Aluminum floc formation. (Metal Toxicity).

When aluminum-rich acidic runoff from mining sites travels through less acidic stream or river water, fluffy flocs of suspended solids form. These flocs carry toxic metals--which normally would have stayed in the streambed--downstream from the mine area, where they can poison aquatic animals and plants. Now scientists have gained further insight into how flocs form.

Gerhard Furrer, a geochemist at the Institute of Terrestrial Ecology in Zurich, and colleagues report in the 27 September 2002 issue of Science that aluminum flocs originate mainly from condensations of the aluminum complex Al[O.sub.4][Al.sub.12][(OH).sub.24][([H.sub.2]O).sub.12.sup.7+], or [Al.sub.13]. These condensations form rapidly and then aggregate as the pH of acidic effluent increases to more than 5. Aluminum-rich acidic solutions form [Al.sub.13] as an intermediate compound, then [Al.sub.13] molecules aggregate to form flocs, says coauthor William Casey, a geochemist at the University of California, Davis.

"Previously it was thought that [[Al.sub.13]] was quite rare and kind of a curiosity that you find in some solutions," says Casey. "We now think that it is very, very common--that is the real important finding of this paper," he says.

Paul Bertsch, director of the Savannah River Ecology Laboratory in Aiken, South Carolina, and colleagues had proposed in a chapter of the 1996 text The Environmental Chemistry of Aluminum that [Al.sub.13] was a common precursor to aluminum solids and that, based on extensive laboratory studies, it could form under a wide range of environmental conditions. The new paper by Furrer and colleagues provides strong support for this hypothesis, Bertsch says.

The international team analyzed flocs from nine polluted streams in Germany and California using a type of nuclear magnetic resonance spectroscopy called [sup.27]Al magic-angle spinning, which detects different types of aluminum complexes in solids. They picked up the signal generated by substantial amounts of so-called 4-coordinated aluminum--aluminum bonded to four oxygen atoms. Only dissolved [Al.sub.13] has that type of aluminum in large concentrations at the pH conditions found in the polluted rivers, says Casey. This suggested that [Al.sub.13] was the key aluminum molecule acting in floc.

About 240,000 square kilometers of the Earth's surface are affected by mining operations, say the authors. With such a large area involved, many watersheds are polluted with acidity, aluminum, and other potentially toxic metals. By identifying [Al.sub.13]--a molecule that is known to be phytotoxic and to have a high affinity for heavy metal cations--as the key molecule in floc formation, researchers now might be able to develop new approaches to attack this weighty pollution problem.