Technology▼Solutions Biopolymer removes mercury from wastewaters
© 2003 American Chemical Society
or parts per billion (ppb), satisfying even the U.S. EPA’s drinking water limits of 2 ppb. The mercury sequestered by the polymer can be recovered easily. Raising the temperature above 25–30 °C, depending on the biopolymer concentration, causes the polymer to WILFRED CHEN
Researchers at the University of California, Riverside, have invented a new and “greener” way to treat mercurycontaminated wastewaters produced by industries such as chlor-alkali, paper, and cement manufacturing. Wilfred Chen and his colleagues believe that the genetically engineered biopolymer they have invented represents the first environmental use of elastin-like polypeptides (ELPs). Elastin is the major protein in the elastic fibers that coat the lungs and blood vessels of all vertebrates, and it is among the most hydrophobic proteins known. It is soluble in cold solutions but forms aggregates above a critical transition temperature that cannot be dissolved in water. This phenomenon, known as coacervation, also characterizes the structurally simpler ELPs, which for this reason can be regarded as temperaturesensitive “switchable” biopolymers whose solubility is controlled by heat. Chen says he first heard of these molecules at the National Academy of Engineering’s Frontiers of Engineering Symposium, where ELPs’ role in pharmaceutical applications such as protein purification and targeted delivery of drugs was described. “I thought, why don’t we do something in the environmental field?” recalls Chen. In the October 1 issue of ES&T (pp 4457–4462), Chen and his colleagues describe how they successfully fused ELPs to the protein MerR, which is the mercury-sensing unit of the detoxification system in mercury-resistant bacterial strains. Because both the ELP and the MerR parts retain their functionalities in the genetically engineered fusion protein, the ELP-MerR polymers are able to bind mercury with high specificity and affinity. The biopolymer was able to reduce mercury concentrations in water samples to well below industrial wastewater standards of 50 micrograms per liter
When these temperature-sensitive polymers are heated, they precipitate (test tube on left), effectively removing the mercury they have scoured from the surrounding wastewater (test tube on right).
precipitate out of the solution. The mercury can then be extracted and the ELP-MerR polymers regenerated with a strong complexing agent. Cooling the material leads to complete resolubilization of the regenerated biopolymers for reuse. Although Chen’s paper discusses recycling the polymer only four times, one of his students is currently testing the system’s performance during additional cycles and on a larger scale, he says. “There is still a long way to go from proving in principle that ELPs can clean up mercury-contaminated water to successfully installing reactors at industrial sites,” says Irene Wagner-Döbler, a project leader with the German Research Centre for Biotechnology (GBF) in Braunschweig, which is developing a bioreactor for mercury removal. If practical solutions are found for immobilizing and
regenerating the biopolymers, then she sees a “good potential for ELPs in combination with other techniques.” Wastewater from chlor-alkali plants contains on average 4000 ppb of mercury, which is a 100-fold higher concentration than in the samples of artificially contaminated water tested in Chen’s laboratory, WagnerDöbler points out. She therefore envisages a two step-system in which the bulk mercury is removed by a technology like GBF’s bioreactor and the remaining mercury is eliminated by ELPs. Wagner-Döbler says that because of their extraordinarily high affinity for the metal, the polymers are also ideally suited for treating mercury-contaminated groundwater in place of activated carbon or ionexchange resins. Chen says that ELPs can be produced for U.S. $3 per gram protein in the lab and suspects they could be produced much more cheaply on a larger scale. The price of the ELPs compares favorably with the price of state-of-the-art regenerable ionexchange resins, he says. He also emphasizes that this is a green technology because “there are no nasty chemicals involved in the production of these polymers” and because the polymers’ degradation will only lead to some discharge of amino acids. Because bacteria have evolved resistances against other heavy metals, fusing ELPs with protein-binding toxics like arsenic or chromium is a logical expansion of Chen’s work. Chen says that the preliminary results with an arsenic-binding fusion protein are promising. One nice thing with the ELP system is its versatility, says Chen, noting that “almost everything we tried worked.” Chen is also investigating how to use ELPs as tunable immunosorbents for detecting low levels of atrazine. He says that his team members are filing patents and look forward to collaborating with industry to commercialize some of their ideas. —ORI SCHIPPER
DECEMBER 1, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 433 A
