Redox Reactions between Mn(II) and Hexagonal Birnessite Change

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Redox Reactions between Mn(II) and Hexagonal Birnessite Change its Layer Symmetry Huaiyan Zhao, Mengqiang Zhu, Wei Li, Evert J. Elzinga, Mario Villalobos, Fan Liu, Jing Zhang, Xionghan Feng, and Donald L. Sparks Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b04436 • Publication Date (Web): 08 Jan 2016 Downloaded from http://pubs.acs.org on January 12, 2016

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Environmental Science & Technology

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Redox Reactions between Mn(II) and Hexagonal Birnessite

2

Change its Layer Symmetry

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Huaiyan Zhao,† Mengqiang Zhu,‡ Wei Li,§,║ Evert J. Elzinga,⊥ Mario Villalobos,# Fan

4

Liu,† Jing Zhang,$ Xionghan Feng,*,†,║ Donald L. Sparks║

5 6

†

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Yangtse River), Ministry of Agriculture, College of Resources and Environment,

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Huazhong Agricultural University, Wuhan 430070, China

9

‡

Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of

Department of Ecosystem Science and Management, University of Wyoming,

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Laramie, WY, 82071, United States

11

§

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Sciences and Engineering, Nanjing University, Nanjing 210093, China

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║

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Department of Plant and Soil Sciences, University of Delaware, Newark, Delaware,

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19716, United States

16

⊥

17

Jersey 07102, United States

18

#

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México (UNAM), Mexico, D.F. 04510, Mexico

20

$

21

Academy of Sciences, Beijing 100039, China

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

Environmental Soil Chemistry Group, Delaware Environmental Institute and

Department of Earth & Environmental Sciences, Rutgers University, Newark, New

Geochemistry Department, Geology Institute, Universidad Nacional Autónoma de

Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese

22 1

ACS Paragon Plus Environment


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Text: 5066 words

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Figures and tables: 4 figures and 1 table

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SI: 5 figures and 1 table

26 27 28 29 30 31 32 33 34 35 36 37 38 39

*Corresponding author:

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Xionghan Feng, Tel: +86 27 87280271; Fax: +86 27 87288618; E-mail:

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[email protected]

42 43 44 2


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TOC Figure

46 (5% - 24% molar)

Mn2+

47 48

e-

HexLayBir

Symmetry Transformation

OrthLayBir

49 50 51

3



52

ABSTRACT

53

Birnessite, a phyllomanganate and the most common type of Mn oxide, affects

54

the fate and transport of numerous contaminants and nutrients in nature. Birnessite

55

exhibits hexagonal (HexLayBir) or orthogonal (OrthLayBir) layer symmetry. The two

56

types of birnessite contain contrasting content of layer vacancies and Mn(III), and

57

accordingly have different sorption and oxidation abilities. OrthLayBir can transform

58

to HexLayBir, but it is still vaguely understood if and how the reverse transformation

59

occurs. Here, we show that HexLayBir (e.g., δ-MnO2 and acid birnessite) transforms

60

to OrthLayBir after reaction with aqueous Mn(II) at low Mn(II)/Mn (in HexLayBir)

61

molar ratios (5 - 24%) and pH ≥ 8. The transformation is promoted by higher pH

62

values, and smaller particle size and/or greater stacking disorder of HexLayBir. The

63

transformation is ascribed to Mn(III) formation via the comproportionation reaction

64

between Mn(II) adsorbed on vacant sites and the surrounding layer Mn(IV), and the

65

subsequent migration of the Mn(III) into the vacancies with an ordered distribution in

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the birnessite layers. This study indicates that aqueous Mn(II) and pH are critical

67

environmental factors controlling birnessite layer structure and reactivity in the

68

environment.

69

4


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70 71

INTRODUCTION

Manganese (Mn) oxides are common minerals in soils and sediments.

1,2

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Birnessite, including its turbostratic nanocrystalline analogue “vernadite”, is regarded

73

as the most reactive phyllomanganate naturally occurring in surficial environments

74

due to its vacant sites, large surface areas and strong oxidizing abilities.

75

octahedral layers of birnessite are negatively charged, owing to the presence of vacant

76

sites and/or Mn(III) in the layers. 5 Birnessite have either hexagonal or orthogonal

77

layer symmetry. 3,6,7 Birnessite with hexagonal layer symmetry (HexLayBir) has up to

78

around 0.25 cation vacancies per layer octahedron with interlayer Mn(III) or Mn(II)

79

cations either above or below the vacancies, 8-11 whereas the ideal birnessite with

80

orthogonal layer symmetry (OrthLayBir) consists of almost vacancy-free Mn

81

octahedral layers arranged alternately by one Mn(III)O6 octahedral chain and two

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Mn(IV)O6 octahedral chains. 12 Orthogonal layer symmetry originates from a portion

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of Mn(IV) substituted by Mn(III) cations and their ordering array along the b axis in

84

the layer causing systematic elongation along the a axis, which creates a cooperative

85

Jahn-Teller distortion effect.

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orthogonal layer symmetry) can greatly influence their reactivity and environmental

87

impacts. HexLayBir commonly exhibits a higher sorption and oxidation reactivity

88

than OrthLayBir.

89

todorokite more readily after exchange with metal cations of large hydrous radii. 18-21

16,17

5,7,13-15

3,4

Mn

The structure of birnessites (hexagonal or

Compared to HexLayBir, OrthLayBir can transform into

90

The poor crystallinity and low abundance of natural birnessites in the

91

environment make it challenging to identify and distinguish between HexLayBir and 5



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OrthLayBir. These natural birnessites are thought to be mainly formed via biological

93

Mn(II) oxidation which is much faster than abiotic oxidation.

94

oxidation products of Mn(II) are mainly birnessite-like phases exhibiting hexagonal

95

symmetry.

96

hexagonal can be achieved by acidification. 5,7,13,27 Equilibration of OrthLayBir at pH

97

4 with dissolved heavy metals, such as Co(II), Cd(II) and Pb(II),

98

OrthLayBir with Co(II) can also result in the formation of HexLayBir. 30 However,

99

much remains unknown about the reverse reaction and its geochemical controls, i.e.,

100

the transformation of birnessite layer symmetry from hexagonal to orthogonal,

101

although this transformation has been observed during biotic Mn(II) oxidation.

102

23,24,27,31

103

8,23-26

Bargar et al.

22,23

Primary biotic

Birnessite layer symmetry transformation from orthogonal to

24

28,29

or doping

observed the formation of 10 Å Na phyllomanganate, which

104

resembled the hydrated form of OrthLayBir, during reaction of aqueous Mn(II) with

105

biogenic HexLayBir in oxic systems at 10 µM Mn(II) and pH 7.7 - 7.8. Orthogonal

106

phyllomanganate was also observed during Mn(II) oxidation by Bacillus sp., strain

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SG-1 in seawater or CaCl2 solutions, which was ascribed to a secondary

108

mineralization product of the primary biogenic HexLayBir. 23,31 Tang et al. 17 found

109

that aging of biogenic Mn oxide produced by a marine bacterium gave rise to the

110

structural ripening from hexagonal to orthogonal layer symmetry, which could be a

111

possible reason for its decreased reactivity towards Cr(III) oxidation. Feng et al. 32

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found OrthLayBir as an intermediate product during transformation of biogenic Mn

113

oxide to todorokite under reflux treatment. However, none of these previous studies 6


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114

revealed the transformation mechanism and how solution chemistry affects the

115

transformation.

116

Our previous study

27

examined the effects of solution chemistry on the

117

formation and structure of biogenic birnessite via biotic Mn(II) oxidation by P. putida

118

GB-1. It was found that OrthLayBir formed in the presence of Na+ and Ca2+ at pH 8,

119

whereas HexLayBir was obtained at lower pHs (6 and 7) or in the presence of Ni2+ at

120

pH 8. These observations were attributed to the sorption and redox reaction between

121

Mn(II) and HexLayBir and the influences of the co-existing cations and H+ on the

122

reaction.

123

aqueous Mn(II) play an important role in the formation of OrthLayBir from

124

HexLayBir. However, organic molecules and microbial activities in their reaction

125

systems interact with Mn redox chemistry and it is hard to ascertain the role of Mn(II)

126

and pH in the formation of OrthLayBir and its underlying mechanisms.

27

This study, along with Bargar et al.

24

, clearly suggests that pH and

127

Recently, Elzinga and his coworkers did examine reactions of aqueous Mn(II)

128

with HexLayBir under anoxic conditions and found reductive transformation of

129

HexLayBir into low valence Mn oxides, such as feitknechtite and manganite at pH 7.0

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and 7.5, and into hausmannite at pH 8.0 and 8.5. 33,34 However, OrthLayBir was not

131

one of the products, contradicting to the results in Bargar et al. 24

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In the present study, we examined reactions of two different sizes of HexLayBir

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with Mn(II) at various pH and molar ratios of aqueous Mn(II) to solid Mn. The Mn(II)

134

to solid Mn ratios of the present study are lower than those used by Elzinga and his

135

coworkers, 33,34 and OrthLayBir product is actually resulted rather than low valence 7



136

Mn phases.

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EXPERIMENTAL SECTIONS

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Mineral synthesis. Acid birnessite and δ-MnO2 were synthesized as two forms

139

of HexLayBir. Acid birnessite was prepared through reduction of sodium

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permanganate solutions by concentrated hydrochloric acid as reported previously. 35

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δ-MnO2 was synthesized via the redox reaction between MnCl2 and NaMnO4 in an

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alkaline medium. 3 Reference triclinic birnessite with orthogonal layer symmetry was

143

synthesized through oxidation of Mn(OH)2 by O2 in an alkali medium. 32,36

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Mineral transformation experiments. Briefly, 3 g of wet HexLayBir (about 1 g

145

of dried HexLayBir) was transferred to 200 mL of deionized water (>18 MΩ, DI)

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containing 0 - 12 mM MnCl2. The molar ratio of Mn(II) to the solid Mn in HexLayBir

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([Mn(II)]/[Mn]) ranged from 0% to 24%. Then the pH of each suspension was

148

maintained at ~ 4.5 for 12 h before the pH was slowly raised to pH 7, 8, 9 or 13 under

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vigorous stirring. The pH of the suspension was adjusted with an automatic

150

potentiometric titrator (Metrohm 907) by addition of 0.1 M NaOH in the first 4 days.

151

Then the suspensions were mixed on a rotary shaker (150 rpm) at 25 ± 0.1 °C for 16 d,

152

during which the pH was manually adjusted three times a day. Aliquots were taken at

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various time intervals and the solids were collected by centrifugation at 16,000 g

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(Neofuge 23R). The collected solids were then rinsed with DI until the supernatant

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conductivity was below 20 µS/cm. A portion of each wet solid sample was directly

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pasted into the groove of an X-ray diffraction (XRD) glass slide for fast XRD data

157

collection. The remaining solid for each sample was freeze-dried and ground into 8


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powder. These dried powders were measured again for powder XRD and used for

159

other characterizations as well.

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To examine the impact of dissolved O2 on the HexLayBir transformation in the

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presence of aqueous Mn(II), parallel experiments were conducted under anoxic

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conditions. The anoxic samples were prepared under the same experimental

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conditions with the oxic experiments, except that the experiments were conducted in

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an N2 atmosphere. Details about the anoxic experiment are described in the

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Supporting Information (S1).

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Physicochemical characterization. The chemical composition of the samples

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was determined by dissolving ~ 0.1 g powder of each sample in 25 mL of 0.25 mol/L

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NH2OH·HCl. The concentrations of dissolved Mn and Na were measured using

169

atomic

170

respectively. Mn average oxidation state (AOS) was measured by a back-titration

171

method using a KMnO4 standard solution. 37 The BET specific surface areas (SSAs)

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of the samples were measured using an Autosorb-1 standard physical adsorption

173

analyzer after degassing 0.1 - 0.2 g samples at 110 °C for 3 h under vacuum.

absorption

spectrophotometry

(AAS)

and

flame

spectrophotometry,

174

Structural characterization. The freeze-dried solid samples were characterized

175

by powder XRD analysis. For the wet samples, a fast scanning with a scanning rate of

176

10°/min was used for XRD data collection to avoid the loss of interlayer water if 10 Å

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Na phyllomanganate, i.e., buserite, is produced. Mn K-edge extended X-ray

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absorption fine structure (EXAFS) spectroscopy was used for local structural

179

characterization, which were collected from the freeze-dried solids at the beamline 9



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1W1B at the Beijing Synchrotron Radiation Facility (BSRF) or at beamline X-11A at

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the National Synchrotron Light Source (NSLS) Brookhaven National Laboratory

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(Upton, NY). Details of these analyses are described in the Supporting Information

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(S2).

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RESULTS

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Physicochemical Properties. Table 1 lists the chemical composition, specific

186

surface area (SSA) and Mn average oxidation state (AOS) of δ-MnO2 and its

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transformation products after treatment with 5 mM Mn(II) ([Mn(II)]/[Mn] = 10%) at

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pH 9, and acid birnessite and its transformation products after treatment with 12 mM

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Mn(II) ([Mn(II)]/[Mn] = 24%) at pH 13 for different time intervals. In spite of some

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difference, the chemical compositions of the 20-d products of δ-MnO2 and acid

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birnessite (Na0.21MnO1.87·1.04H2O and Na0.35MnO1.88·0.23H2O, respectively) (Table 1)

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are close to those reported previously for OrthLayBir. 38,39 The chemical formula of

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triclinic Na-birnessites with orthogonal layer symmetry synthesized by Post and

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Veblen 39 and Giovanoli et al. 38 are Na0.29MnO2·0.75H2O and Na0.29MnO1.93·0.64H2O,

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respectively. The lower Na+ content (in particular for δ-MnO2) can be ascribed to both

196

the exchange of Na+ by Mn(II) and the decrease of charge deficit in octahedral layers.

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The Mn AOS of the transformation products decreased significantly during 20 d of

198

treatment, from 3.90 to 3.53 for δ-MnO2, and from 3.79 to 3.41 for acid birnessite

199

(Table 1). This is opposite to the trend observed with the transformation from Na-rich

200

buserite to H-exchanged hexagonal birnessite at pH 2, in which the Mn AOS was

201

increased from 3.66 5 to 3.72 7. 10


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Transformation of δ-MnO2 based on XRD analysis. The powder XRD pattern

203

of δ-MnO2 shows broad diffraction peaks at d-spacing around 2.42 and 1.41 Å (Fig.

204

1B), which can be respectively indexed to (20, 11) and (31, 02) reflections by a

205

C-centered two-dimensional unit cell. 40 The ratio of d(20,11)/d(31,02) is close to

206

confirming hexagonal layer symmetry.

207

around 7.2 and 3.6 Å (Fig. 1B), assigned to the 001 and 002 reflections, respectively.

208

41

209

(CSD) sizes along the c axis. 11

3,7

3,

Additionally, δ-MnO2 has weak humps

The 001 reflection is significantly shifted due to small coherent scattering domain

210

After treatment with 5 mM Mn(II) at pH 9, the XRD pattern of δ-MnO2 was

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significantly altered (Fig. 1). After 6 h of reaction, in the fast-scanning XRD patterns

212

of the wet product samples three strong diffraction reflections belonging to a 10 Å

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phyllomanganate mineral appeared at ~ 10.00, 5.03 and 3.36 Å (Fig. 1A). The

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diffraction peak amplitudes gradually increased with increasing reaction time,

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implying that the stacking order along the (001) plane increased during the

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transformation. Upon freeze-drying, the Mn(II)-reacted samples showed partial

217

dehydration of the interlayers of birnessite,

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shifting to ~ 7.10 and 3.55 Å in the corresponding powder XRD patterns (Fig. 1B). In

219

the present study, when δ-MnO2 is subject to hydration in suspension, its basal

220

spacing of 7.0 Å is generally non-expandable probably due to the strong interaction

221

between layers and interlayer cations (Fig. 1A). 43

222 223

42

with the former diffraction peaks

Splitting of the (20, 11) and (31, 02) reflections can be used to differentiate orthogonal layer symmetry from hexagonal symmetry. 11


7,13,23,27,30,31,42

A close


224

inspection of (20, 11) (Fig. 1C) and (31, 02) reflections (Fig. 1D) shows splitting of

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these diffraction peaks after δ-MnO2 was treated with 5 mM Mn(II) at pH 9. In

226

addition, HexLayBir usually contains a diagnostic (31, 02) reflection with d-spacing

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of 1.41 ± 0.01 Å, whereas for the products, this reflection is split with (310) at 1.46 ±

228

0.01 Å and (020) at 1.42 ± 0.01 Å (Fig. 1D). A shoulder at ~ 2.55 Å (Fig. 1C),

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resulting from the individualization of (20, 11) reflections, is visible on the low-angle

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side of the products, which is logically absent for HexLayBir. 42 The splitting and

231

position shifts of the diffraction peaks reflect the transformation of δ-MnO2 layer

232

symmetry from hexagonal to orthogonal. The product remains as a birnessite phase

233

rather than transforming to low-valence Mn oxides of feitknechtite, manganite or

234

hausmannite. 33,34 This is consistent with the determined Mn AOS values that are

235

significantly higher than 3 (Table 1).

236

Transformation of acid birnessite based on XRD analysis. In addition to

237

δ-MnO2, acid birnessite, another form of HexLayBir with higher crystallinity and

238

large particle sizes, was tested in the transformation experiments. The XRD patterns

239

of acid birnessite treated with 12 mM Mn(II) at pH 13 for different reaction time

240

intervals are given in Figure 2. After 3 h of reaction, the characteristic reflections of

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OrthLayBir appeared (Fig. 2); meanwhile, the reflections for acid birnessite

242

disappeared. The splitting and position changes of the diffraction peaks occurred

243

(Fig. 2A-D), similar to those observed for δ-MnO2 at pH 9 (Fig. 1A-D), indicative of

244

formation of an OrthLayBir phase. Furthermore, as indicated by the XRD data, the

245

OrthLayBir derived from acid birnessite (Fig. 2B) has lower crystallinity compared 12


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246

to that originating from δ-MnO2 (Fig. 1B), though the latter was processed at a much

247

lower pH (pH 9 vs. pH 13). The estimated crystallite thickness along the c axis using

248

the Scherrer equation 44 is ~ 16 nm for the final transformation product of δ-MnO2 at

249

pH 9, much larger than that (~ 8 nm) of the acid birnessite transformation product at

250

pH 13.

251

Mn K-edge X-ray absorption spectroscopy. EXAFS spectroscopy probes the

252

average local coordination environment around Mn to approximately 6 Å, 3,23,32,45,46

253

which is used to compare the local structure of HexLayBirs and their transformation

254

products. The Mn EXAFS spectra of δ-MnO2 and acid birnessite in k space (Fig. 3A)

255

have two intense and symmetrical resonances at 8.0 Å–1 and 9.3 Å–1, respectively,

256

indicative of hexagonal layer symmetry 23,47-49 and consistent with the XRD results

257

(Fig. 1B and Fig. 2B). Their radial structure function (RSF) (Fig. 3B) show two shells

258

of strong backscattering neighbors corresponding to a first O shell (R + δR ~ 1.5 Å)

259

and the edge-sharing Mn shell (R + δR ~ 2.5 Å). Additionally, there is a strong

260

contribution around 5 Å (R + δR) that corresponds to single scattering from Mn-Mn

261

shells at ~ 5.7 Å and multiple scattering from the same octahedra. 23

262

The positive antinode feature in the EXAFS spectra at ~ 8 and 9.3 Å–1 is affected

263

by the symmetry type of MnO6 layers, which is determined by the content and

264

distribution of Mn(III) and the quantity of vacant sites (fvac) in the layer structure.

265

27,48,49

266

single sharp antinode is typically indicative of hexagonal birnessite.

267

When δ-MnO2 was treated with 5 mM Mn(II) at pH 9, the sharp peak at 8.10 Å–1

A double or blunt antinode at ~ 8 Å–1 occurs for triclinic birnessite, whereas a

13


13,15,23,27,48-50


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268

became blunt and the one at 9.25 Å–1 broadened and shifted to a lower value (Fig. 3A).

269

The obvious left shift of k space spectra of the products compared with that of the

270

starting δ-MnO2 (Fig. S1) indicates that the overall vibration frequency increased

271

significantly. This suggests that the layers were enriched with Mn(III) after δ-MnO2

272

reaction with Mn(II), resulting in the elongation of average Mn-O and Mn-Mn

273

inter-atomic distances. 48 This is also reflected by the decreasing amplitude of Mn-Mn

274

scattering at R + δR ~2.5 Å relative to that of Mn-O, as well as the subtle decreasing

275

scattering at ~5 Å (R + δR) (Fig. 3B). It should be noted that the decrease of Mn-Mn

276

scattering contributions could alternatively be due to the lowering CSD sizes,

277

however, which was not observed after δ-MnO2 reaction with Mn(II) (Fig. S2A and

278

S2B). These results indicate the transformation of the mineral structure toward

279

orthogonal layer symmetry.

280

Mn(II)-treated δ-MnO2 with EXAFS spectra of pure δ-MnO2 and triclinic birnessite

281

(Fig. S3) showed that equivalently about 44.5% of δ-MnO2 transformed into

282

OrthLayBir (Table S1). As to the acid birnessite transformation products, the EXAFS

283

spectra (Fig. 3, S3, and Table S1) show similar trends to those of the δ-MnO2 products,

284

providing further evidence for acid birnessite transformation into OrthLayBir.

285

23,27

40

The linear combination fitting analysis of the

Influence of Mn(II) concentration, pH, and O2. To examine the pH and Mn(II)

286

concentration effects, the transformation experiments were conducted at [Mn(II)]/[Mn]

287

of 0% to 15% and pHs of 7 to 9. The fast-scanning and powder XRD patterns of the

288

samples treated for 20 d are presented in Figure 4 and Figure S4, respectively. At pH

289

9, the diffraction peaks of δ-MnO2 remained unchanged in the absence of Mn(II) (Fig. 14


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290

4f). After reaction with 1 mM Mn(II) ([Mn(II)]/[Mn] = 2%) at pH 9, the diffraction

291

peaks of the product at 7.2 and 3.6 Å became clearly discernible (Fig. 4g). As aqueous

292

Mn(II) increased from 1.0 to 2.5 mM ([Mn(II)]/[Mn] = 5%), the sharpening of the 001

293

and 002 reflections (Fig. 4Ah) suggests an increase in the CSD size along the c axis; 31

294

while concurrent splitting of the (20, 11) and (31, 02) reflections (Fig. 4Bh) indicates

295

the formation of OrthLayBir. As the concentration of Mn(II) increased from 2.5 to 7.5

296

mM ([Mn(II)]/[Mn] = 15%), OrthLayBir was still the only final transformation

297

mineral phase (Fig. 4g-j). But at a high Mn(II) concentration of 7.5 mM, feitknechtite

298

appeared as an intermediate product from 3 h to 1 d (Fig. S5). After 20 d of

299

transformation, no Mn(II) was detected in the supernatants in all the experiments

300

using the formaldoxime method.

301

transformation of δ-MnO2 to OrthLayBir at pH 9 can take place at a very low molar

302

ratio of aqueous Mn(II) to solid Mn, i.e., [Mn(II)]/[Mn] = 5%.

51

The aforementioned results indicate that the

303

Compared to the transformation products at pH 9 with 5 mM Mn(II),

304

low-valence Mn oxides did form at lower pHs with the same Mn(II) concentrations.

305

At both pH 8 and 7, feitknechtite (JCPDS 18-804) appeared at 5 mM Mn(II)

306

([Mn(II)]/[Mn] = 10%), as indicated by the diffraction peak at 4.59 Å (Fig. 4b and e).

307

Elzinga (2011) reported that at pH 7.5 under oxic conditions, both surface-catalyzed

308

oxidation of Mn(II) by O2 and reductive transformation of HexLayBir by aqueous

309

Mn(II) can contribute to feitknechtite formation.

310

responsible for the formation of feitknechtite in our study. In contrast, with 2.5 mM

311

Mn(II) ([Mn(II)]/[Mn] = 5%) at pH 8 (Fig. 4d), OrthLayBir was the only

33,34

These reactions are likely

15



312

transformation product. However, transformation was not observed with either 1 mM

313

Mn(II) at pH 8 or 2.5 mM Mn(II) at pH 7 in the experimental time frame (Fig. 4a and

314

c).

315

Given that atmospheric O2 might also participate in the oxidation reaction of

316

Mn(II), a set of experiments were conducted under anoxic conditions to elucidate the

317

effect of O2 on the transformation. At pH 9 with 2.5 mM Mn(II), the transformation of

318

δ-MnO2 layer symmetry from hexagonal to orthogonal was substantially

319

compromised under anoxic conditions (Fig. 4k) compared to that under oxic

320

conditions (Fig. 4h). Moreover, at pH 9 with 5 mM Mn(II), the low-valence Mn oxide

321

of feitknechtite appeared (Fig. S4l) in addition to OrthLayBir (Fig. 4l). This result

322

implies that with higher Mn(II) concentrations (e.g., ≥ 5 mM) under anoxic conditions,

323

a portion of the Mn(II) was oxidized by HexLayBir to Mn(III) which was

324

incorporated into the vacancies of HexLayBir to produce OrthLayBir, whereas the

325

rest was oxidized by HexLayBir to form feitknechtite. In contrast, with 5 mM and 7.5

326

mM Mn(II) under oxic conditions, OrthLayBir was the only final transformation

327

product (Fig. 4i and j). This might be due to the rapid oxidization of the

328

intermediately formed feitknechtite by O2 to OrthLayBir (Fig. S5). A similar process

329

was observed in our previous experiment where feitknechtite was the metastable

330

intermediate and subsequently converted into OrthLayBir during oxidation of

331

Mn(OH)2 by O2 at room temperature. 52 In addition, oxidation of Mn(II) by O2 to

332

Mn(III) and subsequent incorporation of Mn(III) into the layers may further promote

333

the transformation of the original HexLayBir to OrthLayBir under oxic conditions. 16


Page 16 of 33

Page 17 of 33


334

DISCUSSION

335

Effects of concentrations of Mn(II) and pH. Collectively, these results indicate

336

that the concentration of Mn(II) and pH jointly determine the transformation of

337

birnessite layer symmetry from hexagonal to orthogonal, and their magnitudes affect

338

the extent and rate of the transformation. Figure 4 shows that at 5 mM Mn(II) with pH

339

increased from 7 to 9, the amount of formed feitknechtite gradually decreased and

340

OrthLayBir appeared, indicative of the promotive effect of higher pH on the

341

formation of OrthLayBir. Zhu et al. 27 found that at a higher pH, more Mn(III) was

342

accumulated in Mn octahedral layers, and the resulting biogenic birnessite tended to

343

have an orthogonal layer symmetry, as observed in this study. The possible

344

explanation for the pH effect is that higher pH 1) promotes the adsorption of Mn(II)

345

on vacancy sites of HexLayBir, 2) favors the comproportionation reaction between

346

Mn(II) and Mn(IV) surrounding the vacancies to form Mn(III), and 3) favors

347

incorporation of the Mn(III) adsorbed on vacancies into the Mn octahedral layers. 27

348

Similar enhancement on metal cation adsorption and incorporation into birnessite

349

structure by increasing pH also occurs to Ni(II). 53

350

At pH 8, δ-MnO2 remained unchanged at the lower Mn(II) concentration

351

([Mn(II)]/[Mn] ≤ 2%). It converted to OrthLayBir when Mn(II) concentration was

352

increased to [Mn(II)]/[Mn] = 5%. Further increasing Mn(II) concentration at this pH

353

([Mn(II)]/[Mn] = 10%), feitknechtite was obtained. This is consistent with the

354

observation of Bargar et al., 24 i.e., the formation of 10 Å Na phyllomanganate at 10

355

µM Mn(II) and the formation of feitknechtite at 1 mM Mn(II) during reaction of 17



356

aqueous Mn(II) with biogenic Mn oxide at pH 7.7 - 7.8 in oxic systems. However, in

357

their study OrthLayBir formation occurred at an intermediate pH (7.7 - 7.8) and a

358

lower Mn(II) concentration (10 µM) compared to the present study. The difference

359

could be caused by a higher [Mn(II)]/[Mn] ratio than that of the present study

360

although biological molecules or microbial activities may also contribute. Mn(II)

361

commonly adsorbs on vacant sites as a tridentate corner-sharing complex. 7,40 At a low

362

Mn(II) concentration, each vacant site is probably capped by only one adsorbed

363

Mn(II). Each Mn(II) reacts with adjacent Mn(IV) along the b axis through electron

364

transfer, i.e., a comproportionation reaction, to produce two Mn(III), situated in the

365

layer and above the vacant site, respectively. Then the Mn(III) above the vacant site

366

migrates into the vacant site. The reaction processes between HexLayBir and Mn(II)

367

can be simply denoted as:

368

Mn(IV)layer + □ + Mn(II)(aq) → Mn(IV)layer + □ + Mn(II)sorbed

369

→ Mn(III)layer + □ + Mn(III)sorbed → Mn(III)layer + Mn(III)layer → OrthLayBir

370

in which, □ stands for vacant sites. However, at a high Mn(II) concentration, two

371

Mn(II) ions may sorb below and above a vacant site, respectively, with a minor

372

fraction of Mn(II) adsorbed on the edge sites.

373

produced Mn(III) cations on both sides of a vacant layer site prevents the migration of

374

the Mn(III) into the octahedral layer because of electrostatic repulsion. This situation

375

may favor the formation of separated Mn(III) phases at the surface in an oxic system

376

or even a complete transformation of birnessite to Mn(III) phases. 33,34

377

54

The presence of Mn(II) or the

A recent study by Lefkowitz et al. 34 has demonstrated that reaction of aqueous 18


Page 18 of 33

Page 19 of 33


378

Mn(II) with HexLayBir under anoxic conditions caused reductive transformation of

379

HexLayBir into feitknechtite and manganite at pH 7.0 and 7.5, and into hausmannite

380

at pH 8.0 and 8.5. These bulk structural transformations were induced by interfacial

381

electron transfer from adsorbed Mn(II) to structural Mn(IV), generating Mn(III)

382

which precipitated as Mn(III)OOH and Mn(II)Mn(III)2O4. 33,34 In the present study, no

383

bulk transformation of HexLayBir was observed during reaction with Mn(II) at

384

alkaline pHs. Instead, we demonstrate a change in birnessite layer symmetry from

385

hexagonal to orthogonal, and propose that this is due to interfacial electron transfer

386

from Mn(II) to Mn(IV), generating Mn(III) which is incorporated into the birnessite

387

mineral layers. The lack of bulk mineralogical transformation is attributed to the

388

lower [Mn(II)]/[Mn] molar ratio employed in our experiments (24% or lower) than in

389

the studies of Elzinga and Lefkowitz (50% and up). These results suggest that impacts

390

of aqueous Mn(II) on the structure and composition of Mn-oxide minerals strongly

391

depend on the [Mn(II)]/[Mn] ratio. Since the concentration of aqueous Mn(II) to solid

392

Mn is often quite low in weakly alkaline environment, such as ocean sediments and

393

calcareous soils, the layer symmetry alteration is expected to occur on birnessite

394

rather than bulk structural transformation to low valence Mn oxides.

395

Effects of particle size. Since the complete transformation of δ-MnO2 layer

396

symmetry from hexagonal to orthogonal needs a lower pH and Mn(II) concentration

397

than that of acid birnessite (pH 9 vs. pH 13, 10% vs. 24% [Mn(II)]/[Mn]), the

398

transformation of δ-MnO2 is more facilitated than that of acid birnessite during

399

reaction with Mn(II) in an alkaline medium. This difference can be ascribed to the 19



400

smaller size of δ-MnO2 (Fig. S2A) relative to acid birnessite (Fig. S2C), which may

401

permit a more efficient growth of OrthLayBir particles, probably through an oriented

402

aggregation mechanism (Fig. S2F). Acid birnessite does not necessarily have more

403

vacancies, 55 while having much lower SSA than δ-MnO2 (23 m2/g for acid birnessite,

404

144 m2/g for δ-MnO2, Table 1). An alternative explanation is that less Mn(II) sorbs to

405

acid birnessite, which could lead to less favorable transformation of acid birnessite as

406

compared to δ-MnO2. Furthermore, in the acid birnessite system, more available

407

Mn(II) may be oxidized either by acid birnessite or dissolved O2 to yield separate

408

Mn(III) phases, such as feitknechtite,

409

number of stacked layers of acid birnessite may stabilize the hexagonal structure,

410

making it more difficult to transform into OrthLayBir. Simply put, acid birnessite is a

411

poor precursor for the formation of OrthLayBir. Therefore, it is conceivable that

412

biogenic oxides is subject to the transformation due to their nano-scale particle sizes.

413

23,31,56

33,34

rather than OrthLayBir. Also, a larger

Reversible transformation between HexLayBir and OrthLayBir. Lanson et al.

414 415

7

416

HexLayBir at acidic pHs. In the present study, we observed that HexLayBir (e.g.,

417

δ-MnO2 and acid birnessite) can transform to OrthLayBir via reaction with aqueous

418

Mn(II) at low Mn(II)/Mn (in HexLayBir) molar ratios in an alkaline medium.

419

Therefore, the reverse process of transformation from OrthLayBir into HexLayBir can

420

occur. The transformation from OrthLayBir to HexLayBir occurs via Mn(III)

421

migration into the interlayers and/or Mn(III) disproportionation to Mn(IV) and Mn(II)

observed the transformation of high pH Na-rich OrthLayBir to H-exchanged

20


Page 20 of 33

Page 21 of 33


7

422

with the latter moving to the interlayers.

423

HexLayBir to OrthLayBir can be ascribed to Mn(III) formation via the

424

comproportionation reaction between Mn(II) adsorbed on vacant layer sites and the

425

surrounding layer Mn(IV), and the subsequent migration of Mn(III) into the vacancies

426

with an ordered distribution in the birnessite layers, as proposed previously by Zhu et

427

al.

428

oxidized to Mn(III). This migration mechanism into vacant sites occurring at high pH

429

is similar to the one reported for Ni(II) and Co(II) on birnessite. Ni(II) migrates from

430

being sorbed at the interlayers to structural layer incorporation in HexLayBir at a

431

higher pH, and it is reversible upon decreasing pH to ~ 4. 53 HexLayBir can oxidize

432

the sorbed Co(II) to Co(III) that further enters the layers even under acidic conditions.

433

57

434

the formation of OrthLayBir because neither Ni(II) nor Co(III) can cause Jahn-Teller

435

distortion of the octahedra. 27,28,57

436

ENVIRONMENTAL IMPLICATIONS

27

The transformation mechanism of

The migration process of Mn(II) is promoted at a higher pH after getting

However, unlike Mn(III), the incorporation of Ni(II) and Co(III) does not promote

437

Collectively, our results demonstrate that HexLayBir, e.g., δ-MnO2 and acid

438

birnessite, can transform into OrthLayBir via reaction with appropriate concentrations

439

of Mn(II) in an alkaline medium. Concentration of Mn(II) and pH jointly determine

440

the extent and rate of the birnessites structural transformation and accordingly their

441

environmentally related reactivity. The above transformation will drastically alter the

442

sorption and redox reactivity of birnessites, significantly affecting the biogeochemical

443

cycling of nutrient elements (e.g., C, N, and P) 1,22 and trace metals (e.g., Co, Zn, Pb, 21



Page 22 of 33

9,10,16,17,31,58

444

Cr, As,)

. Although most freshwaters have lower pH values, marine

445

systems and a variety of surficial environments, such as calcareous soils, have slightly

446

alkaline pHs, 59,60 where the transformation of HexLayBir to OrthLayBir may occur

447

when Mn(II) is supplied from soil weathering processes or partial reduction of Mn

448

oxides under transient anoxic or suboxic conditions by organic matter.

449

Furthermore, the high affinity of HexLayBir for Mn(II)

450

accumulate Mn(II) to an interlayer concentration high enough to trigger the

451

transformation. In addition, the transformation could be facilitated by nano-scale sizes

452

of the natural phyllomanganate samples. Thus, a possible origin of natural OrthLayBir

453

6

454

Mn(II) under slightly alkaline conditions. Field studies are further required to testify

455

the effect of environmental conditions on the crystal structure and reactivity of

456

birnessite.

62

34,61

makes it possible to

is the transformation of biogenic HexLayBir by an appropriate input of aqueous

457 458

ACKNOWLEDGMENTS

459

The authors thank the National Natural Science Foundation of China (Grant Nos.

460

41471194 & 41171197) and the Strategic Priority Research Program of the Chinese

461

Academy of Sciences (No. XDB15020402) for financial support of this research. M.Z.

462

is grateful for the support from the U. S. National Science Foundation under Grant No.

463

EAR-1529937. We gratefully acknowledge the associate editor Dr. Daniel Giammar

464

and the five anonymous reviewers for their constructive comments on the manuscript.

465 22


Page 23 of 33


466

ASSOCIATED CONTENT

467

Supporting Information

468

Experimental methods about anoxic condition, spectroscopic analyses and

469

electron microscopy; comparison of k space spectra; Mn EXAFS fitting results using

470

Linear combination fittings; fast-scanning XRD patterns of the Mn(II)-treated

471

δ-MnO2 at various pHs and Mn(II) concentrations and powder XRD patterns of

472

δ-MnO2 and its transformation products after treatment with 7.5 mM Mn(II) at pH 9.

473

These materials are available free of charge via the internet at http://pubs.acs.org.

474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501

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birnessite affected by Zn2+ and Mn2+ pretreatments. J. Soil. and Sediment. 2010, 10 (5), 870-878.

27



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636

Table 1. Chemical composition, specific surface area (SSA) and Mn average oxidation

637

state (AOS) of δ-MnO2 and its transformation products after treatment with 5 mM

638

Mn(II) at pH 9, and acid birnessite (AcidBir) and its transformation products after

639

treatment with 12 mM Mn(II) at pH 13 for different time intervals. Sample

Chemical composition

Mn AOS

SSA(m2/g)

Sample

Chemical composition

Mn AOS

SSA(m2/g)

δ-MnO2

Na0.32MnO2.11·1.50H2O

3.90±0.01

144

AcidBir

Na0.28MnO2.03·0.53H2O

3.79±0.01

23

3h

Na0.14MnO1.94·1.47H2O

3.74±0.01

150

3h

Na0.34MnO1.98·0.69H2O

3.63±0.01

31

6h

Na0.15MnO1.89·1.12H2O

3.62±0.03

128

6h

Na0.33MnO1.90·0.49H2O

3.47±0.02

29

12 h

Na0.18MnO1.93·1.23H2O

3.69±0.01

118

12 h

Na0.34MnO1.97·0.54H2O

3.61±0.01

26

1d

Na0.21MnO1.91·1.31H2O

3.61±0.02

110

1d

Na0.32MnO1.89·0.23H2O

3.46±0.03

30

2d

Na0.20MnO1.92·1.01H2O

3.63±0.01

117

2d

Na0.34MnO1.96·0.49H2O

3.57±0.01

26

6d

Na0.18MnO1.86·0.91H2O

3.54±0.01

120

6d

Na0.33MnO1.92·0.31H2O

3.50±0.01

19

20 d

Na0.21MnO1.87·1.04H2O

3.53±0.02

112

20 d

Na0.35MnO1.88·0.23H2O

3.41±0.03

16

640 641

28


Page 29 of 33


642

Figure captions

643

Figure 1. XRD patterns of δ-MnO2 and its transformation products after treatment

644

with 5 mM Mn(II) at pH 9 for different time intervals. Triclinic birnessite (TricBir)

645

with orthogonal layer symmetry is included as a reference. (A), fast-scanning XRD

646

patterns of wet pastes; (B-D), higher-resolution powder XRD patterns after

647

freeze-drying.

648

Figure 2. XRD patterns of acid birnessite (AcidBir) and its transformation products

649

after treatment with 12 mM Mn(II) at pH 13 for different time intervals. (A),

650

fast-scanning XRD patterns of wet pastes; (B-D), higher-resolution powder XRD

651

patterns after freeze-drying.

652

Figure 3. Mn K-edge EXAFS spectra (A) and the magnitude of their Fourier

653

transforms (B) for δ-MnO2 and its transformation products after treatment with 5 mM

654


655

treatment with 12 mM Mn(II) at pH 13 for different time intervals. A triclinic

656

birnessite standard (TricBir) with orthogonal layer symmetry is also shown for

657

comparison. The changes in the diagnostic features in both k and R spaces are

658

highlighted by arrows, straight lines and shaded areas.

659

Figure 4. Powder XRD patterns of synthetic δ-MnO2 after treatment with different

660

concentrations of aqueous Mn(II) (from 0 to 7.5 mM, or [Mn(II)]/[Mn] from 0% to

661

15%) at pH 7 - 9 for 20 d. The samples were prepared under oxic conditions, except

662

for k and l which were prepared under anoxic conditions. (A), D: 1 - 10 Å; (B), D:

663

1.15 – 2.8 Å; F stands for feitknechtite. 29



664

Figure 1. XRD patterns of δ-MnO2 and its transformation products after treatment

665

with 5 mM Mn(II) at pH 9 for different time intervals. Triclinic birnessite (TricBir)

666

with orthogonal layer symmetry is included as a reference. (A), fast-scanning XRD

667

patterns of wet pastes; (B-D), higher-resolution powder XRD patterns after

668

freeze-drying.

669

30


Page 30 of 33

Page 31 of 33


670

Figure 2. XRD patterns of acid birnessite (AcidBir) and its transformation products

671

after treatment with 12 mM Mn(II) at pH 13 for different time intervals. (A),

672

fast-scanning XRD patterns of wet pastes; (B-D), higher-resolution powder XRD

673

patterns after freeze-drying.

674

31



675

Figure 3. Mn K-edge EXAFS spectra (A) and the magnitude of their Fourier

676

transforms (B) for δ-MnO2 and its transformation products after treatment with 5 mM

677


678

treatment with 12 mM Mn(II) at pH 13 for different time intervals. A triclinic

679

birnessite standard (TricBir) with orthogonal layer symmetry is also shown for

680

comparison. The changes in the diagnostic features in both k and R spaces are

681

highlighted by arrows, straight lines and shaded areas.

682

32


Page 32 of 33

Page 33 of 33


683

Figure 4. Powder XRD patterns of synthetic δ-MnO2 after treatment with different

684

concentrations of aqueous Mn(II) (from 0 to 7.5 mM, or [Mn(II)]/[Mn] from 0% to

685

15%) at pH 7 - 9 for 20 d. The samples were prepared under oxic conditions, except

686

for k and l which were prepared under anoxic conditions. (A), D: 1 - 10 Å; (B), D:

687

1.15 - 2.8 Å; F stands for feitknechtite.

688 689 690 691 692

33


Redox Reactions between Mn(II) and Hexagonal Birnessite Change

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