To really know what's going to happen with radioactive wastes or other contaminants as they move through soil, it's essential to have a good idea of what's happening at the molecular level where minerals and water rub shoulders.
Hoping to overcome some of the current limitations of technology, a team of University of California, Davis, researchers used a novel, indirect strategy to quantify the rate at which minerals and water exchange oxygen molecules in that chemical limbo.
Their findings, which they anticipate will provide a particularly useful test for computer models, will appear in the March 23 issue of the journal Nature.
Virtually all U.S. drinking water is very clean, but there are locations in the world, and in some places within California, where natural poisons and industrial waste exist in natural waters. Oxygens at the surface of aluminum-bearing minerals are key to the elimination of these contaminants.
A technique known as nuclear magnetic resonance (NMR) spectroscopy has long been used to determine molecular structure and rates of reaction. But the technology hasn't proven useful in measuring the rate of oxygen exchange between mineral surfaces and fluids.
So UC Davis geochemist William Casey and NMR spectroscopist Brian Phillips decided to synthesize a representative molecule from one of these aluminum-rich minerals that would be more amenable to NMR analysis. They chose a large, well-known aluminum-containing molecule, the Al13 compound, that resembles a structural fragment of important minerals in soils.
"Many other researchers have used this method to determine oxygen-exchange rates, but in this study we looked in great detail at the rates of oxygen exchanges in a molecule that looks like a fragment of a soil mineral," Casey said. Results from the study are very basic, but promise to have broad implications, he added. They immediately provide researchers with an experimental model from which accurate computer models can be developed that will be able to predict the oxygen-exchange rates where mineral surfaces interact with fluids.
And those models should have very practical applications in dealing with soil contaminants.
"For example, if you want to determine the toxicity of certain contaminants and how quickly they will be destroyed as they move through soils or through a lake, you must understand how the compounds react with minerals and water at the molecular level," Casey stressed. "If you know how quickly the oxygens form a bridge to the contaminant, then you can determine its rate of movement or decomposition."
To study the rate of oxygen exchanges at various locations on the molecule, the researchers first had to grow very pure crystals that dissolved to release the Al13 fragment. The Al13 compound is a collection of aluminums and oxygens configured around a central aluminum oxide (Al(0)4 ) site.
"With this molecule we were able to obtain precise readings on how long it takes for the bonds between aluminum and oxygen to break and reform with oxygen molecules from the water," Phillips said. "We found that the speed of oxygen exchange varied remarkably from site to site. We had guessed that the rates would be different, but were amazed to see just how different they were."
For example, the group found that the outer water molecules of the Al13 were trading oxygens with the water at a rate of about 1000 exchanges per second at room temperature. However, at the oxygens bridging two aluminums, oxygens were swapped only about once every 13 hours. And at the stable core of the big molecule, bonds were broken only when the Al13 molecule finally disintegrated.
He and Phillips are continuing their study with various chemical structures and collaborating with other researchers in developing computer models.
Their work was funded by the National Science foundation and the W.M. Keck Solid State NMR Laboratory at UC Davis.
Media contacts:
William Casey, Land, Air & Water Resources, (530) 752-3211, whcasey@ucdavis.edu
Brian Phillips, Chemical Engineering & Materials Science, (530) 752-4396, blphillips@ucdavis.edu
Patricia Bailey, News Service, (530) 752-9843, pjbailey@ucdavis.edu
Journal
Nature