Effects of sulfate on rhenium incorporation into low-activity waste glass

Journal of Non-Crystalline Solids 521 (2019) 119528

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Effects of sulfate on rhenium incorporation into low-activity waste glass a,⁎

a

Tongan Jin , Dongsang Kim , Albert A. Kruger a b

b

T

Pacific Northwest National Laboratory, Richland, WA 99352, United States U.S. Department of Energy, Office of River Protection, Richland, WA 99352, United States

ARTICLE INFO

ABSTRACT

Keywords: Low-activity waste Borosilicate glass Technetium Rhenium Nuclear waste glass Vitrification Sulfate

Technetium-99 (99Tc) is a major radionuclide of concern in the Hanford low-activity waste (LAW), which will be vitrified into borosilicate glass at the Waste Treatment and Immobilization Plant (WTP). Sulfate in LAW has been known to be a critical factor affecting the volatile loss of 99Tc. We investigated rhenium (a nonradioactive surrogate for 99Tc) incorporation by crucible melting tests of two representative simulated LAW glass feeds, each prepared with varied sulfur concentrations in glass. The slurry feeds were dried and heated to 400–1100 °C. Soluble and insoluble phases of heated feeds were analyzed to profile the partitioning of various components during the melting process. The mechanism of how sulfate affects rhenium incorporation during feed-to-glass conversion reactions, and so final retention of rhenium in glass, is proposed based on the two hypotheses that segregated sulfate-rich phase forms during feed conversion reactions and Re preferentially partitions to this sulfate-rich phase.

1. Introduction At the U.S. Department of Energy's Hanford Site in Washington State, roughly 200,000 m3 of radioactive and chemically hazardous waste is stored in 177 underground tanks. The Waste Treatment and Immobilization Plant (WTP) is currently under construction to immobilize the waste. The WTP design included full pretreatment process to separate the tank wastes into high-volume, low-activity waste (LAW) and low-volume, high-level waste (HLW) before vitrifying them into separate glass waste forms for permanent disposal [1–8]. However, recent changes in the Hanford waste cleanup strategies involve directly feeding LAW to the LAW vitrification facility without full pretreatment [1], called direct-feed LAW (DFLAW), which is currently the first phase of the planned WTP startup and operation. Technetium-99 (99Tc), in the form of water-soluble TcO4−, is a major dose contributor to the performance assessment at the integrated disposal facility designed for various immobilized LAW forms [9–11]. 99 Tc has a long half-life of 2.1 × 105 years. TcO4− is highly soluble and does not adsorb well on the surface of minerals, which could contaminate soil and groundwater [2,12–18]. One of the technical difficulties with vitrifying the large amount of LAW is the high-volatile loss of 99Tc during the feed-to-glass conversion of the waste glass feed (waste mixed with additives consisting of silica, boric acid, and other chemicals/minerals) [19–22]. A series of studies has been conducted to overcome these difficulties through understanding the mechanism of

⁎

Corresponding author. E-mail address: [email protected] (T. Jin).

https://doi.org/10.1016/j.jnoncrysol.2019.119528 Received 15 April 2019; Received in revised form 15 June 2019 Available online 05 July 2019 0022-3093/ © 2019 Published by Elsevier B.V.

technetium volatilization and incorporation [23–29] and through developing technologies that can separate and treat 99Tc from LAW or melter off-gas solution [30–39]. In a previous study with two simulated Hanford LAW glass feeds (AN-102 and AZ-102) [28], using rhenium (Re) as a nonradioactive 99 Tc surrogate [40–42], a deionized (DI) water leach method was developed to investigate the partitioning of components of interest during the feed-to-glass conversion reactions. The results of tracking incorporation and volatilization of Re showed that Re incorporation into glass melt virtually completed by roughly 700 °C with only a small fraction of Re volatilized for both feeds. Between the two feeds, the Re incorporation for AZ-102 feed, which contained higher sulfate but lower nitrates/nitrites than AN-102, was higher at 700 °C than that for AN-102. When heated to above 700 °C, the remaining Re eventually all volatilized without further incorporation into the glass melt for both AZ-102 and AN-102 feeds. As a result, AZ-102 had a much higher final Re retention measured at 1100 °C due to its higher incorporation up to roughly 700 °C; that is, the final Re retention was determined by the fraction incorporated into glass melt up to 700 °C. It was claimed that the primary factor for the difference in incorporation by 700 °C was the composition of the Re-containing salt phase that reacts with additives to form glass melt. The AZ-102 feed with predominantly alkali borate incorporated Re into glass melt faster than the AN-102 feed with predominantly alkali nitrates, although the detailed mechanism on the difference was not understood. Another important observation was

Journal of Non-Crystalline Solids 521 (2019) 119528

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Table 1 Compositions of AN-102 and AZ-102 Melter Feeds per 1 L of LAW Simulant [20,21,28].

LAW simulant components (g/L simulant) Al(NO3)3·9H2O H3BO3 Ca(NO3)2·4H2O Na2CrO4·4H2O KOH NaOHa NiO PbO SiO2 NaCl NaF Na3PO4·12H2O Na2SO4a NaNO2 NaNO3 Na2CO3 Sodium formate (NaHCO2) Sodium oxalate (Na2C2O4) Glycolic acid (C2H4O3) Citric acid (C6H8O7) Oxalic acid (C2H4O4·2H2O) Re2O7 Water (estimated in 1 L of simulant) Additive components (g/L simulant) Kyanite (Al2SiO5) H3BO3 Wollanstonite (CaSiO3) Hematite (Fe2O3) Li2CO3 Olivine (Mg2SiO4) Na2CO3 Quartz (SiO2) Rutile (TiO2) Zincite (ZnO) Zircon (ZrSiO4) Total dry feed mass (g) Target glass mass (g)

AN-102

AZ-102

87.97 0.10 1.46 2.76 7.38 52.88 0.09 0.09 0.10 3.72 1.97 7.56 12.07 64.80 94.65 49.21 24.90 1.44 30.60 8.98

1.75

0.0121 792.96 98.73 200.58 155.51 59.46 89.87 35.54 362.31 15.22 39.63 50.83 1560.4 1145.3

Table 2 Target glass compositions (mass %).

Al2O3 B2O3 CaO Cr2O3 Fe2O3 K2O Li2O MgO Na2O NiO PbO SiO2 TiO2 ZnO ZrO2 Cl F P2O5 SO3 (Original) Sum SO3 (modified “½S”) Suma SO3 (modified “0S”) Suma

3.46 9.21 3.16 0.11 0.11 0.37 4.65 2.46 9.39 20.32 35.51 9.68 40.97

3.21 0.0151 975.01

AN-102

AZ-102

6.02 9.87 6.31 0.08 5.43 0.54 3.17 1.49 13.53 0.01 0.01 44.75 1.38 3.46 2.96 0.20 0.08 0.12 0.59 100.00 0.30 99.70 0 99.41

6.07 9.95 6.96 0.08 5.48 0.54 4.26 2.94 5.72 0.01 0.01 48.92 1.39 3.48 2.99 0.20 0.08 0.12 0.80 100.00 0.40 99.60 0 99.20

a Only SO3 was modified for the ½ S and 0S compositions. With the sum being a little <100%, the mass % of other components should be slightly higher than those of the original compositions. Among the “original,” “½ S,” and “0S” feeds, the mass % differences of the components other than SO3 could be neglected.

149.71 252.97 215.41 71.71 150.93 87.65 34.02 472.28 18.82 49.81 64.17 1719.1 1431.2

prepared. A two-step leach method of heat-treated samples was performed to separate the water-soluble phases from the unreacted mineral and glass-forming phases (insoluble). Partitioning of Re and other components among those phases was profiled by chemical analyses to track Re incorporation and volatilization during the feed-to-glass conversion reactions at a temperature range of 400–1100 °C. The comparisons of Re and other salt components in the same LAW feed with only different sulfate levels were performed to understand how sulfate negatively affects Re incorporation into glass melt during the feed-toglass conversion process and eventually decreases Re retention in final glass product.

a NaOH was added replacing Na2SO4 to supply the same moles of Na for the ½ S and 0S feeds.

that, at the elevated temperatures when most other salt components had converted to glass melt or evolved gases, sulfate was the dominant salt component containing the Re that was not incorporated into the glass melt. Sulfate, a relatively minor salt component found in tank wastes and expected to primarily partition to the LAW stream, has been known to have a strong negative effect on Tc/Re retention. Kim et al. [23] first found that removing sulfate from LAW led to a higher retention of Re and later Matlack et al. [20,21] reported that generally the higher sulfate level in LAW feed decreased the retention of both Tc (using 99m Tc) and Re. AZ-102 feed has been an outlier of this trend as it had the highest sulfate level among the seven representative feeds used in scaled melter tests and yet showed highest Tc/Re retention [18]. Previous work [28] further demonstrated the complex nature of the Re incorporation and volatilization, which occur in the heat-treated LAW feeds that are inhomogeneous mixtures of multiple salt and early glass melt phases with particularly complex chemical reactions among different components. The recent plan changes to DFLAW process from full treatment mentioned above are expected to increase the concentration of sulfur in some LAWs, which can lead to even lower Tc retention in glass. The purpose of the present study is to investigate the correlation between the sulfate concentration in the LAW feeds and the Re partitioning behavior. Three AN-102 feeds and three AZ-102 feeds with original designed and reduced SO3 target concentrations in glass were

2. Material and methods 2.1. Waste simulant and glass composition The LAW glasses used in this study were designed for Hanford LAW in tanks AN-102 and AZ-102 [28]. The feed compositions with the LAW simulants and glass-forming and modifying additives are shown in Table 1. Table 2 shows the target glass compositions of AN-102 and AZ102 along with two variants made with half the sulfate content (½ S) and no sulfate (0S) feeds. For the ½ S and 0S feeds, only the Na2SO4 and NaOH contents were adjusted from the original and all other components were the same. The designed target Re concentration in both glasses was 8.1 ppm. More details of the slurry feed preparation were described previously [20,21,28]. 2.2. Heat treatment After drying at 105 °C for 12 h, the dried feeds were crushed into powder and thermally treated in a Pt crucible (2.5–20 g a batch), heating at 5 K/min to 400, 500, 600, 700, 800, 900, 1000, and 1100 °C. In a previous work [28], 20 g feed was used for all heat-treating temperatures and a 5 g sample was taken for leaching test. Two modifications were made in this work. 2

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Table 3 Samples from leach method. Sample

Phase

Method

RT

Salts

Leach at room temperature in DI water for 1 h.

80C

Salts with limited solubility in water at room temperature and non-durable glass forming melt

Leach at 80 °C in DI water for 24 h. (The 80 °C sample is the part dissolved only at 80 °C for 24 h but not at room temperature, which was calculated by subtracting the chemical analyzed results of RT from the analyzed result of the solution after leaching at 80 °C for 24 h.)

Insol

Insoluble solids

Filter out solution after the 80 °C DI water leach and dry the remaining solid.

Table 4 Mass fraction FijT (mass %, except for Re given in ppm mass) of each (ith) component in each (jth) phase in the dried feed and the sample heated to 1100 °C. Standard deviations (SDs) were calculated from triplicated tests on the dried feed samples. “-” represents the FijT value below analytical reporting limit or < 0.01 mass %, or the SD value <0.001 mass %.

1) Varying feed mass was used: 2.5 g feed was used for heat treatment at 400 and 500 °C, 5 g for 600 and 700 °C, and 20 g for 800, 900, 1000, and 1100 °C. 2) The whole sample taken out from the crucible was used for leaching test.

AN-102_original dried feed, mass %

The purpose of using the whole crucible sample for leach was to minimize potential sampling errors caused by inhomogeneity of the reacting feed and glass samples. The purpose of varied feed mass was to make sure the leach method by 100 to 200 ml DI water can fully dissolve all soluble components. The samples were quenched in air after heated to the target temperature. The mass loss measurement was performed by weighing samples before and after each heat treatment.

Al2O3 B2O3 CaO Cr2O3 Fe2O3 PbO Li2O MgO NiO P2O5 K2O SiO2 Na2O SO3 TiO2 ZnO ZrO2 Cl F N2O5 N2O3 CO2 Re (ppm) Fj (Sum)

2.3. DI water leach After heat treatment, the samples were crushed and sieved through a #40 (420 μm) stainless-steel sieve. A two-step leaching procedure was used as described in Table 3 for the samples heat treated to 600–1100 °C. A sample of the crushed and sieved particles was first leached by 100–200 mL DI water at room temperature (RT) for 1 h. Then, 5 mL of solution was taken for chemical analyses (soluble RT sample) and 5 mL DI water was added to retain the same solution volume. The solids and solution contained in a sealed vessel were placed in an oven at 80 °C for 24 h and then the mixture was filtered. The solution was taken (soluble 80C sample) and the remaining insoluble solid was dried (insol sample). On the other hand, for the dried feed, 400 and 500 °C samples, a modified leach method was used because a clear solution could not be obtained for analyses after 1 h RT leaching caused by fine solid particles suspended in the solution. Two samples with the same size (2.5 g) were taken and leached separately. One was leached at room temperature for 1 h and the other was leached at 80 °C for 24 h; then the two solutions were both filtered to obtain the soluble RT and soluble 80C samples. Only the insoluble solid part filtered out from the 80 °C for 24 h leached sample was used for the chemical analysis. The solid samples after leaching and drying were weighed to determine the mass loss during the leaching process.

AN102_original 1100 °C, mass %

Fi,insol

SD

Fi,RT

SD

Fi,80C

SD

Fi,Insol

4.47 0.125 5.10 0.06 4.77 0.01 0.07 1.45 0.02 0.07 0.02 37.49 0.10 0.01 1.14 2.95 2.34 – 0.01 0.03 0.01 0.19 – 60.41

0.489 0.108 0.285 0.018 0.351 – 0.023 0.037 – 0.028 0.004 0.660 0.075 0.003 0.090 0.127 0.117 – 0.012 0.027 0.009 0.026 –

0.16 7.93 0.04 0.05 0.01 – 2.10 – – – 0.44 – 10.81 0.50 – 0.14 – 0.18 0.18 7.11 2.54 3.16 7.43 35.36

0.037 0.538 0.008 0.004 0.015 – 0.050 – – – 0.006 – 0.410 0.028 – 0.031 – 0.001 0.133 0.034 0.032 0.450 0.244

– 0.21 0.01 – – – 0.48 – – – 0.01 – 0.30 0.01 – – – 0.01 0.01 0.13 0.06 0.71 0.23 1.94

– 0.363 0.006 – – – 0.127 – – – 0.017 – 0.407 0.017 – – – 0.003 0.007 0.079 0.042 0.027 0.223

4.54 7.57 4.98 0.08 4.45 0.01 2.56 1.31 0.02 0.11 0.46 37.29 11.18 0.47 1.03 2.82 2.27 0.11 0.05 – – – 2.55 81.31

AZ-102_original dried feed, mass %

Al2O3 B2O3 CaO Cr2O3 Fe2O3 PbO Li2O MgO NiO P2O5 K2O SiO2 Na2O SO3 TiO2 ZnO ZrO2 Cl F

2.4. Chemical analyses Chemical analyses were performed by Southwest Research Institute1 according to its standard procedures. Two liquid samples from the RT and 80 °C leaching and solution samples prepared from fusion and acid leaching of dried solids were analyzed by inductively coupled plasmamass spectroscopy (ICP-MS) for Re and inductively coupled plasmaatomic emission spectroscopy (ICP-AES) for all other cationic elements. Ion chromatography was used to analyze NO3−, NO2−, F−, and Cl− and the total inorganic carbon method was used for CO32−.

1 Southwest Research Institute (SwRI), 6220 Culebra Rd., San Antonio, Texas 78,238–5166 (http://www.swri.org/).

AZ102_original 1100°C, mass %

Fi,insol

SD

Fi,RT

SD

Fi,80C

SD

5.43 – 6.27 0.04 5.39 0.01 0.04 2.97 0.02 0.03 0.02 44.21 0.05 0.01 1.31 3.22 2.52 – 0.01

0.114 – 0.340 0.001 0.105 – 0.004 0.143 0.001 0.010 0.003 0.792 0.008 0.001 0.019 0.148 0.054 – 0.014

– 8.76 – 0.07 – – 2.08 – – – 0.40 – 4.91 0.77 – – – 0.18 0.05

– 0.351 0.009 – – – 0.291 – – – 0.006 – 0.040 0.016 – – – 0.008 0.007

– 0.33 0.02 – – – 1.75 – – 0.03 0.03 0.02 0.40 0.03 – 0.01 – 0.01 0.01

– 0.194 0.021 – – – 0.334 – – 0.002 0.014 0.022 0.087 0.019 – 0.009 – 0.008 0.010

Fi,Insol 5.56 8.64 6.31 0.10 5.23 0.01 3.94 2.84 0.02 0.04 0.46 43.87 5.36 0.67 1.23 3.23 2.51 – 0.06

(continued on next page) 3

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3. Results

Table 4 (continued) AN-102_original dried feed, mass %

N2O5 N2O3 CO2 Re (ppm) Fj (Sum)

AN102_original 1100 °C, mass %

Fi,insol

SD

Fi,RT

SD

Fi,80C

SD

– – 0.08 – 71.62

– – 0.072 –

0.41 1.21 2.75 8.19 21.60

0.004 0.016 0.435 0.333

0.03 0.03 2.62 0.16 5.32

0.008 0.031 0.405 0.129

3.1. Chemical compositions Out of 54 total dried feed and heat-treated samples (dried feed and eight heat treatment temperatures for six feed compositions) subjected to DI water leach procedure, 16 replicate samples were prepared to estimate the experimental uncertainties. The chemical composition data of FijT and fijT for 70 samples can be found in the Supplementary Materials. In Table 4, the compositions of the dried feeds (Fij0) and final glasses (Fi, insol1100) of AN-102_original and AZ-102_original are shown as examples. The soluble phases in the leached solutions of the samples heated to 1100 °C are negligible. The analytical uncertainties reported by the analytical lab from previous works [28,38] and this work were mostly <5% in relative percent differences (RPDs) of the solid ICP-AES and ICP-MS for Re. In this work, repeat tests were performed for selected samples to evaluate the overall experimental uncertainties. Triplicate tests were performed on the six dried feeds except for AN-102_½ S, which was duplicated. For heat-treated samples, AN-102_original 600 °C, AZ-102_original 700 °C, and AN-102_0S 800 °C were duplicated and AN-102_original 700 °C was triplicated. It should be noted that the six dried feeds were prepared in large batches (over 500 g each) and all the repeat tests were performed with the same feeds. So the experimental uncertainties (reproducibility) in this study represent combined uncertainties from the heat treatment, leach test, and chemical analyses, except for the dried feed samples that involve uncertainties from leach test and chemical analyses only. Table 4 also shows the Fij0 of the selected feed samples with the standard deviation (SD) calculated from repeated samples. The mass distribution (fijT, Eq. 6) is more informative than the mass fraction (FijT, Eq. 5) when profiling the partitioning of components into different phases at different temperatures. The mass distributions of Re, SO3, and Cl in the insol and RT phases, fijT (i = Re, SO3, and Cl. j = insol, RT, and 80C), are given in Table 5, which also shows the SDs for the selected samples with duplicate or triplicate results. The uncertainties of Re in the samples heated to 600–800 °C are larger, with most relative standard deviations (RSDs) <10% except two data points. Generally when Re content is low in a sample, the RSD is high, which is likely caused by both of the uncertainties in sample preparation and the relative high analytical uncertainties with low concentration. Such as fRe, RT800 = 5.33% ± 3.02% (RSD = 57%) for AN-102_0S and fRe, insol600 = 25.58% ± 6.76% (RSD of 26%) for AN102_original are the two samples. However, within the range of Re mass distribution in the insoluble phase, these data are sufficient to evaluate the general trend. The data of SO3 shows overall higher uncertainties as listed in Table 5. It should be noted that even though no sulfate was added to the 0S feeds, there were still ~0.04-mass % for AN-102_0S and ~0.06-mass % AZ-102_0S respectively, both less than one-tenth of the SO3 mass % in the original feeds that were presumably from impurities of other chemicals which might not be sulfate (SO42−). The same sulfur impurities can be seen from the original and ½ S dried feeds (small fraction of insoluble in the dried feed and low-temperature samples), which were all included in the chemical analysis results. The SO3 data of the 0S samples are of large experimental errors. However, the sulfate in both AN-102_0S and AZ-102_0S are negligible. The uncertainties of Cl mass distribution are below 5% in the RT phase for all samples; while the SD is relatively large in the insoluble phase. The larger uncertainties should be caused by the larger experimental errors for measuring low Cl concentrations (mostly <0.1 mass %) in the insoluble samples. As shown in Table 5, SD of selected samples was calculated by replicates. In order to estimate the SDs for all mass distribution (fijT) values, a pooled standard deviation was calculated using:

Fi,Insol – – – 5.79 90.08

2.5. Mass balance For each feed or heat-treated sample a total of 23 components (Table 4) were tracked as in a previous study [28]: Al2O3, B2O3, CaO, Cr2O3, Fe2O3, K2O, Li2O, MgO, Na2O, NiO, PbO, SiO2, TiO2, ZnO, ZrO2, Cl, F, P2O5, SO3, N2O5, N2O3, CO2 and Re. Symbol mij is used for the mass of the ith component in the jth phase (j represents soluble phase leached at room temperature [abbreviated as RT], soluble phase leached at 80 °C [80C], and insoluble phase [insol]). If we include the evolved gas phase [abbreviated as gas], each sample can be profiled by the mass of each component for 23 components in the phases (RT, 80C, insol, and gas): 23

4

mtot =

mij

(1)

i=1 j=1

where mtot is the total mass of each dried feed or heat-treated sample when expressed in terms of 23 components. The mtot0 is obtained from the analytical results of the dried feed by: 23 0 mtot =

mi0

(2)

i=1

where mi0 is the initial mass of the ith component analyzed in the dried feed sample (superscript 0 is used for dried feed), i.e., 3

mi0 =

mij0 = mi0, RT + mi0,80C + mi0, insol

(3)

j =1

where mij0 is the initial mass of the ith component in the jth phase analyzed in the dried feed. For the samples heat-treated at temperature T °C (superscript T is used), the mass of evolved gas is calculated by

miT, gas = mi0

miT, RT

miT,80C

miT, insol

(4)

Then, the compositions in the sample heat treated at T °C or in dried feed are expressed by:

FijT =

fijT =

mijT T mtot

mijT mi0

or Fij0 =

or f ij0 =

mij0 (5)

0 mtot

mij0 (6)

mi0

where and Fij or Fij is the mass fraction of the i component in the jth phase normalized to the initial total mass of all components in the dried T

th

0

feed or heat treated sample (

23

4

i=1 j =1

FijT =

23

3

i=1 j =1

Fij0 = 1) and fijT or fij0 (we

will refer to as “mass distribution” to distinguish from FijT or Fij0) is the mass fraction of the ith component in the jth phase normalized to the initial total mass of the ith component(

4

j=1

fijT =

3

j=1

f ij0 = 1). 4

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Table 5 Mass distribution (%) of Re, SO3, and Cl, fijT for j = insol RT and 80C. “-” represents below analytical reporting limit or the value <0.01%. Averaged values and standard deviations (SDs) are shown for the selected duplicate or triplicate samples. SDs of 80C data are not calculated because the 80C mass fraction values are mostly below 15% with relatively large uncertainties. Small amounts of sulfur were found in the 0S samples, presumably from impurities of other chemicals. The SO3 distribution in the AN-102_0S and AZ-102_0S samples are not listed in this table. The data of the duplicate or triplicate samples before averaging and the SO3 data of the 0S samples can be found in the Supplementary Materials. AN-102 0S

½S

Original

insol

RT

80C

insol

RT

80C

insol

RT

80C

Re

Feed 400 500 600 700 800 900 1000 1100

– – – 21.69 25.47 77.21 ± 2.93 70.84 73.25 66.17

96.42 ± 2.60 96.46 87.72 75.70 63.49 5.33 ± 3.02 0.76 0.29 0.31

3.58 0.73 7.66 7.29 6.34 1.06 0.23 0.41 0.83

– – – 14.18 25.51 59.06 69.03 64.68 57.88

92.11 ± 3.31 92.94 89.07 77.04 69.12 27.01 2.11 0.51 0.26

7.89 1.94 7.07 6.08 2.91 4.16 1.13 1.05 0.20

– – – 25.58 ± 6.76 20.88 ± 0.82 39.79 42.45 34.39 33.24

97.06 ± 2.77 91.69 88.17 66.92 ± 2.45 68.33 ± 5.05 38.93 8.00 0.40 0.32

2.94 3.52 2.92 5.84 3.26 4.64 0.33 0.23 0.05

SO3

Feed 400 500 600 700 800 900 1000 1100

– – – – – – – – –

– – – – – – – – –

– – – – – – – – –

2.14 ± 0.42 – – 9.88 23.28 85.81 99.73 107.25 109.09

97.13 ± 1.46 89.71 89.04 74.48 53.18 15.84 3.95 1.29 –

0.73 2.04 6.65 11.43 9.63 2.39 1.73 1.10 –

2.04 ± 0.41 3.02 3.26 22.55 ± 0.44 25.81 ± 8.38 70.57 76.87 78.23 91.04

95.26 ± 3.28 92.59 87.27 72.85 ± 6.11 60.05 ± 4.08 28.50 22.61 14.42 4.68

2.71 1.86 8.68 6.67 12.92 3.65 1.23 0.75 0.21

Cl

Feed 400 500 600 700 800 900 1000 1100

– – 8.51 16.22 30.16 93.25 ± 10.40 85.87 80.58 68.52

98.87 ± 1.05 99.95 97.20 90.95 54.73 – – – –

1.13 5.25 5.65 5.62 21.52 1.84 – 1.71 1.39

– – – – 14.78 77.90 82.43 77.09 69.14

96.91 ± 0.92 100.44 97.21 91.66 50.07 8.26 – – –

3.09 2.42 5.53 2.60 17.12 2.13 1.87 2.43 –

– – – 18.20 ± 4.32 21.33 ± 18.60 82.41 79.92 70.78 59.24

96.62 ± 1.75 98.35 95.58 89.34 ± 5.05 59.72 ± 7.57 7.96 2.61 – –

3.38 5.95 10.99 8.03 15.92 1.63 0.55 1.78 –

AZ-102 0S

½S

Original

insol

RT

80C

insol

RT

80C

insol

RT

Re

Feed 400 500 600 700 800 900 1000 1100

– – – 15.25 87.62 97.13 94.30 90.26 88.01

96.11 ± 4.63 99.81 82.78 74.61 5.20 1.93 0.75 0.33 0.32

3.89 0.60 17.78 12.52 5.05 0.11 0.34 0.30 0.29

– – – 22.80 96.54 98.65 106.37 101.70 94.27

98.09 ± 1.06 103.65 90.60 71.39 5.88 1.88 0.78 0.48 0.53

1.91 3.19 16.49 9.50 3.76 0.37 0.65 0.17 0.34

– – – 26.35 79.35 79.50 82.48 80.93 69.27

98.08 ± 1.50 104.66 91.22 66.84 14.97 ± 0.33 20.90 3.87 0.34 0.35

1.92 2.49 14.21 10.38 3.45 0.43 0.16 0.13 0.23

SO3

Feed 400 500 600 700 800 900 1000 1100

– – – – – – – – –

– – – – – – – – –

– – – – – – – – –

1.75 ± 0.51 1.59 3.13 13.89 72.43 84.05 88.79 88.68 81.07

92.77 ± 1.59 95.19 83.63 65.44 5.93 – – – –

5.47 5.80 18.70 15.00 8.81 0.81 1.04 – 0.96

1.17 ± 0.15 1.01 1.56 11.53 64.93 ± 4.79 67.65 76.25 78.06 82.32

95.27 ± 2.50 94.18 87.87 69.41 25.35 ± 0.09 18.57 13.86 9.02 0.49

3.56 4.47 8.05 11.36 6.88 0.61 0.55 0.27 0.30

Feed 400 500 600 700

– – – – 40.64

96.30 ± 3.43 91.45 76.70 59.11 5.83

3.70 2.50 18.29 21.31 8.45

– – – – 37.47

95.65 ± 4.02 91.19 78.61 62.45 –

4.35 7.56 17.98 8.91 13.86

– – – – 62.32 ± 18.14

94.34 ± 3.74 89.85 75.92 61.51 4.69 ± 3.31

5.66 5.68 19.02 11.47 11.88

Cl

80C

(continued on next page) 5

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Table 5 (continued) AN-102 0S

800 900 1000 1100

½S

Original

insol

RT

80C

insol

RT

80C

insol

RT

53.91 43.04 47.78 34.05

– – – –

1.49 – – –

47.89 39.80 42.45 43.27

– – – –

– – – –

40.20 43.65 38.98 –

3.88 1.34 – –

80C 0.41 0.22 – –

Fig. 1. Mass fractions (mass %) of all components [Fj, (a) and (b)], Na2O [FNa2O, j, (c) and (d)], and B2O3 [FB2O3, j, (e) and (f)] as a function of temperature. For each temperature or dried feed each column represents, from left to right, 0S, ½ S, and original.

6

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Fig. 1. (continued) N

SDp =

(nk

k=1 N

(nk

all three condensed phases shown in Fig. 1(c) through (f) represent the experimental errors. In all plots the samples with different sulfur levels were grouped together showing in the order of 0S, ½ S, and original, from left to right, for each feed heat treated at different temperatures. During melting of LAW glasses, feed-to-glass conversion (“feed conversion” hereafter) involves reactions within a complex mixture consisting of a soluble salt (simulated LAW and soluble additives such as boric acid) and insoluble mineral additive (SiO2, CaSiO3, etc.) components. As shown in Fig. 1(a) and (b), the total mass fraction changes are basically the same within the three AN-102 feeds and the three AZ-102 feeds; that is, sulfate concentration does not make a measurable difference in overall feed conversion reactions, which is expected as sulfate is only a minor component (<0.8 mass %). Explicitly, all major components, e.g., the components with Fi0 > 1 mass % in the dried feed, show very little difference within the three feeds with different sulfur levels as illustrated in Fig. 1(c) through (f) for Na2O and B2O3 that are initially soluble but nonvolatile. Fig. 1(a) through (f) show that formation of insoluble phase from soluble phase occurred primarily during 700–800 °C for AN-102 and 600–700 °C for AZ-102. For AN-102, the conversion of soluble phase to insoluble phase was nearly complete by 800 °C and for AZ-102 it seemed the near completion of feed conversion was between 700 and 800 °C, probably slightly above 700 °C. To facilitate discussion of relative differences between AN-102 and AZ-102, we will refer to 800 °C for AN-102 and 700 °C for AZ-102 as the temperatures major feed conversion reactions are complete. Above 800 °C, the insoluble phase remained constant and the final total volatile loss after being heated to 1100 °C was 15–17% for AN-102 and 7–9% for AZ-102.

1) SDk2 1)

k=1

(7)

where SDk is the standard deviation of the kth sample with nk replicates (nk = 2 or 3) and N is the number of samples with replicate measurements. For those components of particular interest, the SDp was calculated by the SDs from different feeds heated among different temperatures. In the RT phases, the pooled standard deviations of fRe, RTT, fSO3, RTT, and fCl, RTT are between 3% to 4%; on the other hand, the pooled standard deviations of fRe, insolT, fSO3, insolT, and fCl, insolT are 5%, 6%, and 15% respectively. The uncertainty for the solution phase (RT) are smaller, indicating good reproducibility of the leaching method and ICP-AES/IC. These pooled standard deviations are larger for the leached solid samples which is likely caused by additional uncertainties involved in the solid sample preparation (fusion of the leached insoluble solids). The pooled standard deviations of mass distributions fijT based on replicates on selected samples are used to estimate the uncertainties of the ith component in the jth phase for all samples. 3.2. Feed-to-glass conversion The total mass fraction of each phase, which is the sum of all 23 components, is given as 23

Fj =

Fij i=1

(8)

Fig. 1 shows the mass fractions (Fij) as “stacked” column charts for the sum of all components [Fj, Fig. 1(a) and (b)], for Na2O [FNa2O, j, Fig. 1(c) and (d)], and for B2O3 [FB2O3, j, Fig. 1(e) and (f)] as functions of temperature. The mass fractions of insol (Finsol, Fi, insol), 80C (F80C, Fi, 80C), and RT (FRT, Fi, RT) phases are in different color from bottom to top. The gas phase mass fraction for the sum of all components (Fgas), not shown in Fig. 1(a) and (b), is the balance of total condensed phases and can be seen from the decrease of the total mass fractions with increasing temperature. Na2O and B2O3 are the major contributors to the continuous LAW glass melter off-gas stream [22,43,44]; however their volatile loss fractions are low and expected to be negligible for mass balance of the crucible test. The fluctuation of total mass fractions for

3.3. Partitioning of Re, SO3, and Cl Figs. 2 through 4 show the mass fraction (Fij) and mass distribution (fij) of Re, SO3, and Cl. Rhenium and SO3 are the two components of primary interest in this study and the partitioning of Cl also provides important information on the salt-to-glass conversion reactions as discussed below. The behavior of these three components are similar in that they are initially in soluble forms in dried feed and low-temperature samples and then partially incorporate into glass and partially volatilize as temperature increases. Fig. 2 shows the Re mass fraction (FRe, j left side) and mass 7

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Fig. 2. Mass fractions (FRe, jT, mass %) and mass distribution (fRe, jT, %) of Re as functions of temperature.

distribution (fRe, j right side) in different phases as functions of temperature. As shown in Fig. 2(a) through (d), the major increase of insoluble Re (incorporation into glass forming melt) occurred at 700–800 °C for AN-102 and 600–700 °C for AZ-102, which coincide with the temperature ranges for major feed conversion shown in Fig. 1(a) through (f). There was no further Re incorporation beyond this temperature range for all feeds; instead, slow decreases of Re in insoluble phase were observed from 900 to 1100 °C. This Re loss above 900 °C was not observed in a previous study [28] for the same original AN-102 and AZ-102 samples, which is likely caused by low ratio of sample mass to surface area ratio used for the heat treatments in this study. Fig. 3 shows the SO3 mass fraction (FSO3, j left side) and mass distribution (fSO3, j right side) in different phases as functions of temperature. As shown in Fig. 3(a) and (c) for SO3 mass fractions, small

amounts of SO3 were detected in the 0S samples, which are likely from the impurities present in raw materials. Therefore, data for 0S samples were excluded from the SO3 mass distribution plots in Fig. 3(b) and (d). As shown in Fig. 3, the SO3 behavior in the ½ S and original feeds is similar to that of Re in that the temperature ranges of major SO3 incorporation coincide with those of major feed conversion, 700–800 °C for AN-102 and 600–700 °C for AZ-102. However, unlike Re, the soluble SO3 remaining after major incorporation continues to become incorporated as temperature increases while partially volatizing. As shown in Fig. 4, the main incorporation of Cl also occurred at 700–800 °C for AN-102 and 600–700 °C for AZ-102, just like Re and SO3. However, after the insoluble Cl roughly peaked at 800 °C for both AN-102 AZ-102, the small amount of remaining soluble Cl showed a decreasing trend (volatilized) with temperature without further incorporation, which is similar to Re but different from SO3. 8

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Fig. 3. Mass fractions (FSO3, jT, mass %) and mass distribution (fSO3, jT, %) of SO3 as functions of temperature. fSO3, jT does not include the 0S data for high scattering.

In summary, all three soluble components of major interest, Re, SO3, and Cl, incorporate into insoluble phase mostly in the temperature ranges that major feed conversion reactions occur; that is, incorporation of Re, SO3, and Cl is accompanied by the reactions that lead to the formation of early glass-forming melt. The salt phase remaining after major feed conversion reactions at roughly 700 °C (AZ-102) or 800 °C (AN-102) consists primarily of sulfate (except the 0S samples) mixed with remaining Re and Cl. As temperature increases further, Re and Cl mostly volatilize without further incorporation while SO3 tends to become incorporated. The different behavior of SO3 from Re and Cl can be attributed to the difference in volatility from salt; that is, SO3 is significantly less volatile than Cl and Re and therefore takes more time to become incorporated.

4. Discussion 4.1. AN-102 versus AZ-102 The modified test conditions applied in this study produced the same result as the previous study [28] in that Re retention is higher in the AZ-102 glass than in AN-102 glass, which was observed at all sulfur levels tested in this study. The previous study [28] concluded that the difference in Re retention between AN-102 and AZ-102 glasses was determined by how much Re incorporated at the temperature ranges of major feed conversion reactions. It was further argued that the composition of salt phase is the primary factor that determines the Re incorporation during feed conversion reactions: AN-102 feed forms salt phase of primarily sodium nitrate/nitrite; whereas the salt formed from 9

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Fig. 4. Mass fractions (FCl, jT, mass %) and mass distribution (fCl, 1100 °C was below the detective limit.

j

T

, %) of Cl as functions of temperature. For AZ-102, Cl mass fraction in the insoluble phase at

Cl retention is primarily caused by how much Cl evaporated from the soluble salt phase before the temperature of major feed conversion reactions (incorporation of Re/Cl) is reached. As shown in Fig. 4, the soluble Cl in AZ-102 at 600 °C showed noticeable decrease from 500 °C; that is, significant fraction of Cl volatilized from the soluble salt phase before Cl reaches the temperature to begin to incorporate. Contrast to AZ-102, the AN-102 showed no discernable or a very weak trend of decreasing fraction of Cl in condensed phase (a small fraction of Cl became insoluble when 700 °C was reached, the temperature of major incorporation). It is noted here that there was no measurable Re loss before the start of major incorporation for both feeds (see Fig. 2). In short, for Cl, the different behavior between AN-102 and AZ-102 is originated from the different Cl volatility in different Cl-containing salts; that is, Cl volatility is also dependent on salt composition.

AZ-102 feed is dominated by sodium borate. This signifies that the Re incorporates into the early glass-forming melt (insoluble phase) at different rates during feed conversion reactions depending on the composition of Re-bearing salt. The Re incorporation in AN-102 feed with alkali nitrate as the major component of the Re-containing salt phase is slower than in AZ-102 feed with alkali borate as major salt components, which results in the low retention in the final glass. Section 3.3 showed the similar partitioning behavior of Re and Cl in terms of temperature range of major incorporation and the volatility from the sulfate salt. However, when AN-102 and AZ-102 feeds are compared, the retention of Cl showed opposite results from that of Re at all sulfur levels; that is, final Cl retention at 1100 °C is higher in AN-102 glass than AZ-102 glass (see Fig. 4). When Fig. 4(b) and (d) are examined further it is noted that, unlike the difference in the rate of Re incorporation during major feed conversion reactions, the difference in 10

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Fig. 5. Mass distribution of Re and Cl in the insoluble phase fRe, insolT and fCl, insolT in %. For AZ-102, Cl mass fraction in the insoluble phase at 1100 °C was below the detective limit. Dashed lines are drawn as guides to the eyes. The errors (pooled standard deviation) for Re and Cl in the insoluble phase are ±5% and ± 15% respectively (Section 3.1).

4.2. Effect of sulfate on Re and Cl

much Re incorporates into glass melt before the temperature of surface salt segregation is reached. In other words, the sulfate salt that rises to the surface from which Re quickly volatilizes is not the fundamental cause of low Re retention but is rather a consequence of low incorporation during major feed conversion reactions up to the point sulfate segregates and rises to the surface. Comparing the difference in the effect of sulfate on the behavior of Re and Cl discussed in Section 4.2 suggests that there is likely a strong interaction between Re and SO3. Kim et al. (2006) [24] proposed that a continuous (interconnected) mixed-salt phase is formed after the lowmelting sodium nitrate melts roughly around 300 °C and dissolves other high-melting salt components including sulfate, which migrates through interstitials of additive mineral particles. However, any strong interaction between Re and SO3 is not conceivable if sulfate and perrhenate are dissolved in a continuous salt of one composition before major feed conversion reactions occur. We propose following hypotheses to justify the effect of sulfate on Re behavior observed in this and previous studies. First, a continuous, homogeneous salt phase of one composition is not formed, but rather segregated salts of varied compositions are formed and distributed throughout the reacting feeds. In this case, formation of separated sulfate-rich salt can be envisioned. Second, Re preferentially partitions to this sulfate-rich phase. It is possible that certain precursors for the salts of various compositions may develop, starting from the slurry drying process, which also leads to preferential partitioning of Re into sulfate-rich phase. Various salt components of different aqueous solubility would precipitate at different stages of slurry drying, which can promote segregation as temperature increases. Considering the melting points of Na2SO4, NaReO4, and KReO4 are 884, 420, and 555 °C [40,45], respectively, this sulfate-rich phase would start to melt at a much higher temperature than that of nitraterich phase (note that AZ-102 feed also contains a large fraction of nitrates although dominated by borates). This higher temperature melting of sulfate-rich phase can be one of the reasons for slower reactions with additive minerals to form glassy phase and for remaining unreacted even after all major glass-forming reactions are completed. The sulfate

As shown in Section 3.2, the sulfate variations in both AN-102 and AZ-102 feeds did not make any difference in total phase evolution from feed conversion reactions as expected. To highlight the difference between Re and Cl, Fig. 3 compares Re and Cl fractions in the insoluble phase (fRe, insolT and fCl, insolT). As shown in Fig. 2 and Fig. 5(a) and (c), the decrease of sulfate level increased the insoluble Re fraction achieved after major soluble conversion reactions and led to increased final Re retention for both AN-102 and AZ-102 feeds. The effect of sulfate on Re is less pronounced for AZ-102 feeds likely because of high Re incorporation even at high sulfur content. However, as shown in Fig. 4 and Fig. 5(b) and (d), the variation of sulfur addition did not make any measurable difference in the fraction of Cl in insoluble phase and final retention in glass (treating one data point at 700 °C in original AZ-102 as an outlier). This suggests that Cl has the same fate as other soluble salt components (Na2O, B2O3, N2O5, etc.) that fully react and form early glass-forming melt. The only difference of Cl from other major salt components is its high volatility. In this regard, the lack of effect of sulfur level on Cl behavior is just like the sulfate level did not have any effect on overall feed conversion reactions. In the same direction, Re (perrhenate) salt may have the same or similar fate as sulfate, which is a key in understanding the mechanism of sulfate effect on Re behavior as discussed in the next section. 4.3. Proposed mechanism on the effect of sulfate Kim et al. (2005) [23] proposed that the negative effect of sulfate on Re and Tc retention is because, as temperature increases, the sulfate segregates to the melt surface bringing the Re/Tc to surface of the melt where Re and Tc volatilize faster. However, that mechanism is not supported by the results obtained from this study. It is true that the remaining Re that did not become incorporated into glass melt partitions to the sulfate salt that segregates to melt surface as temperature increases, but the effect of sulfate that makes a difference is on how 11

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salt is not likely to remain unreacted if it was dissolved in the main nitrate or borate dominant salt phase. The plausible reason that the continuous salt phase observed in Kim et al. (2006) [24] was not present in the feeds in this study is two-fold. The S-109 feed used by Kim et al. (2006) [24] contained even higher nitrate than AN-102 in this study and, more importantly, glass composition and additives are very different. The glass used by Kim et al. (2006) [24] was designed for bulk vitrification with a nominal melting temperature of 1350 °C compared to 1150 °C for the glasses tested in this study.

References [1] DOE, River protection project system plan, ORP-11242, Rev. 8, U.S. Department of Energy, Office of River Protection, Richland, WA, 2017https://www.hanford.gov/ files.cfm/ORP-11242_System_Plan_Rev._8.pdf. [2] J.D. Vienna, Nuclear waste vitrification in the United States: recent developments and future options, Int. J. Appl. Glas. Sci. 1 (3) (2010) 309–321. [3] M.I. Ojovan, W.E. Lee, Glassy wasteforms for nuclear waste immobilization, Metall. Mater. Trans. A 42A (4) (2011) 837–851. [4] W.R. Wilmarth, G.J. Lumetta, M.E. Johnson, M.R. Poirier, M.C. Thompson, P.C. Suggs, N.P. Machara, Review: waste-pretreatment technologies for remediation of legacy defense nuclear wastes, Solvent Extr. Ion Exch. 29 (1) (2011) 1–48. [5] D.K. Peeler, J.D. Vienna, M.J. Schweiger, K.M. Fox, Advanced High-Level Waste Glass Research and Development Plan, PNNL-24450, Pacific Northwest National Laboratory, Richland, WA, 2015http://www.osti.gov/scitech/servlets/purl/ 1253884. [6] D.K. Peeler, D.-S. Kim, J.D. Vienna, M.J. Schweiger, G.F. Piepel, Office of River Protection Advanced Low-Activity Waste Glass Research and Development Plan, PNNL-24883 (EWG-RPT-008), Pacific Northwest National Laboratory, Richland, WA, 2015https://www.osti.gov/servlets/purl/1228353. [7] J. Vienna, B. Stanfill, G. Piepel, B. Riley, D. Kim, S. Cooley, J. Crum, T. Jin, C. Lonergan, 2016 Update of Hanford Glass Property Models and Constraints for Use in Estimating the Glass Mass to Be Produced at Hanford by Implementing Current Enhanced Glass Formulation Efforts, Pacific Northwest National Laboratory, Richland, WA, 2016https://www.hanford.gov/files.cfm/Vienna_et_al._ 2016,_Enhanced_Glass_Models_and_Constraints,_PNNL-25835.pdf. [8] D. Kim, Glass property models, constraints, and formulation approaches for vitrification of high-level nuclear wastes at the US Hanford Site, J. Korean Ceram. Soc. 52 (2) (2015) 92–102. [9] B.P. McGrail, D.H. Bacon, J.P. Icenhower, F.M. Mann, R.J. Puigh, H.T. Schaef, S.V. Mattigod, Near-field performance assessment for a low-activity waste glass disposal system: laboratory testing to modeling results, J. Nucl. Mater. 298 (1) (2001) 95–111. [10] J.D. Vienna, D.-S. Kim, D.C. Skorski, J. Matyas, Glass Property Models and Constraints for Estimating the Glass to Be Produced at Hanford by Implementing Current Advanced Glass Formulation Efforts, PNNL-22631 Rev.1; ORP-58289; Other: 830403000 United States, Pacific Northwest National Lab, Richland, WA, 2013, https://doi.org/10.2172/1170502 Other: 830403000 PNNL English https:// www.osti.gov/servlets/purl/1170502https://doi.org/10.2172/1170502. [11] J.D. Vienna, D.-S. Kim, I.S. Muller, G.F. Piepel, A.A. Kruger, Toward understanding the effect of low-activity waste glass composition on sulfur solubility, J. Am. Ceram. Soc. 97 (10) (2014) 3135–3142. [12] R.E. Wildung, T.R. Garland, K.M. McFadden, C.E. Cowan, Technetium sorption in surface soils, in: G. Desmet, C. Myttenaere (Eds.), Technetium in the Environment, Springer Netherlands, 1986, pp. 115–129. [13] D.I. Kaplan, Influence of surface charge of an Fe-oxide and an organic matter dominated soil on iodide and pertechnetate sorption, Radiochim. Acta 91 (3) (2003) 173–178. [14] W. Um, H.-S. Chang, J.P. Icenhower, W.W. Lukens, R.J. Serne, N.P. Qafoku, J.H. Westsik Jr., E.C. Buck, S.C. Smith, Immobilization of 99-technetium (VII) by Fe (II)-goethite and limited reoxidation, Environ. Sci. Technol. 45 (11) (2011) 4904–4913. [15] B.P. Burton-Pye, I. Radivojevic, D. McGregor, I.M. Mbomekalle, W.W. Lukens Jr., L.C. Francesconi, Photoreduction of 99Tc pertechnetate by nanometer-sized metal oxides: new strategies for formation and sequestration of low-valent technetium, J. Am. Chem. Soc. 133 (46) (2011) 18802–18815. [16] W. Um, H. Chang, J.P. Icenhower, W.W. Lukens, R.J. Serne, N. Qafoku, R.K. Kukkadapu, J.H. Westsik Jr., Iron oxide waste form for stabilizing 99Tc, J. Nucl. Mater. 429 (1–3) (2012) 201–209. [17] D. Li, D.I. Kaplan, A.S. Knox, K.P. Crapse, D.P. Diprete, Aqueous 99Tc, 129I and 137Cs removal from contaminated groundwater and sediments using highly effective lowcost sorbents, J. Environ. Radioact. 136 (2014) 56–63. [18] I. Pegg, Behavior of technetium in nuclear waste vitrification processes, J. Radioanal. Nucl. Chem. (2015) 1–6. [19] I. Muller, C. Viragh, H. Gan, K. Matlack, I. Pegg, Iron Mössbauer redox and relation to technetium retention during vitrification, in: E. Kuzmann,K. Lázár (Ed.), Proceedings of the International Symposium on the Industrial Applications of the Mössbauer Effect (ISIAME 2008), Springer, Berlin, Heidelberg, 2009, pp. 347–354. [20] K.S. Matlack, I.S. Muller, I.L. Pegg, I. Joseph, Improved Technetium Retention in Hanford LAW Glass – Phase 1, VSL-10R1920–1, Vitreous State Laboratory, The Catholic University of America, Washington, DC, 2010. [21] K.S. Matlack, I.S. Muller, R.A. Callow, N. D’Angelo, T. Bardacki, I. 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5. Summary and conclusions We investigated partitioning behavior of Re, Cl, and SO3 into different phases during crucible melting of six simulated LAW glass feeds designed for AN-102 and AZ-102 Hanford tank wastes, each with three different sulfate levels. The main findings are summarized. The retention of Re is controlled by how much Re incorporates during major feed-to-glass conversion reactions; whereas Cl retention is differentiated by how much Cl volatilizes before major feed conversion reactions occur. Most Cl not lost before major feed conversion reactions tends to fully incorporate into insoluble glassy phase. Re, however, rarely volatilizes before major feed conversion reactions but incorporates at different rates during major feed conversion reactions, which determines the final retention of Re. Volatilization of Cl before and Re incorporation during major feed conversion reactions are both dependent on the composition of salt that reacts with additive minerals to form glassy phase. Re incorporation and Cl volatilization are slower when the feed forms nitrate-dominant salt (AN-102) compared to borate-dominant salt (AZ-102), resulting in lower Re but higher Cl retention in the AN-102 glass. Sulfate content has no impact on the volatility and retention of Cl that seems to share the same fate as other major salt components during major feed conversion reactions, which are not affected by sulfate level; whereas sulfate content has a strong negative effect on Re incorporation during major feed conversion reactions and so on the final retention of Re. We argue that the effect of sulfate on Re behavior can be explained under the two hypotheses: segregated sulfate-rich phase forms during feed conversion reactions and Re preferentially partitions to this sulfate-rich phase. It was suggested that the segregation of various compositions during melting may develop starting from the slurry drying process caused by differences in the aqueous solubility of the salts. It is also suggested that the assumed sulfate-rich phase with high melting points would react with additive minerals slower compared to the nitrate- or borate-rich major salt phase, which supports the observation that the salt remaining after major feed conversion reactions is dominated by sulfate. Although the proposed mechanism based on these two hypotheses can explain the experimental findings from this and previous studies, there is no direct experimental evidence to support them. Detailed studies are underway to investigate these hypotheses using simplified feeds containing four to five components designed to isolate and test the processes relevant to each hypothesis. Acknowledgements This research was supported by funding provided by the U.S. Department of Energy's (DOE's) Waste Treatment and Immobilization Plant Project of the Office of River Protection. Pacific Northwest National Laboratory is operated by Battelle Memorial Institute for DOE under contract DE-AC05-76RL01830. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jnoncrysol.2019.119528. 12

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T. Jin, et al. Richland, WA, 2006https://www.osti.gov/servlets/purl/903263. [25] J.S. McCloy, B.J. Riley, A. Goel, M. Liezers, M.J. Schweiger, C.P. Rodriguez, P. Hrma, D.S. Kim, W.W. Lukens, A.A. Kruger, Rhenium solubility in borosilicate nuclear waste glass: implications for the processing and immobilization of technetium-99, Environ. Sci. Technol. 46 (22) (2012) 12616–12622. [26] D. Kim, M.J. Schweiger, Incorporation and distribution of rhenium in a borosilicate glass melt heat treated in a sealed ampoule, J. Non-Cryst. Solids 379 (2013) 123–126. [27] T. Jin, D.-S. Kim, M. Schweiger, Effect of sulfate on rhenium partitioning during melting of low-activity-waste glass feeds, Waste Management (WM) Conference, 2014 Phoenix, Arizona. [28] T. Jin, D. Kim, A.E. Tucker, M.J. Schweiger, A.A. Kruger, Reactions during melting of low-activity waste glasses and their effects on the retention of rhenium as a surrogate for technetium-99, J. Non-Cryst. Solids 425 (2015) 28–45. [29] D. Kim, A.A. Kruger, Volatile species of technetium and rhenium during waste vitrification, J. Non-Cryst. Solids 481 (2018) 41–50. [30] B.J. Riley, J.S. McCloy, A. Goel, M. Liezers, M.J. Schweiger, J. Liu, C.P. Rodriguez, D.S. Kim, Crystallization of rhenium salts in a simulated low-activity waste borosilicate glass, J. Am. Ceram. Soc. 96 (4) (2013) 1150–1157. [31] C.Z. Soderquist, M.J. Schweiger, D.S. Kim, W.W. Lukens, J.S. McCloy, Redox-dependent solubility of technetium in low activity waste glass, J. Nucl. Mater. 449 (1–3) (2014) 173–180. [32] D. Banerjee, D. Kim, M.J. Schweiger, A.A. Kruger, P.K. Thallapally, Removal of TcO4− ions from solution: materials and future outlook, Chem. Soc. Rev. 45 (10) (2016) 2724–2739. [33] D. Banerjee, S.K. Elsaidi, B. Aguila, B. Li, D. Kim, M.J. Schweiger, A.A. Kruger, C.J. Doonan, S. Ma, P.K. Thallapally, Removal of pertechnetate-related oxyanions from solution using functionalized hierarchical porous frameworks, Chem. Eur. J. 22 (49) (2016) 17581–17584. [34] D. Banerjee, W. Xu, Z. Nie, L.E.V. Johnson, C. Coghlan, M.L. Sushko, D. Kim, M.J. Schweiger, A.A. Kruger, C.J. Doonan, P.K. Thallapally, Zirconium-based metal–organic framework for removal of perrhenate from water, Inorg. Chem. 55 (17) (2016) 8241–8243. [35] F.N. Smith, W. Um, C.D. Taylor, D.-S. Kim, M.J. Schweiger, A.A. Kruger, Computational investigation of technetium(IV) incorporation into inverse spinels:

[36] [37]

[38] [39] [40] [41] [42] [43]

[44]

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