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Basic Geochemical Research Supports Energy Industries

Water is the key ingredient that promotes and facilitates the transfer of mass and heat from one reservoir to another. The interaction between water and solid phases, whether those phases are natural minerals or metals in pipes, can typically lead to the release of metals and the formation of secondary corrosion or alteration phases. The solutions can range from dilute solutions of a single solute in water to complex systems containing high concentrations of more than 10 dissolved species. These solutions play a crucial role in such geochemical and technological processes as, for example, hydrothermal formation of mineral deposits, hydrothermal crystal growth and materials synthesis, and high-temperature electrochemical processes. Hot, aqueous solutions and mineral deposits can also respectively corrode and clog pipes, leading to inefficient heat transfer in geothermal power systems and reducing the operating capacity and potential lifetime of water desalination plants, boilers, power plants, and nuclear reactor cooling systems.

Chemists and geochemists in ORNL's Chemical and Analytical Sciences Division (CASD) and their collaborators have carried out pioneering studies of the consequences of water interaction with both natural and industrial materials. These studies range from those that address problems of water-solid interactions at the molecular level to more coarse-scaled studies of natural geological systems.

For example, chemists and geo-chemists in CASD are engaged in a major interdisciplinary research project, funded by the Department of Energy's Office of Basic Energy Sciences under the Complex and Collective Phenomena initiative, to quantify behavior at the oxide-water interface. What happens at the interface when water and its contaminants interact with metals and minerals is of great interest to the energy industry.

This schematic model depicts the electrical double layer at the interface between a positively charged metal oxide surface and an aqueous solution containing dissolved cations and anions. In this illustration, adapted from a drawing by Gordon Brown (Stanford University), negatively charged anions are adsorbed directly onto the positively charged metal oxide surface. Note the difference in orientation of water molecules (blue) around anions (green) compared with cations (red). Illustration by LeJean Hardin.

"Most of our world—from industrial materials to minerals in the earth—consists of metal oxides," says ORNL's Dave Wesolowski, the project's principal investigator. "When metal reacts with water or air, metal oxides form. When water and the dissolved ions it contains—such as chlorine, calcium, iron, lithium, potassium, sodium, strontium, and zinc—come in contact with these oxides, considerable reaction takes place at the interface. Secondary minerals can be formed there, plugging up pipes. Corrosion can result in the failure of pressurized pipes. Metal-oxide nanoparticles can spall off from the surface through contact with circulating water at high pressures and temperatures."

According to Wesolowski, when a metal-oxide surface reacts with water molecules and accompanying ions, an electric charge develops on the surface. If the surface charge is positive and if the ions in solution are negatively charged, the ions will deposit on the metal surface to which they are attracted, causing the buildup of corrosion products.

"Particles can be adsorbed onto the metal-oxide surface or can be released from the surface, depending on pH, temperature, and the chemical composition and concentrations of ions in the water," Wesolowski says. "When water reacts with the oxygen atoms bonded to atoms at the surface, hydrogen and hydroxide ions from the water cause the surface to be charged up. This surface charge is the principal driving force for both the adsorption of contaminant ions in subsurface aquifers and the formation and transport of colloidal particles in steam generator systems. It is this molecular behavior at surfaces we are focusing on in this project, through the use of sophisticated tools such as neutron reflectometry, standing-wave X rays (with partners at Argonne National Laboratory), molecular dynamic simulations, and high-temperature pH cells."

A representative portion of a rutile titanium oxide (TiO2) surface is shown interacting with a strontium cation (Sr2+). In situ X-ray reflectivity is used to measure the atomic-scale positions of ions such as Sr2+, rubidium (Rb+) and bromine (Br-) in aqueous solutions adjacent to the oxide surface. This work involving ORNL researchers is conducted in collaboration with scientists at the Advanced Photon Source at DOE's Argonne National Laboratory. Illustration by LeJean Hardin.

In fact, CASD researchers Wesolowski, Don Palmer, and Pascale Benezeth-Gisquet, in collaboration with a group of visiting researchers, including Mike Machesky (Illinois State Water Survey) and Moira Ridley (Texas Tech), have pioneered the study of the chemical properties of oxides and other minerals as they exist in states from ambient conditions (10 to 50°C) to the extreme temperatures and pressures encountered in geothermal systems and commercial power plants (350°C, 150 atmospheres). These researchers have—for the first time—directly measured the sorption of ions on the surfaces of minerals that form naturally in geological environments, as well as the corrosion products that form in steam generators and other industrial settings, at temperatures exceeding 100°C.

The participants in this study of metal-oxide surface chemistry at high temperatures include Wesolowski, Palmer, Ariel Chialvo, Lawrence Anovitz and Bénézeth-Gisquet, all of CASD; Baohua Gu and Liyuan Liang, both of ORNL's Environmental Sciences Division; Bill Hamilton of ORNL's Solid State Division; Peter Cummings of the University of Tennessee at Knoxville (UTK); Jim Kubicki and Serguei Lvov, both with Penn State University; Paul Fenter and Neil Sturchio (ANL); and Machesky and Ridley.

The Geochemistry Group in CASD's Physical and Materials Chemistry Section has made its mark in other areas, as well. One such area is in the determination of the effects of temperature, pressure, and chemical composition on the redistribution of ratios of stable isotopes (rare over common) of light elements, such as oxygen (¹⁸O/¹⁶O), carbon (¹³C/¹²C), hydrogen (D/H), nitrogen (¹⁵N/¹⁴N), and sulfur (³⁴S/³²S) in fluids and rocks. Water (H₂O) in fluids deep within the earth has a stable isotope signature that can be very different from that of water in a river, lake, or the ocean. This is also true for gases such as carbon dioxide (CO₂) and methane (CH₄).

CASD's David Cole, Juske Horita, and Lee Riciputi are using sensitive techniques, including gas source isotope ratio mass spectrometry and secondary ionization mass spectrometry (ion microprobe) to determine shifts in isotopic signatures (isotope redistribution) that occur during the interaction of water with minerals, as a function of changes in temperature, pressure, and different concentrations of dissolved salts. Naturally occurring isotopes of O, H, C, S, and N provide built-in tracers for monitoring the interaction of fluids and solids in both natural and industrial settings.

In 1999, Juske Horita, postdoctoral fellow Thomas Driesner (now at Eidgenossische Technische Hochschule in Zurich, Switzerland), and Cole published a paper in Science magazine that refuted one fundamental assumption of stable isotope geochemistry. For decades it was believed that temperature, not pressure, had a dominant effect on the hydrogen-isotopic composition of hydrous minerals (e.g., brucite, or magnesium hydroxide) in the presence of water. Theoretical calculations performed by Driesner indicated that this might not be the case. So, the group set out to investigate experimentally the effect of pressure on hydrogen-isotope redistribution (changed D/H ratios) between brucite and pure water at elevated temperatures. They found that equilibrium D/H partitioning between brucite and water systematically increases by 1.24% as pressure increases from 15 to 800 MPa at 380°C. "We concluded that increasing pressure sometimes has more impact than increasing temperature on hydrogen-isotope ratio shifts," Cole says.

Also in 1999, ORNL's Horita and Michael Berndt (University of Minnesota) published a paper in Science magazine that explored the underground formation of methane, the earth's most abundant natural gas. Most methane is formed either by the digestion of organic compounds by microorganisms or by the thermal decomposition of organic matter. However, there is also evidence that some "abiogenic" methane is formed from inorganic matter in the earth's crust. In laboratory experiments, Horita and Berndt found that abiogenic methane can be produced rapidly from dissolved CO₂ in the presence of a naturally occurring nickel-iron alloy under hydrothermal conditions similar to those present in a mid-ocean ridge.

"We found that abiogenic methane can be formed rapidly under reducing conditions encountered in the earth's crust and that the ratio of carbon-13 and carbon-12 isotopes is not a clear-cut criterion for distinguishing biogenic from abiogenic methane," Horita says. "An important implication is that abiogenic methane could be far more widespread in nature than currently thought, especially in crystalline rocks on land and on ocean floors, including in gas hydrates."

Lee Riciputi shows an obsidian souvenir depicting a native Mexican god of thunder and lightning and an obsidian sample that he analyzed using the ion microprobe (secondary ionization mass spectrometer). The analysis casts doubt on the validity of a traditional method for dating glassy artifacts. (Photo by Curtis Boles; enhanced by LeJean Hardin)

Isotopic distributions can be determined in situ in minerals, using the microbeam capability of CASD's Cameca 4f ion microprobe. A tiny beam of ions (such as Cs⁺ or O^-) is delivered to the mineral surface, liberating a cloud of secondary ions that is then accelerated down the flight tube of a mass spectrometer. This technology, expanded and refined by CASD's Lee Riciputi, allows determination of the concentrations and isotopic distributions of most elements in the periodic table, both laterally and vertically (depth profiling), for spot sizes on the order of a few microns, in phases such as oxides, sulfides, silicates, and glasses. In one application, Riciputi and Cole, along with Larry Anovitz and Mike Elam of UTK, debunked the long-standing industry practice used to date ancient glassy artifacts, such as prehistoric arrowheads, knives, and spear points made of obsidian (volcanic glass). Traditionally, these artifacts have been dated by observing through an optical microscope how far water from air and soil has migrated into the glass. Research using ORNL's ion microprobe demonstrated that the true position of the hydration front (penetration of hydrogen atoms in the absorbed moisture) was not at all coincident with the optical front (refracted light thought to represent the hydration front), as was originally assumed. Armed with experimental diffusion data on water in glass, which determines the rate of water migration into the glass, as well as measurements of water's true distribution in the glass, a new, more reliable dating method was proposed that capitalizes on the sensitivity of the ion probe.

Using a one-of-a-kind, vibrating-tube densimeter (VTD), CASD's Jim Blencoe is investigating the thermophysical properties (e.g., density and volume) of volatile species such as CO₂ and CH₄ at subsurface conditions. The VTD measures the density of an individual gas or gas mixture (e.g. CO₂-CH₄, CO₂-H₂O) at unprecedented precision and accuracy at a given temperature (up to 500°C) and pressure (up to 100 MPa). This activity is supplying a new body of the pressure-volume-temperature data for high temperatures and high pressures that is crucial to the development of equations of state (EOS) for geologically and industrially important fluids. These EOS are used to predict fluid behavior in a diverse range of environments, such as oil, gas, and geothermal reservoirs; volcanic systems; pipelines; the high-temperature treatment of wastes (supercritical oxidation processes); and CO₂ disposal in geological formations, such as depleted oil and gas reservoirs and coal beds. The results Blencoe obtains are directly relevant to the use of CO₂ to displace CH₄ from unmineable coal seams, a value-added technology that is being seriously considered by DOE for both producing an energy-rich gas and sequestering a greenhouse gas.

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