LIKE its namesake, the messenger of the gods, mercury is notoriously mobile in the environment. Water-soluble and toxic mercury readily leach out of landfills and even wastes solidified with cement. In recent years, environmental scientists and regulators have focused on the development of new processes to remove mercury ions from solutions more efficiently and cheaply than present methods.|
The technical challenge is formidable because any method must be impervious to the corrosive nature of the waste streams that contain these ions. In addition, these waste streams can contain a variety of other metal ions (sometimes in much higher concentrations)-some of which also possess the same +2 charge as mercury ions. So an effective removal process must be selective of only mercury ions.
In response to the need for a better method for mercury removal, a team of Lawrence Livermore chemists (Glenn Fox, John Reynolds, and Ted Baumann) has designed an organic polymer called Mercaptoplex that demonstrates an unusually strong affinity for mercury ions in solution. Tests at Livermore show that Mercaptoplex extracts more than 95 percent of mercury ions and does so faster and more selectively than other techniques such as precipitation and activated carbon absorption. Originally developed for use in processing nuclear fuel rods at the Department of Energy's Idaho National Engineering and Environmental Laboratory, the molecule can also remove mercury from both industrial waste streams and public water supplies.
Mercaptoplex has demonstrated a remarkable capacity for removing mercury ions under a broad range of conditions, including those currently found in government and industrial waste streams. In addition, the molecule can be reused indefinitely after the bound mercury is removed, making the process cost-effective. Because of its ability to be recycled, the molecule minimizes the amount of secondary waste generated during extraction, a major challenge in waste treatment.
Three Molecules in One
Strong Polystyrene Backbone|
The Livermore chemists chose a backbone of polystyrene because its chemistry is well understood and its simple cross-links of divinylbenzene transform the molecule into a highly entangled and thereby insoluble repeating unit (or polymer) that does not dissolve in water. The Livermore team postulates that other materials, such as polymers of polyethylene, may also prove effective as backbones.
In solution, because of the entangled nature of the Mercaptoplex polymer, it is probable that neighboring crowns combine to trap mercury ions. For example, two sulfur atoms from one crown may combine with three sulfur atoms from a nearby crown to bind to a mercury ion. Studies using spectroscopic techniques are under way at Lawrence Livermore to gain a better understanding of the bonding mechanism.
The Livermore chemists have shown that Mercaptoplex is effective at pH ranging from 1.5 (extremely acidic) to 7.0 (neutral). In contrast, precipitation, a common technique of mercury removal, requires constant pH adjustment. If the pH gets too low (too acidic), the precipitation process produces hydrogen sulfide, a highly toxic gas, and does not remove the mercury. The other popular mercury removal process, activated carbon, also requires continuous adjustment of pH.
Mercaptoplex is also faster and more selective in removing mercury than other techniques. When mixed with solutions containing mercury ions, it captures virtually all of the mercury within 30 minutes. This extraction rate is much faster than that seen in other systems, which can take up to 20 hours to do their job. Baumann says the ultimate goal is to use Mercaptoplex as packing for large columns to speed up the waste treatment process. In this design (shown below), the waste stream would simply flow through the Mercaptoplex without the need for mixing.
Fox notes that because typical mixed waste streams (those combining both toxic and radioactive materials) contain a variety of other metal ions, such as aluminum, iron, cadmium, and lead, removal of mercury requires a highly selective process. The chemists have tested Mercaptoplex in solutions of mercury ions ranging from 4 to 200 parts per million and when concentrations of other ions outnumber mercury by 100 to 1. In every case, Mercaptoplex has selectively removed mercury with an efficiency of 95 percent or greater. (Baumann says mercury removal is probably greater than 99 percent, but the amount of mercury left in solution after treatment is too small for the chemists to measure accurately.)
Recycling Is a Big Advantage|
Because Mercaptoplex is insoluble in water, it can be easily separated from solution by filtration once the extraction is complete. The mercury can then be recovered, and the Mercaptoplex regenerated by a variety of treatments. One method developed by the Livermore team is to use chloroform solutions of diphenylthiocarbazone to strip the bound mercury from the polymer. Under these regeneration conditions, the diphenylthiocarbazone has an even greater affinity for mercury ions than does the sulfur crown. Once rinsed and dried, Mercaptoplex has been used to effectively treat additional volumes of mercury. The team is investigating other methods of stripping the mercury ions from the polymer such as electro-chemically reducing the ions to the safer metallic mercury.
In comparison to Mercaptoplex, other techniques typically require additional treatment steps and generate large amounts of secondary waste. Precipitation generates mercury sludges that require further treatment. Activated carbon columns loaded with mercury are rarely regenerated, and the spent columns require additional processing.
At the Idaho National Engineering and Environmental Laboratory, where mercaptoplex was first used, mercury is used as a catalyst to treat spent fuel rods from U.S. Navy submarines. The Livermore process is also applicable at other DOE sites that need selective and cost-effective treatments for mixed waste.
The process should prove useful in treating industrial waste streams and water supplies that contain mercury. For example, the Livermore team has discussed the process with representatives from the bleach manufacturing and oil industries, who must meet strict federal regulations concerning mercury levels in their waste streams.
By simple substitution of the sulfur atoms, the molecule can be tailored to target other metal ions, such as cadmium, silver, and lead, commonly found in mixed waste streams and water supplies. "There is a lot of synthetic chemistry you can do with crowns," says Fox, "such as modifying the number of noncarbon atoms in the ring to better bond to the ion in the solution of interest. In this way, chemists can target a particular metal pollutant through careful molecular design."
Key Words: activated carbon, crown polymers, Idaho National Engineering and Environmental Laboratory, Mercaptoplex, mercury, precipitation.
For further information contact Glenn Fox (925) 422-0455 (firstname.lastname@example.org).