Incorporation Modes of Iodate in Calcite - Environmental Science

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Incorporation Modes of Iodate in Calcite Sebastien Kerisit, Frances N Smith, Sarah A. Saslow, Megan Hoover, Amanda R. Lawter, and Nikolla P. Qafoku Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00339 • Publication Date (Web): 26 Apr 2018 Downloaded from http://pubs.acs.org on April 27, 2018

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Environmental Science & Technology

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Incorporation Modes of Iodate in Calcite

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Sebastien N. Kerisit,†* Frances N. Smith,‡ Sarah A. Saslow,‡ Megan E. Hoover,§ Amanda R.

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Lawter,‡ and Nikolla P. Qafoku‡ †

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Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory,

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Richland, Washington 99352, United States ‡

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Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States

§

Environmental Engineering and Earth Sciences Department, Clemson University, Anderson,

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South Carolina 29625, United States

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April 20th 2017

1 ACS Paragon Plus Environment


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ABSTRACT

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Iodate (IO3−) incorporation in calcite (CaCO3) is a potential sequestration pathway for

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environmental remediation of radioiodine-contaminated sites (e.g., Hanford Site, WA), but the

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incorporation mechanisms have not been fully elucidated. Ab initio molecular dynamics (AIMD)

15

simulations and extended X-ray absorption fine structure spectroscopy (EXAFS) were combined

16

to determine the local coordination environment of iodate in calcite, the associated charge

17

compensation schemes (CCS), and any tendency for surface segregation. IO3− substituted for

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CO32− and charge compensation was achieved by substitution of Ca2+ by Na+ or H+. CCS that

19

minimized the I−Na/H distance or placed IO3− at the surface were predicted by density functional

20

theory to be energetically favored, with the exception of HIO3, which was found to be metastable

21

relative to the formation of HCO3−. Iodine K-edge EXAFS spectra were calculated from AIMD

22

trajectories and used to fit the experimental spectrum. The best-fit combination consisted of a

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significant proportion of surface-segregated IO3− and charge compensation was predominantly

24

by H+. Important implications are therefore that pH should strongly affect the extent of IO3−

25

incorporation and that IO3− accumulated at the surface of CaCO3 particles may undergo

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mobilization under conditions that promote calcite dissolution. These impacts need to be

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considered in calcite-based iodate remediation strategies.


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INTRODUCTION

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Iodine is a contaminant of significant environmental concern at facilities where irradiated

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nuclear fuel is stored, reprocessed, or disposed of because of its high toxicity, high

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bioaccumulation factor, extremely long half-life, and rapid mobility in subsurface environments.1

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129

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Hanford Site in southeastern Washington State and Savannah River Site in South Carolina.

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Some of the challenges with predicting iodine behavior in the environment stem from its high

35

solubility, multiple oxidation states, dynamic aqueous speciation across the entire pH range, and

36

potential for interacting with organic matter.1

I (half-life = 1.6×107 years) is present in groundwater at the U.S. Department of Energy’s

37

Potential remediation technologies for iodine contamination are currently being studied as part

38

of environmental remediation activities at the Hanford Site. A speciation study2 of the

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groundwater at the Hanford Site determined that iodine was present mostly as iodate (IO3−);

40

however, small percentages of iodide (I−) and organically-bound iodine were also detected.

41

Moreover, iodine was found in association with calcite particles that precipitated following CO2

42

degassing during removal of groundwater from the deep surface,2 and a significant fraction of

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the total iodine content of sediments from the Hanford Site was associated with the carbonate

44

fraction.3 These findings are consistent with results from the literature on natural calcium

45

carbonates4 and led to further research into the ability of calcite, and other calcium carbonate

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polymorphs, to sequester iodine as IO3−.5 Precedent for iodine association with calcium

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carbonate phases in nature exists in speleothems (i.e., cave features6, 7). Other studies have used

48

I/Ca ratios in microscopic ocean-dwelling organisms, foraminiferas, or in marine carbonate

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deposits as a proxy for paleo-redox conditions.4, 8 Although not a calcium carbonate phase, the



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calcium iodate phase lauterite (Ca(IO3)2) is found in arid climates9 and serves as a useful

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benchmark for studying IO3− incorporation in calcium carbonate phases.4

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Calcium carbonate is able to incorporate a wide range of metals and radionuclides,10, 11 and a

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variety of studies have also examined oxyanion substitution into calcium carbonate phases.12-14, 5

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However, one of the challenges with the incorporation of IO3− into calcium carbonate is the

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resulting charge imbalance from the aliovalent substitution of the CO32− group. On the basis of

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X-ray absorption spectroscopy measurements and density functional theory (DFT) calculations,

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Podder et al.5 concluded that IO3− substituted for CO32− in calcite and formed ionic bonds with

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two or three additional oxygen atoms. Although their electron microprobe analysis showed a

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positive correlation between sodium and iodine concentrations, Podder et al.5 were not able to

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determine the nature and location of the charge compensating species from their extended X-ray

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absorption fine structure spectroscopy (EXFAS) measurements. Questions regarding the role of

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IO3− segregation to the calcite surface also remain unanswered.

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The objective of this work was therefore to determine the local environment around IO3−

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incorporated in calcite, the most likely charge compensation scheme(s) (CCS), and any tendency

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for surface segregation. This objective is critical, not only for evaluating the energetics and

66

mechanisms of iodine incorporation, but also as a basis for determining its effects on calcite

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stability and solubility. The approach employed in this work consisted in first undertaking a

68

systematic evaluation of IO3− incorporation schemes in bulk and surface environments to identify

69

key representative configurations. Calculated EXAFS spectra were then generated from ab initio

70

molecular dynamics (AIMD) trajectories of these configurations and used as components in a

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direct fit to the EXAFS measurements performed in this work. This approach has been shown to


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be a powerful method for extracting more structural information from EXAFS spectra than via

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traditional shell-by-shell fitting.15, 16



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METHODS

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DFT calculations. All of the plane-wave DFT calculations were performed with VASP

76

(Vienna Ab-initio Simulation Package)17-20 using the projector augmented-wave (PAW)

77

approach21,

78

Perdew, Burke, and Ernzerhof23, 24 (PBE) with Grimme dispersion corrections (G).25 For all solid

79

phases, a constant-pressure energy minimization was first performed to determine the optimized

80

crystal structure (Table S1) at the PBE+G level of approximation and the optimized unit cell was

81

then scaled to achieve the desired supercell size.

22

and the generalized gradient approximation exchange-correlation functional of

82

A first series of calculations was performed to determine incorporation energies (Einc) using a

83

generalized “products minus reactants” approach and solid-state reference phases.26 One CO32−

84

was substituted by one IO3− in the supercell and the net charge thus introduced was compensated

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by replacing one Ca2+ cation either by Na+ or H+. Multiple initial positions of Na+ and H+ relative

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to IO3− were considered. Surface calculations were also performed, whereby slabs representing

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the lowest-energy and morphologically dominant (104) surface were “cleaved” from the

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optimized calcite unit cell.27 Hereafter, bulk incorporation configurations are labeled as BX,

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where X is a digit used to differentiate between the different positions of the charge

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compensating species, and surface incorporation configurations are labeled as SXd/h, where X

91

represents the surface atomic layer iodate is incorporated in and d/h indicates whether the surface

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is dry (d, no water adsorbed) or hydrated (h, 1 water monolayer adsorbed). In each case, the label

93

is preceded by either Na or H to indicate the charge compensating species.

94

A second series of calculations employed key configurations determined in the first series to

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perform AIMD simulations and calculate EXAFS spectra. NVT (constant number of particles,

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constant volume, and constant temperature) AIMD simulations were performed and a minimum


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of 100 configurations were collected from each simulation at 50 fs intervals to calculate the I K-

98

edge EXAFS spectrum. For each configuration, a cluster centered on the I atom was generated to

99

calculate all the scattering paths using FEFF928-30 and the spectra of all configurations were

100

averaged for comparison with experiment. The Fourier transform (FT) was applied to the

101

averaged EXAFS spectra using IFEFFIT.31 The same approach was applied to standard iodate

102

compounds (e.g. NaIO3). Computational details and additional information can be found in the

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Supporting Information (SI) document.

104

Coprecipitation and EXAFS measurements. An iodate-doped calcite sample was synthesized

105

at room temperature from CaCl2, (NH4)2CO3, and NaIO3 solutions (without calcite seed crystals)

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using the approach detailed in the SI and was prepared for EXAFS analysis immediately after

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synthesis completion. EXAFS spectra for the iodate-doped calcite sample and three iodate

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standards, NaIO3, Ca(IO3)2, and KIO3, were collected at the Stanford Synchrotron Radiation

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Lightsource (SSRL) beam line 11-2 at the iodine K edge (33,169 eV). Measurements were made

110

at 8.0 ± 0.2 K using an Oxford Instruments cryostat cooled with liquid helium.

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X-ray absorption data were obtained from 240 eV below the edge to 1100 eV above the edge.

112

The data from 30 eV below the edge to 20 eV above the edge were obtained with 0.5 eV spacing.

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The data beyond 20 eV above the edge were obtained with a k-spacing of 0.05 and a k2-weighted

114

collection time. The monochromator was detuned 20% to reduce the harmonic content of the

115

beam. Transmission data (iodate standards; NaIO3: 7 scans, Ca(IO3)2: 6 scans, KIO3: 5 scans)

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were obtained using Ar filled ion chambers. Fluorescence data (iodate-doped calcite sample; 9

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scans) were obtained using a 100 element Ge detector and were corrected for detector dead time.

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A NaIO3 reference (diluted in boron nitride) was used to account for minor shifts in energy



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between samples. Spectra were generated from raw data using SIXPack32 and then normalized

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using Athena.33


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RESULTS AND DISCUSSION

Figure 1. Atomistic models illustrating IO3− incorporation at the CO32− position in calcite and charge compensation by Na+ in three nearest-neighbor positions (Na B1, B2, and B3) and one distant position (Na B4) in the bulk and in one nearest-neighbor position (Na S1d ) at the surface (the inset shows a top view of position Na S1d ). The corresponding atomic positions in pure bulk calcite are also shown (0). Calcium is shown in blue, oxygen in red, carbon in brown, iodine in purple, and sodium in yellow. 122

Energetics of Na+/IO3− co-substitution. In the pure calcite structure (model 0 in Figure 1),

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each carbonate oxygen is coordinated to two calcium ions. Because of the planar configuration

124

of the carbonate ion, these two nearest-neighbor calcium positions are symmetrically equivalent

125

(C−Ca distance of ~3.2 Å). In contrast, IO3− assumes a trigonal pyramidal geometry with its

126

oxygen atoms in the same plane as neighboring carbonate groups. As a result, the local

127

symmetry is broken when CO32− is substituted by IO3− and the two positions are no longer

128

equivalent, whereby Ca2+ substitution by Na+ (Figure 1) can occur at a first nearest-neighbor

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position (Na B1), with a long optimized I−Na distance of ~3.7 Å, or at a second nearest-neighbor 9 ACS Paragon Plus Environment


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position (Na B2), with a short optimized I−Na distance of ~3.3 Å. Similarly, a calcium atom is

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positioned directly above and below a carbon atom along the [001] direction in the calcite

132

structure (C−Ca distance of ~4.3 Å), but introducing IO3− breaks the local symmetry resulting in

133

two inequivalent positions for Na+ substitution with the one leading to the shortest optimized

134

I−Na distance (~3.4 Å) being considered here (Na B3). When Na+ is in a distant position,

135

calcium ions occupy the three nearest-neighbor positions (Na B4).

Figure 2. Incorporation energy (Einc) as a function of I−Na/H distance for the co-substitution of H+/IO3− or Na+/IO3− for Ca2+/CO32− in bulk calcite and at the (104) surface. For H+/IO3−, configurations in which H+ is on the IO3− or CO32− group are shown with different colors. Labels indicate the configurations used in the AIMD simulations (blue labels: Figure 1; red and dark yellow labels: Figure 3). See Methods section for nomenclature of incorporation configurations. 136

2×2×1 supercell calcite models were built with increasing I−Na separation distances, including

137

the three nearest-neighbor positions shown in Figure 1 and a number of distant positions (Figure

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2). Positions Na B2 and B3, which minimized the I−Na distance, were the lowest-energy

139

positions differing only by 0.02 eV. In contrast, position Na B2 was more stable than position Na


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B1 by approximately 0.4 eV. Podder et al.5 only considered position Na B1 and a distant position

141

and, therefore, did not consider the global energy minimum for the IO3−−Na+ pair.

142

At I−Na distances greater than ~5 Å, averaged Einc values were on par with position Na B1

143

(1.3 eV), suggesting that energy-lowering effects due to defect clustering are minimized at

144

greater distances. Variations in Einc above and below 1.3 eV are attributed to the number of

145

effective carbonate layers separating iodine from sodium in the [001] direction. Energies are

146

higher when at least 2 carbonate layers separate the defects and lower when only one carbonate

147

layer is between the iodine and sodium, again pointing to the energetic preference for defects to

148

cluster.

149

IO3− incorporation at the (104) calcite surface, which dominates the morphology of calcite

150

crystals, was also considered. A first series of calculations (Figure S1) determined the

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energetically-favored position of Na+ for IO3− incorporated in the topmost atomic layer (position

152

Na S1d in Figure 1). When at the surface, the iodine atom moved toward the free surface with its

153

oxygen atoms remaining in a relatively planar configuration and, as in the bulk case, the lowest-

154

energy position for Na+ was the one that minimized the I−Na distance. A second series

155

determined the depth-dependent energetics of Na+/IO3− co-substitution for the lowest-energy Na+

156

position (Figure 2). Incorporation was much more favorable at the topmost atomic layer and

157

rapidly converged with depth to the bulk values, indicating a strong preference for segregation to

158

the calcite surface, in agreement with previous electronic structure calculations of selenite

159

incorporation in calcite.34, 35 The impact of surface hydration was also considered by adsorbing a

160

water monolayer at the (104) surface, which had the effect of lowering the incorporation energy

161

further (Figure 2).



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Energetics of H+/IO3− co-substitution. As with the first co-substitution scheme, a series of

163

calculations was performed to evaluate the effect of proton placement on Einc for H+/IO3− co-

164

substitution in calcite. Two distinct cases were tested (graphical description in Figure S2): (1) H+

165

associated with oxygen atoms of nearest-neighbor carbonate groups – either in the same

166

carbonate layer as IO3− or in a nearest-neighbor carbonate layer; and (2) H+ associated with each

167

of the three iodate oxygen atoms.

Figure 3. Atomistic models illustrating IO3− incorporation at the CO32− position in calcite and charge compensation by H+ in three nearest-neighbor positions (H B1, B2, and B3) in the bulk and in one nearest-neighbor position (H S1d ) at the surface (the inset shows a top view of position H S1d ). Calcium is shown in blue, oxygen in red, carbon in brown, iodine in purple, and hydrogen in white. Calcium vacancies are shown by green dashed circles. 168

The incorporation energies indicated a preference for H+ to associate with oxygen atoms of

169

nearest-neighbor CO32− groups over oxygen atoms of IO3−, even though the I−H distance was

170

shortest for the latter (Figure 2). Key configurations labeled in Figure 2 and used in the AIMD


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simulations are shown in Figure 3. As for Na+/IO3− co-substitution, the iodate oxygen atoms

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remained in plane with the carbonate groups in all cases.

173

In the lowest-energy configuration (position H B2 in Figure 3), H+ was associated with the

174

nearest-neighbor CO32− group thus forming HCO3−, the calcium vacancy was closest to the

175

iodine atom, and IO3− accepted a hydrogen bond from HCO3−. In position H B1 (Figure 3), IO3−

176

also accepted a hydrogen bond from HCO3− but the calcium vacancy and iodine atom were on

177

opposite sides of the plane formed by the iodate oxygen atoms and were thus further apart,

178

resulting in a less favorable incorporation energy. In cases where the H-bearing IO3− oxygen was

179

too far from the Ca2+ vacancy to allow for proton transfer, the hydrogen atom pointed towards a

180

neighboring CO3− instead.

181

Configurations in which H+ was associated with iodate oxygen atoms yielded the least

182

favorable incorporation energies (Figure 3), by at least 1 eV with respect to position H B2

183

(Figure 2). In these cases, the I−O(H) bond distance elongated to approximately 2.05 Å from

184

1.84 Å in IO3−. To compensate for the weakening of the I−O bond, one of the carbonate groups

185

in the plane above that containing HIO3 distorted to reduce the I−O second-nearest-neighbor

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bond distance to approximately 2.4 Å compared to 2.7 Å in the case of IO3−.

187

In two of the six cases in which it was initially positioned within 1 Å of an iodate oxygen

188

(Figure S2), the distance between the H-bearing IO3− oxygen and the vacancy was short (Table

189

S3), and the hydrogen atom was able to move to form a bond with a carbonate oxygen during the

190

course of the energy minimization, indicating that there was no energy minimum associated with

191

the formation of HIO3 for these configurations (Figure 2). These calculations therefore indicate

192

that, depending on the relative positions of IO3−, the calcium vacancy, and H+, HIO3 may be

193

metastable or may spontaneously dissociate to lead to the formation of HCO3−. This could



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explain the results of Podder et al.,5 who apparently only considered the case where HIO3 was

195

metastable when H+ was in a nearest-neighbor position, and thus only reported the formation of

196

HIO3 and not the global minimum involving HCO3− (position H B2).

197

As for co-substitution with Na+, IO3− incorporation at the (104) calcite surface was also

198

considered. Here again, a first series of calculations (Figure S3) was performed to determine the

199

lowest-energy position for H+ charge-compensating IO3− incorporated in the topmost atomic

200

layer. A second series evaluated the depth-dependence of the incorporation energy for this

201

charge-compensating position (Figure 2). Consistent with the calculations already discussed,

202

substitution of H+ on a nearest-neighbor CO32− was favored, the magnitude of the incorporation

203

energy was greatly reduced when IO3− was present in the topmost atomic layer, and the presence

204

of an adsorbed water monolayer reduced the incorporation energy further (Figure 2).

205

Incorporation energies obtained for the lowest-energy configurations of the Na+/IO3− co-

206

substitution scheme were more favorable than those obtained for H+ associated with carbonate

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groups. However, the values of Einc were calculated using solid-state reference phases. To better

208

reflect the aqueous conditions found in nature and in laboratory synthesis experiments, the

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energetics of the reaction + CaCO3 ∙NaIO3 +H3 O+ aq ↔ CaCO3 ∙HIO3 +Na aq +H2 Ol

(2)

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were evaluated by performing AIMD simulations of aqueous Na+ and H3O+ and of liquid water.

211

The results of these simulations point to a negative energy of reaction and thus a preference for

212

charge compensation of the incorporated IO3− by H+ over Na+ with respect to the aqueous ions

213

(see Table S4 and associated text in the SI).

214

EXAFS standard compounds. Simulated EXAFS spectra of four standard iodate compounds

215

at various temperatures were compared to experiment to evaluate the simulation approach,


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develop an understanding of the effect of temperature on the EXAFS spectra, and determine how

217

the local environment around IO3− might influence the EXAFS spectra.

Figure 4. Experimental and calculated I K-edge EXAFS spectra (left; ∆E0 = 7 eV) and corresponding Fourier transform magnitudes (right; Hanning window, dk = 1 Å−1) for I in NaIO3 (top; 3.5 Å−1 ≤ k ≤ 14.7 Å−1), Ca(IO3)2·nH2O (n = 0 or 1; middle; 3.5 Å−1 ≤ k ≤ 14.7 Å−1), and KIO3 (bottom; 3.0 Å−1 ≤ k ≤ 14.0 Å−1) for temperatures ranging from 8 to 200 K. EXAFS data collected in this work at 8 K were complemented by data from Laurencin et al.36 and Yagi et al.37.



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In addition to the EXAFS spectra collected at 8 K for NaIO3, Ca(IO3)2, and KIO3, data for

219

NaIO3 and Ca(IO3)2·H2O obtained at 77 K by Laurencin et al.36 and data for KIO3 collected at

220

10, 100, and 200 K by Yagi et al.37 were also included in the comparison (Figure 4). Two main

221

discrepancies were observed. Firstly, a small offset between the calculated and experimental

222

k3χ(k) spectra was apparent for high values of k, which was attributed to the slightly longer I−O

223

bonds predicted by DFT compared to experimental data. Comparison of the I−O bond lengths

224

obtained from an energy minimization of NaIO3 and Ca(IO3)2 with X-ray diffraction (XRD) data

225

from the literature confirmed this result (Table 1). Because this difference was small (< 2%), its

226

effect was very slight in the FT magnitudes, which led to an overall good agreement with

227

experiment regarding the first-shell peak, albeit with a slight overestimation of its magnitude

228

(Figure 4).

229

Secondly, the simulations often overestimated the amplitude of the EXAFS signal at high

230

values of k, although experimental uncertainties may play a role at high k and the k3 weight will

231

accentuate these differences. The FT magnitudes showed that this effect stemmed from a slight

232

overestimation of the stiffness of the I−O covalent bonds as well as from enhanced scattering

233

from second-shell ions some of the simulations, but that the positions of the second-shell peaks

234

were well reproduced. This discrepancy was most evident for NaIO3 and KIO3 at 8-10 K and

235

greatly diminished with temperature, even for a temperature as low as 77 K (Figure 4).

236

Table 1. Comparison of calculated I−O bond lengths (Å) in NaIO3 and Ca(IO3)2 with XRD data. Comp. Pair XRD DFT ∆ (%) a

NaIO3 Ca(IO3)2 I−O(1) I−O(2) I(1)−O(1) I(1)−O(2) I(1)−O(3) I(2)−O(1) I(2)−O(2) I(2)−O(3) 1.802a 1.811a 1.825b 1.796b 1.801b 1.814b 1.795b 1.804b 1.827 1.838 1.856 1.829 1.832 1.840 1.823 1.828 1.4 1.5 1.7 1.8 1.7 1.4 1.6 1.3

Svensson and Ståhl 1988 38; bGhose et al. 19789


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The EXAFS spectra of NaIO3 and Ca(IO3)2 collected at 8 K by Podder et al.5 were much less

238

structured than both those measured in this work and those predicted by the simulations (Figure

239

S4), indicating that the temperature of their measurements was likely higher than that reported.

Figure 5. Effect of Na+ (1) and H+ (2) positions on the calculated I K-edge EXAFS spectra at 8 K (left; ∆E0 = 10 eV) and corresponding FT magnitudes (right; 3.5 Å−1 ≤ k ≤ 16.3 Å−1, Hanning window, dk = 1 Å−1). The effects of temperature (3) and surface segregation (4) for position Na B2 are also shown as examples. The experimental spectrum collected in this work at 8 K and its FT are also displayed for comparison in each panel.



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EXAFS signal of IO3− incorporated in calcite. The first two sections focused on determining

241

the minimum-energy configurations for the two CCS, but entropy and/or kinetic effects may lead

242

the charge compensating species to not occupy its minimum-energy site. Therefore, this section

243

investigates how the calculated EXAFS spectra depend on the nature and position of the charge

244

compensating species. The effect of temperature is also discussed. The calculated spectra are

245

then used in the next section to fit the EXAFS spectra collected in this work and in the work of

246

Podder et al.5

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For Na+ and H+ positions in the bulk (panels 1 and 2 of Figure 5), notable differences between

248

the calculated spectra were evident in the fine structure of the EXAFS spectra, and the FT

249

magnitudes (right panel of Figure 5) showed that those differences stemmed mostly from the

250

geometry of the second coordination shell around I. One exception is position H B3, in which

251

the proton formed a covalent bond with an iodate oxygen, thereby significantly elongating that

252

bond and greatly distorting the first-shell peak. The calculated spectra demonstrate that, despite

253

significant scattering from the first shell, structural disorder in the second shell can be identified

254

at low temperature where thermal disorder is minimized.

255

The amplitude of first-shell peak varied little (saved for position H B3) and only slightly

256

overestimated the FT of the experimental spectrum. In contrast, the amplitude of the second-shell

257

peak was much higher in the calculated FT. Although the analysis of the iodate standards

258

indicated that the calculations could overestimate the strength of scattering from the second shell,

259

the differences in amplitudes were much larger for the case of IO3− incorporated in calcite.

260

Additionally, when the temperature of the calculations was raised, as a way to artificially

261

increase thermal disorder, a temperature close to room temperature was required to fully account


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for the experimental result solely on the basis of temperature (panel 3 of Figure 5; position Na

263

B2 is used as an example).

264

Another possible phenomenon that would influence the signal from the second shell is IO3−

265

segregation to the surface. Indeed, comparison of the EXAFS spectra and corresponding FT

266

magnitudes of positions Na B2 (bulk) and Na S1d (surface) (panel 4 of Figure 5) showed no

267

difference in the position and intensity of the first-shell peak but a significant reduction in

268

scattering from second-shell ions when IO3− was located in the topmost atomic surface layer.

269

However, differences remained and it follows that the experimental data likely arose from a

270

significant proportion of surface IO3− ions together with incorporation in the bulk with various

271

CCS.

272

It should be noted that additional simulations indicated the I K-edge EXAFS spectra of IO3−

273

and I− were mostly out of phase (data not shown) and a small amount of iodine present as I−

274

could, therefore, significantly affect the measured EXAFS spectra. Although the presence of

275

iodide was not expected in the measured sample due to the lack of a reductant in its synthesis,

276

this consideration is particularly relevant to the groundwater at Hanford, which is expected to

277

contain iodine in both oxidation states.1

278

Linear combination fits to experimental EXAFS data. Linear combination fits (LCF) to the

279

experimental spectrum collected in this work were performed using the EXAFS spectra

280

calculated at 8 K for the nine configurations shown in Figure 2 and Figure 3 (using the largest

281

supercell available in each case and dry surface configurations, see Table S2). Average distances

282

and thermal disorder parameters for the nine configurations are provided in Table S5. The fits

283

were performed in reciprocal space with both k2 and k3 weights and the results are summarized in

284

Table S6. The single largest contribution to the fit with k3 weight (Figure 6) was the surface



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configuration involving H+ as the charge compensating species (position H S1d , 43%). Na+

286

position S also contributed, but to a much lesser extent (8%). Surface configurations therefore

287

accounted for half of the iodate inventory. Bulk Na+ configurations summed to 16% and, despite

288

its unfavorable incorporation energy relative to other bulk H+ configurations, position H B3

289

accounted for 31%.

Figure 6. Linear combination fits (k3 weight) to the experimental EXAFS spectrum collected in this work and to that of Podder et al.5 in reciprocal (left) and real (right) space using the EXAFS spectra calculated from AIMD trajectories of IO3− incorporated in calcite at 8 K and 77 K (including S1d configurations). Contributions from each configuration are listed in Table S6. 290

As was shown in Figure 5, the calculated FT magnitude of the first-shell peak for all CCS was

291

higher than that obtained from the measured EXAFS spectrum. Position H B3 was the exception

292

because of the lengthening of the I−O(H) bond, which explains its significant contribution to the

293

fit. Figure 5 also showed that bulk configurations led to second-shell peaks with higher FT

294

magnitudes than the experimental spectrum, which is why surface configurations accounted for

295

approximately half of the fit. Based on the analysis of the iodate standards, the magnitude of the

296

EXAFS signal at high k might have been overestimated due to the possible overestimation of the


Page 21 of 29


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stiffness of I−O bonds and of the scattering from second-shell ions. The fit was therefore

298

repeated first with k2 weight (Figure S5), which de-emphasizes the high k values relative to k3

299

weight, and then using spectra calculated at 77 K (Figure 6), to compensate for these effects by

300

artificially increasing the thermal disorder. The k2-weight fit showed increased contributions of

301

positions Na B2 and Na S1d to the detriment of positions H B3 and H S1d , but remained otherwise

302

similar (Table S6). The contributions from positions H B3 and H S1d also remained high in the

303

77-K fit.

304

The EXAFS spectrum of iodate-doped calcite reported by Podder et al.5 was fitted in the same

305

way (Figure 6) and resulted in a fit that was broadly similar, with positions H B3 and H S1d as

306

major contributors (Table S6), although the contribution from position Na S1d was significantly

307

increased in this case. Fits using the spectra calculated at 77 K (Figure 6), which are particularly

308

relevant given the likely increased temperature in the measurements of Podder et al. (see

309

discussion of the iodate standards above), still showed contributions from positions H B3 and H

310

S1d and resembled the fits to the spectrum collected in this work.

311

The effect of surface hydration was evaluated by repeating the fits with configurations Na and

312

H S1d replaced by configurations Na and H S1h (Figure S6). The fits remained generally similar

313

with positions H B3 and H S1h accounting for 65% to 81% of the iodate inventory. There was a

314

slight systematic improvement of the goodness of fit based on the reduced χ2 (Table S7). The

315

fact that positions H B3 and H S1d remained major contributors when the k weight, simulation

316

temperature, hydration state of the surface, and experimental spectrum were varied minimizes

317

the likelihood of this prediction being the result of simulation artifacts, such as those highlighted

318

in the discussion of the iodate standards. The combination of low-temperature measurements to

319

reduce thermal disorder and AIMD-EXAFS standards therefore enabled the characterization of a 21 ACS Paragon Plus Environment


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system with significant configuration disorder and despite the strong scattering from iodate

321

oxygen atoms, which dominates the EXAFS spectrum. Also noteworthy is the fact that

322

metastable configurations, such as position H B3, which might have been ruled out on the basis

323

of bulk thermodynamics alone, can play a significant role, likely due to kinetic limitations or

324

entrapment.34

325

Implications for iodate sequestration in calcite. Because of its implications for environmental

326

remediation of radioiodine-contaminated soils and sediments, iodate incorporation in calcite was

327

studied in this work to gather crucial information on the local coordination environment around

328

IO3−, the associated CCS, and the potential role of surface segregation. Best-fits to the

329

experimental data collected in this work and those reported by Podder et al.5 consistently showed

330

significant contributions from surface IO3− and a preference for charge compensation by H+. It

331

follows that pH should strongly affect the extent of iodate incorporation in calcite during

332

coprecipitation, as alluded to by Zhang et al.2 and Podder et al.5 Moreover, synthesis routes that

333

result in small calcite particles (i.e., large specific surface areas) are expected to enhance iodate

334

incorporation. However, IO3− ions accumulated at the surface of calcite crystals are likely to be

335

easily re-mobilized under conditions that might promote calcite dissolution. Calcite-based iodate

336

remediation strategies need to consider these impacts. How the presence of incorporated IO3−

337

ions might influence calcite dissolution is not yet known, but the identified CCS and extent of

338

surface segregation can be used as starting points for calculations of the effect of iodate

339

incorporation on calcite stability and solubility.

340

ACKNOWLEDGMENTS

341

This work was performed under the Deep Vadose Zone − Applied Field Research Initiative at

342

Pacific Northwest National Laboratory (PNNL) and funded by the U.S. Department of Energy


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(DOE) Richland Operations and Environmental Management Offices. PNNL is operated by

344

Battelle Memorial Institute for the DOE under Contract DE-AC05-76RL01830. MEH

345

acknowledges DOE Office of Nuclear Energy University Program grant number DE-NE0008568

346

for enabling her summer internship at PNNL. A portion of the research was performed using the

347

Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility

348

sponsored by the U.S. DOE’s Office of Biological and Environmental Research and located at

349

PNNL in Richland, WA. Use of the Stanford Synchrotron Radiation Lightsource, SLAC

350

National Accelerator Laboratory, is supported by the DOE, Office of Science, Office of Basic

351

Energy Sciences under Contract No. DE-AC02-76SF00515. The authors acknowledge Micah P.

352

Prange for fruitful discussions.

353

AUTHOR INFORMATION

354

Corresponding Author

355

*

356

SUPPORTING INFORMATION

357

Optimized lattice parameters of solid phases; computational details for DFT and FEFF

358

calculations; approach used to compute incorporation energies; synthesis of iodate-doped calcite

359

sample; preparation of iodate-doped calcite sample and iodate standards for EXAFS analysis;

360

atomistic models of iodate incorporation in bulk calcite and at the (104) surface; competitive

361

incorporation of NaIO3 and HIO3 with respect to aqueous ions; calculated and experimental

362

EXAFS of NaIO3 and Ca(IO3)2 at 8 K; average distances and thermal disorder parameters from

363

AIMD simulations of iodate incorporation; results of linear combination fits to experimental

364

EXAFS spectra, including effect of surface hydration.

Phone: (509) 371-6382; e-mail: [email protected].



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TOC

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Incorporation Modes of Iodate in Calcite - Environmental Science

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