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Effects of NH4+, K+, and Mg2+ and Ca2+ on the Cesium Adsorption/Desorption in Binding Sites of Vermiculitized Biotite Xiangbiao Yin, Xinpeng Wang, Hao Wu, Hideharu Takahashi, Yusuke Inaba, Toshihiko Ohnuki, and Kenji Takeshita Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04922 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 14, 2017

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Page 1 of 35

Environmental Science & Technology

1

Effects of NH4+, K+, and Mg2+ and Ca2+ on the

2

Cesium Adsorption/Desorption in Binding Sites of

3

Vermiculitized Biotite

4

Xiangbiao Yin,*, † Xinpeng Wang,‡ Hao Wu,

5

Toshihiko Ohnuki,

†

†

Hideharu Takahashi,

†

Yusuke Inaba,

†

Kenji Takeshita†

6 7

†

8

Ookayama, Meguro-ku, Tokyo 152-8550, Japan

9

‡

Laboratory for Advanced Nuclear Energy, Tokyo Institute of Technology, 2-12-1,

College of Resources and Metallurgy, Guangxi University, 100 Daxue East Road, Nanning

10

530004, PR China

11

AUTHOR INFORMATION

12

Corresponding Author:

13

*

14

Telephone number: +81-3-5734-3845

15

Fax number: +81-3-5734-3845

[email protected]

16 17

1

ACS Paragon Plus Environment


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18

ABSTRACT: The reversibility of cesium adsorption in contaminated soil is largely

19

dependent on its interaction with micaceous minerals, which may be greatly influenced by

20

various cations. Herein, we systematically investigated the effects of NH4+, K+, Mg2+, and

21

Ca2+ on the adsorption/desorption of Cs+ into different binding sites of vermiculitized biotite

22

(VB). Original VB was initially saturated by NH4+, K+, or Mg2+; we then evaluated the

23

adsorption of Cs+ on three treated VBs, and the desorption by extraction with NH4+, K+, Mg2+,

24

or Ca2+ was further evaluated. Our structural analysis and Cs+ extractability determinations

25

showed that: NH4+ and K+ both collapsed the interlayers of VB, resulting in the dominant

26

adsorption of Cs+ to external surface sites on which Cs+ was readily extracted by NH4+, K+,

27

Mg2+, or Ca2+ irrespective of their species, whereas Mg2+ maintained the VB with expanded

28

interlayers, leading to the overwhelming adsorption of Cs+ in collapsed interlayer sites on

29

which the Cs+ desorption was difficult and varied significantly by the cations used in

30

extraction. The order of Cs+ extraction ability from the collapsed interlayers was

31

K+ >> Mg2+ ≈ Ca2+ >> NH4+. These results could provide important insights into Cs migration

32

in soil and its decontamination for soil remediation.

33 34 K

K+

Cs

Cs+

H2O

Cs

Cs T O T

2:1 Layer Cs

Cs

T O T

2:1 Layer

T O T

2:1 Layer

K

Vermiculitized biotite

Cs

K

δCs

δ-

Cs

δ- δ-

δ-

δ-

Cs

Cs

Cs

δ- δ-

δ-

δ-

Cs

K

K

δ-

δ- δ-

K

K

K

δ-

δ-

Mg2+ Ca2+ 35 36 37

Cs

Cs Cs

10.1 Å

Cs

Cs Cs

11.0 Å

Cs

Cs

TOC GRAPHICS

38 39 2


Cs Cs

Cs

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40


INTRODUCTION

41

Since the serious accident at Fukushima Dai-ichi Nuclear Power Plant triggered by an

42

earthquake and tsunami in March 2011, the decontamination and volume reduction of a huge

43

amount of polluted soil (18.7–28 million m3) via the removal of radioactive cesium (i.e., 134Cs

44

and 137Cs) has become an urgent problem.1 The existing investigations of the dynamics of Cs

45

in contaminated soil due to a nuclear disaster (e.g., Chernobyl, Soviet Union and Fukushima,

46

Japan) or the accidental release of high-level nuclear waste (e.g., the Hanford Site,

47

Washington, U.S.) suggest that weathered micaceous minerals are responsible for the

48

retention and migration of Cs into soil.2-7 Generally, three types of binding sites with distinct

49

affinities are considered for Cs adsorption in micaceous minerals: (1) the external sites

50

including edge/planar sites on the outer surface of clay particles, which are associated with

51

low-affinity sites through the outer-sphere complexation of hydrated Cs with a negatively

52

charged basal surface, and (2) interlayer sites or (3) frayed edge sites (FES), which are

53

associated with high-affinity sites via an inner-sphere coordination of partially or fully

54

dehydrated Cs in the ditrigonal cavities near a partially opened edge.8-10 Depending on the

55

particle size, the capacities of these binding sites and their relative proportions may differ

56

greatly for different micaceous minerals.8, 11, 12 Typically for the vermiculite in specific size

57

fraction (e.g., < 75 μm), the site capacities of FES and interlayer sites has been estimated to be

58

~0.1% and ~40% of the total cation exchange capacity (CEC), respectively.8

59

In view of the complexity of the clay composition in different areas and the

60

heterogeneous distribution of Cs on single clay particles, trace amounts of deposited Cs

61

(~10−12–10−10 mol kg−1) in contaminated soil could be heterogeneously adsorbed on different

62

binding sites.13, 14 As such, Cs sorbed on the external sites may be readily replaced by other

63

cations, whereas Cs intercalated into FES or interlayer sites can be replaced only by particular

64

cations with a similar hydrated radius (e.g., K+ and NH4+) and such replacement would

65

become increasingly difficult from the edge to the collapsed core along the interlayers.15-18 3



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66

Thus, the extractable fraction of sorbed Cs in micaceous minerals/soil is expected to depend

67

on the relative composition of the sites involved in Cs adsorption and the cation species used

68

for Cs desorption.8

69

However, on one hand, the nature of these binding sites in regard to Cs affinity can be

70

altered by interlayer changes. In natural surroundings, various cations (e.g., NH4+, K+, Mg2+

71

and Ca2+) are released into soil as fertilizer components or as a result of the decomposition of

72

organic matter and the partial dissolution of minerals in acidic environments.19,

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released cations will be transferred in geological material, partially exchange with the

74

interlayer cations of micaceous minerals, and induce their structural change. As such,

75

adsorption of cations with low hydration energy (e.g., NH4+, K+) tends to collapse the

76

interlayers, whereas intercalation of cations with high hydration energy (e.g., Mg2+, Ca2+)

77

allows the expansion of the interlayers.15 As a result, these structural changes (i.e., interlayer

78

collapse or expansion) will significantly affect the Cs adsorption characteristics by favoring

79

Cs sorption on the specific type of binding sites and leading to a change in the Cs

80

extractability.19, 21

20

These

81

On the other hand, several studies have investigated the Cs extractability (i.e., desorbility,

82

corresponding to the ability of Cs+ to be expelled from initially Cs+-adsorbed materials) from

83

micaceous minerals/soil at relatively low loaded amounts due to trace amounts of Cs

84

deposition in contaminated soil.22-24 However, under the low loadings, the detailed cation

85

exchange process underlying the Cs extraction from FES by other cations can hardly be

86

detected due to the relative low density of FES and its associated nano-scale size for the

87

wedge-shaped transition zone.25 Moreover, a considerable proportion of adsorbed Cs at trace

88

amounts may heterogeneously distribute on external sites rather than adsorbed to collapsed

89

FES/interlayer sites, although the latter are generally accepted as the major sites for the

90

irreversible Cs adsorption under field conditions.26, 27 Therefore, a distinct Cs extractability

91

measurement — which would confirm that the vast majority of binding sites involved in

4


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92

desorption are from the collapsed FES/interlayer sites (i.e., so-called irreversible sites) — will

93

provide insights into Cs extraction from the strongest fixation state in contaminated soil and

94

thus promote Cs decontamination for soil remediation in Fukushima and other regions.12, 28

95

Our main goal in conducting the present study was to clarify the effect of different

96

cations (i.e., NH4+, K+, Mg2+ and Ca2+) on the structural change of a specific partially

97

weathered micaceous mineral called vermiculitized biotite (VB) and how this affects the

98

reversibility (adsorption and desorption) of Cs into different binding sites, especially in the

99

collapsed interlayer sites. Herein, the original VB was initially treated by saturation with

100

NH4+, K+, or Mg2+ solutions. The adsorption of Cs on treated VB was investigated, and its

101

extractability was subsequently evaluated by extended treatment with various solutions

102

containing the same cations.

103

MATERIALS AND METHODS

104

Materials. Mg-rich vermiculized biotite (VB) obtained from Vermitech Co. Japan was used

105

as the original material. The chemicophysical and mineralogical properties of this type of 2:1

106

phyllosilicate has been characterized elsewhere.29-33 The VB was used in experiments after

107

simple milling in a mortar and sieving through a mesh (size fraction 100–800 μm). This larger

108

size fraction was chosen for the purpose of intercalation more adsorbed Cs into interlayers as

109

the coarser fractions generally have the higher proportion of interlayer sites over the total

110

binding sites.11,

111

meq/100 g by displacing cations in the interlayers (essentially Mg2+, Ca2+, and Na+) and on

112

external sites using Cs+ through the method as reported by Reinholdt et al.11 Analytical-grade

113

reagents of chloride salts, including CsCl, NH4Cl, KCl, MgCl2 and CaCl2 were purchased

114

from Wako Pure Chemical Industries (Osaka, Japan) and were used as received, unless

115

otherwise noted.

12

The CEC value for this particle-sized clay was measured to be 30.1

116

5



117

Pretreatments of the vermiculized biotite. To prepare the homogeneous specimens with

118

monoionic form that allowed us to identify the Cs sorption behaviors on known properties of

119

VB, a pretreatment of the original VB particles was performed by a well-known method

120

described in specific tests reported by Coleman and Sikalidis,16, 34 according to the following

121

procedures: 10 g of VB in the 100–800-μm size fraction was packed into a 250-ml conical

122

flask and saturated three times by 0.5 mol L−1 chloride electrolytes (100 ml) of mono- or

123

divalent cations (NH4+, K+ and Mg2+) lasting 2 days for each pretreatment step. After being

124

washed free of excess afore-mentioned cations by several rinses with distilled water, these

125

NH4+-, K+-, and Mg2+-pretreated VBs (hereafter referred to as NH4-VB, K-VB and Mg-VB)

126

were dried at 75°C overnight prior to characterization and the subsequent Cs sorption

127

experiments.

128 129

Cs adsorption on pretreated vermiculized biotite. The adsorption of Cs on the three

130

pretreated VBs was examined by a batch method. In detail, 1 g of a pretreated sample of

131

NH4-VB, K-VB or Mg-VB was equilibrated with 0.1 L of 7.510−3 mol L−1 CsCl solution

132

maintained at 25°C for 2 weeks with slow stirring. This Cs+ concentration and the duration

133

time were shown to be sufficient to achieve equilibrium of the saturated sorption capacity for

134

similar vermiculite-like minerals due to the involved rapid cation exchange kinetics,

135

especially in the cases of Mg- or Ca-vermiculite.12, 31, 32, 35

136

After reaction time was met, the mixtures were centrifuged and the clay particles were

137

filtered through a 0.22-μm membrane and dried under extremely gentle atmospheric

138

conditions for more than two days prior to subsequent characterization. The concentration of

139

Cs in the supernatant before and after equilibrium was quantified by atomic absorption

140

spectroscopy (AAS, SpectrAA-6200, Shimadzu Corp., Kyoto, Japan), and the Cs adsorption

141

capacity (mg Kg−1) was then calculated.

142

6


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143

Cs desorption experiments. Desorption of Cs from Cs-NH4-VB, Cs-K-VB, and Cs-Mg-VB

144

was applied according to a well-established semicontinuous batch approach.15 In each cycle

145

of treatment, 0.1 g of as-prepared samples were suspended in 10 mL 10−2, 10−1, 1, and 3 mol

146

L−1 of NH4+, K+, Mg2+ or Ca2+ solutions for a specific time (50 hr in the first three cycles and

147

250 hr in the fourth cycle), separated from the liquid without drying, and then directly

148

dispersed into the same freshly prepared solution. The Cs concentration in the filtrate was

149

quantified to calculate the Cs desorption ratio based on its initial adsorption capacity. The

150

entire above-described semicontinuous desorption process was performed a total of four times

151

and the overall desorption ratio for each cation was then summed for its value in each single

152

extraction. To assure data quality, all tests were conducted in triplicate for each single

153

identical situation.

154 155

Crystal structure characterization. To confirm the cation exchange process in crystal

156

during pretreatment, Cs adsorption and desorption, we performed an X-ray diffraction (XRD)

157

analysis of pretreated, Cs saturated and desorbed VB specimens to observe the variation of

158

the basal spacing referring to structural change. The XRD can be rather sensitive to

159

characterize a cation exchange process involved in interlayer sites, as a slight change of the

160

crystal structure would be distinguished from the shift of peak for 001 reflections once the

161

original interlayer cations are replaced by other cations.12 Prior to the XRD analysis, all

162

samples were sufficiently heated at 75°C, milled to fine powder in a mortar and passed

163

through a 280-mesh sieve (< 53 μm). The patterns were recorded at 25°C and room humidity

164

with a diffractometer with CuKα radiation (λ = 0.15406 nm) from 3°to 10°at the rate of 1°

165

(2θ)/min at a step-interval angle of 0.02°, operating at 40 kV and 20 mA (MultiFlex, Rigaku

166

Co., Tokyo). For the sample characterizations, it has been reported that Cs+ would readily

167

dehydrate and be tightly fixed in collapsed interlayers of vermiculite-like minerals even in

168

aqueous conditions at room temperature, while divalent cations with high hydration energy

7



Page 8 of 35

169

(e.g., Mg2+) were expected to keep their hydrous states and maintain the clays with the similar

170

structure until 200°C.12, 36 Therefore, it was expected that the drying temperature, grinding

171

treatment, and sample humidity during the sample preparation in present study might

172

negligibly affect the results of XRD analysis.

173

 RESULTS AND DISCUSSION

174

Interlayer change of VB by treatment with major cations 12.0Å 12.4Å

Ori VB

Normalized intensity (arb unit)

10.4 Å

K+

N

T O T

2:1 Layer

δ-

N

10.1 Å

10.1 Å

T O T

10.1 Å

12.0Å

Mg-VB

K T O T

12.4Å

25Å

10.1Å

6

10.1 Å

K+

2:1 Layer

K

2:1 Layer

δ-

K

K

δ-

K

(B)

K

δ-

δK

K

δ-

M

δ-

δ-

δ-

H2O δ- δ-

M

δ-

δ-

K

K

(A)

K

δ-

K

K

K

δ-

K

H2O δ- δ-

M Mg2+ δ- δ- δ-

2:1 Layer K

T O T

δ-

K+

M

M

14.3 Å T O T

δ-

N

δ-

K

δ- δ- δ-

2:1 Layer

δ-

N

K

K

K

K

K

δ-

N

2:1 Layer δ- δK K K K K 2:1 Layer δ- δK

T O T

N

K

K T O T

Ori

δ-

K

δ- δ- δ-

2:1 Layer δ-

T O T

K

NH4+ H2O δ- δ- δ- δ-

N

K

K

10.1Å

K-VB

4

N

2:1 Layer

M

δ-

δ- δ- δ-

2:1 Layer

T O T

K

K

T O T

K

M

M

K

K

K

H2O δ- δ-

δ- δ- δ-

2:1 Layer

10.4Å 10.1Å

NH4-VB

14.3Å

175

10.1 Å

M

M T O T

10.1Å

M Mg2+ δ- δ- δ-

2:1 Layer

14.3 Å

14.3Å 25Å

K+

K T O T

K

δ- δ- δ-

M

δK

δ-

δ-

(C)

K

δ-

10

8

2/ o (CuK)

176

Fig. 1. Comparison of XRD patterns (left) and assumed conformational cartoons (right)

177

between the original VB (Ori) and the treated VBs by saturation with NH4+ (NH4-VB, A),

178

K+ (K-VB, B), or Mg2+ (Mg-VB, C). For the XRD patterns, characteristic basal spacing was

179

assigned to the corresponding d001 value as denoted above each peak in Å.

180

8


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181

We firstly conducted an XRD analysis to investigate the structural change of VB following

182

treatment with different cations (i.e., NH4+, K+, and Mg2+) (Fig. 1). Since these cations have

183

distinct ionic properties (Table 1), they will maintain a highly hydrated (i.e., Mg2+ with high

184

hydration energy) or weakly hydrated (i.e., NH4+ and K+ with low hydration energy) state in

185

the interlayers after their saturation and thus induce contrasting basal spacing. Therefore, an

186

XRD analysis can reveal the composition of different types of interlayers and can be used to

187

characterize the cation exchange process involved in interlayer sites.

188

Regarding the original VB, its XRD pattern exhibited five characteristic peaks at the 2θ

189

range of 3°–10°. Among these peaks, the diffraction peaks located at 6.1°(14.3 Å) and 8.7°

190

(10.1 Å) were separately distinguished as the interlayers containing hydrous Mg2+ with 2

191

sheets of water molecules (2 W, 14.6-14.3 Å) and anhydrous K+ (0 W, 10.3-9.8 Å), and the

192

one at 3.5°(~25Å) was reasonably interpreted as the regular interstratification of Mg2+- and

193

K+-interlayers (i.e., the periodic 25 Å d-spacing consists of regular alternation of 14.3 and

194

10.1 Å repeat units).32, 37, 38 Since a completely regular interstratified phase would be expected

195

to give rise to sharp peaks only, thus the nature of the peaks at 7.1°, and 7.3°, which

196

corresponded to intermediate basal spacings of ~12.4 Å and 12.0 Å, were less clear. They are

197

likely to have arisen from the interlayers of Mg-hydrate with 1 sheet of water molecules (1 W,

9



Page 10 of 35

198

12.4-11.6 Å),38 or a random interstratification of the Mg-interlayers (2 W) with K-interlayers

199

(0 W) in the case of their disordered alternation.32, 39 Therefore, our detected peaks were

200

indeed relatively consist with their historically reported values in the literatures within the

201

uncertainties of ±0.3 Å.

202

Compared with the original VB, a negligible modification of pattern was observed for

203

the Mg2+-treated VB (Mg-VB), suggesting that it has the same crystal structure as that of the

204

original VB (Fig. 1C). In contrast, the NH4+- and K+-treated VBs (NH4-VB and K-VB) both

205

showed the absence of all Mg-related peaks (i.e., ~25, 14.3, 12.4, 12.0 Å), accompanied by

206

the separate appearance of peaks at ~10.4 and ~10.1 Å, the characteristic basal spacing for the

207

NH4+- and K+-interlayers, respectively (Fig. 1A,B).40 The explanation of these results is as

208

follows. NH4+ and K+ have smaller ionic radii and lower hydration energy compared to those

209

of divalent cations, as is the case for Mg2+ (Table 1), which can result in higher affinity to the

210

size-fitted hexagonal cavity (2.6 Å) on the basal surface, allowing the substitution of NH4+

211

and K+ for the original interlayer Mg2+ and inducing the interlayer collapse through their

212

facile dehydration. As a result, these evolutions of the variation in peak positions

213

demonstrated that the structural transformations of VB from the original Mg2+-interlayer to

214

NH4+- and K+-replaced interlayers were achieved upon the treatment with NH4+ or K+ (right

10


Page 11 of 35

215 216


cartoons in Fig. 1). Table 1. The ionic radius, hydrated radius and hydration energy of the cations41, 42

Cation

Ionic radius (Å)

Hydrated radius (Å)

Hydration energy (kJ/mol)

NH4+

1.48

3.31

307

K+

1.33

3.31

321

Mg2+

0.72

4.28

1922

Ca2+

1.01

4.12

1577

Cs+

1.81

3.29

264

217 Effect of interlayer change on the Cs adsorption on VB Normalized intensity (arb unit)

218

10.4Å10.1Å 10.5Å

NH4-VB

100

(A)

Cs saturated NH4-VB

K+

K

10.4 Å

50

T O T

N T O T

10.1 Å 0

3

4

5

6

7

8

9

Cs

2:1 Layer N

N

N

N

δ-

δ-

δ-

K

K

K

K

K

N

δδ-

2:1 Layer

δ-

K

N

K

δ-

δ-

δK

T O T


K-VB Cs saturated K-VB

10.5Å

10.1 Å

50

T O T

3

4

5

6

7

8

9

Cs

K

K

K

K T O T

10.1 Å

K+ δ-

2:1 Layer

δ-

2:1 Layer 2:1 Layer

δ-

K

K

δK

K

δ-

K

δ-


219

50

0

T O T

K

δ-

δ-

Cs

K

δ-

T O T

10.1Å

4

6

8

10.1 Å

10

M

K T O T

K

K

2:1 Layer δ-

δK

K

δ-

Cs

δ-

δ-

δ-

δ-

δ-

Cs

δK

Cs

δ-

10.4 Å 10.1 Å

Cs

Cs

T O T

Cs

Cs

K

K

δ-

δ-

δ-

δ-

K

K

K

K

δ-

δ-

δ-

δ-

Cs

2:1 Layer K

2:1 Layer Cs

δK

δ-

δ-

Cs

K

Cs

Cs

δK

δ-

10.5 Å

10.1 Å

Cs

K

δ-

M

δK

δ-

δ-

Cs

Cs T O T

2:1 Layer Cs

Cs

K

δ-

Mg-VB

2/ o (CuK)

11

K

(Cs: 35.77 mg/g)

M

δ-

δ-

K

δ-

Cs+ Saturated K-VB

M

δ-

2:1 Layer

δ-

K

δ-

Cs

M T O T

δ-

K

δ-

K

Cs+ M Mg2+ H2O 2:1 Layer δ- δ- δδ- δ-

14.3 Å

14.3Å

25Å

K+

K

10.1Å

12.4Å

N

2:1 Layer

(C)

Cs saturated NH4-VB

N

δ-

Cs T O T

K-VB

10

δ-

(Cs: 9.16 mg/g)

K

K

K

K

K T O T

12.0Å 11.0Å

NH4-VB

2:1 Layer

N

δ-

Cs+ Saturated NH4-VB

2/ o (CuK)

100

T O T

Cs

δ-

N

Cs

Cs+ H2O δ- δ- δ- δδ-

K

K Cs

δ-

δ-

N

2:1 Layer

(B)

10.1Å

K

0

T O T

Cs

2/ o (CuK)

100

2:1 Layer Cs

NH4-VB

10

Cs

Cs

Cs+ N NH4+ H2O δ- δ- δ- δ- δ -

2:1 Layer

T O T

(Cs: 4.11 mg/g)


T O T

Cs

2:1 Layer K

K Cs

T O T

2:1 Layer Cs

δCs

δCs

Cs

δ-

δ-

δ-

Cs

Cs

Cs

δ-

δ-

δ-

δ-

K

K

K

K

δ-

δ-

δ-

δ-

Cs

δK

δ-

11.0 Å 10.1 Å

Cs

Cs+ Saturated Mg-VB


Page 12 of 35

220

Fig. 2. Comparison of XRD patterns (left) and assumed conformational cartoons (right)

221

between before and after Cs adsorption for NH4-VB (A), K-VB (B), and Mg-VB (C). The

222

d001 values were assigned to the corresponding peaks in Å. The number value at the top of

223

each cartoon is the Cs saturation adsorption capacity of the treated sample.

224 225

To understand the Cs adsorption behaviors on known characteristics of VB and thus to assess

226

the effect of interlayer change on the distribution of sorbed Cs into various binding sites, we

227

further investigated the Cs ion exchange properties with the three pretreated materials. As a

228

result, the saturated Cs adsorption amounts for the three treated VBs, i.e., NH4-VB, K-VB and

229

Mg-VB, were revealed to be approx. 4.11, 9.16 and 35.77 mg g−1 (corresponded to 10.3%,

230

22.9% and 89.4% of CEC), respectively. Notably, the nearly homogeneous distribution of Cs

231

on the cross-sectional surface of the Cs adsorbed Mg-VB particles was observed, suggesting

232

the complete Cs penetration into the interlayers and thus a saturated adsorption (see details in

233

the Supporting Information of Figs. S1). Subsequently, the varied XRD patterns of the

234

samples before and after Cs saturation are compared in Figure 2.

235

Regarding the Cs saturated NH4-VB (Cs-NH4-VB), its pattern was overlapped by that

236

of NH4-VB, exhibiting a negligible position shift for the detected peaks (i.e., 10.4 and 10.1 Å).

237

The overlapping of these two profiles suggested that (1) little of the NH4+ was replaced by

238

sorbed Cs from interlayers, (2) a binary cation exchange of Cs+-for-NH4+ was rather difficult

12


Page 13 of 35


239

to achieve in NH4+-collapsed interlayers, and (3) most of this sorbed Cs could be confidently

240

ascribed to its binding on external sites (planar/edge sites) rather than interlayer sites (right

241

cartoon in Fig. 2A).

242

In contrast to this scenario, a slight shift of the characteristic peak from 10.1 Å to 10.5

243

Å was detected for K-VB after Cs saturation (Cs-K-VB) (Fig. 2B). This observation indicated

244

that the cation exchange of Cs+-for-K+ proceeded viably to some extent and that a minor

245

fraction of Cs+ indeed intercalated into the K+-collapsed interlayers for partial K+ substitution

246

(at least in part) (right cartoon in Fig. 2B). A significant structural pattern change of Mg-VB

247

was induced by Cs saturation (Fig. 2C), for which the peak near 10.1 Å remained whereas all

248

Mg-related peaks (i.e., 25, 14.3, 12.4, 12.0 Å) disappeared, coupling with the emergence of a

249

new peak at ~11.0 Å, the characteristic basal spacing for the Cs+-interlayers.43 This result

250

clearly suggested that the transformation from original Mg2+-K+ interstratified Mg-VB to

251

subsequent Cs+-K+ interstratified Cs-saturated Mg-VB (i.e., Cs-Mg-VB) was associated with

252

the Cs+-for-Mg2+ exchange.

253

In detail, the collective adsorption of Cs during Cs saturation proceeds by its very high

254

degree of substitution for the originally occupied hydrous Mg2+ rather than anhydrous K+ in

255

interlayers and then readily induce interlayer collapse via partially or fully Cs+ dehydration

256

due to the relative low hydration energy of Cs+ (Table 1), thus allowing the retention of

257

numerous sorbed Cs+ in Cs-collapsed interlayers (right cartoon in Fig. 2C).30, 32 On the basis

13



Page 14 of 35

258

of the observed shifts in basal spacing, we suspect that an increasing amount of adsorbed Cs+

259

was distributed into interlayer sites on the three treated VB samples in the order of

260

NH4-VB < K-VB < Mg-VB, which is completely consistent with their varied Cs saturation

261

adsorption amounts.

262 Cs extractability in the different binding sites of VB (C)100

80

80

80

1st 2nd

60

3rd 4th

60 40 20 0

NH4Cl

264

KCl

MgCl2

Reagent (1M)

CaCl2

Desorption ratio (%)

(B) 100


(A)100


263

60 40 20 0

NH4Cl

KCl

MgCl2

CaCl2

Reagent (1M)

40 20 0

NH4Cl

KCl

MgCl2

CaCl2

Reagent (1M)

265

Fig. 3. Semicontinuous extraction of Cs from Cs-NH4-VB (Cs: 4.11 mg/g) (A), Cs-K-VB

266

(Cs: 9.16 mg/g) (B) and Cs-Mg-VB (Cs: 35.77 mg/g) (C) with 1 mol L−1 NH4+, K+, Mg2+,

267

or Ca2+.

268 269

As described in the above section, adsorption experiments enabled us to qualitatively compare

270

distribution of sorbed Cs on different binding sites (i.e., external and interlayer sites), and

271

subsequent desorption testing was necessary to determine the contribution of different sites to

272

the regulation of Cs reversibility and thus to identify an effective cation exchanger for Cs

273

removal from these high-affinity sites. In the present work, we conducted Cs desorption

274

experiments by performing semicontinuous extractions with either monovalent (i.e., NH4+ and

275

K+) or divalent cations (i.e., Mg2+ and Ca2+). As is frequently reported and generally accepted,

14


Page 15 of 35


276

monovalent cations such as NH4+ and K+ have small hydrous radii and may readily access

277

FES, and they are thus expected to replace Cs plausibly from these sites; by contrast, divalent

278

cations such as Mg2+ and Ca2+ are generally thought to desorb Cs mostly from accessible

279

external sites but are considered to be less effective in desorbing Cs from FES due to their

280

larger hydrous radii and limited entry into wedge zone of FES.16, 44 Nevertheless, it has been

281

reported that FES are orders of magnitude more selective to Cs+ than K+.45 Even when trace

282

amounts of Cs+ is present in solutions, these sites could remain Cs-saturated, and thus the

283

desorption of Cs is difficult. Therefore, considering the different ionic properties of these

284

cations and their relative selectivity to different binding sites, distinct desorption patterns

285

could be expected. In addition, semicontinuous extraction was conducted herein to evaluate

286

the insufficiency of a single treatment as the re-sorption of desorbed Cs may occur, and to

287

distinguish the after-effects during extractions (e.g., the crystal structural change), as well as

288

to compare the summed desorption ratio achieved for the four cations used herein.15

289

Figure 3 provides the results of Cs desorption from Cs-NH4-VB, Cs-K-VB, and

290

Cs-Mg-VB by semicontinuous extraction with different cations. For Cs-NH4-VB with the

291

smallest Cs saturation adsorption capacity among three treated VBs, relative high desorption

292

ratios (72%–94%) were achieved in the first cycle of extraction with all cations (Fig. 3A),

15



Page 16 of 35

293

again suggesting that most of the adsorbed Cs+ on the specimens was devoted to external sites

294

and was thus easily desorbed by surplus replacing cations via an accessible cation exchange

295

that was nearly irrespective of their species. With the increase of cycle numbers, the summed

296

total desorption ratio improved greatly and approx. 100% Cs removal was realized after four

297

rounds of the semicontinuous desorption processes (except for the NH4+ extraction), which

298

clearly indicated the limited efficiency of a single extraction to completely remove all of the

299

interacted Cs on the accessible sites due to the reversible reaction.46 Nevertheless, a single

300

extraction could be useful to make a comparison between the achieved desorption patterns for

301

different cations. As such, the order of abilities of the four cations for Cs desorption from

302

external sites was K+ > NH4+ > Mg2+ ≈ Ca2+, which is quite consistent with the order of their

303

ion selectivity to the planar sites in the literature.9

304

In contrast to Cs-NH4-VB, the initial extractions with each given cation separately

305

achieved an intermediate and fairly low Cs desorption ratio for Cs-K-VB (43%–79%) and for

306

Cs-Mg-VB (8%–52%) (Fig. 3B,C), keeping a reverse trend to the amounts of Cs intercalated

307

into the interlayers of these three specimens (Fig. 2). These results suggested that adsorption

308

involved in collapsed interlayer sites had played a significant role in regulating the

309

reversibility of Cs on the vermiculitized biotite. In other words, desorption of Cs + that is

16


Page 17 of 35


310

sorbed mostly in interlayers, especially collapsed interlayers, seemed to be rather difficult and

311

varied notably by the cation used in the extraction.

312

In our closer examination of Cs-Mg-VB (which intercalated the largest amount of

313

sorbed Cs into collapsed interlayers), we noted that the summed desorption ratio after

314

four-times treatment varied widely for the different cations and showed the order K+

315

(~100%) >> Mg2+ ≈ Ca2+ (~40%) >> NH4+ (~13%) (Fig. 2C), suggesting the overall abilities

316

of the four cations for Cs extraction from collapsed interlayers. On the basis of the varied

317

desorption patterns achieved for the four cations used herein, different extraction processes

318

for Cs removal could be distinguished. In detail, most of the extracted Cs by NH4+ was

319

obtained in the first cycle of treatment, whereas subsequent extractions with NH4+ desorbed a

320

negligible amount of Cs (Fig. 3C). Similar desorption patterns were also found for Cs

321

extraction from Cs-NH4-VB and Cs-K-VB by the NH4+ cation (Fig. 3A,B). By contrast, Cs

322

desorption by Mg2+ and Ca2+ was limited in the initial treatment but sustained in the

323

subsequent extractions, reaching a higher summed desorption ratio than that shown by NH4+

324

(Fig. 2A–C). Compared with the above-mentioned two desorption patterns, Cs extraction by

325

K+ revealed an effective desorption in the first cycle that continued in the subsequent

326

treatments, leading to achieve the highest total desorption ratio among the four cations (Fig.

17



327

Page 18 of 35

2A–C).

328 Cationic exchange pathways of Cs desorption from different binding sites (B) 10.4Å

NH4-VB

10.5Å10.1Å

b

11.4Å 10.1Å

14.3Å 11.2Å

10.1Å

1M Ca

d e

10.4Å10.1Å

f

1M K 4

6

8

2/ o (CuK)

330

a

c

1M NH4 1M Mg

10.1Å

10.5Å10.1Å

Cs-NH4-VB



(A)

10

(C) 10.1Å

K-Verm 10.5Å 10.1Å

Cs-K-Verm 10.3Å10.1Å

a b c

1M NH4 11.4~10.7Å 10.1Å

1M Mg

d 11.0Å

10.1Å

1M Ca

e 10.1Å

f

1M K 4

6

8

10


329

2/ o (CuK)

12.4Å 14.3Å

Mg-VB 25Å

12.0Å

a

10.1Å 11.2~10.7Å 10.1Å

Cs-Mg-VB 11.2~10.5Å

b

10.1Å

1M NH4

c 11.8Å

1M Mg 14.3Å

11.5Å

10.1Å 10.1Å

1M Ca

d e

10.4Å 10.1Å

1M K 4

f 6

8

10

2/ o (CuK)

331

Fig. 4. Comparison of typical XRD patterns for Cs-NH4-VB (A), Cs-K-VB (B) and

332

Cs-Mg-VB (C) before and after different cationic extractions, with spacing values in Å. a:

333

pretreated VBs, i.e., NH4-VB, K-VB, or Mg-VB. b: post-Cs adsorbed VBs. c–f: post-Cs

334

desorbed VBs after cationic extractions with 1 mol L−1 NH4+ (c), Mg2+ (d), Ca2+ (e) and K+

335

(f).

336 337

To clarify the mechanism underlying the cation exchange processes in different binding sites

338

of VB during Cs extraction, we compared the XRD patterns of Cs-desorbed Cs-NH4-VB,

339

Cs-K-VB, and Cs-Mg-VB after extraction with different cations to those recorded for each of

340

the pretreated VB samples before and after Cs adsorption (Fig. 4).

341

We focused on the structural modification induced by the three groups of cations (i.e.,

342

NH4+; Mg2+/Ca2+; and K+) in view of their distinct desorption patterns achieved in the

18


Page 19 of 35


343

aforementioned Cs extraction section. First, for NH4+, the extraction of Cs-NH4-VB,

344

Cs-K-VB, and Cs-Mg-VB by this cation hardly altered their crystal structures (Fig. 4b,c). This

345

result suggested both the negligible entry of NH4+ into K+ or Cs+ collapsed interlayers to

346

replace them and the occurrence of a dominant cation exchange for Cs desorption on external

347

sites, which is also in good accordance with our observation that NH4+ facilely desorbed most

348

of the Cs from Cs-NH4-VB (Fig. 3A) but was limited or null in desorbing Cs from Cs-K-VB

349

and Cs-Mg-VB in light of the interlayer retention of a higher proportion of adsorbed Cs in the

350

latter two specimens (Fig. 3 B,C).

351

As mentioned above, this is inconsistent with the general impression that NH4+ (which

352

has a hydrous radius and an affinity to the hexagonal cavity on the basal surface that are

353

similar to those of Cs+ and K+) is thought to be capable of entry into Cs+/K+-collapsed

354

interlayers and would thus likely replace partial fixed Cs from these positions. However, our

355

results indeed suggested that the extraction of Cs+ by NH4+ from interlayers was rather

356

difficult and limited. To explain the reasons of the limited Cs extractability by NH4+, two

357

possible hypotheses would be proposed. One was attributable to the thermodynamic factor

358

arising from the structural immobilization by NH4+ treatment. In detail, once the NH4+-for-Cs+

359

exchange in interlayers near the edge side is initiated, the concentrated NH4+ would readily

19



Page 20 of 35

360

cause more extensive interlayer collapse (~10.4 Å) compared to that by Cs+ (~11.0 Å)

361

because of its smaller ionic radius and the relatively low hydration energy, thus hindering the

362

subsequent Cs release. The other one was suggestive of a kinetic factor involving the

363

intraparticle

364

chemical-mechanical coupling of the Cs+-NH4+ neighboring may produce a limited driving

365

force at exchange front, thus allowing a subsequent inefficient interlayer migration of Cs+ or

366

NH4+ and leaving a considerable proportion of residual Cs being trapped in collapsed

367

interlayers. Consequently, both of these two factors may cause a significant reduction of the

368

Cs desorption ratio in the subsequent NH4+ extractions and eventually achieve the lowest total

369

desorption ratio among the four cations (Fig. 3A–C).

diffusion

within

the

collapsed

interlayers.

To

be

specific,

the

370

To further identify the main controls (i.e., thermodynamic or kinetic factors) on Cs

371

extractability, we subsequently performed additional semicontinuous extractions of

372

Cs-Mg-VB by NH4+ at ionic strengths ranging from 0.01 to 5 mol L−1. The results revealed no

373

differences in desorption patterns among the various ionic strengths, and a total summed

374

desorption ratio

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