News Briefs
Clays modify pesticide toxicity in soils
In the lab, reduced smectite clays, similar to this blue clay (left), altered the toxicity of pesticides. Oxidized clay (right) did not affect pesticide toxicity.
JOSEPH STUCKI
Reduced iron-bearing clays commonly found in soil can react with herbicides to alter their toxicity in vitro, according to research published in this issue of ES&T (4383– 4389). The paper is the first to report that such smectite clays can modify pesticide toxicity. Interaction with soils is generally thought to make pesticides less harmful to nontarget species, explains soil mineralogist Joseph Stucki at the University of Illinois at Urbana– Champaign, the paper’s corresponding author. The mechanisms responsible are thought to be microbial degradation and sequestration by organic materials and clays—not chemical reactions with clays. This report is noteworthy for showing that pesticide transformation products can be produced solely through reactions with clays, and that the transformation products may sometimes be more toxic, he says. Stucki and his colleagues mixed four different herbicides—2,4-dichlorophenoxyacetic acid (2,4-D), alachlor, dicamba, and oxamyl— with either reduced- or oxidizedferruginous smectite. Both occur naturally but the scientists wanted to test the hypothesis that reduced clays affect pesticide toxicity. They then compared the toxicity of the pesticide on its own to the toxicity of the pesticide that had reacted with the smectite. To assess toxicity to cells they employed a widely used mammalian cell line, Chinese hamster ovary (CHO) cells, and an assay developed by coauthor Michael Plewa, a University of Illinois geneticist, to measure changes in cell viability and metabolism. Treatment with reduced smectite substantially decreased the toxicity of oxamyl—it took three times as much of the clay-treated pesticide to suppress cell growth by 50% as
the original compound—and slightly decreased the toxicity of alachlor. However, contact with the reduced smectite increased the toxicity of dicamba—33% less was required to suppress cell growth by 50%. The toxicity of 2,4-D stayed about the same. These results were confirmed by the metabolic assay. The scientists found degradation products but were unable to identify them. These results may be applicable to fields where pesticides are applied to wet smectite-bearing soils, says Stucki. University of Berkeley biogeochemist Javiera Cervini-Silva notes that the pesticide concentrations in the experiments are for the most part similar to the concentrations found in soils, and that smectite clays are abundant in soils and sediments worldwide. Although the clay content in soil is typically between 8 and 15%, some soils contain as much as 70–80% clay. Of this clay fraction, smectite clays can account for up to 50% of the total mass, she says. Because soils are inherently dynamic, a portion of the smectite clays near the top of a swath of soil can sometimes be chemically reduced, Cervini-Silva says. For example, common events like rainfall and irrigation can cause swelling and generate more reducing conditions by changing the soil’s oxygen content, she says. In addition, microorganisms can chemically reduce the structural iron in clays. “Stucki and colleagues have shown that this redox cycling in smectites influences the bioavailabitiy of pesticides. This means that we cannot understand the behavior of pesticides in soils if we consider that the clay remains unchanged. Soils are inherently dynamic and that dynamic affects bioavailability,” says Cervini-Silva. Stucki’s group is currently investigating the degradation products and looking at other compounds with similar functional groups to try to explain the behavior of the different pesticides. —REBECCA RENNER
Renewables setback The Australian government has decided not to extend mandatory targets for renewable energy production. This decision, contained within an energy white paper published in June, has disappointed renewable energy supporters who say it will dramatically reduce future investment across Australia. The current target is 9500 additional gigawatt hours of renewable electricity per year by 2010. However, proposals include A$134 million for developing commercial renewable technologies and A$75 million for Solar Cities, clusters of solar energy and energy-efficiency projects incorporated into existing and new city buildings. Securing Australia’s Energy Future is at http://energy.dpmc.gov.au/ energy_future/contents.htm.
Electricity beats hydrogen Directly using electricity remains more efficient than making hydrogen for storing and transporting energy, according to a study from the Institute for Lifecycle Environmental Assessment, an environmental advocacy group. “Hydrogen is not an energy source” but “an energy storage medium and carrier,” remind the report’s authors. Their assessment indicates that even using renewable sources like the sun to create electricity that converts water to hydrogen provides about half of the energy of just transmitting that electricity. For example, using electricity to charge electric vehicles provides twice the miles per kilowatt hour compared with the same amount of electricity to make hydrogen fuel. In addition, hydrogen storage returns around 47% of original energy, while batteries can return up to 85%. Carrying the Energy Future: Comparing Hydrogen and Electricity for Transmission, Storage, and Transportation is available at www.ilea.org.
AUGUST 15, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 305A
