terça-feira, 29 de julho de 2008

Arsênio e chumbo são liberados a partir de fertilizante popular

Arsênio e chumbo são liberados a partir de fertilizante popular

Algumas mineradoras, tentando se livrar dos resíduos da mineração e obter lucros, vendem estes resíduos como adubos para a agricultura. Um artigo publicado na revista Environmental Science and Technology mostra que quantidades suficientemente grandes de arsênio e chumbo são liberadas a partir de um fertilizante amplamente usado nos Estados Unidos, ironita, a ponto de fazer com que esse produto seja considerado um rejeito perigoso para a saúde e o meio ambiente.

Os resultados deste estudo, realizado por Bradjesh Dubey e Timothy Townsend, da Universidade da Flórida em Gainesville, oferecem evidência crucial para uma avaliação de risco feita pela U.S. EPA (Agência de Proteção Ambiental dos Estados Unidos) que poderá levar ao banimento ou restrições a este tipo de produto, em nível nacional.

Os manufaturadores da ironita, Ironite Products Co. em Scottsdale, Arizona, afirmaram que seu produto é seguro porque o chumbo e o arsênio que eles contêm não são biodisponíveis. Um relatório técnico comissionado pela companhia indica que estes metais estão presentes no fertilizante como os minerais galena e arsenopirita e que os elementos nestas formas são “muito estáveis no meio ambiente e não biodisponíveis numa forma que é tóxica.”

Mas, os resultados de Dubey e Townsend refutam este argumento. “Nossos resultados mostram que quando a ironita é aplicada sob condições muito comuns, levemente acídicas, os metais pesados realmente são liberados. Nós também mostramos que certa proporção de chumbo e arsênio é biodisponível se a ironita for consumida por uma criança”, disse Townsend, que é um engenheiro de rejeitos.

Veja o artigo original, em inglês (original article):

Science News - September 22, 2004

Arsenic and lead leach out of popular fertilizer

Ironite, a widely available commercial and home plant fertilizer, releases enough lead and arsenic in standard leaching tests to be classified as a hazardous waste. The fertilizer’s heavy metals also appear to be bioavailable, according to the results of some of the first independently conducted standard leachability tests, which were published recently on ES&T’s Research ASAP site (ES0493392). These results provide crucial evidence for an ongoing risk assessment by the U.S. EPA that could lead to nationwide bans or restrictions on such products, according to EPA officials involved with the assessment.

Ironite is produced from the tailings of a former mine, now a proposed Superfund site, in Humboldt, Ariz. The fertilizer—a mixture of mine tailings, sulfuric acid, and urea—is popular with gardeners in the United States because the iron in the mine tailings greens up lawns and plants without promoting heavy growth, according to gardening specialists. A federal exemption allows mine-derived wastes to be sold as fertilizers, and fertilizer producers are only required to list nutrients on the labels of their products. As a result, gardeners using Ironite and other waste-derived fertilizers are not informed about potentially hazardous ingredients.

Several other studies have highlighted Ironite’s exceedingly high concentrations of heavy metals—up to 3600 milligrams of arsenic per kilogram of fertilizer (mg/kg) and up to 2900 mg/kg of lead. Bradjesh Dubey and Timothy Townsend of the University of Florida in Gainesville break new ground by presenting the leaching and bioavailability data.

Ironite’s manufacturer, Ironite Products Co. in Scottsdale, Ariz., has maintained that its products are safe because the lead and arsenic they contain are not bioavailable. A technical report commissioned by the company indicates that these metals are present in the fertilizer as the minerals galena and arsenopyrite and that the elements in these forms are “very stable in the environment and not available in a form which is toxic.”

But Dubey and Townsend’s results refute this claim. “Our results show that when Ironite is land-applied under very common, slightly acidic conditions, the heavy metals do leach out. We’ve also showed that some proportion of the lead and arsenic is bioavailable if the Ironite’s consumed” by a child, says Townsend, who is a waste engineer.
Bioavailability is one of the main aspects of EPA’s assessment, says an anonymous agency official. “The ES&T study is very valuable to us because it confirms and expands on some other evidence we have about the bioavailability of heavy metals in Ironite.”

Townsend and Dubey bought Ironite at several local stores and conducted leaching experiments following standard protocols. In addition, they evaluated the fertilizers for bioavailability using a test designed to simulate the gastric fluid in a child’s digestive system.

Both lead and arsenic showed leaching behavior that varied by pH, with the highest amounts released under low- or high-pH conditions. The fertilizer itself is acidic, so leaching in water will result in high lead and arsenic concentrations in the leachate.

The fertilizer issue dates back to 1997, when an investigation by The Seattle Times created an uproar among environmental groups and state regulators by revealing that a number of fertilizers used by farmers and consumers contained high levels of lead, arsenic, and other heavy metals. Ironite contains the highest amounts of arsenic of all fertilizers tested by EPA as part of a preliminary risk assessment.

The presence of these trace metals has led some state governments, including those of Washington and Minnesota, to investigate potential safety and environmental concerns associated with Ironite, and EPA is soon to finish its two-year-long risk assessment.

Canada limits toxic metals in fertilizers, and Washington, California, Texas, and Minnesota are among the U.S. states that set limits. U.S. federal law requires that all hazardous waste be properly disposed of in regulated landfills. But Congress exempted mining-industry wastes from hazardous landfill disposal laws by means of a 1980 loophole called the Bevill Exemption, which allows mining companies to dispose of waste by turning it into a useful product—in this case, fertilizer. —REBECCA RENNER

Fonte: Environmental Science and Technology online. http://pubs.acs.org/subscribe/journals/esthag-w/2004/sep/science/rr_arsenic.html
Copyright © 2008 American Chemical Society, 1155 Sixteenth Street N.W., Washington, DC 20036