Introduction: Dissolution and precipitation of feldspar is an important reaction in many environmental systems, as plagioclase feldspar is the most common mineral in the crust. However, the mechanism of dissolution of feldspar has remained in dispute despite twenty or more years of analysis of feldspar dissolution. We are pursuing an approach toward better understanding of feldspar dissolution by investigating reactivity of aluminosilicate glasses of variable composition. In particular, to investigate the effects of Al/Si ratio on plagioclase dissolution without complications of varying Na/Ca content or exsolution, three glasses with varying Al/Si ratio (albite, jadeite, nepheline) were synthesized. Glass powders were then prepared and dissolved at various values of pH in batch reactors under ambient conditions. Solution chemistry was analyzed by inductively coupled plasma -optical emission spectroscopy or -mass spectrometry. In addition, alteration of the glass surface was investigated using Xray photoelectron spectroscopy, secondary ion mass spectrometry, and Fourier transform infrared spectroscopy. Dissolution results: Many similarities in dissolution behavior between plagioclase crystals and this suite of glasses were observed: 1) dissolution was slowest at near-neutral pH and increased under acid and basic conditions, 2) dissolution rate at all pH values increased with increasing Al/Si ratio, 3) the pH dependence of dissolution was higher for the phase with Al/Si = 1 than the phase with Al/Si = 0.3, 4) after acid leaching, the extent of Al-depletion of the altered surface increased with increasing bulk Al/Si ratio from Al/Si = 0.3 (albite) to 0.5 (jadeite), but then decreased in nepheline (Al/Si = 1.0), which dissolved stoichiometrically with respect to Al, and 5) little to no Al depletion of the surface of any glass occurred at pH > 7. However, in contrast with some observations for plagioclase dissolution, log(rate) increased almost linearly with Al content, and n, the slope of the log(rate)pH curve at low pH, varied smoothly from albite to jadeite to nepheline (n = 0.3, 0.6, 1.0 respectively). At high pH, the slope of this curve, m, did not differ between glasses (m = -0.4 + 0.1). Mechanistic interpretation: These results are consistent with an identical mechanism controlling dissolution of nepheline, albite, and jadeite glass, although no Si-rich layer can develop on nepheline because of the lack of Si-O-Si linkages. Such a conclusion is consistent with a transition state for these aluminosilicates at high pH consisting of a deprotonated Q3 hydroxyl group (where Qv refers to an x atom in a tetrahedral site with v bridging oxygens) or a five-coordinate Si site after nucleophilic attack by OH, and, at low pH, a protonated Q4OQ4. At low pH, we infer that Q4OQ4 Al linkages are ratelimiting because they are presumed to hydrolyze more slowly than Qv OQw Si (v,w < 3). According to this model, dissolution rate increases from albite to jadeite to nepheline because hydrolysis of Al-O-Si linkages becomes more energetically favorable as the number of Al atoms per Si tetrahedral increases, a phenomenon documented by geometry optimizations using ab initio methods (for example, Figure 1). However, a model wherein Q4OQ4 Si linkages are faster to hydrolyze than lower connectivity linkages between Si atoms (Qv OQw , v,w < 3) may also explain aspects of this data.

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Role of Polymerization in the Dissolution of Nepheline, Jadeite, and Albite Glasses: Toward Better Models for Aluminosilicate Dissolution

Efficient and cost-effective removal of radioactive pertechnetate anions from legacy nuclear waste is a key challenge to mitigate long-term nuclear waste storage issues. Traditional materials such as resins and layered double hydroxides (LDHs) were evaluated for their pertechnetate or its non-radioactive surrogate perrhenate removal capacity, but there is room for improvement in terms of capacity, selectivity and kinetics. A series of functionalized hierarchical porous frameworks were evaluated for their perrhenate removal capacity in the presence of other competing anions.

Removal of Pertechnetate-Related Oxyanions from Solution Using Functionalized Hierarchical Porous Frameworks.

Towards high-efficiency sorptive capture of radionuclides in solution and gas

Technetium-99 (99Tc) is one of the most-problematic radioisotopes in used nuclear fuel owing to its intrinsic features of a high fission yield, long half-life, high environmental mobility, volatile nature during waste vitrification, and its redox interface capability with actinides during used fuel repossessing. The selective separation of pertechnetate (TcO4–) from legacy nuclear waste and contaminated natural water is therefore highly desirable but still a significant challenge, because the conditions of a strong radiation field, high ionic strength, high acidity/alkalinity, and large amounts of competing anions are often involved in these systems. Until now, there are a handful of functional materials that can efficiently remove TcO4– from nuclear waste solutions with high uptake capacities, fast kinetics, and good selectivity, but room still remains to further improve our capabilities for controlling the contamination/separation of TcO4–. In this Perspective, we discuss the current state of the art Tc...

Separation and Remediation of 99TcO4– from Aqueous Solutions§

Comparison of adsorption equilibrium models for the study of CL−, NO3− and SO42− removal from aqueous solutions by an anion exchange resin

Fluidized Bed Steam Reforming (FBSR) is being considered as a potential technology for the immobilization of a wide variety of high sodium aqueous radioactive wastes. The addition of clay and a catalyst as co-reactants converts high sodium aqueous low activity wastes (LAW) such as those existing at the Hanford and Idaho DOE sites to a granular ''mineralized'' waste form that may be made into a monolith form if necessary. Simulant Hanford and Idaho high sodium wastes were processed in a pilot scale FBSR at Science Applications International Corporation (SAIC) Science and Technology Applications Research (STAR) facility in Idaho Falls, ID. Granular mineral waste forms were made from (1) a basic Hanford Envelope A low-activity waste (LAW) simulant and (2) an acidic INL simulant commonly referred to as sodium-bearing waste (SBW). The FBSR waste forms were characterized and the durability tested via ASTM C1285 (Product Consistency Test), the Environmental Protection Agency (EPA) Toxic Characteristic Leaching Procedure (TCLP), and the Single Pass Flow Through (SPFT) test. The durability of the FBSR waste form products was tested in order to compare the measured durability to previous FBSR waste form testing on Hanford Envelope C waste forms that were made by THOR Treatment Technologiesmore » (TTT) and to compare the FBSR durability to vitreous LAW waste forms, specifically the Hanford low activity waste (LAW) glass known as the Low-activity Reference Material (LRM). The durability of the FBSR waste form is comparable to that of the LRM glass for the test responses studied.« less

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Durability testing of fluidized bed steam reformer (fbsr) waste forms

Batch equilibrium measurements were conducted at 25±2 °C with a granular resorcinol-formaldehyde (RF) resin to determine the distribution coefficients (K
d
for cesium. In the tests, Hanford Site actual waste sample containing radioactive cesium (137Cs) and a pretreated waste sample that was spiked with non-radioactive 133Cs were used. Initial concentrations of non-radioactive cesium in the waste sample were varied to generate an equilibrium isotherm for cesium. Majority of the tests were conducted at 25±2 °C using a liquid to solid phase ratio of ∽100 ml/g and at a contact time of 72 hours. Two additional tests were conducted at a liquid to solid phase ratio of 10 and at contact time of 120 hours. The measured distribution coefficient (Kd) for radioactive cesium (137Cs) was 948 ml/g, the Kd for non-radioactive cesium (133Cs) was 1039 ml/g. The Kd for nonradioactive cesium decreased from 1039 to 691 ml/g with increased initial cesium concentration from ∽8 to 64 μg/ml. Very little change of the Kd was observed at initial cesium concentration above 64 ∽g/ml. The maximum sorption capacity for cesium on granular RF resin was 1.17 mmole/g dry resin. This value was calculated from the fit of the equilibrium isotherm data to the Dubinin-Radushkevich equation. Previously, a total capacity of 2.84 mmole/g was calculated by Bibler and Wallace for air-dried RF resin.

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Resorcinol - formaldehyde adsorption of cesium from Hanford waste solutions Part I. Batch equilibrium study

Hanford waste solutions-part i: batch equilibrium study

The Department of Energy (DOE) plans to immobilize a portion of the excess weapons useable plutonium in a ceramic form for final geologic disposal. The proposed immobilization form is a titanate based ceramic consisting primarily of a pyrochlore phase with lesser amounts of brannerite, rutile, zirconolite, vitreous phases and/or other minor phases depending on the impurities present in the feed. The ceramic formulation is cold-pressed and then densified via a reactive sintering process. Cerium has been used as a surrogate for plutonium to facilitate formulation development and process testing. The use of cerium vs. plutonium results in differences in behavior during sintering of the ceramic form. The phase development progression and final phase assemblage is different when cerium is substituted for the actinides in the form. However, the physical behavior of cerium oxide powder and the formation of a pyrochlore-rich ceramic of similar density to the actinide-bearing material make cerium an adequate surrogate for formulation and process development studies.

Cerium as a Surrogate in the Plutonium Immobilized Form

Column ion exchange tests have been used to demonstrate the ability of SuperLig® 644 resin to remove 1 3 7 Cs from highly alkaline radioactive waste. The column test resulted in 1 3 7 Cs removal and overall decontamination factor of 99.6% (lead column) and 517000 (lag column) respectively. The concentration of 1 3 7 Cs in the lag column effluent (0.000423 μCi 1 3 7 Cs/mL) was two orders of magnitude below the low-activity waste vitrification limit of 0.087 μCi 1 3 7 Cs/mL. The bed volume (BV = 175) at the onset of breakthrough exceeded the requirement of 100BV at 50% breakthrough. Cesium elution with 0.5 M HNO 3 was adequate with a peak BV of 3.5. C/C 0 = 0.01 occurred at 10BV of eluant, which is less than the target of 15BV. Further, 99.99% of sorbed cesium was eluted from the resin. The residual 1 3 7 Cs on the regenerated resin after the column operation was 23 μCi/g dry Na + form resin.

James C. Marra

Papers

Durability testing of fluidized bed steam reformer (fbsr) waste forms

Resorcinol - formaldehyde adsorption of cesium from Hanford waste solutions Part I. Batch equilibrium study

Hanford waste solutions-part i: batch equilibrium study

Cerium as a Surrogate in the Plutonium Immobilized Form

Removal of cesium from alkaline waste solution: Part I – Batch ion exchange study