Immobilization of 99-technetium (VII) by Fe(II)-goethite and limited reoxidation.

During the nuclear waste vitrification process volatilized (99)Tc will be trapped by melter off-gas scrubbers and then washed out into caustic solutions, and plans are currently being contemplated for the disposal of such secondary waste. Solutions containing pertechnetate [(99)Tc(VII)O(4)(-)] were mixed with precipitating goethite and dissolved Fe(II) to determine if an iron (oxy)hydroxide-based waste form can reduce Tc(VII) and isolate Tc(IV) from oxygen. The results of these experiments demonstrate that Fe(II) with goethite efficiently catalyzes the reduction of technetium in deionized water and complex solutions that mimic the chemical composition of caustic waste scrubber media. Identification of the phases, goethite + magnetite, was performed using XRD, SEM and TEM methods. Analyses of the Tc-bearing solid products by XAFS indicate that all of the Tc(VII) was reduced to Tc(IV) and that the latter is incorporated into goethite or magnetite as octahedral Tc(IV). Batch dissolution experiments, conducted under ambient oxidizing conditions for more than 180 days, demonstrated a very limited release of Tc to solution (2-7 µg Tc/g solid). Incorporation of Tc(IV) into the goethite lattice thus provides significant advantages for limiting reoxidation and curtailing release of Tc disposed in nuclear waste repositories.

Uranium phases in contaminated sediments below Hanford's U tank farm.

Macroscopic and spectroscopic investigations (XAFS, XRF, and TRLIF) on Hanford contaminated vadose zone sediments from the U-tank farm showed that U(VI) exists as different surface phases as a function of depth below ground surface (bgs). Secondary precipitates of U(VI) silicate precipitates (boltwoodite and uranophane) were present dominantly in shallow-depth sediments (15-16 m bgs), while adsorbed U(VI) phases and polynuclear U(VI) surface precipitates were considered to dominate in intermediate-depth sediments (20-25 m bgs). Only natural uranium was observed in the deeper sediments (> 28 m bgs) with no signs of contact with tank wastes containing Hanford-derived U(VI). Across all depths, most of the U(VI) was preferentially associated with the silt and clay size fractions of sediments. Strong correlation between U(VI) and Ca was found in the shallow-depth sediments, especially for the precipitated U(VI) silicates. Because U(VI) silicate precipitates dominate in the shallow-depth sediments, the released U(VI) concentration by macroscopic (bi)carbonate leaching resulted from both desorption and dissolution processes. Having different U(VI) surface phases in the Hanford contaminated sediments indicates that the U(VI) release mechanism could be complicated and that detailed characterization of the sediments using several different methods would be needed to estimate U(VI) fate and transport correctly in the vadose zone.

The effect of gravel size fraction on the distribution coefficients of selected radionuclides.

This manuscript addresses the consequences of the common practice of assuming that the gravel fraction of sediments does not participate in sorption reactions and thus sorption quantified by the distribution coefficient (K(d)) construct can be estimated from laboratory tests on sediments less than 2 mm size fraction. However, this common assumption can lead to inaccurate estimates of the mobility and sorption affinity of many radionuclides (e.g., Tc, U, and Np) on gravel dominated sediments at the Hanford Site and other locations. Laboratory batch sorption experiments showed that the distribution coefficients measured using only sediment less than 2 mm size fraction and correcting for inert gravel fraction were not in agreement with those obtained from the bulk sediments including gravel (larger than 2 mm size fraction), depending on the radionuclide. The least reactive radionuclide, Tc had K(d) values for bulk sediment with negligible deviations from the inert gravel corrected K(d) values measured on less than 2 mm size fraction. However, differences between measured K(d) values using sediment less than 2 mm size fraction and the K(d) values on the bulk sediment were significant for intermediately and strongly reactive radionuclides such as U and Np, especially on the sediment with gravel fractions that contained highly reactive sites. Highly reactive sites in the gravel fraction were attributed to the presence of Fe oxide coatings and/or reactive fracture faces on the gravel surfaces. Gravel correction factors that use the sum of the K(d)(,<2 mm) and K(d)(,>2 mm) values to estimate the K(d) for the bulk sediment were found to best describe K(d) values for radionuclides on the bulk sediment. Gravel correction factors should not be neglected to predict precisely the sorption capacity of the bulk sediments that contain more than 30% gravel. In addition, more detailed characterization of gravel surfaces should be conducted to identify whether higher reactive sorbents are present in the gravels.

Effect of saline waste solution infiltration rates on uranium retention and spatial distribution in Hanford sediments.

The accidental overfilling of waste liquid from tank BX-102 at the Hanford Site in 1951 put about 10 t of U(VI) into the vadose zone. In order to understand the dominant geochemical reactions and transport processes that occurred during the initial infiltration and to help understand current spatial distribution, we simulated the waste liquid spilling event in laboratory sediment columns using synthesized metal waste solution. We found that, as the plume propagated through sediments, pH decreased greatly (as much as 4 units) at the moving plume front. Infiltration flow rates strongly affect U behavior. Slower flow rates resulted in higher sediment-associated U concentrations, and higher flow rates (> or =5 cm/day) permitted practically unretarded U transport. Therefore, given the very high Ksat of most of Hanford formation, the low permeability zones within the sediment could have been most important in retaining high concentrations of U during initial release into the vadose zone. Massive amount of colloids, including U-colloids, formed at the plume fronts. Total U concentrations (aqueous and colloid) within plume fronts exceeded the source concentration by up to 5-fold. Uranium colloid formation and accumulation at the neutralized plume front could be one mechanism responsible for highly heterogeneous U distribution observed in the contaminated Hanford vadose zone.

Synthesis of nanoporous zirconium oxophosphate and application for removal of U(VI).

Uniformly arrayed zirconium-phosphate nanoporous material was synthesized, characterized, and used as an adsorbent for removal of U(VI) in a NaNO3 solution with varying background conditions including pH, ionic strength, U(VI) concentrations, and carbonate concentrations. Batch U(VI) adsorption results showed that U(VI) adsorption reached steady-state condition within 48 h, and all the dissolved U(VI) (10(-6)M) was removed by this material at neutral pH and closed conditions to atmospheric CO2(g). The U(VI) adsorption followed a traditional Langmuir adsorption isotherm, and the distribution coefficient (K(d)) calculated from the linear region of the Langmuir isotherm was 105,000 mL g(-1). Because this phosphate-bearing nanoporous material exhibits high thermal stability and has a very high Kd value, it can be applied as an efficient adsorbent for removing U(VI) from various contaminated waste streams, such as those present at the US Department of Energy defense sites and the proposed geologic radioactive waste disposal facility at Yucca Mountain in Nevada.

Surface complexation modeling of U(VI) sorption to Hanford sediment with varying geochemical conditions.

A series of U(VI) sorption experiments with varying pH, ionic strength, concentrations of dissolved U(VI), and alkalinity was conducted to provide a more realistic database for U(VI) sorption onto near-field vadose zone sediments at the proposed Integrated Disposal Facility (IDF) on the Hanford Site, Washington. The distribution coefficient (Kd) for U(VI) in a leachate that is predicted to result from the weathering of vitrified wastes disposed in the IDF is 0 mL/g due to the high sodium and carbonate concentrations and high pH of the glass leachate. However, when the pH and alkalinity of the IDF sediment native pore water increases during mixing with the glass leachate, U(VI) uptake is observed and the value of the U(VI) Kd increases 4.3 mL/g, because of U(VI) coprecipitation with newly formed calcite. A nonelectrostatic, generalized composite approach for surface complexation modeling was applied and a combination of two U(VI) surface species, monodentate (SOUO2+), and bidentate (SO2UO2(CO3)2-), simulated the measured U(VI) sorption data very well. The generalized composite surface complexation model, when compared to the constant or single-valued Kd model, more accurately predicted U(VI) sorption under the varying geochemical conditions expected at the IDF.

U(VI) adsorption on aquifer sediments at the Hanford Site.

Aquifer sediments collected via split-spoon sampling in two new groundwater wells in the 200-UP-1 operable unit at the Hanford Site were characterized and showed typical Ringold Unit E Formation properties dominated by gravel and sand. High iron-oxide content in Fe oxide/clay coatings caused the highest U(VI) adsorption as quantified by batch K(d) values, indicating iron oxides are the key solid adsorbent in the 200-UP-1 sediments that affect U(VI) fate and mobility. Even though U(VI) adsorption on the gravel-sized fraction of the sediments is considered to be negligible, careful characterization should be conducted to determine U(VI) adsorption on gravel, because of presence of Fe oxides coatings and diffusion-controlled adsorption into the gravel particles' interior surfaces. A linear adsorption isotherm was observed up to 10(-6) M (238 microg/L) of total U(VI) concentration in batch U(VI) adsorption tests with varying total U(VI) concentrations in spiked groundwater. U(VI) adsorption decreased with increasing concentrations of dissolved carbonate, because strong anionic aqueous uranium-carbonate complexes formed at high pH and high alkalinity conditions. Noticeable uranium desorption hysteresis was observed in a flow-through column experiment, suggesting that desorption K(d) values for aged uranium-contaminated sediments at the Hanford Site can be larger than adsorption K(d) values determined in short-term laboratory experiments and slow uranium release from contaminated sediments into the groundwater is expected.

Enhanced radionuclide immobilization and flow path modifications by dissolution and secondary precipitates.

Caustic radioactive wastes that have leaked at Hanford Site (Richland, WA) induce mineral dissolution and subsequent secondary precipitation that influence the fate and transport of contaminants present in the waste solutions. The effects of secondary mineral precipitates, formed after contacting solids with simulated caustic wastes, on the flow path changes and radionuclide immobilization were investigated by reacting quartz, a mixture of quartz and biotite, and a Hanford sediment (Warden soil: coarse-silty, mixed, superactive, mesic Xeric Haplocambids) with simulated caustic tank waste solution. Continuous Si dissolution and concomitant secondary mineral precipitation were the principal reactions observed in both batch and flow-through tests. Nitrate-cancrinite was the dominant secondary precipitate on mineral surfaces after 3- to 10-d reaction times in batch experiments. X-ray microtomography images of a reacted quartz column revealed that secondary precipitates cemented quartz grains together and modified pore geometry in the center of the column. Along the circumference of the packed column, however, quartz dissolution continuously occurred, suggesting that wastes that leaked from buried tanks in the past likely did not migrate vertically as modeled in risk assessments but rather the pathways likely changed to be dominantly horizontal on precipitation of secondary precipitate phases in the Hanford vadose zone. Based on batch equilibrium sorption results on the reacted sediments, the dominant secondary precipitates (cancrinites) on the mineral surfaces enhanced the sorption capacity of typical Hanford sediment for radionuclides 129I(-I), 79Se(VI), 99Tc(VII), and 90Sr(II), all of which are of major concern at the Hanford Site.