The research will be presented at the 223rd national meeting of the American Chemical Society, April 7-11 in Orlando.
The interaction of toxic metals in water with mineral surfaces in soils and rocks plays a major role in the quality of the world's fresh water and the mobility, bioavailability, persistence, and ultimate fate of dangerous environmental pollutants.
Lead (Pb), which is toxic, is one of the most common of the heavy metals in the earth's crust and therefore a common pollutant. But it can bind to several minerals, effectively reducing its availability to plants and animals.
S. Erin O'Reilly, a doctoral student in geological sciences at Virginia Tech is examining the lead sorption efficiency of several manganese (Mn) and iron (Fe) oxides. The aim is to clarify the understanding of which phases of Mn and Fe and which mineral properties of those phases are most influential in controlling lead availability in the environment. "We also hope to increase the understanding of the mineral surface properties that determine the lead absorption efficiency of a solid," says O'Reilly.
Iron oxide minerals are common in soils, so plentiful that they are responsible for the orange or red rusty color of many soils. Manganese oxide minerals are also common, but not all forms are easily studied. Birnessite, for instance, is "poorly crystalline" andis often mixed with other soil components and difficult to isolate for laboratory study.
The comparisons are based upon the amount of lead per surface area that the minerals "sorb" or remove from solution. The minerals are reduced to particles and placed in solution, which is injected into a filter chamber called a "reactor" -- one reactor per mineral type. The same amount of lead in solution is passed through each reactor. The results are measured in several ways -- the amount of lead in the water after it passes through the reactor being the obvious measure.
Since the scientists are also interested in the surface dynamics of the mineral with the dissolved lead, the surface chemistry of each mineral sample with its lead baggage is measured with X-ray photoelectron spectroscopy (XPS). Then the researchers look at the surface with the scanning electron microscope (SEM), which provides a good view of the mineral surfaces and certain mineral-water-lead interactions.
Previous studies have been done with natural and synthetic forms of iron oxide minerals, but fewer studies have been done with natural manganese oxide minerals, since it is less available. However, O'Reilly was able to augment natural manganese oxides obtained from the Smithsonian Institution with ones from Virginia Tech's geology museum and from the donations of collectors.
Results to date show that while synthetic manganese oxides are highly efficient at taking up lead, the naturally occurring manganese oxides (ad)sorb lead in similar amounts as naturally occurring iron oxides.
An interesting finding is that the synthetic Birnessite form of manganese oxide is particularly efficient. "It has a layered structure and the lead moves within the layer, so the lead doesn’t show up on the surface of the mineral when we look at it with XPS," O'Reilly says. But with reduced levels of lead in the water that was passed through the Birnessite as a clue, a closer examination of the mineral revealed what was happening to the lead.
O'Reilly will present "Pb sorption efficiencies on natural and synthetic Mn- and Fe-oxides: A new look at an old problem" (Geoc 121) on Wednesday, April 10, at 4:05 p.m. in Convention Center room 206A, level two. Co-author is Michael F. Hochella Jr., professor of geological sciences.
The work is funded by the NSF. The study may also have application for removal of other heavy metals from the environment.
O'Reilly, who grew up in Hockessin, Delaware, did her undergraduate and master's degree work at the University of Delaware in Newark, DE. She is the daughter of John T. and Susan K. O'Reilly of Newark.
Contact Information: S. Erin O'Reilly, 540-231-3358 or soreilly@vt.edu
Michael F. Hochella Jr. is presently in Germany on a Humboldt Fellowship, but can be reached by e-mail at hochella@vt.edu
PR CONTACT: Susan Trulove (540) 231-5646 STrulove@vt.edu