In situ spectroscopic evidence for neptunium(V)-carbonate inner-sphere and outer-sphere ternary surface complexes on hematite surfaces.

Abstract

Np(V) surface speciation on hematite surfaces at pH 7-9 under pC2 = 10(-3.45) atm was investigated using X-ray absorption spectroscopy (XAS). In situ XAS analyses suggest that bis-carbonato inner-sphere and tris-carbonato outer-sphere ternary surface species coexist at the hematite-water interface at pH 7-8.8, and the fraction of outer-sphere species gradually increases from 27 to 54% with increasing pH from 7 to 8.8. The results suggest that the heretofore unknown Np(V)-carbonato ternary surface species may be important in predicting the fate and transport of Np(V) in the subsurface environment down gradient of high-level nuclear waste respositories.

Full text

Page 1

In Situ Spectroscopic Evidence for
Neptunium(V)-Carbonate
Inner-Sphere and Outer-Sphere
Ternary Surface Complexes on
Hematite Surfaces
Y U J I A R A I , *, †P . B . M O R A N ,‡ , §
B . D . H O N E Y M A N ,‡A N D J . A . D A V I S†
Department of Entomology, Soils and Plant Sciences, 270
Poole Agricultural Center, Clemson University, Clemson,
South Carolina 29634-0315, Department of Environmental
Science & Engineering, Colorado School of Mines, Coolbaugh
Hall, Golden, Colorado 80401-1887, and Department of
Chemistry and Geochemistry, Colorado School of Mines,
Golden, 1500 Illinois Street, Golden, Colorado 80401
Np(V) surface speciation on hematite surfaces at pH 7-9
under pCO2) 10-3.45atm was investigated using X-ray
absorptionspectroscopy(XAS).InsituXASanalysessuggest
that bis-carbonato inner-sphere and tris-carbonato outer-
sphere ternary surface species coexist at the hematite-
waterinterfaceatpH7-8.8,andthefractionofouter-sphere
species gradually increases from 27 to 54% with increasing
pH from 7 to 8.8. The results suggest that the heretofore
unknown Np(V)-carbonato ternary surface species may be
important in predicting the fate and transport of Np(V) in
the subsurface environment down gradient of high-
level nuclear waste respositories.
Introduction
237Neptunium(Np)ispresentinlowconcentrationsinhigh-
level nuclear waste, but because of its long half-life, high
radiotoxicity, and low sorption reactivity, it is considered an
important radionuclide that may govern radiation exposure
tohumansseveralthousandsofyearsafterrepositoryclosure
(1). Proposed underground storage of long-lived actinide
isotope waste (2) raises questions about the long-term fate
of transuranic elements in deep geologic repositories (e.g.,
Yucca Mountain, NV). Np(V) is expected be highly soluble
and mobile in low-temperature aqueous-geochemical en-
vironments(3).Thus,knowledgeofmineral-waterinterfacial
speciation of Np is critical in assessing the long-term risk
related to the fate and transport of this element. In oxic
environments, Np is typically present in the pentavalent
oxidation state (2, 4), and adsorption of the neptunyl
oxocation (Np(V)O22+) on mineral surfaces presumably
retards Np(V) transport in subsurface environments (5).
Several researchers have investigated Np(V) adsorption on
aluminum oxide, silica, kaolinite, montmorillonite, magne-
tite, hematite, hydrous aluminum oxide, hydrargillite, man-
ganite, and hausmanite (6-16) and natural sediments/soils
(10, 17-20). Effects of pCO2and pH on Np(V) adsorption on
hematite/amorphous iron oxyhydroxide/manganite/haus-
manite surfaces were previously studied. Whereas Np(V)
sorption generally increased with increasing pH in a carbon-
ate-free system, adsorption only occurs at pH 7-9.5 in air-
equilibrated systems (7, 9, 10, 15, 16). Previous surface
complexation model studies by Kohler et al.(9) indirectly
suggested the presence of inner-sphere Np(V)-carbonate
ternary species on hematite/goethite surfaces in air-
equilibratedsystems.However,nomolecular-scaleevidence
has been reported to support the theory, while several XAS
studieshavefrequentlydocumentedtheformationofU(VI)-
carbonato ternary surface species on hematite/imogolite/
calcitesurfaces(21-24).Carbonateisoneofmostimportant
ligandsthataffectsNp(V)speciationingroundwaterbecause
ofitscommonabundanceanditspropensitytoformaqueous
carbonate complexes (25, 26). In this study, we hypothesize
the formation of Np(V)-carbonato ternary surface species
on hematite surfaces. To test the hypothesis, surface spe-
ciation was examined as a function of pH using extended
X-ray absorption fine structure spectroscopy.
Materials and Methods
Hematite was synthesized using the method described by
MatijevicandScheiner(27).Hematiteprecipitateswerekept
in low ionic strength solution (e0.001 M NaClO4) prior to
use.TheN2Brunauer-Emmett-Teller(BET)surfaceareaof
thefreeze-driedpowderswas43.8m2/g.PowderXRDshowed
that the hematite was crystalline with no evidence for
goethite. The isoelectric point occurs at pH ∼ 9.8.
The neptunium-237 (237Np) stock solution, in secular
equilibrium with its daughter protactinium-233 (233Pa), was
preparedin4MnitricacidbyIsotopeProductsLaboratories.
With the exception of233Pa, the237Np solution was free of
radioimpurities.
SampleswereanalyzedusingaBeckmanCoulterDU800
UV/vis spectrophotometer with a wavelength interval of 0.1
nmandascanningspeedof120nmmin-1.Eachsamplecell
was made of optical glass with a 10 mm path length and a
septumsealableopening.Near-infrared(NIR)spectroscopic
analysis showed that a dilute237Np stock solution (2 × 10-4
Min20mMnitricacid)consistsofthepentavalentoxidation
state. A characteristic band at 980 nm and a small band at
617 nm confirm the presence of the uncomplexed neptunyl
(NpO2+) species. The spectrum lacked any evidence of the
characteristic absorption bands for Np(IV) at 723 and
960 nm.
SpeciationCalculations.Aqueousspeciationcalculations
were completed with the equilibrium speciation program,
FITEQL 4.0 (28), and the neptunium thermodynamic data
given in Guillamont et al. (29). Supporting Information (SI-
1) lists the relevant Np(V) thermodynamic data used in the
calculations.
ExtendedX-rayAbsorptionFineStructureSpectroscopic
Measurements. Np(V) surface speciation on hematite sur-
faces was studied as a function of pH (7, 8, and 9) under the
following reaction conditions: suspension density ) 0.3 g
L-1, [Np]T) 4-5 µM, 22 ((2) °C, and I ) 0.1 M NaClO4. In
addition to samples, Np(V) retention in controls and filter-
ation equipment was tested. We found negligible amount of
adsorption in these systems. Sufficient amounts of 10 mM
NaHCO3solution were added to achieve the desired bicar-
bonateconcentrationinequilibriumwiththepartialpressure
of carbon dioxide gas in air (pCO2) 10-3.45atm) at specified
experimental pH values.
* Correspondingauthorphone: (864)656-2607;fax: (864)656-3443;
e-mail: yarai@clemson.edu.
†Clemson University.
‡DepartmentofEnvironmentalScience&Engineering,Colorado
School of Mines.
§Department of Chemistry and Geochemistry, Colorado School
of Mines.
10.1021/es062468t CCC: $37.00
Published on Web 05/04/2007
 xxxx American Chemical SocietyVOL. xx, NO. xx, xxxx / ENVIRON. SCI. & TECHNOL.9A
PAGE EST: 5

Page 2

Withuseofaqueousspeciationcalculations,thesesystems
were calculated to be undersaturated with respect to
amorphousNpO2OH(s)andcrystallineNaNpO2CO3‚3.5H2O
(s) at all three pH values (Table 1), and neptunyl hydroxide
NpO2(s) at pH 6.98, but slightly supersaturated for neptunyl
oxide, NpO2(s), at pH 8.28 and 8.81 (Table 1). However,
Np(V) hydroxide precipitates were not observed in any of
the samples, as indicated by the results of our XAS analyses
(see the Results and Discussion section), and this result is
likelybykineticlimitationsintheprecipitationofcrystalline
Np(V) oxides.
Samples were prepared in 1 L polycarbonate bottles.
Samples were stirred under humidified air for 24 h prior to
Np(V) addition. During this pre-equilibration, each sample
pH was periodically adjusted to the desired value. After 24
h, a 0.036 M acidic Np(V) stock solution was slowly added
toeachsample(30µLorlessattime)toavoidlocalsaturation.
The samples were stirred for an additional 24 h while being
maintained at the chosen pH values. Small aliquots were
passed through 0.1 µm Acrodisc syringe filters and analyzed
for alpha activity by a Packard 2500TR liquid scintillation
counting (LSC) system and Ultima Gold XR LSC cocktail.
Sorption was determined by difference using the total and
final Np(V) solution activities with a relative standard
deviation of (1%. A Thermo Orion Ross combination
electrode calibrated with NIST certified pH buffers (4.000,
7.000,and10.000)wasusedforallpHmeasurements.Sample
pH was adjusted using hydrochloric acid and sodium
hydroxide. The bulk of the solution phase was decanted and
theremainingsamplesuspensionswerecentrifugedat10000
rpm for 60 min.
Thesampleswereloadedincustom-designedsamplecells
that consist of 3 mm Delrine sample holders, which were
then sealed with Mylar/polycarbonate windows and two
additionalaluminumcontainments.Room-temperatureNp
LIIIedge (17610 eV) fluorescence spectra were collected at
the Stanford Synchrotron Radiation Laboratory (SSRL)
beamline11.2(Si(220)doublecrystalmonochromator)using
a Canberra 13-element Ge detector array equipped with a Y
6µx filter and several Al foils. A zirconium reference foil was
used to calibrate at the Zr K-edge absorption edge energy
positions (17998 eV).
The program FEFF 6 (30) was used to estimate back-
scatteringphasesandamplitudefunctionsofsinglescattering
(SS) Np-Oax, Np-Oeq, Np-C, and transdioxo Np-Oax
multiple scattering paths (MS), which were derived from
structural refinement data for Np substituted andersonite
(Na2CaNpO2(CO3)3‚5.33H2O) (31). A Np-Fe SS path was
estimated from the structural refinement data of Np/Fe
substituted phuralumite (Fe2(OH)2(PO4)2(NpO2)3(OH)4‚10H2O)
(32).XASdatareductionandanalyseswereperformedusing
theIFEFFITenginebasedinterface,SixPACK(33).k3-weighted
Fourier-transformed XAS spectra were fit in R-space over
the range of 1-4.5 Å.
In the fit, fixed values for CN and R values for transdioxo
neptunium MS paths (NpdOax1dNpdOax2) was correlated
with SS of NpdOax, and σ2of transdioxo MS paths were
estimatedusingthesimilartheoreticalcalculationdescribed
inHudsonetal.(34)(2×σ2oaxand2×Roax).∆Eowasallowed
tofloatandlinkedtoallshellsduringthefit.Tocomparethe
differences in fit, unfixed σ2values for Np-C and Np-Fe
shellsfromthepreliminaryfitwereaveraged,andthesevalues
(i.e., Np-C: 0.007 and Np-Fe: 0.005) were used for all
samples. To account for the changes in amplitude of two
Np-OdistMSpaths(3and4legged),SSandMScontributions
of distal oxygen (Odist) atom of carbonate ions ((1) Np-Odist
ss, (2) Np-C-Odist (3 legged MS), and (3) Np-C-Odist (4
legged MS)) were included in the fit with a Np-OdistSS path.
Theσ2forNp-OdistMSpathswereexplicitlycalculatedfrom
σ2of Np-OdistSS path by summing the disorder parameters
ofeachSSpath(Hudsonetal.,1996).Basedonthefollowing
assumptions, σ2c-o,, σ2odist) σ2Np-c(0.007 Å2), σ2for Np-
OdistMS paths was fixed at 0.008 Å2.
A maximum of four putative surface species (binary,
mono-carbonato,andbis-carbonatoinner-sphere,andtris-
carbonato outer-sphere surface species) were considered in
thefitasAraiandco-workershavepreviouslydemonstrated
in the XAS analyses of U(VI) adsorbed on imogolite surfaces
(21). Amplitude reduction factors for Np-C and Odistshells
were assumed to originate from four different fractions (F)
of Np surface species (i.e., binary (Fbin), mono-carbonato
(Fmono), and bis-carbonato inner-sphere (Fbis), and tris-
carbonato outer-sphere surface species (Ftris)). Based on the
assumption,wesetthe“totalfraction(F))1”asasummation
of all fractions of surface species (i.e., 1 ) Fbin+ Fmono+ Fbis
+ Ftris). The fractions for each mono- and bis-carbonato
species (i.e., Np-C SS and Np-Odist SS) were defined as
“Fmono*So2*1” and “So2*Fbis*2”, respectively. For the tris-
carbonatoouter-spheresurfacespecies,“So2*Ftris*3”wasused
only for defining the Np-C SS and Np-OdistSS paths. Based
onapreliminaryfit,wefoundaNp-Feinteratomicdistance
(∼3.45 Å) corresponding to inner-sphere Np bidentate
mononuclearcoordinationwiththeironoctahedralstructure
(i.e., one Fe atom is coordinated). Therefore, we used
“So2*1*Fbin”,“So2*1*Fmono”and“So2*1*Fbis”fortheNp-Fepath
for non-, mono-, and bis-carbonato inner-sphere surface
species, respectively.
To facilitate the comparison under different reaction
conditions,coordinationnumbersandDebye-Wallerfactors
forsomeshellswerefixed(i.e.,CNofNp-Oax: 2,σ2ofNp-C;
0.007 Å2, σ2of Np-Fe; 0.005). So2was fixed at 0.9. The other
coordination numbers, Debye-Waller factors, and inter-
atomic distances were allowed to vary.
Results and Discussion
Neptunyl Aqueous Speciation Calculations. Equilibrium
speciation calculations were conducted for the reaction
TABLE 1. Least-Squares Analyses of Np LIII-Edge Bulk XAS Spectraa
samplesOax
Oeq
C FeOaxMSOdistMSFbin
Fmono
Fbis
Ftris
R factorlog SI
pH 6.98
[Np(V)]i) 5
Γ ) 2052
pH 8.28
[Np(V)]i) 4
Γ ) 1636
pH 8.81
[Np(V)]i) 5
Γ ) 944
CN
R (Å)
σ2(Å2)
CN
R (Å)
σ2(Å2)
CN
R (Å)
σ2(Å2)
2*
1.875(5)
0.001(3)
2*
1.886(5)
0.0012(3)
2*
1.887(6)
0.0012(4)
6(1)
2.47(1)
0.009(2)
5.1(9)
2.49(1)
0.0081(2)
5(1)
2.49(1)
0.009(2)
----
2.95(3)
0.007*
----
2.90(3)
0.007*
----
2.93(3)
0.007*
1*
3.45(2)
0.005*
1*
3.47(3)
0.005*
1*
3.41(3)
0.005*
2*
3.750
0.002
2*
3.772
0.0023
2*
3.775
0.0025
----
4.26(6)
0.008*
----
4.21(5)
0.008*
----
4.23(5)
0.008*
000.73((0.25)0.27((0. 25)0.0158
-0.124
-2.769
-3.624
0.708
-0.63
-2.792
0.454
-0.32
-3.046
000.61((0.25)0.39((0.25) 0.0034
000.46((0.23)0.54((0.23)0.0134
aCN: Coordination number. R: Interatomic distance (Å). σ2: Debye-Waller factor (Å2). [Np(V)]I: Initial Np(V) concentrations (µM). Γ: Surface
coverage (mg kg-1). Fit quality confidence limit for parameters: Np-C/Fe shell CN: (10% (Clark et al., 1996), *: Fixed parameter. Parameters for
three OdisMS paths are summarized into one. Fbin, Fmono, Fbis, and Ftris: Fraction of binary, mono-, bis-, and tris-carbonato Np(V) ternary surface
species. Saturation index values (Log SI) were estimated for crystalline NpO2(s), (Log K ) -1.8 (37)), crystalline NaNpO2CO3‚3.5H2O(s) (Log K )
-11.156 (29)) and am. NpO2OH(S) (Log K ) -5.3 (37)) (from top to bottom; NpO2(S), crystalline NaNpO2CO3‚3.5H2O(S), and am. NpO2OH(S)).
B9ENVIRON. SCI. & TECHNOL. / VOL. xx, NO. xx, xxxx

Page 3

conditions ([Np(V)] ) 4 µM, I ) 0.1 M NaClO4, 25 °C, and
pCO2) 10-3.5atm) in XAS sample preparation. The results
showthatthepredominantNp(V)aqueousspeciesisNpO22+
at pH 4-8, and the activity of carbonato complexes (i.e.,
NpO2(CO3)2OH4-, NpO2(CO3)23-, NpO2CO3-, and NpO2-
(CO3)35-) gradually become significant aqueous species at
pH > 7.5 (SI-2).
Extended X-ray Absorption Fine Structure Analyses.
Figures 1a and 1b show the k3-weighted EXAFS spectra of
Np(V)-adsorbed hematite samples. Fit results are shown in
Table 1 (in units of Å). Interatomic distances are corrected
for phase shift unless otherwise mentioned in the text. The
structuralparametersofallsamplescontaintwoaxialoxygen
distances at approximately 1.88 Å and five to six equatorial
oxygen distances at ∼2.5 Å, indicating the presence of a Od
Np(V)dOtransdioxostructure.Nolossofaxialoxygenatoms
indicatethatbeam-inducedreductionofNp(V)didnotoccur
duringtheXASdatacollection.TheseNp-Oax/Oeqdistances
can be clearly observed in the Fourier transforms (FTs) at
∼1.75 and 2 Å, R + ∆R (Figure 1b). Additional FT peaks are
also observed at ∼2.6, 3.1, and 3.8 Å, R + ∆R, which can be
attributedtoC,Fe,anddistalO(incarbonategroups)shells,
as described below.
As reported in a previous XAS study of Np(V)-carbonate
(aq) complexes at pCO2) 10-3.45atm (25), the distance at
∼2.6 Å, R + ∆R, corresponds to a carbonate ligand coor-
dination of the Np(V) atom in a bidentate fashion having a
Np-C distance of ∼2.9 Å. Estimated distances are in good
agreement with the results of Np(V)-carbonate aqueous
species (i.e., NpO2(CO3)-, NpO2(CO3)23-, and NpO2(CO3)35-)
(25). The presence of inner-sphere surface species can also
besuggestedintheresidualFouriertransformNp-Fepeaks
(SI-3) which were prepared by subtracting the SS contribu-
tions of Np-Oax, Np-Oeq, Np-C, and Np- Odist, and the MS
contributions of one NpdOax and two Np-Odist from
normalized spectra. All three samples contain the Np-Fe
peaks (SI-3).
The FT frequency at ∼3.1 Å, R + ∆R, is fit well with Fe
neighbors at ∼3.45 Å, suggesting the formation of inner-
sphere bidentate mononuclear surface species on iron
octahedral structures. The FT frequency at ∼3.8 Å, R + ∆R,
is attributed to SS and MS paths of the Np-Odistat ∼4.2 Å.
Clarkandco-workershaveshowntheimportanceoftheNp-
Odistscattering paths in Np(V)-carbonate aqueous species
(25). Our observation of the Np-Odistcontribution from the
Np(V)-carbonato ternary species at the mineral-water
interfaceaddsnewinsighttothetypesofNp(V)surfacesthat
may form in low-temperature geochemical environments.
Priortoincorporatingthevariouscombinationsofbinary
andcarbonatoternarysurfacespeciesinthefit,weattempted
a fit with the analogous predominant aqueous species at
each pH value. For example, based on the Np(V)(aq) spe-
ciation diagram (SI-2), we assumed that 100% of the surface
species were binary at pH ∼ 7-8 and 100% of mono-car-
bonato or bis-carbonato surface species (Figures 2a and 2b)
at pH ∼ 9. The results of these fit attempts are presented in
Figures 2a and 2b. k3-weighted EXAFS spectra clearly in-
dicate a poor fit at k ∼ 8 Å-1in pH 6.98 and 8.82 spectra
(indicated by arrows in Figure 2a) and at k ∼ 11 Å-1in the
pH 8.2 spectrum (arrow in Figure 2a). The poor fits are
clearly observed in all FT spectra (Figure 2b). Np-Fe shells
are overestimated (as indicated by solid-line arrows in
Figure2b),suggestingthatthefractionofinner-spheresurface
species are overestimated. This indicates that some fraction
of outer-sphere surface species (e.g., tris-carbonato outer-
sphere surface species) might need to be considered in
the fit.
In pH 6.98-8.81 samples, underestimated Np-C shells
suggest that Np coordination by carbonate ligand in the
FIGURE 1. (a) Normalized, background-subtracted k3-weighted Np
LIII-edgeEXAFSspectraofNp(V)-adsorbedhematitesamplesinair;
(b) Fourier-transformed k3-weighted Np LIII-edge EXAFS spectra of
Np(V)-adsorbed hematite (solid lines) and nonlinear least-squares
fits (dotted lines).
FIGURE 2. (a) Normalized, background-subtracted k3-weighted Np
LIII-edgeEXAFSspectraofNp(V)-adsorbedhematitesamplesinair;
(b) Fourier-transformed k3-weighted Np LIII-edge EXAFS spectra of
Np(V)-adsorbed hematite. Solid lines are the experimental data
and the dotted lines represent fits with fixed fractions of binary,
mono-carbonato, and bis-carbonato inner-sphere surface species.
Dotted, dash, solid, and dash-dot arrows indicate Np-C, Np-C/Fe,
Np-Fe/OtransMS, and Np-OdistSS/MS shells.
VOL. xx, NO. xx, xxxx / ENVIRON. SCI. & TECHNOL. 9 C

Page 4

surface species might be greater than observed in aqueous
solution at the same pH value. Furthermore, the fit quality
for distal oxygen shells becomes poor as pH increases from
7to8.8(indicatedbydash-dotarrowsinFigure2b).Previous
XAS studies suggested inner-sphere binary surface species
occurred on goethite surface at pH ∼ 7 (35); however, our
preliminaryXASanalysesindicatesthatneptunyl-carbonato
ternary species occur on hematite surfaces. The chemical
environment at the mineral-water interface is apparently
sufficiently different (e.g., electrical double layer, solvation
energy, and dielectric constant of solvent at interface) to
favor these species at pH values lower than those observed
inwater.Consequently,Np-C/Fe/Odistcoordinationnumber
correlated analyses were conducted to simultaneously con-
sider the four putative species mentioned earlier.
To compensate for the overpredicted Np-Fe shell (i.e.,
inner-spheresurfacespecies)andunderpredictedU-Cshell
atpH∼7(thebottomspectruminFigure3b),weconsidered
tris-carbonatosurfacespeciesalongwiththreeinner-sphere
species(binary,mono-carbonato,andbis-carbonato)inthe
fit. A combination of bis-carbonato inner-sphere and tris-
carbonato outer-sphere surface species yielded the best fit
(R factor: 0.0158) (Figures 1a and 1b), resulting in 73% of
bis-carbonatoinner-sphereand27%oftris-carbonatoouter-
sphere surface species (Table 1).When binary or mono-
carbonato inner-sphere surface species were included with
the tris-carbonato surface species, the amplitude of Np-C
shell was slightly underpredicted.
Similarly, the best fit (R factor ) 0.0034) was obtained for
the pH 8.28 sample when a combination of bis-carbonato
inner-sphereandtris-carbonatoouter-spheresurfacespecies
were considered (Figures 1a and 1b). The fraction of inner-
spherebis-carbonatosurfacespeciesslightlyreducedto61%
and the fraction of outer-sphere surface species increased
to 39%. An increase in the fraction tris-carbonato surface
species (i.e., an increase in carbonate ligand coordination)
can be seen in the enhanced FT feature at ∼3.8 Å, R + ∆R
(Figure 1b).
In the pH 8.82 sample, the strong distal oxygen SS/MS
contribution at ∼3.8 Å, R + ∆R, is clearly seen (Figure 1b).
54%ofthesurfaceNp(V)wasadsorbedasthetris-carbonato
outer-sphere surface species, with approximately 46% of
adsorbed Np(V) present as bis-carbonato inner-sphere
ternary species (Table 1). As was demonstrated for U(VI)
sorption on imogolite (21), an outer-sphere tris-carbonato
species forms at alkaline pH values where a combination of
factors contribute to its relative stability: (1) the activity of
carbonate is increasing in solution, (2) the stability of inner-
sphere species is decreasing due to increasing pH, and (3)
the surface charge is still positive because the pH is below
the PZC.
As Arai and co-workers have previously demonstrated,
the use of coordination number-correlated XAS analyses on
U(VI)-reactedimogolite(21),asimilarmodelwassuccessfully
applied here to elucidate Np(V) surface species at the
hematite-water interface. We found that Np(V)-carbonato
species, which have previously only been documented as
aqueous species (25), readily form on hematite surfaces as
ternary species at pH 7-9 in air-equilibrated systems. The
stabilitiesofbothsurfacespecies(relativetoaqueousspecies)
are likely to be very dependent on aqueous chemical
conditions, especially the concentration of bicarbonate in
groundwaterandothersolutesthatinfluencesurfacecharge
at the mineral-water interface. The variable chemistry of
the groundwater environments down gradient of the Yucca
Mountain high-level waste disposal site and its potential
impact on Kdvalues for Np(V) sorption have been discussed
by Turner et al. (1). The results presented here demonstrate
thatthestabilityofsurfacespeciesforNp(V)sorptionwillbe
sensitivetoaqueouschemicalconditionsinthegroundwater
because the carbonate anion is a component of the pre-
dominant surface and aqueous complexes. As in the case of
U(VI) (36), this means that reactive transport model simula-
tions that describe the fate of transport of Np(V) in the
environmentdowngradientoftheYuccaMountainsitewould
have a stronger technical basis if the conceptual model for
sorptionwasrepresentedbysurfacecomplexation(coupled
withaqueouschemistry)ratherthanaconstantKd-modeling
approach.
Acknowledgments
AuthorsthankDr.J.RogersandDr.J.R.Bargarforassistance
withtheXASmeasurementsandDr.M.Kohlerforsupplying
synthetic hematite. Portions of this research were carried
out at the Stanford Synchrotron Radiation Laboratory, a
national user facility operated by Stanford University on
behalf of the U.S. Department of Energy, Office of Basic
Energy Sciences. The SSRL Structural Molecular Biology
Program is supported by the Department of Energy, Office
of Biological and Environmental Research, and by the
National Institutes of Health, National Center for Research
Resources,BiomedicalTechnologyProgram.Thisprojectwas
partially supported by the U.S. Nuclear Regulatory Com-
mission Interagency Agreement RES-03-006 with the U.S.
Geological Survey (USGS) and by funding from the National
Research Program of USGS.
Note Added after ASP Publication
This paper was published ASAP May 4, 2007 with a minor
error in the y-axis labels of Figures 1 and 2; the corrected
version was published ASAP May 9, 2007.
Supporting Information Available
In SI-1, -2, and -3, formation constants for aqueous species,
Np(V)aqueousspeciationdiagram,andFourier-transformed
FIGURE 3. Ball-and-stick representation of Np(V) surface species
on the iron octahedral structure of hematite based on the results
of XAS analyses shown in Table 1. (a) Bis-carbonato-Np(V) inner-
sphere ternary complex via bidentate mononuclear Np(V)-O2-Fe
linkage. (b) Tris-carbonato-Np(V) outer-sphere ternary complex.
D9ENVIRON. SCI. & TECHNOL. / VOL. xx, NO. xx, xxxx

Page 5

XAS spectra of residual Np-Fe shells, respectively, are
provided. This material is available free of charge via the
Internet at http://pubs.acs.org.
Literature Cited
(1) Turner, D. R.; Bertetti, F. P.; Pabalan, R. T. Role of radionuclide
sorption in high-level waste performance assessment: Ap-
proachesfortheabstractionofdetailedmodels.InGeochemistry
ofSoilRadionuclides;Zhang,P.-C.,Brady,P.V.,Eds.;SoilScience
Society of America: Madison, WI, 2002; pp 211-252.
(2) Dozol, M.; Hagemann, R.; Radionuclide migration in ground-
waters: Review of the behavior of actinides. Pure Appl. Chem.
1993, 65, 1081-1102.
(3) Nitsche, H.; Lee, S. C.; Gatti, R. C. Determination of plutonium
oxidation states at tracer levels pertinent to nuclear waste
disposal. J. Radioanal. Nucl. Chem.-Art. 1988, 124, 171-185.
(4) Runde, W. The chemical interactions of actinides in the
environment. Los Alamos Sci. 2000, 26, 392-411.
(5) Viswanathan, H. S.; Robinson, B. A.; Valocchi, A. J.; Triay, I. R.
A reactive transport model of neptunium migration from the
potential repository at Yucca Mountain. J. Hydrol. 1998, 209
(1), 251-280.
(6) Aksoyoglu,S.;Burkart,W.;Goerlich,W.SorptionofNeptunium
onClays.J.Radioanal.Nucl.Chem.-Art.1991,149(1),119-122.
(7) Girvin, D. C.; Ames, L. L.; Schwab, A. P.; McGarrah, J. E.
Neptunium adsorption on synthetic amophous iron oxyhy-
droxide. J. Colloid Interface Sci. 1991, 141 (1), 67-78.
(8) Bradbury, M. H.; Baeyens, B. Modelling the sorption of Mn(II),
Co(II), Ni(II), Zn(II), Cd(II), Eu(III), Am(III), Sn(IV), Th(IV), Np-
(V) and U(VI) on montmorillonite: Linear free energy relation-
ships and estimates of surface binding constants for some
selected heavy metals and actinides. 2005, 69, 875-892.
(9) Kohler, M.; Honeyman, B. D.; Leckie, J. O. Neptunium(V)
sorption on hematite (R-Fe2O3) in aqueous suspension: The
effect of CO2. Radiochim. Acta 1999, 85 (33-48).
(10) Li, W. J.; Tao, Z. Y.; Guo, L. T.; Li, S. S. Sorption and desorption
of neptunium(V) on loess: batch and column experiments
Radiochim. Acta 2003, 91 (10), 575-582.
(11) Nakata, K.; Fukuda, T.; Nagasaki, S.; Tanaka, S.; Suzuki, A.;
Tanaka, T.; Muraoka, S. Sorption of neptunium on iron-
containing minerals. Czech. J. Phys. 1999, 49 (1), 159-166.
(12) Nakata, K.; Nagasaki, S.; Tanaka, S.; Sakamoto, Y.; Tanaka, T.;
Ogawa, H. Sorption and desorption kinetics of Np(V) on
magnetite and hematite. Radiochim. Acta 2000, 88 (8), 453-
457.
(13) Nakata, K.; Nagasaki, S.; Tanaka, S.; Sakamoto, Y.; Tanaka, T.;
Ogawa, H. Sorption and reduction of neptunium(V) on the
surface of iron oxides Radiochim. Acta 2002, 90 (9-11), 665-
669.
(14) Tochiyama, O.; Endo, S.; Inoue, Y. Sorption of neptunium(V)
on various iron-oxides and hydrous iron-oxides. Radiochim.
Acta 1995, 68 (2), 105-111.
(15) Turner, D. R.; R. T. P.; Bertetti, F. P. Neptunium(V) sorption on
montmorillonite; an experimental and surface complexation
modeling study. Clays Clay Miner. 1998, 46 (3), 256-269.
(16) Wilk, P. A.; Shaughnessy, D. A.; Wilson, R. E.; Nitsche, H.
Interfacial Interactions between Np(V) and Manganese Oxide
Minerals Manganite and Hausmannite. Environ. Sci. Technol.
2005, 39 (8), 2608-2615.
(17) Mincher, B. J.; Fox, R. V.; Cooper, D. C.; Groenewold, G. S.
NeptuniumandplutoniumsorptiontoSnakeRiverPlain,Idaho
soil Radiochim. Acta 2003, 91 (7), 397-401.
(18) Braithwaite,A.;Richardson,S.;Moyes,L.N.;Livens,F.R.;Bunker,
D. J.; Hughes, C. R.; Smith, J. T.; Hilton, J. Sorption kinetics of
uranium-238andneptunium-237onaglacialsediment.Czech.
J. Phys. 2000, 50 (2), 265-269.
(19) DelNero, M.; Made, B.; Bontems, G.; Clement, A. Adsorption of
neptunium(V) on hydrargilite. Radiochim. Acta 1997, 76 (4),
219-228.
(20) Hart, K. P.; Payne, T. E.; Robinson, B. J.; Vaniseghem, P.
Neptunium uptake on Boom Clay-time dependence and
association of Np with fine particles. Radiochim. Acta 1994, 66
(7), 19-22.
(21) Arai, Y.; McBeath, M.; Bargar, J. R.; Joye, J.; Davis, J. A. Uranyl
adsorption and surface speciation at the imogolite-water
interface: Self-consistent spectroscopic and surface complex-
ation models. Geochim. Cosmochim. Acta 2006, 70 (10), 2492-
2509.
(22) Bargar,J.R.;Reitmeyer,R.;Davis,J.Spectroscopicconfirmation
of uranium(VI)-carbonate adsorption complexes on hematite.
Environ. Sci. Technol. 1999, 33 (14), 2481-2484.
(23) Bargar, J. R.; Reitmeyer, R.; Lenhart, J. J.; Davis, J. A. Charac-
terization of U(VI)-carbonato ternary complexes on hematite:
EXAFS and electrophoretic mobility measurements. Geochim.
Cosmochim. Acta 2000, 64 (16), 2737-2749.
(24) Elzinga, E. J.; Tait, C. D.; Reeder, R. J.; Rector, K. D.; Donohoe,
R.T.;Morris,D.E.SpectroscopicinvestigationofU(VI)sorption
atthecalcite-waterinterface.Geochim.Cosmochim.Acta2004,
68 (11), 2437-2448.
(25) Clark, D. L.; Conradson, S. D.; Ekberg, S. A.; Hess, N. J.; Neu,
M. P.; Palmer, P. D.; Runde, W.; Tait, C. D. EXAFS studies of
pentavalent neptunium carbonate complexes. Structural elu-
cidation of the principal constituents of neptunium in ground-
water environments. J. Am. Chem. Soc. 1996, 118, 2086-2090.
(26) Clark, D. L.; Hobart, D. E.; Neu, M. P. Actinide carbonate
complexes and their importance in actinide environmental
chemistry. Chem. Rev. 1995, 95, 25-48.
(27) Matijevic, E.; Scheiner, P. Ferrous hydrous oxide sols III.
PreparationofuniformparticlesbyhydrolysisofFe(III)-chloride,
-nitrate, and -perchlorate solutions. J. Colloid Interface Sci.
1978, 63, 509-524.
(28) Herbelin, A. L.; Westall, J. C. FITEQL: A computer program for
the determination of chemical equilibrium constants from
experimental data, Version 4.0, 99-01; Oregon State University:
Corvallis, OR, 1999.
(29) Guillaumont, R.; Fanghanel, T.; Neck, V.; Fuger, J.; Palmer, D.
A.;Grenthe,I.;Rand,M.H.ChemicalThermodynamics5: Update
on the chemical thermodynamics of uranium, neptunium,
plutonium,americium,andtechnetium ;Elsevier: Amsterdam,
2003.
(30) Rehr, J. J.; Albers, R. C.; Zabinsky, S. I. High-order multiple
scattering calculations of x-ray absorption fine structure. Phys.
Rev. Lett. 1992, 69, 3379-3400.
(31) Mereiter, K. Neue kristallographische daten ueber das uran-
mineral andersonit. New crystalographic data of the uranium
mineralandersonite.Anz.Oesterr.Akad.Wiss.,Math.-Naturwiss.
Kl. 1986, 123 (3), 39-41.
(32) Piret, P.; Declercq, J.-P.; Wauters-Stoop, D. Structure of phuralu-
mite. Acta. Crystallogr., Sect. B: Struct. Sci. 1979, 35, 1880-
1882.
(33) Webb, S. M. A graphical user interface for XAS analysis using
IFEFFIT. Phys. Scr. 2005, (T115), 1011-1014.
(34) Hudson, E. A.; Allen, P. G.; Terminello, L. J.; Denecke, M. A.;
Reich, T. Polarized x-ray absorption spectroscopy of the uranyl
ion: Comparison of experiment and theory. Phys. Rev. B 1996,
54, 156-165.
(35) Combes,J.;Chisholm-Brause,C.J.;Brown,G.E.J.;PArks,G.A.;
Conradson, S. D.; Eller, P. G.; Trlay, I. R.; Hobart, D. E.; Meijer,
A. EXAFS spectroscopic study of neptunium(V) sorption at the
R-FeOOH/waterinterface.Environ.Sci.Technol.1992,26,376-
382.
(36) Curtis, G. P.; Davis, J. A.; Naftz, D. L. Simulation of reactive
transportofuranium(VI)ingroundwaterwithvariablechemical
conditions. Water Resour. Res. 2006, 42, W04404 ; doi:10.1029/
2005WR003979.
(37) Lemire, R. J.; Fuger, J.; Nitsche, H.; Potter, P.; Rand, M. H.;
Rydberg, J.; Spahiu, K.; Sullivan, J. C.; Ullman, W. J.; Vitorge, P.;
Wanner, H. Chemical Thermodynamics 4. Chemical Thermo-
dynamicsofNeptuniumandPlutonium;Elsevier: Amsterdam,
2001.
Received for review October 14, 2006. Revised manuscript
received March 7, 2007. Accepted March 28, 2007.
ES062468T
PAGE EST: 5 VOL. xx, NO. xx, xxxx / ENVIRON. SCI. & TECHNOL. 9 E