area – but without the microchannels – they were actually able to improve the photoreaction efficiency by more than 100 times.
While the space between the plates is very small, Zhang intends to increase the reactor’s capacity by scaling the plate size up to two square meters (21.5 square feet), with an ultimate goal of being able to process 1,000 liters (264 US gallons) per hour – the current version has plates that measure just 5 x 1.8 cm (about 2 x 0.7 inches). If the larger version works as well as hoped, he envisions multiple devices being used together in industrial applications. Before that can happen, however, he wants to increase the oxygen availability in the reactor chamber, and boost the light sensitivity of the Ti02.
The research was recently published in the journal Biomicrofluidics.
Images courtesy Xuming Zhang