SCIENCE & TECHNOLOGY FLOWER POWER Canna lilies play a role in removing pesticides from nursery runoff.
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RIDDING THE WORLD OF UNWANTED CHEMICALS Innovative approaches can destroy or detoxify insecticides, herbicides, and germicides PAMELA S. ZURER, C&EN WASHINGT Ν
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on their ability to make new molecules. But when the need presents itself, chemists devise ways to destroy molecules, too. The need to safely get rid of unwanted pesticides and germicides was the driving force behind a daylong symposium in the Division of Environmental Chemistry at the American Chemical Society meeting in Boston last month. Electrochemical meth ods, immobilized enzymes, and even liv ing plants deployed as filters were among the approaches presented. In organizing the symposium, Peter Zhu, a principal scientist at Johnson & Johnson, hoped to draw attention to the importance of planning to remove chem icals from the environment. "Green chem istry in most cases focuses only on the pro duction of chemicals by environmentally benign methods," he says. "This is not enough. It's perhaps equally important to look at how we can deal with the toxic chemicals—such as pesticides, herbicides, and biocides—that we already have." 34
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In an ideal world, Zhu says, chemists would design and make only those toxic chemicals that would "turn good" or that could be easily neutralized. Since that's not yet the case, speakers at the symposium talked about ways to address existing com pounds that are considered a threat to the environment or human health. The problem of pesticides in aqueous systems has been a focus of professor Ann T. Lemley and her colleagues in Cornell University's graduate field of environmen tal toxicology They have been investigat ing an electrochemical version of the Fenton reaction for degrading water-soluble pesticides, such as atrazine and carbaryl. In the future, farmers might use a flowthrough system in the farmyard to treat water contaminated when, say, they rinse their spray applicators. In the Fenton reaction, ferrous ion re acts with hydrogen peroxide, producing ferric ion, hydroxyl ion, and hydroxyl rad ical: Fe2+ + H 2 0 2 — Fe3+ + O H > O H "Hydroxyl radicals react with everything,"
Lemley says. "They're good at breaking compounds down." In the classic Fenton reaction, ferrous sulfate is the usual starting material. Over the past decade, an electrochemical Fen ton method, in which the source of the fer rous ion is a sacrificial iron anode, has been shown by a number of research groups to degrade a variety of pesticides. Control ling the current controls the rate of pro duction of Fe2+. A drawback, however, is that the reaction tends to be slow. It runs best at acidic pH, Lemly notes, but the cathode reaction in the electrochemical method produces hydroxyl ions as water is reduced, raising the p H of the system. Lemley's group has addressed that dif ficulty by using two half cells, a method the Cornell researchers call anodic Fenton treatment. Hydrogen peroxide is contin ually pumped into the anodic half cell, which contains an iron plate immersed in aqueous solution of the pesticide to be de graded. The other halfcell consists of an in ert graphite cathode in water. The half cells are connected by a salt bridge or— in more recent work at Cornell—an ion-exchange membrane [J.Agric. Food. Chem., 50,2331 (2002)}. "YOU GET ALL the efficiency of electro chemical delivery of Fe2+ at the right p H for the Fenton to occur," Lemley says. Lemley and postdoc Qiquan Wang have modeled the kinetics for the anodic Fen ton method. "The model gives us rate pa rameters," she says. "We expect to use it as a probe for the reaction rates of hyroxyl radical with all kinds of pesticides." At the University of Arizona, Tucson, associate professorJames Farrell in the de partment of chemical and environmental engineering also studies electrochemical treatment of wastewater. In Boston, he de scribed the destruction of triclosan. That antibacterial agent—found in household products such as cosmetics, deodorants, and disinfectants—has become a ubiqui tous contaminant in streams in the U.S. Boron-doped diamond electrodes are the key to the system in Farrell's experi ments. Their use allows cost-efficient gen eration of the powerful hydroxyl radical without the expense of added reagents such as the hydrogen peroxide required in the Fenton reaction. The inert electrodes provide a surface on which oxidation re actions proceed. Their use is being ex plored widely, Farrell says. HTTP://PUBS.ACS.ORG/CEN
UNWANTED Safe disposal of agents like these poses increasing challenges
H3CO^/R HoCO
coli that express both Ο Ρ Η and a cellu lose-binding protein on the surface. "We can now immobilize the cells on inex pensive cellulose supports," Mulchandani says. "They are tightly attached and don't wash away unless deliberately cleaned with a surfactant." Mulchandani and coworkers have de signed an immobilized cell bioreactor, based on hollow cellulose fibers, that ex hibits good stability and long life. "We can flow pesticide-contaminated water through and collect decontaminated water downstream," he says.
THE RECENT INVASION of imported red fire ants into California has created the pesticide problem thatJohn N. Kabashima, an environmental horticultural adviser with the University of California Cooperative "In the case of triclosan, you can remove world. For example, from Australian chem Extension in Irvine, and his colleagues are 1 mole using just 3 moles of electrons," Far- ical company Orica, Agentase has received trying to address. Plant nurseries in certain rell says. "That costs you just 1.8 cents." enzymes originally isolated from soil bac counties are being required by the state to Enzymes can also be harnessed to de teria. Those proteins may eventually prove treat their soil with bifenthrin to check the stroy unwanted pesticides. The enzyme useful in managing the huge pesticide spread of the ant. The pesticide is toxic not strategy described by Bryan T. Allinson, re waste problem generated by Australia's only to the fire ants, however, but also to search engineer at Agentase in Pittsburgh, millions of sheep dip units. aquatic organisms, so preventing runoff was first developed to detoxify chemical Another approach to using Ο Ρ Η to from the nurseries is important. warfare agents (C&EN, Sept. 7,1998, page degrade organophosphates is under de 30). The company uses a variety of en At a commercial nursery, Kabashima velopment at the University of Califor zymes, including organophosphorus hy and colleagues are developing low-cost nia, Riverside. Ashok Mulchandani, drolase (ΟΡΗ), incorporated into poly methods to reduce pesticide runoff At the chairman of the department of chemimers, to cleave the phosphate field site, runoff is directed into ester in nerve agents and in pes £ a sediment trap, a settlement ticides such as cHorpyrifos and | pond, and finally into a végétamethyl parathion. it tive filter consisting of rows of £ canna lilies. Polyacrylamide, a "We incorporate enzyme pro £ long-chain anionic polymer that teins into a variety of polymers I causes the sediment particles to for a variety of applications," ex ύ floculate, is added along the way plains Agentase CEO Keith E. | Analyses show that over 9 0 % of Lejeune. "Examples include g the bifenthrin entering the syssponges for cleaning up small I tern is trapped in the sediment, spills of pesticides, hand-wash ™ which is recaptured and used ing towelettes, and cartridge de S again in plant containers. vices through which you could pump water that's been used to "The system is very inexpen clean agricultural equipment." sive compared to other mitiga In the formulation of the poly ELECTRIFYING Wang (left) and Lemley developed tion systems that have been pro mers, the enzymes actually par anodic Fenton method to destroy pesticides. posed," Kabashima says. "Most ticipate in the polymerization of the cost is labor, and sale of the process as one of the monomers. In mak plant materials offsets that cost. The can cal and environmental engineering, and ing hydrophilic polyurethanes, for in na lilies thrive here. associate professor Wilfred Chen are stance, the amine groups in the side chains "For real-world applications, having the seeking to avoid the tedium and high cost of the enzymes' lysine residues react with right technology is just part of the answer," of isolating enzymes. Instead, they are the isocyanate functionality in the preworking with whole cells that can be im Kabashima notes. "Psychological aspects polymer to give urea linkages, Lejeune says. mobilized on supports. are important, too. The end users have to "The enzymes become incorporated in be convinced that the system will work Last year, the Riverside group reported to the polymer at many points on their sur without being too expensive or cumber they had genetically engineered an Es faces, tying down the tertiary structure," cherichia coli cell that expresses Ο Ρ Η on its some." That combination of technology Lejeune says. "They become extremely sta and psychology likely will come into play surface (C&EN, Dec. 17,2001, page 49). ble, with extended lifetimes." That feat eliminates the need for pesti with other pesticide neutralization strate cides to pass into the cell's interior before T h e firm is studying a range of en gies presented at the symposium as well, as the enzyme can go to work. zymes—both native and engineered—that scientists try to move their findings from they get from collaborators around the the lab to the real world. • Now, the researchers have created E. Carbaryl
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