Iron- and manganese-oxide minerals remove lead from water and soil but how? And how much? Virginia Tech researchers are beginning to answer these questions.
The interaction of toxic metals in water with mineral surfaces in soils and rocks plays a major role in the quality of the worlds 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 earths crust and therefore a common pollutant. But it can bind to several minerals, effectively reducing its availability to plants and animals.
S. Erin OReilly, 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 OReilly.
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 and is 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, OReilly was able to augment natural manganese oxides obtained from the Smithsonian Institution with ones from Virginia Techs 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 doesnt show up on the surface of the mineral when we look at it with XPS, OReilly 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.
The work is funded by the NSF. The study may also have application for removal of other heavy metals from the environment.
OReillys major professor is Michael F. Hochella Jr. (email@example.com).
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S. Erin Reilly received her Ph.D. in 2002. Her dissertation is available online.
— Written by Susan Trulove