Borohydride reduction of an aqueous iron salt in the presence of a support material gives supported zerovalent iron nanoparticles that are 10-30 nm in diameter. The material is stable in air once it has dried and contains 22.6% iron by weight. The supported zero-valent iron nanoparticles ("Ferragels") rapidly separate and immobilize Cr(VI) and Pb(II) from aqueous solution, reducing the chromium to Cr(III) and the Pb to Pb(0) while oxidizing the Fe to goethite (R-FeOOH). The kinetics of the reduction reactions are complex and include an adsorption phase. About 10% of the iron in the material appears to be located at active surface sites. Once these sites have been saturated, the reduction process continues but at a much lower rate, which is likely limited by mass transfer. Rates of remediation of Cr(VI) and Pb(II) are up to 30 times higher for Ferragels than for iron filings or iron powder on a (Fe) molar basis. Over 2 months, reduction of Cr(VI) was 4.8 times greater for Ferragels than for an equal weight of commercial iron filings (21 times greater on the basis of moles of iron present). The higher rates of reaction, and greater number of moles of contaminant reduced overall, suggest that Ferragels may be a suitable material for in situ remediation.

We reviewed the chemistry of radioactive technetium species and their nonradioactive rhenium surrogates in the context of Hanford Site low-level radioactive waste processing and vitrification. Information concerning the hydrolysis, precipitation, phase transformation, volatilization, and redox chemistries of these species during the drying, calcining, and vitrification of aqueous waste slurries is condensed and extrapolated from the literature. The similarities between the chemistry of technetium and rhenium species were highlighted to evaluate the performance of rhenium as a surrogate for technetium in laboratory and engineering-scale low-level radioactive waste vitrification experiments.

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The microstructure, physical characteristics, corrosion behavior, and reactivity of zerovalent iron nanoparticles synthesized on a support (primarily a nonporous, hydrophobic polymer resin) were studied. The remediation of groundwater by zerovalent iron in in situ permeable reactive barriers relies on the redox reaction between metallic iron and a reducible contaminant. Decreasing the size of the iron particles and dispersing them on a support increases the specific surface area of the iron, as well as the ratio of surface to bulk iron atoms, and should thereby increase both the reaction rate and the fraction of iron atoms available for the reaction. Borohydride reduction of aqueous ferrous sulfate gives supported iron nanoparticles, 10−30 nm in diameter, which consist of 85% zerovalent iron by weight. These materials (“ferragels”) are stable in air and have corrosion behavior comparable to iron filings. Interestingly, the presence or absence of a support, as well as the boron remaining from the borohydride reduction process, influences the electrochemical corrosion rate of the composite materials. Supported and unsupported zerovalent iron nanoparticles are superior to iron filings in both terms of initial rates of reduction and total moles of contaminants (Cr(VI), Pb(II), TcO4 -) reduced per mole of iron. The enhanced reactivity and passive corrosion behavior of these materials should make them good candidates for use in permeable reactive barriers.

We present the first direct measurement of ion hydration in supercritical water using X-ray absorption fine structure (XAFS). Radial structure functions were determined for strontium ions in supercritical water at 385 "C and 269-339 bar at a concentration of strontium of 0.2 M. For supercritical water, at a temperature of 385 "C and density of 0.54 g/cm3, the number of waters of hydration was a factor of 0.52 of the number in liquid water under ambient conditions. The radius of the first solvation shell changes very little at these elevated temperatures. This large local depletion of water around the ion would affect the short-range interactions with counterions and may increase the ion reactivity. We also report XAFS results for krypton in supercritical water and show that, in contrast to strontium ions, the local solvent environment is more gaslike in the first few solvation shells of the krypton atom. IntroductionSupercritical water (SCW) is an interesting solvent for chemical reactions and hazardous waste At temperatures above 375 "C, the solubility behaviors of two important classes of compounds reverse: most organic species have high solubility whereas the solubility of inorganic salts is limited. The high solubility of organics and the aggressive oxidizing environment are attractive for organic reactions and waste-destruction reactions. The low salt solubility presents both obstacles to and opportunities in developing SCW oxidation technology. Salt precipitation may lead to equipment fouling and erosion. However, the ability to control solubility with temperature and pressure may provide a way of separating organic from inorganic wastes and may provide a way of selectively separating different salts. A better understanding of the mechanism of salt solubility in SCW is required to fully utilize this unique solvent. In this Letter, we report X-ray absorption fine structure (XAFS) measurements of the extent of ion hydration in SCW. From these results, one can build a picture of the radial distribution of water around the ion, which is important information needed in the development and testing of models of these systems.In the supercritical state, all of the properties of the fluid, including the solvent strength, are "tunable" through adjustments in the fluid density. It is this variable nature of the fluid that makes SCW (Z', = 374 "C, Pc = 220 bar) interesting from a fundamental point of view. For liquid-phase systems, there is only a very narrow latitude for changing the properties of the solvent. In the supercritical fluid or near-critical liquid state, the density of the solution can be changed over a considerable range, offering the opportunity to fully test models of these systems over a wide range of thermodynamic conditions.To a large extent, the dielectric constant of water controls the solubility of ionic species. In the supercritical region, the dielectric constant, E , spans the range from about 5 to 20,6s7 a value that is greatly reduced from the liquid phase ( E 80).Because of the inability of...

X-ray absorption fine structure (XAFS) spectroscopy was used to measure the coordination structure about Cu2+, Cu1+, and Br- in water at temperatures up to 325 °C. The hexaaqua Cu2+ species maintains its distorted octahedral structure up to 325 °C, whereas at higher temperatures, dehydration reactions occur producing CuO. Under reducing conditions, the dibromo Cu1+ species, [CuBr2]-, is predominant at 200 °C and above for systems having excess Br-. Even for a very high salt concentration of 2.0 m NaBr, only the dibromo Cu1+ species, [CuBr2]-, is observed with no evidence of higher Br- coordination. For this dibromo-species there are no tightly bound hydration waters in the first shell. In the absence of excess Br-, a monoaqua monobromo Cu1+ species, [Cu(H2O)Br] is observed. For certain systems, both Cu and Br XAFS were acquired, and a global model was used to fit the two independent sets of XAFS data. Thus, the results represent a complete picture of the coordination structure about Cu1+ including the coordination numbers, distances for the ion−ion and water-ion associations and also a high-quality measurement of the binding strength and amount of disorder (Debye−Waller factor and the anharmonicity) of the Cu1+/Br- association. Molecular dynamics (MD) simulations were used to further explore the structure and the binding forces for the [CuBr2]- species under hydrothermal conditions. We found quantitative agreement for the Cu−Br interactions, but the simulation has difficulty predicting the experimental Cu−H2O interaction. In particular, the amount of scattering from the water in the dibromo Cu1+ complex was highly over-predicted, so that it is clear that simple intermolecular potential models do not adequately capture this structural feature.

The coordination structure about Ni2+ in water at temperatures up to 525 °C was measured by the X-ray absorption fine structure (XAFS) technique. Solutions containing 0.2 m NiBr2 and 0.2 m NiBr2/0.8 m NaBr were explored at pressures up to 720 bar. For certain systems, both Ni and Br XAFS data were acquired and a global model was used to fit the two independent sets of XAFS data. These two independent measurements gave excellent agreement on the coordination structure of the Ni2+/Br- contact-ion pairing. The result is a complete picture of the coordination structure of the contact-ion pairing including the coordination numbers, distances for the water−ion and ion−ion associations, and also a high-quality measurement of the binding strength and amount of disorder (Debye−Waller factor and the anharmonicity) of the Ni2+/Br- association. The XAFS measurements strongly indicate a transitional change in the coordination of Ni2+ from the octahedral Ni2+(H2O)6 species at room temperature to the 4-coordinate structures at supercritical conditions (e.g., T > 375 °C). Specifically, the equilibrium structure at 425 °C is Ni2+(Br-)3.3(H2O)1.0 for the aqueous solution of 0.2 m NiBr2 with 0.8 m NaBr. At 325 °C, the octahedral species still exists but it is in equilibrium with new species of lower coordination. Above 425 °C, at moderate pressures up to 700 bar, the stable structures are a family of 4-coordinated species (NiBr(H2O)3·Br, NiBr2(H2O)2, NiBr3(H2O)·Na), where the degree of Br- adduction and replacement of H2O in the inner shell depends on the overall Br- concentration. The most likely symmetry of these species is a distorted tetrahedron. Measurements of the Ni preedge 1s → 3d and to 1s → 4d transitions using X-ray absorption spectroscopy confirm that a symmetry change occurs at high temperature, and the results are consistent with the XAFS and molecular dynamics (MD) conclusion of a distorted tetrahedral structure. This observation is further confirmed by near-infrared (NIR) spectra of the same system. The MD simulations under identical conditions were used to verify the experimental findings. Although we found qualitative agreement between the experimental and simulated first-shell coordination structure, it is clear that refinements of the intermolecular potentials are required to quantitatively capture the true high-temperature structure.

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