SEATTLE --A new technique for reducing waste from chemical processes involved in everything from petroleum refining to pharmaceutical manufacturing also may hold the key to cleaning up radioactive remains at eastern Washington's Hanford nuclear site.
Using electricity, instead of concentrated chemicals, to drive a key step in many chemical separation processes could greatly reduce waste byproducts, says Dan Schwartz, a University of Washington chemical engineering professor who is exploring ways to improve this environmentally-friendly approach for a variety of scientific and industrial applications.
Schwartz and a team of four undergraduate and graduate students are collaborating with scientists at Pacific Northwest National Laboratories who are pilot testing the new process for use in radioactive waste cleanup efforts at Hanford. Schwartz also is teaching an interdisciplinary laboratory course on the technique this quarter as part of a major UW effort to integrate leading-edge research on environmental technology into the undergraduate curriculum.
"Environmentally-benign chemical processing is an area of growing importance for chemical industries, and it's being driven as much by economics as by regulation," Schwartz says. "If you don't produce waste, you're using your raw materials more efficiently and you're not paying for disposal."
A significant amount of waste generated by industry and at Hanford occurs in recharging ion exchange systems which are used to separate the byproducts of chemical and nuclear processes Traditional ion exchange systems employ charged compounds to attract specific chemicals with opposite charges in order to remove them from a solution.
In water softening, for example, a negatively-charged ion exchange material acts like a sponge for positively-charged calcium ions in hard water. Eventually, however, the ion exchange material becomes filled with calcium and must be regenerated by a series of chemical treatments. A concentrated salt- water solution is used to remove the calcium and replace it with sodium, which is then rinsed away with fresh water to leave the ion-exchange material ready to again attract calcium. In the process, the relatively innocuous calcium and saltwater byproducts are washed down the sewer system.
Waste byproducts from industrial ion exchange processes, on the other hand, can be environmentally hazardous and costly to contain. Radioactive wastes at Hanford are even more troublesome. During the Cold War, Hanford produced most of the nation's nuclear bomb material and now is left with millions of gallons of liquid wastes containing sodium and radioactive cesium. Hanford scientists are using traditional ion exchange systems to separate the cesium from the sodium for easier disposal. But each time the ion exchange materials are refreshed with chemical treatments, Schwartz says, the volume of radioactive byproducts is increased.
Three years ago, Pacific Northwest National Laboratory began studying an innovative technique that uses electricity rather than chemicals to replenish ion exchange systems and thus reduces the volume of waste byproducts. The technique takes advantage of iron compounds called metal hexacyanoferrates that have a strong affinity for cesium and also conduct electricity. By attaching electrodes to the hexacyanoferrate material, the negative charge in the iron compound can be increased or decreased. When placed in the cesium- sodium solution and given a highly negative charge, the iron compound attracts cesium ions. When removed from the solution and given a more positive charge, it drives off the radioactive cesium for disposal.
Pacific Northwest National Laboratory is developing a continuous processing system to test the potential of using this innovative approach for large-scale cleanup efforts at Hanford. Schwartz and his team of student researchers are supporting the project by conducting experiments to explore the fundamental properties and limits of the new hexacyanoferrate materials being used.
"One problem is that this material can only be re-used for a certain number of cycles before it dies; its molecular structure apparently changes so it no longer binds and releases cesium using electricity," Schwartz explains. "Some of the issues we're studying are how to make these materials live longer, regenerate easier, and attract cesium more readily."
Schwartz and his students also are examining various industrial applications for this new technique. Potential uses include removing potassium chloride from pulp and paper processing streams to protect equipment and reduce costs, treating water to guard against contamination in computer chip manufacturing operations, removing magnesium in sugar-refinery processing plants to yield more high-grade product, and reducing hazardous waste byproducts from mining and metal production.
The research is supported by grants from the Department of Energy, Environmental Protection Agency, and the National Science Foundation.
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