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Effects of mineral compositions on matrix diffusion and sorption of Se(IV) in granite 75

Xiaoyu Yang, Xiangkun Ge, Jiangang He, Chunli Wang, Liye Qi, Xiang-Yun Wang, and Chunli Liu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05795 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on January 1, 2018

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Page 1 of 35

Environmental Science & Technology

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Effects of mineral compositions on

2

matrix diffusion and sorption of 75Se(IV) in granite

3

Xiaoyu Yang†, Xiangkun Ge‡, Jiangang He†, Chunli Wang†§, Liye Qi†, Xiangyun Wang†, Chunli

4

Liu*†

5

† Beijing National Laboratory for Molecular Sciences, Fundamental Science Laboratory on

6

Radiochemistry & Radiation Chemistry, College of Chemistry and Molecular Engineering,

7

Peking University, Beijing, 100871, China

8

‡ Beijing Research Institute of Uranium Geology Analtical Laboratory of BRIUG, Beijing,

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100029, China

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§Nuclear and Radiation Safety Center, Beijing, 100082, China

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ACS Paragon Plus Environment

1


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ABSTRACT

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Exploring the migration behaviors of selenium in granite is critical for the safe disposal of 75

Se(IV) (analogue for

79

15

radioactive waste. The matrix diffusion and sorption of

Se) in granite

16

were systematically studied to set reliable parameters in this work. Through-diffusion and batch

17

sorption experiments were conduct with four types of Beishan granite. The magnitudes of the

18

obtained apparent diffusion coefficient (Da) values are of the following order: monzogranite >

19

granodiorite-2 > granodiorite-1, which is opposite to the sequence of the Kd values obtained from

20

both the diffusion model and batch sorption experiments. The EPMA results of the granitic flakes

21

showed that there was no obvious enrichment of Se(IV) on quartz, microcline and albite. Only

22

biotite showed a weak affinity for Se(IV). Macroscopic sorption behaviors of Se(IV) on the four

23

types of granite were identical with the sequence of the granitic biotite contents. Quantitative

24

fitting results were also provided. XPS and XANES spectroscopy data revealed that bidentate

25

inner-sphere complexes were formed between Se(IV) and Fe(III). Our results indicate that biotite

26

can be representative of the Se(IV) sorption in complex mineral assemblages such as granite, and

27

the biotite contents are critically important to evaluate Se(IV) transport in granite.

28 29 30 31 32 33 34


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1. INTRODUCTION

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With the development of nuclear energy, high-level radioactive waste disposal has become one

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of the major concerns for humanity. As a fission product of nuclear materials, 79Se is one of the

38

critical radionuclides for the long-term safety of radioactive waste repositories due to its long half-

39

life ( ~ 3.28 × 105 years)1, radiological toxicity2 and high mobility3. Five possible oxidation states

40

of selenium including -II, -I, 0, +IV and +VI determine its mobility. Under well-oxidized

41

geochemical environments, the most frequently encountered Se species is in the form of selenate4,

42

Se(+VI)O42-. In less oxidizing conditions, selenite (Se(+IV)O32-) and biselenite (HSe(+IV)O3-)

43

dominate3, 5. Upon more reducing environments, Se is reduced to elemental selenium(0)6. In highly

44

reducing conditions, oxidation states –I and -II dominate and usually present as solid states that

45

have low mobility, such as FeSe2(-I) and FeSe(-II)7, 8. Overall, the higher oxidation states Se(IV)

46

and Se(VI) prevail as more mobile aqueous oxyanions9. Thus, it is important to investigate the

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migration of 79Se(IV) in the geologic media.

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The species of 79Se(IV) can diffuse into the pore network of a rock matrix (i.e., matrix diffusion),

49

and sorb onto the pore walls of the minerals, thereby delaying their migration through the near-

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surface environment10, 11. Recently, deep geological repositories which composed of crystalline

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rocks as potential host rocks have been considered in many countries12-14. In China, Beishan area

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(in northwest China’s Gansu province) has been selected as an important research site for the

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potential HLW repository, and Beishan granite, as an abundant, tough, and low porosity igneous

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rock, is considered as a candidate host rock for the potential repository15. Host rocks as the final

55

barrier in the repository system have been studied for years because they assuredly act as the

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natural barriers providing retention for radionuclides presented in the groundwater flow16, 17. With

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the purpose of obtaining parameters and developing reliable models for the pre-safety assessment


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of the deep geological repositories, ascertaining the diffusion and sorption behaviors of Se(IV) in

59

granite is important.

60

However, there is still little information about the diffusion of Se(IV) in granite. This is largely

61

owing to the reason that diffusion experiments are time consuming and cost intensive. Wen

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Ruiyuan et al.18 and Jussi Ikonen et al.4 acquired effective diffusion coefficient (De) by through-

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diffusion experiments. Nevertheless, these De values were not well understood on account of the

64

different mineral compositions, porosities and initial concentrations of selenium. Granite is

65

generally composed of quartz, albite, microcline and biotite19. These minerals exhibit various

66

porosities and pores’ surface sorption properties that have different effects on diffusion. Withal

67

several diffusion experiments that have been conducted with relatively large selenium

68

concentrations (approximately mmol/L magnitude) to obtain the diffusion parameters may lead to

69

an overestimation or an inadequate mechanism of matrix diffusion4, 5, 20. Conducting a series of

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diffusion experiments with concentrations close to realistic situations is vital to an understanding

71

of the diffusion behaviors of selenium in granite.

72

On the contrary, the sorption of Se(IV) on granite has been studied under a wide range of

73

conditions21-24. Ticknor et al. found that Se(IV) sorption on geological materials decreased in the

74

order of hematite > goethite > biotite > montmorillonite > granite21. Zhijun Guo et al. investigated

75

Se(IV) sorption on granite as a function of pH22. Most sorption data were measured by batch

76

sorption experiments using crushed rock samples, which can provide useful information related to

77

the quantitative sorption behaviors and sorption mechanisms. However, crushing has been

78

identified as the main reason for increases in the rock samples’ sorption capacity by creating

79

additional surfaces and reaction sites25; Therefore, the Kd values obtained from different minerals

80

cannot be compared and well analyzed. It is necessary to investigate the intact rock’s sorption


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properties of Se(IV). Moreover, granite is invariably composed of several mineral assemblages,

82

not a single mineral phase. Thus, a more microcosmic mechanism for sorption is needed.

83

To our best knowledge, studies focused on the effects of mineral compositions on Se(IV)

84

diffusion and sorption behaviors in granite have not been previously published. The objective of

85

this work was to systematically study the diffusion and sorption behaviors of Se(IV) in different

86

granite for setting reliable parameters. The diffusion parameters were determined by through-

87

diffusion experiments. The sorption of Se(IV) was investigated by both batch sorption experiments

88

using crushed granite macroscopically and EPMA using intact rocks microscopically. Furthermore,

89

XPS and XANES were used to speculate the retardant mechanism for Se(IV).

90 91

2. MATERIALS AND METHODS

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2.1 Materials:

93

All granitic rocks used in this work were collected from the boreholes at 380-550 m depth in

94

Beishan area, Gansu province. Mineralogical compositions were assessed for 6 times by powder

95

X-ray diffraction (PXRD, Rigaku D/max-rA). In addition, different positions of the granitic slices

96

were chosen for characterization to acquire more accurate data. The results are shown and listed

97

in SI Figure S1 and Table 1. The main minerals in the four types of granite are same, namely,

98

quartz, albite, biotite and microcline. Nonetheless, their contents are different. No peaks of other

99

minerals were collected by XRD patterns. Secondary minerals were observed by EPMA analyses

100

(Ca minerals were associated with biotite parts, SI Figure S2). With the absences of phases from

101

XRD patterns, the contribution of secondary minerals was deemed to be negligible. Granitic slices

102

used in through-diffusion experiments were 64.0±0.6 mm in diameter and 5.0±0.3 mm in thickness.

103

They were immersed and washed repeatedly with ultrapure water to simplify the system. Then


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granitic slices were air-dried for porosity measurements. Water loss porosities (ε) of granitic slices

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used for diffusion experiments were obtained according to the technique of Emerson et al. (1990)26,

106

as porosity is one of the most important factors influencing matrix diffusion (Table 1). The porosity

107

values were much less than 1% for all types of granite with no significant difference, and 1% was

108

regarded as a very low porosity27, 28. For batch sorption experiments, granite was crushed, dry

109

sieved and the fractions within size of less than 0.074 mm were collected.

110 111

Table 1. The mean value (N=6) of mineralogical compositions and physical properties for

112

different granite types used in this work. mineralogical composition wt%a

granodiorite-1

granodiorite-2

monzogranite

quartzdiorite

quartz

34.5

36.0

40.5

36.0

albite

42.8

51.0

34.0

38.9

biotite

15.2

12.7

10.0

18.1

microcline

7.2

-

15.3

6.6

ρ(g/cm3)

2.618

2.569

2.534

2.613

ε (%)

0.844

0.420

0.864

0.613

physical properties

113

a

The relative error was about 5%.

114 115

The Na275SeO3 in 0.1 mol/L HCl(＞400MBq/mg Se) used for through-diffusion and sorption

116

experiments was purchased from POLATOM (Poland). The stable selenium (Na2SeO3) was

117

purchased from AMRESCO (Solon, Ohio, USA). All other reagents were analytically pure.

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2.2 Through-diffusion experiments

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A PMMA through-diffusion cell was designed and processed (patent number: CN201177591). It

121

was ameliorated from our previous work29, 30. Figure 1. schematically represents the through-

122

diffusion cell, which contained two reservoirs. A larger source reservoir (1475.0 mL) was filled

123

with 0.1 mol/L NaClO4 solution initially, while the smaller sampling reservoir had a volume of

124

51.0 mL that contained only a 0.1 mol/L NaClO4 solution. The granitic slice was placed between

125

the two reservoirs, and it was sandwiched between two rubber seal rings so that the radionuclide

126

75

127

source reservoir was spiked with 2.46×106 Bq (negligible volume) Na275SeO3 solution to achieve

128

a Se(IV) concentration of 7.03×10-8 mol/L. Then, 1.00 mL of the solution was taken from the

129

sampling reservoir regularly for the radioactivity measurements. The radioactivity of

130

measured by an autoγ-counter (Perkin Elmer 2470). To maintain the volume of the sampling

131

reservoir at constant level, 1.00 mL 0.1 mol/L NaClO4 solution was added after each sampling. Eh

132

and pH were monitored regularly by a Mettler Toledo LE501 ORP electrode and Mettler Toledo

133

FE20/EL20 pH meter, respectively.

Se(IV) could only diffuse from the granitic slice. After pre-equilibration for four weeks, the

75

134 135 136


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Se was


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Figure 1. Schematic diagram of the diffusion cell (1. sampling port, 2. cap, 3. perspex rings, 4.

139

seal ring, 5. granitic slice, 6. stirrer, 7. liquid level, 8. source reservoir, 9.sampling reservoir)

140 141

2.3 Diffusion Modelling

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According to the set up of our experiments, the diffusion of

75

Se(IV) can be described

143

approximately by the constant inlet concentration–increasing outlet concentration through-

144

diffusion model (CC–IC)31. Considering the decay of 75Se and dilution by sampling, the diffusion

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of selenium in granitic slice can be described by the following one-dimensional diffusion equation:

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C(x ,t ) D e  2C(x ,t ) 1   S(x ,t )       C(x ,t ) 2 t   t x

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Where C(x, t) [cpm/mL] represents the selenium concentration in pore water of granite at

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diffusion distance x [m] and at time t [s]; De [m2/s] is the effective diffusion coefficient for 75Se(IV)

149

in granite; ε[-] is the porosity and ρ is the dry density[kg/dm3] of the granitic slice; λ [s-1] is the

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decay constant of 75Se. S(x, t) [cpm/g] represents the sorbed concentration of 75Se(IV) in granite

151

at time t and position x.


(1)

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152 153

154


When reversible instantaneous sorption with a linear adsorption model is assumed, S(x, t) is related with the distribution coefficient, Kd (L/ kg):

Kd 

S(x ,t ) C(x ,t )

(2)

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S(x ,t ) C(x ,t )  Kd t t

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Then eq (1) is transformed into:

157

C(x ,t ) D e  2C(x ,t ) 1 C(x ,t )     K    C(x ,t ) d t   t x 2

158

Let

159

  R    (1   )K d

160

Da 

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R[-] is the retardation factor, [-] is the capacity factor and Da[m2/s] is called apparent diffusion

162

R  1 

(3)

(4)

1 Kd 

(5) (6)

Dp D  e R 

(7)

coefficient.

163

For granite, ε granodiorite-2. This trend also

273

represents the order of the formation factor (Ff, listed in Table 2), as Ff= ε ∙ 𝜏2 = 𝐷 𝑒 (Dw at 25℃

274

=8.87×10-10 m2/s)37. The Ff only depends on the inner structure of the porous media, and the values

275

we obtained are comparable to those found in the literature data38-40.

𝛿

𝐷

𝑤

276

In the case of Da, the magnitudes of Da are not identical with the magnitudes of De, and the

277

sequence of Da is the reverse of the sequence of the Kd values obtained from the diffusion model,

278

suggesting that the properties of the pores could be considered as one of the factors influencing

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Se(IV) diffusivity in various types of granite, and the contribution of

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surface of diffusion pores to its diffusion is thus vital as well. Therefore, the suppositional order

281

of the retardant ability to

282

monzogranite, which is basically in agreement with the order of Kd obtained from the diffusion

283

model.

75

Se(IV) sorption on the

75

Se(IV) is quartzdiorite > granodiorite-1 > granodiorite-2 >

284 285

3.2 Batch sorption

286

To better understand the migration behaviors of selenite in four types of granite, batch sorption

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experiments were carried out to distinguish the retardant ability of different types of granite to

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75

Se(IV).

289

In the past, the modeling experts used Kd from batch sorption experiments to predict the

290

diffusion of some radionuclides in the environmental media, such as granite or clay41, and the

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effective diffusion coefficient was calculated directly by the batch sorption Kd : 𝐷𝑒 = 𝐷𝑎 ∙ (𝜀 +

292

𝐾𝑑 ∙ 𝜌). Since we know that the batch sorption Kd is obtained at equilibrium conditions, while the

293

sorption of radionuclides in the media during the diffusion experiments usually occur at a non-

294

equilibrium conditions (for example, we obtained diffusion coefficients from all data points by


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295

using the difference schema method). Therefore, the prediction of the diffusion with batch sorption

296

Kd usually provides a non-conservative result42. Hence, here we focus on comparing the sorption

297

properties of different granite.

298

Since the specific surface areas of the four types of granite are different, we listed both Kd values

299

and surface area corrected Kd values, namely, Ka to compare the sorption properties43. The sorption

300

results were presented in Table 3 and Figure 4.

301 302

Figure 4. Kd values (L/kg) and surface area corrected Kd values (Ka, L/m2) for different types of

303

granite.

304 305 306 307 308 309 310 311


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Table 3. The specific surface areas (particle size ＜0.074 mm), Kd values (L/kg) and surface area

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corrected Kd values (Ka, L/m2) for different types of granite.

granite types

specific surface area (m2/g)

Kd (L/kg)

Ka (×10-3 L/m2)

granodiorite-1

2.8

83.97±3.24

30.04±1.16

granodiorite-2

2.7

72.02±3.56

26.43±1.31

monzogranite

2.6

20.77±2.63

8.12±1.03

quartzdiorite

3.6

128.3±13.7

35.95±3.83

314 315

It can be seen from Table 3 and Figure 4. that both Kd values and Ka values are in the same order: 75

Se(IV) (128.3 L/kg, 35.95×10-3L/m2),

316

quartzdiorite showed the best sorption property towards

317

followed by granodiorite-1 (83.97 L/kg, 30.04×10-3L/m2), granodiorite-2 (72.02 L/kg, 26.43×10-

318

3

319

sorption experiments were higher than the results obtained from the diffusion model, which can

320

be explained by the increased specific surface areas due to milling. Nonetheless, all sorption data

321

follow the same sequence. Moreover, the magnitudes of Kd or Ka values confirmed the order of

322

the retardant ability deduced from diffusion experiments: quartzdiorite > granodiorite-1 >

323

granodiorite-2 > monzogranite.

L/m2) and monzogranite (20.77 L/kg, 8.12×10-3L/m2). The Kd values obtained from the batch

324 325

3.3 EPMA

326

To identify the host minerals that generated different sorption properties to Se(IV) in the four

327

types of Beishan granite, EPMA technique is applied and practically avoids the sorption difference

328

of the different minerals arising from various the specific surface areas.


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Back scattered electron (BSE) images containing the major minerals of Beishan granite

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(microcline, biotite, quartz and albite) before and after the reaction with Se(IV) are shown in Figure

331

5. No prominent differences were found in the textures and compositions of microcline, quartz,

332

and albite before and after the reaction. The cracks in biotite which increased after the reaction

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could be identified as the water channel in the granitic flakes under solution conditions19, and the

334

entrance of Na after the reaction was most likely due to the contact with the electrolyte solution(0.1

335

mol/L NaClO4).

336 337

Figure 5. BSE, K, Na, Fe, Se distribution images of the area showing quartz, microcline, biotite

338

and albite in Beishan granite before (a−e) and after (f−j) the reaction with Se(IV).

339 340

Figure 5. also shows that only biotite exhibited a weak affinity for Se(IV), while the point

341

chemical analyses of quartz, microcline and albite showed almost no detectable Se(IV) contents

342

(Table 4). Although biotite showed Se enrichment properties by EPMA analyses, the quantity of

343

Se(IV) enrichment was relatively low. The average SeO2 content in the biotite was 0.17 wt %.

344


19


345 346

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Table 4. Averaged chemical compositions of the minerals in granite before and after the reaction with Se(IV) obtained from point chemical analyses. before the reaction

after the reaction

biotite

albite

microcline

quartz

biotite

albite

microcline

quartz

N=5

N=3

N=3

N=3

N=4

N=3

N=4

N=3

SiO2

37.18

60.74

64.68

100.12

35.09

61.39

64.73

99.83

FeO

18.24

0.02

0.07

0.01

19.99

0.11

0.05

0.08

K2O

10.23

0.23

16.36

0.00

8.57

0.17

16.10

0.00

Na2O

0.10

8.07

0.76

0.01

0.78

8.29

0.63

0.02

MgO

11.18

0.01

0.00

0.00

9.51

0.00

0.00

0.00

TiO2

2.93

0.03

0.02

0.02

2.83

0.02

0.02

0.01

CaO

0.00

6.10

0.00

0.02

0.00

5.86

0.00

0.01

Al2O3

15.31

24.13

17.90

0.01

16.21

24.07

18.14

0.00

SeO2

0.02

0.01

0.00

0.02

0.17

0.00

0.01

0.00

total

95.20

99.35

99.80

100.20

93.14

99.92

99.67

99.94

347 348

Our EPMA results basically agree with the results from Mervi et al.43 who found that quartz and

349

potassium feldspar (microcline belongs to potassium feldspar) showed no sorption of selenite,

350

while biotite and plagioclase (albite belongs to plagioclase) presented some sorption properties by

351

the batch sorption experiments. Note that Mervi and coworkers attributed the sorption on

352

plagioclase to impurity minerals.

353 354

3.4 Diffusion and pores’ surface sorption mechanisms

355

Comparing the sorption properties of the four types of granite with biotite contents, we can see

356

that they follow the same orders. According to the PXRD results (Table 1), the magnitudes of the


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357

biotite contents are monzogranite < granodiorite-2 < granodiorite-1 < quartzdiorite, which follow

358

the order of the Kd or Ka values.

359 360 361

Figure 6. Quantitative fitting results for sorption. Quantitative fitting results were also provided (Figure 6). The R2 was relatively good, which

362

reconfirms that the Se(IV) enrichment in granite was dominated by biotite, and higher biotite

363

contents in granitic rocks exhibit stronger sorption properties to Se(IV) so as to greatly attenuate

364

diffusion, assuming the granite is composed of the four major minerals.

365

On the granitic surface, most of the ≡Fe-OH groups remain in their protonated form at a pH

366

value of approximately 8, while the deprotonation of the Si-OH groups occurs at pH values of 3-

367

444. Therefore, the ≡Fe-OH group in granitic surface is positively charged at pH=7, and the

368

sorption of Se(IV) is favorable under the conditions of diffusion and sorption experiments.

369

M. Soderlund et al. proposed that the structural Fe(II) in biotite was capable of immobilizing

370

SeO32-, whereas muscovite showed no sorption to Se(IV) based on the absence of structural Fe in

371

its composition43. Biotite is a Fe(II)-rich trioctahedral mica45 and in our studies, iron was


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372

detected accessibly only in biotite from the granitic flakes by EPMA (Figure 5). The Fe(2p3/2)

373

XPS spectra of the fresh quartzdiorite (Figure 7) shows a strong peak between 711.0 eV and

374

712.0 eV, displaying the characteristics of Fe(III) due to surface oxidation46. Two peaks at

375

∼709.1 eV and ∼712.5 eV correspond to Fe(II)-O and Fe(III)-F in biotite, respectively23. Our

376

results reveal that most of the Fe(II) on the biotite’s edge surface/pore surface originally may be

377

oxidized to Fe(III), which has a certain sorption capacity for selenite47.

378 379

Figure 7. Fitted Fe(2p3/2) XPS spectra of the quartzdiorite.

380 381

Figure 8. shows the XPS Fe(2p3/2) spectrum of quartzdiorite after the reaction. Only Fe(III)-O

382

could be fitted, indicating that Fe(III) was still the dominant oxidation state. The relatively broad

383

peak suggests it is facilitated for the formation of Fe(III)-oxyhydroxide. In addition, the

384

disappearance of the Fe(II)-O peak after the reaction was also observed. Under ambient conditions,

385

the reaction between Fe(II) and oxygen is easily occurred, only under anaerobic conditions selenite


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386

is the main oxidant with a weaker reactivity towards Fe(II)48. For this study, as all the experiments

387

were conducted under ambient conditions, the Fe(II) could be easily oxidized by oxygen. And this

388

interpretation was supported directly by the XANES spectra.

389 390

Figure 8. Fitted Fe(2p3/2) XPS spectra of the quartzdiorite after the reaction.

391 392

The XANES spectrum of the granodiorite-1 powder interacting with Se(IV) are shown in Figure

393

9, as well as FeSe, FeSe2, grey Se(0), FeSeO3, and Na2SeO3 reference spectra. The sample shows

394

the same peak with Se(IV) at the energy of 12664 eV. Thus, the dominant species of Se did not

395

change, Se(IV) cannot be reduced to Se, FeSe2 or FeSe under ambient conditions. The good fit

396

with FeSeO3 can be ascribed to a similar coordination structure between the FeSeO3 and the

397

selenite surface complexes on the Fe(III)-bearing minerals: Bidentate inner-sphere complexes can

398

be observed on the surface of the Fe(III)-bearing minerals, such as goethite49 and α-Fe2O350, while

399

FeSeO3 possesses similar bidentate oxygen bridging between Fe and Se51.


23


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400 401

Figure 9. Se K-edge XANES spectra of sample and reference compounds. The red line represents

402

the linear combination fit (LCF).

403 404

Table 5. Compositional values for sample, derived from the LCF results. ∑ is the concentration

405

sum (%), Χν2 is the reduced Χ square. The value of Χν2 indicates good match to the experimental

406

spectra. Amounts lower than 5% are uncertain.

sample

FeSeO3(%)

Na2SeO3(%)

∑(%)

Χν2×10-3

82.4

17.6

100

6.4

407 408

Although selenite interacts strongly with aluminum minerals52, which exist both in albite and

409

microcline, the surface Al: Si ratio(defined as Ras below) may also play an important role in the


24

Page 25 of 35


410

sorption43, 53. M. Soderlund et al. determined that minerals consisting mainly of SiO2 (e.g., quartz

411

and potassium feldspar) with a low surface Ras showed low immobilization of Nb by a presumed

412

inner sphere complexation mechanism54. As the sorption of Se(IV) onto Beishan granite did not

413

vary with ionic strength22, the main sorption mechanism for selenite is also an inner sphere

414

complexation. Therefore, the same reasoning should apply to albite and microcline, in which these

415

minerals with the Ras =1 : 3 lead to no obvious sorption capacity for Se(IV). Studies from Heini et

416

al. also confirmed this phenomenon. They showed that the Kd values of Se(IV) for illite (Ras =1 :

417

2) were from (0.4–60) × 10-3 m3 kg-1, while the Kd values were from (11–720) × 10-3 m3 kg-1 for

418

kaolinite (Ras =1 : 1) at a pH range from 5.8 to 955.

419 420 421

4. ENVIRONMENTAL IMPLICATIONS Granite as potential host rocks has been considered in many countries12-14, and the diffusion and

422

sorption behaviors of Se(IV) in granite is of concern. Granite is invariably composed of several

423

mineral assemblages, and the minerals exhibit various porosities and pores’ surface sorption

424

properties so as to have different influences on diffusion. It is crucial to explore the retardant

425

mechanism for Se(IV) in granite on the molecular scale.

426

The different sequences between De and Da of Se(IV) in four granitic types imply that the

427

contribution of 75Se(IV) sorption on the surface of the diffusion pores to its diffusion is vital. The

428

order of the retardant ability to 75Se(IV) is same as the magnitudes of biotite contents in four types

429

of granite. The EPMA results confirmed that only biotite originating in granite exhibited a weakly

430

retardant ability to Se(IV). Bidentate inner-sphere complexes were formed between Se(IV) and the

431

structural Fe(III) on the biotite’ edge surface/pore surface.

432

This study revealed that nuclide enrichment on a single mineral can be representative of bulk


25


Page 26 of 35

433

enrichment in complex mineral assemblages to a large degree. The compositions of granite will

434

greatly affect the sorption behaviors of Se(IV) and therefore provides an information for

435

selecting the sites and the host rock’s type for HLW repository, since many nuclides can be

436

immobilized on biotite10, 56, 57. For the host rock surrounding the HLW repository, the deposited

437

level of Se is relatively low. Some previous studies about diffusion were conducted with

438

relatively large selenium concentrations to obtain the diffusion parameters 4, 5, 20, which may lead

439

to an overestimation or an inadequate mechanism of matrix diffusion. In this work, the diffusion

440

experiments were conducted with a 7.03×10-8 mol/L Se(IV) concentration to provide relatively

441

reliable diffusion and sorption parameters. Furthermore, it is expected that the retardant

442

mechanism of Se(IV) in granite on a molecular scale will ensure a more reliable evaluation for

443

the pre-safety assessment of the deep geological repositories.

444 445

SUPPORTING INFORMATION

446

XRD patterns of four granite types (Figure S1); BSE image and elemental distributions of

447

granodiorite-2 (Figure S2).

448 449

AUTHOR INFORMATION

450

Corresponding Author

451

*E-mail: [email protected].

452

Notes

453

The authors declare no competing financial interest.

454 455


26

Page 27 of 35


456

ACKNOWLEDGMENTS

457

Funding for this research was provided by the Special Foundation for High-level Radioactive

458

Waste Disposal (2007-840, 2012-851) and the National Natural Science Foundation of China

459

(NSFC, No. 11475008, U1530112). In addition, we are grateful to the 1W2B beamline at the

460

Beijing Synchrotron Radiation Facility (Beijing, China) and the BL14W1 beamline at Shanghai

461

Synchrotron Radiation Facility (Shanghai, China) for providing beam time and assistance during

462

the XAS measurements.

463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478


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Page 28 of 35

479 480

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