Can
Green Chemistry Promote Sustainable Agriculture? The rewards are higher yields and less environmental contamination.
DENNIS L. HJERESEN AND RANGEL GONZALES
uman population is increasing. Demand for food is rising. Agricultural production must expand. Pests that destroy crops must be eliminated. Natural resource demands are increasing. Environmental impacts are worsening. Taken together, few issues reflect the difficulties of sustainable development more than the problem of controlling pests and increasing food production while protecting the environment and conserving natural resources. To address this problem, new approaches are needed, and particularly for pest control and the agricultural chemicals industry, green chemistry may provide opportunities (1–7). Green chemistry approaches reduce and, in some instances, eliminate the use and generation of hazardous substances in the design, manufacture, and application of chemical products. A growing trend in industry, green chemistry is motivated by simultaneous requirements for environmental improvement, economic performance, and social responsibility.
MICK WIGGINS
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© 2002 American Chemical Society
As green chemistry moves into mainstream academic research and industrial practices, it is increasingly having an impact on important global environmental problems such as sustainable food production, a multidimensional problem. According to the United Nations, world population will expand to 7.9–10.9 billion by 2050 (8). Presently, it is about 6.2 billion people. The U.N. Food and Agriculture Organization (FAO) estimates that current annual crop losses due to pests cost more than $300 billion worldwide (9). In addition to these losses, resource demands associated with food production generate significant environmental impacts. Worldwide agricultural operations are the largest consumer of water (69% of all consumption). Demand for agricultural land, housing, and fuel results in deforestation. Also problematic are the loss from unsustainable agricultural practices of biological species in forests and in waters; desertification, erosion, and salination of farmland; and the pollution of fresh and marine waters. MARCH 1, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Adding to this panoply of negative environmental impacts are the growing amounts of pesticides, herbicides, and fertilizers being used in agricultural applications. Pesticides and fertilizers presently account for 7% of all chemical production worldwide, with total annual consumption of pesticides (including insecticides, fungicides, and herbicides) in Organization for Economic Cooperation and Development (OECD) countries (see box below) alone totaling ~1.2 million tons (active ingredients only) (10). The OECD’s member countries presently account for 62% of global fertilizer and pesticide production (10). The organization reports that the estimated $12 billion annual market for insect control chemicals is driven by the global economic and human costs of crop losses (10). Many of these chemicals persist in the environment and produce adverse biological impacts. Clearly, having to make a choice between sufficient food and clean water and ecosystem survival is not acceptable.
OECD member countries Australia, Austria, Belgium, Canada, Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Japan, Korea, Luxembourg, Mexico, the Netherlands, New Zealand, Norway, Poland, Portugal, Slovak Republic, Spain, Sweden, Switzerland, Turkey, United Kingdom, United States
A vicious cycle Collectively, this complex situation exemplifies a sustainable development dilemma. Modern synthetic organic pesticides, fertilizers, herbicides, fungicides, and biocides are responsible for increasing the yield of agricultural production, decreasing human suffering, and enabling a world population of more than 6 billion people. Thus, few chemical applications are more important than those used for agricultural purposes, but widespread application of agricultural chemicals has proven costly to the environment. First, the persistence of many pesticides, herbicides, and fertilizers in the ecosystem; their bioaccumulation and distribution; and evidence of their biological impacts are a significant consequence of the synthetic organic chemical approach to the problem. Evidence of carcinogenic, mutagenic, and reproductive effects extends from manufacturing processes, to field workers, to local and regional ecosystems, and finally to consumers. Second, genetically transmitted resistance among insect species to these pesticides and damage to nontarget species indicate that the current approach may have limited long-term efficacy. Insect resistance cycles as short as five years are being reported in Africa (11). Third, the use of synthetic chemical fertilizers and pesticides by poorly trained workers poses a significant health risk. The World Health Organization (WHO) estimates that every year 3 million people suffer acute, severe pesticide poisoning, and that deaths worldwide from pesticide poisoning number 300,000 a year, primarily in developing countries (12). 104 A
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Fourth, where and how pesticides and fertilizers are used and produced also raise environmental concerns. In recent years, competitive pressures in the global chemicals market have led companies in developed countries to shift production from basic chemicals to specialty chemical, life sciences, and high value-added products (10). Concurrently, domestic chemical industry in developing countries has increased its production of fertilizers and other agricultural chemicals, along with other basic chemical products. The OECD report indicates that production has moved from countries with tight environmental standards to countries without standards or vigorous enforcement. Finally, the accumulation of obsolete and abandoned pesticides, especially in the developing world, has emerged as an environmental disaster with as yet unassessed consequences. Although many pesticides have been banned by treaty because of their health and ecological consequences, few steps have been taken to remove existing stocks. The FAO estimates that at least 100,000 tons of obsolete pesticides are located in developing countries (12), including 20,000–30,000 tons in Africa; Morocco, for example, has 3000 tons constituting 127 varieties. The largest volumes of abandoned pesticides include organochlorine compounds such as DDT and dieldrin, and organophosphate compounds such as parathion, methyl-parathion, dichlorvos, and monocrotophos. Although an international campaign is under way to remove and destroy these obsolete pesticides, the costs are staggering. FAO estimates that it costs $3500–$4000 to remove 1 ton of obsolete pesticides from Africa, far exceeding the original cost of production and distribution (12). The issue, however, is not the need to make a choice between protecting crops critical to human sustainability and a healthy environment. Rather, it is a challenge to the world chemical, biological, and agricultural communities to devise new methods to protect and enhance plant growth and yield while eliminating downstream consequences. This is why green chemistry is important. It provides tools to protect environmental quality in the face of increasing global pressures on food production. Toward this end, several green chemistry approaches are under development. U.S. EPA’s annual Presidential Green Chemistry Challenge Award has recognized several contributions to sustainable agriculture, and many of the examples that follow are drawn from summaries of these awards.
Biomimetic approaches Long before the advent of synthetic organic chemical pesticides, nature evolved numerous mechanisms for plant protection. One approach for controlling insects uses synthetic pheromones to disrupt the reproductive cycles of target species. Pheromones are precisely specific to the target species and are nontoxic, but because the purity required for their use makes them expensive to synthesize, they make up only $50 million of the $12 billion annual market. To increase yield and decrease costs, Douglas Knipple of Cornell University has developed a pheromone synthesis process
Toward less fertilizer Although anhydrous ammonia has traditionally been the leading chemical for providing nitrogen in fertilizer, it has been overtaken by urea, which has numerous favorable properties as a solid form of nitrogen and in mixtures with other fertilizers, such as diammonium phosphate and potassium chloride. However, urea has a high rate of ammonium volatilization—it loses 30% or more of its nitrogen content—and requires higher application rates. In addition, its volatilization rate makes urea unsuitable for no-till applications now favored in many climates to reduce soil loss. To address ammonium loss from urea, IMC-Agrico Co. has developed AGROTAIN with N-(n-butyl) thiophosphoric triamide (NBPT) (15). Once applied in relatively small quantities (0.4 lb/acre), NBPT acts as a urease enzyme inhibitor, slowing the hydrolysis of urea, which supports no-till agriculture and encourages less urea application. Another green chemistry approach is to introduce thermal polyaspartate (TPA) in small amounts to increase plant uptake of nutrients and fertilizers, reducing reliance on agricultural chemicals. The increased nutrient uptake improves crop yield without compromising ecological integrity. TPA was originally developed by Donlar as an environmentally preferable substitute for polyacrylic acid (PAC), an important anionic polymer (17). The TPA manufacturing process uses a new solvent-free polymerization step for converting aspartic acid to polysuccinimide with a 97% effective yield and water as the sole byproduct. Further base hydrolysis of polysuccinimide to polyaspartate is similarly efficient. TPA is biodegradable and less toxic than PAC. TPA is also used in water treatment, detergent, oil, and gas applications. AGRICULTURAL RESEARCH SERVICE
AGRICULTURAL RESEARCH SERVICE
based on biocatalysis (13, 14). The process uses molecular genetics to create recombinant yeast strains that express pheromone biosynthetic enzymes. These catalyze the formation of specific pheromone intermediates from inexpensive fatty-acid feedstock. Extensive experimentation has produced pheromone desaturases with unique stereo- and regioselective properties. Such specificity is critical to bringing down manufacturing costs; in this case, by two- to fivefold compared with current processes for five different commercial pheromone products. In another interesting development, Dow Agrosciences has systematically searched for natural microorganisms with insecticidal properties by establishing a large-scale testing program to ferment and screen samples. The program led to the discovery of Saccaropolyspora spinosa, a microorganism isolated from a Caribbean island soil sample. The product name, Spinosad, derives from the combination of two forms of the aerobic, gram-positive, nonacid, fast, nonmotile, nonfilamentous bacteria spinosyn A and spinosyn D. By disrupting insect nicotinic and gammaaminobutyric acid receptor mechanisms through a novel neurotoxicity mechanism that remains to be elucidated, the pesticide exhibits neurotoxic effects on a range of insects that attack trees, fruits, cotton, vegetables, and other plants (15). Spinosad represents a major improvement over synthetic organic pesticides. It does not leach, bioaccumulate, volatilize, or persist in the environment. Dow studies with the material find that 70–90% of beneficial insects are not harmed, and because of decreased mammalian toxicity, worker risks are lowered. This is also the case with aquatic species. Elsewhere, Rohm and Haas has developed a new class of anti-insect compounds based on dicylhydrazines, including CONFIRM, an anticaterpillar compound that has been classified by the U.S. EPA as a reduced-risk pesticide. The insecticide mimics the naturally occurring hormone 20-hydroxyecdysone, which initiates the molting sequence in the insects—but the insects do not eat during this stage and quickly die of dehydration and starvation. Because the physiological action is specific to caterpillar molting, no secondary effects occur in nontarget mammalian, avian, or aquatic organisms. Beneficial predatory in-
sects are not affected. Moreover, worker safety is enhanced by the low mammalian toxicity, and risk assessments indicate no carcinogenic, mutagenic, or reproductive effects (16). Another major category of biomimetic pesticides is azadirachtin (Azadirachta indica, Melia azadirachta). This botanical insect growth regulator, extracted from the kernels of the neem tree, disrupts insect molting by interfering with ecdysone, the key molting hormone discussed above. Several products are derived from the neem to control aphids, armyworms, and a variety of dipteran and lepidopteran pests on food crops and cotton—see www.agrobiologicals. com/glossary/G2401.htm for a product listing. These developments demonstrate two important environmental principles. First, finding the S. spinosa sample in the forest of a Caribbean island highlights the need to protect natural ecosystems. Second, the fusion of microbiology with chemistry to produce these products is indicative of the multidisciplinary collaboration promoted by green chemistry.
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New impacts from biotechnology
AGRICULTURAL RESEARCH SERVICE
New approaches to sustainable agriculture, based on biochemistry, have been emerging over the last decade. This transition began with the introduction of biopesticides, which encompass several types of pesticides created from natural materials such as organic materials, bacteria, and derived proteins. The EPA had 195 registered biopesticide-active ingredients and 750 products at the end of 2001 (18). The first of these were the microbial pesticides in which a microorgan-
ism (e.g., a fungus, virus, or bacterium) is the active agent. Microbial pesticides were identified by their specific action on weeds and insects, for example, strains of Bacillus thuringiensis (Bt). Specific strains and subspecies of Bt yield a different combination of proteins and are effective against specific species of insect larvae. By applying specific Bt strains to crops, is it possible to kill target insects in the larval stage. The newer extension of this technology is to genetically engineer plant-incorporated-protectants— plants are induced to produce their own pesticidal compounds from added genetic material. When expressed by the host plant, the Bt bacterium produces proteins that kill by destroying the digestive systems of target insect pests. This technology has seen rapid application. EPA estimates adoption of Bt field corn was 0.4 million acres (1% of crop planting) in 1996, 4.4 million acres (6%) in 1997, 14.5 million acres (18%) in 1998, 19.8 million acres (26%) in 1999, and 19.5 million acres (25%) in 2000 (18). Applications have been increasing in cotton, potato, and corn crops. Another genetic engineering approach involves creating food crop tolerance to herbicides. Monsanto’s Roundup is the world’s largest selling herbicide. It is not thought to be carcinogenic and is effective in controlling weeds without damaging food crops. Monsanto has genetically engineered crop plants to enhance their resistance to the herbicide (19). Roundup Ready technology was introduced for soybeans in the United States in 1996 and for cotton and corn in 1997 and 1998, respectively. Herbicide use was decreased 10–35% in soybean fields with better weed control. Additional applications are planned for sugarbeets, rice, vegetables, salad crops, and in forestry. Eden Bioscience Corporation has commercially developed an entirely new class of plant proteins 106 A
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called harpins, which were originally developed at Cornell University (20). The premise for this work is to understand the natural response of plants to pathogens, isolate the proteins responsible for the response, and genetically engineer their large-scale production. Harpin is a protein produced by the plant pathogenic bacterium erwina amylorva (20). Trials on more than 40 plant varieties indicate effective resistance to a range of fungal, viral, and bacterial diseases. Yields in these species increased 10– 20% and eliminated the use of traditional chemicals. The reported effective doses range is 0.004–0.14 lb/acre per season, drastically reducing chemical use and worker exposure. The first generation of harpin-based products is sold as Messenger. Harpins are nontoxic and act to trigger a plant’s defense systems against disease and pests but also act to stimulate growth. These proteins cause increased plant mass, nutrient uptake, photosynthesis, and root development and are ultimately biodegradable. Tests indicate no adverse effects on any other species, including mammals, avians, plants, algae, and aquatic invertebrates. It’s important to note that the production of harpins takes place in a water-based fermentation system without the use of any chemical reagents and generates no chemical waste products. There are several advantages to using biopesticides and herbicides compared with traditional organic chemical pesticides. First, biopesticides are usually inherently less harmful than conventional pesticides, as they are specific to the target pest and closely related organisms, in contrast to broad-spectrum, conventional pesticides that impact on other species, including fish, birds, and mammals. Second, biopesticides often are effective in smaller quantities and, because they comprise plant proteins, decompose quickly. This helps reduce pesticide persistence in the environment. Third, because EPA has determined that biopesticides pose fewer risks than conventional pesticides, the registration process for new biopesticides is nearly two years shorter than for conventional pesticides. Finally, there are numerous economic advantages to using biopesticides. Benefits include reduced pesticide use, reduced production costs, better crop yield (often in marginal growing conditions), and increased profit—EPA estimates that growers of Bt products gained approximately $350 million in 2000 (18). EPA also cites indirect benefits, such as improved populations of beneficial insects and wildlife in cotton fields, reduced pesticides runoff, reduced air pollution and waste from the use of chemical insecticides, improved farm worker and neighbor safety, and reduction of fossil fuel use (18). It would, however, be inaccurate to suggest that these biotechnology innovations are not without new risks (21), which continue to be evaluated. Chief concerns include the transfer of genetic material to other plant species, including wild variations of crop plants, damage to nontarget species, long-term insect resistance, and the possibility that the newly produced proteins may be allergens. One high-profile case involves the claim of higher Monarch butterfly mortality in regions heavily planted with Bt corn, and there is serious debate about labeling food produced with
the assistance of genetic engineering. Although EPA has generally approved the use of Bt crops, it has limited planting in regions where plant diversity is an issue (18). Concerns have also led EPA to actively consider longer-term issues, such as insect resistance, when approving new pesticides. Such an integrated pest management strategy is a significant evolution from past pesticide approval processes.
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Reducing manufacturing risks It is also important to reduce worker exposure and mitigate environmental consequences during agricultural chemical manufacturing. One example of risk reduction is Monsanto’s catalytic dehydrogenation chemistry for preparing amino acid salts (17). Disodium iminodiacetate (DSIDA), a key intermediate in Roundup, has been produced using ammonium chloride, formaldehyde, and hydrogen cyanide to form iminodiacetonitrile. This synthesis created 1 kg of waste for every 7 kg of product. The waste contained cyanide and formaldehyde and required treatment before disposal. Monsanto’s new method for obtaining DSIDA uses a copper catalyst and cuts down on waste, producing a product so pure that it does not require further refinement—selectivity is increased to more than 97%, and yield is increased to more than 95%. In addition, the catalyst can be recovered and reused.
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Solving erosion problems Soil loss from erosion annually removes up to 20 tons of soil per acre from lands under furrow irrigation. Scientists at the Northwest Irrigation and Soils Research Laboratory of the U.S. Department of Agriculture are exploring a polyacrylamide technology for reducing soil erosion (16). Mixing polyacrylamide with soil reduced sediment loss by an average of 94% in tests. By creating a water-soluble polyacrylamide solution that can be applied through irrigation systems, doses as small as 10 mg/L can be applied, corresponding to only 1–2 lb/acre compared with 500 lb/acre for dry application. Improving soil retention also ensures that fertilizers and herbicides will remain on fields.
Green chemistry prospects Green chemistry offers an array of innovative approaches to pest management, food production, and ecosystem protection. Most examples in this article are based on commercially viable products. This is not to discount the importance of an impressive body of academic research on this subject. Rather, this discussion demonstrates that ecosystem protection is not necessarily associated with the loss of economic benefits. The examples highlighted in this article indicate opportunities for increased agricultural yield, economic benefits for manufacturers and end users, and enhanced environmental performance. Such cases are not isolated but rather indicate the path forward.
References (1) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice; Oxford University Press: New York, 1998. (2) Benign by Design: Alternative Synthetic Design for Pollution
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Prevention; Anastas, P. T., Farris, C. A., Eds.; Oxford University Press: New York, 1994. Green Chemistry: Challenging Perspectives; Anastas, P. T., Tundo, P., Eds.; Oxford University Press: New York, 2000. Green Chemistry: Designing Chemistry for the Environment; ACS Symposium Series No. 626; Anastas, P. T., Williamson, T. C., Eds.; Oxford University Press: New York, 1996. Green Chemical Syntheses and Processes; ACS Symposium Series No. 767; Anastas, P. T., Heine, L. G., Williamson, T. C., Eds.; American Chemical Society: Washington, DC, 2000. Green Chemistry: Frontiers in Benign Chemical Synthesis and Processes; Anastas, P. T., Williamson, T. C., Eds.; Oxford University Press: New York, 1998. Hjeresen, D. L.; Anastas, P. T.; Kirchhoff, M.; Ware, S. Environ. Sci. Technol. 2001, 35, 114 A–119A. World Population Prospects: The 2000 Revision; Population Division Department of Economic and Social Affairs, United Nations: New York, 2000. Public Health Impact of Pesticides Used in Agriculture; World Health Organization: Geneva, 1991. OECD Environmental Outlook for the Chemicals Industry; Organization for Economic Cooperation and Development: Paris, 2001. Diop, A. Study of Well Water Level of Pollution in the District DeKhombole: Prospecting of Contamination by the Organochlorine Pesticide Residues and by Organic Substances (Faeces). In U.S. Africa Workshop on Environmental Chemistry and Water Quality: Exploring Collaborative Opportunities for Research, Applications and Education; ACS International Programs, American Chemical Society: Washington, DC, 2001. Prevention and Disposal of Unwanted Pesticide Stocks in Africa and the Near East; Report W8419; Plant Production and Protection Division, Food and Agriculture Organization of the United Nations: Rome, 1994. Knipple, D. C.; Rosenfield, C.-L.; Miller, S. J.; Liu, W.; Tang, J.; Ma, P. W. K.; Roelofs, W. L. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 15,287–15,292. The Presidential Green Chemistry Challenge Awards Program: Summary of 2000 Award Entries and Recipients; EPA744-R-00-001; U.S. Government Printing Office: Washington, DC, 2001. The Presidential Green Chemistry Challenge Awards Program: Summary of 1999 Award Entries and Recipients; EPA744-R-00-001; U.S. Government Printing Office: Washington, DC, 2000. The Presidential Green Chemistry Challenge Awards Program: Summary of 1998 Award Entries and Recipients; EPA744-R-98-001; U.S. Government Printing Office: Washington, DC, 1998. The Presidential Green Chemistry Challenge Awards Program: Summary of 1996 Award Entries and Recipients; EPA744-K-96-001; U.S. Government Printing Office: Washington, DC, 1996. The Environmental Protection Agency’s White Paper on Bt Plant-Pesticide Resistance Management; Biopesticides and Pollution Prevention Division, Office of Pesticide Programs, Office of the Assistant Administrator for Prevention, Pesticides and Toxic Substances, U.S. Environmental Protection Agency: Washington, DC, 1998. The Presidential Green Chemistry Challenge Awards Program: Summary of 1997 Award Entries and Recipients; EPA744-S-97-001; U.S. Government Printing Office: Washington, DC, 1998. Wei, Z.; Laby, R. J.; Zumoff, C. H.; Bauer, D. W.; He, S. Y.; Collmer, A.; Beer, S. Science 1992, 257, 85–88. Rissler, J.; Mellon, M. The Ecological Risks of Engineered Crops; MIT Press: Cambridge, MA, 1996.
Rangel Gonzales researched these issues as a Green Chemistry Institute intern and is currently a chemical engineering graduate student at Humboldt State University, Arcata, CA. Dennis L. Hjeresen is Director of the Green Chemistry Institute, American Chemical Society, Washington, DC; e-mail:
[email protected] and Senior Program Manager in the Environmental Program at Los Alamos National Laboratory in New Mexico. MARCH 1, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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