May 5, 1997, p.59. Rapid Reactions Put The Brakes on Metals in Soil Researchers at the University of Delaware, Newark, have discovered a pathway by which metal cations are immobilized in soil. A team led by soil chemistry professor Donald L. Sparks has observed that metals rapidly react with the aluminum in soil minerals to form complexes that are resistant to degradation. Trapped in these stable formations, the metals stay in place, with a greatly diminished ability to leach into groundwater or be taken up by plants. The findings will affect the modeling of metal transport in the environment. "Almost all models used to predict the mobility and fate of metals are based on the assumption that one is dealing only with adsorption phenomena," notes Sparks. If models are to make accurate predictions, these reactions with aluminum also have to be taken into account. The stable species are "multi-nuclear complexes," says André M. Scheidegger, a postdoctoral research associate who examined the reaction of nickel with different clay types. With nickel, the complexes consist of mixed nickel and aluminum hydroxide phases. Analysis by X-ray absorption fine-structure spectroscopy, in collaboration with Geraldine M. Lamble at Brookhaven National Laboratory, indicates structures similar to the mineral takovite, Ni6Al2(OH)16CO3·H2O [J. Colloid Interface Sci., 186, 118 (1997)]. The findings are quite unexpected. It appears nickel cations promote the release of aluminum from the clay lattice, and the mixed-metal hydroxide phase quickly forms, in some cases within just 15 minutes. Scheidegger says similar stable complexes form with cobalt, zinc, and chromium. | |
