TECHNOLOGY UPDATE Bioengineered bacteria show promise in mercury removal A Cornell University scientist is searching for a site to field-test a new method for bioremediating water contaminated with low mercury concentrations. The technology ultimately could prove useful for publicly owned water treatment facilities, landfills, cleanup sites, and some industries, according to EPA scientists. The new biotechnology was developed by a team led by David B. Wilson, a professor of biochemistry, molecular and cell biology at Cornell, and director of the university's Institute for Comparative and Environmental Toxicology. The technology revolves around Escherichia coli bacteria that have been genetically engineered to take up mercury, which is a persistent, bioaccumulative, toxic substance. Scientists have been working toward developing methods for bioremediating metals using transgenic organisms over the past decade, but only recentiy have they managed to combine the various building blocks into a usable technology. Although other researchers have succeeded in cloning the needed mercury transport genes into bacteria, Wilson believes this is the first time that bacteria have succeeded in taking up large amounts of Hg2* mercury without being killed by it. Shaolin Chen, a graduate toxicology student at Cornell, induced the E. coli bacteria to sequester mercury by adding genetic material that caused them to overexpress production of metallothionein (MT), a protein capable of binding metals like mercury. Previous attempts to get bacteria to produce MT were unstable, Wilson said. "We had to work out a method of stabilizing [the genetically engineered bacteria] to get the high level of production of the protein in the cell." Wilson's team is currently maintaining cells in an antibiotic solution so that they do not expel their additional genetic material; however, he said that other © 1999 American Chemical Society
known methods could be employed to achieve that end. Pilot tests of the bacteria's ability to take up metals in a hollow-core bioreactor showed it to be capable of consuming 99.75% of the mercury in a solution containing 2 mg/L. If the reactors were placed in series, Wilson's team calculated that they could clean mercury down to the 6.3 ng/L. This level is well below the 144 ng/L level in drinking water recommended to protect human health, on a par with the 12 ng/L level in water bodies recommended to protect wildlife, and near the 0.18 ng/L level suggested for the mercury-contaminated Great Lakes, Wilson said.
Time course of Hg 2 + bioaccumulation In pilot tests, bacteria genetically engineered to remediate mercury took up 94% of the toxic metal in five miniutes. (Courtesy Cornell University, Ithaca, N.Y.)
If it is developed into a viable product, the technology's ability to clean mercury to the parts-per-trillion level could become valuable in coming years in light of EPA's goal to reduce environmental mercury concentrations, said Elwood Forsht, chief of the Chemicals and Metals Branch of EPA's Office of Water's Engineering and Analysis Division. Wilson has approached the aluminum and chloralkali industries about the technology because they use electrodes that can leave trace amounts of mercury in their waste streams. The technology "could be worth its weight in gold" to industries mat must
stringendy limit their mercury discharges into water bodies with very strict water quality requirements, Forsht said. Biological treatment requires sufficient organic material in a waste stream to sustain biological growth, he explained. Landfill leachates often have enough biological material, Forsht noted, adding that the technology might also be useful for remediating certain types of groundwater. Mercury also is becoming an issue that concerns local governments, because mercury waste from hospitals can end up in publicly owned water treatment plants, said Shari Zuskin, a chemical engineer in the Engineering & Analysis Division of EPA's Office of Water. Wilson said he believes that the technology could ultimately be useful for cleaning mercury-contaminated water bodies, sediments, and soil. Compared to existing methods for removing low levels of mercury, the technology is resistant to environmental conditions, Wilson stressed. Unlike conventional chemical-precipitation, carbon-adsorption, and ionexchange technologies, the new method is selective in removing mercury and can operate at varying salinities and pHs. Wilson believes that it would be relatively inexpensive to use in cases where water was contaminated with trace amounts of mercury. "You could treat a very large amount of water before you saturate the cells because they have a relatively high capacity," he explained. Wilson and his fellow researchers currently are attempting to use the same approach to target other metals. "The general idea can be applied to any metal to which you can find a transport system that works in bacteria, because MT has a very broad metal-binding specificity," he explained. The team is currently trying to apply the technique to cadmium and nickel and hopes to have results by the end of the summer. —KELLYN S. BETTS
AUGUST 1, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 2 9 9 A
