Characterization of Lead Uptake by Nano-Sized Hydroxyapatite: A

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Characterization of lead uptake by nano-sized hydroxyapatite: A molecular scale perspective Dong-Xing Guan, Chao Ren, Jingzhao Wang, Yinian Zhu, Zongqiang Zhu, and Wei Li ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00020 • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018

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ACS Earth and Space Chemistry

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Characterization of Lead Uptake by Nano-Sized Hydroxyapatite: A

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Molecular Scale Perspective

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Dong-Xing Guan1, Chao Ren1, Jingzhao Wang1, Yinian Zhu2, Zongqiang Zhu2, Wei Li1, *

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1

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and Engineering, Nanjing University, Nanjing 210023, China

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2

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Guilin University of Technology, Guilin, Guangxi 541004, China

Key Laboratory of Surficial Geochemistry, Ministry of Education, School of Earth Sciences

Collaborative Innovation Center for Water Pollution Control and Water Safety in Karst Area,

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* Corresponding author, Tel: +0086-25-89681539; Fax: +0086-25-83686016; Email:

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[email protected]

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ACS Paragon Plus Environment

ACS Earth and Space Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract

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Page 2 of 31

Sequestration of toxic metals (e.g., Pb) by hydroxyapatite (HAP) has attracted wide

21

attentions in soil remediation.

22

HAP was

23

Fourier-transform infrared (FTIR) and high-resolution transmission electron microscopy

24

(HRTEM).

25

precipitation but also incorporation, depending on initial Pb concentration.

26

concentrations (e.g., 0.1 mM), surface adsorption may contribute considerably to the total Pb

27

uptake, as no changes were observed in the XRD analysis. At medium concentration of 0.5–

28

5.7 mM, formation of lead phosphate precipitates was evidenced by the XRD analysis that

29

new peaks at 2θ of ~30.3o and can be indexed to crystalline hydroxypyromorphite (HPY).

30

This was also consistent to the ratios (0.9–1.0) of Pbsorbed/Cadissolved at Pb level of 2.5–5.7 mM.

31

At higher Pb concentration (≥ 6.6 mM), larger ratios (1.2–1.9) of Pbsorbed/Cadissolved was

32

observed; and surprisingly the XRD signal denoting to HPY was weakened.

33

that Pb has incorporated through substituting the Ca sites into HAP crystal lattice, different

34

from the dissolution-precipitation process occurring at the medium Pb concentration range.

35

Direct observation of PbxCa5-x(PO4)3OH solid solutions with high Ca content from HRTEM

36

further supported this argument.

37

fundamental geochemistry for Pb interaction with HAP provided by this research could

38

improve the efficiency of the application of HAP material for environmental remediation.

39

Keywords: Lead; Hydroxyapatite; Adsorption; Precipitation; Incorporation;

40

Hydroxypyromorphite; Solid solution

conducted

using

In this study, a comprehensive investigation on Pb uptake by combined

batch

method,

X-ray

diffraction

(XRD),

Results revealed that Pb uptake mechanism involves not only surface adsorption, At the low Pb

This may imply

Overall, the molecular level understanding of the

41

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1. Introduction

42 43

Sequestration and stabilization of Pb and other toxic metals (including radionuclides) by

44

low-cost phosphate minerals, such as hydroxyapatite [HAP, Ca5(PO4)3OH], is a promising and

45

effective strategy for remediation of contaminated soils, groundwater, and wastes.1-5

46

has a robust hexagonal atomic framework based on two distinct metal-cation sites (Ca(I),

47

Ca(II)), a tetrahedral-phosphate site, and an anion column along four edges of the unit cell

48

(Figure S1).6

49

substitutions and complete replacement of Ca by bivalent metals (e.g. Pb2+, Cd2+, Cu2+,

50

Zn2+).6,7

51

crystal structure of HAP facilitates metal immobilization.

52

sequestering toxic metals has motivated extensive fundamental research to elucidate the

53

geochemical reaction mechanisms involved in the remediation processes.8-13

HAP

The unique crystal structure of HAP makes it tolerant to many ionic

The formation of highly insoluble and non-bioavailable metal fraction bound in the The efficiency of HAP in

54

Owning to the soluble nature of HAP mineral, interaction between HAP and Pb2+ is more

55

complex than traditional adsorption reaction between metals and insoluble adsorbents (e.g.,

56

Fe/Al hydroxides) that can be easily modelled using classic surface complexation theory.

57

Conceptually, during the uptake of Pb2+ by HAP, several processes may be involved, such as

58

cation exchange (also known as incorporation or lattice substitution), metal adsorption on

59

HAP surface, HAP dissolution followed by generation of hydroxyls to increase solution pH,

60

and a new metal phase precipitation via either OH- or PO43-.13,14

61

influence and contribute to the Pb sequestration from aqueous solutions by HAP.

All of these reactions may

62

The presence of dissolved Ca ion in many natural settings raises the possibility that

63

phases intermediate in composition between pure HAP and its substitution product

64

(Pb5(PO4)3OH, hydroxypyromorphite or HPY) may form in preference to end-member

65

compositions,15 i.e. PbxCa5-x(PO4)3OH solid solutions.

66

role in geochemical and environmental sciences because metal-containing solid solutions can

Solid solutions play a very important

3 / 31



Page 4 of 31

67

commonly form on mineral surfaces when aqueous toxic metal ions react with minerals, thus

68

affecting the transport and fate of these toxic metals in waters, soils and rocks.16

69

formation of solid-solution as a possible mechanism for Pb uptake by HAP was not clearly

70

addressed in previous studies17 due to the lack of good solid solution standards.

However,

71

Because of the complexity discussed above, to gain a clear understanding of the

72

interfacial processes during Pb uptake by HAP is quite challenging, especially at the

73

molecular level.

74

observed the formation of a secondary precipitate (i.e. HPY), which was further confirmed by

75

the scanning electron microscopy (SEM) characterization on the solid reaction products.

76

was then proposed a dissolution-precipitation mechanism to interpret this process, based on

77

the analysis of solution Ca ([Ca2+]), P ([PO43-]), Pb ([Pb2+]) and pH changes observed during

78

Pb uptake.8,18,19

79

precipitates, such as PbxCa5-x(PO4)3OH solid solutions, based on the Rietveld analysis on the

80

powder XRD data.

81

mechanism.

82

surface complexes), the estimate of contributions from different mechanisms was not

83

discussed.

84

pH-controlled condition, where the dissolution of HAP caused high fluctuation in solution

85

pH,13,18,19 which in turn affected the uptake mechanisms.

86

mechanisms of Pb onto HAP, it is prerequisite to fix the solution pH20 and expand the range of

87

Pb concentrations.

Using powder X-ray diffraction (XRD), Ma et al.8 for the first time

It

However, Mavropoulos et al.13 suggested the formation of a more stable

This implies that Pb2+ would occupy the Ca sites via an ion-exchange

Although these works did not exclude the presence the adsorption species (e.g.,

This is mainly because many previous studies were not performed in a

To unravel the complexity of

88

In this work, the mechanisms of Pb uptake by HAP nanoparticles under a wide range of

89

initial metal concentrations (0.1- 9.0 mM) were investigated by combining thermodynamic

90

simulation with solution chemistry and solid-state crystallographic analysis from a batch

91

experiment.

The experiments were performed in acidic environment (e.g., pH 5) in order to

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avoid the hydrolysis of Pb2+ and for comparison with previous research.

93

equilibrium modelling of HAP dissolution in the presence of Pb (0.1 - 9.0 mM) was

94

performed to assist interpretation of batch uptake experiments.

95

spectroscopic methods, such as XRD, Fourier-transform infrared (FTIR), and high-resolution

96

transmission electron microscopy (HRTEM), were applied to gain a molecular scale

97

understanding.

98

in aid of interpreting the uptake mechanisms.

99

hypothesis that precipitation, adsorption and incorporation would co-exist during the uptake

Chemical

A series of advanced

Solid solutions (PbxCa5−x(PO4)3OH) were synthesized and used as references The objective of this research was to test the

100

processes and make shifting contributions to Pb uptake.

Fundamental information provided

101

in this research would improve the current understanding of Pb sequestration by HAP, leading

102

to a better use of HAP material for environmental remediation.

103 104

2. Experimental Section

105 106 107

2.1. Reagents, materials, and solutions. Deionized water (DI, 15.0 Ω cm) was used to prepare all solutions.

All chemicals were

108

of analytical grade or better.

109

using Pb(NO3)2·4H2O.

110

from Nanjing Emperor Nano Material Co. LTD, China.

111

±0.02,

112

Brunauer-Emmett-Telle method to the N2 isotherm (ASAP 2020 HD88, micromeritics, USA).

and

specific

A 500 mM Pb stock solution (pH < 4) was prepared freshly

Hydroxyapatite (HAP) nanoparticles (20 nm, 97.5%) were bought

surface

area

was

69.37

The Ca/P ratio of HAP was 1.67

m2

g-1

as

determined

using

a

113 114

2.2. Equilibrium modelling of HAP dissolution in the presence of Pb.

115

To predict the feasibility of using HAP material for Pb uptake, the theoretical dissolution

116

behaviors of HAP in the presence of Pb in solution were simulated using Visual MINTEQ 5 / 31



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117

version 3.1.

Specifically, the simulation was performed at a fixed solution pH of 5.0 with

118

2.5 g L-1 HAP and 0, 0.1, 0.5, 1.0, 2.5, 5.0, 5.7, 6.6, 7.4 or 9.0 mM Pb in the suspension

119

systems.

120

of Visual MINTEQ.

121

common distribution of the Na+ ion in the natural environment and the widespread presence

122

of the NO3- strength in aquatic systems.

123

0.01 M to simulate the conventional ionic strength in aquatic system.21,22

124

of 0.00038 atm was also input to simulate the partial pressure of dissolved CO2 in aquatic

125

systems.

126

precipitation equilibrium of the HAP material. MINTEQ was also used to determine the

127

saturation state of possible solid precipitates for initial solution compositions.

128

indices (SI) for potential precipitates are listed in Tables S1–S4.

The specific concentrations of Pb and NaNO3 were input in the simulation window A 0.01 M NaNO3 was used as the background electrolyte due to the

The concentration of NaNO3 was maintained at A pressure value

The thermodynamic calculation was based on the theoretical dissolution–

Saturation

129 130 131

2.3. Batch uptake experiments. The macroscopic experiments for the HAP−Pb systems were conducted in 40 mL

132

centrifuge tubes under ambient conditions.

The HAP suspensions and NaNO3 electrolyte

133

solution (0.01 M) were mixed in the tubes and pre-equilibrated for ≥ 24 h.12

134

pre-equilibrium, solution pH was about 7 and dissolved Ca and P in solutions measured using

135

inductively coupled plasma optical emission spectroscopy (ICP-OES, iCAP 6300,

136

ThermoFisher Scientific, Waltham, USA) were about 0.2 and < 0.1 mM, respectively.

137

the Pb stock solution were added to the tubes to obtain the initial Pb concentrations of 0.1, 0.5,

138

1.0, 2.5, 5.0, 5.7, 6.6, 7.4 or 9.0 mM.

139

the reaction period of 1 h adjusted with diluted HNO3 and/or NaOH solutions.

140

pre-experiments, we noticed the phenomenon of a sudden decrease in solution pH, especially

141

when high Pb concentrations were added to the system.

After

Then

A constant pH value (5.0 ± 0.1) was maintained during In our

The measure we took is to adjust

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142

the pre-equilibrated suspensions to a considerable high pH (6–9), the extend depends on the

143

Pb amount added. When Pb was added to the system, the solution pH would drop to about 5,

144

and then we would adjust the solution pH to 4.9–5.1.

145

solutions were rigorously stirred and pH was monitored and adjusted when necessary to

146

maintain in the range of 4.9–5.1.

147

centrifuged immediately at 4000 g for 5 min.

148

µm filters; the solids were collected, stored at -80 oC before freeze drying to obtain the

149

powder for characterization.

150

experiments were conducted in duplicate.

151

analyzed for total Ca, P, and Pb using ICP-OES.

152

solid solutions [PbxCa5−x(PO4)3OH] (x = 0, 0.5, 1, 2, 3, 4, or 5) were synthesized following

153

the procedure published in previous literature.23

154

different Pb/(Pb + Ca) molar ratios were prepared while maintaining Pb + Ca in each solution

155

at 0.4 M, which were mixed with 250 mL of 4.4 M CH3COONH4 buffer solution and 500 mL

156

of 0.12 M NH4H2PO4 solution.

During the 1-h uptake process, the

After the reaction at 25 oC, the suspensions were The supernates were then filtered through 0.22

To confirm the experimental repeatability, the batch The filtrates were diluted using 0.3 M HNO3 and The hydroxypyromorphite–hydroxyapatite

In short, a series of 250 mL solutions of

157 158

2.4. Characterization of the uptake samples.

159

The crystal structures of the synthesized PbxCa5−x(PO4)3OH solid solutions and the

160

reaction solid products were characterized through XRD and FTIR. The XRD patterns were

161

obtained using a Bruker D8 ADVANCE X-ray diffractometer equipped with a LynxEye

162

detector using Ni-filtered Cu Kα radiation (λ = 0.15418 nm).

163

operated at a tube voltage of 40 kV and a tube current of 40 mA with a scanning rate of 1°

164

min-1 and a step size of 0.02°.

165

features, which result from the detection of vibrational modes, i.e. lattice vibrations and/or

166

molecular group vibrational modes.24

The diffractometer was

FTIR spectra of minerals display characteristic absorption

To obtain the FTIR spectra, the solids in KBr pellets

7 / 31



Page 8 of 31

167

were analyzed in a Thermo Nicolet 6700 FTIR with a diffuse reflectance attachment from

168

wave numbers 4000–400 cm-1.

169

HRTEM with energy dispersive X-ray spectrometry (EDS) were performed to

170

characterize the uptake products with initial Pb concentrations of 2.5 and 9.0 mM.

171

Transmission electron microscopy (TEM) specimens were prepared by dispersing the powder

172

sample slurry on a holey carbon mesh supported by a Cu grid.

173

properties at the reaction front were examined using an FEI TECNAI-F20 with an

174

acceleration voltage of 200 kV under both the conventional diffraction contrast [bright field

175

(BF)] mode and HRTEM mode.

176

charge-coupled device (CCD) camera attached on a Gatan image filter (Gatan GIF 2000).

177

The selected area diffraction (SAD) was also performed in areas of interest.

Their crystallographic

All TEM images were recorded using a slow-scan

178 179

3. Results and discussion

180 181 182

3.1. Batch uptake of Pb onto HAP. The Pb sorption isotherm by HAP, which covered a wide concentration range (0.1–9.0

183

mM) was presented in Figure S2.

Nearly 100% Pb was adsorbed by HAP at lower Pb

184

concentrations (i.e. 0.1–1.0 mM), whereas only 50% of the total Pb was sorbed by HAP at

185

medium. When [Pb] reached 5.0 mM or even higher, the maximum adsorption level at ~0.9

186

mmol g-1 was observed, which converts to a surface density of 13 µmol m-2.

187

excellent Pb sequestration capacity among many adsorbents (Table S5). Note that potential

188

material loss through filtering may exist owning to the larger pore size (0.22 µm) syringe

189

filter over the HAP nanoparticles (~20 nm), but negligible phosphorus was detected in the

190

clear supernatants.

191 8 / 31


This reveals an

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192 193

3.2. HAP dissolution in the solution. Both theoretical simulation and experiment measurement were conducted to investigate

194

the dissolution behavior of HAP in the presence of Pb in the system.

The theoretical

195

concentrations of dissolved HAP in the solution under the coexistence of 0–9.0 mM Pb are

196

shown in Figures 1a.

197

Pb concentrations.

198

HAP–Eu(III) system.12

Generally, for [Pb] 0–1.0 mM, the dissolved HAP was similar

199

maintaining at 1.08 mM.

When [Pb] increased to 2.5–9.0 mM, the dissolved HAP increased

200

visibly to 1.32–2.96 mM.

Through this simulation, we found that no more than 60% of the

201

total HAP was dissolved, leaving substantial amount of HAP material in the solid phase.

202

The undissolved HAP may provide active sites for complexation/adsorption of Pb onto the

203

material surface or further substitution of complexed Pb into the HAP crystal lattice by

204

diffusion.

The dissolution of HAP was gradually enhanced with the increment of Such enhanced dissolution was also theoretically predicated for the

205

In acidic environment (pH 5), HAP solids in the suspensions were considerably

206

dissociated, causing Ca and P dissolved into the solution (Figures 1b and 1c). Without Pb in

207

the system, the molar ratio of the dissolved Ca:P in the solution kept constant at 1.4, slightly

208

lower than the molar ratio (1.67) of Ca:P in the HAP materials.

209

analysis, no new precipitate phases could have been formed at this condition (Table S1).

210

The dissolved P and Ca in the solution was slightly lower at [Pb] 0.1–1.0 mM than without Pb,

211

generally consistent with the modelled results (Figures 1b and 1c).

212

mM), solution P and Ca decreased considerably or to almost undetectable level measured

213

using ICP-OES, generally in agreement with previous studies,13,25 but slightly lower than 9 / 31


Inconsistent with the bulk

At higher [Pb] (2.5–9.0


Page 10 of 31

214

predicated concentrations using Minteq modeling (Figure 1b).

The difference between

215

predicted and experimental values probably suggested re-adsorption of dissolved phosphate

216

by HAP or formation of an additional co-precipitates other than Pb3(PO4)2 considered in the

217

models (Table S2–S4), for instance, PbxCa5−x(PO4)3OH solid solution.13 The co-presence of

218

Ca-P precipitates in the system could influence signal height and width of peaks indexed to

219

HYP and HAP in XRD and FTIR. However, according to the SI for potential precipitates, the

220

formation of Ca-P precipitates, such as amorphous calcium phosphate, tricalcium phosphate

221

(β-Ca3(PO4)2) and brushite (CaHPO4·2H2O), was not favored under experimental conditions.

222 223

3.3. Relationship between HAP dissolution and Pb uptake.

224

The mechanisms of Pb uptake by HAP probably reply on the initial Pb concentrations in

225

the system. When [Pb] was 0.1–2.5 mM, the ratio of sorbed Pb versus dissolved Ca (Pb/Ca)

226

increased linearly from 0.02 to 0.9 (Figure 1d).

227

close to the unity, indicating precipitation is the main mechanism accordingly (Eqs. 1 and 2).

228

Ca5(PO4)3OH + 7H+ → 5Ca2+ + 3H2PO4- + H2O

(1)

229

5Pb2+ + 3H2PO4- + H2O → Pb5(PO4)3OH + 7H+

(2)

At [Pb] 2.5–5.7 mM, the Pb/Ca ratio was

230

At even higher [Pb] (6.6–9.0 mM), the removal of Pb from the solution not only formed

231

HPY, but could be adsorbed or complexed at the HAP or HPY surface to a large content.

232

Interestingly, the Pb/Ca ratio increased linearly from 1.2 to 1.9, implying an incremental

233

contribution of surface immobilization to the overall mechanism.

234

Pb was almost unchanged at [Pb] > 5 mM, the varied Pb/Ca ratio suggested an additional

235

reaction occurring, because formation of HPY precipitates only yields a Pb/Ca of 1:1 (i.e. at

10 / 31


Consider that the sorbed

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236

[Pb] = 2.5-5.7 mM) and both surface adsorption and Pb substitution into HAP crystal lattice

237

would cause elevated Pb/Ca ratio.

238

Taken together, these results demonstrate that the mechanisms of Pb uptake by HAP

239

highly reply on the initial metal concentrations.

It seems that the dominant mechanisms of

240

Pb uptake by HAP were probably different at different [Pb] levels (i.e., surface adsorption for

241

0.1 mM; precipitation as HPY for 0.5–5.7 mM; and probably formation of PbxCa5−x(PO4)3OH

242

solid solution for 6.6–9.0 mM).

243

further investigated in the following sections.

Experimental evidence for the specific mechanism was

244 245

3.4. XRD analysis.

246

To unravel the uptake mechanism, the XRD patterns of both PbxCa5-x(PO4)3OH solid

247

solution standards (x = 0, 0.5, 1, 2, 3, 4 or 5) and Pb-loaded HAP samples were

248

synthesized/collected and analyzed in detail.

249

PbxCa5-x(PO4)3OH standards belong to the apatite group of the hexagonal system P63/m

250

differing only in peak location, peak width and absolute intensity. Without Pb doping in the

251

structure (x = 0), the solid HAP showed the calculated unit cell parameters of a = b = 0.944

252

nm and c = 0.686 nm.

253

(0.119 nm), the solid was HPY (x = 5), whose lattice parameters a (= b) and c increased to

254

0.989 and 0.748 nm, respectively.

255

values increases from 0 to 5), the reflection of the PbxCa5-x(PO4)3OH solid shifted gradually

256

to a lower-angle direction (Figure 2a).

257

continuous solid solution within the whole range of x = 0–5.23

The XRD patterns in Figure 2a showed that all

For the standard with substitution of all Ca (0.100 nm) with large Pb

With the Pb percentage increased in the standards (x

This indicated that PbxCa5-x(PO4)3OH was a

11 / 31



258

Page 12 of 31

XRD patterns for the Pb sorbed HAP samples are shown in Figure 2b.

There was clear

259

evidence to indicate the formation of a new phase at 2θ of 30.1o–30.5o with one exception.

260

For sample with the lowest initial [Pb] at 0.1 mM, the XRD pattern was almost identical to

261

that of HAP (x = 0, Figure 2a), indicating no new phase (precipitate, HPY or

262

PbxCa5-x(PO4)3OH) formed in the product.

263

phenomenon: 1) Pb was sorbed mostly onto the HAP surface and no precipitate produced; 2)

264

HPY precipitate formed but below the detection limit of XRD; or 3) the newly formed solid

265

precipitate was poorly crystalline.

266

was HPY, its dry weight would account for ~1.5% of the total solids.

267

HPY in the solids should be able to be detected by XRD (Figure S3), therefore the 2nd reason

268

was excluded.

269

precipitate may form, similar to that at higher Pb concentrations (Table S2); but XRD did not

270

detect any signal of Pb3(PO4)2 (Figure 2a, also see the analysis below).

271

lowest Pb concentration (0.1 mM), surface adsorption may contribute considerably to the total

272

Pb uptake.

Three possible reasons could account for this

From available solution data, if the precipitate formed Such a percentage of

According to SI of possible precipitates from modelling, Pb3(PO4)2

Therefore, at the

273

With initial [Pb] increased from 0.5 mM to 5.7 mM, an additional peak at 2θ of 30.1o–

274

30.5o formed, corresponding to the diagnostic peak for HPY or PbxCa5-x(PO4)3OH solid

275

solution with low content of calcium in the reference standards (Figure 2a).

276

peak height (i.e., intensity) was in agreement with the elevated Pb loading along the uptake

277

processes(Figure S2).

278

the characteristic signals from diffraction of HPY and HAP in the solids, respectively, the

279

ratio of the two peaks could be used to semi-quantify the transformation of HAP to HPY.

The increasing

Because the peaks at ~30.3o and ~31.8o were identified/assumed as

12 / 31


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280

Note that no peaks representing HPY at [Pb] 0 (Figure 2a, x=0) and 0.1 mM were identified

281

visually in the XRD patterns, the maximum height of intensity within the range of 2θ of

282

30.1o–30.5o was assumed as HPY.

283

consistent with early result obtained from batch uptake experiments that Pb uptake almost

284

equals to the dissolved Ca at the initial [Pb] ranging from 2.5 mM to 5.0 mM, which

285

suggested a co-precipitation process of phosphate with Pb at the stoichiometry of 3:5 to form

286

HPY after the dissolution of HAP.

287

dissolved Ca was quite close to the stoichiometry of 1:1, thus the associated HPY/HAP ratio,

288

together with that assumed at [Pb] = 0 mM, was thought as the end member produced via the

289

precipitation mechanism.

290

dotted line in Figure S4) was used to estimate the contribution of other mechanism at [Pb]

291

0.1–2.5 mM.

292

to contribute to the uptake.

293

Np(V), Pu(VI), Eu(III)) at low initial metal concentrations (or low metal loadings, assuming

294

the HAP loading is fixed) on the HAP surfaces via surface adsorption has been

295

reported.10,12,26-30

296

contributed to the bulk uptake of Pb.

297

The XRD evidence for the HPY formation was

At [Pb] of 5.0 mM, the ratio (0.98) of sorbed Pb versus

The deviation from the charted line using two end members (the

A percentage of about 7%–20% (20% at [Pb] of 0.1 and 0.5 mM) was thought The expected sequestration of other metals (Zn(II), U(VI),

Therefore, we estimated that at low [Pb] (0.1 and 0.5 mM), adsorption

However, as [Pb] further increased from 6.6 to 9.0 mM, the relative height of HPY peak

298

started to decrease (Figure 2b).

Therefore, a potential shift from that at lower [Pb] in the

299

uptake mechanism(s) was expected.

300

weakened, indicating a substantial contribution of incorporation to the overall mechanism.

301

The almost linear decrease in the HPY/HAP ratio (Figure S4) with [Pb] (6.6–9.0 mM) further

Concurrently, the XRD signal denoting to HPY was

13 / 31



Page 14 of 31

302

supported this speculation.

If we reckoned that the weakness of the HYP signal was caused

303

by the replacement of precipitation mechanism by incorporation mechanism, the estimated

304

contribution of incorporation to the overall uptake mechanism was 40%–55% at [Pb] 6.6–9.0

305

mM (Figure 3).

306

HAP-Pb system by calculation of “ion-ion interaction potential”, the direct incorporation of

307

Pb into the HAP crystal lattice by the substitution of Ca ions is not probable.31

308

the lattice substituted Pb probably from the subsequent diffusion of the surface sorbed-Pb

309

toward the particle core.

310

recognized by aligning the patterns with the references (Reference codes 00-029-0773 and

311

00-025-1394).

According to a theoretical approach for determination of stability of the

Therefore,

In all uptake samples, species of PbHPO4 or Pb3(PO4)2 were not

312 313

314

3.5. FTIR analysis. FTIR analysis was performed to identify the presence of chemical functional groups in the

315

PbxCa5-x(PO4)3OH solid solution standards and Pb-loaded HAP samples.

Generally, the

316

tetrahedral PO43- has four vibrational modes, i.e., the symmetric P–O stretching (v1), the O–P–

317

O bending (v2), the P–O stretching (v3), and the O–P–O bending (v4).23

318

of HAP were visible at 472 cm-1 (v1), 640–566 cm-1 (v4), 963 cm-1 (v2) and 1100-1040 cm-1

319

(v3) (Figure S5).

320

presented moderate intensities compared to that of HAP (Figures 4 and S5).

321

Pb, the frequencies of most IR peaks changed very little, indicating the main IR signal were

322

still contributed from bulk HAP, which can be hardly to be used to deduce the Pb uptake

323

mechanisms.

The phosphate bands

In general, the spectra of the uptake samples showed that all bands After uptake of

The spectra of the standards and the uptake samples had intense peaks at a

14 / 31


Page 15 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


324

frequency level of (3600 to 3300) cm-1, representing -OH stretching.

The band at 1455 cm-1

325

for CO32- vibration and the band at 871 cm-1 for HPO42- were not visible in the FTIR spectra

326

of the present work.

327

the formation of HPY or PbxCa5-x(PO4)3OH solid solution with high content of calcium, in

328

agreement with the XRD analysis.

However, peaks at 3571–3567, 1094–1035 and 962 cm-1 may indicate

329 330 331

3.6. HRTEM analysis. At [Pb] 2.5 and 9.0 mM, different uptake mechanisms dominated the uptake of Pb by

332

HAP (Figure 3).

333

chosen for morphology and element distribution analysis.

TEM showed the rod-like shape

334

of mineral crystals in HAP (Figures 5a, 6a, S6a and S7).

The length of the crystals was

335

generally 20–200 nm, with the width of 10–20 nm.

336

consistent with previous literature.8,32-34

337

precipitate was the main mechanism at [Pb] = 2.5 mM.

338

HPY mineral was clearly visualized.

339

the absence of Ca and presence of Pb and P in the crystal domain were observed (Figure

340

S6b-d), verifying the formation of pure HPY precipitate.

341

and Pb using ImageJ gave even clearer evidence (Figure S6e).

342

Therefore, two uptake samples with initial [Pb] 2.5 and 9.0 mM were

The shapes of the minerals were

As confirmed above, the formation of HPY In the TEM micrograph (Figure S6a),

Through scanning of an area with rod-like HPY crystal,

The co-localization analysis of Ca

At [Pb] of 9.0 mM, two types of uptake mechanism seemed to exist.

A small subarea

343

(Zone 1, Figure 5a) in the map was much more bright than other areas, resulting from the

344

enrichment of Pb and the depletion of Ca.

345

PbxCa5-x(PO4)3OH solid solution with low content of calcium.

This probably suggested the formation of HPY or

15 / 31


In contrast, other areas (e.g.,


Page 16 of 31

346

Zones 2 & 3, Figure 5a) showed a much more homogeneous brightness, consistent to the even

347

distribution patterns for elements of Ca, Pb, O and P (Figure 5b-e), probably implying a

348

surface adsorption mechanism or Pb incorporation to the bulk HAP.

349

distinguished color contrasts, indicating different percentages of Pb in the locals, were chosen

350

for EDS analysis (Figures 6g and Figure S8).

351

and P in the three zones were shown in Table S6.

352

solids were the final products, the chemical formula for Zones 1–3 would be

353

Pb1.5Ca3.5(PO4)3OH, Pb0.3Ca4.7(PO4)3OH, and Pb0.5Ca4.5(PO4)3OH, respectively.

354

percentage of Pb in the solids of Zones 2 and 3 was so low (≤ 10%) and the amount of Pb

355

existed in the solution phase was still very high (0.3 mmol, Figure S2), that the supposititious

356

chemicals would not be the crystals in the TEM images.

357

crystals with high content of Ca are unstable, and would be dissolved in order to form a more

358

stable structure, with higher content of Pb.13

359

incorporation, together with adsorption, dominated the uptake processes.

360

Three areas with

The relative atom percentages of Ca, Pb, O If we assumed that PbxCa5-x(PO4)3OH

The

The PbxCa5-x(PO4)3OH (x < 5)

One reasonable explanation was that

For [Pb] = 9.0 mM, an area with discrete crystals was selected for detailed SAD and

361

high-resolution TEM analyses (Figure 6a).

Using an acceleration voltage of 200kV, the fine

362

crystals may be damaged, which influences C and O content and finally the determination of

363

the diffraction ring.

364

voltage, three measures were taken: (1) choosing a relatively large area (~200 nm in diameter)

365

for spotting; (2) adjusting the spotsize (ranging from 1 to 10) from normally 3 to 8 to maintain

366

the light intensity at a relatively low level; (3) shortening the exposure time to 0.2 s.

367

strongest diffraction ring identified in Figure 6b (the calculated d value is 2.82 Å), was

To minimize the damage of the fine crystals by the high acceleration

16 / 31


The

Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


368

assigned to the (211) diffraction for HAP.

Another diffraction with only several visible

369

bright floccule (the calculated d value is 2.96 Å) can probably be assigned to the (211)

370

diffraction for HPY.

371

was also consistent with the previous d value based on XRD and FTIR results.

372

probably sorbed onto the HAP surface, without altering its structures.

The well-defined

373

lattice fringes were visible in the high-resolution TEM mode (Figure 6c).

The sum of 20 (A

374

to B), 5 (C to D) and 5 (E to F) times the d value was 5.781, 1.420 and 1.480 nm based on the

375

distance of adjacent lattice fringes (Figure 6c and 6d), which leaded to the d values of ∼2.89,

376

2.84 and 2.96 Å for the crystals.

377

diffraction for HAP and HPY, respectively, and also matched the values of 2.84 and 2.96 Å

378

based on SAD analysis (see Figure 6b).

379

of 2.84 Å (HAP) and 2.96 Å (HPY), indicated a ∼40% doping of Pb in HAP and can be

380

explained by the formation of PbxCa5-x(PO4)3OH solid solutions; supports can be found from

381

the TEM images (Figure 6a, especially Zone 1).

382

ion-ion interaction potential value for the HAP-Pb system was the lowest compared to that at

383

other doping ratios according to a theoretical predication,31 indicating the remarkably

384

thermodynamic stability at 40% doping.

385

mechanism that both incorporation and adsorption contribute considerably to the bulk uptake,

386

because incorporation mechanism alone would result in lower Pb doping ratio in the lattice.

This indicated the main structures in the crystals were still HAP, which Lead was

The d values of 2.84 and 2.96 Å matched the (211)

The d value of 2.89 Å, between two end-members

Interestingly, at such a doping ratio, the

This result further supported the aforementioned

387 388

3.7. New insights into Pb sequestration mechanisms.

389

The dissolution of HAP followed by HPY or PbxCa5-x(PO4)3OH precipitation is the

390

proposed mechanism for Pb uptake by HAP, but other mechanisms, such as incorporation 17 / 31



Page 18 of 31

391

(forming PbxCa5-x(PO4)3OH solid solutions with high Ca content) and surface adsorption may

392

also contribute to the metal uptake.

393

The combination of the equilibrium modelling, solution chemical analysis, and solid

394

XRD and TEM studies showed that the reaction of Pb uptake by HAP at pH 5 involved

395

several mechanisms, i.e. adsorption, precipitation and incorporation.

396

concentrations (0.1–9.0 mM), the classic sequential process of HAP dissolution and HPY or

397

PbxCa5-x(PO4)3OH precipitation always existed and was the dominant mechanism at low and

398

medium initial Pb concentration ([Pb] = 0.1–5.7 mM). At the lowest [Pb] (0.1 and 0.5 mM),

399

adsorption, though probably not the main mechanism, contributed considerably (~20%) to the

400

bulk uptake, similar to that at [Pb] of 6.6 mM.

401

contributed substantially to the overall uptake. At even higher [Pb] (7.4–9.0 mM), however,

402

incorporation and adsorption probably contributed most to the uptake of Pb.

403

pointed out that, both the precipitation process through co-precipitation of Ca and Pb and the

404

incorporation process can form PbxCa5-x(PO4)3OH solid solutions.

405

lies in the Ca content.

406

mechanism owns a wide range of Ca contents, from nearly 100% to 0%; during the uptake

407

process or due to the introduction of new sources of Pb into the system, the precipitate with

408

high Ca content would be further re-dissolved and form solid solutions with lower Ca content

409

until complete replacement of Ca by Pb.13

410

PbxCa5-x(PO4)3OH products with high Ca content (≥ 60%), as observed in the TEM images at

411

[Pb] of 9.0 mM.

412

apatite minerals were linked with initial Pb concentrations.

At the tested initial Pb

At high [Pb] (6.6 mM), all three mechanisms

It should be

The difference usually

The PbxCa5-x(PO4)3OH precipitate obtained through the former

However, the latter mechanism usually results in

Overall, this work demonstrated that the mechanisms of Pb uptake onto

413

The mechanisms of adsorption, incorporation and precipitation controlling the uptake of

414

Pb by soluble phosphate minerals (e.g. HAP), are also responsible for Pb sequestration by

415

soluble carbonate minerals (e.g. calcite, Table S7).35-40 18 / 31


The phenomenon of uptake

Page 19 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


416

mechanism depended on initial [Pb] was also revealed for the calcite/aragonite-Pb

417

systems.38,41,42

418

al.38 concluded that at low [Pb] (1 µM), surface complexation contributes to the uptake of Pb

419

onto calcite, and at higher [Pb] (5 and 10 µM), both surface complexation and precipitation of

420

PbCO3 (cerussite) account for the mechanism, whereas at even higher [Pb] (20 and 60 µM),

421

precipitation of hydrocerussite and cerussite dominates the uptake.

422

uptake mechanism, although obtained at alkaline conditions (pH 8.3), is to some extent

423

similar to what we observed in the HAP-Pb system at acidic environment, with [Pb] ranging

424

from 0.1 to 2.5 mM.

425

optical microscopy and SEM, to be transformed into polycrystalline cerussite through reaction

426

with acidic Pb-bearing solutions (pH = 2.8–5.2, [Pb] = 1 and 5 mM).36

427

replacement reactions began with the growth of a cerussite shell on top of calcite surfaces

428

followed

429

dissolution-recrystallization mechanism.36

430

mechanism for the HAP-Pb system we observed at initial [Pb] = 1.0–5.0 mM in this study.

431

Earlier studies using

432

complexation mechanism, i.e. Pb was sorbed as a reversibly bound adsorbed surface complex

433

on calcite in a nonoctahedral coordination geometry.38,43

434

also contributes substantially to the uptake of Pb by HAP, at both low and high [Pb] (0.1–0.5,

435

and 5.0–10 mM).

436

precipitation dominates the uptake of Pb by calcite and aragonite at acidic conditions (pH

437

4.75–6.80), whereas at higher [Pb] (5 mM), incorporation mechanism also contributes to the

438

Pb uptake.

439

was also revealed in our HAP-Pb system at higher [Pb] (e.g. 9.0 mM). Recently, the formation

440

of Pb-rich calcite solution solutions through a incorporation mechanism was reported at

by

Combining batch uptake and in situ X-ray absorption spectroscopy, Rouff et

the

Such evolution of Pb

In a recent study, single-crystal calcite was observed visually, using

replacement

210

of

the

remaining

calcite

core,

The mineral

indicating

a

This is similar to the dissolution-precipitation

Pb radiotracer and X-ray absorption spectroscopy indicated a surface

In another study, Godelitsas et al.

The surface adsorption mechanism

41

found that at [Pb] = 0.5 mM,

The increasing contribution of incorporation to the overall uptake mechanism

19 / 31



Page 20 of 31

441

alkaline conditions (pH = 8.8) using in situ atomic force microscopy imaging.35 The

442

comparison here clearly reflected the prevalence of three mechanisms existed in the uptake

443

processes of Pb onto soluble minerals, and the occurrence of dominant mechanisms were

444

linked somewhat to solution composition, such as the initial metal concentrations in the

445

systems.

446 447

4. Conclusion

448 449

As reflected by solution chemistry and solid-state chemical analysis, mechanisms of Pb

450

uptake by HAP at pH 5 were linked to the initial metal concentrations and involves several

451

processes, i.e. adsorption, precipitation and incorporation.

452

(e.g., 0.1 mM), surface adsorption may contribute considerably to the total Pb uptake. At

453

medium concentration of 0.5–5.7 mM, formation of crystalline HPY precipitates was

454

evidenced by the XRD analysis.

455

together with adsorption, contributed to the overall uptake mechanisms.

456

mechanisms result in sorbed Pb species with different availabilities. The surface immobilized

457

Pb may be easily desorbed into the solution phase again due to environmental change, such as

458

pH variation, whereas the precipitated and incorporated Pb would be quite stable.

459

Measurements should be taken to investigate the long-term stabilities of various bound-Pb

460

species onto phosphate minerals. In the future, the presence of potentially competing ions (e.g.

461

Mg and Zn) and natural organic matter, and the variation in pH levels under environmental

462

conditions, should be considered in identification of the specific contribution of each

463

mechanism to Pb sorption by phosphate minerals.

464

insoluble phosphate minerals should be combined together, and the immobilization should be

465

avoided in heavily arsenic-contaminated soils.44

At the low Pb concentrations

At higher Pb concentration (≥ 6.6 mM), incorporation, Different uptake

In field applications, soluble and

20 / 31


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466 467

Supporting Information

468

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

469 470

at DOI:.

471

Ionic activity products and SI of possible precipitates for initial solution compositions

472

used for Pb uptake by HAP (Table S1-S4), adsorption capacities of Pb2+ ion by various

473

adsorbents (Table S5), the relative atom percentages (%) of Ca, Pb, O and P in the three zones

474

obtained using EDS analysis (Table S6), the mechanisms of Pb uptake by carbonate minerals

475

(Table S7), HAP structure (Figure S1), uptake amount of Pb by HAP (Figure S2), XRD

476

patterns of the mixtures of HPY and HAP (Figure S3), Effect of initial Pb concentrations on

477

the molar ratio of heights of two characteristic peaks representing HPY and HAP from XRD

478

patterns (Figure S4), FTIR spectra of the standards PbxCa5-x(PO4)3OH (x=0, 1, 2, 3, 4 or 5)

479

(Figure S5), and TEM micrographs of HAP nanoparticles after reaction with 2.5 or 9.0 mM

480

Pb2+ (Figures S6–S8)

481

Acknowledgements

482 483

This work was funded by the National Natural Science Foundation of China (41722303),

484

Jiangsu Province Distinguished Young Scientists Program (BK20150018), National Key

485

R&D Program of China (2017YFD0800303), and the Young Thousand Talented Program in

486

China.

487

Science Foundation (2016M601770 and 2017T100350).

488

Jiani at the State Key Lab for Mineral Deposits Research for assistance of the TEM analysis.

Dr. Dong-Xing Guan is indebted to the support from and the China Postdoctoral We also wish to thank Ms. Chen

489 490

References

491

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aqueous solution on nano-sized hydroxyapatite: Examination of a fluoridated surface layer. Environ. Sci. Technol. 2012, 46, 802-809. 21. Garcia-Perez, P.; Pagnoux, C.; Rossignol, F.; Baumard, J. F., Heterocoagulation between SiO2 nanoparticles and Al2O3 submicronparticles; influence of the background electrolyte. Colloids Surf., A 2006, 281, 58-66. 22. Guan, D. X.; Williams, P. N.; Luo, J.; Zheng, J. L.; Xu, H. C.; Cai, C.; Ma, L. Q., Novel precipitated zirconia-based DGT technique for high-resolution imaging of oxyanions in waters and sediments. Environ. Sci. Technol. 2015, 49, 3653-3661. 23. Zhu, Y.; Huang, B.; Zhu, Z.; Liu, H.; Huang, Y.; Zhao, X.; Liang, M., Characterization, dissolution and solubility of the hydroxypyromorphite–hydroxyapatite solid solution [(PbxCa1−x)5(PO4)3OH] at 25 °C and pH 2– 9. Geochem. Trans. 2016, 17, 2-19. 24. Ji, J.; Ge, Y.; Balsam, W.; Damuth, J. E.; Chen, J., Rapid identification of dolomite using a Fourier Transform Infrared Spectrophotometer (FTIR): A fast method for identifying Heinrich events in IODP Site U1308. Mar. Geol. 2009, 258, 60-68. 25. Kaludjerovic-Radoicic, T.; Raicevic, S., Aqueous Pb sorption by synthetic and natural apatite: Kinetics, equilibrium and thermodynamic studies. Chem. Eng. J. 2010, 160, 503-510. 26. Fuller, C. C.; Bargar, J. R.; Davis, J. A.; Piana, M. J., Mechanisms of uranium interactions with hydroxyapatite: implications for groundwater remediation. Environ. Sci. Technol. 2002, 36, 158-65. 27. Betts, A. R.; Chen, N.; Hamilton, J. G.; Peak, D., Rates and mechanisms of Zn2+ adsorption on a meat and bonemeal biochar. Environ. Sci. Technol. 2013, 47, 14350-14357. 28. Moore, R. C.; Holt, K.; Zhao, H.; Hasan, A.; Awwad, N.; Gasser, M.; Sanchez, C., Sorption of Np(V) by synthetic hydroxyapatite. Radiochim. Acta 2003, 91, 721-728. 29. Thakur, P.; Moore, R. C.; Choppin, G. R., Np(V)O2+ sorption on hydroxyapatite-effect of calcium and phosphate anions. Radiochim. Acta 2006, 94, 645-649. 30. Moore, R. C.; Gasser, M.; Awwad, N.; Holt, K. C.; Salas, F. M.; Hasan, A.; Hasan, M. A.; Zhao, H.; Sanchez, C. A., Sorption of plutonium(VI) by hydroxyapatite. J. Radioanal. Nucl. Chem. 2005, 263, 97-101. 31. Raicevic, S.; Kaludjerovic-Radoicic, T.; Zouboulis, A. I., In situ stabilization of toxic metals in polluted soils using phosphates: theoretical prediction and experimental verification. J. Hazard. Mater. 2005, 117, 41-53. 32. Kamiishi, E.; Utsunomiya, S., Nano-scale reaction processes at the interface between apatite and aqueous lead. Chem. Geol. 2013, 340, 121-130. 33. Li, Z.; Tang, L. Y.; Zheng, Y. F.; Tian, D.; Su, M.; Zhang, F.; Ma, S. J.; Hu, S. J., Characterizing the mechanisms of lead immobilization via bioapatite and various clay minerals. ACS Earth Space Chem. 2017, 1, 152-157. 34. Mavropoulos, E.; Rocha, N. C. C.; Moreira, J. C.; Rossi, A. M.; Soares, G. A., Characterization of phase evolution during lead immobilization by synthetic hydroxyapatite. Mater. Charact. 2004, 53, 71-78. 35. Callagon, E.; Fenter, P.; Nagy, K. L.; Sturchio, N. C., Incorporation of Pb at the calcite (104)-water interface. Environ. Sci. Technol. 2014, 48, 9263-9269. 36. Yuan, K.; Lee, S. S.; De Andrade, V.; Sturchio, N. C.; Fenter, P., Replacement of calcite (CaCO3) by cerussite (PbCO3). Environ. Sci. Technol. 2016, 50, 12984-12991. 37. Rouff, A. A.; Elzinga, E. J.; Reeder, R. J.; Fisher, N. S., The effect of aging and pH on Pb(II) sorption processes at the calcite−water interface. Environ. Sci. Technol. 2006, 40, 1792-1798. 38. Rouff, A. A.; Elzinga, E. J.; Reeder, R. J.; Fisher, N. S., X-ray absorption spectroscopic evidence for the formation of Pb(II) inner-sphere adsorption complexes and precipitates at the calcite−water interface. Environ. Sci. Technol. 2004, 38, 1700-1707. 39. Rouff, A. A.; Elzinga, E. J.; Reeder, R. J.; Fisher, N. S., The influence of pH on the kinetics, reversibility 23 / 31



580 581 582 583 584 585 586 587 588 589 590 591 592 593

and mechanisms of Pb(II) sorption at the calcite-water interface. Geochim. Cosmochim. Acta 2005, 69, 5173-5186. 40. Sturchio, N. C.; Chiarello, R. P.; Cheng, L.; Lyman, P. F.; Bedzyk, M. J.; Qian, Y.; You, H.; Yee, D.; Geissbuhler, P.; Sorensen, L. B.; Liang, Y.; Baer, D. R., Lead adsorption at the calcite-water interface: Synchrotron X-ray standing wave and X-ray reflectivity studies. Geochim. Cosmochim. Acta 1997, 61, 251-263. 41. Godelitsas, A.; Astilleros, J. M.; Hallam, K.; Harissopoulos, S.; Putnis, A., Interaction of calcium carbonates with lead in aqueous solutions. Environ. Sci. Technol. 2003, 37, 3351-3360. 42. Chada, V. G. R.; Hausner, D. B.; Strongin, D. R.; Rouff, A. A.; Reeder, R. J., Divalent Cd and Pb uptake on calcite {101¯4} cleavage faces: An XPS and AFM study. J. Colloid Interface Sci. 2005, 288, 350-360. 43. Elzinga, E. J.; Rouff, A. A.; Reeder, R. J., The long-term fate of Cu2+, Zn2+, and Pb2+ adsorption complexes at the calcite surface: An X-ray absorption spectroscopy study. Geochim. Cosmochim. Acta 2006, 70, 2715-2725. 44. Xu, M.; Zhou, S.; Chen, S., Remediation of heavy metal-contaminated soils by phosphate fertilizers. In Twenty Years of Research and Development on Soil Pollution and Remediation in China, Luo, Y.; Tu, C., Eds. Springer Singapore: Singapore, 2018; pp 545-562.

594 595

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Page 24 of 31

Page 25 of 31

Figures

596

5

0.20

(a)

(b) Dissolved P (mmol)

Dissolved HAP (mM)

6

4 3 2 1

0.15 0.10 0.05 0.00

0 0

0.1 0.5

1

2.5

5

5.7 6.6 7.4

0

9

2

0.6

0.4 0.3 0.2 0.1

8

10

(d) 1.5

1.0

0.5

0.0

0.0 0

2

4

6

8

10

0

-1

Initial Pb concentration (mg L )

597

6

2.0

(c)

Sorbed Pb / dissolved Ca

0.5

4

Initial Pb concentration (mg L-1)

Initial Pb concentration (mM)

Dissolved Ca (mmol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

4

6

8

10


598

Figure 1. Theoretical calculation of dissolved HAP at pH 5 (a), modelling and experimental

599

measurement of dissolved P (b) and Ca (c) in the solution, and the molar ratio of sorbed Pb

600

versus dissolved Ca (d). The theoretical calculation of dissolved HAP and dissolved P in the

601

presence of 0–9.0 mM Pb in solution was simulated using Visual MINTEQ version 3.1. The

602

dotted line in (a) indicates the maximal concentration (4.98 mM) when 0.1 g HAP is

603

incongruently dissolved in 40 mL solution. The hollow circles with a solid line and solid

604

circles in (b) and (c) were the modelled and experimentally measured concentrations of P and

605

Ca in the solutions. The solid line in (c) indicates the ratio of 1.00, whereas the two dotted

606

lines indicate the ratios of 0.85 and 1.15. The experiment was performed by reacting 0.100 g

607

hydroxyapatite with 0, 0.1, 0.5, 1.0, 2.5, 5.0, 5.7, 6.6, 7.4 and 9.0 mM Pb(NO3)2 for 1 h at pH

608

5. The solution volume was 40 mL with 0.01 M NaNO3 as the matrix. The pH of the solutions

609

was adjusted with diluted HNO3 or NaOH. Scattered circles represent the average values of

610

two replicates.

611 25 / 31



(a)

(b)

612 613

Figure 2. XRD patterns of the standards PbxCa5-x(PO4)3OH (x = 0, 0.5, 1, 2, 3, 4 or 5) (a) and

614

the reaction products of hydroxyapatite (HAP) with 0.1, 0.5, 1.0, 2.5, 5.7, 7.4 and 9.0 mM Pb

615

at pH 5 (b). Two references of HAP (code 00-024-0033) and hydroxypyromorhpite (HPY,

616

code 01-087-2477) were included. The (200), (002), (102), (210), (211) and (130) diffraction

617

peaks of HAP were marked.

618

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Page 26 of 31

Page 27 of 31

10

Pb sorbed onto HAP (mmol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8

precipitation adsorption incorporation

6 4 2 0 0.1

2.5

9


619 620

Figure 3. A diagram showing the contribution of three mechanisms to the overall uptake of

621

Pb by HAP. The deviation from the charted line using two end members (the dotted line in

622

Figure S4) was used to estimate the contribution of adsorption mechanism at [Pb] 0.1 and 2.5

623

mM. The contribution of adsorption mechanism at [Pb] 9.0 mM was inferred roughly from

624

the fractions of Pb/Ca ratio higher than the stoichiometry of 1:1, whereas the incorporation

625

mechanism was estimated from the attenuation degree of the diagnostic HYP/HAP signal

626

compared to the highest at [Pb] 5.0 mM (Figure S4), which was reckoned to be caused by the

627

replacement of precipitation by incorporation mechanism.

628

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629 630

Figure 4. Fourier transform infrared (FTIR) spectra of the reaction products of hydroxyapatite

631

with 0.1, 0.5, 1.0, 2.5, 5.7, 7.4 and 9.0 mM Pb at pH 5. The symbols v1, v2, v3 and v4 indicate

632

the four vibrational modes of tetrahedral PO43-, i.e., the symmetric P–O stretching, the O–P–O

633

bending, the P–O stretching, and the O–P–O bending, respectively.

634 635

28 / 31


Page 28 of 31

Page 29 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(b)

Zone 1

Ca

(c) (c)

Pb

Zone

Zone

(e) (d)

O

(e)

P

(f)

Ca+Pb

(b)

(g)

Zone 1

636 637

Figure 5. High-resolution TEM electronic image (a) of hydroxyapatite nanoparticles after

638

reaction with 9.0 mM Pb2+. The images of Ca (b), Pb (c), O (d) and P (e) were obtained by

639

scanning the whole area in (a). The last figure (f) indicates the co-localization of Ca and Pb,

640

represented by the red and green colors, respectively. Three zones in (a) were scanned by EDS

641

to obtain the relative atom percentages of Ca, Pb, O and P. Only the EDS spectrum of zone 1#

642

(g) was shown here, whereas the other two spectrums were displayed in Figure S8, supporting

643

information. Carbon, Cu and Si were background elements from carbon meshes supported by

644

a Cu grid.

645

29 / 31



(a)

Page 30 of 31

(b)

2.82 Å

2.96 Å 200 nm

(c)

10 1/nm

(d) 450

20 cycles 400

350

300

250

5.781 nm

10 nm

646

5.839 nm

200

A

0

1

2

3

4 nm

5

6

B

7

647

Figure 6. High-resolution TEM of the mineralization on the surface of hydroxyapatite

648

nanoparticles after reaction with 9.0 mM Pb2+. (a) Morphology of the uptake products. (b)

649

SAD pattern shows the diffraction of 2.82 Å (strong) and 2.96 Å. (c) Representative grain

650

shows the polycrystalline nature of the crystals. (d) Pixel intensity profile from the line trace

651

(A to B) as marked in the white band in panel c. The sum of 20 (A to B), 5 (C to D) and 5 (E

652

to F) times the d value is 5.781, 1.420 and 1.480 nm, respectively.

653

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Characterization of Lead Uptake by Nano-Sized Hydroxyapatite: A

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