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The Making of a Geochemist: Q&A with Andrew Stack

Released: 4-Mar-2015 2:05 PM EST
Source Newsroom: Oak Ridge National Laboratory
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Mar. 6, 2015 - Scientists who bridge disciplines often take research in new directions. Andrew Stack of the Department of Energy’s Oak Ridge National Laboratory calls on his expertise in geology, chemistry and computing to advance understanding of the dynamics of minerals underground. Working in the Geochemistry and Interfacial Sciences Group of ORNL’s Chemical Sciences Division, he investigates chemical processes that take place on mineral surfaces at scales ranging from individual atoms to entire rocks. These processes can trap contaminants, such as nuclear waste, carbon dioxide and toxic by-products from hydraulic fracturing. Fundamental knowledge of chemical transformations is crucial to many DOE missions and serves as a basis for developing ways to lessen the environmental impacts of energy use.

Q: How did your multidisciplinary career begin?

A: I was good at science in intermediate school and went to a high school that specialized in it (Thomas Jefferson High School for Science and Technology). It probably helped my interest in science that my parents worked for the Food and Drug Administration in Washington, D.C. (I have my Dad’s old balance in my office). One summer in high school, I volunteered at the Smithsonian National Air and Space Museum, helping to preserve their space suit collection and creating a file of space shuttle missions for their research library. What an amazing experience that was! That alone would get somebody interested in science. In my senior year I really liked an Earth science course in which we read a novel by John McPhee (Rising from the Plains) about a geologist in Wyoming. Just when I was starting to think about a career path, my mother arranged for me to talk to a paleontologist at the Smithsonian National Museum of Natural History for a couple of hours. The novel Jurassic Park had just come out, and I wanted to talk to him about the feasibility of resurrecting dinosaurs. I thought that whole concept was really cool, and we talked about problems with the novel and what careers in geosciences were like.

Q: What was your college experience like?

A: I got into Virginia Tech in 1993 and decided to major in geology. I liked geology because you could work outdoors as well as do high-tech science. But freshman year I also discovered that I liked chemistry—so much so that I was agonizing about whether to change majors when my department offered this new geochemistry degree, geology with a chemistry minor. I thought, hey, that solves my problem!

I got a job during summers and breaks in the Water Resources Division of the US Geological Survey with a geochemist who needed a computer programmer with some geochemistry sense. What he got was me, a geochemist who had taken a computer programming course and liked computers. It turned out—and this was random luck— that my supervisor was a famous geochemist, Niel Plummer. My mineralogy and geochemistry professors at Virginia Tech, Mike Hochella and Don Rimstidt, were also really good and opened my eyes to how interesting minerals and their reactions are. I ended up deciding to go to graduate school with one of Mike’s former students, Carrick Eggleston, at the University of Wyoming.

Q: You had other offers, though. Why Wyoming?

A: Of the schools I applied to, the University of Wyoming was the lowest ranked, but my advisor was great and doing the science that interested me the most out of the schools I applied to. I also remember looking out over the plains in Wyoming and seeing snow-capped mountains in the distance and thinking, ‘Gosh! This is an incredible place!’

Q: How did graduate school go?

A: I did both my master’s and doctoral degrees with Carrick in Wyoming. The master’s degree was on a laser technique called second harmonic generation. I used it to look at the surface charge of a mineral called corundum, which is aluminum oxide. Rubies and sapphires are made of corundum. The difference between them is they have some different impurities that give each of them their nice colors. The surface charge of minerals is important in groundwaters and soils because similar minerals affect the transport of contaminants, such as toxic metals. If the dissolved metal ion has a positive charge and the mineral ion has a positive charge, too, the like charges repel and you don’t tend to get a lot of adsorption.

Q: What big scientific question were you exploring?

A: A big question that geochemists want to answer is whether the minerals present in a rock or in a soil are going to be positively or negatively charged in groundwater. The laser technique allowed us to isolate a specific crystal surface of that mineral to see how charge might change from crystal face to face. What I found was that the surface charge might change a lot, and you should account for this.

Q: How did you decide on a PhD topic?

A: I decided to stay for a PhD with Carrick but on a different subject—scanning tunneling microscopy of surfaces of hematite, a common iron oxide mineral. When you look at the Grand Canyon, its redness is due to hematite. For the first part of my degree, Carrick and I wanted to know what the structure of this mineral looked like at the atomic scale, specifically whether the surface was covered in iron or oxygen atoms. We found that the mineral could have mixtures of both. For the second half of my PhD, I was interested in an organic compound chemically similar to those that might be used by bacteria to transfer electrons to hematite. There are several species of what are known as iron-reducing bacteria. Similar to the way we breathe in oxygen and breathe out carbon dioxide after transferring electrons to the oxygen, these organisms transfer electrons to iron minerals. So they’re essentially “breathing” on the iron minerals. The mineral becomes unstable and dissolves during this process, and I wanted to understand the reaction mechanisms that were controlling it. I found that the electron-transfer process was much faster than the rate at which the minerals dissolved and couldn’t be controlling the dissolution.

Q: Where did your career take you after you received your doctorate?

A: I obtained a postdoctoral research position at the University of California–Davis to look at reaction mechanisms of water that’s bound to mineral surfaces. One can synthesize molecules that have similar chemical structures as minerals but that are easier to characterize. My advisor, Bill Casey, and I were interested in understanding how quickly water molecules bound to a mineral surface exchange with each other, with the idea that this process is connected to how fast the mineral reacts generally. When you expose a mineral to water, one water molecule will be bound to a metal site on the mineral surface for a little while, and then it will get pushed out by another one; or it will leave and another one will come in. I simulated that process using quantum chemical calculations. I knew what the answer was on this particular molecule because it had been measured by someone else in my advisor’s group. If I could simulate the reaction on the molecule correctly, then I could simulate the same thing on a mineral surface and be confident that I got the right answer. I found that I could get mostly the right answer, but I needed to simulate many more water molecules than we could do at the time to get a better answer.

Q: What other topics did you explore as a postdoctoral researcher?

A: I also did some work related to the National Ignition Facility (NIF), where they use lasers to create fusion. They were having trouble growing crystals of potassium dihydrogen phosphate to use as optics in the lasers. I was doing fundamental work to understand how that type of crystal grew. NIF researchers needed to grow crystals several feet wide, and it takes a long time to grow a crystal that big. They wanted to grow them more rapidly but with a low number of defects in the crystal. Defects make the crystal not last as long, and too many defects make it useless. I found that, while I couldn’t determine what they were precisely, the atomic-level mechanisms for reaction on the crystal surface were important for determining the shape of the crystal and how fast it grew.

Q: What job opportunities were available to you next?

A: In 2005 I decided to take a position at the Georgia Institute of Technology as an assistant professor. I felt I could do more advanced science there as opposed to my other job offers. Whenever I’ve been faced with a career decision, I have always picked the path that lets me do the science that’s the most interesting.

Q: What science interested you most at Georgia Tech?

A: Regarding those iron-reducing bacteria that I had mentioned from my PhD, I started working on the bacteria themselves, trying to understand more about what controls how much of the iron oxide mineral they can access. I also continued working on simulating water exchange. This time, instead of working on an idealized molecule, I was simulating the water exchange on the mineral surfaces themselves. Finally, I kept working on the atomic-scale mechanisms for crystal growth to find out what they really were. I started working on barium sulfate, which is called barite, and calcium carbonate, which is calcite. Barite is not as common geochemically as calcite, but it is important because it precipitates in wells and pipes during the production of oil and natural gas, and people are thinking about using it to stop radium from contaminating the flowback water from hydrofracturing operations.

Q: Why is calcite interesting?

A: Calcite is really common in soils and rocks and interesting for a couple of reasons. One is those atomic-level mechanisms of crystal growth I mentioned earlier. Also, calcite is an important biomineral. Many organisms in the ocean make their shells out of calcite, like corals and most of the shells you find on the beach. People try to use the isotope ratios in those corals and shells to infer what past ocean conditions were like. Calcite (and barite) are also important because we’d like to be able to control precipitation reactions under the ground. Let’s say a contaminant behaves very similarly to calcium or barium. For example, strontium-90 is a problem contaminant on some DOE sites. Strontium substitutes for calcium in calcite. You’ve got some dissolved strontium sitting in groundwater. If you could cause calcite to precipitate and incorporate that strontium into its crystal structure, you could trap that strontium. If the mineral remains intact for about 100 years, long enough for the strontium to decay, it won’t be a threat anymore. The contamination will not be in danger of spreading from the original site. A scientist working with me at Georgia Tech, Meg Grantham, and I measured the rate at which calcite grew when you varied the ratio of calcium and carbonate in the solution. We found this ratio was very important in determining how fast the mineral grew but this information hadn’t been incorporated into geochemical models at the time. Later at ORNL, my student Jacky Bracco and I measured the effects of the calcium-to-carbonate ratio on strontium incorporation in calcite. Another example of the importance of calcite is that carbon dioxide injected into deep geological formations might be permanently sequestered in the form of carbonate minerals like calcite.

Q: Did following your scientific curiosity bring you to ORNL in 2010?

A: Yes. DOE’s Office of Science has been very supportive of research on the kinds of atomic-scale mineral reactions that I am interested in. Another thing is that collaboration with other scientists at the national labs is so much easier than at a university. At a university you might talk to your friend or somebody across campus and get a collaboration going between two or three of you. But here at ORNL, papers often have a whole bunch of co-authors, even from other national labs or universities, because the labs are about large, collaborative projects. The scope of the problems that you can tackle or that you can try to understand is larger at a national lab than what you can do by yourself at a university.

The really amazing thing to me about the lab is that the opportunities for research topics are so diverse. I’ve got five projects right now, and they include things like neutron and X-ray scattering, which I had never done before I came to ORNL. We’re working now on rare earth element minerals, geologic carbon storage, hydraulic fracturing, mineral reactions during nuclear waste disposal and other things. If I were back at Georgia Tech, I’m not sure how much of that I would have been able to work on. The lab has helped me realize my potential and allows me to work on important problems for society.—interview by Dawn Levy

IMAGE 1, 2015-P00289
Andrew Stack of Oak Ridge National Laboratory calls on his expertise in geology, chemistry and computing to advance understanding of the dynamics of minerals underground. Image credit: Oak Ridge National Laboratory, U.S. Dept. of Energy; photographer Carlos Jones


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