(Photoelectrocatalytic Degradation and Removal)
The University of Wisconsin-Madison (UW-Madison) is developing a photocatalytic technology that uses titanium dioxide (TiO2) suspensions to coat various supporting materials used in treatment applications. For this application, the suspensions are used to coat a conductive metallic or carbon mesh. Coating the mesh with a suitable thickness of TiO2 catalyst provides the basis for a photoreactor that uses most of the available ultraviolet (UV) radiation. An electrical field can also be applied across the catalyst to improve its performance.
The figure below shows a possible photoreactor design that uses a ceramic film. In this design, the TiO2 coating on the porous metal acts as a photoanode. An electric potential can then be placed across the coating to direct the flow of electrons to a porous carbon counter-electrode that has a high surface area and would collect any heavy metal ions present in the liquid. In addition, an applied electric potential can improve the destruction efficiency of organic contaminants by reducing electron-hole recombination on the catalyst surface. This recombination is seen as a primary reason for the observed inefficiency of other UV/TiO2 systems when treating organics in groundwater. Lastly, the electric potential has been shown to reduce the interference of electrolytes on the oxidation process. Electrolytes such as the biocarbonate ion are known hydroxyl radical scavengers and can be problematic in the UV/TiO2 treatment of contaminated groundwater.
This technology improves on liquid-phase photocatalytic technologies by distributing radiation uniformly throughout the reactor. Also, the technology does not require additional oxidants, such as peroxide or ozone, to cause complete mineralization or to improve reaction rates. In addition, it eliminates the need for an additional unit to separate and recover the catalyst from the purified water after the reaction is complete.
This particular technology is designed to treat groundwater and dilute aqueous waste streams contaminated with organics and heavy metals. Organics are completely oxidized to carbon dioxide, water, and halide ions. Heavy metals are subsequently stripped from the cathode and recovered.
The UW-Madison photocatalytic technology was accepted into the SITE Emerging Technology Program in 1995. The overall objective of the Emerging Technology Program study is to refine the reactor design, enabling it to treat heavy metals as well as organic contaminants. Testing of a bench-scale unit is currently underway.
UW-Madison has tested its photocatalytic reactor at the laboratory scale on aqueous solutions of several organic contaminants, including polychlorinated biphenyls, chlorosalicylic acid, salicylic acid, and ethylenediamine tetraacetate. UW-Madison has also used similar reactors to remove volatile organic compounds, such as trichloroethene, tetrachloroethene, benzene, and ethylene from air streams. Photooxidation of trichloroethene and tetrachloroethene has been successfully field-tested at low flow rates (<0.1 standard cubic feet per minute).
EPA PROJECT MANAGER:
Vince Gallardo
U.S. EPA
National Risk Management Research Laboratory
26 West Martin Luther King Drive
Cincinnati, OH 45268
513-569-7176
Fax: 513-569-7620
TECHNOLOGY DEVELOPER CONTACTS:
Marc Anderson
Water Chemistry Program
University of Wisconsin-Madison
660 North Park Street
Madison, WI 53706
608-262-2674
Fax: 608-262-0454
Charles Hill, Jr.
Department of Chemical Engineering
University of Wisconsin-Madison
Engineering Hall
1415 Engineering Drive, Room 1004
Madison, WI 53706
608-263-4593
Fax: 608-262-5434