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When a Sticky Pollutant Is a Good Pollutant

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Franz Geiger

Physical/Analytical and Environmental Chemist

Northwestern University

Franz Geiger heads up a research group at Northwestern that uses lasers to see how tightly pollutants stick to other substances. This new field's called environmental surface science--and research in it may help improve our groundwater.

Harold Henderson: Why "surface science"?

Franz Geiger: Environmental pollutants interact with the surfaces of particles in the air or soil. We ask how the surfaces control this interaction. In the projects graduate student Christopher Konek and postdoctoral student Hind Al-Abadleh are working on, we want to learn what exactly happens at the interface between soil and a number of pollutants in the groundwater. How strongly do the pollutants stick to the soil particles, and under what conditions?

HH: But there are lots of different kinds of soil.

FG: We start out with the most abundant mineral, silica, found in sand. We set up an interface where water meets the silica. That's a platform we can control and make more complex, one step at a time. Instead of starting with a real soil, which has who knows how many different things in it and is hard to generalize from, we start with a simplified soil and see how each new addition affects it.

HH: I understand chromium is one pollutant you've been investigating.

FG: Actually, we see chromium in three different forms. Its toxicity changes completely depending on its oxidation state. Metallic chromium, or chromium 0, doesn't corrode, doesn't react. Chromium 3 is an essential nutrient that helps you digest sugars. Without it you die. Chromium 6, or chromate, originates mainly from fossil-fuel combustion. It's also used in the metal, alloy, and wood industries and in petrochemical cooling towers.

HH: And that's the bad one?

FG: Terrible, because it's such a good oxidant. If you're exposed to it on a daily basis, it breaks up cells and may react with your DNA, possibly damaging it and causing cancer. That's what the movie Erin Brockovich showed so well. We are very interested in examining what happens when this kind of chromium percolates through the groundwater. So we add a known amount of chromium 6 to the water and see what happens when it meets the silica.

HH: But don't we already have some idea of that? By measuring it in the field or by pouring it into a column of soil in the laboratory?

FG: Yes, but these measurements don't tell you exactly what happens at the interface, and they can take a long time. Nonlaser methods for looking at interfaces aren't very sensitive either. In order to get results scientists have to use chromate concentrations that are 100 or 1,000 times greater than occur in actual situations. With concentrations like that the surface processes may be different.

HH: So you aim to be more realistic.

FG: We aim to be more precise. In our experiments we can turn the chromium flow on and off, and we know exactly how much is going onto the surface. These are molecular-level snapshots. The laser shows us what's actually happening where the solid and water meet, in real time and at realistic levels of contamination. When the chromium flow is on we see more chromium at the interface between the solid and the water. When the chromium is turned off and it's just water flowing by, the chromium 6 goes away. It doesn't stay with the surface.

HH: That's not good news if you plan to drink the water.

FG: No. Chromium 6 turns out to be highly mobile. In a silica-rich soil we find that it moves 90 percent as fast as water itself does. That is, the soil only slows it down about 10 percent.

HH: How big a surface do you look at?

FG: We work with a quartz hemisphere about one inch in diameter. Quartz is ultrapure silica, containing only silicon and oxygen atoms.

HH: And how do you "see" what's going on between particles there?

FG: It's easier to think of light interacting with matter as different ways of hitting a tuning fork. Take low-intensity light from a lamp, say. When it hits a surface it reflects, or refracts, or shows colors. It's like hitting a tuning fork gently and hearing its ground tone. When you aim a high-powered laser at a surface it's like whacking a tuning fork really hard. Besides a very loud ground tone you also hear a lot of ringing overtones. The laser makes matter ring at certain frequencies according to what's there. We listen for the very faint ringing at a certain frequency that is unique to surface-bound chromate or whatever other substance we might be working on.

HH: Just how faint is that ringing?

FG: For every trillion photons we shoot from the laser we get only one photon signal back out. We use detectors called single-photon counters, which are able to listen for that very faint response. It's like taking the entire amount of cash in the U.S., investing it, and getting a dollar in return.

HH: How much does all this cost?

FG: The laser itself costs $300,000. Running my lab and my group costs about $25,000 a month. That includes everything, from salaries and benefits to supplies and optics. This is a fairly small budget--we're just starting out.

HH: Where does the money come from?

FG: I spend 80 to 90 percent of my time writing grant applications to support the research. It takes time because the success rate is about 20 percent. Funders include the National Science Foundation, the Department of Energy, the Camille and Henry Dreyfus Foundation, Shell Oil, NASA, the Petroleum Research Fund of the American Chemical Society, and Northwestern.

HH: Let me ask the question another way. How much of this work could be done if you weren't bringing in any grants?

FG: None.

Art accompanying story in printed newspaper (not available in this archive): photo/Lloyd DeGrane.

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