Mechanistic Understanding of Uranyl Ion Complexation on

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Environmental Modeling

A mechanistic understanding of uranyl ion complexation on montmorillonite edges: a combined first principles molecular dynamics-surface complexation modeling approach Chi Zhang, Xiandong Liu, Ruth M. Tinnacher, and Christophe Tournassat Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02504 • Publication Date (Web): 27 Jun 2018 Downloaded from http://pubs.acs.org on June 28, 2018

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Environmental Science & Technology

1

A mechanistic understanding of uranyl ion

2

complexation on montmorillonite edges: a combined

3

first principles molecular dynamics-surface

4

complexation modeling approach

5

Chi Zhang 1, Xiandong Liu 1*, Ruth M. Tinnacher2, Christophe Tournassat 3,4,5 * 1

6 7 8

State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, P.R. China

2

Department of Chemistry & Biochemistry, California State University East Bay, Hayward, CA

9

94542, USA 3

10 4

11 12

Université d’Orléans−CNRS/INSU−BRGM, UMR 7327 Institut des Sciences de la Terre d’Orléans (ISTO), France

5

13 14 15 16

BRGM, Orléans, 45060, France

Energy Geosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA

*

Corresponding author: [email protected], [email protected].

1 Environment ACS Paragon Plus


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TOC

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Abstract

22

Systematic first principles molecular dynamics (FPMD) simulations were carried out to study

23

the structures, free energies, and acidity constants of UO22+ surface complexes on

24

montmorillonite in order to elucidate the surface complexation mechanisms of the uranyl ion

25

(UO22+) on clay mineral edges at the atomic scale. Four representative complexing sites were

26

investigated, that is, ≡Al(OH)2 and ≡AlOHSiO on the (010) surface, and ≡AlOHOa and ≡SiOOa

27

on the (110) surface. The results show that uranyl ions form bidentate complexes on these sites.

28

All calculated binding free energies for these complexes are very similar. These bidentate

29

complexes can be hydrolyzed and their corresponding derived pKa values (around 5.0 and 9.0 for

30

pKa1 and pKa2respectively) indicate that UO2(OH)+ and UO2(OH)2 surface groups are the

31

dominant surface species in the environmental pH range. The OH groups of UO2(OH)2 surface

32

complexes can act as complexing sites for subsequent metals. Additional simulations showed

33

that such multinuclear adsorption is feasible and can be important at high pH. Furthermore,

34

FPMD simulation results served as input parameters for an electrostatic thermodynamic surface

35

complexation model (SCM) that adequately reproduced adsorption data from the literature.

36

Overall, this study provides an improved understanding of UO22+ complexation on clay mineral

37

edges.



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1. INTRODUCTION

39

Uranium is an important radionuclide in natural environments and nuclear waste 1. In the

40

recent decades, due to the activities of mining and milling at uranium mines, considerable

41

amounts of uranium has been leached into subsurface systems, which will continuously

42

contaminate the water and soil in the vicinity of these sites 2.

43

Adsorption of radionuclides onto mineral surfaces is a key process affecting their migration in

44

the environment 3-5. Clay minerals usually play significant roles in the transport and retention of

45

metal cations in soils and aquifers because of their high adsorption capacity and high abundance

46

in nature 6-9. Clay materials have also been foreseen as engineered barriers in future repositories.

47

In addition, argillaceous rocks are the potential host rocks for deep geological disposal sites in

48

some countries including France, Belgium, and Switzerland

49

understanding of the adsorption of uranyl on clay minerals is not only helpful for predicting the

50

environmental behavior of radionuclides but also important for the long-term risk assessment of

51

nuclear waste disposal in deep geological disposal repository sites.

10, 11

. Therefore, a mechanistic

52

Extensive experimental studies using techniques such as batch sorption experiments, EXAFS

53

(extended X-ray absorption fine structure), and TRLFS (time resolved laser-induced

54

fluorescence spectroscopy) measurements have been conducted to investigate the surface

55

complexation of uranyl on clay minerals

56

form outer-sphere complexes (i.e., by cation exchange in the interlayer space) at lower pH and

57

ionic strength, and inner-sphere complexes (i.e., complexation on the edge surfaces) at higher pH.

58

It has also been proposed that at high pH or U(VI) concentrations, polynuclear adsorption or

59

precipitation is possible 30, 31.

12-29

. These studies have confirmed that uranyl ions


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60


Among the EXAFS studies, Hennig et al. 25 proposed that UO22+ forms bidentate complexes at 23

61

aluminol groups; Křepelová et al.

62

formation of edge-sharing complexes with Al octahedra and/or Si tetrahedra. Schlegel and

63

Descostes

64

of edge surfaces. Furthermore, it was proposed that Si tetrahedra are possible binding sites, as

65

well as Al octahedra, on montmorillonite edges

66

complexing sites and identified that bidentate complexes are the major surface species.

27

ascribed the uranyl species adsorbed on kaolinite to the

concluded that UO22+ forms mononuclear bidentate complexes on the Al octahedra

15

. These studies have suggested possible

67

Molecular simulation has been proven an effective tool for probing the chemical processes on

68

mineral-water interfaces 32-40. Quantum chemistry techniques have been used to calculate uranyl

69

adsorption on clay edges 41-44. For example, by using DFT-based static calculations, the possible

70

complexing sites have been revealed: ≡AlOHOH and ≡AlOHSiO on (010) edges and ≡AlOHOa

71

(Oa means the apical O linking both Al and Si), ≡AlOHSiO and ≡SiOOa on (110) edges

72

These studies employed geometry optimization in vacuum or just including several H2O

73

molecules, which oversimplified solvent effects and thus could not accurately estimate

74

adsorption free energies.

42

.

75

In the past decades, surface complexation models (SCMs) based on macroscopic sorption

76

experiments have been exploited to interpret and model the adsorption of UO22+ on clay minerals

77

over a wide range of experimental conditions

78

to some extent, rests on the specific parameters used, such as site types and numbers, surface

79

potential models, deprotonation constants, and binding constants

80

Fernandes et al.

81

Cation Exchange (2SPNE SC/CE) model to describe uranyl sorption on montmorillonite in the

82

absence and presence of CO32- . More recently, Tournassat et al.

15

15, 22, 45-48

. The accuracy of such modeling results,

18

. For instance, Marques

applied the Two Site Protolysis Non-Electrostatic Surface Complexation and


49

developed a new SCM for


83

montmorillonite edge surfaces taking into account the spillover effect of the electrostatic

84

potential at the basal surface on the electrostatic potential at the edge surfaces. With this model,

85

Tournassat et al.

86

single site and three adsorption reactions that correspond to the hydrolysis of surface complexed

87

UO22+ with increasing pH

88

could also be more than one site with those sites possessing similar surface complexation

89

energies, since SCMs alone cannot discriminate sites with similar affinities.

50

found that experimental adsorption data for UO22+ can be well fitted by a

50

. The single-site of this model could be really one type of site, but

90

On the basis of the above-mentioned studies, one can summarize that multiple binding sites

91

exist on edge surfaces including octahedral and tetrahedral positions. However, the relative

92

stabilities of the complexes on different sites have not been revealed. Surface complexed UO22+

93

is very likely to become hydrolyzed with increasing pH (e.g., UO22+ has a pKa1 of ~5.6 in water

94

51

), but the pH-dependent speciation of adsorbed UO22+ is still unclear.

95

In this study, we carried out first principles molecular dynamics (FPMD) simulations

96

systematically to investigate the complexing structures and free energies of UO22+ ions on

97

montmorillonite edge surfaces, as well as the hydrolysis constants of montmorillonite-UO22+

98

surface complexes. This atomic-level information was then implemented in a SCM for

99

montmorillonite edge surface sites. The overall objective of this study was to test the feasibility

100

of combining atomistic simulations with thermodynamic modeling in order to build a physically-

101

grounded methodology for future adsorption modeling studies.

102 103 104

2. METHOD 2.1. FPMD models


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105


The (010) and (110) surface models were taken from our previous studies

52, 53

. The unit cell

106

formula of the clay used was Li0.5[Si8] [Al3.5Mg0.5]O20(OH)4. Two unit cells were contained in

107

the clay framework (Fig. 1). Therefore, one Mg for Al substitution was in the octahedral sheet. A

108

lithium counterion was placed in the interlayer for compensating the negative charge of the clay

109

framework. Li+ was selected because of its lower computational cost compared to Na+ and K+.

110

All of the surface models were placed in a 3D periodically repeated box (10.36Å×28Å×12Å)

111

with a solution region of 18 Å. Ten water molecules were inserted into the interlayer to create a

112

one-layer hydrate, and 67 water molecules were placed in the solution region, which

113

approximately reproduced the density of bulk water at room temperature. Initially, UO22+ was

114

placed at the complexing sites according to previous static density functional theory (DFT)

115

calculations42. Four complexing sites were investigated, that is, ≡Al(OH)2 and ≡AlOHSiO on the

116

(010) edge and ≡AlOHOa and ≡SiOOa (Oa means the apical O linking Al and Si) on the (110)

117

edge. For each site configuration, the overall charge is +1. For the simulation of multinuclear

118

complexes, two UO22+ were placed in the systems. The first UO22+ is adsorbed on the ≡AlOHOH

119

site; the second UO22+ is adsorbed on the two OH groups of the first one created through

120

hydrolysis. (Fig. 1B). The overall charge of the multinuclear complex system is +1. A uniform

121

background charge was added to neutralize the overall system charge.54

122 123 124



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125 126

Fig. 1. The model used for uranyl complexation on the (010) edge surface. A: Model for

127

mononuclear complex. B: Model for binuclear complex. The snapshots are derived from

128

simulations. Si = yellow; Al = purple; Mg = green; O = red; H = white; U = blue; Li = cyan.

129

2.2. FPMD setups

130

The CP2K/QUICKSTEP package

55

was employed to carry out all calculations in this study.

131

In QUICKSTEP, electronic structures were calculated with density functional theory (DFT)

132

implemented by using a hybrid Gaussian plane wave approach (GPW)

133

Ernzerhof (PBE) functional

134

applied. Double-zeta valence polarized (DZVP) basis sets

135

Al, Si, and U. The plane wave density cutoff was set to be 400 Ry. Our previous study has

57

56

. The Perdew–Burke-

and analytic Goedecker-Teter-Hutter pseudopotentials 59


58

were

were employed for H, Li, O, Mg,

Page 9 of 29


136

shown that this setup reproduces the pKa of the reaction UO22+ + H2O = UO2(OH)+ + H+ with an

137

accuracy of one unit 60.

138

Born-Oppenheimer molecular dynamics (BOMD) simulations were performed with a wave

139

function optimization tolerance of 10-6 and a time step of 0.5 fs. The temperature was controlled

140

at 330 K with the Nosé-Hoover chain thermostat. This elevated temperature was intended to

141

avoid the glassy behavior of Generalized Gradient Approximation (GGA) liquid water at lower

142

temperature.61 For each simulation, a production run was carried out for approximately 5.0-20.0

143

ps after an equilibration stage of at least 2.0 ps.

144

2.3. Method of constraint

145

To quantitatively estimate the stability of each surface complex, a series of constrained FPMD

146

simulations were conducted to calculate their respective desorption free energies. In this method,

147

the free energy change (∆F) was computed by integrating the mean force (f) along the reaction

148

coordinate (Q) according to the thermodynamic integration relation 62, 63.

149

∆F (Q) = −∫ dQ' f (Q')

150

(1)

Q

Q0

151

For the (010) surface, in the free energy calculations of desorption from ≡Al(OH)2, the

152

coordination number (CN) of U with respect to the O of the two ≡AlOH groups was chosen as Q.

153

For the desorption from ≡AlOHSiO, we divided this U(VI) desorption process into two steps: (1)

154

the transformation from a bidentate complex on ≡AlOHSiO to a monodentate complex on ≡SiO

155

and (2) from a monodentate complex on ≡SiO to an outer-sphere complex. In step (1), the



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156

distance between U and the O of ≡AlOH was selected as Q; in step (2), the distance between U

157

and the O of ≡SiO was chosen as Q.

158

For the (110) surface, in the free energy calculations of U(VI) complexation on ≡AlOHOa, the

159

desorption was controlled by selecting the Al-U distance as Q. The desorption from ≡SiOOa site

160

was divided into two steps: (1) from a bidentate complex on ≡SiOOa to a monodenate complex

161

on ≡SiO by choosing the ≡Oa-U distance as Q and (2) from a monodentate complex to outer-

162

sphere complex by choosing the distance between U and the O of the ≡SiO group as Q.

163

164

The CN was determined by:

n=∑ i

165

1 − (ri / rc )12 1 − (ri / rc ) 24

(2)

Here n, denotes the CN, ri, the distance between U and the O of ≡AlOH groups, and rc the

166

cutoff value. In this study, rc was set to be 3.1 Å.

167

2.4. Acidity constant calculations

168 169

170

The acidity constants values (Ka) of the four uranyl surface complexes were evaluated for the surface reactions: ≡UO2(H2O)3+

=

≡UO2(H2O)2(OH) + H+

Ka1

(3)

≡UO2(H2O)2(OH)

=

≡UO2(H2O)(OH)2- + H+

Ka2

(4)

≡UO2(H2O)(OH)2-

=

≡UO2 (OH)32- + H+

Ka3

(5)

where ≡ depicts a surface site, and with:

‫ܭ݌‬௔ଵ = −logଵ଴ ‫ܭ‬௔ଵ = − logଵ଴

൫ୌశ ൯ሺ≡୙୓మ ሺୌమ ୓)మ ሺ୓ୌ)) ൫≡୙୓మ ሺୌమ ୓)శ య൯


(6)

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‫ܭ݌‬௔ଶ = −logଵ଴ ‫ܭ‬௔ଶ = − logଵ଴

൫ୌశ ൯ሺ≡୙୓మ ሺୌమ ୓)ሺ୓ୌ)ష మ)

(7)

ሺ≡୙୓మ ሺୌమ ୓)మ ሺ୓ୌ)) ൫ୌశ ൯൫≡୙୓మ ሺ୓ୌ)మష య ൯

‫ܭ݌‬௔ଷ = −logଵ଴ ‫ܭ‬௔ଷ = − logଵ଴ ሺ≡୙୓

(8)

ష మ ሺୌమ ୓)ሺ୓ୌ)మ )

171

where values in brackets represent activities. The pKa values were calculated through the half-

172

reaction scheme of vertical energy gap method 64, 65. This technique has been validated on many

173

molecular acids with an accuracy of 2 pKa units

174

mineral surface groups of oxides

175

Supplementary Data 1 in the Supplementary Information (SI) for the details of the method.

176

2.5. Surface complexation modeling

177

68, 69

64-67

. It has also been applied successfully to

, hydroxides

70

, and clay minerals

53, 71, 72

. See

An in-house version of PHREEQC, which was modified to handle the spillover effect of the 49, 50

178

basal surface potential on the edge surface potential

179

complexation calculations, together with the database THERMOCHIMIE v.9b0 for

180

thermodynamic parameters of the solute species 73. This database is available in various formats

181

including the PHREEQC format at the following address: https://www.thermochimie-tdb.com/.

182 183

, was used to carry out the surface

3. RESULTS AND DISCUSSION 3.1. Structures and free energies

184

The structures of UO22+ complexes adsorbed on edge sites are illustrated in Fig. 2. It can be

185

seen that UO22+ formed bidentate complexes on the ≡Al(OH)2 and ≡AlOHSiO sites of the (010)

186

surface and ≡AlOHOa and on the ≡SiOOa site of the (110) surface.

187

For the (010) edge, ≡Al(OH)2-UO2 has two equivalent U-O bonds with an average bond

188

length of 2.37 Å (Fig. 2A). For the complex on the ≡AlOHSiO site, the bond lengths of ≡AlOH-



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189

UO2 and ≡SiO-UO2 are 2.48 Å and 2.25 Å, respectively (Fig. 2B). For the (110) edge, in the

190

≡AlOHOa-UO2 complex , the bond length between UO22+ and ≡AlOH is 2.33 Å and ≡Oa-UO2

191

has a bond length of 2.50 Å (Fig. 2C). This complex has similar bond lengths to ≡AlOHSiO-

192

UO2(H2O)3 on the (010) surface. For the complex on ≡SiOOa, the bond lengths of ≡SiO-UO2

193

and ≡Oa-UO2 are 2.25 Å and 2.79 Å, respectively (Fig. 2D). The latter is longer than the ≡Oa-

194

UO2 bond in ≡AlOHOa-UO2.

195

In experiments, it is difficult to assign the obtained U-Al/Si distances to explicit sites because

196

the derived structure is the average of multiple scattering, which as a result can cause

197

considerable uncertainty. The distances between U and their closest Al/Si neighbors obtained

198

from our calculations and previous DFT static calculations

199

(see values in Fig. 2). It can be seen that each distance agrees with its counterpart to within 0.1 Å.

200

Among the available spectroscopic studies of UO2-montmorillonite systems, Hennig et al.

201

detected a U-Al pair at 3.40-3.44 Å; Schlegel and Descostes 27 found an atomic shell of U-Al/Si

202

at 3.31 Å; the EXAFS measurements by Fernandes et al. 15 resolved two shells: one Si/Al shell at

203

3.09 Å and the other at 3.29 Å. These experimentally measured distances can be divided into two

204

categories of values: one is at ~3.1 Å and the other between 3.29 Å and 3.44 Å. Our FPMD-

205

derived U-Al/Si distances also fall into two regions, 3.17 Å and ~3.47 Å-3.62 Å, which roughly

206

correspond to the two experimental ranges.

42


are also provided for comparison

25

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207 208

Fig. 2. The structures of uranyl surface complexes on the (010) (left figure) and (110) (right

209

figure) edge sites. For clarity, solvent water molecules and interlayer Li+ are hidden. All bond

210

lengths and distances are in Å. The values in parenthesis are the counterpart distances from DFT

211

static calculations.42

212 213

Figure 3 provides the calculated desorption free-energy curves for UO22+ ions. The details of

214

the desorption processes are described in the SI (Supplementary Data 2). The corresponding

215

values are collected in Table 1. The free energies on all of the investigated sites are very close

216

(within 1.7 kcal/mol), which means that the four complexes have very similar thermodynamic

217

stability.

218



219 220

Fig. 3. The desorption free-energy profiles of UO22+ complexing on edge surface sites. Lines

221

added for improved visualization. In B and D, the offset in x-axis refers to each of the two steps.

222

CN in A means the coordination number of U with respect to the two O of Al(OH)2. Distance in

223

B means the distance between U and the O of AlOH (2.5-3.9Å for step 1) and between U and the

224

O of SiO (2.3-3.1Å for step 2), respectively. Distance in C means the distance between U and the

225

Al of AlOHOa. Distance in D means the distance between U and Oa (2.8-4.0Å for step 1) and

226

between U and the O of SiO (2.3-3.0Å for step 2), respectively.

227

Table 1. Desorption free-energy values of uranyl from edge surface sites.

Surface 010

Site

∆F (kcal/mol)

≡Al(OH)2

21.5±1.7

≡AlOHSiO

21.1±2.4

≡AlOHOa

20.0±1.5

≡SiOOa

19.8±1.3

110


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228 229 230 231

3.2. Complexation mechanism of uranyl ion The simulations show that uranyl ions form bidentate complexes on edge sites, which is consistent with the conclusions derived from spectroscopic studies 15, 25, 27.

232

Due to the relatively large cross-sectional area of the UO22+ ion, adsorbed UO22+ influences

233

the availability of neighboring sites. For instance, as shown in Fig. 2A, the UO22+ adsorbed on

234

the ≡Al(OH)2 site of the (010) surface has occupied the octahedral position, and this will prevent

235

complexation on the ≡AlOHSiO site in the same unit cell. Similarly, for the (110) surface, the

236

adsorption on the ≡AlOHOa site will prevent any binding to the ≡SiOOa site in the same unit

237

cell (see Fig. 2C). Likewise, Fig. 2B and Fig. 2D also clearly show that the UO22+ on these sites

238

will prevent complexation reactions on neighboring sites. This site limitation is in sharp contrast

239

with transition metal binding. For example, as shown in our previous simulation studies on Cd2+

240

and Ni2+ binding74-76, each unit cell can adsorb two Cd2+ or Ni2+ ions 76 but only one UO22+ ion.

241

It is very likely that this steric site limitation is also relevant for other actinyl species.

242

3.3. pKa values and surface speciation

243

Table 2. pKa values of UO22+ complexes on (010) and (110) edge sites according to FPMD

244

results. Surface complexes

pKa1

pKa2

pKa3

[010]≡Al(OH)2-UO2(H2O)3+

5.8±0.8

9.9±1.4

10.9±0.8

[010]≡AlOHSiO-UO2(H2O)3+

4.6±1.0

8.6±0.7

10.1±1.0



Page 16 of 29

[110]≡AlOHOa-UO2(H2O)3+

5.8±1.7

9.7±2.0

-

[110]≡SiOOa-UO2(H2O)3+

5.5±1.5

9.0±2.0

-

245 246

The obtained energy gaps and free energy changes of uranyl complexes are listed in the SI

247

(Table S2), and the calculated pKa values are summarized in Table 2. It is clear that the four

248

complexes have very similar pKa values: all pKa1 and pKa2 values are about 5 and 9, respectively.

249

The pKa3 values calculated for (010) are also similar (values higher than 10). It can therefore be

250

inferred that the complexes on the (110) edge should also have similarly high pKa3 values. In

251

aqueous solution, pKa1, pKa2, and pKa3 values are 5.3, 6.9, and 8.1, respectively 77. So the surface

252

complexation of UO2+2 has little effect on the pKa1 value, but slightly increases the pKa2 and

253

pKa3 values.

254

As the amount of UO2(OH)+ and UO2(OH)2 groups gradually accumulate at clay-water

255

interfaces, these OH groups can serve as complexing sites for other UO22+ ions. This makes

256

polynuclear complexation and even surface-enhanced precipitation possible. Additional

257

simulations showed that the ≡Al(OH)2-UO2(OH)2 complex can adsorb a second UO22+ ion by

258

forming a binuclear complex (see Fig. 4). The U-U distance in this complex derived from a

259

trajectory of 30 ps is 3.92 Å, matching well with the experimentally obtained value by EXAFS at

260

pH 9.7 for the UO2-gibbsite system (~3.9 Å) 31.


Page 17 of 29


261 262

Fig. 4. Structure of the binuclear complex on the (010) edge.

263

3.4. Implications for SCM of U(VI) adsorption on montmorillonite edge surfaces

264

The FPMD results can be further used to constrain a SCM, which is necessary to accurately

265

predict uranium mobility in clay-rich environments. A SCM makes it possible to capture the

266

complex uranium solution and surface speciation, which allows the prediction of changes in

267

adsorption and desorption as a function of chemical solution conditions over time and space.

268

Recently, Tournassat et al. 50 published a SCM for montmorillonite that specifically accounted

269

for the spillover effect that takes place on montmorillonite edge surfaces due to the interaction of

270

its electrostatic potential with that of the neighboring basal surfaces. The consideration of this

271

spill-over effect is necessary to consistently simulate the best acid-base titration data available

272

for montmorillonite, and more importantly, to reproduce the absence of a point of zero salt effect

273

or isoelectric point in experimental potentiometric titration data

274

describing edge surface protonation and deprotonation reactions was obtained from acidity

275

constant values calculated in our previous FPMD work

276

to a charge / potential relationship that is specific to montmorillonite edge surfaces and that takes

277

into account the spillover effect 49. No fitted parameters were necessary for this part of the SCM.

53, 71, 78


49

. The set of parameters

. These parameters were coupled


278

U(VI) adsorption data were then adequately described over a wide range of pH, ionic strength

279

and dissolved inorganic carbon (DIC) concentration conditions with the addition of one uranyl

280

adsorption reaction and three hydrolysis reactions 50. The parameters of this adsorption model are

281

given in Supplementary Data 3 and 4 in the SI, together with PHREEQC input files, which allow

282

to run the model for montmorillonite particles dispersed in NaCl electrolyte and in the absence or

283

presence of dissolved inorganic carbon. An example U(VI) adsorption curve as a function of pH

284

is also provided, which shows the relative contributions of the three U(VI) surface species

285

(Figure S3 in the SI).

286

In this new study, we will show how FPMD simulation results can help to (1) constrain the

287

values taken by the parameters in the SCM, especially the hydrolysis reactions, and (2) infer new

288

conclusions concerning the main mechanisms leading to U(VI) retention on montmorillonite

289

surfaces (nature of the adsorbing surface site; U(VI) polymerization at the surface).

290

In the SCM of Tournassat et al.49, 50, one edge surface functional group corresponds to the

291

combination of one octahedral and two tetrahedral cations together with their associated OH

292

groups. With this scheme it is possible to define surface complexation reactions that correspond

293

to the reactions probed with FPMD calculations, i.e. U(VI) adsorption reaction on (010) and

294

(110) edge surfaces and hydroxylation of U(VI) surface complexes. Unfortunately, the binding

295

free energy values computed with FPMD calculations (Table 1) cannot directly be used to

296

determine surface complexation reaction constants because, for such a conversion, the free

297

energy profile needs to be calculated from inner-sphere adsorption to a completely solvated state

298

in bulk solution 79. Such calculations are currently hindered by the expensive computational cost

299

of the FPMD technique. Hence, the present study just focuses on the transformation from inner-

300

sphere to outer-sphere complexes. In addition, the adsorption energies calculated from FPMD


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301

relate to adsorption reactions on four different sites having the same formal charge, i.e. -1, before

302

U(VI) adsorption. Consequently, the computed U(VI) adsorption energy must be corrected for

303

differences in energy required to meet these site charge conditions as a function of adsorption

304

site configurations. The maximum number of H+ complexed to each of the functional sites is 4,

305

and the functional site charge is +1 if no isomorphic substitutions are present. The site can thus

306

be depicted as ≡SiteOH4+. U(VI) adsorption reactions were thus defined for each site:

307

≡SiteOH4+ + UO22+ = ≡SiteOH2UO2+ + 2 H+

308

The types of groups bearing the two remaining H in ≡SiteOH2UO2+ depend on the nature of

309

the complexing site. In the case of configuration A: ≡Al(OH)2 on the (010) surface (see Fig. 2),

310

the two Si-OH groups remain protonated. In the case of configuration B: ≡AlOHSiO on the (010)

311

surface, only one Si-OH group remains protonated and the Al-(OH)2 group remains singly

312

protonated. The FPMD calculation results correspond to the reaction:

313

≡SiteOH2- + UO22+ = ≡SiteOH2UO2+

log KsiteOH4, U

log KsiteOH2, U

(9)

(10)

314

and accordingly, the log KsiteOH2, U should have similar values for all types of sites. In a first step,

315

we fixed the log KsiteOH2, U at the same arbitrary value, and derived the log KsiteOH4, U values from

316

the relationship:

317

log KsiteOH4, U = log KsiteOH2, U – pKa1, site – pKa2, site

318

where pKa1, site and pKa2, site are the pKa values of the groups that were deprotonated in the FPMD

319

calculations. On each site complexed with U(VI), we considered that the deprotonation reaction

320

constants of remaining protonated groups were not influenced by U(VI) adsorption. In addition,


(11)


321

FPMD calculations showed that the complexation of UO22+ on these sites will prevent

322

complexation on neighboring sites. This effect was taken into account in the SCM with the

323

combination of two functional sites, one bearing the complex with U(VI) and the other being free

324

of U(VI). The adsorption extents of U(VI) in each of the four site configurations were evaluated

325

independently with a log KsiteOH2, U value of 7.3 (Figure 5A).

326

Under these modeling conditions, ≡AlOHOa groups on the (110) surface had the highest

327

affinity for U(VI) followed by ≡AlOHSiO groups on the (010) surface, ≡SiOOa groups on the

328

(110) surface, and ≡Al(OH)2 groups on the (010) surface in decreasing affinity order. It was not

329

possible to reproduce the U(VI) adsorption edge with the (110)≡AlOHOa groups, while a good

330

fit was obtained between pH 5 and 8.5 with the (010)≡AlOHSiO groups alone. ∆F and pKa

331

values calculated with FPMD indicated that the various surface sites had similar affinity for

332

U(VI). However, the differences among the values and their associated uncertainties allowed us

333

to vary the affinity constants in the SCM over a limited range of values.

334

A sensitivity analysis on pKa values indicated that the SCM results were not very sensitive to

335

these parameters, while the affinity constants for the adsorption of U(VI) on the functional sites

336

were the main contributors to the position and maximum height of the adsorption curves. A 2.2

337

log unit decrease of the log KsiteOH2, U values for (110)≡AlOHOa groups, together with a 0.1 log

338

unit decrease of the log KsiteOH2, U values for (010) ≡AlOHSiO made it possible to match the

339

prediction of the empirical model of Tournassat et al. 49 between pH 5 and 8. This decrease was

340

compatible with the lower ∆F value of desorption at the (110)≡AlOHOa site compared to the

341

(010) ≡AlOHSiO site and the associated error bands (Table 1). Another possible explanation to

342

the discrepancies could also be the structure of the montmorillonite used. The presented

343

simulations are based on the so called trans-vacant model (Tournassat et al., 2016). Another


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344

structural polymorph (cis-vacant one) was not considered here. The structure of edges on cis-

345

and trans-vacant structures is different and may result in different sorption energies for U-

346

complexes.

347

Data above pH 8.5 were underestimated by our new SCM (Figure 5B). In their original 50

348

model, Tournassat et al.

needed to include a U(VI) surface complex with a -3 charge that

349

corresponds to a fourth deprotonation of the U(VI) surface complex. These deprotonation

350

reactions are constrained in our model by the FPMD results. SCM sensitivity analysis conducted

351

on pKa values within the error bands estimated from FPMD calculations revealed that only

352

neutral and singled negatively charged surface sites significantly contributed to U(VI)

353

adsorption. U(VI) adsorption at pH above 9 must be explained with a mechanism other than the

354

hydrolysis of adsorbed U(VI). This additional mechanism may be the formation of U(VI)

355

binuclear complexes as suggested by our FPMD results. Consequently, we added a single

356

reaction on the (010) ≡AlOHSiO site for the formation of this polynuclear complex (for clarity

357

and completeness of further model evaluation, the full details of the reactions are available in the

358

form of a commented PHREEQC file in Supplementary Data 5 in the SI).

359

As a final step, we tested our model against data published in the literature. As expected from

360

the good match of our new model to the predictions of the previous model from Tournassat et al.,

361

(2018)

362

(Supplementary Data 6 in SI). We cannot prove that there is a unique set of parameters that

363

makes it possible to fit U(VI) adsorption data. However, the combination of our SCM with

364

thermodynamic and structural constraints gained from FPMD calculations made it possible to

365

gain important insights related to the mechanisms responsible for U(VI) adsorption on clay

366

mineral edge surfaces. In particular, adsorption properties at pH values above 8 cannot be

50

, we were able to adequately predict U(VI) adsorption data from the literature



367

explained by the deprotonation of adsorbed U(VI) species alone, as frequently hypothesized in

368

previous SCM studies

369

necessary to further assess the importance of the formation of polynuclear surface complexes on

370

the montmorillonite edges at pH values above 8.

15, 50, 80

. Additional spectroscopic and/or microscopic characterization is

371 372

Fig. 5. A: Comparison of U(VI) adsorption modeling results according to the empirical model

373

from Tournassat et. al (2018) (circles)

374

results (lines). The log KsiteOH2, U value was fixed at 7.3 for all four U(VI) adsorption reactions

375

(see text for details). B: Same model with log KsiteOH2, U value of ≡AlOHOa groups decreased by

376

2.2 log units (red dash line: contribution of the (010) surface; blue dotted line: contribution of the

377

(110) surface). The black line (Total) is indicative of the model result with the consideration of

50

and according to parameters constrained by the FPMD


Page 22 of 29

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378

binuclear complex formation. Montmorillonite concentration: 0.5 g⋅L-1; electrolyte background:

379

NaCl, 0.1 mol⋅L-1; total U(VI) concentration: 1 µmol⋅L-1.

380



381

ACKNOWLEDGMENT

382

The authors acknowledge the National Science Foundation of China (Nos. 41222015, 41273074,

383

41425009, and 41572027), Special Program for Applied Research on Super Computation of the

384

NSFC-Guangdong Joint Fund (the second phase) under Grant No. U1501501, the Newton

385

International Fellow Program and financial support from the State Key Laboratory for Mineral

386

Deposits Research. We are also grateful to the High Performance Computing Center of Nanjing

387

University for use of the IBM Blade cluster system. This research is being performed using

388

funding received from the U.S. DOE Office of Nuclear Energy's Nuclear Energy University

389

Program (Federal Grant No. DE-NE0008683). CT acknowledges funding from Andra in the

390

framework of the Andra-BRGM partnership and the program CTEC (P.I. Jean-Charles Robinet,

391

Mélanie Lundy and Benoît Madé).

392

SUPPORTING INFORMATION

393

Two tables, seven figures with additional information on FPMD and SCM results. Ready-to-use

394

PHREEQC input file scripts to reproduce our SCM results.

395 396


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397

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