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Remediation and Control Technologies

Structural incorporation of manganese into goethite and its enhancement of Pb(II) adsorption Huan Liu, Xiancai Lu, Meng Li, Lijuan Zhang, Chao Pan, Juan Li, Rui Zhang, and Wanli Xiang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05612 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018

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Environmental Science & Technology

Structural incorporation of manganese into goethite and its enhancement of Pb(II) adsorption Huan Liu†‡, Xiancai Lu†*, Meng Li†, Lijuan Zhang§, Chao Pan∥, Rui Zhang†, Juan Li†, Wanli Xiang†

†

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

Engineering, Nanjing University, Nanjing, Jiangsu 210023, China ‡

Department of Earth and Planetary Sciences, Washington University in St. Louis, 1 Brookings

Drive, St. Louis, MO 63130, USA §

Chinese Academy of Sciences, Shanghai Institute Applied Physics, Shanghai Synchrotron

Radiation Facility, Shanghai 201204, People’s R China ∥

Department of Energy, Environmental and Chemical Engineering, Washington University in St.

Louis, St. Louis, Missouri, 63130 United States

* Corresponding author: Prof. Xiancai Lu Email: [email protected] Tel: +86 25 89681065

Revised manuscript submitted to Environmental Science & Technology March 2018

ACS Paragon Plus Environment


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Abstract:

2

Natural goethite (α-FeOOH) commonly accommodates various metal elements by substituting

3

for Fe, which greatly alters the surface reactivity of goethite. This study discloses the enhancement

4

of Mn-substitution for the Pb2+adsorption capacity of goethite. The incorporated Mn in the

5

synthesized goethite presents as Mn(III) and causes a slight decrease in the a and c of the unit cell

6

parameters and an observable increase in the b direction due to the Jahn-Teller effect of the

7

Mn(III)O6 octahedra. With the Mn content increasing, the particle size decreases gradually, and the

8

surface clearly becomes roughened. The Pb2+ adsorption capacity of goethite is observably enhanced

9

by

Mn

substitution

due

to

the

modified

surface

complexes.

And

the

increased

10

surface-area-normalized adsorption capacity for Mn-substituted goethite indicated that the

11

enhancement of Pb adsorption is not only attributed to the increase of surface area but also to the

12

change of binding complexes. Extended X-ray Absorption Fine Structure (EXAFS) analysis

13

indicates that the binding structures of Pb2+ on goethite presents as edge-sharing complexes with a

14

regular RPb-Fe=3.31 Å. In the case of Mn-goethite, Pb2+ is also bound with the Mn surface site on the

15

edge-sharing complex with a larger RPb-Mn=3.47 Å. The mechanism for enhancing Pb2+ adsorption

16

on Mn-goethite can be interpreted as the preferred Pb2+ binding on the Mn site of Mn-goethite

17

surface. In a summary, the Mn-goethite has great potential for material development in

18

environmental remediation.

19


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20


Graphical Abstract

21 22



23

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1. Introduction

24

The existence of toxic metals in water and soils imposes great environmental challenges. Lead

25

is one of the most toxic heavy metals that have caused serious harm to ecological systems and the

26

health of human beings worldwide.1, 2 In the last decades, numerous of iron oxides such as goethite,

27

magnetite, hematite, ferrihydrite, and lepidocrocite have been applied to adsorb lead, arsenic and

28

other heavy metals.3-7 Among these iron oxides, goethite (α-FeOOH), which is ubiquitous in soils

29

and sediments, has been used successfully to remove heavy metals due to its high chemical affinity.5,

30

6, 8-12

31

Natural goethite commonly accommodates various foreign elements such as divalent (Ni2+,

32

Cu2+, Zn2+, etc.), trivalent (Al3+, Cr3+, Co3+, Mn3+, etc), and occasionally tetravalent (Pb4+, Tc4+, Si4+,

33

etc) cations by substituting the iron.10, 12-25 The content of Al(III) in natural goethite can be as high

34

as 33% in molar ratios, while other elements are less than that.19, 26, 27 By using Raman and Fourier

35

transform infrared spectroscopy (FTIR) as well as microscopic techniques, the cation substitutions

36

have been shown to alter the structure and surface reactivity of goethite to a certain extent, which

37

further affects adsorption behavior of the heavy metals on it.28, 29 Among these cations, manganese

38

is often substituted for Fe in many natural iron-bearing minerals, especially in goethite, due to its

39

high affinity with Fe and almost equal radii of Fe (rFe3+ = rMn3+ = 0.645 Å).30 The incorporated Mn

40

predominantly presents as Mn3+ and/or Mn4+ in an oxidizing environment.13,

41

incorporation normally alters the structure of goethite and its environmental performance.31,25 Mn

42

substitution observably enhances the oxidation of arsenite (As3+) to arsenate (As5+) by goethite,

43

which suppresses the toxicity of arsenic in the environment.32 However, the valence of Mn on the

44

goethite surface layer has not yet been accurately determined, which is crucial for understanding the

45

surface reactivity. Extended X-ray absorption fine structure (EXAFS) analysis could only provide

46

the valence information of the bulk Mn and the X-ray photoelectron spectroscopy (XPS) is hard to

47

identify because of the high noise with low Mn content. Total electron yield (TEY) is a promising

48

technique to identify the valence state as well as the electronic state of the elements in a thickness of

49

dozens of angstroms33,

50

transition of Mn-goethite.35,

51

adsorption capacity and surface binding mechanisms of pure iron oxides, such as ferrihydrite,

34

19, 22

Such

by investigating the absorption spectra of the Mn 2p → 3d electron 36

For the adsorption of heavy metals, most studies focus on the


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52

goethite, and hematite.37-39 Bargar, Brown Jr and Parks 37 revealed the surface complexation of Pb(II)

53

on goethite in mononuclear bidentate complexes to the edges of FeO6 octahedra. The substitution of

54

Mn in the goethite could change the surface reactivity which may cause differences in the binding

55

structure. How heavy metals bind with the substituted goethite has seldom been discussed.

56

In this study, a series of goethite samples with various contents of substituted Mn have been

57

synthesized and characterized by Synchrotron radiation-based X-ray diffraction (SR-XRD), FTIR,

58

Raman spectroscopy and electron microscopy. TEY, X-ray absorption spectroscopy (XAS) of the

59

L-edge of Mn and Fe, was used to reveal the chemical states of the surface manganese and iron.

60

Batch experiments of Pb2+ adsorption have been performed with Mn-goethite samples, and the

61

effects of Mn substitution on Pb2+ adsorption are discussed. The EXAFS spectra of the surface Pb

62

has been measured to disclose the binding structure of Pb2+ on Mn-goethite. All the findings provide

63

significant insights for developing high-performance goethite-based materials for restoration of

64

heavy metal contaminations.

65

2. Materials and Experimental Methods

66

2.1. Preparation of goethite and Mn-substituted goethite

67

Goethite and a series of Mn-goethite samples were synthesized according to the reported

68

procedure.13 First, 25 mL 1 mol/L Fe(NO3)3, x mL 0.5 mol/L (the x varies with the target ratio of

69

Mn/(Fe+Mn) mol%) Mn(NO3)2 and 5 mol/L 45 mL KOH solutions were mixed in Teflon bottles.

70

Then, deionized water was added to reach a final KOH concentration of 0.3 mol/L. The suspensions

71

obtained were aged for 15 days at 60 °C, with opening, recapping and shaking by hand each for 5 s

72

daily. Obtained suspensions were centrifuged at 5000 rpm for 10 min, and washed with deionized

73

water, then dried at 60 °C. The collected samples were treated with dilute HCl solution at room

74

temperature for 4 h in the dark to remove the poorly crystallized materials. Finally, samples were

75

washed 6-10 times with deionized water and air-dried for further analysis.

76

2.2. Characterization techniques

77

SR-XRD analysis of Mn-substituted goethite was performed using the beamline 14B of the

78

Shanghai Synchrotron Radiation Facility (SSRF). The effective X-ray energy was set as 10 keV, and

79

the wavelength of the incident monochromatic X-ray was 1.2438 Å.40 XRD patterns were taken in

80

the range of 2θ of 12-72° at a step of 0.01° and integration time of 1 s. The collected data were



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81

analyzed by using the GSAS software package41 with EXPGUI interface.42 The initial unit-cell

82

parameters for goethite were taken from Alvarez, Sileo and Rueda

83

pressed into a pellet with powered KBr, then the FTIR spectra in the range of 400-4000 cm-1 were

84

collected using a Nexus 870 FT-IR spectrometer (Nicolet, USA). Raman analyses were performed

85

using at a JY/Horiba LabRAM HR Raman system with a 532.11 nm laser excitation and 50WL×

86

objective with an 1800- groove/mm grating. The spectra were collected from 100 cm-1 to 1600 cm-1

87

with a resolution of 0.5 cm-1. To improve the signal-noise ratio, the spectra were acquired for as

88

long as 50 s with three to five accumulations per spectrum.

14

. The goethite samples were

89

The Mn and Fe contents of the synthesized goethite samples were measured by an inductively

90

coupled plasma- optical emission spectrometry (ICP-OES) (Perkin-Elmer, San Diego, USA) with a

91

relative standard deviation of ≤ 2%. The samples (50 mg) were completely dissolved at 80 °C in a 6

92

mol/L HCl solution. Each sample analysis was performed in duplicate. The zeta potentials of the

93

samples were measured by a Malvern Zetasizer (nano-ZS) in 0.01 mol/L NaNO3 solution. The pH

94

values were adjusted by adding 0.01 mol/L NaOH or HNO3. Based on nitrogen adsorption

95

isotherms (77 K, P/P0 range of 0.05-0.20) measured on a Micrometrics ASAP 2020 apparatus

96

(USA), their specific surface areas (SSA) were calculated by employing the multi-point BET

97

method.

98

The morphology of the goethites was observed using a Zeiss Supra 55 field emission scanning

99

electron microscopy (FE-SEM) (Carl Zeiss, Oberkochen, Germany) equipped with an energy

100

dispersive spectrometer (EDS) (Oxford Aztec X-Max 150). An acceleration voltage of 15 kV was

101

used. The samples were coated by sprayed platinum of approximately 25 nm thick before

102

observation. High resolution transmission electron microscopy (TEM) observation was carried out

103

with an FEI TF20 electron microscope operated at 200 kV acceleration voltage. A drop of the

104

suspension of goethite sample, which was ultrasonically dispersed in aqueous ethanol solution, was

105

deposited on a carbon-coated Cu grid and then air-dried before observation.

106

XAS spectra of Mn L-edge and Fe L-edge were collected in the TEY model at Beamline

107

08U1A of SSRF.43 Powders of samples were dispersed on an aluminum foil on a plastic support.

108

The energy step of Mn L3, 2-edge and Fe L3, 2-edge was set at 0.1 eV and 0.2 eV respectively with 3

109

averages per spectrum. The collected spectra were normalized with an absorption current that

110

passed through a high transmission gold mesh in the X-ray path. ACS Paragon Plus Environment

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2.3. Pb2+ adsorption experiments

112

The adsorption isotherm of the Pb2+ ions on goethite samples (1.0 g/L) at 25 °C were obtained.

113

A stock solution of Pb2+ was prepared at 1000 mg/L by dissolving lead chloride (PbCl2) in deionized

114

water. The initial Pb2+ concentration range was 10-500 mg/L. The ion strength was controlled by

115

0.01 mol/L NaNO3. The pH value was adjusted and stabilized at 5.0 (±0.02) by adding 0.1 or 0.01

116

mol/L HNO3 or NaOH over each run. All the adsorption experiments were conducted in duplicate.

117

After shaking at 120 rpm for 12 h, the suspensions were filtered through 0.22 µm membrane filters

118

and the Pb2+concentration of the filtered solution was determined immediately by inductively

119

coupled plasma atomic emission spectroscopy (ICP-OES). The collected solids were dried followed

120

by FTIR, XPS, and extended X-ray absorption fine structure (EXAFS) analysis. XPS analysis

121

(UIVAC-PHI, Japan) was performed to characterize the speciation of binding Pb on the goethite

122

surface and the binding energy was calibrated according to adventitious contamination C 1s at 284.6

123

eV for all spectra.

124

A series of adsorption experiments with varying pH was conducted to investigate the effect of

125

Mn incorporation. Pure goethite and two Mn-goethite samples were selected for this study.

126

Adsorption experiments were performed over a wide pH range of 1.0-9.0, with a pH interval of ~0.5.

127

Each experimental system contained 0.4 µmol/L (82.88 µg/L) Pb2+, 1 g/L goethite, 0.01 mol/L

128

NaNO3 for ionic strength, and 1 mmol/L MES to buffer the pH. The pH of each sample was

129

adjusted to the target pH using HCl and NaOH, then checked and adjusted for any pH drift if

130

necessary. After 24 h, samples were centrifuged and filtered using 0.22 µm syringe filters. The Pb2+

131

concentration in equilibrated solution was determined by ICP-OES analysis.

132

2.4 EXAFS measurements and data processing

133

EXAFS data for Pb2+-adsorbed goethites were collected at the beamline 14W at SSRF. The

134

electron beam energy of the storage ring was 3.5 GeV, and the maximum stored current was

135

approximately 250 mA. The monochromator using a Si (111) crystal was calibrated with a Pb foil

136

for collecting Pb L-edge spectra. The Pb L-edge spectra were collected in the fluorescence mode

137

with a 32 Ge solid-state detector. EXAFS data were acquired from -200 eV to 800 eV with three to

138

five spectra to reduce the noise. Data were processed using the Athena and Artemis from the

139

IFEFFIT software package.44 The backscattering phase and amplitude functions for the structural

140

fitting of the EXAFS data were calculated using FEFF 7.0245 from the crystal structure of PbO46 ACS Paragon Plus Environment


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with partial Fe-for-Pb substitution. During the fitting, the amplitude reduction factor (S02) was fixed

142

to 1.0 as used in previous studies on Pb2+ adsorption38, 47. The parameter ∆E0, the difference between

143

the threshold energy (E0) relative to FEFF phase shift function, varied during fitting but was a

144

common value for all shells in a sample. The Debye-Waller factor (σ2) for all O and Fe atoms were

145

set to 0.0137, 48. The interatomic distance (R) and coordination number (CN) were allowed to float

146

during fitting.

147

3. Results and Discussion

148

3.1. Structural and surface properties of Mn-goethite

149

3.1.1 SR-XRD patterns of goethite samples

150

The Mn content in synthesized Mn-goethite increases from 3.4 mol% in sample MnGoe1 to

151

12.9 mol% in MnGoe4 (Table 1) and the color was darker with increasing Mn contents. No new

152

phase can be identified in the XRD patterns of the goethite samples. However, observable shifts of

153

the main peaks of goethite due to Mn substitution can be found (Figure 1). As the Mn content

154

increases, the b-dimension increases from 10.0014 Å to 10.0336 Å. Meanwhile, the a- and c-values

155

decrease from 4.6304 Å to 4.6176 Å and from 3.0368 Å to 3.0275 Å, respectively (Table 1, Figure

156

S1), which is consistent with the reported cases of other substituted goethites.49, 50 The decrease of

157

all the parameters, a, b, and c, can be observed in Al-, Cr- and Co-goethite samples due to their

158

smaller radius compared to Fe ions.14, 28, 51 However, the radii of Fe(III) and Mn(III) herein are

159

almost equal, and the changes of unit cell parameters due to Mn substitution can be attributed to the

160

Jahn-Teller distortion of the Mn(III)-O octahedra.52 For the Mn(III)-O octahedra, the apical

161

Mn(III)-O distance was elongated (2.47 Å compared to the 2.09 Å in Fe(III)-O octahedra). The

162

introduction of apical Mn-O bonds with longer length relative to the equatorial Mn-O/OH bond

163

prolongs the unit cell along the b direction and shrinks slightly along the a and c directions.24, 31


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164 165

Figure 1. SR-XRD patterns of pure goethite and Mn-goethite samples (λ=1.2438 Å), which

166

correspond well with the standard card of goethite (JCPDS: 29-0713). (a). The magnified XRD

167

patterns in a 2-theta range of 26°- 30° (b) and 40°- 44° (c) show the shifts of diffraction peaks. The

168

vertical lines marked in (b) and (c) are the positions of standard XRD peaks of pure goethite.

169 170

Table 1

171

Chemical compositions and their refined unit cell parameters based on SR-XRD patterns Samples Mn contents (mol%)

Goe

MnGoe1

MnGoe2

MnGoe3

MnGoe4

0

3.4%

5.7%

10.8%

12.9%

Formula

FeOOH

Fe0.966Mn0.034OOH

Fe0.943Mn0.057OOH

Fe0.892Mn0.108OOH

Fe0.871Mn0.129OOH

SSA (m2/g)

20.9

25.3

24.6

49.0

71.3

pHpzc

7.2

7.4

7.1

6.5

6.4

a (Å)

4.6304 (2)

4.6268 (2)

4.6242 (4)

4.6212 (2)

4.6176 (3)

b (Å)

10.0014 (3)

10.0073 (3)

10.0117 (6)

10.0281 (4)

10.0336 (5)

c (Å)

3.0368 (1)

3.0342 (1)

3.0328 (2)

3.0297 (1)

3.0275 (1)

140.637 (12)

140.490 (11)

140.405 (20)

140.403 (10)

140.267 (19)

*WRp

9.41

8.61

10.6

5.11

5.16

Rp

6.99

6.29

7.26

3.97

4.22

Chi2

0.52

0.465

0.6

0.268

0.32

3

Volume (Å )

172

Notes *: WRp=100×Σ(Io-Ic)/ΣIo; Rp=100[Σ[wi×(Io-Ic)2]/Σ(wi×Io)2]0.5. Io and Ic = observed and calculated intensities; wi =

173

weight assigned to each step intensity.

174 175 176

3.1.2. XAS spectra of Fe L3,2-edge and Mn L3,2-edge The L3,2-edge XAS spectra feature the transition from the initial Fe 2p core level to the final



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177

unoccupied Fe 3d level (Figure 2a). The spectra are regarded as total unoccupied Fe 3d states. Two

178

peaks I (707.6 eV) and II (709 eV) in the L3 region can be assigned to Fe t2g and eg orbitals from Fe

179

2p3/2, and the other two peaks III (720.6 eV) and IV (722.2 eV) in the L2 region assigned to Fe t2g

180

and eg orbitals from Fe 2p3/2, respectively.53, 54 The ∆eV and the quotient (intensity ratio) between

181

peak I and peak II in the L3 range can be taken as an ideal probe to investigate the structural

182

information of Fe oxides.55, 56 The ∆eV for all the goethites are identical to 1.38 eV, and the intensity

183

ratio is 0.63 for pure goethite. The ratio decreases to 0.56-0.57 for the Mn-goethite samples,

184

indicating a considerable Fe polyhedral distortion due to Mn incorporation.56 All the TEY spectra of

185

the Mn L3,2 adsorption edges of the Mn-goethite samples shown in Figure 2b present a major peak

186

at 639.7 eV, which is the same as the state of Mn3+ of Mn2O3. Although the TEY spectra of Mn2O3

187

shows a small shoulder peak at ~638 eV, the MnGoe samples with a small shoulder indicate that the

188

surface Mn contains minor amount of Mn(II). But Mn(III) is the predominant Mn speciation in the

189

Mn-goethite (peak at 639.7 eV). Therefore, the surface manganese of Mn-goethite is exactly in the

190

same state of Mn3+ as the bulk disclosed by bulk EXAFS analysis.13

191 192

Figure 2. Synchrotron-based XAS of the Fe L3,2-edge (a) and the Mn L3,2-edge (b) of Mn-goethite

193

and reference samples. The TEY spectra of the references are in good agreement with the studies of

194

Laan and Kirkman

195

manganese minerals. The inset in (a) is the changes in the intensity ratio of peak I and peak II for

196

the Fe L3-edge with Mn contents.

197

3.1.3. FTIR and Raman spectra

57

on Mn2+ ions and Schofield, Henderson, Cressey and Van der Laan

36

on

198

The structural change induced by the substitution of Mn in goethite could also be observed

199

from the investigation of FTIR and Raman vibration spectra. The shapes of the FTIR spectra of all

200

Mn-goethites are all similar to the shape of the FTIR spectrum of pure goethite (Figure S2 and Table


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201

S2).50, 58, 59 However, the intensity of a shoulder band at 3374.9 cm-1, which is assigned to the O-H

202

stretching vibration of non-stoichiometric hydroxyl groups,29 increases with the Mn content,

203

indicating an enhancing effect on the hydroxyl units. There are also observable blueshifts of the

204

representative bands of Fe-O(H) (890.29 and 795.55 cm-1) and a redshift at 642.72 cm-1, which are

205

linear with the Mn content and imply symmetrical effects of the Fe-O bands (Figure S3). The band

206

shifts at 890.29 and 795.55 cm-1 can be explained as the dilation of hydrogen bonds in the a-b plane,

207

and the blueshift of the band at 642.72 cm-1 is due to the Mn-O bonds being shorter than the Fe-O

208

bonds in the b-c plane, caused by the isomorphous substitution of Mn.29, 50

209

The peaks in the Raman spectra of goethite samples (Figure 3 and Table S2) correspond with

210

the goethite reported along with the assignment of Fe-O and Fe-O-Fe vibration signatures.21, 60-63

211

The incorporation of Mn induces structural disorder, so the Raman intensity decreases with the Mn

212

content (Figure 3 and Table S2). Interestingly, the intensity ratio of 386 and 400 cm-1 (I386/I400), as

213

well as the shifts of these peak positions, changes with the Mn content. The fitting results show that

214

the band at 386 cm-1 blueshifts while the band at 400 cm-1 redshifts and I386/I400 decreases linearly

215

with the Mn content (Figure S4). The broadening and displacement of all the peaks indicated that

216

the incorporation of Mn increases the structural disorder in goethite.62 The enhanced intensity of the

217

Raman band at 400 cm-1 assigned to Eg Fe-OH symmetric stretching could result from the increase

218

in the hydroxyl units.29 The substitution of Mn for Fe causes the structural defects of goethite and

219

increases the hydroxyl units on the surface, which also occurred in the Al-substituted goethite and

220

Mn-substituted magnetite.4, 29 Furthermore, the good linear relationship between Raman shifts and

221

Mn content can be used to estimate the content of substituted Mn in goethite.

222 223

Figure 3. Raman spectra of pure and Mn-substituted goethite samples (a) and the magnified Raman



224

spectra in the wavenumbers of 200-800 cm-1 (b).

225 226

3.2. Morphology and surface characteristics of Mn-goethite

227

3.2.1 FE-SEM and TEM observations

228

In the FE-SEM images of Mn-goethite samples, the pure goethite exhibits a uniformly acicular

229

shape with an average length of 1.95 µm and width of 180 nm, and the average length/width (L/W)

230

ratio is approximately 10.7 (Figure S5). The incorporation of Mn significantly decreases the crystal

231

size while increasing the L/W ratio (Figure S5 and Table S1), which is consistent with a previous

232

finding.64

233

From the HR-TEM images, the lattice fringes are observable for all pure goethite and

234

Mn-substituted goethite (Figure 4). All the three electron diffraction patterns of pure goethite,

235

MnGoe2 and MnGoe4 (Figures 4c, f and i) obtained from Fast Fourier Transform (FFT) of

236

high-resolution images are identical with each other, probably due to an isomorphous substitution of

237

Fe by Mn.65 In addition, the surface of pure goethite is exactly smooth, whereas the surface of the

238

Mn-goethite becomes roughened and presents a sawtooth shape as the Mn content increases. The

239

EDS mapping of Fe, Mn, and O in MnGoe3 indicate a homogeneous distribution of Mn in all

240

goethite (Figure S6).


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241 242

Figure 4. HR-TEM images of goethite samples. (Goe: a, b and c; MnGoe2: d, e and f; MnGoe4: g, h

243

and i). The insets (c, f and i) show the electron diffraction pattern obtained by FFT transformation of

244

high-resolution images (rectangle areas). White dotted lines denote the edge of the goethite crystals.

245 246

3.2.2. Surface properties of Mn-substituted goethite

247

Mn substitution can obviously affect the surface properties of goethite. The SSA of Mn-goethite

248

increases from 20.9 m2/g for pure goethite to 71.3 m2/g for MnGoe4 (Table 1), consistent with the

249

decrease in particle size and surface roughening caused by Mn substitution (Figure 4). Meanwhile,

250

the point of zero charge (PZC) value is also altered by the Mn substitution. Although the PZCs of

251

MnGoe1 (7.4) and MnGoe2 (7.1) are similar to the PZC of pure goethite (7.2 in this study and 7.3

252

of64) (Figure S7 and Table 1), the PZC values of MnGoe3 and MnGoe4 clearly decrease to 6.5 and

253

6.4, between the PZC value of pure goethite and manganite (MnOOH, PZC=5.4 of Kosmulski 66).

254

Such a decrease probably resulted from the replacement of Fe-OH groups by Mn-OH groups on the

255

Mn-goethite surface.

256

3.3. Pb(II) adsorption behavior on Mn-goethite



257

3.3.1. Pb2+ adsorption isotherm and adsorption capacity

258

The Pb2+ adsorption isotherm indicates that the Mn substitution greatly promotes the Pb2+

259

adsorption (Figure 5a). For example, in the equilibrated solution with the Pb2+ concentration of

260

300~400 mg/L, the Pb2+adsorption increases from 18 mg/g to 90 mg/g as the Mn content increases

261

from 0 (Goe) to 12.9 mol% (MnGoe4). The Pb2+ adsorption on Mn-goethite is well fitted by using

262

the Langmuir model (the details are provided in Section S1 of the supporting information), which is

263

comparable to the adsorption of Ni and Zn on goethite.11, 21, 67 The calculated adsorption capacity

264

(Qmax) for Goe, MnGoe1, MnGoe2, MnGoe3 and MnGoe4 are 16.34, 20.70, 27.47, 60.61 and 90.09

265

mg/g, and the surface-area-normalized Qmax values are 0.78, 0.82, 1.12, 1.24 and 1.26 mg/m2,

266

respectively (Table S3). For SSA-normalized adsorption behavior of goethite, the substitution of Mn

267

clearly increased the adsorption of Pb compared to pure goethite (Figure 5b). The inverse decrease

268

of SSA-normalized Qe for MnGoe4 compared to MnGoe3 at low concentration may be attributed to

269

the measured surface area, while in the solution the particle may aggregate and decrease the

270

effective adsorption site of MnGoe4. Clearly, the greatly promoted adsorption capacity of goethite

271

for Pb2+ by Mn substitution is not only attributed to the increase of SSA but also to the change in

272

surface properties.

273

The effect of the Mn substitution on enhancing the adsorption of Pb2+ is substantial at a low

274

Pb2+ concentration (Figure 6), probably because Pb2+ prefers to bind with the surface Mn sites. Pb(II)

275

in solution is dominantly Pb2+ and less Pb(OH)+ at pHRFe-O=2.09 Å), the binding distance of Pb on the Mn(III)(O, OH)6 site should be larger than

329

RPb-Fe=3.31 Å. Another evidence comes from the comparison to Pb adsorbed on Mn(IV) oxides. The

330

binding distance of Pb on Mn(IV) oxides in the edge-sharing complex is 3.3 Å,71 while the radii of

331

Mn(III) is larger than that of Mn(IV), the binding distance of Pb on Mn(III) should also be larger

332

than that on Mn(IV) sites. Therefore, an edge-sharing complex with the binding distance of 3.47 Å

333

is the most reliable complex of Pb binding on the Mn-substituted goethite surface at the Mn surface

334

site.

335


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Page 17 of 32


336 337

Figure 7. EXAFS spectra of k3-weighted (left) and Fourier transformation magnitudes and the

338

imaginary components (right) of the Pb adsorbed samples. Dots are the raw data and solid lines are

339

structural fits.

340 341

Table 2 Fitting results of Pb LIII-edge EXAFS spectra in Pb adsorbed samples Samples Pb/Goe

Pb/MnGoe2

Pb/MnGoe3

Atom

CN

R (Å)

σ2 (Å2)

∆E (eV)

R-factor (%)

O

1.75 ±0.14

2.31 ±0.01

0.01

-9.47 ±2.02

1.4

Fe

0.34 ±0.20

3.31 ±0.03

0.01

O

1.42 ±0.16

2.30 ±0.02

0.01

-10.79 ±2.86

4.7

Fe/Mn

0.41 ±0.28

3.34 ±0.03

0.01

O1

1.85 ±0.33

2.26 ±0.02

0.01

-11.43 ±3.58

4.0

O2

1.45 ±0.37

2.46 ±0.02

0.01

Fe

1.06 ±0.53

3.29 ±0.03

0.01

Mn

0.93 ±0.62

3.47 ±0.03

0.01

342 343

XPS results of the Pb (4f) of the Pb adsorbed on goethite samples (Figure S12) suggested that

344

after adsorbed onto Mn-substituted goethites, binding energy of Pb 4f shift to higher binding energy,

345

confirming the possible strong interactions between goethite and Pb(II) with the substitution of

346

Mn.72 The evidence of surface groups changed by adsorbing Pb was also shown in the FTIR spectra

347

of Pb-adsorbed Mn-goethite samples (Figure S13 and Table S5). Both peaks at 890.29 and 642.72

348

cm-1 shift to higher wavenumbers, especially for the samples of MnGoe3 and MnGoe4 with higher

349

Pb(II) adsorption. It was shown that the adsorption of Pb on the surface occurred by reacting with

350

surface OH groups, inducing the dehydrolyzed binding to form edge-sharing complexes.37, 73-75 The



351

deprotonating of a surface hydroxyl group indicates that Pb2+ acts as a Lewis acid to form Lewis

352

salt-type compounds with surface functional groups.76

353

Since the Fe-O vibration is affected by the surface complexation of Pb and (≡FeOH), the

354

deprotonation at a high Pb uptake by Mn-goethite (MnGoe3 and MnGoe4) leads to the shifts of

355

FTIR peaks. The exposed hydroxyl functional groups (Mn-OH) of Mn-goethite can bind directly

356

with Pb2+ ions and form inner-sphere complexes, which is the same as Pb2+ on manganese oxides.68,

357

69

358

4. Environmental implications of Mn substitution in goethite

359

Goethite is one of the most stable and common iron oxides in nature, and has been applied in

360

environmental restorations of heavy metals in contaminated soil and waters. The incorporation of

361

Mn into goethite observably affects the lattice structure and surface properties. Its enhancement of

362

Pb2+ adsorption highlights the development of novel environmental materials for heavy metals

363

removal. The spectroscopic investigation of Pb2+ uptake on Mn-goethite promotes the understanding

364

of the modified surface reactivity of Mn-substituted iron oxides and the retention mechanism of

365

heavy metals on iron oxides.

366

Our findings clarify the important role of the Mn substitution into goethite on the removal of

367

heavy metals. The incorporation of foreign metals in iron oxides such as goethite is common,18, 19

368

which can produce new surface sites quite different from the pure phase. Based on this study, it is

369

reasonable to expect that the Mn incorporation into iron oxides can similarly enhance the adsorption

370

of other heavy metals, such as Cu, Cd, and Zn. It was also found that in natural Fe-Mn crusts, the

371

incorporation of Mn into iron oxides enhances the retention of arsenic.77 Therefore, in subsurface

372

environments with the coexistence of Mn and Fe, the precipitation of Mn-bearing iron oxides favors

373

entrapping the heavy metals from contamination. Furthermore, based on this laboratory

374

investigation, the method of inducing the substitution of Mn into iron oxides could have potential

375

application in the development of environmental materials in water treatment for heavy metals.

376 377


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378

Supporting information

379

The supporting information includes the unit parameters changing with Mn substitution,

380

FTIR/Raman/FE-SEM results, Zeta potential of samples, Pb2+ adsorption behavior, and XANES

381

results. This information is available free of charge via the Internet at http://pub.acs.org.

382



383

Acknowledgements

384

We acknowledge National Science Foundation of China (No. 41425009) and National Basic

385

Research Program of China (No. 2014CB846004). H. Liu is also supported by the China

386

Scholarship Council and the program B for Outstanding Ph.D. candidate of Nanjing University. We

387

gratefully acknowledge beam line of BL08U, BL14B and BL14W in Shanghai Synchrotron

388

Radiation Facility (SSRF) for providing us the beam time for TEY, SR-XRD, and EXAFS

389

measurements. We are also grateful for the help from Ms. Jiani Chen in TEM observation at the

390

State Key Laboratory for Mineral Deposits Research.


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