How minerals react in the environment depends on particle size

One of the most common groups of minerals on earth is the iron oxides, found in soils, rusting iron, and the dust of Mars. Due to their importance in the environment, iron oxide minerals have been widely studied, providing insight into their properties and reactivities. But when the size of minerals decreases to 1 to 10 nanometers (billionths of a meter), many of their properties change.

Due to their importance in the environment, iron oxide minerals have been widely studied, providing insight into their properties and reactivities. But when the size of minerals decreases to 1 to 10 nanometers (billionths of a meter), many of their properties change.

Andrew Madden, of Blacksburg, a Ph.D. student in geosciences at Virginia Tech and a Michigan State alum, will report on the nanoscale properties of iron oxide at the 227th national meeting of the American Chemical Society, being held in Anaheim, Calif., March 28 through April 1, 2004.

"Geoscientists now recognize that there are small particles in our environment, but we don't know their properties at the nanoscale," says Madden. He is doing his experiments using hematite, the same iron oxide associated with Mars.

He is studying the reaction between dissolved manganese and oxygen, a process known as manganese oxidation. The rate of the reaction is greatly enhanced by minerals such as hematite. This process is responsible for removing dissolved manganese from water and forming manganese oxide minerals, which are extremely important in adsorbing and transforming a variety of pollutants, such as lead, nickel, cobalt, and pesticides.

Manganese is everywhere – in soils, rivers, oceans, and lakes – and its oxidation depends upon the solid with which it interacts. How is this interaction different at the nanoscale? So far, Madden has found that nanoparticles are 30 times more efficient at promoting the manganese oxidation reaction than the same material in bulk.

One consequence of the research is the questioning of a long-held assumption about manganese oxidation – that the process requires bacteria because it is much slower in the absence of bacteria. But maybe it is particle size and not bacteria that influences the speed of the process in some environments.

"Reactivity is controlled by the electrons and electronic structure of the particles, which changes as the particle gets smaller," Madden explains. In a smaller hunk of matter, more of the atoms are at the surface. In the research circumstances, the iron oxide gave up electrons to the manganese, making them more susceptible to reaction with dissolved oxygen.

Madden says he can't say yet what might happen as a result of such interactions. "We expect to synthesize smaller particles and see an even more efficient reaction."

Madden will present the paper, "Testing geochemical reactivity as a function of mineral size: Manganese oxidation promoted by hematite nanoparticles (GEOC 92)" at 5:30 p.m. Tuesday, March 30, at the Marriott -- Marquis NW as the last presentation of the symposium on “Interfacial Phenomena: Linking Atomistic and Macroscopic Properties.” Co-author is Virginia Tech professor of geosciences Michael F. Hochella Jr.

Madden is a member of the Hochella NanoGeoscience and mineral-microbe research group. He became a Ph.D. candidate in geosciences in fall 2000 and was awarded a National Science Foundation fellowship in 2001. His undergraduate degree is from Michigan State University, and he worked at Dart Oil and Gas in Mason, Mich., while at MSU.

Abstract:

The rate of Mn2+(aq) oxidation on hematite surfaces in the presence of oxygenated water has been studied as a function of the hematite particle size, where the particles are in the nanometer size regime. Experimental results from this study suggest that the surface area normalized initial heterogeneous manganese oxidation rate is approximately one to one and a half orders of magnitude greater on hematite particles with average diameter and thickness of 7.3 nm and 1.5 nm than those with average dimensions of 37 nm by 8 nm. The acceleration of electron transfer rate for the reaction promoted by the smallest particles was rationalized in the framework of electron transfer theory, considering the effects of the changing surface geometric and electronic structure on the coupling between solid-Mn-O2, the reduction in reorganization energy for distorted octahedral Mn coordination environments, and the redox potential of the adsorbed Mn.