New research reveals how contaminants can be "locked" into soil For many years, agricultural chemists have known that herbicides applied to soils can become so tightly bound to humus that even strong solvents cannot recover them. Now scientists are investigating the chemical reactions that create these "bound residues" in an effort to develop soil treatment techniques that permanently lock contaminants into soil. The formation of bound residues is an important natural process that helps to limit the effects of contaminants in soils, according to Jean-Marc Bollag. Bollag, who is director of Pennsylvania State University's Center for Bioremediation and Detoxification and the leader of one of the groups investigating this binding, estimates that "40 to 50% of chemicals applied to the soil, including pesticides and. other potentially harmful compounds, stay bound in the soil." The environmental impact of this binding is that it reduces the toxicity, mobility, and bioavailability of contaminants. However, if bound residues are not long-lasting, their formation could also have the undesirable effect of protecting contaminants from remediation methods such as microbial biodegradation. Current research is aimed at determining the permanence of the bonds and the conditions diat promote their formation. The question of permanence is being addressed by the Penn State group, which has used 13C nuclear magnetic resonance (NMR) to show that bound residues involve covalent bonding between humic substances and pesticides. Such
Darwin Sorenson and Ron Sims, Utah State University environmental engineers, are studying soil samples from the Champion International Superfund Site, Libby, Mont, to better understand the formation of bound residues. (Coortees ycott G. Huling, EPA, Ada, Okla.)
strong bonds are likely to be permanent, according to Jerald Schnoor, an environmental engineer at the University of Iowa. "If these bound residues are covalently bonded, then this may be considered a treatment technique for some contaminated soils." Bollag, speaking at the American Chemical Society (ACS) meeting in San Francisco in April, described two transformations of the fungicide Cyprodinil in the presence of humus constituents. The Cyprodinil molecule is first split, and then the two parts bind to humic acid. To achieve this result, the Penn State group had to surmount difficulties in analyzing the structure of bound residues. After they spiked
REGULATION Dioxin may be added to TRI Dioxin and dioxin-like compounds may be added to the Toxics Release Inventory, EPA announced (Federal Register, ,997, 62(55), 24887-24896). AA sart oo the announcement, published in response to a petition from the environmental group Communities for a Better Environment, EPA is for the first time asking for public comment on whether it should reduce the reporting threshold for TRI-listed chemicals that bioaccumulate and are highly toxic. Agency scientists believe that dioxin meets the TRI reporting criteria, but most facilities emit far below the reporting threshold of 25,000 pounds for manufacturing. Before it adds dioxin to the roster, EPA would like to formally lower the reporting threshold, said Denise Keams, EPA spokesperson. A rulemaking to lower the reporting threshold should be released within the year.
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the pesticide with 13C, this radiolabeled pesticide was aged in soil to form bound residues. Used in NMR, 13C generates signals that reflect its specific chemical environment. By comparing the pesticide spectrum with that of the bound residue, humic substances, and other reference chemicals, the structural changes were delineated. To enhance the binding process, Bollag's group has also been investigating the role of enzymes and nonbiological catalysts in the formation of bound residues. In another ACS talk, Jang-Eok Kim described how the enzyme laccase promoted the binding of Bentazon, an herbicide that is otherwise unlikely to bond with humic compounds. Kim is a Penn State visiting professor from Kyungpook National University, Taegu, Korea. Among the nonbiological catalysts that could enhance the formation of bound residues, manganese oxide is a good candidate because of its ability to exchange electrons with other chemicals. Ronald Sims and colleagues at the Utah Water Research Laboratory at Utah State University have been investigating the role of manganese oxides and the creation of bound residues involving polycyclic aromatic hydrocarbons (PAHs). Sims has found that manganese oxides can oxidize and
