Newswise — Andrew Stack, a geochemist at the Department of Energy’s Oak Ridge National Laboratory, advances understanding of the dynamics of minerals underground. He earned his doctorate in geology from the University of Wyoming in 2002 and went on to become a postgraduate researcher at the University of California–Davis and assistant professor at the Georgia Institute of Technology before joining ORNL in 2010 to investigate chemical reactions of minerals in soils and rocks. He is a member of the American Chemical Society and the Geochemical Society. Stack and his team make discoveries that will help to improve our understanding of a wide range of energy-related issues, such as geologic storage of carbon dioxide, oil and gas discovery and development, and remediation of toxic contaminants. Following is Part II of a Q&A with Stack [Part I is posted here] detailing his current research, which spans three disciplines—geology, chemistry and computing.
Q: What is geochemistry and why is it critical to the Department of Energy?
A: Geochemistry is literally the chemistry of the Earth, but there are a couple of different types. One type uses the chemical composition of minerals to help interpret geology or past climate. The kind of work that I do is different—I want to understand the reaction mechanisms that occur on minerals from a fundamental perspective. Understanding those reaction mechanisms turns out to be very useful for predicting groundwater composition, trapping of contaminants underground or predicting the suitability of a waste repository, which are important to DOE.
Q: What big scientific questions interest you?
A: I’ll quote a friend, George Redden at Idaho National Laboratory, who said that when dealing with reactions underground, you’ve got control over only three things: what you inject, how fast you inject it, and where you inject it. And that’s all you can really control. Coupled with limited ability to control things at the beginning is that predicting what is going to happen in geological systems in general is a really large-scale problem, especially when it’s something like certifying a nuclear waste repository to store waste safely for a million years. In addition to the large time and length scales of natural processes, there are variations in the minerals, their reactivity and the distributions of pores in rocks that control fluid flow, which may mean that what you study in the laboratory is difficult to extrapolate to the real world. Given all that, my feeling is that a better understanding of geochemistry, while not a complete answer, is going to be useful any time things happen, or you want things to happen or not happen, under the ground, whether during hydraulic fracturing or re-injecting the waste fluid from that process, geologic carbon storage (a.k.a. carbon sequestration) or nuclear waste disposal.
Q: Does fundamental research create understanding that would help develop a better ability to predict geochemical reactions?
A: Absolutely! Right now a lot of computational models are being run to understand transport of fluids like water and hydrocarbons through rock and their interactions with the surrounding rock. These models don’t include very realistic geochemical reactions, though. The geochemistry has been simplified to make the problem easier to simulate, but it is really over-simplified. It’s becoming increasingly clear that the ability to model subsurface processes is essential to obtaining an accurate predictive capability for contaminant transport. Making these kinds of predictions is a daunting task because of the wide range of time and length scales involved.
Q: If that task is daunting, what will success look like?
A: If I could look back on my career and declare success, it would be because some expressions for mineral growth and dissolution that I developed were used inside a large-scale model to predict contaminant transport over time, for example at a spent nuclear fuel storage facility or a carbon sequestration site. Even better than just predicting contaminant transport would be using these kinds of models to show that a toxic contaminant can and will be immobilized.
Q: You won a recent award for a talk about atomic- to pore-scale probes and predictions of mineral reactivity. What approach does that research take?
A: In our core geosciences project funded by the US Department of Energy’s Office of Basic Energy Sciences, our group is interested in understanding mineral reactions going from atoms to pores, particularly mineral precipitation and dissolution. Say you want to find out if a mineral is going to form in a groundwater. You pull up some well water, analyze its composition and see how it compares with the stability of various minerals. The problem with that approach is that you have a very difficult time accounting for all the important factors that affect the mineral reactions and why. A mineral grain’s roughness, morphology, how many defects and impurities it has—all play a role in how it reacts. These issues are important, not only to see if contaminants might be sequestered, but also because mineral precipitation and dissolution can block or enhance fluid flow in porous geologic formations, which is the primary control on contaminant transport.
The approach that our group takes in our core project is to try to understand growth or dissolution from the mineral’s perspective. We study things like the specific chemical reactions it undergoes and how those atomic-scale processes affect the overall water composition and the porosity of the rock. There are a bunch of different types of atomic sites on a mineral’s surface, and each has potentially different chemical reactivity. All of these sites act in concert to create the net reactivity of the mineral, and that in turn determines the water composition. Predicting if a mineral will precipitate is more difficult in rocks. Because rocks are porous, water (or other fluids) moves through them slowly, and so the transport of reactants and products in the water to the mineral becomes important in controlling its growth. That presents some additional challenges, to try and understand what mineral reactions will take place, and how they occur. But the basic question we’re trying to answer is: when you have a given solution composition moving through a rock, what minerals will precipitate or dissolve and how fast will it happen?
Q: Can you tell me a bit about your latest research?
A: When I was a postdoc I had discovered that crystal shape depended on the atomic-level reaction mechanisms controlling the mineral’s growth, but I didn’t know what those were precisely. In a recent paper in the Journal of the American Chemical Society, I and two researchers from Curtin University in Australia— Julian Gale and Paolo Raiteri—finally got it right. We found a mechanism for barite growth and dissolution with a variation in rate as a function of temperature that matched the experimental measurements. To discover this, we simulated the attachment and detachment process of a barium ion onto an atomic-level feature of a barite surface called a “step.” These steps are important because their advance or retreat is the mechanism by which the mineral grows or dissolves and takes its shape. Each one is made of one or two molecular rows of the mineral, and a crystal grows because ions are constantly incorporated at the step edges. We simulated the mechanism of how the barium ions attach to or detach from the steps and the amount of energy it takes to do that. Finally, we related those parameters to the experimental measurements and found they matched.
Q: Why is barite important?
A: It’s mostly known geochemically because it has a low solubility that causes it to clog pores in rocks, wells, pipes and equipment. We’re interested in avoiding precipitation where it blocks permeability, but taking advantage of that property and using it in other subsurface environments to trap radium. Radium is a problem for some proposed spent nuclear fuel storage sites because it doesn’t adsorb well onto a lot of minerals, especially the clay minerals normally used to stop contaminants from spreading. But if you can trap the radium as an impurity in barium sulfate, it won’t be going anywhere. Another issue is that—during hydraulic fracturing—wells have salty water that comes up along with the natural gas or oil. A few wells have dissolved barium, radium and strontium coming up to the surface too. In fact, radium is the dominant component in terms of the total radioactivity of these brines. These are so-called RCRA compounds, for the Resource Conservation and Recovery Act of 1976. You are not allowed to dump these metals in streams because they’re toxic and/or radioactive. Since you don’t want the radium coming up to the surface, our group is working on the idea of trapping the radium and barium by controlling the precipitation of barium sulfate. That is the topic of our Laboratory Directed Research and Development (LDRD) project. LDRD awards support cutting-edge research and enhance the lab’s ability to obtain external funding to address DOE missions.
Q: Is geochemistry important for carbon capture and storage?
A: That’s what we’re trying to find out. For carbon sequestration, you want to pump carbon dioxide into the ground and you want it to stay there. The safest way to ensure that is to know if the carbon dioxide will dissolve in the surrounding groundwater and react to form a carbonate mineral like calcite, which is calcium carbonate. The idea is that if you can trap the contaminant as a solid mineral, it’s not going anywhere for a long time. That is where geochemistry comes in. Our job is to find out how much carbonate mineral will form, how long it will take and how it affects the communication of fluids in the rock.
Q: How are government researchers focusing their efforts in geologic carbon storage?
A: We explore aspects of this problem in an Energy Frontier Research Center (EFRC) called the Center for Nanoscale Control of Geologic CO2. It’s run out of Lawrence Berkeley National Laboratory, but Oak Ridge National Laboratory is the largest partnering institution. This EFRC was initially funded for 5 years, and was renewed in June 2014 and awarded $12.8 million over an additional 4 years to characterize and understand nanoscale to pore-scale processes that control the trapping of carbon dioxide in subsurface rock formations.
Q: Recently you wrote a paper in the journal Environmental Science & Technology with implications for carbon storage. What did you discover?
A: Our finding adds a new wrinkle to the question: what is the size of the pores in which minerals prefer to form? The answer will have implications for carbon storage, because if you want to convert the carbon dioxide to a mineral, you would like to make efficient use of all the pores. What I and other researchers from the Center for Nanoscale Control of Geologic CO2 showed in this paper is that you can get precipitation in the smallest pores, but only if there’s a favorable chemical interaction between the mineral and the host rock. Otherwise, you only see minerals forming in the larger pores, where they might block large-scale fluid flow.
Q: What are the next grand challenges in geochemistry and what’s your vision for addressing them?
A: Earlier I mentioned this atom-to-pores concept— translating atomic-level reactions on a mineral into how these reactions occur in the pores that control fluid flow in geologic formations. Our group has made enough progress that, while we haven’t got the puzzle finished yet, we have just enough pieces to see what the finished picture might look like. For the next 5 or 10 years, we’re going to try to finish that puzzle. If it works, it will be incredibly useful; but it will require cooperation among scientists working at different scales and areas of expertise. Geochemistry can be useful for addressing a lot of different grand challenges for society, which all fall under the general heading of “how do we maintain or even enhance our standard of living without degrading the environment and endangering ourselves in the process?”
Q: Can you tell me about some of the techniques you use to push the frontiers of geochemistry?
A: Working at a DOE lab gives me access to advanced national research facilities that maintain capabilities that individual universities or companies do not operate. Some are the Spallation Neutron Source (SNS) and the High Flux Isotope Reactor (HFIR), DOE User Facilities at ORNL. SNS is an accelerator-based neutron source that provides the most intense neutron beams in the world for scientific research and industrial development, and HFIR is a powerful reactor-based source. We use these in a couple of ways. One way (at HFIR) relies on the fact that neutrons have no net electric charge so they pass through relatively thick sections of rock. By measuring how much the neutrons scatter at smaller angles we can get information about the distribution of pores in a rock (how big they are) and how that changes as minerals dissolve and precipitate. Another way that we use neutrons at SNS is to examine the atomic structure of ions dissolved in solution, or the dynamics of water adsorbed to minerals. These studies help us in building the puzzle pieces I just mentioned.
Q: What geochemistry challenges keep you up at night?
A: I’m constantly asking myself what is the most interesting science that I could do that is also useful for society. If you want to know what I worry about most though, it’s not the science. I worry about society continuing to take advantage of the benefits of funding basic research. I think part of the problem is that we as scientists have to do a better job of explaining to people how the things that we’re doing and learning are helpful to them. Mineral precipitation in rocks is not exactly dinner table conversation, because it’s outside of most people’s everyday experience. But this and other such geochemical processes largely determine things like the discovery and extraction of oil and gas, the safe disposal of energy-related contaminants and even the huge amounts of clean fresh water from underground aquifers that we rely so heavily on. I don’t think we scientists do a good enough job of explaining to non-scientists why we should study something. It’s also not enough to obtain funding and do good science—for that science to be really useful there has to be a way for society to take advantage of it too. That becomes especially difficult when the science contradicts someone’s interests or deeply held beliefs. What is the best way for scientists to deal with that?—interview by Dawn Levy
IMAGE 1, Stack_2015-P00294CAPTION/CREDITOak Ridge National Laboratory geochemist Andrew Stack and his team make discoveries that will help to improve geologic storage of carbon dioxide and remediation of toxic contaminants. Image credit: Oak Ridge National Laboratory, U.S. Dept. of Energy; photographer Carlos Jones
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