Top Papers in Environmental Science, First Runner-Up: Minerals get more complex
COURTESY OF CHONGZHENG NA AND SCOT MARTIN
the team was sampling the mineral faces, for example, or the me“Interfacial Forces Are Modified rhodochrosite, MnCO3, which is chanics of filtration in soils. by the Growth of Surface NanoBecause nanomaterials have feastructures” by Chongzheng Na common in anoxic soils. The retures in the same size range, the and Scot Martin, Harvard Universearchers’ interests focused on the same heterogeneity may apply, sity, School of Engineering and interface between anoxic rhodothe team hypothesizes. Martin Applied Sciences, 2008, 42 (18), chrosite and the oxic structures and Na also suggest that 6883-6889; DOI 10.1021/ the tiny probe tip can es800839a. stand in for many differScot Martin and ent materials that might Chongzheng Na of Harinteract with soil vard University published surfacesseven bacteria, a study about a simple theoretically. method that has shed The new work shows new light on the variability that nanostructures with extreme detail the create on mineral sursize of these nanostrucfaces. This work is part of tures and how they may a series of studies examhave oxidized differently. ining the interfaces of This could lead to a more minerals common to realistic view of minerals some soils. The nanoand how they weather, as structure maps, described well as “why they sorb in one of ES&T’s best paspecies we don’t expect,” pers of 2008, could lead to explains Tracy Bank, a new ideas about how Chongzheng Na (left) and Scot Martin, pictured during a whalebiogeochemist at the Unisorption and other inter- watching trip in Boston, mapped the nanostructures of soil versity at Buffalo, State minerals for the first time by using AFM. The images revealed actions in soils actually University of New York. canyons and plateaus of heterogeneous force potential on the work. Until now, hints of this surface. “I happened to be at heterogeneity at soil surthe right place at the faces existed only because (MnOx) that form on top, several right time,” says Na, a postdoctorcertain behaviors did not seem to nanometers high. al researcher in Martin’s group fit into the expected fate and transDifferences in forces allowed Na and coauthor of the paper. “Scot port of materials. “At the nanosand Martin to minutely map out has been working on these nanocale, surfaces aren’t anything like the surface of the rhodochrosite structures for many years, and the what you expect at the larger and to monitor the shifts in those team had been modeling what scale,” Bank says. forces caused by the nanostructhey saw in the lab with atomic The finding that surface charges tures sitting on top. The resulting force microscopy, or AFM. AFM is differ radically between nanoscale images look like continents of very good at picking up contrasts” versus bulk particles of MnOx “is MnOx atop an ocean of MnCO3. for electrical properties, surface important when you wish to un“When we look at the map, we are morphology, and more, Na says. derstand how certain contaminants looking for contrast,” Na says, both The method uses a tiny cantileor pathogens will move through an in forces working at the surface, vered arm, with a probe at one aquifer,” says Steven Lower of Ohio such as van der Waals attraction, freely moving tip and the other end State University. and in structures. anchored. As the forces at the sur“It’s really cool how they used “This is basic research on funface change, the probe is deflected, this [atomic force microscope] to damental science: we used a sysand the atomic force microscope determine” the shifts in forces at tem that we think is relevant to arm bends. A laser reflecting off the the mineral surface, Bank adds. geochemistry,” Na continues. The back side of the arm measures that “And they’ve done it so surfaces that they’ve mapped are movement, producing vertical prosimplyswhy didn’t we think of highly heterogeneous. These diffiles that allow the researchers to that before?” ferences could help to elucidate —NAOMI LUBICK map the forces acting at the surthe fundamental physics of how face of the samples. In this case, contaminants sorb to soil sur2196 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / April 1, 2009
10.1021/es900412t
2009 American Chemical Society
Published on Web 02/25/2009
