Direct Observation of Simultaneous Immobilization of Cadmium and

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Direct Observation of Simultaneous Immobilization of Cadmium and Arsenate (V) at the Brushite Interface Hang Zhai, Lijun Wang, Lihong Qin, Wenjun Zhang, Christine V Putnis, and Andrew Putnis Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06479 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 1, 2018

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Page 1 of 36

Environmental Science & Technology

1

Direct Observation of Simultaneous Immobilization of

2

Cadmium and Arsenate at the Brushite-Fluid Interface

3 4

Hang Zhai,† Lijun Wang,*,† Lihong Qin,† Wenjun Zhang,*,† Christine V. Putnis,‡,§

5

and Andrew Putnis‡,¶

6 7 †

8

College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, China

9 ‡

10 §

11

Institut für Mineralogie, University of Münster, 48149 Münster, Germany

Department of Chemistry, ¶The Institute for Geoscience Research (TIGeR), Curtin University, Perth, Western Australia 6845, Australia

12 13 14 15

*

To whom correspondence should be addressed.

16 17

Lijun Wang

18

College of Resources and Environment

19

Huazhong Agricultural University

20

Wuhan 430070, China

21

Tel/Fax: +86-27-87288382

22

Email: [email protected]

23 24

Wenjun Zhang

25

College of Resources and Environment

26

Huazhong Agricultural University

27

Wuhan 430070, China

28

Email: [email protected] 1

ACS Paragon Plus Environment


Page 2 of 36

ABSTRACT

29 30 31

Cadmium (Cd2+) and Arsenate (As5+) are the main toxic elements in soil environments

32

and are easily taken up by plants. Unraveling the kinetics of the adsorption and

33

subsequent precipitation/immobilization on mineral surfaces is of considerable

34

importance for predicting the fate of these dissolved species in soils. Here we used in

35

situ atomic force microscopy (AFM) to image the dissolution on the (010) face of

36

brushite

37

Na2HAsO4-bearing solutions over a broad pH and concentration range. During the

38

initial dissolution processes, we observed that Cd or As adsorbed on step edges to

39

modify morphology of etch pits from the normal triangular shape to a four-sided

40

trapezium. Following extended reaction times, the respective precipitates were formed

41

on brushite through a coupled dissolution-precipitation mechanism. In the presence of

42

both CdCl2 and Na2HAsO4 in reaction solutions at pH 8.0, high-resolution

43

transmission electron microscopy (HRTEM) showed a coexistence of both amorphous

44

and crystalline phases, i.e. a mixed precipitate of amorphous and crystalline

45

Cd(5-x)Cax(AsO4)(3-y)(PO4)yOH

46

observations of the transformation of adsorbed species to surface precipitates may

47

improve the mechanistic understanding of the calcium phosphate mineral

48

interface-induced simultaneous immobilization of both Cd and As and subsequent

49

sequestration in diverse soils.

(dicalcium

phosphate

dihydrate,

phases

was

CaHPO4·2H2O)

detected.

These

in

CdCl2-

direct

or

dynamic

50

2


Page 3 of 36


51

INTRODUCTION

52 53

Cadmium (Cd2+) and Arsenate (As5+) are ubiquitously present in the soil

54

environment due to anthropogenic influences.1-4 They can be easily taken up by plants

55

and subsequently accumulated, causing chronic or severe acute toxicity to plants and

56

are consequently potentially health-threatening to humans via food chains.5,6 Given

57

that the toxicity of Cd2+ and As5+ is mainly related to their bioavailability rather than

58

to the total concentrations in soils, the efficient and promising strategy can focus on

59

lowering their chemical reactivity and mobility by the adsorption and precipitation at

60

the mineral-water interface.7-9 For example, the immobilization of Cd or As has been

61

demonstrated on birnessite,10 calcite,11-13 and goethite surfaces14,15 to induce the

62

formation of more stable precipitation products, such as CdCO3 (Ksp = 10-13.7).16

63

Moreover, the use of calcium phosphates (Ca-Ps), for example hydroxyapatite (HAP),

64

Ca10(PO4)6(OH)2, Ksp = 10-116.8, to induce the formation of cadmium phosphates

65

including Cd5H2(PO4)4·4H2O (Ksp = 10-30.9) and Cd5(PO4)3OH (Ksp = 10-42.5),17,18 and

66

calcium arsenates including Ca4(OH)2(AsO4)2·4H2O (Ksp = 10-27.5) and Ca5(AsO4)3OH

67

(Ksp = 10-40.1)19,20 has been extensively studied.21-24 However, efficient Cd and As

68

immobilization using relatively soluble Ca-P minerals compared with HAP may

69

provide more Ca2+ and PO43- ions at the mineral interface as an alternative approach

70

for more effectively precipitating Cd and As. Moreover, brushite (DCPD,

71

CaHPO4·2H2O, Ksp = 10-6.59)25 has been widely used as an available form of

72

phosphorus (P) fertilizers26 due to its relatively high solubility among Ca-P minerals.

3



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73

Despite the Cd and/or As adsorption and immobilization at various Ca-P mineral

74

surfaces that have been widely observed by macroscopic investigations, the behavior

75

and surface reaction kinetics of solutions containing mixed Cd and As with brushite at

76

the nanoscale remain largely unidentified. Therefore, the objective of this study was

77

to observe the kinetics of brushite dissolution in the presence of aqueous solutions

78

containing both Cd and As and coupled precipitation of a more stable phase and

79

thereby reveal the fundamental mineral interfacial phenomenon in controlling the fate

80

of contaminants. To achieve these goals, in situ atomic force microscopy (AFM) was

81

used to provide the real-time kinetics of brushite dissolution and subsequent

82

formation of new phases in the presence of CdCl2 and Na2HAsO4 at various

83

concentrations. The precipitates were isolated from brushite surfaces by ultrasonic

84

vibration in alcoholic solutions, and then characterized by high-resolution

85

transmission electron microscopy (HRTEM). To our knowledge, this is the first direct

86

observation of both Cd and As immobilization on Ca-P minerals at the nanoscale and

87

the characterization of mineral interfacial precipitates by ultrasonically separating the

88

precipitates from brushite substrates and performing HRTEM and selected area

89

electron diffraction (SAED) pattern analyses on them. It defines the potential role of

90

brushite in controlling immobilization of mixed Cd and As contaminants through

91

adsorption, co-precipitation and isomorphic substitution. These direct observations

92

may improve fundamental understanding of the interfacial interaction between

93

contaminants and Ca-P minerals, with implications for practical soil remediation.

94

4


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EXPERIMENTAL SECTION

95 96 97

Reagents. All the reagents were purchased from Sigma-Aldrich (St. Louis, Missouri).

98

Ultrahigh purity water from two-step purification treatment including triple

99

distillation (YaR, SZ-97A, Shanghai, China) and deionization (Milli-Q, Billerica, MA)

100

was used for the solution preparation. All experimental solutions were prepared

101

immediately before AFM experiments, and the pH was measured by a glass pH

102

electrode coupled with a single-junction Ag/AgCl reference electrode (Orion 4 Star

103

ISE meter, Thermo).

104

Brushite crystal synthesis. Brushite single crystals were synthesized by a gel

105

method.27 The synthesized crystals were characterized by Bruker D8X-ray diffraction

106

(Billerica, Massachusetts) to identify the crystals as a single phase.

107

In situ AFM imaging of brushite dissolution. In situ dissolution experiments were

108

performed using Agilent-AFM (Agilent 5500, Phoenix) equipped with a fluid cell (1

109

mL) working in contact mode. All solution conditions can be found in Supporting

110

Information (Tables S1-S5). A fresh cleavage (010) surface of brushite was exposed to

111

each experimental solution. Solutions of 5-500 µM Na2HAsO4·7H2O and CdCl2 were

112

injected into the fluid cell with a Master Flex C/L pump (Cole Parmer Instrument) and

113

the flow rate was approximately 1 mL/min which has been shown in previous studies

114

to make surface-control the rate determining step in the reaction, rather than diffusion

115

through the fluid at the interface.27 All images were collected with Si3N4 tips with a

116

force constant of 0.2 N/m at scan rates of 3-5 Hz. Minimizing tip−surface force

5



Page 6 of 36

117

interactions during the flow-through of solutions reduced artifact effects on step edge

118

morphology and measured velocities. Quantitative analyses of AFM images were

119

made by using PicoScan 5 software. All data were presented as their mean value ±

120

standard deviation (SD) of three independent experiments. Significant difference was

121

analyzed at P < 0.01 by the SPSS software.

122

Precipitate identification. The reacted brushite crystals were removed from the

123

reaction cell, rinsed with alcohol solutions and placed onto filter paper to remove the

124

residual solutions for SEM (SEM, JSM 6390 LV) observations. Moreover, the

125

precipitates were also isolated from the brushite surfaces by ultrasonic vibration in

126

alcohol solutions, and suspension samples were deposited on a carbon-coated copper

127

grid to characterize crystalline phases by a FEI Titan G2 60-300 probe Cs-corrected

128

TEM, as well as using the EDX detector for qualitative chemical analyses. Data were

129

collected using an acceleration voltage of 200 kV. Liquid nitrogen was used to cool

130

down the precipitate sample to room temperature during the HRTEM observations in

131

order to decrease the influence of the electron beams on the phase transformation and

132

prevent decomposition of possible hydrous phases in the electron beam. Parameters of

133

d-spacing values for phases Cd5H2(PO4)4·4H2O (23-0091), Ca5(AsO4)3OH (26-0296),

134

Ca4(OH)2(AsO4)2·4H2O (18-0289) and Cd5(PO4)3OH (12-0442) were found in

135

JCPDF.

136 137

RESULTS

138

6


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139

Dissolution Features of the Brushite (010) Surface. a. Morphology modification

140

of etch pits.

141

Prior to injecting the reaction solutions, the brushite (010) cleavage surfaces were

142

exposed to pure water at pH 5.5-6.0, and dissolution occurred through the formation

143

of shallow etch pits with a monomolecular step of 7.6 Å (Figure 1A), corresponding

144

to half the size of the (010) lattice plane spacing.27 Triangular etch pits are bound by

145

the 101

146

1A). Following 5 min of injection of 5 µM CdCl2 solutions at pH 6.0, the etch pit

147

shape transformed to a trapezium shape with the newly formed 101

148

1B). This morphology modification by low concentration CdCl2 occurred at solution

149

pH 4.0-8.0 (Figure S1), and the trapezium recovered to the normal triangle with

150

increasing CdCl2 concentrations up to 500 µμM at pH 6.0 (Figure S2). In addition, the

151

emergence of another new direction of the 102

152

dissolution in 50 µμM Na2HAsO4 solutions at pH 4.0 (Figure S3A), leading to the

153

generation of a new triangle with angles 29-72-79 degrees (Figure S3A). With the

154

increase of pH, the etch pits changed to the four-sided shape with another new step

155

along the 301

156

pH 10.0 (Figure S3C).

157

b. Dissolution rates.

158

Brushite crystals dissolved in pure water along the 101

159

directions and exhibited anisotropic step retreat velocities of 0.52 ± 0.18 (n = 3, the

160

number of the crystals that were observed), 2.49 ± 0.25 nm/s (n = 3), and 5.29 ± 0.24

Cc,

101

Cc

Cc

and 100

Cc

directions with angles 29−55−96 degrees (Figure

Cc

Cc

steps (Figure

was observed following the

direction at pH 8.0 (Figure 1C), and even to the fan-like shape at

Cc,

101

Cc,

and 100

Cc

7



Page 8 of 36

161

nm/s (n = 3) at pH 6.0, respectively (Figure S4). Retreat velocities of three steps were

162

promoted in the presence of different concentrations of NaCl, CdCl2 or Na2HAsO4

163

(Figure S5). In addition to the step retreat velocity, the presence of 5 µM CdCl2 at pH

164

6.0 resulted in the formation of deep etch pits (Figure 1B) with a deepening velocity

165

of 3.06 ± 0.19 nm/min (n = 3) that increased with increasing CdCl2 and/or salt

166

concentrations (Figures 2A, C and S6). Similarly, the presence of 50 µM Na2HAsO4

167

at pH 8.0 increased the etch pit deepening velocities (Figure 2B). When the

168

Na2HAsO4 concentration was increased to 500 µM, the deepening velocities increased

169

up to 3.30 ± 0.12 nm/min (n = 3) (Figures 2B and S7), and NaCl (> 0.01 mM) also

170

promoted the deepening velocities (Figures 2D and S8).

171 172

Precipitation at the Brushite-Water Interface.

173

a. The occurrence and growth of nanosized particles in Cd-containing solutions.

174

During the dissolution of brushite crystals, Ca2+ and HPO42- ions were released into

175

the brushite-water interface and subsequently precipitation occurred on the brushite

176

surface after different induction periods (Figure 3). Following 15 min of dissolution

177

reactions in the presence of 5 µM CdCl2 at pH 6.0, nanoparticles with average heights

178

of about 110 nm were observed, and they grew to form bigger particles (about 440 nm)

179

after 60 min (Figure 3A and A1). According to the induction time that is needed for

180

the occurrence of nucleated particles, low concentration NaCl (10-2-10-1 mM) added

181

to CdCl2 solutions (5 µM, pH 6.0) solutions shortened the induction times, promoting

182

the formation of precipitates (Figure 4A and F), whereas a relatively high

8


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183

concentration of NaCl (10-1 -102 mM) inhibited the nucleation through an increase in

184

the induction times (Figures 4B, F and S6). We also note that it is impossible to form

185

CdCO3 precipitates even in 500 µM CdCl2 solutions at pH 5.5-6.0 according to

186

concentration calculations (Table S6).

187

b. The occurrence and growth of nanosized particles in As-containing solutions.

188

Similar phenomena occurred in Na2HAsO4 solutions (50 µM, pH 8.0) (Figure 4C-F).

189

Following the exposure to only 50 µM Na2HAsO4 solutions at pH 8.0 (Figure 3B), the

190

size distribution of nucleated particles remained unchanged within 60 min of reaction

191

time (Figure 3B1), compared to that in CdCl2 solutions (Figure 3A1). However, no

192

precipitates were observed at pH 4.0-6.0 (Figure S3A and B).

193 194

Precipitate Identifications.

195

Newly formed precipitates were ultrasonically isolated from the brushite surface,

196

observed and characterized by SEM and HRTEM (Figures 5-7). In the presence of

197

500 µM CdCl2 alone, the precipitates with a size of about 438 nm (Figure 5A and B)

198

covered the whole (010) surface of brushite crystals (Figure 5A), and the EDX with

199

the spatial resolution of about 50 nm showed that precipitates consisted of Cd, Ca, P,

200

and O elements (Figure 5C). HRTEM analyses demonstrated the existence of rounded

201

particles of a crystalline phase (particle II) within an amorphous matrix phase (area I)

202

(Figure 5D). The measured d-spacing (8.62 Å) of lattice planes of these particles and

203

comparison with possible phases with appropriate composition suggests that the most

204

likely phase is Cd(5-x)CaxH2(PO4)4·4H2O. This is based on the data base with d = 8.83

9



Page 10 of 36

205

Å for the (200) lattice spacing of Cd5H2(PO4)4·4H2O and the assumption that the

206

difference between the measured spacing and data base is due to Ca2+ substitution for

207

Cd2+ in the structure, although Ca2+ (100 pm) and Cd2+ (95 pm) have almost identical

208

ionic radius.

209

In the presence of 500 µM Na2HAsO4 alone, ultrasonically isolated precipitates

210

(Figure 6A) consisted of Ca, As, P and O (Figure 6B), and HRTEM (Figure 6C)

211

showed that the precipitates contained two crystalline phases (II and III) within an

212

amorphous matrix phase (I). The identification of the crystalline phases was again

213

based on matching lattice plane spacings with the data base for possible phases. The

214

best match for phase II was Ca4(OH)2(AsO4)2·4H2O (7.824 Å for measured value and

215

7.82 Å for characteristic d-spacing of (100) planes in the data base) and

216

Ca5(AsO4)(3-x)(PO4)xOH for phase Ⅲ for which the data base d spacing for (210)

217

planes of Ca5(AsO4)3OH is 3.14 Å in comparison with the measured d spacing of

218

3.034 Å. The difference between the data base and the measured spacing is assumed

219

to be due to the partial substitution of AsO43- (46 pm for the ionic radius of As5+) by

220

PO43- ions (38 pm for the ionic radius of P5+).

221

For a mixed solution of CdCl2 and Na2HAsO4, the precipitates isolated from the

222

brushite surface (Figure 7A) showed the presence of Cd, Ca, As, P, and O (Figure 7B)

223

with two crystalline phases (II and III) within an amorphous matrix phase (I). The

224

identification of phase II as Cd5(PO4)3OH is based on d = 2.673 Å for the (220) lattice

225

spacing (2.73 Å in the data base), and phase III as Cd(5-x)Cax (AsO4)(3-y)(PO4)yOH

226

based on d = 2.251 Å for the (311) face (2.18 Å in the data base) with partial

10


Page 11 of 36

227


substitution of AsO43- by PO43- ions and Cd2+ substituted by Ca2+ (Figure 7).

228

DISCUSSION

229 230 231

The Adsorption of Cd and As on the Brushite (010) Surface.

232

For all low concentrations of CdCl2 or Na2HAsO4 (≤ 50 µM), a rapid adsorption of Cd

233

and As on brushite steps was observed based on the changes of the etch pit

234

morphology that were captured immediately following the injection of the reaction

235

solutions into the AFM fluid cell (Figure 1B and C). The changes result from the

236

direction-specific adsorption of Cd and As species on the step edges. From the atomic

237

structure of brushite, the [101]Cc step is the Ca-terminated polar step, both the 100

238

and 101

239

preferentially bind to the [101]Cc/ 101

240

The similar morphological modifications of etch pits have been frequently observed

241

during the interaction between minerals and additives. 28-30

Cc

Cc

steps are mixed charge.27 Therefore, HAsO42- and Cd2+ ions may Cc

and the 100

Cc/

101

Cc

steps, respectively.

242

With the increase of CdCl2 and Na2HAsO4 concentrations, the dissolution rates

243

rapidly increased (Figure 2A and B), and this resulted in less adsorption along step

244

edges and the four-sided etch pits recovered to triangular shapes (Figure S2B),

245

suggesting that the adsorption of Cd and/or As depends on the timescale for

246

adsorption and the lifetime of the steps. If the timescale for the Cd and/or As

247

adsorption on the crystal surface to achieve equilibrium values is longer than the

248

timescale for crystal components to be at the equilibrium state, Cd and/or As

11



Page 12 of 36

249

adsorption will not occur.31 Thus, the dissolution rates of brushite including the step

250

retreat and deepening rates affect Cd and/or As adsorption. In addition, the desorption

251

of Cd and/or As from brushite step edges was also rapid when CdCl2 or Na2HAsO4

252

solutions were replaced by pure water according to the modified etch pits that

253

returned immediately to normal triangular shapes (Figure 1A).

254

Coupled Dissolution-Reprecipitation Reactions.

255

The dissolution of brushite in all solutions provides a reliable source of Ca2+ and

256

HxPO4(3-x)- (x depending on the pH of interfacial solutions) ions which result in

257

supersaturation with respect to Cd-P or Ca-As phases at the brushite (010) interface.

258

The major chemical reactions related to the coupled dissolution/precipitation include:

259 260

H+ + CaHPO4(s) ®Ca2+ + H2PO4-

(1) (pH £6.0, H+-promoted dissolution27)

261

H2O + CaHPO4(s) ®Ca2+ + HPO42-

(2) (pH ³8.0, H2O-promoted dissolution27)

262

Ca2+ + H2PO4- + Cd2+ ® Cd(5-x)CaxH2(PO4)4·4H2O

(3) (pH £ 6.0)

263

Ca2+ + HPO42- + HAsO42- ®Ca4(OH)2(AsO4)2·4H2O + Ca5(AsO4)(3-x)(PO4)xOH

(4) (pH ³8.0)

264

Ca2+ + HPO42- + Cd2++ HAsO42- ®Cd5(PO4)3OH + Cd(5-x)Cax (AsO4)(3-y)(PO4)yOH

(5) (pH ³8.0)

265 266

Precipitation does not occur until the critical supersaturated condition is reached at the

267

brushite-water interface after the induction time (τ) that is defined by eq 6

268 γ3SL 3 3 kB T (lnS)2

269

ln τ = C1 +C2

(6)

270

where C1 and C2 are independent constants, and C2 contains only geometric 12


Page 13 of 36


271

parameters and C1 is controlled by entropy changes.32 kB is Boltzmann constant, T is

272

absolute temperature, S is supersaturation (S =

273

activity product and Ksp is its value at equilibrium), and γSL is the interfacial free

274

energy between the mineral surfaces and the liquid.32 In the present study, increasing

275

the Cd and/or As concentration shortened the induction time (Figure 4E), probably

276

due to the increase of the S value which depends on the activity of the Cd2+, PO43-,

277

and AsO43- ions at the brushite interface. A net increase in the dissolution rates

278

including the step retreat (Figure S5) and deepening velocities (Figure 2) of etch pits

279

can increases S. This implies that there is a build-up of ions in the interfacial solution,

280

then transport (via diffusion) may be playing some role in the solution chemistry.

281

Moreover, CdCl2 and Na2HAsO4 (acting as background electrolytes) may alter the

282

hydration layer near the (010) face, effectively lowering the desolvation barrier by

283

displacing water molecules to remove Ca2+ or HPO42- ions from the steps and

284

promoting dissolution.30,

285

increases the dissolution of brushite, resulting in a decrease in induction time.

286

a. pH. Amorphous and Cd-Ca-P-containing phases can form in a wide pH range

287

(4.0-10.0).34 The Cd-Ca-P nucleation occurred within 5 min at pH 4.0 (Figure S1),

288

much shorter than at pH 6.0 and 8.0 (Figures 3A and S2) due to higher dissolution

289

rates at pH 4.0 (Figure S4). This can be related to the protonation of oxygen sites

290

along the steps, resulting in the formation of H2PO4-35 because the solubility of

291

brushite increases at lower pH where H2PO4- formation is favorable. Rapid dissolution

292

contributes to more Ca2+ and HxPO4(3-x)- ions that are released into the brushite-water

33

%&' ()*

, where IAP is the actual ion

Thus, an increase in the Cd and/or As concentration

13



Page 14 of 36

293

interface and shortens the induction time. In contrast, few precipitates were observed

294

on the brushite (010) surfaces in 5 µM CdCl2 solutions at pH 8.0 after 60 min reaction

295

time due to lower dissolution rates of brushite at higher pH. Moreover, aqueous Cd2+

296

at pH 8.0 could be converted into CdOH+ (log Ka1 = -7.9, log Ka2 = -10.6 and log Ka3

297

= -14.3),36 resulting in a decrease in concentrations of free Cd2+ ions (Figure S9).

298

For the influence of pH on precipitation in the Na2HAsO4 solution, various pH

299

values alter the relative concentrations of the four protonated forms of arsenate

300

including H3AsO40, H2AsO4-, HAsO42-, and AsO43- (log Ka1 = -2.3, log Ka2 = -6.8 and

301

log Ka3 = -11.6),37 thus both the chemical composition and the ratio of Ca/As are

302

increased by the deprotonation process. In general, the higher the Ca/As ratio at high

303

pH (³ 8.0), the less soluble the Ca-As phases (the solubility product for a given Ca−

304

As phase, Ksp = 10-4.68, 10-18.91, 10-29.20, and 10-38.04 for pure CaHAsO4·2H2O,

305

Ca3(AsO4)2·4H2O, Ca4(AsO4)2(OH)2·4H2O, and Ca5(AsO4)3OH, respectively, at

306

25 °C).19 The precipitation in Na2HAsO4 solutions on brushite only occurred at pH ³

307

8.0.

308

b. NaCl. In the present study, NaCl ranged from 10 µM to 100 mM in 5 µM CdCl2 or

309

50 µM Na2HAsO4 solutions to adjust the ionic strength (IS =

310

and 𝑧. are the concentration of the species and the ionic charge, respectively). The

311

dissolution rates in 5 µM CdCl2 or 50 µM Na2HAsO4 solutions with NaCl were

312

significantly higher than that in the same solutions without NaCl. This trend is

313

consistent with the results from Ruiz-Agudo et al. for the case of calcite dissolution in

314

the presence of NaCl, suggesting that background electrolytes enhance dissolution

+ ,

0 , . 𝑐. 𝑧. ,

in which 𝑐.

14


Page 15 of 36


315

rates by changing water structural dynamics and solute surface hydration.38 At

316

relatively low IS ([NaCl] = 10-2-10-1 mM), the solvent structure around Ca2+ is

317

determined by Cl- ions and influences Ca2+ removal from the surface structure, the

318

rate limiting step for brushite dissolution.38 In addition, the increase in brushite

319

dissolution rates in the presence of NaCl has been attributed to the enhancement of

320

brushite crystal solubility through the strong long-range electric fields emanating from

321

the ions of the background electrolyte to screen the charges of the hydrated ions

322

building the crystal, thereby shifting the chemical equilibrium.39 Therefore, the higher

323

dissolution rate, the more Ca2+ and HPO42- released into the brushite-water interface,

324

leading to the shorter induction time that is needed for the nucleation of Cd-Ca-P or

325

Ca-As-P phases. On the other hand, the interfacial energy (γSL ) between the mineral

326

surface and the solution has been verified to be decreased with increasing IS, resulting

327

in a shorter induction time during nucleation.40, 41

328

At relatively high IS ([NaCl] > 1 mM), NaCl dramatically inhibits nucleation

329

(Figure 4F). The most likely explanation is that background electrolytes stabilize

330

water molecules in the hydration shell of Cd2+ and Ca2+ ions,42 inhibiting the

331

formation of precipitates. Moreover, the interactions between Na+ and HAsO42- or Cl-

332

and Cd2+ ions will be progressively increased with high NaCl concentrations, resulting

333

in an increase in the volume of the hydration shell.43 In this case, the hydrated size of

334

the ion, that is larger than the electric field of the ion, controls the solvent behavior,

335

i.e., the point charge eventually becomes distant enough due to the water molecules at

336

the surface, resulting in ion to ion (Cd2+ to HPO42- and Ca2+ to HAsO42-) interactions

15



Page 16 of 36

337

being weaker at the brushite-water interface in higher NaCl concentration (> 1 mM).43

338

Thus, low concentration NaCl (10-2-10-1 mM) shortens the induction time and

339

promotes nucleation, and high concentration NaCl (> 1 mM) prolongs the induction

340

time and inhibits nucleation (Figure 4F).

341

Phase Transformation of the Precipitates.

342

In CdCl2 solutions, AFM measurements showed that the size of the newly formed

343

precipitates was about 110 nm and increased to about 440 nm in 60 min (Figure 3A1),

344

consistent with the sizes measured by SEM and TEM (Figure 5A and B). The

345

long-time existence of these particles ultrasonically isolated from brushite surfaces

346

suggests that these particles (about 440 nm) are stable. The random formation of small

347

amorphous or crystalline particles/clusters and subsequent aggregation44 may occur at

348

the brushite interface. Also, the mixed phases of amorphous (nanoparticle I) and

349

crystalline (nanoparticle Ⅱ, Cd(5-x)CaxH2(PO4)4·4H2O) (Figure 5D) suggest that an

350

amorphous phase may transform to a thermodynamically stable phase. A similar result

351

was also reported in the removal of Cd by apatite.45 This transformation can be

352

divided into three steps: (1) the dissolution and hydration of Cd2+ and PO43- ions from

353

the amorphous phase; (2) the transfer of these hydrated ions; (3) the nucleation and

354

subsequent growth of Cd5H2(PO4)4·4H2O and Cd(5-x)CaxH2(PO4)4·4H2O after the Ca2+

355

substitution.46-49 In the present study, liquid nitrogen was used to cool down the

356

precipitate sample to room temperature during the HRTEM observations in order to

357

decrease the influence of the electron beams on the phase transformation. Thus, this

358

transformation is possible at pH 6.0 at room temperature. Moreover, the more

16


Page 17 of 36


359

thermodynamically stable phase is Cd5(PO4)3OH at pH 8.0 (Figure 7C2). However, it

360

cannot form directly but through the dissolution of an intermediate phase

361

Cd5H2(PO4)4·4H2O50

362 363

Cd5H2(PO4)4·4H2O → Cd5(PO4)3OH + 3 H3O+ + PO43-

(7)

364 365

At pH 8.0, the Ca4(OH)2(AsO4)2·4H2O and Ca5(AsO4)3OH are the main phases51 with

366

solubility

367

Ca4(OH)2(AsO4)2·4H2O may transform to Ca5(AsO4)3OH or their coexistence is

368

possible (Figure 6C).

products

of

10-27.49 and

10-40.12,

respectively,19

suggesting

that

369

During the transformation processes, isomorphic substitution may occur, i.e. Cd2+

370

and AsO43- may be substituted by Ca2+ and PO43-, respectively.49 Making an

371

assumption of a binary substitution and two simultaneous substitutions mainly

372

depends on the relative sizes of ions. Based on the measured d-spacing of 8.62 Å

373

(Figure 5D) that is smaller than the characteristic value of 8.830 Å (the (200) face) for

374

the phase Cd5H2(PO4)4·4H2O, as well as EDX results showing the presence of Ca

375

(Figure

376

Cd(5-x)CaxH2(PO4)4·4H2O. This may occur through the adsorption of Ca2+ at the

377

precipitate’s surface, then the Ca2+ diffusion into the precipitates and the subsequent

378

substitution of Cd2+ by Ca2+.52, 53 Also, this could be by a dissolution-precipitation

379

mechanism at the precipitate-fluid interface. According to the same analyses, the

380

substitution of arsenate by phosphate in the Ca5(AsO4)3OH lattices may also occur to

5C),

we,

therefore,

reasonably

assign

this

crystalline

phase

as

17



381

Page 18 of 36

form particles of Ca5(AsO4)(3-x)(PO4)xOH (Figure 6C).

382

In the mixed CdCl2 and Na2HAsO4 solutions, the isomorphic substitution of both

383

cations and oxyanions may result in the formation of Cd(5-x)Cax(AsO4)(3-y)(PO4)yOH

384

(Figure 7C). In general, this structural distortion and chemical substitutions are known

385

in apatite-like minerals with the general formula with M5(ZO4)3X (M = Ca, Cd, or Pb

386

etc., Z = P, As, or Si etc., and X = F, OH or Cl etc.).49,

387

substitution mechanism has been widely applied to the efficient removal of Cd and As

388

contaminants by HAP which can be formed from brushite at relatively high pH (³

389

8.0).58 Due to the limitation/lacking of appropriate thermodynamic data in PHREEQC,

390

molecular modeling and simulations using density functional theory (DFT) for the

391

formation of three solid phases/solid solutions (Figures 5-7) have been used for

392

further investigations.

54-57

This isomorphic

393 394

Implications. Cd and As pollutants in arable soils have attracted extensive attention

395

and their immobilization is the key strategy to decrease bioavailability from soil to

396

plant.59-61 Ca-P minerals can adsorb Cd and As and offer an economically and

397

practically benign method for pollution mitigation and remediation.62 The adsorbed

398

Cd and As combine with the released Ca2+ ions during dissolution and form sparingly

399

soluble precipitates, which provide a possibility of permanent immobilization. In the

400

present study, we indicate that the dissolution of brushite, one of the precursor phases

401

of HAP, could induce simultaneous precipitation of both Cd and As phases by an

402

interface-coupled dissolution-precipitation mechanism,63 and high concentrations of

18


Page 19 of 36


403

salts delay and inhibit Cd and As immobilization on brushite. These in situ

404

observations may provide a fundamental understanding of how both Cd and As can be

405

immobilized and subsequently sequestered through mineral interfacial reactions in

406

soils.

407

ASSOCIATED CONTENT

408

409

Supporting Information

410

The Supporting Information is available free of charge on the ACS Publications

411

website. AFM experimental conditions (Tables S1-S5); Speciation calculations in 500

412

µM CdCl2 solutions (Table S6); Brushite dissolution-precipitation (Figures S1-S3 and

413

S6-S8); Step retreat velocities (Figures S4 and S5) in various solutions and the

414

relative speciation distributions in As and Cd solutions (Figure S9).

415

AUTHOR INFORMATION

416 417 418

Corresponding Authors

419

*Phone/Fax:

420

[email protected]

+86-27-87288382.

E-mails:

[email protected];

421 422

ACKNOWLEDGEMENTS

423

19



Page 20 of 36

424

This work was supported by the National Natural Science Foundation of China

425

(41471245 and 41071208 to L.J.W.), the Fundamental Research Funds for the Central

426

Universities (2662015PY206 and 2662017PY061 to L.J.W; 2662017JC020 to W.J.Z.).

427

C.V.P. and A.P. acknowledge funding through the EU seventh Framework Marie S.

428

Curie ITNs: Minsc; CO2 react; and Flowtrans.

429

REFERENCES

430 431 432 433

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pH. Environ. Sci. Technol. 2017, 51 (19), 11156-11164.

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Zhu, Y. Occurrence and partitioning of cadmium, arsenic and lead in mine

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impacted paddy rice: Hunan, China. Environ. Sci. Technol. 2009, 43 (3), 637-642.

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Pb, and phosphorus and their availability in the phosphate-amended contaminated

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21128-21137.

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610 611 612 613 614 615 616 617 618 619 620 621

28


Page 29 of 36


622 623

Figure 1. AFM deflection and height images of etch pits formed in (A) pure water, (B)

624

5 µM CdCl2 at pH 6.0, and (C) 50 µM Na2HAsO4 at pH 8.0. Height profiles show the

625

depth of etch pits measured along the white dashed lines 1®2 in (A-C) height images.

626 627

29



Page 30 of 36

628 629

Figure 2. Deepening velocities of etch pits formed at the brushite (010) surface in

630

solutions of (A) 5 µM CdCl2 (pH 6.0), (B) 50 µM Na2HAsO4 (pH 8.0), (C) NaCl +

631

CdCl2 (pH 6.0), and (D) NaCl + Na2HAsO4 (pH 8.0). The deepening velocities were

632

presented as mean value ± SD (n =3). Different uppercase letters indicate significant

633

difference at P < 0.01.

634 635 636 637 638

30


Page 31 of 36


639 640

Figure 3. Time sequence of AFM deflection images reveals that precipitation of a

641

new phase occurs on the brushite (010) surface in solutions of (A) 5 µM CdCl2 at pH

642

6.0 and (B) 50 µM Na2HAsO4 at pH 8.0. Darker lines are dissolution steps down to

643

right while lighter lines are steps up to right. (A1, B1) The particle size distributions

644

of the precipitates formed at different time periods.

645 646 647 648 649

31



Page 32 of 36

650 651

Figure 4. AFM deflection images show that the precipitation and inhibition in

652

solutions of (A, B) 5 µM CdCl2 (pH 6.0) and (C, D) 50 µM Na2HAsO4 (pH 8.0) in the

653

presence of different concentrations of NaCl after 60 min of the dissolution reaction.

654

(E, F) The induction time for the formation of the precipitates at various solution

655

conditions.

656 657 658

32


Page 33 of 36


659 660

Figure 5. (A) SEM image of the precipitates formed on the brushite (010) surface

661

after the AFM experiments in the presence of 500 µM CdCl2 (pH 6.0). (B) TEM

662

image showing the precipitate particles (about 438 nm) ultrasonically isolated from

663

brushite surfaces. (C) EDX spectrum demonstrating that the particles consist of Cd,

664

Ca, P and O. (D) HRTEM image taken from the dotted red square in (B) shows the

665

coexistence of amorphous (I) and crystalline phase (Ⅱ). The fast fourier transform

666

(FFT) diffraction patterns of (D1) particle I and (D2) particle Ⅱ.

667

33



Page 34 of 36

668 669

Figure 6. (A) TEM image showing ex situ formation of precipitates ultrasonically

670

isolated from the brushite surfaces in the presence of 500 µM NaHAsO4 at pH 8.0. (B)

671

EDX spectrum of the precipitates demonstrating the Ca, As, P, and O peaks. (C)

672

HRTEM image taken from a dotted red rectangle in (A) and corresponding FFT

673

patterns of particles I, II and III of precipitates show the presence of the mixed

674

amorphous and crystalline phases.

675

34


Page 35 of 36


676 677

Figure 7. (A) TEM image showing ex situ formation of precipitates ultrasonically

678

isolated from brushite surfaces in the presence of both 5 µM CdCl2 and 50 µM

679

NaHAsO4 at pH 8.0. (B) EDX spectrum demonstrates that the precipitates consist of

680

the Cd, Ca, As, P and O. (C) HRTEM image taken from the dotted red rectangle in (A)

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and corresponding FFT patterns of particles I, II and III of precipitates show the

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presence of the mixed amorphous and crystalline phases.

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Direct Observation of Simultaneous Immobilization of Cadmium and

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