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An international collaborative team of researchers at the Stanford Synchrotron Radiation Laboratory (SSRL) is currently working to learn how to keep uranium soil contamination out of circulation in ground water.

Previous experiments show that soil bacteria, with enough fuel, will convert the soluble kind of uranium (hexavalent uranium) into a dramatically less soluble kind (tetravalent uranium).

Now researchers at SSRL want to know how to keep the uranium in that latter state, which should stay put in soils. If it were to revert back to the hexavalent form, which can seep far through the groundwater, the uranium could potentially spread contamination over larger areas. The bacteria chemically reduce hexavalent uranium by giving it extra electrons. But in the presence of oxygen, the uranium oxide could subsequently lose these electrons and be oxidized back to the more soluble form.

John Bargar, head of Molecular Environmental and Interface Science at SSRL, and postdoc Ellie Schofield have been carefully characterizing the molecular-scale of bacteriogenic uranium oxide to understand its structure. Knowing the structure could help in engineering a uranium oxide that is more stable.

The bacteria produce uranium oxide in very small pieces, called nanoparticles. In general, nanoparticles can react differently than their bulkier counterparts. In part this is due to their very high surface areas compared to their volume. Chemical reactions occur more quickly, for example, making researchers wonder whether nanoparticles are more susceptible to being dissolved by groundwater. In addition, the structures of nanoparticles can be strained and distorted (in comparison to their bulkier counterparts), which can significantly destabilize the materials. It is also possible that nanoparticles made by bacteria may be unique as compared to chemically synthesized analogs.

To find out, Bargar and Schofield examined a pure culture of biogenic (bacteria-made) uranium oxide prepared by collaborators at Ecole Polytechnique Federal de Lausanne. Up to half the atoms are on the surface of nanoparticle uranium oxide, potentially creating instability in the entire nanoparticle.

So far, the experiments have found no evidence for significant stress on the nanoparticles, and the structures of the particle interiors are surprisingly pristine. "This is a good thing, it may mean they're less likely to dissolve—that is, that the small particle size may not render them intrinsically unstable," said Bargar.

In the next set of experiments, underway now, the group will repeat measurements on real soil samples from the old rifle site in Colorado where uranium ore had been milled. With added bacteria and oxygen, the scientists want to record the rate at which uranium oxide reoxidizes to the dissolvable kind, all the while looking for factors that might slow this reaction. This work will help to guide our understanding of how molecular- and nano-scale structure controls the behavior of uranium at the field scale.

Note for Uranium
Uranium is a silvery gray metallic chemical element in the actinide series of the periodic table that has the symbol U and atomic number 92. It has 92 protons and electrons, 6 of them valence electrons. It can have between 141 and 146 neutrons, with 143 and 146 in its most common isotopes. Uranium has the highest atomic weight of the naturally occurring elements. Uranium is approximately 70% more dense than lead and is weakly radioactive. It occurs naturally in low concentrations (a few parts per million) in soil, rock and water, and is commercially extracted from uranium-bearing minerals such as uraninite (see uranium mining).

In nature, uranium atoms exist as uranium-238 (99.284%), uranium-235 (0.711%), and a very small amount of uranium-234 (0.0058%). Uranium decays slowly by emitting an alpha particle. The half-life of uranium-238 is about 4.47 billion years and that of uranium-235 is 704 million years, making them useful in dating the age of the Earth. Many contemporary uses of uranium exploit its unique nuclear properties. Uranium-235 has the distinction of being the only naturally occurring fissile isotope. Uranium-238 is both fissionable by fast neutrons, and fertile (capable of being transmuted to fissile plutonium-239 in a nuclear reactor). An artificial fissile isotope, uranium-233, can be produced from natural thorium and is also important in nuclear technology. While uranium-238 has a small probability to fission spontaneously or when bombarded with fast neutrons, the much higher probability of uranium-235 and to a lesser degree uranium-233 to fission when bombarded with slow neutrons generates the heat in nuclear reactors used as a source of power, and provides the fissile material for nuclear weapons. Both uses rely on the ability of uranium to produce a sustained nuclear chain reaction. Depleted uranium (uranium-238) is used in kinetic energy penetrators and armor plating.

Uranium is used as a colorant in uranium glass, producing orange-red to lemon yellow hues. It was also used for tinting and shading in early photography. The 1789 discovery of uranium in the mineral pitchblende is credited to Martin Heinrich Klaproth, who named the new element after the planet Uranus. Eugène-Melchior Péligot was the first person to isolate the metal, and its radioactive properties were uncovered in 1896 by Antoine Becquerel. Research by Enrico Fermi and others starting in 1934 led to its use as a fuel in the nuclear power industry and in Little Boy, the first nuclear weapon used in war. An ensuing arms race during the Cold War between the United States and the Soviet Union produced tens of thousands of nuclear weapons that used enriched uranium and uranium-derived plutonium. The security of those weapons and their fissile material following the breakup of the Soviet Union in 1991 along with the legacy of nuclear testing and nuclear accidents is a concern for public health and safety.

When refined, uranium is a silvery white, weakly radioactive metal, which is slightly softer than steel, strongly electropositive and a poor electrical conductor. It is malleable, ductile, and slightly paramagnetic. Uranium metal has very high density, being approximately 70% more dense than lead, but slightly less dense than gold.

Uranium metal reacts with nearly all nonmetallic elements and their compounds, with reactivity increasing with temperature. Hydrochloric and nitric acids dissolve uranium, but nonoxidizing acids attack the element very slowly. When finely divided, it can react with cold water; in air, uranium metal becomes coated with a dark layer of uranium oxide. Uranium in ores is extracted chemically and converted into uranium dioxide or other chemical forms usable in industry.

Uranium was the first element that was found to be fissile. Upon bombardment with slow neutrons, its uranium-235 isotope will most of the time divide into two smaller nuclei, releasing nuclear binding energy and more neutrons. If these neutrons are absorbed by other uranium-235 nuclei, a nuclear chain reaction occurs and, if there is nothing to absorb some neutrons and slow the reaction, the reaction is explosive. As little as 15 lb (7 kg) of uranium-235 can be used to make an atomic bomb. The first atomic bomb worked by this principle (nuclear fission).

Note for Nanoparticle
A nanoparticle is a small particle with at least one dimension less than 100 nm. This definition can be fleshed out further in order to remove ambiguity from future nano nomemeclature. A nanoparticle is an amorphous or semicrystalline zero dimensional (0D) nano structure with at least one dimension between 10 and 100nm and a relatively large (≥ 15%) size dispersion. A nanocluster is an amorphous/semicrystalline nanostructure with at least one dimension being between 1-10nm and a narrow size distribution. This distinction is an extension of the term "cluster" which is used in inorganic/organometallic chemistry to indicate small molecular cages of fixed sizes. A nanopowder is an agglomeration of noncrystalline nanostructural subunits with at least one dimention less than 100nm. A nanocrystal is any nanomaterial with at least one dimension ≤ 100nm and that is singlecrystalline. Any particle which exhibits regions of crystllinity should be termed nanoparticle or nanocluster based on dimensions. Nanoparticle research is currently an area of intense scientific research, due to a wide variety of potential applications in biomedical, optical, and electronic fields. The National Nanotechnology Initiative of the United States government has driven huge amounts of state funding exclusively for nanoparticle research.

Nanoparticles are of great scientific interest as they are effectively a bridge between bulk materials and atomic or molecular structures. A bulk material should have constant physical properties regardless of its size, but at the nano-scale this is often not the case. Size-dependent properties are observed such as quantum confinement in semiconductor particles, surface plasmon resonance in some metal particles and superparamagnetism in magnetic materials.

The properties of materials change as their size approaches the nanoscale and as the percentage of atoms at the surface of a material becomes significant. For bulk materials larger than one micrometre the percentage of atoms at the surface is minuscule relative to the total number of atoms of the material. The interesting and sometimes unexpected properties of nanoparticles are partly due to the aspects of the surface of the material dominating the properties in lieu of the bulk properties.

Nanoparticles exhibit a number of special properties relative to bulk material. For example, the bending of bulk copper (wire, ribbon, etc.) occurs with movement of copper atoms/clusters at about the 50 nm scale. Copper nanoparticles smaller than 50 nm are considered super hard materials that do not exhibit the same malleability and ductility as bulk copper. The change in properties is not always desirable. Ferroelectric materials smaller than 10 nm can switch their magnetisation direction using room temperature thermal energy, thus making them useless for memory storage. Suspensions of nanoparticles are possible because the interaction of the particle surface with the solvent is strong enough to overcome differences in density, which usually result in a material either sinking or floating in a liquid. Nanoparticles often have unexpected visible properties because they are small enough to confine their electrons and produce quantum effects. For example gold nanoparticles appear deep red to black in solution.

About Stanford Synchrotron Radiation Laboratory
The Stanford Synchrotron Radiation Laboratory, a division of Stanford Linear Accelerator Center, is operated by Stanford University for the Department of Energy. SSRL is a National User Facility which provides synchrotron radiation, a name given to x-rays or light produced by electrons circulating in a storage ring (SPEAR) at nearly the speed of light. These extremely bright x-rays can be used to investigate various forms of matter ranging from objects of atomic and molecular size to man-made materials with unusual properties. The obtained information and knowledge is of great value to society, with impact in areas such as the environment, future technologies, health, and education.

The SSRL provides experimental facilities to some 2,000 academic and industrial scientists working in such varied fields as drug design, environmental cleanup, electronics, and x-ray imaging.

Tags: Stanford Synchrotron Radiation Laboratory (SSRL) , uranium , soil contamination , bacteria , fuel , hexavalent uranium , tetravalent uranium , John Bargar , bacteriogenic uranium oxide , nanoparticles ,

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