Mobility of Aqueous and Colloidal Neptunium Species in Field

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Mobility of aqueous and colloidal neptunium species in field lysimeter experiments Kathryn M. Peruski, Melody Maloubier, Daniel I Kaplan, Philip Almond, and Brian A. Powell Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05765 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018

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Environmental Science & Technology

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Mobility of aqueous and colloidal neptunium species

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in field lysimeter experiments

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Kathryn M. Peruski(1), Melody Maloubier(1), Daniel I. Kaplan (2), Philip Almond (2), Brian A.

4

Powell(1,3)*

5

(1)

6

29625

7

(2)

8

(3)

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KEYWORDS: Neptunium, colloid, oxidation, actinide, transport, NpO2

Environmental Engineering and Earth Sciences, Clemson University, Anderson, SC, USA

Savannah River National Laboratory, Aiken, SC, USA 29808 Department of Chemistry, Clemson University, Clemson, SC USA 29634

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ABSTRACT Due to its radiotoxicity, long half-life, and potentially high environmental

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mobility, neptunium transport is of paramount importance for risk assessment and safety.

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Environmental transport of neptunium through field lysimeters at Savannah River Site was

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observed from both oxidized (Np(V)) and reduced (Np(IV)) source materials. While transport

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from oxidized neptunium sources was expected, the unexpected transport from reduced

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neptunium sources spurred further investigation into transport mechanisms. Partial oxidation of

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the reduced neptunium source resulted in significant release and transport into the mobile

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aqueous phase, though a reduced colloidal neptunium species appears to have also been present,

ACS Paragon Plus Environment

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enhancing neptunium mobility over shorter distances. These field and laboratory experiments

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demonstrate the multiple controls on neptunium vadose zone transport and chemical behavior, as

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well as the need for thorough understanding of radionuclide source terms for long-term risk

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prediction.

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Introduction

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Neptunium (Np) is a radioactive element produced through both commercial nuclear 237

24

reactor operations and nuclear weapons production. The longest-lived Np isotope,

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half-life of 2.14 million years, making it an environmental risk-driver for nuclear waste disposal

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and legacy nuclear waste management. Under environmental conditions, Np can occur in a

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range of oxidation states, including (IV), (V), and (VI)

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similar for all elements across the series, with oxidation state governing major chemical behavior

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such as sorption to sediments1, 2. Sorption affinity of actinide elements to sediment follows the

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trend: (IV) > (III) ≈ (VI) > (V)2, 3, making (V) the most mobile of the oxidation states and (IV)

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the least mobile. This is due to the high effective charge of Np(IV) and relatively low effective

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charge of Np(V) as the neptunyl dioxycation, (NpO2+), which limits sorption and complexation

33

affinity2, 3. Vadose zone conditions create an oxidizing environment in which Np(V) would be

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expected and in aqueous solution would likely occur as NpO2+.

1, 2

Np, has a

. Behavior of actinide elements is

35

Actinide elements have also been shown to form colloids in natural waters, either through

36

hydrolysis of actinide cations to form “intrinsic” colloids or through adsorption of actinide

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cations to inorganic or organic constituents in groundwater to form “pseudo” or “associative”

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colloids2, 4-6. Actinides in the (IV) oxidation state show the greatest affinity for formation of


2

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intrinsic colloids, and the (IV) oxidation state is also likely to form pseudo colloids due to its

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high sorption affinity to organic or inorganic matter2, 3, 7. Colloidal actinide species may have

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different environmental behavior than simple cationic actinide species, necessitating that

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environmental transport models account for such potential colloidal actinide species1.

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The Radionuclide Field Lysimeter Experiment (RadFLEx) is an ongoing field study of

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radionuclide transport in the vadose zone at the U. S. Department of Energy Savannah River Site

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(SRS). Four of the 48 lysimeters at the facility contain

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NpIVO2(s) and two of which have NpVO2(NO3). The field lysimeter studies at SRS focus on a

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wide range of radionuclides. While previously published work from RadFlEx has mostly focused

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on plutonium transport8-12, this is the first report of neptunium transport from this field study. A

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difficulty in field-scale studies of radionuclides is a lack of information regarding the source

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material, including origin, chemical form, or physical form, and studies are forced to look at

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environmental transport or source alteration in a retrospective manner, making transport and

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performance assessment modeling more difficult4, 5, 13, 14. Therein lies the unique aspect of the

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RadFLEx experiments, since the initial source materials are well-characterized before and after

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environmental exposure, but are still subject to natural conditions, including variability that

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cannot be accurately represented in a lab scale study.

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Np sources, two of which have

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Overall, these studies at RadFLEx seek to determine the effect of initial source material

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characteristics, particularly oxidation state, on transport of neptunium through the vadose zone

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and to identify potential transport mechanisms. Our conceptual model initially proposed that

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significant Np would be transported from the Np(V) source in the porewater due to the low

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sorption affinity and high mobility of aqueous NpO2+, while Np transport from the Np(IV)


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source would be limited and controlled by the oxidative dissolution of the relatively insoluble

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source material15-17 and subsequent transport of NpO2+ aqueous species in the porewater.

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However, our studies of the Np(IV) lysimeter sediment from both field and laboratory work

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indicate that Np is released from the Np(IV) source not only as a result of oxidative dissolution

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and ion transport, but also in the form of mobile Np(IV) colloidal species. This work captures

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transport of a mobile colloid and a mobile aqueous species in the same environmental system.

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Furthermore, the observations of colloid and redox facilitated transport of aqueous ions in the

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subsurface have direct analogies to the transport of other elements that have both soluble and

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insoluble oxidation states (e.g., iron, manganese, chromium). The objective of this work is to

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compare transport of neptunium in a field lysimeter experiment from Np(IV) and Np(V) sources.

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The relative solubility of Np(V) and insolubility of Np(IV) potentially allow for the comparison

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of the impacts of oxidative dissolution and colloid formation from the NpO2 source. While this

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study specifically focuses on neptunium chemistry, examination of redox mediated transport

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processes extends beyond the general interest in nuclear waste disposition.

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Materials and Methods

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Field Lysimeter Experimental Set-Up

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The lysimeters at RadFLEx are constructed out of 60 cm tall and 10 cm diameter PVC

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pipes, packed with sediment and with a source placed halfway down the sediment column. The

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PVC pipes are maintained in a vertical orientation using secondary containment pipes and are

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housed in an above-ground metal dumpster18. The lysimeters are open at the top and have been

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exposed to natural conditions since 2013, with the lysimeters presented in this work having been

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buried and exposed to rainfall for 3 years, then removed from the facility for sediment and


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source analysis. Tubing at the base of the PVC pipe collected effluent from lysimeters, with

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effluent from the lysimeters monitored quarterly for aqueous neptunium, with measurement of

85

237

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measureable

87

Information).

Np in the effluent of the Np(V) lysimeter as early as 1 year after deployment and no 237

Np in the effluent of the Np(IV) lysimeter after 3 years (See Online Supporting

The lysimeters were packed with a representative sandy clay loam sediment from SRS18,

88 89

19

90

SI Table 1, with full details given in Montgomery et al.19. Notable properties include the organic

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matter content, which is less than 1%, and the sand/silt/clay ratio, which is 66/14/20. Of the clay

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fraction, greater than 95% is kaolinite. The average pH of the collected porewater is 5.12, and

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the dissolved oxygen content of the porewater is consistently close to or at the saturation value of

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8 mg/L. The neptunium source materials used in the field lysimeters were obtained from

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Savannah River National Laboratory, with the NpVO2(NO3) made from an aqueous Np(V) nitrate

96

stock solution, pipetted and dried onto a filter paper18, and the NpIVO2(s) procured from the HB-

97

line production.

98

temperatures >700°C18. A small amount of NpIVO2(s) (~2 mg) was added to a filter paper from a

99

larger batch of the HB-line product18.

100

. Selected physical and chemical properties of the soil used in these experiments are listed in

The oxides produced on the HB-line are high-fired, with calcination

Sediment Analysis

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To determine the spatial distribution of neptunium in the field lysimeters, the lysimeters

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were segmented into approximately 1 cm sections from top to bottom in a HEPA-filtered glove

103

box, with each segment placed into a sealed bag and homogenized by hand before analysis.


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Approximately 20 grams of homogenized sediment (exact weight determined gravimetrically)

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was removed from each bag, distributed evenly across a 4-inch diameter petri dish, and dried for

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2 days at 50 degrees Celsius. Activity of

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direct measurement on a low-energy high purity germanium detector (HPGe). Petri dishes

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containing dried sediment samples were placed directly on the detector and counted for 1 day

109

(86,400 seconds). The activity (A) of

110

from Knoll20:

237

237

Np (in Bq) in each sediment sample is then calculated

=

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Np in each sediment sample was determined using

( ) ∗∗

112

Where E is the detector efficiency, F is the emission fraction, t is the count time, and nG and nB

113

are gross and background counts in the region of interest, respectively. The detector calibration

114

and efficiency, as well as the calculation of the minimum detectable concentration according to

115

Currie21 are presented in the Supplemental Information. The activity is normalized per mass of

116

the sediment sample by dividing by the mass of dry sediment (in grams) to get Bq/g.

117

Imaging

118

The source material from the extracted Np(IV) lysimeter source and from an unexposed

119

NpO2 source that had been maintained in an oxygen-free atmosphere were studied using a

120

Hitachi S4800 high resolution Scanning Electron Microscope (SEM). SEM stubs were prepared

121

by transferring small amounts of source material from the source filter paper using a pipette tip

122

to an SEM stub with carbon tape on top. Stubs were not sputter coated.


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Sediment samples from the Np(IV) lysimeter were also imaged using autoradiography

124

plates. Small samples of sediment (~10 grams) were placed on the autoradiography plate in a

125

plastic bag, attempting to create a flat layer of soil on the imaging plate. The autoradiography

126

plates were sealed and allowed to expose in a sealed, dark environment for 1 week.

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Autoradiography images were altered using photo-editing software to adjust the brightness and

128

contrast of the image. The autoradiography image does not capture alpha decay from

129

which could not travel through the plastic bag containing the sediment, but rather captures the

130

beta decay from 233Pa that grew in from 237Np decay. Thus, the 233Pa beta activity is a signal for

131

the presence of 237Np in the sediment.

132

XANES/EXAFS data acquisition and analysis

237

Np,

133

The initial source and the source recovered after 3 years of exposure were analyzed by X-

134

ray absorption spectroscopy (XAS). Small pieces of filter paper containing the Np sources were

135

cut with a razor blade. The autoradiography of the sources were used to locate the sample.

136

X-ray absorption experiments at the Np LIII edge (17 610 eV) were performed on the 11.2

137

beamline of the Stanford Synchrotron Radiation Lightsource (SSRL) facility. The

138

monochromator was set with the Si(220) crystals. Harmonics were rejected with a Rh-coated

139

mirror with a cutoff energy of 23 100 keV. Energy calibrations were performed simultaneously

140

from the transmission spectrum of a Zr foil (K edge at 17998 eV). EXAFS measurements were

141

performed in fluorescence mode using a 100-element germanium detector (Canberra). Due to the

142

possibility of redox reactions working with actinides under large photon flux, sample were

143

cooled using a liquid nitrogen filled cryostat.


7


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Data processing was carried out using the Six Pack and Athena code22,

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. Dead time

145

corrections was performed measuring the relationship between the incoming count rate and

146

selected channel analyzer readings for each channel using SixPack software22. The Eo energy

147

was identified as the first inflection point. Fourier transformation (FT) was performed between

148

2.0 and 9.7 Å-1 with Hanning windows using the ARTEMIS code23. Spectral noise was

149

calculated using the CHEROKEE code24, 25 by using the Fourier back transform filter above 6 Å

150

corresponding to the noise spectrum. The R factor (%) and the quality factor (QF, reduced χ²) of

151

the fits were provided from ARTEMIS. Phases and amplitudes were calculated using the FEFF9

152

simulation code2625 using the Fm-3m fluorite structure models27.

153

Desorption Studies

154

Six representative sediment samples were chosen for desorption studies, three from the

155

Np(V) source lysimeter and three from the Np(IV) source lysimeter. For the Np(V) lysimeter,

156

sediments from above and below the source were used, but for the Np(IV) lysimeter,

157

concentrations above source were too low for desorption studies. Between 0.25 and 0.5 grams of

158

dry sediment from each segment was placed in 15 mL centrifuge tubes with 10 mL of 10 mM

159

NaCl background solution and allowed to desorb for times ranging from 15 minutes to 1 month.

160

All experiments were performed in triplicate and were performed on the benchtop. At each

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sampling time point, the samples were centrifuged to remove particles >145 nm according to

162

Stoke’s Law, as described by Jackson28(15 minutes at 4500 rpm in a Allegra X22R centrifuge

163

and 4250 rotor). , at which point the aqueous phase was removed for analysis. The aqueous

164

samples were diluted to 2% nitric acid for analysis using inductively coupled plasma mass

165

spectrometry (ICP-MS). Concentration of 237Np in aqueous samples was converted to activity of


8

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166

237

167

was determined for each sample and the Kd was determined using the equation:

Np in the sample for comparison to solid phase activity. The total aqueous activity of

(

237

Np

( − ) ∗ () )= ∗ !"# ()

237

168

Where Nps and Npaq are the activity of

169

phase, respectively. Initial solid phase activity of

170

measurements of the sediment samples. The standard deviation applied to Kd values is based on

171

triplicate measurements.

172

Results and Discussion

173

237

Np in the initial solid phase and the final aqueous 237

Np was taken from low-energy HPGe

Np transport from oxidized and reduced sources

174

Transport of neptunium was observed from both Np(IV) and Np(V) sources after the 3237

175

year field study (Figure 1). For the Np(V) source lysimeter,

176

lower half of the lysimeter with an average concentration of 3.4 Bq/g, and upward transport of

177

neptunium also occurred at least 13 cm above the source, likely due to diffusion through the pore

178

space.

179

experiments8, 9, 11, 29. Unsaturated hydraulic conductivity measurements of this sediment indicate

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two domains where upward diffusional transport of neptunium could have occurred in a low

181

hydraulic conductivity domain (Online Supporting Information, Figure S1). However, this

182

hypothesis is currently under investigation. While below source concentrations are relatively

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high and consistent with distance from the source, the concentration of

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exponentially above the source and reaches detection limits.

Np was detected throughout the

Similar upward transport of Pu and Sr has been observed in previous lysimeter

237

Np decreases


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27 Np(IV) Source

24

MDC

21

Np(V) Source 18

Source

15 12 9

Distance from Source(cm)

6 3 0 -3 -6 -9 -12 -15 -18 -21 -24 -27 -30 -33 1.00E-03

1.00E-02

1.00E-01

1.00E+00

1.00E+01

Concentration of Np-237 in soil (Bq/g)

186 237

187

Figure 1. Sediment

Np concentration profile cores recovered from the Np(V) and Np(IV)

188

amended lysimeters. MDC = Minimum detectable concentration and error bars represent HPGe

189

gamma counting error (some error bars are hidden by the data symbol),. except for those samples


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with symbol “X”, which have error bars that represent the standard deviation of triplicate sample

191

measurement in the Np(IV) source lysimeter.

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For the Np(IV) source lysimeter, peak concentrations of 237Np occurred 2.5 cm below the

193

source, with concentrations decreasing toward the base of the lysimeter and reaching detection

194

limits approximately 20 cm below the source (Figure 1). Upward transport was also observed in

195

the Np(IV) lysimeter as far as 3.5 cm above source, though samples are near detection limits so

196

transport may be more extensive than measured. It is notable that concentrations of 237Np in the

197

Np(IV) lysimeter are 1-2 orders of magnitude lower than the Np(V) lysimeter, despite having

198

initially equal activity of

199

expected due to the low solubility of NpO2(s). Transport of Np(IV) aqueous species 20 cm below

200

the source would be unlikely due to the high sorption affinity of Np(IV) to sediment, so the

201

presence of elevated neptunium concentrations 5 to 20 cm and 27 to 30 cm below the source is

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supportive of the process in which the Np(IV) source underwent oxidation to the more mobile,

203

oxidized NpO2+ aqueous species. Oxidation of Np(IV) to Np(V) is consistent with speciation

204

modeling assuming atmospheric conditions at pH 5 (Geochemist Workbench v11,

205

thermo.com.v8r6+.dat database). Desorption experiment described below will demonstrate that

206

the sediment-bound neptunium in these distal samples behave more similarly to Np(V) than

207

Np(IV). The significantly lower concentrations of 237Np in the Np(IV) lysimeter sediment profile

208

suggest only partial oxidation of the source. This was confirmed using x-ray absorption

209

spectroscopy (XAS) discussed below.

210 211

The

237

237

Np in the sources. Very little transport from the Np(IV) source was

Np sediment concentrations in the Np(IV) lysimeter showed a large degree of

variability in the near-source region. Triplicate measurements were performed for samples


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directly above and below source, with replicates having the largest standard deviations in the

213

samples directly below the source, decreasing with distance away from the source (Figure 1).

214

Above-source samples had almost no variability in replicates. The variability in near-source

215

sediment samples were due to “neptunium hot spots” in the sediment. The heterogeneous

216

distribution of

217

sediments (Figure 2). The autoradiography image of a sediment sample 3 cm below the Np(IV)

218

source shows many small, dark spots, spread randomly throughout the sample (Figure 2),

219

consistent with individual particles with high levels of radioactivity, rather than homogenous

220

activity throughout the sediment samples.

237

Np below the Np(IV) source is supported by autoradiography images of these

4 cm

221 222 223

Figure 2. Autoradiography image (left) of the sediment sample (right) collected 3 cm below the

224

Np(IV) source. Areas high activity are circled in both the radiography image and photograph.

225

Alteration of the NpIVO2 source


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The SEM image of the initial NpO2 source shows well-defined cubic crystals, while the

227

source material that was in the field shows mostly amorphous surfaces with the underlying cubic

228

structure remaining intact (Figure 3). The SEM images demonstrate that morphological changes

229

occurred at the surface of the NpO2 source as a result of environmental exposure. The reduced

230

crystallinity and structural changes at the surface of source particles suggest potential oxidation

231

of the surface of NpO2. Formation of an oxidation layer on actinide metal or oxide solid phases

232

has also been observed in a number of other environmental systems. The oxidation layer both

233

produces a surface with more labile (i.e,, prone to dissolution or desorption) and soluble

234

oxidation states and also protects the inner reduced core of the particles30-32. Surface oxidation of

235

Np(IV) solids to soluble and labile Np(V) species could account for observed mobility of

236

neptunium in the Np(IV) lysimeter. If so, the sediment associated Np should have similar

237

mobility to that from the Np(V) source, which is explored in the desorption tests described

238

below.

239

240

Figure 3. SEM images of NpO2 before (right) and after (left) being buried in a vadose zone

241

sediment for three years.


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X-ray absorption spectroscopy data further probes the alteration of the NpO2 source due

243

to environmental exposure. The Np LIII XANES data for both initial and exposed NpO2 sources

244

is compared to a Np(IV) reference (Figure 4), with no significant difference between sources,

245

confirming the predominance of Np(IV) in the source even after 3 years of environmental

246

exposure. The first derivatives presented in the insert of Figure 4 suggests the presence of both

247

Np(IV) and Np(V) in the sample. However, Denecke et al.,33 also showed that this multiple

248

scattering contribution is not easy to distinguish and XANES analysis can be inconclusive,

249

especially with a heterogeneous sample. The confirmation of Np(V) requires EXAFS analysis.

250

251

Figure 4. Np LIII edge XANES spectra of the initial NpO2 sources and the one exposed to

252

lysimeter for 3 years with Np(IV) reference. The inset shows the first derivative curves.


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253

EXAFS analyses were limited by the unintended presence of near 500 ppm plutonium in

254

the sample (Pu LIII edge at 18057 eV), meaning that only the first shell can be fitted with

255

accuracy. Figure 5 shows the EXAFS spectra of the both sources with their corresponding

256

Fourier transforms (FT). The best fit parameters are also reported in Table 1. The EXAFS FT

257

moduli of the samples display a coordination shell at R+∆R = 1.8 Å (non phase shift corrected

258

distance), which is fitted by 8 oxygen atoms at 2.35 Å. Moreover, a peak appears at R+∆R = 3.7

259

Å, this shell could be characteristic of the Np-Np path at a distance of 3.85 Å. These radial

260

distances are in good agreement with the data reported in the literature for NpO2(s)34-36 even after

261

exposure to the field for 3 years. Chollet et al.,35 have demonstrated by fitting only the first shell

262

that the presence of NpO2+x could be determined by an increase of the Debye-Waller factor σ2

263

from 0.002-0.004 to 0.01. In our study, the Debye-Waller factor are equal to 0.005 – 0.0065

264

(Table 1). There is no evidence of a shorter distance at R+∆R = 1.4 Å featuring the presence of

265

Np(V) in the sample. The addition of an additional scattering shell at around 1.85 Å

266

(coordination number around 0.5) leaded only to a decrease of the reduced χ- squared from 0.02

267

to 0.013. Tayal et al.,37 have demonstrated NpO2+x existence using EXAFS and XRD with an O

268

shell around 1.85 – 1.9 Å, multisite Np-O. Tayal et al.,37 have demonstrated NpO2+x existence

269

using EXAFS and XRD with an O shell around 1.85 – 1.9 Å, multisite Np-O.

270

Table 1. Best-fit parameters for EXAFS analysis Sample E0 (eV)

CN 8O

First shell R(Å) s2 (Å2) 2.35

17613.0

8O

2.35(1)

NpO2 crystal structure 34

NpO2, initial source

CN 12 Np 24 O

Second shell R(Å) s2 (Å2) 3.84 4.51

0.0065

∆E0(eV)

ε

Rfactor (%)

3.390

0.0020

2.0


15


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NpO2, after 3 y. in field exposure

17613.2

8O

2.35(1)

0.0051

3.390

0.0035

1.1

Np(IV) aquo

17614.1

-

-

-

-

-

-

271

While the presence of a small amount of Np(V) cannot be definitively ruled out, the

272

initial source material is composed primarily of NpO2, as is the environmentally exposed source.

273

The SEM image of the initial source (Figure 3) supports the idea that the source material is NpO2

274

with the cubic structure38. It is likely that the sources appear to have similar compositions

275

because if any Np(IV) were oxidized on the surface over time in the lysimeter, the NpO2+

276

aqueous species formed would rapidly be leached by the porewater and transported through the

277

sediment, limiting the amount of Np(V) in the source.

278

Figure 5. k-weighted EXAFS spectra and corresponding FT at the Np LIII edge of the initial

279

source and after 3 years field exposure. Experimental spectra in black lines and fitting in dots.

280

Role of colloids and oxidation in transport from reduced neptunium source


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281

Observed desorption coefficients from the Np(V) source lysimeter are between 16-72

282

mL/g (Figure 6), which agrees with previous measurements of Np(V) sorption to SRS clayey

283

sediments19, 39. The Np(V) lysimeter sediments show consistent behavior both above and below

284

the source and develop a baseline of behavior for Np(V) in the SRS sediment used in the

285

lysimeters. Observed desorption coefficients from the Np(IV) lysimeter do not show the same

286

consistent behavior as a function of depth in the lysimeter. The desorption coefficients are 810

287

and 1333 mL/g for 0.5 cm and 2.5 cm below source, respectively. The desorption coefficient for

288

the sediment 15 cm below the Np(IV) source is 10 mL/g (Figure 6), which is in agreement with

289

previous measurements of Np(V) sorption to SRS clayey sediments19,

290

measured Kd values in the Np(V) lysimeter, supporting the hypothesis that Np transported farther

291

away from the Np(IV) source is Np(V) aqueous species. The higher Kd values near the source

292

indicate a stronger sorbing species of Np, likely Np(IV), and these values in the Np(IV) near-

293

source region are similar to Pu(IV) Kd values40, 41, suggestive of the presence of Np(IV) just

294

below the source.

39

and is similar to

1.0E+04 Np(IV) Lysimeter Np(V) Lysimeter

Kd (mL/g)

1.0E+03

1.0E+02

1.0E+01

1.0E+00 0.5 cm below source

2.5 cm below source

15 cm below source

5 cm above source

15 cm below source

25 cm below source

295


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Figure 6. Sediment-water distribution coefficients, Kd values, of Np after 1-month desorption of

297

lysimeter sediments

298

Behavior of Np on the contaminated sediments was also explored using acid leaching, in

299

which sediment samples from various locations in the lysimeters were allowed to leach in either

300

1M or 8M HNO3 for 7 days. Samples from the Np(V) source lysimeter show leachable fractions

301

greater than 0.70 in 1M HNO3 (Online Supporting Material Figure S2). One sample from the

302

Np(IV) lysimeter, approximately 15 cm below the source, also has a leachable fraction greater

303

than 0.80 (Online Supporting Material Figure S2), which agrees well with the Np(V) lysimeter

304

samples, supporting the idea that Np in the lower region of the Np(IV) source lysimeter is

305

Np(V). Samples from the near-source region in the Np(IV) lysimeter had leachable fractions of

306

neptunium less than 0.07 for both 1M and 8M HNO3 (Online Supporting Material Figure S2).

307

High-fired actinide oxides are known to be very insoluble, even in strong acid, with reports of

308

NpO2 requiring the use of hot 10M HNO3 or even HF for dissolution42. The inability to leach the

309

Np present on near-source sediments from the Np(IV) lysimeter, even with high molarity acid,

310

suggest the presence of colloidal Np(IV) species that are strongly associated with sediment

311

particles.

312

Environmental Significance

313

Observed mobility from initially oxidized and reduced Np sources in field-scale studies at

314

Savannah River Site highlight the mobility of Np(V) in the subsurface, but also emphasize the

315

potential mobility of Np from reduced solid phases, such as NpO2, due to source oxidation and

316

colloidal behavior. The NpO2 source system displays multiple mobile species, each with their

317

own mobility through SRS sediments.

Though direct measurement of the proposed NpO2


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318

colloids is not possible due to the low concentration of Np in the system, we proposed the

319

following conceptual model of Np transport in the Np(IV) lysimeter:

320

1. Formation of an amorphous layer on the NpO2 surface which could lead to release of NpO2 colloids or oxidation to form a surface layer of Np(V)

321 322

2. Transport of NpO2 colloids and mobile NpO2+ to produce the observed downward

323 324

mobility. 3. Retention of the NpO2 colloids within the first 2 cm of depth followed by surface

325

oxidation and leaching as described in #2 above.

326

Transport of neptunium as far as 20 cm below the Np(IV) source is likely due to

327

oxidation of Np(IV) to the more mobile Np(V) aqueous species, which is readily desorbed from

328

the sediment in laboratory studies and behaves similarly to the Np(V) lysimeter sediments.

329

Closer to the source region, transport in the Np(IV) lysimeter can be attributed to colloidal

330

Np(IV) species that have been removed from the source, strongly sorbing to sediments just

331

below the source in a heterogeneous manner. Data collected in this work cannot differentiate

332

between Np(IV) transport as an intrinsic or pseudo-colloid and future studies will attempt to

333

differentiate between the two. The behavior of the likely Np(IV) contaminated sediments is

334

distinctly different from Np(V)-contaminated sediments and is more similar to that of Pu(IV)-

335

contaminated sediments40, supporting the hypothesis that Np(IV) species are present. Transport

336

of Np(IV) species over a few centimeters is in agreement with plutonium transport studies in this

337

sediment, which report movement of Pu to a few centimeters of depth below the source as well8-

338

11, 29

339

zone is a process controlled not only by oxidation state, but presence of reduced colloidal species

. Laboratory and field work support the concept that mobility of neptunium in the vadose


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Page 20 of 24

340

as well, stressing the importance of continued understanding of source terms in the environment,

341

their interaction with sediment and groundwater, and potential for multiple aqueous species with

342

different mobility, for prediction of environmental transport of actinides.

343 344 345 346 347

ASSOCIATED CONTENT

348

Supporting Information.

349

AUTHOR INFORMATION

350

Corresponding Author

351

*Brian A. Powell, [email protected], office (864) 656-1004

352

Author Contributions

353

The manuscript was written through contributions of all authors. All authors have given approval

354

to the final version of the manuscript.

355

ACKNOWLEDGMENTS

356

Soil and source analysis along with model development is based upon work supported by the

357

U.S. Department of Energy (DOE) Office of Science, Office of Basic Energy Sciences and

358

Office of Biological and Environmental Research under Award Number DE-SC-00012530.

359

Effluent analysis is based upon work supported by Savannah River Remediation under project

360

SRRA021685SR. SRNL personnel also receive support through contract DE-AC09-08SR22470


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with the DOE. XAS experiments were all performed at the Stanford Synchrotron Radiation

362

Lightsource (Beamline 11.2), which is supported by the U.S. Department of Energy, Office of

363

Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515.

364 365 366 367 368

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Table of contents figure 152x117mm (150 x 150 DPI)


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Mobility of Aqueous and Colloidal Neptunium Species in Field

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