Tikalon Blog by Dev Gualtieri

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Water Purification 'CD'

October 14, 2013

As other scientists with an interest in computing , I was an early user of the Internet , both at work, and at home. This was at the time when access to the World Wide Web was through the Mosaic Web Browser , and there was no Google Search . I used AltaVista .

I had an Ethernet connection at work, but only dial-up at home, initially at 1200 baud , migrating notably to 9,600 baud, then to V.34 , the limit of what the plain-old-telephone-service (POTS) could provide (about 33,600 baud). I had an additional telephone line installed at my home exclusively for dial-up Internet, and I used a local Internet service provider (ISP).

In those dial-up days, non-computer people were led to believe that a company called AOL was the Internet. AOL blitzed people with copies of their interface software by direct mail , first via floppy disks , then compact disks (CDs). These offered free connection hours before access rates began. It's been reported that at one time, half of manufactured CDs contained AOL software, and the total cost of AOL CDs was more than $300 million.[1]

Although I didn't keep an accurate count, I personally received more than twenty AOL CDs over the years. Others had this same experience, and you can find web sites that list uses for AOL CDs other than their intended purpose.[2] CDs are made from polycarbonate , a useful structural material , so the CDs were used as beverage coasters , ice scrapers and grill scrapers, and vegetable garden markers. Since CD grooves form an optical diffraction grating , you can use a CD to make a crude spectrometer .[3]

Lexan polycarbonate structural diagram

Chemical structure of Lexan™ polycarbonate. Lexan is formed in a reaction of bisphenol-A with phosgene . (Image via Wikimedia Commons.)

Using a CD as a drink coaster is fairly obvious, but scientists have found some technically exquisite uses for CDs. Professor Gang Logan Liu of the University of Illinois Micro and Nanotechnology Lab and his research team have developed a "lab-on-a-chip " medical diagnostic system using CDs formed as microfluidic devices which can be read by standard computer CD drives (see image).[4] Such low cost devices can enable enhanced health care in remote or impoverished regions .

Microfluidic CD (Liu/University of Illinois)

Microfluidic CD system for lab-on-chip medical diagnostics.
(Image courtesy of Professor Gang Logan Liu of the University of Illinois Micro and Nanotechnology Lab.)[4]

Another need in remote or impoverished regions is clean water , and CDs have again been employed to solve this technical problem. Din Ping Tsai , a physicist at National Taiwan University , and his colleagues from the National Applied Research Laboratories in Taiwan , and the Research Center for Applied Sciences in Taiwan, have developed a water treatment photocatalytic reactor based on a zinc oxide coated CD.[5-6] Just as in its more typical application, the CD spins.

They prepared a sea of upright zinc oxide nanorods on the surface of a CD (see photos). Zinc oxide is a well-known photocatalyst for the decomposition of organic pollutants , and it will break apart organic molecules in sewage under ultraviolet illumination.[5] The essential trick is that the rapidly-spinning CD will spread droplets of fluid dripped onto it into a thin sheet of liquid which allows more efficient photocatalysis.[5]

A CD coated with a zinc oxide nanorod photocatalyst

A CD coated with a zinc oxide nanorod photocatalyst (left), and a scanning electron micrograph of the zinc nanorods(right). The scale of the micrograph is about ten micrometers across the width of the image. (OSA/Din Ping Tsai/National Taiwan University images.)[5]

The treatment reactor, as shown in the photograph, is about a cubic foot in volume . The reactor includes the UV light sources and a water recirculating system. As a test, the Taiwanese research team used a solution of methyl orange dye , an organic compound often used to test the efficiency of photocatalytic reactions. It was found that 95% of the methyl orange dye had been broken down after treatment of a half-liter solution for 60 minutes. Based on this observation, it appears that the reactor could treat 150 mL of waste water per minute.[5]

Photocatalytic reactor using a ZnO nanorod catalyst on a spinning compact disk.

Photocatalytic reactor using a ZnO nanorod catalyst on a spinning compact disk.

(Din Ping Tsai/National Taiwan University, a still image from a video supplied by The Optical Society )

Of course, one possible improvement of the system is to have a stack of spinning disks spun by a single motor . The system could find utility in the treatment of not only domestic sewage , but also farm waste , urban run-off , and industrial effluents .[5] I can see how this type of system can be used, also, for air purification when shaped impellers are used. Considering the current environmental problems of mainland China , this might be a useful export across the Taiwan Strait .

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