6 Effects of Hanford High-Level Waste Components on the Solubility and Sorption of Cobalt, Strontium, Neptunium, Plutonium, and Americium C. H. DELEGARD, G. S. BARNEY, and S. A. GALLAGHER Rockwell International, Energy Systems Group, Richland, WA 99352
High-level radioactive defense waste solutions, originating from plutonium recovery and waste processing operations at the U.S. Department of Energy's Hanford Site, currently are stored in mild steel-lined concrete tanks located in thick sedimentary beds of sand and gravel. Statistically designed experiments were used to identify the effects of 12 major chemical components of Hanford waste solution on radionuclide solubility and sorption. The chemical components with the most effect on radioelement solubility and sorption were NaOH, NaAlO , EDTA, and HEDTA. The EDTA and HEDTA increased Co, Sr, and Am solubility and decreased sorption for almost all radioelements studied. Sodium hydroxide and NaAlO increased Pu solubility and decreased Np and Pu sorption. Sodium nitrite decreased Np solubility, while Na CO and HEDTA increased it. These observations give evidence for the formation of radioelement complexes which are soluble and are not strongly sorbed by the sediments near the waste tanks. 2
2
2
3
High-level defense waste solutions resulting from plutonium recovery and waste processing activities currently are stored in mild steel-lined concrete tanks located underground at the U.S. Department of Energy's Hanford Site. Low radioélément solubility and extensive radioélément sorption on surrounding sediment help maintain isolation of hazardous radionuclides from the biosphere in the event of tank failure. Chemical components in the waste solutions potentially could affect radioélément solubility and sorption reactions, and thus enhance or reduce radionuclide transport. The effects of 12 chemical components on the solubility and sorption of cobalt, strontium, neptunium, plutonium, and americium were studied to 0097-6156/84/0246-0095S06.00/0 © 1984 American Chemical Society
GEOCHEMICAL BEHAVIOR OF RADIOACTIVE WASTE
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aid in judging the feasibility of continued storage of Hanford highlevel waste solutions in existing tanks, as well as in predicting the effects of possible future waste processing operations. The results of the solubility and sorption studies are presented in this report. Experimental Experimental Design, To identify which of the 12 high level waste (HLW) components significantly affected radioélément solubility and sorption, the Plackett-Burman design, a statistical 20-run screeningtest,was employed (1). The Plackett-Burman design is a two-level fractional factorial design having internal replication and is effective for screening variables (in this case, chemical components) for significance when the interactions between the variables are not important. The effects of the significant HLW components on solubility and sorption were quantified using three-level statistical designs. Full factorial designs were run if only two variables (HLW components) were to be studied and fractional factorial BoxBehnken designs were run for three or four variables (2). The 2-variable design required 12 experiments while the 3- and 4-variable Box-Behnken designs required 15 and 27 experiments, respectively. The three-level designs yielded quadratic equations predicting solubility and sorption values in terms of the concentrations of the significant components. Materials. Twelve chemical components of Hanford alkaline HLW solutions were studied. These were NaNCL, NaN0 , NaOH, NaA10 , NaJC0 , Na S0 , Na^PO^, NaF, Na HEDTA (N-2-hydroxyethylethylenediaminetriacetic acid, trisodium salt), NaJSDTA (ethylenediaminetetraacetic acid, tetrasodium salt), Na0 CCH OH (sodium hydroxyacetate), and N a C H 0 , (trisodium citrate). Reagent chemicals and distilled and deionizea water were used to prepare all test solutions. The 12 waste components were selected for study based on their quantities in the Hanford HLW solutions and their abilities to complex, or influence the complexation of metallic radioéléments. The range of chemical component concentrations studied, given in Table I, was broadly representative of concentrations found in HLW. Three Hanford sediments were used in the sorption studies. Each of these sediments contained significant quartz, feldspar, vermiculite, mica, and montmorillonite and were typical of the Hanford sediment in which the HLW tanks are located. Properties of the sediments used are given in Table Π. 2
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3
2
2
4
3
3
6
5
6. DELEGARD ET AL.
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Five radioéléments were selected for study: cobalt, strontium, neptunium, plutonium, and americium. These radioéléments were selected because of their presence in the HLW, their potentials to form complexed species, and their radiological hazards. Table ΠΙ summarizes the radioélément concentrations estimated and measured in some HLW solutions and the concentrations used in these experiments. Procedures. Each solubility experiment was conducted by adding dried tracer solids to 5.00 mL portions of test solution, and by capping and equilibrating the mixture for 2 weeks. The tracer solids were obtained by drying precise volumes of stock nitric acid solutions of the radioélément under a heat lamp. Thus, the radioéléments were present initially as Co(H), Sr(H), Np(VI), Pu(IV), and Am(lH). Following equilibration, the mixtures were filtered through 0.003um pore size ultrafilters to remove undissolved tracer solids. Spectrometry of gamma and low energy X-rav emissions was used to determine 60Co, 85Sr, N p , 238Pu, and 241 Am concentrations. Each sorption experiment was conducted by adding 5.0 mL of the appropriate traced solution, prepared as described above, to a weighed (-1 g) portion of Hanford sediment. To simulate advancement of a radioélément plume from a failed tank through previously waste-wetted sediment, each sediment sample was preequilibrated twice with the relevant untraced solution prior to introduction of the traced solution. Each pre-equilibration lasted at least 2hr. Following a 7-day equilibration with the traced solution, each sediment-solution mixture was centrifuged, the solution was filtered through an ultrafilter, and the radionuclide solution concentration was determined. Distribution coefficients and fractions of radionuclides sorbed were determined for each sorption experiment. The distribution coefficient, Kd, is the activity per gram of sediment divided by the activity per mL of solution at equilibrium. 2 3 7
Results Solubility Screening Tests. Table IV summarizes the results of the solubility screening tests performed using the Plackett-Burman statistical design. Cobalt was found to be significantly more soluble in the presence of EDTA and HEDTA, and, to a lesser extent, Na SO^. Cobalt probably is present as Co(H) in strongly basic solution (3). As expected, high formation constant values are known for Co(H) complexes of EDTA and HEDTA (4). These stable complexes enhanced the solubility of cobalt. The effect of N a S 0 on increasing cobalt solubility was unexpected since the complex CoSO. has a very low formation constant (5). Strontium, present as Sr(II), also showed increased solubility in the presence of EDTA and HEDTA due to the formation of stable soluble complex species (4). 2
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GEOCHEMICAL BEHAVIOR O F RADIOACTIVE WASTE
Table I. Component Concentration Values Concentration Range Studied (M)
Component NaN0
3
0-2
NaN0
2
0-2
NaOH
1-4
NaA10
2
0-0.5
Na C0
3
0-0.05
2
Na S0 2
Na P0 3
0-0.01
4
0-0.01
4
NaF
0-0.01
Na HEDTA
0-0.1
Na EDTA
0-0.05
Na hydroxyacetate
0-0.1
Na citrate
0-0.03
3
4
3
Table Π. Properties of Hanford Sediments Studied Texture (wt%) Sediment
CEC (meq/100g)
CaCO.3
(mg/g)
Clay
Silt
Sand
L
1.9
8.3
89.8
3.6
34
Ρ
1.3
6.9
91.8
3.5
19
S
3.0
10.3
86.7
6.8
0
DELEGARD ET AL.
High-Level Waste Components
Table EQ. Radioélément Concentrations
Radionuclide
Radionuclide concentration (uCi/L)
Total element concentration (μΜοΙ/L)
Concentration in Hanford HLW 6OC0
153
90Sr
3,190-4,800
a
b
0.002 0.75
-l.l
b
2.8
17
238,240p
0.46-6.8
0.023-0.38
241Am
0.12-21
0.00016-0.025
237Np u
b
b
Maximum Possible Concentration in Solubility Experiments (no precipitation) 6OC0
-
3.8
85Sr
-
3,000
237Np
-
6,600
238,239,240P
-
90
24lAm
-
8.6
U
Concentration in Sorption Experiments 6OC0
-
0.08
85Sr
-
0.8
237Np
-
30
238Pu
-
0.002
0.0006 24iAm Filtered (0.45μπι) solutions. No neptunium analyses available. It was assumed that all Np inventory in HLW was dissolved uniformly in waste liquor. Assume total Co/eoCo = 1. Ratios of total/active radioélément are 3,1, and 1 for strontium, plutonium, and americium, respectively. a
b
+3
Plutonium
3
-1
NaN0
2
+1
NaOH
+2
+7
NaA10 2
+3
2
Na C0 3
+8
+3
2
Na S0 4
+2
+5
3
Na P0 4
NaF
+1
+5
-2
+1
+2 +4
+1
EDTA
+2
HEDTA
+4
+6
Sodium hydroxy -acetate
+3
Sodium citrate
•Significant at >80% confidence interval. Numbers indicate rank in importance; + indicates component increased solubility, indicates component decreased solubility.
Americium
-9
NaN0
Neptunium
Strontium
Cobalt
Radioélément
Table IV. Significant Components in Radioélément Solubility *
s
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The oxidation-reduction behaviors of neptunium, plutonium and americium in basic solution have been determined via polarographic and coulometric studies (6-9). These studies, which showed that the more soluble (V), (VI), and (VŒ) oxidation states of these actinides are stable in alkaline solution under certain redox conditions, helped identify possible actinide species and oxidation states in our experiments. Actual identification of radioélément oxidation states was not done in the present experiments. As shown in Table IV, neptunium exhibited a complicated solubility behavior in HLW. Nine of the 12 waste components studied were significant in affecting neptunium solubility. By far, NaNCL had the greatest influence and significantly decreased the solubility of neptunium. According to the neptunium redox potentials determined in alkaline solution (6-7) and the potential established by the NO«~-N0 " couple (3), nitrite apparently reduced Np(VI) to Np(V). The(V) state of neptunium has been shown to be less soluble than the (VI) state in NaOH solution (6). The probable existence of both Np(V) and (VI) in the screening tests complicated the interpretation of the data. The increase in neptunium solubility caused by Na CO., Na S0 , Na P0 , HEDTA, and hydroxyacetate probably was due to complexation. The decreased solubility of neptunium in the presence of EDTA was not anticipated since EDTA generally forms stable soluble complexes with metal ions. Sodium nitrate also decreased neptunium solubility while NaA10 increased it. Electromigration studies have shown that neptunium (V), (VI), and (VU) are anionic species when in alkaline solutions (10). Sodium salts of Np(V) have precipitated from alkaline solution (10). Thus, N a N 0 may have precipitated Np(V) as a sodium neptunate salt in the screening tests. However, an increase in sodium hydroxide concentration should have led to increased neptunium concentrations in the screening tests due to increased formation of the more soluble anionic hydroxide complexes of neptunium. Detailed analysis of the test data showed that in reducing conditions (i.e., with NaNOJ, NaOH did increase neptunium solubility; without NaN0 , NaOH decreased solubility. Further studies must be done to deduce specific neptunium solubility dependencies with respect to the oxidation state. The plutonium solubility increased in the presence of increased NaN0 , NaOH, and NaA10 concentrations. According to the literature, Pu(V) should be the stable oxidation state in alkaline NO ~-NO ~ solutions (8). It has been observed that the solubility of Pu(V) increases as the NaOH concentration increases (8,11); probably this occured due to formation of the more soluble anionic hydroxide complexes of Pu(V) such as Pu0 (OH) " (11). Sodium nitrate and NaA10 may have increased Pu(V) solubility through complexation. Sodium nitrate also may have increased plutonium solubility by oxidizing the less soluble Pu(IV), initially present in the tracer solids, to Pu(V). 3
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3
4
2
3
2
3
2
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2
2
2
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GEOCHEMICAL BEHAVIOR OF RADIOACTIVE WASTE
The screening tests showed that americium solubility increased in the presence of the four organic components tested: HEDTA, EDTA, sodium citrate, and sodium hydroxyacetate; as well as with NaJP0 . According to electrochemical studies, Am(HI) should be the oxidation state present in the screening tests (9). In light of the high formation constants found for the respective americium complexes (12), the strong effect of the organic complexants increasing Αιη(ΙΠ) solubility was reasonable. Literature data were not sufficient to confirm the effect of phosphate on americium solubility. 4
Solubility Prediction Equations. Using results obtained in the solubility screening tests, three-level statistically designed solubility prediction equation tests were run for strontium, plutonium, and americium. As shown above, strontium solubility depended principally on the presence of HEDTA and EDTA. Plutonium solubility depended on NaN0 , NaOH, and NaA10 concentrations. Americium solubility depended on concentrations of Na P0L, HEDTA, EDTA, sodium hydroxyacetate, and sodium citrate. Tests were designed using concentrations of these HLW components as variables while the remaining components were maintained at constant intermediate concentrations representative of HLW. The component EDTA was not included in the americium tests because the number of experiments (46) required in the 5-variable Box-Behnken design would have been too large and because EDTA probably would have behaved similarly to HEDTA. The solubility prediction equations derived from the threelevel tests are given in Table V with their respective correlation coefficients (R ) to assess goodness of fit. Judging from the R values, plutonium solubility was described adequately by its prediction equation while the strontium and americium solubility equations were less satisfactory. Analysis of the individual test data was found to be more useful than the prediction equations in assessing strontium and americium behavior. Inspection of individual test runs showed that the strontium spike, corresponding to 3xlO" M total strontium, dissolved entirely when HEDTA or EDTA concentrations were 0.1M or 0.05M, respectively. However, in the presence of 10" M or lower HEDTA or EDTA concentrations, strontium was at saturation with concen trations of about 4xlO" M (-0.17 Ci 90Sr/L in Hanford HLW). Strontium concentrations in Hanford HLW, as shown in Table ΙΠ, are about 0.004 Ci 90Sr/L and probably not solubility limited. Inspection of the americium test data showed that the entire spike (about 8xlO' M americium) dissolved in the presence of 0.1M HEDTA. Subsequent synthetic HLW test solutions haying 0.1M HEDTA completely dissolved an 8xlO~ M americium spike (0.6 24lAm/L), thus confirming the strong effect of HEDTA on americium solubility. Citrate and hydroxyacetate also increased americium solubility. At 0.03M citrate, with ^10" M HEDTA, 3
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High-Level Waste Components
Table V. Radioélément Solubility Prediction Equations Radioélément Sr
2
Solubility prediction equation (logM)
R
log (Sr) = -1.12 + 0.50 log (HEDTA) + 0.483 log (EDTA) + 0.050 [log (HEDTA)] + 0.045 [log (EDTA)] -0.023 [log (HEDTA)] [log (EDTA)]
0.92
log (Pu) = -5.67 + 0.14 log (NaNOJ 0.18 log (NaOH) + 0.121og (NaAlOJ + 0.012 [log (NaNOJ] + 2.10 [log NaOH)] + 0.0090 [log NaA10 )] log (Am) = -3.22 + 0.31 log (HEDTA) + 0.15 log (OAcr + 0.51 log (citrate) + 0.39 [log (HEDTA)] + 0.21 [log (OAc)] + 0.048 [log (citrate)] -0.030 [log (HEDTA)] [log (OAc)] - 0.043 [log (HEDTA)] [log (citrate)]
0.98
a
2
2
Pu
2
2
2
2
Am
0.96
2
2
2
a
90
To convert mol/L total strontium to Ci Sr/L, change the constant in equation from -1.12 to 2.52. To convert mol/L total plutonium to Ci239,240p /L, change the constant in equation from -5.67 to -4.42. To convert mol/L total americium to Ci 4l Am/L, change the constant in equation from -3.22 to -0.30. OAc represents hydroxyacetate. u
2
b
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GEOCHEMICAL BEHAVIOR O F RADIOACTIVE WASTE
americium concentrations were solubility limited to about 3x10" M . For test solutions containing 0.1M hydroxyacetate, with
