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Impact of Natural Organic Matter on Plutonium Vadose
Zone Migration from an NH 4 Pu(V)O 2 CO 3 (s)
Source
Melody Maloubier, Hilary Emerson, Kathryn Peruski, Annie Kersting, Mavrik
Zavarin, Philip Almond, Daniel Kaplan, Brian Powell
To cite this version:
Impact of natural organic matter on plutonium
1
vadose zone migration from an NH
4
Pu(V)O
2
CO
3
(s)
2
source
3
Melody Maloubier a, Hilary Emerson b, Kathryn Peruskia, Annie B. Kersting c, Mavrik Zavarin c,
4
Philip M. Almond d, Daniel I. Kaplan d, Brian A. Powella,d,e
5
a
Department of Environmental Engineering & Earth Sciences, Clemson University, Clemson, 6
SC, 29634, USA 7
b
Subsurface Science and Technology, Energy & Environment, Pacific Northwest National 8
Laboratory, Richland, WA, 99354, USA 9
c
Glenn T. Seaborg Institute, Physical & Life Sciences, Lawrence Livermore National Laboratory, 10
Livermore, CA 94550, USA 11
d
Savannah River National Laboratory, Aiken, SC, USA 12
e
Department of Chemistry, Clemson University, Clemson, SC, 29634, USA 13
Abstract: 14
We investigated the influence of natural organic matter (NOM) on the behavior of Pu(V) in the 15
vadose zone through a combination of field lysimeter and laboratory studies. Well-defined solid sources 16
of NH4Pu(V)O2CO3(s) were placed in two 5-L lysimeters containing NOM-amended collected from
17
Savannah River Site or unamended vadose zone soils and exposed to three years of natural South 18
Carolina, USA meteorological conditions. The lysimeter soil cores were removed from the field, used in 19
desorption experiments, and characterized by wet chemistry methods and X-ray absorption spectroscopy 20
(XAS). For both lysimeters, Pu migrated slowly with the majority (>95%) remaining within 2 cm of the 21
source. However, without the NOM amendment, Pu was transported significantly farther than in the 22
presence of NOM. Downward Pu migration appears to be influenced by the initial source oxidation state 23
and composition: These Pu(V) sources exhibited significantly greater migration than previous studies 24
by the lysimeter soil on the order of hours, indicating that downward migration of Pu may be due to 26
cycling between Pu(V) and Pu(IV). Under the conditions of these experiments, NOM appeared to both 27
enhance reduction of the Pu(V) source as well as enhance Pu sorption to soils. This indicates that NOM 28
will tend to have a stabilizing effect on Pu migration under Savannah River Site (SRS) vadose zone field 29
conditions. 30
1. Introduction 31
Radionuclides have been deposited in the surface worldwide due to nuclear weapons fallout 32
and from nuclear power plant activities and accidents. Actinides such as Pu are of significant concern 33
when released to the environment because of their long half-lives (24,130 years for 239Pu) and high 34
radiotoxicity. The Savannah River Site (SRS) in South Carolina produced approximately 36 metric 35
tons of Pu from 1953 to 1988. Much of this Pu is destined for long-term disposition in a stable 36
subsurface repository. However, it is estimated that the SRS released 60 Ci of 238Pu and 239Pu to the 37
vadose zone primarily through direct disposal into seepage basins (maximum contaminant level of 15 38
pCi/L for -particle activity in drinking water).1, 2 39
Evaluation of Pu migration in the environment is particularly complex due to the fact that Pu 40
may be simultaneously present in multiple oxidation states (from +III to +VI)3-6. Previous studies 41
have shown that subsurface migration of actinides is influenced by many factors including redox 42
behavior, sorption, and complexation7-10. The speciation and mobility of Pu is highly dependent on 43
the initial depositional environment and form of plutonium.11 For example, Batuk et al. examined the 44
speciation of Pu in soil from different contaminated field sites (Hanford Site, Rocky Flat and Los 45
Alamos) through a combination of XAS and X-ray fluorescence element maps (XRF).11 They 46
identified different forms, such as, mononuclear Pu, PuO2+x precipitates that can incorporate other
47
elements (Fe, P) and particles. Various mechanisms leading to the observed species were proposed 48
based on an a priori knowledge from the site (e.g., the initial form and disposal conditions), 49
especially to explain incorporation with other elements. To understand the fate of Pu previously 50
discharged to the seepage basins and to mitigate potential future releases, it is important to 51
investigate the behavior of Pu under field conditions. An improved understanding of Pu behavior in 52
the field will help to develop more robust geochemical models for predicting the long-term fate of Pu 53
at SRS and others. The knowledge of the behavior of Pu according to its initial source material and 54
the site conditions will allow to determine the transport mechanisms and predict a future behavior in 55
other sites. 56
Previous field lysimeter studies at the SRS have demonstrated reduction of PuVIO2(NO3)2(s)
57
and oxidation of PuIIICl3(s) sources to unspecified Pu(IV) solid phases 12, 13. This study has also
shown that Pu originating from a PuVI(NO3)2(s) source was transported significantly further than
59
PuIV(NO3)4(s) and Pu(III) sources 1, 12, 13. Based on these data and speciation modeling, Pu(V) is
60
expected to be the most mobile oxidation state in natural waters. 10, 14, 15 However, the behavior of 61
Pu(V) sources and the impact of organic matter were not previously studied.The current work seeks 62
to monitor the chemical and physical changes of well characterized NH4PuVO2CO3(s) sources to
63
compare with these earlier results. In contrast to measurements of Pu bearing samples recovered from 64
legacy management sites, the initial source material and experimental conditions of the current study 65
are well known, allowing for a direct evaluation of the transformations of Pu solid phases under 66
environmental conditions. 67
Following release from the initial Pu source, migration of Pu can be influenced by colloids, 68
organic matter, and redox reactions. Formation of pseudo-colloids (e.g., Pu bound to mineral colloids 69
or organic colloids) can have a strong impact on Pu migration in the subsurface. Several studies have 70
shown that the fate of Pu in the environment may be controlled by transport on colloidal iron oxide 71
and clay mineral particles.16-18 Additional studies have demonstrated that association of Pu with 72
organic colloids can enhance pore water concentrations 19-27. Moreover, the presence of organic 73
matter can affect the oxidation state as it is known that Pu is rapidly reduced from Pu(VI)/Pu(V) to 74
Pu(IV)/Pu(III).3, 4, 28, 29 The overall influence of organic matter on Pu migration appears to be 75
dependent on the nature of the organic matter 17, 30, 31 and the Pu-organic matter interactions and 76
further studies are warranted. Most previous research has been conducted via laboratory experiments 77
that are not representative of natural conditions (wet/dry cycles, preferential flow, rainfall…). 78
However, there is a need to supplement these data with field scale experiments under more 79
representative environmental conditions. In most vadose zone contaminated soils, the Pu 80
concentration is too low to determine the speciation spectroscopically. 81
In 2012, a large experimental program was initiated to study the long-term migration 82
behavior of Pu and neptunium (Np) in SRS vadose zone soil.32 As part of this program, 22 field 83
lysimeters containing Pu and Np sources were deployed at the Radiological Field Lysimeter 84
Experiment (RadFLEx) facility at the US Department of Energy Savannah River Site (USDOE, 85
SRS). Lysimeters containing soil from the SRS and well-defined Pu sources with different oxidation 86
states are currently open to natural rainfall and temperature fluctuations. The lysimeters are housed 87
within secondary containment and kept in a roll-off container. Triplicate lysimeters have been 88
prepared to allow for depth-discrete destructive sampling after 3, 10 and 20 years. The study reported 89
here focuses on two lysimeters containing NH4Pu(V)O2CO3(s) sources and exposed to the same
90
of the experiments. A third NH4Pu(V)O2CO3(s) source, named archived source, was kept after its
92
synthesis under a nitrogen atmosphere in the lab. The objectives were to evaluate the effect of NOM 93
on the release of Pu in the vadose zone and to determine how source speciation changes (oxidation 94
state, chemical species) affect migration. The release from sources was quantified by measuring total 95
Pu concentrations in lysimeter leachate during the 3-year study and, at the end of the study, as a 96
function of soil depth. X-ray absorption spectroscopy (XAS) was conducted on the sources to 97
characterize the alteration of the Pu over time. Because the soil Pu concentrations were too low to 98
use direct speciation techniques, information on the Pu behavior in the soil was obtained through 99
laboratory based batch desorption experiments. 100
101
2. Materials and Methods 102
2. 1. Radionuclide Field Lysimeter Experiment (RadFLEx) 103
2. 1. 1. Source Preparation 104
Clemson University and Savannah River National Laboratory (SRNL) have installed and are 105
currently operating a radionuclide lysimeter facility at the Savannah River Site (SRS) in South Carolina. 106
Out of 48 total lysimeters at RadFLEx, 22 lysimeters were installed to evaluate the long-term migration 107
of Pu and Np through vadose zone soils. Six lysimeters contained NH4Pu V
O2CO3(s) sources. Each
108
lysimeters consists of a 60 cm × 10 cm PVC pipe with a representative sandy clay loam soil from SRS. 109
Three of the six lysimeters contained soil amended with 10% by weight NOM prepared by mixing a dried 110
sample of organic matter collected from a wetland area upgradient of the field lysimeter facility. The 111
NOM was isolated from a 20 kg leaf litter sample collected by hand from a forested wetland floor along 112
the edge of Tims Branch in Aiken South Carolina (33o 19’ 58” N, 81o 43’ 09” W). The forest contained 113
black gum (Nyssa sylvatica), black willow (Salix nigra), red maple (Acer rubrum), and water tupelo 114
(Nyssa aquatica). The litter was dried at 60 oC in a forced air oven for 1 week. The dried leaf litter was 115
ground to a fine powder in a Thomas Wiley Mill Model 4, equipped with a 40 mesh sieve. The ground 116
leaf litter was then mixed by shovel with lysimeter sediment at a leaf litter:sediment weight ratio of 117
1:10.For radiation protection when transferring the Pu source into the field, the source (held between filter 118
papers as described below) was packed into the center a 5 cm diameter x 8 cm tall cylinder which was 119
extruded into the field lysimeter then surrounded with additional 10% by weight NOM amended soil. 120
The main properties of the soil are detailed in Table 1. The average pH for the two lysimeters 121
was 5.1. The ratio of sand/silt/clay was determined by particle size analysis and found to be 66:14:20 122
primarily (>95 %) kaolinite (Al2Si2O5(OH)4). The dithionite-citrate-bicarbonate (DCB) extractable
124
iron was 6.01 mgFe/gsoil measured using the method developed by Mehra and Jackson.33
125
To monitor for the lability of NOM, a desorption experiment was performed by mixing 250 126
mg of 10% amended sediment with 50 mL 10mM NaCl in pH 3, 5.5, and 10 solutions. Samples at 127
each pH were prepared in triplicate and mixed end-over-end for 24 hours before sampling aqueous 128
phase. Samples were centrifuged to remove particles <100 nm, according to Stoke's Law, then a 129
small aliquot of aqueous phase was sampled and diluted in deionized water. Samples were analyzed 130
for dissolved organic carbon on a Shimadzu total organic carbon analyzer. 131
Soil was packed into the lysimeters in 5 cm lifts and the lysimeters were lightly tapped on the 132
ground in between lifts to pack the soil. The radioactive sources were secured between two glass 133
fiber filter papers and placed in the center of the 50 cm tall lysimeters. The NH4PuVO2CO3(s) sources
134
were prepared by pipetting an aliquot containing 50 mg of 239/240Pu from a 53 g/L stock solution into 135
a vial containing Ag(NO3) and Na2S2O8. The solution was then heated at 60-80 C for 20 min.
136
Following oxidation to Pu(V/VI), the NH4PuO2CO3(s) was precipitated by adding (NH4)2CO3 (180
137
mg) which formed a green solid over 10 minutes. After filtration and washing, the solid was 138
deposited on a filter paper (47 mm GRR, Whatman). The NH4PuO2CO3(s) composition was verified
139
using XRD (see Figure S1 in the SI). The final amount of Pu added to each source was 140
approximately 3.02 × 107 Bq of total Pu (⁓2 mg of 239/240Pu).. One archived source was stored in a 141
nitrogen purged vessel during the study. The source materials placed in the lysimeters were left 142
exposed to natural weather conditions as noted below. During the experiment, the leachates from the 143
lysimeters were periodically sampled and analyzed for total aqueous Pu concentrations using ICP-144
MS, the results indicated that no breakthrough had occurred. 145
146
2. 1. 2. Plutonium Lysimeter Coring Process 147
After 3 years, two lysimeters (NOM-amended and unamended) were removed to analyze soils 148
and sources. The lysimeters were sectioned in one cm slices in a glove box for containment and total Pu 149
concentrations were measured using inductively coupled plasma mass spectrometry (ICP-MS, Thermo X 150
Series II). The soil cores were stored moist, in the dark, and at room temperature. The Pu sources were 151
removed from the lysimeters for further analysis. Total Pu concentration in each depth-discrete section of 152
soil was determined in triplicate. Each 1 cm soil section was contained in a separate bag, and soil was 153
homogenized by hand before sub-sampling. A 0.5 gram soil sample was taken from each segment and 154
dried in an oven at 50 °C for 2 days. Dried soil samples were then placed in Teflon 155
was spiked into each sample. A 7 mL aliquot of concentrated HNO3 and 3 mL of concentrated HF were
157
added to each Teflon tube. Samples were run in a MARS microwave digestion system (CEM 158
Corporation) using the EPA 3052 protocol.34 Samples near the source were run in triplicate and the 159
average and standard deviation are reported below. 160
After microwave digestion, samples were further processed using extraction chromatography to 161
separate plutonium isotopes from iron in the digestate prior to chemical analysis. Digestate samples were 162
diluted to 8 M HNO3/0.01 M ascorbic acid prior to chromatography. Plutonium isotopes were extracted
163
from solution using extraction chromatography on pre-packed TRU resin columns (Eichrom) with a 2 mL 164
bed volume. Columns were conditioned with two column volumes of 8 M HNO3/0.01 M ascorbic acid
165
before sample was added, then washed with three column volumes of 8 M HNO3/0.01 M ascorbic acid
166
and followed by three column volumes of 8 M HNO3. Iron was eluted from the column using one column
167
volume of 0.01 M HNO3, while Pu was eluted from the column in a final step using one column volume
168
of 0.1 M HNO3/0.1 M oxalic acid solution. The extraction chromatography method was tested with
iron-169
amended plutonium (242Pu and 239Pu) standards prior to application, confirming that 242Pu and 239Pu were 170
effectively separated from Fe using different elutions from the column. Chemical yield for 242Pu was 171
tracked throughout digestion and extraction chromatography of sediment samples to further confirm 172
methodology. The final, plutonium-containing elution was directly analyzed by ICP-MS. ICP-MS 173
calibration standards for 239Pu and 242Pu were prepared using National Institute of Standards and 174
Technology (NIST) standards 4330C and 4334I, respectively, and stock solutions traceable to these NIST 175
standards. Samples were analyzed for 242Pu and 239Pu to determine the chemical yield and 239Pu content in 176
the lysimeter soil sample, respectively.. 177
The concentration of 239Pu in the soil sample was determined using equation 1: 178
179
Equation 1 180
where [239Pu]ICPMS is the concentration of 239
Pu measured by ICP-MS (mol/L), VICPMS is the eluate volume
181
(L), Y is the chemical yield of 242Pu, and msoil is the mass of soil (g). The purpose of the 242
Pu tracer was 182
to determine the efficiency of Pu separation during the digestion and ion exchange process. This recovery 183
yield was used to correct for potential losses of 239Pu during the separation. Thus, the final reported Pu 184
concentration is in molPu/gsoil. Using the isotope ratios of 239
Pu and 240Pu in the original source term (6% 185
240
Pu), the total Pu concentration can be determined. For comparison with the activity levels reported in 186
the original source, the mass of 239Pu was converted to activity using a 239Pu half-life of 24,110 years. 187
2. 1. 3. Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge 189
Structure (XANES) 190
The speciation of Pu in the archived source material (stored in nitrogen atmosphere) and the two 191
lysimeter sources deployed in the field was determined using X-ray absorption spectroscopy (XAS). The 192
XAS analyses were performed on small sections of the filter papers containing the Pu sources cut with a 193
razor blade13. XAS at the Pu LIII edge (18057 eV) and LII edge (22266 eV) was conducted on the 11.2
194
beamline of the Stanford Synchrotron Radiation Lightsource (SSRL) facility. The monochromator was set 195
with the Si(220) crystal and the harmonics were rejected with a Rh-coated mirror. Energy calibrations 196
were conducted at the Zr K edge at 17998 eV for the Pu LIII edge and at the Ru K edge at 22117 eV for
197
the Pu LII edge. EXAFS measurements were conducted in fluorescence mode using a 100-element
198
germanium detector (Canberra). Due to the possibility of redox reactions under large photon flux, the 199
samples were held at low temperature using a cryostat stage that was cooled with liquid nitrogen. XANES 200
spectra were recorded to verify the oxidation states in the samples. No differences were observed during 201
the experiment that could be induced by the beam flux. 202
Data processing was carried out using the Six Pack35 and Athena code36 with five scans per 203
samples. Dead time corrections were performed by measuring the relationship between the incoming 204
count rate and selected channel analyzer readings for each channel using SixPack software 35. The Eo
205
energy was identified as the first inflection point. Fourier transformation (FT) in k2 was performed 206
between 2.25 and 9.5 Å-1 with Hanning windows using the ARTEMIS code 36. Spectral noise was 207
calculated using the CHEROKEE code 37, 38 using the Fourier back transform filter above 6 Å 208
corresponding to the noise spectrum. The r factor (%) and the quality factor (QF, reduced χ²) of the fits 209
were provided from ARTEMIS. Phases and amplitudes were calculated using the FEFF9 simulation 210
code39 using PuO2 40, 41
and NH4PuO2CO3 crystallographic models 42
. 211
212
2. 1. 4. Desorption of Plutonium-amended Soil Under Oxidizing Conditions 213
Desorption experiments were conducted on soils from six segments of the lysimeter ranging from 214
2.5 cm above to 4.5 cm below the source region (distances reflect the midpoint of each segment of the 215
lysimeter). A 5 g aliquot of pre-homogenized soil was placed in a 50 mL centrifuge tube containing 50 216
mL of 10 mM NaCl; samples were run in triplicate. Samples were not pH adjusted but the soil buffering 217
capacity produced a pH of 4.78 +/- 0.07 in all samples. The soil suspensions were placed on a slow 218
moving platform shaker for equilibration. After 8 weeks of mixing, the samples were centrifuged to 219
remove particles greater than 100 nm from solution based on Stokes law43. A 5 mL aliquot of supernatant 220
was removed as the unfiltered solution. A second 5 mL aliquot of the supernatant was filtered through a 221
nm). The unfiltered and filtered samples were acidified to 2% HNO3 for analysis using ICP-MS.
223
Concentration of 239Pu (in parts per billion) was measured on ICP-MS, then converted to Bq/mL using the 224
239
Pu half-life of 24,110 years for comparison with the soil concentrations in Bq/g. For each sample, the 225
apparent Kd was calculated by taking the ratio of the Pu concentration in the soil (Bq/g) and in the
226
aqueous phase (Bq/mL) after 8 weeks of desorption. 227
228
2. 2. Laboratory Experiment of Plutonium(V) Reduction by SRS Soil 229
2. 2. 1. Pu(V) Working Solution Preparation 230
A Pu(V) stock solution was prepared by ozonation according to the method used by Conroy et al. 231
44
. Briefly, the appropriate amount of a 2.0×10-3 M 242Pu(IV) solution in 0.01 M HCl or a 2.2×10-9 M 232
238
Pu(IV) solution in 0.01 M HCl was pipetted into a flat-bottomed, round flask containing 75 ml of 233
deionized water to obtain a total activity of 700 dpm/ml. The initial pH was adjusted to between 3 and 4. 234
The diluted solution was treated with O3 gas (Aqua-6 ozone generator; A2Z Inc.) for 20 min every hour
235
for a total of 72 h. After ozonation, the pH was adjusted to 5 by dropwise addition of NaOH. No 236
significant reduction of Pu(V) were observed in the stock solution during the experiment. The spike 237
solutions contained 98 ± 5 % Pu(V) at time zero and 95 ± 5 % Pu(V) at the end of the batch experiments 238
as determined by solvent extraction employing 0.5 M bis-(ethylhexyl)-phosphoric acid (HDEHP) in 239
hexane and 0.025 M 4-benzyol-3-methyl-1-pheyl-2-pyrazolin-5-one (PMBP) in hexane as the extractants 240
as previously reported45-48. At pH 0, the PMBP solution selectively extracts Pu(IV) into the organic phase 241
with Pu(V) and Pu(VI) remaining in the aqueous phase, and the HDEHP solution selectively extracts 242
Pu(IV) and Pu(VI) into the organic phase, leaving Pu(V) in the aqueous phase. The Pu concentration in 243
the organic and aqueous phases obtained after extraction were determined using liquid scintillation 244
counting (LSC, Perkin Elmer Tricarb 2910). The oxidation state analysis results were verified by 245
comparison with similar results obtained from lanthanum fluoride coprecipitation49. LaF3 coprecipitates
246
Pu(III) and Pu(IV), leaving Pu(V) and Pu(VI) in the aqueous phase. Because these techniques both 247
determine the Pu oxidation states indirectly, the uncertainty is higher than with direct methods; thus, a 248
conservative uncertainty of 15% was assumed which was greater than typical LSC counting statistics. 249
250
2. 2. 2. Batch Sorption and Leaching Experiments 251
Batch experiments were conducted to determine the impact of soil (<53 µm size fraction of SRS 252
lysimeter soil) on the reduction of Pu(V) to Pu(IV). A 2.5 ml aliquot of Pu(V) working solution was 253
added to suspensions containing approximately 25 gsoil/L soil and adjusted to pH 5. The final
254
concentrations of Pu(V) in the suspensions were 1.2 x 10-10 M (238Pu) and 3.1 x 10-7 M (242Pu). The 255
equilibration time, the samples were centrifuged at 8000 rpm for 30 min (Beckman Coulter Allegra X-257
22R Centrifuge, 2402 rotor) which was calculated to remove particles >100 nm from solution based on 258
Stokes law 43. Pu concentrations in the supernatant were measured using LSC and the oxidation state 259
distribution of the aqueous phase Pu was determined using the oxidation state analysis technique 260
described in section 2.2.1. 261
An acid extraction approach was used to indirectly monitor the oxidation state of sorbed Pu. The 262
wet solid phase was suspended in 0.3 M HCl and mixed for 15 min to leach sorbed Pu into the aqueous 263
phase. The suspension was again centrifuged under the same conditions as before, followed by an 264
oxidation state analysis. Similar to previous work, the acid leaching step was assumed to remove Pu(V) 265
and Pu(VI) from the soil surface and the unleachable Pu was assumed to be Pu(IV)50-54. All the aliquots 266
(supernatants from the sorption step and leachates) were filtered through 3k MCWO filters (Microsep 267
Advance Centrifugal device; Pall Corporation) before oxidation state analysis. Total Pu concentrations 268
before and after filtration were compared to monitor for the presence of colloids. 269
270
3. Results and Discussion 271
3.1. Plutonium Depth Profiles 272
The total Pu concentration in the two lysimeter soils as a function of depth after 3 years of 273
exposure are presented in Figure 1, and they are compared to previous data obtained after an 11-year field 274
study of PuIII andPuIV sources and a 2-year field study of PuVI sources12, 13 (Figure S2 Supporting 275
Information). In regards to Pu mass balance, negligible Pu was measured in the field collected effluent in 276
all lysimeters. Therefore, Pu was only present in the lysimeter in the associated with the soil and the 277
source. There are four major observations: 278
1. The Pu concentration profiles are different (Figure 1). The downward migration of Pu was 279
hindered by the presence of NOM. 280
2. Pu migration after three years is very limited, with more than 95% of the Pu remaining within 281
2 cm of the source material. More specifically, the highest soil Pu concentration was found 282
within 1 cm of the source and corresponding to 2-5% of the initial Pu mass. Pu concentrations 283
in the soil decreased by approximately an order of magnitude per cm in the first 2 cm and 284
decreased more gradually with depth, approaching a concentration of 1 Bq/g 239Pu below 12 285
cm. 286
3. Plutonium migration was similar to a previously published two year Pu(VI) lysimeter 287
experiment. Both the Pu(V) and Pu(VI) sources traveled farther than previous 11 year Pu(IV, 288
III) lysimeters experiments (though NOM was absent in all previous lysimeter experiments) 289
12
4. Pu migration in this NOM-amended lysimeter experiment was similar to the previously 291
published Pu(IV) and Pu(III) lysimeter experiments with an order of magnitude decrease in 292
Pu concentration per cm for the first 3 cm below the source 12. 293
294
Based on these observations, the presence of NOM in SRS soil and under vadose zone field 295
conditions limits Pu migration. While this study only compares two lysimeters, running multiple 296
replicates was not possible due to the scale of these field experiments. However, all experimental 297
parametesr except the NOM amendment were the same, allowing for direct comparison. Several studies 298
have found that organic matter may enhance radionuclide migration but can also rapidly reduce Pu(VI) 299
and Pu(V) to Pu(IV) and Pu(III)30. In this study, the slower migration of Pu in the presence of NOM can 300
be explained by an enhanced reduction of Pu(V) to the less mobile Pu(IV) species by the organic matter 301
or by soil-bound NOM providing additional Pu sorption sites. Laboratory studies have demonstrated that 302
ternary surface-NOM-actinide complexes can enhance Pu sorption at low pH26, 27. These studies 303
hypothesize that Pu sorption is enhanced at low pH via complexation with NOM that is sorbed to the 304
mineral surface, thus forming a ternary surface complex. Such ternary surface complexes would reduce 305
the mobility of Pu under low pH conditions as observed in these field data. Thus, two working hypotheses 306
for the observed Pu migration in the presence of NOM are 1) formation of stable ternary surface-NOM-307
Pu complexes and/or 2) inhibition of Pu(IV) oxidation to more mobile Pu(V) in the pore water. Previous 308
studies observed enhanced migration of Pu(V) from Pu(IV) lysimeter sources due to redox cycling 309
between Pu(IV) and Pu(V) states55. Pu(V) formed via oxidation of Pu(IV) from infiltrating, oxidizing 310
rainwater was transported a short distance then reduced to Pu(IV) via surface mediated reduction. The 311
formation of Pu(IV)-NOM surface complexes may prevent this oxidation step vis-à-vis working 312
hypothesis #2. Evaluation of the chemical/physical changes of the Pu(V) sources over time provides 313
additional insight into these working hypotheses. 314
Retardation of plutonium migration due to formation of ternary surface-NOM-Pu complexes 315
requires that NOM sorption to mineral surfaces is sufficiently strong that some fraction of the NOM 316
remains bound to mineral surfaces. To exam, ine this phenomena, desorption of NOM from the 10% 317
amended sediments was monitored at pH 3, 5, and 10. The fraction of organic carbon desorbed after 24 318
hours was 4.8% ± 0.5%, 12.2% ± 5.9%, 15.9% ± 0.3% at pH 3, 5, and 10, respectively. Thus, in all cases 319
a large fraction of the NOM remained associated with the mineral surfaces or as an insoluble fraction. The 320
increase in NOM desorption with increasing pH is indicative of formation of deprotonated acidic 321
functional groups that facilitate increased desorption with increasing pH. Thus, the natural NOM used in 322
enhanced retardation of Pu in the presence of NOM under the low pH conditions of these experiments, 324
further studies are needed to understand the fraction of NOM causing this effect. 325
326
3.2. Oxidation State Changes in the Sources 327
The NH4PuO2CO3 sources in the two lysimeters were characterized using XAS and compared to
328
the archived source that was preserved under a nitrogen atmosphere. Figure 2 shows the Pu LIII XANES
329
spectra of the three sources compared to PuO2 and Pu(V) references 41
. No significant differences are 330
apparent among the two lysimeter sources and the XANES spectra indicate the presence of Pu(IV). This 331
is based on the fact that white line peak heights are higher for the Pu(IV) complexes, and that a shoulder 332
above the white line typical of the Pu transdioxocation is not present in the lysimeter spectra41. However, 333
the first derivative curve seems to indicate the presence of Pu(V) in the archived source, with a shoulder 334
above the edge which is a multiple scattering feature of the transdioxocation. To verify this hypothesis, 335
solvent extraction was conducted on the sources as previously done for solid phase oxidation state 336
analysis51,54. In the archived source, approximately 50% Pu(IV) and 50% Pu(V) was detected; whereas, 337
more than 80% of the Pu in both lysimeter sources was present as Pu(IV). A linear combination fitting of 338
the XANES spectra confirmed these results and yielded an estimate of 65% Pu(IV) and 35% Pu(V) in the 339
archived source (Rf = 0.008) compared to an estimate of 98% Pu(IV) and 2% Pu(V) in the source after 3 340
years in field exposure with NOM (Rf = 0.008) (Figure S3 and S4). Because the archived source is a 341
mixture, its EXAFS spectrum was not analyzed in detail. However, the archived source spectra is clearly 342
different from the EXAFS spectra of the two lysimeter sources. The presence of Pu-Pu backscattering at 343
3.5 Å in the non phase shifted EXAFS Fourier transform confirms the contribution of PuO2+x type species
344
in the archived source (see Figure S5 in the SI). The formation of Pu(IV) observed in the archived source 345
might be due to alpha radiation, disproportionation, or the absence of oxygen needed to stabilize the 346
Pu(V) compound. Nevertheless, the difference between the archived source and the lysimeter sources 347
suggests that reduction is accelerated in the environment (potentially due to the presence of water, organic 348
matter, and iron). 349
Figure 3 shows the EXAFS spectra of both lysimeter sources with their corresponding Fourier 350
transforms (FT). The best fit parameters are reported in Table 2. At the Pu LIII edge, the presence of
351
zirconium in the lysimeter sample without NOM prevented data acquisition at the LIII edge because the Zr
352
K edge (17998 eV) is too close to the Pu LIII edge (18057 eV). The presence of Zr can be explained by the
353
existence of Zr in the soil and the soil grains found on the filter paper. In this case, the measurement was 354
taken at the LII edge (22266 eV), but the lower absorption cross section led to a weaker signal to noise
355
ratio. Nevertheless, for both lysimeter sources, the spectra are similar. The EXAFS FT moduli of the 356
is fitted by 8 oxygen atoms at 2.30-2.33 Å. Both spectra depict the characteristic Pu-Pu contribution at 358
R+R = 3.5 Å, fitted at 3.78-3.82 Å. A small coordination number is determined for the Pu-Pu peak 359
(around 4), which is a feature of high disorder compared to the well crystallized PuO2. These samples do
360
not show any Pu=O contribution from an oxo shell around 1.80 Å. However, Conradson et al. have 361
demonstrated that samples with small Pu-Pu contributions and several oxygen shells at non-362
crystallographic distances (>2.6 Å) exhibit a negligible number of oxo atoms56. These radial distances are 363
in good agreement with the data reported in the literature for PuO2+x-y(OH)2y.zH2O compounds 56
. A better 364
fit could be obtained by using several Pu-O distances, a shorter distance of approximately 2.2 Å; 365
however, the bond length resolution is limited at 0.22 Å (R = /2k). The shorter distance observed for 366
the unamended lysimeter source (3.30 Å vs 3.33 Å) suggests the presence of shorter Pu-O distances (Pu-367
OH type). Moreover, the inherent disorder at this first oxygen shell, illustrated by a high Debye-Waller 368
factor (around 0.01 Å2) and the small number of Pu atoms (3-4), may be an indication of the presence of 369
colloids56-59. 370
371
3.3. Plutonium(V) Sorption and Reduction Rates on SRS Soils 372
Because the concentration of Pu in the soil was too low for direct spectroscopic analysis, 373
laboratory studies were combined to field study to investigate Pu interaction with the soil. This step also 374
allows to predict Pu behavior at shorter time scale (hours) that is not possible to monitor in field 375
conditions. First, oxidation state changes were monitored over time after adding Pu(V) to the soil to 376
evaluate the hypothesis that downward migration of Pu from the Pu(V) source was hindered due to Pu(V) 377
reduction to Pu(IV). The percentage of Pu remaining in the aqueous phase of the soil suspension (after 378
filtration) as a function of time can be seen in Figure 4. No significant differences were observed between 379
the concentration of Pu in filtered (3k MWCO, ⁓ 4 nm) and unfiltered (⁓ 60 nm) samples, indicating no 380
colloidal particles were present (see Figure S6 in the SI). The fraction of Pu in the aqueous phase 381
decreased with time, regardless of Pu concentration. The data in Figure 4 indicate that Pu is quickly 382
sorbed to the soil, with only 40% remaining in solution after 6 hours and only 3% after 1 day. Hixon et al. 383
and Kaplan et al. studied Pu sorption and reduction by SRS soils with a composition similar to the one 384
used in this study but with a higher Fe concentration (15.9 g/kg). 1, 60 The trend obtained here is in 385
agreement with the data obtained in their studies. After 6 hours, 50% of Pu remained in solution for 386
Kaplan et al. compared to 70 % for Hixon et al. The lower sorption observed in the the Hixon et al. study 387
can be explained by a lower amount of soil in the suspension (5 g/L instead of ⁓25 g/L). 388
At pH 5, it was verified in all samples that > 90% of the Pu in the aqueous phase was present as 389
Pu(V) (Figure S7, Supporting Information). While the aqueous Pu was in the form of Pu(V) at all times, 390
fraction of Pu(IV) leached from the soil concurrently increased over time. This trend confirmed that the 392
reduction of Pu(V) to Pu(IV) is mediated by the soil.51, 54 Similar reduction rates were obtained at both 393
initial Pu(V) concentrations: at 1.1 × 10-10 M Pu the reduction rate was 0.13 ± 0.01 h-1 and at 3.1 × 10-7 394
M Pu, the reduction rate was 0.15 ± 0.02 h-1 (see Figure S8 in the SI). These values are in good 395
agreement with the rate reported by Kaplan et al. at 0.112 ± 0.007 h-1for the same soil concentration (25 396
g/l). In the previous studies using soil from SRS, Pu(V) reduction was assumed to be induced by Fe(II) or 397
Fe(III) bearing minerals and especially the presence of Fe(II) in phyllosilicates.1, 60 In this study, the same 398
behavior wasobserved at a similar rate, even though the total iron concentration was almost two times 399
lower. However, while the overall concentration of iron in the soil was lower compared to the other SRS 400
soil, the concentration of iron extractable by dithionite-citrate-bicarbonate (DCB) is almost the same (⁓ 6 401
mg/g). The DCB treatment determines the amount of amorphous and crystalline iron oxides and 402
phyllosilicate Fe(III). In this soil, this percentage corresponds to 75% of the total iron as confirmed by 403
TEM analysis (see Figure S9 in the SI). These results are consistent with the hypothesis that Pu(V) 404
reduction is induced by iron in the phyllosilicate or the iron oxides. Therefore, the limited migration of Pu 405
from the Pu(V) source in the absence of NOM appears to be due to reduction of the initial source to 406
PuO2+x-y(OH)2y .
zH2O phases and reduction of any Pu(V) released from the source to Pu(IV) by the soil.
407
Similar behavior was previously observed for a field lysimeter containing an initial PuO2(NO3)2(s)
408
source.1 409
410
3.4. Desorption of Plutonium from SRS Soil 411
After confirming the reduction of Pu(V) in the lysimeter soil, desorption experiments using soil 412
sections retrieved from the lysimeters were conducted to examine the influence of NOM on Pu sorption 413
affinity (and by extension mobility). Results from desorption experiments conducted in triplicate after 56 414
days are shown in Figure 5. The concentration of Pu in the filtered (30k MWCO, ⁓ 12 nm) fraction is 415
slightly lower than the concentration in the unfiltered (⁓100 nm) fraction. However, after accounting for 416
uncertainties, the values are similar indicating most Pu was dissolved. An average of all values was used 417
(filtered and unfiltered) to obtain an average equilibrium Pu concentration (values close to the detection 418
limit (6 × 10-12 M) were omitted from the average). Desorption was quantified using an apparent 419
desorption Kd construct, which does not distinguish between solubility and surface desorption processes.
420
The 56-day apparent desorption Kd values (7.0 × 10 4
mL/g with NOM and 1.2 × 103 mL/g without NOM) 421
were greater for the NOM amended soils near the source. Therefore, NOM appears to increase the 422
sorption affinity of Pu, potentially due to the formation of ternary surface complexes as discussed above. 423
Note that soils from these two segments were in contact with the source. Furthermore, spatial 424
Autoradiography images performed on a small samples of soil from both lysimeters indicated the 426
presence of “hot spots” (see Figure S10 in the SI) which may explain the high standard deviation of 427
triplicate measurements. 428
After desorption, the average percent of plutonium remaining in the soil fraction was 99.9%, and 429
log Kd values varied between 3.2 and 4.6 for the NOM-free and NOM-amended soils, respectively. The
430
values are in good agreement with the Kd values obtained previously from batch sorption experiments at a
431
similar pH with SRS soil and desorption experiments from previous lysimeter field studies 61-63. In those 432
studies, the estimated log Kd values based on desorption from batch experiments were 3.95-4.26, while
433
those based on the lysimeters were 3.2-4.2, respectively. Moreover, this is in agreement with values 434
determined by desorption of Pu from kaolinite (log Kd = 3.5-4 at pH 5) 64, 65
. Emerson and Powell 435
observed more Pu extracted after 3 days of aging than after 32 years63. Given there was little change in 436
the apparent desorption Kd values for Pu aged on these soils for approximately 3 years, it appears any
437
aging processes occur on the timescale of decades rather than days. 438
439
4.0 Environmental Significance 440
This study demonstrates that the presence of NOM can have a significant impact on the mobility 441
of Pu in the subsurface through formation of ternary surface-NOM-Pu complexes and/or stabilization of 442
Pu(IV) by NOM that prevents further downward migration of reoxidized Pu(V) formed from incoming 443
rainwater. Overall, the downward migration in these Pu lysimeters was influenced by the initial source 444
oxidation state where the Pu(V) lysimeters in the current work exhibited greater migration than previous 445
studies using Pu(IV) or Pu(III) sources16. The mobilized Pu(V) appears to be reduced to Pu(IV) on the 446
order of hours indicating that downward migration of Pu may be due to cycling between Pu(V) and 447
Pu(IV) as previously observed.1, 55 The form of reduced Pu within the soil may differ depending on the 448
initial oxidation state of the source. Moreover, the presence of NOM in the soil accelerates the reduction 449
of Pu(V) to Pu(IV) and enhances immobilization due to the strong sorption of Pu(IV) onto mineral 450
surfaces leading to behavior like Pu(IV) and Pu(III) amended lysimeters. EXAFS analyses of the sources 451
showed that Pu(V) was reduced to Pu(IV) after 3 years of exposure including in the archived source 452
stored in an inert nitrogen atmosphere, likely as colloids or other PuO2+x-y(OH)2y .
zH2O type compounds
453
with high disorder. Therefore, the mobility of Pu in similar soils at the SRS will be controlled by 1) 454
transformation of the initial source to a PuO2+x-y(OH)2y .
zH2O type phase on the timescale of months to
455
years and 2) reduction of soluble Pu(V) to Pu(IV). Additionally, the presence of NOM appears to both 456
enhance reduction of the Pu(V) source as well as enhance Pu sorption to soils. Therefore, Pu is expected 457
to be less mobile in the presence of NOM under the SRS vadose zone geochemical conditions. 458
Pu migration in surface and groundwater.17, 30 This difference may be attributable to the relative 460
immobility and nature of the organic matter to which Pu is associated (i.e. soluble versus mineral-bound 461
organic matter). The findings from this study can be used to evaluate the potential for Pu mobility in other 462
subsurface environments with organic matter. A critical factor to determine if Pu mobility will be 463
enhanced or retarded in the presence of organic matter is the pH of the pore water which influences Pu 464
speciation and the sorption of organic matter. Sorption of organic matter to mineral surfaces is necessary 465
in order to form the ternary surface-NOM-Pu complexes that were found to retard Pu mobility in this 466 work. 467 468
Acknowledgements
469This research is funded by the Office of Biological and Environmental Research of the U.S. 470
Department of Energy as part of the Subsurface Biogeochemical Research Program under Work Proposal 471
Number SCW1053, Subsurface Biogeochemistry of Actinides. SRNL personnel also received support 472
through contract DE-AC09-08SR22470 with the DOE. XAS experiments were conducted at the Stanford 473
Synchrotron Radiation Laboratory (Beamline 11.2), which is supported by the U.S. Department of 474
Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. 475
We thank Dr. Dai (LLNL) for TEM analysis.
477 478
479
Figure 1: Normalized soil Pu concentrations in the lysimeters containing NH4PuO2CO3(s) sources with
480
and without an NOM ammendment as a function of distance from source. A photograph of the soil 481
column taken prior to segmentation in the NOM amended lysimeter is shown to the right. Note that the 482
dark ring of NOM above the source region. This ring was only near the walls and was not present as a full 483
cross-sectional layer. Detection limits for the NOM amended lysimeter and NOM free lysimeters are 2.3 484
× 10-6 and 1.4 × 10-5, respectively. The x-axis scale is normalized to the maximum Pu concentration 485
18020 18040 18060 18080 18100 18120 N o rma lize d a b so rb a n ce Energy (eV)
NH4PuO2CO3 source/OM; 3 yr in lys. NH4PuO2CO3 ; 3 yr in lys. NH4PuO2CO3; 3 yr in dessicator PuO2+ PuO2 NH4PuO2CO3 source/OM; 3 yr in lys. NH4PuO2CO3 source ; 3 yr in lys NH4PuO2CO3; 3 yr in dessicator PuO2+ PuO2 18020 18040 18060 18080 18100 18120 A/ E Energy (eV) 487
Figure 2: XANES (normalized absorbance, left; first derivative, right) of the two field lysimeter sources 488
and the archived NH4PuO2CO3(s) source compared to Pu(V) aquo complex 41
and PuO2 standards.
489
0 2 4 6 8 10 12
NH4PuO2CO3 source/OM; 3 yr in lys. NH4PuO2CO3 source; 3 yr in lys.
EXAFS amplitud e in k 2 . (k) Wave number k (Å-1) 0 2 4 6
NH4PuO2CO3 source/OM; 3 yr in lys.
FT amplitude
k2
.
(k)
Non phase shift corrected distance(Å) NH4PuO2CO3 source; 3 yr in lys.
Figure 3: k2-weighted EXAFS spectra (left) and corresponding Fourier transform (right) of the two field 490
lysimeter sources following 3 years of field exposure. Experimental spectra in black lines, adjustment in 491
10-1 100 101 102 103 0,0 0,2 0,4 0,6 0,8 1,0 F ra ct io n a q u e o u s Pu (C/C 0 ) time (hours) 493
Figure 4: Plutonium(V) sorption to unamended SRS soils [25 gsoil/L] as a function of time at [ 238
Pu] = 10 -494
10
M (square) and at [242Pu] = 3 × 10-7 M (triangle). 495
Figure 5: Average apparent distribution coefficient (Kd) from desorption experiments in the presence of
497
10 mM NaCl from 100 gsoil/L suspension after 8 weeks. The amount of NOM in the unamended lysimeter
498
is 0.9 % compared to 10% (by weight) for the amended one. Error bars represent the standard deviation of 499
3 replicates. 500
501
Table 1: Principal properties of the lysimeter soils 502
Parameters Lysimeter soil Lysimeter NOM-amended
pH 5.27 4.89
Organic matter % 0.9 10
Sand/slit/clay (wt %) 66/14/20 66/14/20
Surface area (m2.g-1) 14.1 Not measured
Clay mineralogy >95% kaolinite, quartz
Total Fe (mg/g) 8.1 13.2
DCB extractable Fe (mg/g) 6.01 Not measured
503 504
Table 2: EXAFS best fit parameters for both NH4PuO2CO3 exposed sources. Numbers in italics
505
have been fixed. s0 2
is the EXAFS global amplitude factor; e0 is the energy threshold; ε is the
506
average noise, and r(%) is the agreement factor of the fit. 507
508
Sample First shell Second shell
CN R(Å) s2 (Å2) CN R(Å) s2 (Å2) E0(eV) Rfactor (%)
PuO2 crystal structure 40, 56 8 O 2.337 12 Pu
24 O 3.816 4.474 NH4PuO2CO3, after 3 y. in field exposure 8 O 2.30(1) 0.012 3.9 Pu 3.779 0.0033 -3.29 0.005 0.90 NH4PuO2CO3, after 3 y. in
509
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