Effective Removal of Selenite and Selenate Ions from Aqueous

Subscriber access provided by UNIVERSITY OF CONNECTICUT

Article

Effective removal of selenite and selenate ions from aqueous solution by barite Kohei Tokunaga, and Yoshio Takahashi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01219 • Publication Date (Web): 07 Jul 2017 Downloaded from http://pubs.acs.org on July 7, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 49

1

Environmental Science & Technology


2

Effective removal of selenite and selenate ions from aqueous solution by barite

3

Kohei Tokunaga1, 2* and Yoshio Takahashi1*

4 5

1

Department of Earth and Planetary Science, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

6 7 8

2

Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan

9 10

Correspondence: Email: [email protected], [email protected]

ACS Paragon Plus Environment


11

Page 2 of 49

Abstract

12

In the present study, we explore a new application of barite (BaSO4) as a sequestering phase for

13

selenite (Se(IV)) and selenate (Se(VI)) ions from aqueous solutions due to the low solubility and

14

high stability of barite with its ability to selectively incorporate a large amount of various ions.

15

uptake of Se(IV) and Se(VI) during coprecipitation with barite was investigated through batch

16

experiments to understand the factors controlling effective removal of Se(IV) and Se(VI) from

17

polluted water to barite.

18

complexation between barite surface and Se(IV)/Se(VI) ion and (ii) structural similarity related to

19

the structural geometry of incorporated ions into the substituted site.

20

barite is dependent on pH, coexistent calcium ion, and sulfate concentration in the initial solution,

21

possibly due to their effects on the chemical affinity and structural similarity.

22

the uptake of Se(VI) by barite was strongly dependent on sulfate concentration in the initial solution,

23

which is only related to the structural similarity.

24

distribution between barite and water, thereby providing a good estimate of its ability to effectively

25

remove Se(IV) and Se(VI) from aqueous solutions under optimized experimental parameters

26

examined here.

The

The factors include (i) chemical affinity related to the degree of surface

The uptake of Se(IV) by

On the other hand,

This study describes the mechanisms for Se


Page 3 of 49

27

28


1. Introduction Selenium (Se) is generally a trace element in nature, but frequently occurs in the environment

29

by anthropogenic activities such as mining and fossil fuels combustion.

30

element but can also a toxic to organisms depending on its concentration and chemical form in water

31

(e.g., drinking water limits of 0.05 mg/L and 0.01 mg/L in the United States and Japan,

32

respectively).1 In addition, the element is an important radionuclide with a long half-life (Se79:

33

about 105 years) found in radioactive wastes.2

34

aqueous solutions (-2, 0, +4, +6) and dissolved as oxyanions (selenite: SeIVO32-; selenate: SeVIO42-)

35

with high solubility and mobility in an aquatic environment.

36

organic Se, and the toxicity of inorganic selenite is higher than inorganic selenate.

37

transport of Se in contaminated sites are influenced by the chemical form and speciation of the

38

element: hence, Se(IV) is strongly adsorbed by soil particles, whereas Se(VI) is weakly adsorbed and

39

leaches easily.3

40

It is known as an essential

In nature, Se exists in four oxidation states in

Inorganic Se is more toxic than The fate and

Several techniques can be used to reduce the Se level in solutions such as ion-exchange,

41

bioremediation, adsorption, and coprecipitation.4-7

42

for the selective separation of Se(IV) or Se(VI), an increasing concentration of competitive ions,

43

such as sulfate, significantly reduces adsorption ability during for the ion-exchange separation.4,5

44

Retention by adsorption on mineral surface is also unstable in the long run because changes in the

45

surrounding environment could release the adsorbed ions back into the water.7

Although ion-exchange resins can be available


Therefore, this


Page 4 of 49

46

study investigated the immobilization of a trace element in mineral during crystal growth, a process

47

known as coprecipitation, to develop effective removal methods of Se(IV) and Se(VI) from a

48

polluted solution.

49

capable of preserving substituent ions in the crystal lattice for a long time, which makes it work as

50

engineered barriers for the retention of various ions, including Se(IV) and Se(VI).

51

The advantages of the method are simplicity, short treatment time, low cost, and

In the present study, we designed and optimized methods using barite (BaSO4) for this purpose

52

and analyzed their efficiencies in incorporating Se(IV) and Se(VI).

53

many geological environments and can be used to remove toxic and/or radioactive elements from

54

polluted waters.

55

solubility8 (ca. Ksp = 10-9.98 at 25 °C, 1 atm), (ii) incorporation of numerous elements because of the

56

large ionic radii of substituted ions9, 10 (Ba2+: 1.68 Å; SO42-: 1.48 Å), (iii) high density compared with

57

other minerals (4.5 g/cm3), which is an advantage for rapid sedimentation during coprecipitation

58

process, and (iv) high crystal stability under wide ranges of pH, Eh, temperature, and pressure

59

conditions.4,11-21

60

radioactive elements from polluted solutions.

61

Ra2+ in fresh or brine water because of its high stability and larger incorporation of Ra2+ relative to

62

other minerals.12-15

63

coprecipitation of trace elements with barite has not been conducted in previous studies.

64

Barite is a common phase in

Barite as a sequestering phase shows following characteristics: (i) extremely low

Thus, barite serves as a sequestering phase for the removal of toxic and/or For example, barite acts as an ideal host mineral for

However, except for the Ra2+ uptake by barite, an experiment on the

Hence, this study examined and compared the coprecipitation capacities under controlled


Page 5 of 49


65

experimental conditions to optimize the effective removal of Se(IV) and Se(VI) by barite.

Even

66

though the degree of immobilization depends on various factors, the most critical are the charge and

67

the size of the ions relative to the substituted site.22 In the present study, the following two factors

68

were mainly investigated: (i) the affinity of the surface complex between barite surface and

69

Se(IV)/Se(VI) ion (= i.e., chemical affinity) and (ii) the affinity of the substituent ion in the

70

substituted site (= i.e., structural similarity).

71

investigated through a series of batch experiments in barite-equilibrated solutions.

72

experimental conditions such as pH, saturation state, ionic strength (IS), coexistent cation

73

concentrations ([Ca2+] and [Mg2+]), and sulfate concentration ([SO42-]) were investigated.

74

constructed method was also applied to Se(IV) and Se(VI) in an artificial seawater (ASW) system.

75

The ASW system was compared with a Milli-Q (MQ) water system for its use as a remediation

76

technique of Se in the seawater system, because the removal of ions from seawater by

77

adsorption/coprecipitation with mineral is not effective in most cases compared with freshwater.23-24

The uptake of Se(IV) and Se(VI) by barite was


The effects of

The


78

2. Materials and Methods

79

2.1. Experiment procedure In the present studies, Se(IV) and Se(VI) stock solutions were prepared from NaHSeO3 and

80

81

Na2SeO4 (Wako, Japan), respectively.

Barite was precipitated from a mixture of (i) Na2SO4

82

solution and (ii) BaCl2·2H2O solution.25

83

Se(IV) or Se(VI) was added to the sulfate solution.

84

SI = log(IAP/Ksp), where IAP and Ksp are the ion activity product and solubility product of the

85

mineral, respectively), and aqueous concentration of sulfate were fixed at pH 8.0, SI 4.2, and [SO42-]

86

= 27 mM, respectively, as a basic system.

Additional experiments were conducted by changing one

87

of the parameters from the basic system.

The Se concentration in the initial solution (using 1 mM

88

Se solutions) was unsaturated with respect to the solid phases of barium selenite and barium selenate

89

to avoid the formation of Ba-Se precipitates in the system.

Right before the addition of the BaCl2·2H2O solution, The pH, saturation index of barite (defined as

90

The precipitates of barite and the aqueous phase were separated by filtration with a 0.20 µm

91

membrane filter (mixed cellulose ester, Advantec, Tokyo, Japan) and then rinsed three times with

92

MQ water.

93

X-ray diffractometer (MultiFlex, Rigaku Co., Tokyo, Japan), in which the mineral phase was

94

identified by comparing the XRD patterns with those in the International Center for Diffraction Data

95

file.

96

plasma-mass spectrometry (7700cs, Agilent, Tokyo, Japan) after dilution by a 2 wt.% HNO3 solution.

The X-ray diffraction (XRD) patterns of the precipitates were measured using a powder

The total Se concentrations in the solution and solids were analyzed by inductively coupled


Page 6 of 49

Page 7 of 49


97

A part of the solid sample was dried in an oven at 60 °C and then dissolved in water by adding

98

sodium carbonate to determine the Se concentration in the precipitates.26

99

coefficient of Se between barite and water was calculated on the basis of the Se concentrations in the

The distribution

100

aqueous and solid phase.

In order to evaluate the stabilities of Se(IV) and Se(VI) sorbed on barite,

101

the extraction experiment was also carried out.

102

phosphate and Se oxyanions on adsorption sites, and adsorption capacity can be calculated based on

103

the amount of Se released into solution using phosphate as an extractant.3,7

104

of each sample was added to 10 mL of 1.0 M Na2HPO4 solution and shaken for 24 hours prior to

105

measurement of Se concentration using ICP-MS.

106

measurements are described in detail in Supporting Information.

Previous studies showed the competition between

Approximately 10 mg

The analytical methods for XAFS and XRD

107

108

109

2.2. Effect of pH, SI, IS, coexistent ions, and sulfate concentration The effect of pH on coprecipitation was studied by determining the amount of Se with barite

110

within the pH range of 2.0-10.0 (fixed at SI = 4.2, [SO42-] = 27 mM).

111

solution was initially adjusted with a HCl or NaOH solution.

112

experiments was examined to account for the pH dependence of Se species: Se(IV) is mainly

113

dissolved as H2SeO30 at pH 2.0, HSeO3- in the pH range of 2.0-8.0, and SeO32- at pH above 8.0,

114

whereas Se(VI) is mainly dissolved as HSeO4- and SeO42- at pH 2.0, SeO42- above at pH 2.0,

115

respectively (see Figure SI1 of Supporting Information).

The pH of the starting

The pH condition during the

The procedure was carried out for the MQ



Page 8 of 49

116

water and ASW systems.

117

[Na+] = 54 mM, [SO42-] = 27 mM, [CO32-] = 2.2 mM) and ASW (IS = 0.534; [Na+] = 440 mM,

118

[Mg2+] = 50 mM, [Ca2+] = 9.6 mM, [Cl-] = 440 mM, [SO42-] =27 mM, [CO32-] = 2.2 mM).

119

The chemical compositions of the solutions were MQ water (IS = 0.076;

The effect of saturation state on the coprecipitation in the MQ water and ASW systems was

120

studied within the SI range of 2.9-4.2 (fixed at pH = 8.0, [SO42-] = 27 mM).

121

initial solution was adjusted with initial Ba2+ concentration.

122

The SI of barite in the

The effect of the IS on the coprecipitation in the MQ water system was investigated within the

123

IS range of 0.08-0.52 M (fixed at pH = 8.0, SI = 4.2, [SO42-] = 27 mM).

124

adjusted by adding NaCl.

The IS was initially

125

The effect of coexistent cations on coprecipitation in the MQ water system was studied at

126

different [Ca2+] and [Mg2+] from 0.1-10 mM (fixed at pH = 8.0, SI = 4.2, [SO42-] = 27 mM), because

127

of the larger concentrations of Mg2+ and Ca2+ in the seawater system.

128

The effect of SO42- on coprecipitation in MQ water was also investigated at different [SO42-]

129

from 1-27 mM (fixed at pH = 8.0).

130

barium concentration was also changed as a function of [SO42-].

131

132

The SI of barite in the initial solution was fixed at 4.2.

Thus,

The effect of reaction time was also examined in this study, which was described in the Supporting Information.


Page 9 of 49


133

3. Results

134

3.1. Effect of pH on Se removal by coprecipitation

135

Batch experiments were conducted under different pH conditions to understand the pH effect on

136

the distribution coefficients of Se(IV) and Se(VI) between barite and water.

137

that the uptake of Se(IV) by barite increased to 20.8 mmol/kg (MQ water system) and 239.5

138

mmol/kg (ASW system) as the pH was increased from 2.0 to 10.0 (Fig. 1 and Table SI1 of

139

Supporting Information).

However, the uptake of Se(VI) by barite was almost constant in both

140

systems regardless of pH.

Barite had greater incorporation of Se(IV) than that of Se(VI) at the

141

range of pH 2.0-10.0.

142

The results showed

In the present study, we mainly studied coprecipitation capacity of barite for developing the

143

direct Se removal technique from solution instead of adsorption method.

The solubility of BaSO4

144

(Ksp= 10-9.97) is significantly lower than BaSeO3 (Ksp = 101.8) and BaSeO4 (Ksp = 10-7.5), thus barite

145

with selenite/selenite can be readily recovered from solution than precipitations of pure solids of

146

BaSeO3 and BaSeO4.

147

for the immobilization of Se, suggesting the high applicability of barite coprecipitation method.

148

Coprecipitation is a structural incorporation process during crystal growth, and the precipitates do

149

not release the immobilized ions unless the host mineral is dissolved, whereas adsorption is a surface

150

accumulation process that easily releases the adsorbed ions back to water.

151

both mechanisms can be distinguished whether Se is released into the solution or not using

In addition, barite is resistant to dissolution and has strong crystal stability


In the present study,


Page 10 of 49

152

phosphate as an extraction agent.

To identify the amount of Se adsorption on surface site in

153

comparison to coprecipitation into the crystal lattice, the adsorption capacity was also determined.

154

The results showed the similar trend in removal efficiency between adsorption and coprecipitation

155

experiments, although the amount of Se coprecipitated with barite was relatively larger compared

156

with that adsorbed on the surface because of the incorporation of target ions within the crystal

157

structure (Fig. 1).

158

crystal lattice of barite rather than by adsorption at the surface site.

159

fixed Se was more resistant to release than those adsorbed at the mineral surface, suggesting that

160

adsorption is less effective as a Se removal mechanism compared with coprecipitation.

161

difference in distribution behavior between Se(IV) and Se(VI) shows the importance of chemical

162

affinity on the Se incorporations to barite, which is related to the degree of proton dissociation of

163

Se(IV) and Se(VI) in water.

164

ions than for monovalent ones, because of the high stability of the surface complex to ions with large

165

charges. 3 Selenium(IV) is dissolved as monovalent (HSeO3-) and divalent (SeO32-) species from pH

166

2.0 to 10.0 (Fig. SI1).

167

mainly dissolved as divalent species.

168

constant because Se(VI) is mainly dissolved as a divalent (SeO42-) species in the experimental pH

169

range.

170

depending on pH, which is controlled by the affinity for the adsorption site on the surface rather than

Most of Se in the solutions can be removed by coprecipitation possibly into the In addition, the structurally

The

Previous studies showed the higher adsorption affinity for divalent

Thus, the amount of Se(IV) in barite increased at higher pH where Se(IV) is Conversely, the amount of Se(VI) in barite was almost

The uptake of Se(IV) by barite was affected by the degree of dissociation of Se(IV) in water


Page 11 of 49


171

structural similarity in the crystal lattice.

172

at pH 10.0.

Thus, we can efficiently remove Se(IV) from the solution

173

174

175

3.2. Effect of saturation state on Se removal by coprecipitation Batch experiments were conducted to understand the effect of precipitation rate on the

176

distribution coefficients of Se(IV) and Se(VI) between barite and water.

177

that the precipitation rate is correlated with the degree of SI in solutions.26, 27

178

study, the SI values of barite in the initial solution were changed (SI = 2.9, 3.2, 3.5, 3.8, or 4.2) to

179

understand the incorporation mechanism associated with structural geometry between substituted

180

(SO42-) and substituent (SeO32- or SeO42-) ions.

181

Previous studies showed Thus, in the present

The results showed that the amounts of Se(IV) and Se(VI) in barite were relatively unaffected

182

by the change in SI (Fig. 2 and Table SI2).

Previous studies showed the dependence of structural

183

similarity between substituted- and substituent-ion on the partition behavior of trace elements,

184

resulting in differences in the amount of incorporation as a function of the precipitation rate. 21, 27-32

185

However, in the present study, the difference in geometry between SeO32- and SeO42- had little

186

influence on the amount of their incorporations, possibly because of the minimal crystal lattice

187

distortion in the SI-changed system (Fig. SI2, discussed in 4.1).

188

the amount of Se removal from the solution by adjusting SI in the initial solution when SI is lower.

Thus, it is not effective to increase

189



190

191

Page 12 of 49

3.3. Effect of IS on Se removal by coprecipitation Batch experiments were conducted at different IS (0.08, 0.15, 0.32, or 0.52 M) to understand the

192

effect of IS on the distribution coefficients of Se(IV) and Se(VI) between barite and water.

193

results showed that the amounts of adsorbed- and coprecipitated-Se(IV) and Se(VI) were relatively

194

unaffected by the change in IS by one order of magnitude (0.05-0.6 M) (Fig. 3, Table SI3).

195

effect of IS on the partitions can be explained by distinguishing inner-sphere and outer-sphere

196

surface complexes.33, 34 For example, Hayes and Leckie (1988)33 showed that the adsorption of

197

Se(IV) on hydrous ferric oxide is slightly influenced by IS because Se(IV) forms an inner-sphere

198

complex, whereas that of Se(VI) is markedly decreased by increasing IS because Se(VI) forms an

199

outer-sphere complex. In the present study, the uptake of Se(IV) and Se(VI) by barite was not

200

affected by IS, possibly owing to the strong specific binding as an inner-sphere complex between

201

Se(IV) or Se(VI) and barite.

202

The

The

Extended X-ray absorption fine structure (EXAFS) was performed to characterize the local

203

coordination structure of adsorbed- and incorporated-metals in a host mineral.

The Se K-edge

204

EXAFS and the Fourier transforms (FTs) of the barite samples are shown in Fig. SI3.

205

the positions and intensities of peaks roughly correspond to the interatomic distances and

206

coordination numbers (CNs), respectively.

207

barite can be explained by the three shells of one Se-O and two Se-Ba shells named as Se-Ba1 and

208

Se-Ba2 shells (Table SI4).

In the FTs,

Fitting results showed that both Se(IV) and Se(VI) on/in

The CN and distance of Se-O shows that Se(IV) or Se(VI) is


Page 13 of 49


209

incorporated into barite as SeO32- in a trihedral coordination with oxygen or SeO42- in a tetrahedral

210

coordination with oxygen, respectively.

211

and Se(VI) are similar to those of S-Ba in barite,35 suggesting that Se is incorporated into the barite

212

structure by substitution in the sulfate site.

213

adsorption samples, indicating the strong specific binding of Se(IV) or Se(VI) with barite as an

214

inner-sphere complex. An inner-sphere surface complex has no water molecule interposed between

215

the adsorbed species and adsorption site.

216

between adsorption and coprecipitation, suggesting similar bonding of Se with barite and there is no

217

water molecules present between the ions and the surface or crystal lattice of barite.

218

of CNs in the second shell (Se-Ba2) between adsorption and coprecipitation samples also suggests

219

that we can distinguish adsorption/coprecipitation reactions in the experiments.

220

Se(IV) and Se(VI) by barite was not affected by IS variation because of the formation of the

221

inner-sphere complex.

222

using the present method.

The CNs and distances of Se-Ba1 and Se-Ba2 for Se(IV)

The Se-Ba1 and Se-Ba2 shells were also observed in the

The observed Se-O interatomic distances are nearly equal

The difference

The uptake of

Thus, we can remove Se(IV) and Se(VI) from solution regardless of IS by

223

224

3.4. Effect of coexistent cations on Se removal by coprecipitation

225

Artificial seawater contains larger amounts of Mg2+ and Ca2+ ions compared with MQ water.

226

Batch experiments were conducted at different coexistent ion concentrations ([Mg2+] or [Ca2+]) to

227

understand their effects on the distribution coefficients of Se(IV) and Se(VI) between barite and



Page 14 of 49

228

water (Fig. 4, Table SI5).

The results showed that the uptake of Se(IV) by barite greatly increased

229

in the presence of Ca2+ in the solution, whereas that of Se(VI) was almost constant.

230

presence of Mg2+ in the solutions had little effect on its partitions of Se(IV) and Se(IV) to barite.

231

The uptake of Se(IV) by barite was only affected by the variation of [Ca2+] in the solutions.

232

present study, we can ignore the removal of Se(IV) by calcium selenite precipitate because of the

233

high solubility of calcium selenite precipitate (Ksp = 102.8) and undersaturated in experimental

234

solutions.

235

Ca-added barite and there is no Ca-Se shell in all samples (Fig SI3 and Table SI4), suggesting that

236

Se(IV) is removed from solution by coprecipitation with barite, but not as calcium selenite

237

precipitate.

238

the solutions.

By contrast, the

In the

The Se K-edge EXAFS and the FTs spectra were generally similar between pure and

Thus, we can efficiently remove Se(IV) from solution in the presence of high [Ca2+] in Further information will be discussed in 4.1.

239

240

241

3.5. Effect of sulfate concentration on Se removal by coprecipitation Our present study showed that both Se(IV) and Se(VI) are substituted to the sulfate site in the

242

barite structure as a trihedral or tetrahedral coordination with oxygen, respectively.

In other words,

243

sulfate works as a substituted ion for Se(IV) and Se(VI) in the barite structure and may control the

244

extent of trace element incorporation in barite.

245

[SO42-] to understand the effect on the distribution coefficients of Se(IV) and Se(VI) between barite

246

and water.

Batch experiments were conducted at different

The SI of barite in the initial solution was fixed at 4.2, thus barium concentration was


Page 15 of 49

247

248


also changed as a function of [SO42-] ([Ba2+] = 0.7~6.4 mM; [SO42-] = 1~27 mM). The results showed that both Se(IV) and Se(VI) were incorporated to a large degree into barite

249

when the sulfate level was low (Fig. 5, Table SI6).

The removal of Se(IV) and Se(VI) by barite was

250

relatively higher compared with those under other conditions, suggesting that we can efficiently

251

remove Se from the solution by adjusting sulfate level in the solutions. XRD analysis showed that

252

the barite transformed to barium selenate at lower sulfate levels on the basis of based on the larger

253

peak shift in XRD patterns to low 2θ (KBaSeO3 = 101.8 and KBaSeO4 = 10-7.5, at 25 °C, 1 atm) (Fig. SI4).

254

However, the amount of released Se in 1 M Na2HPO4 solution was significantly lower in these

255

samples, suggesting that Se(IV) and Se(VI) were structurally fixed in the precipitates, thereby

256

preventing the release of Se to the solution when the surrounding environment was changed.



257

4. Discussion

258

4.1. Se removal by coprecipitation in artificial seawater system

Page 16 of 49

259

The degree of Se uptake during coprecipitation with barite showed significant difference

260

between the MQ water and ASW systems (Fig. 1 and 2). Results showed that (i) the uptake of

261

Se(IV) by barite in ASW was relatively higher compared with that in MQ water, but (ii) that of

262

Se(VI) was similar between the two systems.

263

on its partitions to minerals, such as goethite24 and apatite36.

264

from seawater is negligible compared with that in freshwater because of the inhibition of

265

sorption/crystallization by high IS or competitive ions in the ASW system.

266

study showed that the uptake of Se by barite was enhanced in the presence of [Ca2+] in the solution,

267

causing the greater incorporation of Se(IV) by barite from ASW compared with that from MQ water.

268

To validate the result, we investigated the cause of the greater incorporation of Se in the presence of

269

[Ca2+] in the solution by using XRD analysis to determine the unit-cell dimensions of barite and to

270

understand the dependence of the degree of structural distortion caused by substituent ion in the

271

barite structure.

272

cell, relative angles of sides to each other and the volume of the cell, which describes the degree of

273

substitution of foreign ions into the crystal lattice.

274

greater expansion/shrinkage of unit-cell volume in the crystal structure, which is related to the ionic

275

size of the substitution atom.

Previous studies showed the effect of seawater matrix They reported that the removal of ions

However, the present

The unit-cell dimension is defined by three parameters: length of the sides of the

At higher level of ion incorporation causes

In the present study, the shrinkage in the unit-cell parameters by the


Page 17 of 49


276

incorporation of foreign ions (SeO32-, SeO42-, and Ca2+) was observed in the crystal structure of

277

barite (Fig. SI5).

278

barite35 to calculate the unit-cell dimensions of a-, b-, and c-axes for each sample (which are defined

279

as ka, kb, and kc) by the difference between pure and Ca-added barite.

280

The initial unit-cell parameter was determined based on the Pnma space group of

The unit-cell parameters of the non-substituted, Se(IV)-substituted, and Se(VI)-substituted

281

samples in the presence of [Mg2+] or [Ca2+] are listed in Table SI7.

282

changes in the b-axis but considerable differences in the a- and c-axes.

283

the solution, the ka values of these samples changed as a function of [Ca2+], and the order of degree

284

of distortion is as follows: non-substituted < Se(VI)-substituted < Se(IV)-substituted sample (Fig. 4a).

285

On the other hand, in the presence of Mg2+ in solution, the ka values were constant regardless of

286

[Mg2+] (Fig. 4b).

287

behavior of Se(IV) and Se(VI) to barite.

288

the barite structure with varying degrees of crystal distortion, and the shrinkage of the unit cell

289

volume controls the amounts of Se(IV) and Se(VI) in barite.

290

strongly increased in the presence of [Ca2+] in the solution possibly because of the larger crystal

291

lattice distortion by Ca2+ than by Mg2+ into the barite structure.

292

with Ca2+ promoted the incorporation of Se(IV) into the barite structure.

293

Se(VI) in barite was almost constant with increasing [Ca2+].

294

between Se(IV) and Se(VI) can be explained by the lattice mismatch between substituted- and

These parameter shows little In the presence of [Ca2+] in

These findings show the dependence of structural distortion on the partition It is considered that Ca2+ and Mg2+ are incorporated into

Thus, the amount of Se in barite

This structural distortion associated However, the amount of

The difference in the distribution



Page 18 of 49

It is considered that SeO32-, which has different geometry from SO42-, is

295

substituent-ion.

296

incompatibly substituted to the SO42- site in the barite structure, thereby inducing larger

297

incorporation when the crystal lattice is distorted by Ca2+.

298

substituted to the SO42- site because of the similar geometry, which has little influence on the amount

299

of its incorporation regardless of structural distortion.

300

SeO32- -substituted samples were changed by the degree of incorporations (Fig. SI5).

301

showed that the distortion effect of Ca2+ was stronger than that of SeO32- or SeO42-, indicating that

302

the structure of barite became more distorted in the presence of Ca2+.

303

distortion was observed in the coexistent system of Ca and Se than that in the single system of Ca or

304

Se (Fig 4a and SI5).

305

the SI, but the extent of change was smaller in the SI-change system compared with the Ca2+-added

306

system (Fig. SI2).

307

Se by barite because of the minimal crystal lattice distortion in the SI-change system.

308

By contrast, SeO42- is compatibly

The unit-cell dimensions of Ca2+, SeO32-, and The results

A larger degree of crystal

The unit-cell dimension of non-substituted barite was also linearly changed by

Thus, we can ignore the effect of SI in the present study for effective removal of

Similar phenomena were observed in other oxyanions (H2AsO3-, HAsO42-, TeO32-, HTeO4-,

309

MoO42-, and WO42-) coprecipitated with barite at various [Ca2+] (Fig. 6, Table SI7).

310

showed the large incorporation and distortion of incompatible elements (H2AsO3-, HAsO42-, TeO32-)

311

with varying degree of [Ca2+], and the order of distortion is as follows: Mo(VI)-substituted

Effective Removal of Selenite and Selenate Ions from Aqueous

Recommend Documents