Mechanism of Arsenic Adsorption on Magnetite Nanoparticles from

Jun 9, 2015 - Mechanism of Arsenic Adsorption on Magnetite Nanoparticles from Water: Thermodynamic and Spectroscopic Studies. Cheng-Hua Liu†‡, Ya-...
0 downloads 7 Views 2MB Size
Subscriber access provided by NEW YORK UNIV

Article

Mechanism of Arsenic Adsorption on Magnetite Nanoparticles from Water: Thermodynamic and Spectroscopic Studies Cheng-Hua Liu, Ya-Hui Chuang, Tsan-Yao Chen, Yuan Tian, Hui Li, Ming-Kuang Wang, and Wei Zhang Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 09 Jun 2015 Downloaded from http://pubs.acs.org on June 9, 2015

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

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

Page 1 of 32

Environmental Science & Technology

1

Mechanism of Arsenic Adsorption on Magnetite Nanoparticles from Water:

2

Thermodynamic and Spectroscopic Studies Cheng-Hua Liu,†,‡ Ya-Hui Chuang,† Tsan-Yao Chen,§ Yuan Tian,† Hui Li,† Ming-Kuang

3

Wang,# and Wei Zhang†,‡,*

4 5 6 7 8 9 10



Department of Plant, Soil and Microbial Sciences, Michigan State University, East Lansing,

Michigan 48824, United States ‡

Environmental Science and Policy Program, Michigan State University, East Lansing,

Michigan 48824, United States §

Department of Engineering and System Science, National Tsing Hua University, Hsinchu,

30013, Taiwan

11

#

12

*Corresponding author: Dr. Wei Zhang, Address: 1066 Bogue ST RM A516, East Lansing,

Department of Agricultural Chemistry, National Taiwan University, Taipei 10617, Taiwan

13

MI 48824, United States; Tel: 517-353-0471; Fax: 517-355-0270; Email: [email protected]

14

TOC/ABSTRACT ART

15 1

ACS Paragon Plus Environment

Environmental Science & Technology

16

Page 2 of 32

ABSTRACT

17

Removal of arsenic (As) from water supplies is needed to reduce As exposure through

18

drinking water and food consumption in many regions of the world. Magnetite nanoparticles

19

(MNPs) are promising and novel adsorbents for As removal, due to their great adsorption

20

capacity for As and easy separation. This study aimed to investigate adsorption mechanism of

21

arsenate As(V) and arsenite As(III) on MNPs by macroscopic adsorption experiments in

22

combination with thermodynamic calculation and microspectroscopic characterization using

23

synchrotron radiation-based X-ray absorption spectroscopy (XAS) and X-ray photoelectron

24

spectroscopy (XPS). Adsorption reactions are favorable endothermic processes as evidenced by

25

increased adsorption with increasing temperatures, and high positive enthalpy change. EXAFS

26

spectra suggested predominant formation of bidentate binuclear corner-sharing complexes (2C)

27

for As(V), and tridentate hexanuclear corner-sharing (3C) complexes for As(III) on MNP surfaces.

28

The macroscopic and microscopic data conclusively identified the formation of inner-sphere

29

complexes between As and MNP surfaces. More intriguingly, XANES and XPS results revealed

30

complex redox transformation of the adsorbed As on MNPs exposed to air: Concomitant with the

31

oxidation of MNPs, the oxidation of As(III) and MNPs was expected, but the observed As(V)

32

reduction was surprising, likely due to the role played by the reactive Fe(II).

33

INTRODUCTION

34

Contamination of natural waters by inorganic arsenic (As) has dire consequences to public

35

health worldwide, due to potential toxic and carcinogenic effects from As exposure via

36

consumption of As-tainted drinking water and food.1 Short-term exposure to high level of As is 2

ACS Paragon Plus Environment

Page 3 of 32

Environmental Science & Technology

37

fatal, whereas chronic exposure to trace levels of As can cause skin, bladder, and lung cancers.2, 3

38

As can be released from both natural and anthropogenic sources such as rock and soil weathering,

39

wood preservation, pesticides, mining, and industrial wastewater discharge, and is therefore

40

commonly found in natural waters as inorganic oxyanions of trivalent arsenite (AsO33−, As(III))

41

and pentavalent arsenate (AsO43−, As(V)).4 Crop irrigation with As-contaminated water allows

42

for trophic transfer of As through food chain.5 Exposure through contaminated drinking water is

43

another direct pathway to humans. Many regulatory authorities thus impose a maximum

44

contamination level of 10 µg L−1 in drinking water.6, 7 However, human populations in certain

45

regions of the world are constantly over-exposed to As, especially in developing countries,8, 9 due

46

to high As levels in water supplies, insufficient water treatment, or lack of water quality

47

surveillance and public awareness.10, 11 Therefore, effective As removal from water supplies is

48

essential to protecting environmental and human health.

49

Among many techniques currently available for As removal from water,12,

13

adsorption

50

process is considered one of most promising techniques because it is economical, effective, and

51

socially acceptable.14,

52

nanoparticles, zero-valent iron nanoparticles, carbon nanotube, and iron oxide nanoparticles are

53

novel adsorbents, due to greatly enhanced As removal efficiency at nanometer scale.16

54

Nonetheless, significant challenges remain regarding post-treatment separation of adsorbent

55

nanoparticles from treated water.16 Because superparamagnetic magnetite (Fe3O4) nanoparticles

56

(MNPs) can be easily separated from aqueous solution with a low external magnetic field, there

57

have been growing interest in As removal by MNPs.17, 18 This is particularly well-suited for

15

In particular, engineered nanomaterials such as titanium oxide

3

ACS Paragon Plus Environment

Environmental Science & Technology

Page 4 of 32

58

applications in water treatment facilities or in-situ groundwater remediation. To better develop

59

the magnetite-based As removal technique, more fundamental knowledge on both macroscopic

60

and microscopic aspects of As adsorption by MNPs is needed.

61

Over last several years, macroscopic adsorption studies have provided valuable insight on the

62

effects of adsorbent particle size, solution pH and ionic strength on As adsorption by MNPs.17,

63

19-21

64

by magnetite have been sparse with conflicting reports in the literature.22-25 As shown by Figure

65

S1 in Supporting Information, four types of As surface complexation with iron oxide surface

66

have been proposed: 22-37 (i) bidentate mononuclear edge-sharing (2E), (ii) bidentate binuclear

67

corner-sharing (2C), (iii) monodentate mononuclear corner-sharing (1V), and (iv) tridentate

68

hexanuclear corner-sharing (3C) complexes. Jönsson and Sherman22 suggested that both As(III)

69

and As(V) form 2C complexes on the {100} surfaces of magnetite at As–Fe distance (RAs-Fe) of

70

3.39 and 3.42 Å, respectively, using extended X-ray absorption fine structure (EXAFS)

71

spectroscopy. However, Morin et al.23 and Wang et al.24 reported formation of 3C complexes of

72

As(III) on {111} surfaces of magnetite at RAs–Fe of 3.47–3.52 Å. More recently, Wang et al.25

73

suggested that As(V) forms 2C complexes on the {100} surfaces of magnetite at RAs–Fe of 3.35–

74

3.39 Å with a lower coordination number of 0.8–1.3 than the expected value of 2, explained by

75

the presence of outer-sphere complexes of As(V). Thus, the type of As inner-sphere complexes

76

formed on the magnetite surface has yet to be settled.

77 78

However, convincing and indisputable evidences on molecular mechanism of As adsorption

Furthermore, since both mobility and toxicity of As(III) are greater than As(V),1,

3

any

potential reductive transformation of As(V) to As(III) should be given due consideration. Drying 4

ACS Paragon Plus Environment

Page 5 of 32

Environmental Science & Technology

79

is an important sludge management process in water treatment facilities to substantially reduce

80

the volume and mass of sludge for easier transport and disposal. During drying, the MNPs (a

81

mixed-valence iron (II, III) oxide) and adsorbed As species are exposed to oxygen, which may

82

result in complex surface redox reactions. As expected, Ona-Nguema et al.38 reported rapid

83

oxidation of As(III) to As(V) at magnetite surface by dissolved oxygen in aqueous solution.

84

However, the reduction of adsorbed As(V) to As(III) on magnetite/maghemite nanoparticles

85

reported by Chowdhury et al.39 was rather surprising, but unfortunately no mechanistic

86

explanation was provided. Therefore, the potential redox transformation of adsorbed As on

87

MNPs and underlying mechanisms need to be further elucidated. This information is required for

88

more effective and safer disposal of the MNP sludge produced in the As removal processes.

89

This study aimed to elucidate As adsorption mechanisms by MNPs through spectroscopic

90

techniques involving X-ray absorption near edge structure (XANES), EXAFS, and X-ray

91

photoelectron spectroscopy (XPS), in addition to batch sorption experiments and thermodynamic

92

calculations. The redox transformation of As adsorbed on MNPs during the drying process was

93

also investigated. We chose arsenate As(V) and arsenite As(III) as our model As species, and

94

performed As adsorption experiments under a range of solution pH, ionic strength, and

95

temperature. Thermodynamic parameters were calculated using As adsorption isotherm data at a

96

temperature range of 10–55 °C. Synchrotron radiation-based XANES, EXAFS and XPS analyses

97

were employed to probe As molecular binding mechanism and redox reactions on MNP surfaces.

98

Specifically, As-adsorbed MNPs samples with and without drying were analyzed to study the

99

transformation of As species and MNPs upon exposure to oxygen. 5

ACS Paragon Plus Environment

Environmental Science & Technology

100

Page 6 of 32

MATERIALS AND METHODS

101

General. All chemicals used were of analytical grade. Since As(III) and MNPs are

102

oxygen-sensitive, special care was required to maintain anoxic conditions to minimize the

103

potential effect of atmospheric and dissolved oxygen. In this study, all deionized (DI) water was

104

deaerated by purging with N2 gas overnight prior to use. All containers for solutions, suspensions,

105

or solid samples were purged with N2 gas for 10 seconds before they were tightly capped or

106

sealed so as to remove oxygen. Transferring of solutions, suspensions, or solid samples during

107

the experiments was performed as quickly as possible to reduce air exposure time. Unless noted

108

otherwise, the above steps were taken during the experiments. These anti-oxidation measures

109

appeared to be effective, as minimal oxidation of MNPs and As(III) were observed during the

110

adsorption experiments demonstrated by subsequent spectroscopic analyses.

111

Magnetite Synthesis and Characterization. Magnetite nanoparticles (MNPs) were

112

synthesized using a modified iron (II) and iron (III) co-precipitation method under N2

113

protection.40 The methods for MNP synthesis and characterization were detailed in Supporting

114

Information S2. X-ray diffraction (XRD) pattern confirmed the presence of standard magnetite

115

(JCPDS 19-0629) without other crystalline iron oxide phase (Figure S2). Transmission electron

116

microscopy images showed near-spherical primary particles with an average diameter of 34 nm

117

(Figure S3). However, MNPs existed as larger aggregates under adsorption experimental

118

conditions (Figure S3), and the aggregate size was 2.57 µm following a 30-min sonication, and

119

increased to 5.1 µm after 24-hr shaking (Figure S4).41 Additionally, the MNPs had a N2-BET

120

specific surface area of 39 m2 g−1 and an isoelectric point (IEP) of 7.2 (Figure S5). 6

ACS Paragon Plus Environment

Page 7 of 32

Environmental Science & Technology

121

Adsorption Experiments. Batch adsorption experiments were conducted under anoxic

122

conditions to measure As adsorption by MNPs in triplicate. As(V) and As(III) solutions were

123

freshly prepared in DI water from disodium hydrogen arsenate (Na2HAsO4·7H2O, J.T. Baker)

124

and sodium arsenite (NaAsO2, J.T. Baker), respectively. Sodium nitrate (NaNO3) was added as

125

background electrolyte to control solution ionic strength. Dilute HNO3 and NaOH solutions were

126

used to adjust pH of As solutions and MNP suspensions. Adsorption kinetics of As(V) and As(III)

127

described in Supporting Information S2 was first examined to determine the equilibration time

128

needed for adsorption isotherm experiments. As(V) and As(III) adsorption on MNPs occurred

129

rapidly and reached equilibrium around 120 min (Figure S6). Consequently, the adsorption

130

equilibration time was set at 24 hours to ensure complete reactions.

131

For adsorption isotherm experiments, MNPs (50 mg) were mixed with 25 mL of DI water in

132

polypropylene bottles, and the pH of MNP suspensions was adjusted to 5.0. After 30-min

133

sonication in an ultrasonic bath, 25 mL of As(V) or As(III) solution was added into the MNP

134

suspensions to reach initial As concentrations of 0.05, 0.125, 0.25, 0.5, 0.75, or 1.0 mM, ionic

135

strength of 0.01 M NaNO3 and solution pH 5.0. The mixtures were horizontally shaken at 180

136

rpm in a shaking water bath (BT-350 PID, Yih Der, Taiwan) for 24 h at 10, 25, and 40°C for

137

As(III) experiments, and at 10, 25, 40, and 55 °C for As(V) experiments. After equilibration, the

138

suspensions were filtered through a 0.22-µm syringe filter with mixed cellulose esters membrane

139

(Millipore, USA), and the final As concentrations in the filtrates were determined by inductively

140

coupled plasma optical emission spectrometry (ICP-OES, Perkin Elmer Optima 2000DV,

141

Waltham, MA, USA). The difference between the initial and final As concentrations was 7

ACS Paragon Plus Environment

Environmental Science & Technology

Page 8 of 32

142

assumed to be adsorbed by MNPs. The surface coverage were further calculated by dividing the

143

adsorbed As concentration with the BET specific surface area of MNPs.25, 35 The adsorption

144

isotherms were plotted as function of the adsorbed As concentrations vs the equilibrium As

145

concentrations in the solution.

146

More adsorption experiments were conducted following the similar procedure as described

147

above. To examine the effect of solution pH on As adsorption, the MNP suspensions were mixed

148

with 1 mM As(V) or As(III) solutions at solution pH of 5.0, 6.0, 7.0, 8.0, and 9.0, and ionic

149

strength of 0.01 M NaNO3. The MNP suspensions were also mixed with 1 mM As(V) or As(III)

150

solution at ionic strength of 0, 0.005, 0.01, 0.05, or 0.1 M NaNO3 and pH 5.0 to examine ionic

151

strength effect.

152

Thermodynamic Calculations. The MNPs were homogeneous crystalline solids shown by

153

the XRD pattern (Figure S2), and As(V) and As(III) each formed only one type of monolayer

154

complexes on the MNP surface revealed by the XAS study next. By assuming no lateral

155

movement of adsorbed As and no interactions between neighboring adsorbed As, the Langmuir

156

equation could be derived for the As adsorption on the MNPs in the aqueous solution.42, 43 The

157

isotherm data were fitted with the linearized Langmuir equation as follows:

158

Ce 1 1 = Ce + qe qmax K L qmax

159

where Ce (mmol L−1) is the equilibrium As concentration in solution, qe (mmol g−1) is the

160

adsorbed As amount on MNPs, qmax (mmol g−1) is the maximum As adsorption capacity, and KL

161

(L mmol−1) is the Langmuir constant. Thermodynamic parameters for As adsorption were first

162

estimated using the Gibbs free energy equation and the linearized Van′t Hoff equation (i.e., the

(1)

8

ACS Paragon Plus Environment

Page 9 of 32

Environmental Science & Technology

163

Van′t Hoff plot) as follows:44-47

164

∆G 0 = − RT ln K

165

∆H 0 ∆S 0 ln K = − + RT R

166

where ∆G0 (kJ mol−1) is the change of free energy, ∆H0 (kJ mol−1) is the change of enthalpy, ∆S0

167

(kJ mol−1) is the change of entropy, T (K) is the absolute temperature, R is the ideal gas constant

168

(0.008314 kJ mol−1 K−1), and K is the dimensionless equilibrium coefficient. K can be estimated

169

from the Langmuir constant (KL) as:48

170

K = K L × Cw

171

where Cw is the water concentration (5.56 × 104 mmol L−1).

(2) (3)

(4)

172

Since adsorption enthalpy changes with surface coverage of adsorbed As, assessing the

173

enthalpy changes with increasing As adsorption (qe) would be useful to understand the

174

interaction between As and MNPs. Thus, the observed molar differential enthalpies (∆Hobs) of As

175

adsorption on MNPs were further estimated using the differential Van′t Hoff equation:49 qe Ce = −R 1 d T d ln

176

∆H obs

(5)

177

At any given qe, Ce was computed using the Langmuir parameters obtained at different

178

temperatures. ∆Hobs was then calculated from the slope of the linear plots of ln(qe/Ce) versus 1/T.

179

Synchrotron Radiation-based XAS and XPS Analyses. XAS was used to investigate the

180

oxidation state and local molecular environment of adsorbed As(V) or As(III) on MNPs, and

181

XPS to study the oxidation state of adsorbed As and Fe on the outer surface of MNPs. The 9

ACS Paragon Plus Environment

Environmental Science & Technology

Page 10 of 32

182

As-adsorbed MNPs for the XAS and XPS analyses were prepared at pH 5.0 under anoxic

183

conditions, as described in Supporting Information S2. The As-adsorbed MNP samples were

184

either kept in the anoxic wet-paste or oven-dried at 40 °C overnight. The anoxic wet-pasted

185

samples labeled as As(V)-MWP and As(III)-MWP were used for investigating actual adsorption

186

mechanisms in aqueous solution, and the oven-dried samples labeled as As(V)-MOD and

187

As(III)-MOD for studying surface redox reactions upon exposure to oxygen under simulated

188

drying process.

189

Detailed XAS and XPS analysis procedures were provided in Supporting Information S2.

190

Briefly, As K-edge XAS spectra of As-adsorbed MNP samples were acquired at the beamline

191

BL17C1 in the National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan.

192

Electron storage ring was operated at 1.5 GeV with 360 mA at a top-up mode. The As-adsorbed

193

MNP samples were scanned in an energy range of 11667 to 12867 eV to obtain a full As K-edge

194

(11867 eV) XAS spectrum. The XAS data analysis was performed using the ATHENA and

195

ARTEMIS interfaces to the IFEFFIT version 1.2.11 program package.50 The XPS spectra of

196

As-free and As-adsorbed MNPs were collected at the beamline BL24A1 in NSRRC.

197

High-resolution scans were carried out in an energy range of 70 to 35 eV for Fe3p and As3d XPS

198

spectra. The XPS data analyses were performed using the XPSpeak 4.1 software.

199

RESULTS AND DISCUSSION

200

Adsorption and Thermodynamics Calculations. As(V) and As(III) adsorption occurred

201

rapidly within the first 5 minutes and reached a plateau at 120 minutes, which fitted well with a

202

pseudo second order kinetic model (Table S1 and Figure S6). The estimated initial adsorption 10

ACS Paragon Plus Environment

Page 11 of 32

Environmental Science & Technology

203

rate was 7.7 mmol g−1 h−1 for As(V) and 6.7 mmol g−1 h−1 for As(III), indicating that As

204

adsorption by MNPs was much faster than bulk magnetite that reached As adsorption

205

equilibrium around two days.51 The observed and fitted adsorption isotherms of As(V) and As(III)

206

at various temperatures and pH 5.0 are presented in Figure 1, and the fitted isotherm parameters

207

in Table 1. The Langmuir equation fitted the isotherm data well (R2 ≥ 0.97), suggesting

208

monolayer As adsorption onto the MNP surface. The maximum adsorption capacity (qmax)

209

slightly increased from 0.20 mmol g−1 at 283 K to 0.25 mmol g−1 at 328 K for As(V), and from

210

0.21 mmol g−1 at 283 K to 0.23 mmol g−1 at 313 K for As(III), respectively, demonstrating As

211

adsorption as an endothermic process.

212

The calculated thermodynamic parameters including ∆G0, ∆H0, and ∆S0 are given in Table 1,

213

and ∆Hobs as a function of qe in Figure 2. The negative ∆G0 values suggest spontaneous

214

adsorption reactions.52 The positive ∆S0 likely resulted from the release of orderly structured

215

hydration water from and subsequent increase in randomness with increased concentration of

216

adsorbed As on the solid surface.52, 53 The positive ∆H0 again indicates the endothermic nature of

217

As adsorption (Table 1). An increase of ∆Hobs with increasing qe (i.e., more energy was needed

218

for As adsorption at greater qe) could be attributed to the fact that greater repulsion between the

219

adsorbed As and the free As needs to be overcome at higher qe. Overall, the large positive

220

enthalpy change and the entropy increase collectively contribute to the strong spontaneous As

221

adsorption by MNPs.

222

Solution pH and Ionic Strength Effects and IEP Measurements. As(V) adsorption

223

decreased monotonically with increasing pH from 0.21 ± 0.01 mmol g−1 at pH 5.0 to 0.14 ± 0.00 11

ACS Paragon Plus Environment

Environmental Science & Technology

Page 12 of 32

224

mmol g−1 at pH 9.0; As(III) adsorption started to decrease at pH greater than 7.0 (Figure S7a).

225

These results are consistent with the pH-dependent As speciation and surface charge of MNPs. In

226

the experimental pH range, the predominate species of As(V) is H2AsO4− and HAsO42−, and that

227

of As(III) is H3AsO30. As(V) adsorption would be facilitated by electrostatic attraction between

228

negatively charged As(V) species and positively charged MNP surface at pH below the IEP of

229

MNPs (i.e., 7.2, Figure S5), but inhibited by electrostatic repulsion at higher pH. Conversely,

230

neutral As(III) species rendered electrostatic interaction insignificant at pH below 7.2. At pH 8.0

231

and 9.0, As(III) adsorption slightly decreased due to competitions from hydroxyls (OH−) for the

232

adsorption sites as well as the deprotonation of MNP surfaces.

233

The above observations agreed with the previously reported adsorption of multi-protonated

234

oxyanion species on metal oxides.54-56 The adsorption of oxyanions such as As(V) and As(III)

235

involves two-step ligand exchange reaction: the hydroxyl group of metal hydroxide is first

236

protonated, and then the H2O ligand is replaced with the oxyanion. Consequently, the adsorption

237

is affected by the pH-dependent surface protonation of metal hydroxides. Since the differences of

238

adsorption affinity among oxyanion species are usually small, this trend is primarily attributed to

239

deprotonation of metal hydroxide surface with increasing pH, 56 but As speciation is expected to

240

play a role in electrostatic interaction that modulates As flux toward the MNP surface, a crucial

241

step prior to ligand exchange. While ligand exchange forms inner-sphere complexes, this

242

evidence alone cannot rule out the possibility of outer-sphere surface complexation between As

243

species and protonated hydroxyl group at acidic pH.

244

Nonetheless, given that As(V) and As(III) adsorption remained relatively unchanged from 0 12

ACS Paragon Plus Environment

Page 13 of 32

Environmental Science & Technology

245

to 100 mM NaNO3 (Figure S7b), the outer-sphere complexation was likely to be minor because

246

outer-sphere complexation is typically suppressed with increasing ionic strength.57 Compared to

247

that of As-free MNPs, the IEP of As(V)-adsorbed MNPs decreased from 7.2 to 4.9 and that of

248

As(III)-adsorbed MNPs from 7.2 to 6.1, respectively (Figure S5). This significant IEP shift to

249

lower pH after As adsorption could result from shielding of positively charge sites on the MNP

250

surface by the formation of inner-sphere complexes, with a greater IEP shift from As(V) than

251

As(III) due to greater negative charge number of As(V).

252

Arsenic Adsorption and Transformation Mechanisms Studied by XAS. The

253

abovementioned macroscopic adsorption and thermodynamic evidences present a convincing

254

case for specific chemical adsorption of As(V) and As(III) on MNPs, but does not provide insight

255

on coordination and configuration of formed inner-sphere complexes, i.e, whether inner-sphere

256

complexes are formed through monodentate, bidentate, or tridentate bonds (Figure S1). Also, the

257

oxidation state of As after adsorption on MNPs has yet to be determined. Therefore, we would

258

use the spectroscopic data to indisputably pinpoint the As adsorption and redox transformation

259

mechanisms.

260

XANES spectroscopy was used to investigate the oxidation state of adsorbed As(V) and

261

As(III) on MNPs. According to the As K-edge XANES spectra, the absorption edge of

262

As(III)-MWP and As(V)-MWP were located at the same absorption edge of the As(V) and As(III)

263

reference spectra with an adsorption maximum at 11875.0 eV and 11871.8 eV, respectively

264

(Figure 3). The Linear Combination Fitting (LCF) analysis revealed only trace levels of As(V)

265

(i.e., 2%) in As(III)-MWP and As(III) (i.e., 4%) in As(V)-MWP (Table S2). Thus, no major redox 13

ACS Paragon Plus Environment

Environmental Science & Technology

Page 14 of 32

266

reaction occurred for the As(V) or As(III) adsorbed on MNPs under anoxic experimental

267

conditions. Conversely, dramatic redox reactions occurred upon exposure to air during the

268

overnight drying process, as evidenced by the white line energy shifts in the XANES spectra of

269

As(III)-MOD and As(V)-MOD (Figure 3), and the pronounced increases of As(V) fraction for

270

As(III)-MOD (i.e., 34%) and As(III) fraction for As(V)-MOD (i.e., 12%) (Table S2). Since

271

minimal redox transformation of adsorbed As on MNPs was observed in anoxic waste-paste

272

samples, the direct redox reaction of As by MNPs could be excluded. Thus, the redox reaction of

273

As in oven-dried samples more possibly occurred during the drying process, due to the exposure

274

to atmospheric oxygen. These As redox reactions were further corroborated with the XPS results

275

in next section.

276

Previously, four types of As surface complexes forming on FeO6 octahedral sites have been

277

proposed (Figure S1);22-37 this is instrumental to analyzing the EXAFS data for determining local

278

molecular coordination geometry surrounding the adsorbed As atoms. The k3-weighted As

279

K-edge EXAFS spectra and Fourier-transformed radial distribution function (RDF) profile for

280

As-adsorbed MNPs samples are shown in Figure 3, and the fitted structural parameters including

281

the coordination number (CN), interatomic distance (R), and Debye–Waller factor (σ2) in Table

282

S3. The Fourier-transformed RDF profile of all samples showed one domain signal of As–O

283

first-neighbor contribution and relatively weak signals of As–Fe second-neighbor contributions.

284

The As–O first coordination shell was attributed to 3.9 and 2.9 oxygen atoms at 1.69 and 1.79 Å

285

in As(V)-MWP and As(III)-MWP, corresponding to the molecular structure of AsO4 tetrahedron

286

and AsO3 trigonal-pyramid, respectively. In addition, the As–O shell was fitted by 3.5 and 3.2 14

ACS Paragon Plus Environment

Page 15 of 32

Environmental Science & Technology

287

oxygen atoms at 1.69 and 1.74 Å in As(V)-MOD and As(III)-MOD, respectively. In agreement

288

with the XANES results, the changes of R and CN reflect the partial reduction of As(V) to As(III)

289

in As(V)-MOD and oxidation of As(III) to As(V) in As(III)-MOD.

290

In the As-Fe second coordination shells of As(V)-adsorbed samples, the As atom was

291

surrounded by Fe atoms at 3.35 Å with a CN of 1.5 for As(V)-MWP and at 3.39 Å with a CN of

292

1.4 for As(V)-MOD, suggesting the formation of the bidentate binuclear corner-sharing (2C)

293

complexes. These As(V)–Fe distances of 2C complexes are close to the values reported in the

294

previous studies of As(V) adsorption on magnetite,22,

295

lepidocrocite.27, 34 In agreement with Jönsson and Sherman22 and Wang et al.25, the dominant 2C

296

surface complexes were formed between one adsorbed AsO4 tetrahedron and two FeO6 octahedra

297

on {100} surfaces of magnetite (Figure 4). The As–Fe distance of As(V)-MOD is slightly longer

298

than that of As(V)-MWP, due to the sum of contributions from 2C complexes of both As(V) and

299

As(III). Importantly, the transformation of adsorbed As(V) to As(III) on MNPs surface would not

300

change the ligand complexation type.

25

maghemite,32 goethite,27,

34

and

301

Conversely, the EXAFS fitting on the As–Fe shells of As(III)-adsorbed samples revealed

302

different surface complexes, compared with that of the As(V)-adsorbed sample. The As–Fe shells

303

of As(III)-MWP were contributed from 3.8 Fe atoms at 3.49 Å and 1.0 Fe atoms at 3.76 Å, which

304

does not match that of 2C complexes (~3.3–3.4 Å) and monodentate mononuclear corner-sharing

305

(1V) complexes (~3.5–3.6 Å) of As(III) adsorption on iron oxides.29-31, 33 The EXAFS results of

306

As(III)-MWP essentially corroborated with that of Morin et al.23 and Wang et al.,24 indicating the

307

formation of tridentate hexanuclear corner-sharing (3C) complexes between adsorbed As(III) and 15

ACS Paragon Plus Environment

Environmental Science & Technology

Page 16 of 32

308

2 to 6 FeO6 octahedra on the vacancy and defect sites of {111} magnetite surfaces (Figure 4).

309

The As–Fe distance of 3.76 Å was attributed to the adjacent FeO4 tetrahedra around 3C

310

complexes (Figure 4). Although Jönsson and Sherman22 suggested that the adsorbed As(III)

311

would form 2C complexes at As-Fe distance of 3.4 Å on the {100} magnetite surfaces, this was

312

not observed in this study and that of Morin et al.23 and Wang et al.24 In As(III)-MOD, the As–Fe

313

shells of were fitted by 3.8 Fe atoms at 3.39 Å and 1.3 Fe atoms at 3.67 Å. These As–Fe

314

distances of As(III)-MOD were shorter than that of As(III)-MWP, but the coordination numbers

315

were similar. Interestingly, these As-Fe distances were close to the 3C complexes (~3.4–3.5 Å

316

and ~3.6–3.7 Å) of As(V)-magnetite co-precipitates on the {111} surface,25 which was not

317

observed in As(V) adsorption in our study. Thus, the As-Fe shells at 3.39 and 3.67Å was more

318

likely to due to the mixed contribution from the 3C complexes of both As(V) and As(III) instead

319

of the 2C and 1V complexes, implying the transformation of adsorbed As(III) to As(V) would not

320

change the ligand complexation type.

321

Arsenic Redox Transformation on MNPs Studied by XPS. The XAS analysis of

322

As-adsorbed MNP samples showed the oxidation of adsorbed As(III) and reduction of adsorbed

323

As(V) occurred during the drying process. Since Fe atoms in magnetite are in Fe(II) and Fe(III)

324

mixed valence states, the possible role of reactive Fe(II) atoms in the redox transformation of

325

adsorbed As should not be ignored. Because As K-edge XAS analysis cannot provide

326

information on Fe oxidation state, the XPS analysis was further conducted to elucidate the

327

change of both Fe and As oxidation status on the outer surface of MNPs.

328

The XPS spectra of Fe3p and As3d peaks for As-free MNPs, As(III)-MOD, and As(V)-MOD 16

ACS Paragon Plus Environment

Page 17 of 32

Environmental Science & Technology

329

samples are shown in Figure 5. By deconvoluting the Fe3p peak into Fe(III) and Fe(II)

330

component peaks and the As3d peak into As(V) and As(III) component peaks, as well as

331

analyzing each component peak area (Table S4), the changes in the oxidation state of As and Fe

332

atoms after exposure to air could be assessed. The As-free MNPs sample had 67% of Fe(III) and

333

33% of Fe(II), which agrees well with theoretical value of magnetite (Fe(III)/Fe(II) = 2.0). For

334

the As(V)-MOD sample, the Fe(III)/Fe(II) ratio of Fe3p peak was 3.2, indicating a significant

335

oxidation of Fe(II) to Fe(III) on magnetite surface. Interestingly, approximately 32% of adsorbed

336

As(V) was reduced to As(III). For the As(III)-MOD sample, the Fe(III)/Fe(II) ratio of Fe3p peak

337

was 2.9, and about 49% of adsorbed As(III) was oxidized to As(V), suggesting a parallel

338

oxidation of As(III) and Fe(II).

339

Also noted were greater fractions of As(V) in As(III)-MOD and As(III) in As(V)-MOD

340

detected by the XPS analysis than the XANES analysis (Table S2 and Table S4). This likely

341

resulted from the differential probing regions by these two techniques. The XPS analysis focuses

342

on the outer surface (< 10 nm) of MNPs, but the XANES analysis detects the signals from the

343

adsorbed As on the external and interior surfaces of MNP aggregates. Thus, the redox signature

344

in the XANES analysis was dampened because the adsorbed As in the interior of MNP

345

aggregates were protected from redox reactions occurring on the external surfaces.

346

The XPS and XANES analyses suggest that complex redox reactions occurred on the outer

347

surface of As-adsorbed MNPs after exposure to air, and both As(III) oxidation and As(V)

348

reduction were concomitant with magnetite surface oxidation, whereas minimal redox reactions

349

of adsorbed As on MNPs occurred during the anoxic adsorption experiments (Table S2 and S4). 17

ACS Paragon Plus Environment

Environmental Science & Technology

Page 18 of 32

350

These surface redox reactions are illustrated in Figure 6. The oxidation of magnetite to

351

maghemite (γ-Fe2O3) and As(III) to As(V) was expected in the presence of oxygen. Moreover,

352

the oxidation of adsorbed As(III) was not only caused by direct reaction with oxygen, but also

353

facilitated by highly oxidizing species (i.e. O2●−) produced from the oxidation of Fe(II) in

354

magnetite surface due to a Fenton-type reaction (Figure 6b).38, 58, 59 While As(III) oxidation is

355

expected, the reduction of adsorbed As(V) to As(III) was surprising. A previous XPS study

356

reported the reduction of adsorbed As(V) to As(III) on magnetite/maghemite nanoparticles, but

357

provided no mechanistic explanation.39 Here we propose a mechanism responsible for this novel

358

surface redox reaction, illustrated in Figure 6a. Based on the XPS results, we postulate that a thin

359

maghemite layer is formed when magnetite surface is exposed to oxygen. During this process,

360

the octahedral center Fe(II) atoms are oxidized to Fe(III), and cation vacancies are subsequently

361

created at the octahedral sites. Then, electroneutrality must be balanced by either the diffusion of

362

Fe(II) or electron migration from internal bulk magnetite to oxidized surface.60, 61 Consequently,

363

the adsorbed As(V) could be reduced to As(III) via reacting with the migrated Fe(II) or electrons.

364

Finally, the presence of inner-sphere complexes, evidenced by EXAFS data, may further

365

facilitate the electron transfer process between As and Fe through binding ligand. Nonetheless,

366

this proposed mechanism needs to be further studied.

367

IMPLICATIONS

368

The findings of this study demonstrate that MNPs have high affinity to both As(V) and As(III)

369

from water. This is distinctively advantageous over conventional technologies (i.e., coagulation,

370

ion-exchange, and activated alumina adsorbent), for which the pre-oxidation of As(III) to As(V) 18

ACS Paragon Plus Environment

Page 19 of 32

Environmental Science & Technology

371

is needed to enhance efficiency.14, 62 Moreover, the application of MNPs for remediation of

372

As-contaminated waters is further facilitated by easy-separation of MNPs from treated water by

373

applying a low external magnetic field. Therefore, the As removal by the MNPs could be used

374

for in-situ groundwater remediation, or retrofitted into existing water treatment facilities during

375

the coagulation process or one of final clean-up processes. Subsequently, the As-adsorbed MNP

376

sludge produced during the As removal will likely undergo a typical sludge drying process.

377

Therefore, the redox transformation of adsorbed As, especially the reduction of As(V) to As(III),

378

likely occur on the MNP surfaces. Consequently, the release of adsorbed As, especially As(III),

379

from the MNPs under fluctuating redox conditions needs to be further evaluated in order to

380

design safe disposal strategies for post-treatment MNP sludge.

381

ASSOCIATED CONTENT

382

Supporting Information

383

S1. Schematic of As inner-sphere complexes; S2. Supplemental Materials and Methods,

384

including MNP synthesis and characterization, adsorption kinetics experiments, and XAS and

385

XPS sample preparation, data collection, and analyses; S3. Supplemental Results and Discussion,

386

including MNP characterization, adsorption kinetics, solution pH and ionic strength effects, and

387

XAS and XPS results. This material is available free of charge via the Internet at

388

http://pubs.acs.org.

389

ACKNOWLEDGMENT

390

This research was partly supported by the AgBioResearch of Michigan State University (MSU)

391

through the Hatch Act Formula Grant from the US Department of Agriculture–National Institute 19

ACS Paragon Plus Environment

Environmental Science & Technology

Page 20 of 32

392

of Food and Agriculture (Project No. MICL02248). We express our special thanks to Dr. Jyh-Fu

393

Lee and Dr. Yaw-Wen Yang for providing the beamtime of BL17C1 and BL24A1 for XAS and

394

XPS data collection at NSRRC, Dr. Volodymyr V. Tarabara at MSU for providing the

395

measurement of aggregate size distribution of magnetite nanoparticles, and Dr. Brian J. Teppen

396

at MSU for assisting in spectroscopic analyses and interpretations. We would also like to thank

397

the three anonymous reviewers for their insightful and constructive comments that helped us

398

improve an earlier version of this paper.

20

ACS Paragon Plus Environment

Page 21 of 32

Environmental Science & Technology

399 400 401

References (1) Mandal, B. K.; Suzuki, K. T., Arsenic round the world: A review. Talanta 2002, 58, (1), 201-235.

402

(2) Hughes, M. F.; Beck, B. D.; Chen, Y.; Lewis, A. S.; Thomas, D. J., Arsenic exposure and

403 404

toxicology: a historical perspective. Toxicol Sci 2011, 123, (2), 305-332. (3) Bissen, M.; Frimmel, F. H., Arsenic - a review. - Part 1: Occurrence, toxicity, speciation,

405 406

mobility. Acta Hydroch Hydrob 2003, 31, (1), 9-18. (4) Smedley, P. L.; Kinniburgh, D. G., A review of the source, behaviour and distribution of

407 408

arsenic in natural waters. Appl Geochem 2002, 17, (5), 517-568. (5) Meharg, A. A.; Rahman, M., Arsenic contamination of Bangladesh paddy field soils:

409 410 411

Implications for rice contribution to arsenic consumption. Environ Sci Technol 2003, 37, (2), 229-234. (6) Smith, A. H.; Lopipero, P. A.; Bates, M. N.; Steinmaus, C. M., Public health - Arsenic

412 413 414

epidemiology and drinking water standards. Science 2002, 296, (5576), 2145-2146. (7) WHO, Guidelines for Drinking-water Quality. 4th ed.; World Health Organization: Geneva, Switzerland, 2011.

415

(8) Smith, A. H.; Lingas, E. O.; Rahman, M., Contamination of drinking-water by arsenic in

416 417 418

Bangladesh: a public health emergency. B World Health Organ 2000, 78, (9), 1093-1103. (9) Berg, M.; Tran, H. C.; Nguyen, T. C.; Pham, H. V.; Schertenleib, R.; Giger, W., Arsenic contamination of groundwater and drinking water in Vietnam: A human health threat. Environ

419 420 421

Sci Technol 2001, 35, (13), 2621-2626. (10) Ahmed, M. F.; Ahuja, S.; Alauddin, M.; Hug, S. J.; Lloyd, J. R.; Pfaff, A.; Pichler, T.; Saltikov, C.; Stute, M.; van Geen, A., Epidemiology - Ensuring safe drinking water in

422 423 424 425

Bangladesh. Science 2006, 314, (5806), 1687-1688. (11) Rahman, M. M.; Chowdhury, U. K.; Mukherjee, S. C.; Mondal, B. K.; Paul, K.; Lodh, D.; Biswas, B. K.; Chanda, C. R.; Basu, G. K.; Saha, K. C.; Roy, S.; Das, R.; Palit, S. K.; Quamruzzaman, Q.; Chakraborti, D., Chronic arsenic toxicity in Bangladesh and West Bengal,

426 427

India - A review and commentary. J Toxicol-Clin Toxic 2001, 39, (7), 683-700. (12) Choong, T. S. Y.; Chuah, T. G.; Robiah, Y.; Koay, F. L. G.; Azni, I., Arsenic toxicity, health

428 429 430

hazards and removal techniques from water: An overview. Desalination 2007, 217, (1-3), 139-166. (13) Bissen, M.; Frimmel, F. H., Arsenic - a review. Part II: Oxidation of arsenic and its removal

431 432

in water treatment. Acta Hydroch Hydrob 2003, 31, (2), 97-107. (14) Mohan, D.; Pittman Jr, C. U., Arsenic removal from water/wastewater using adsorbents—A 21

ACS Paragon Plus Environment

Environmental Science & Technology

Page 22 of 32

433 434

critical review. J Hazard Mater 2007, 142, (1–2), 1-53. (15) Sharma, V. K.; Sohn, M., Aquatic arsenic: Toxicity, speciation, transformations, and

435

remediation. Environ Int 2009, 35, (4), 743-759.

436 437

(16) Ali, I., New generation adsorbents for water treatment. Chem Rev 2012, 112, (10), 5073-5091.

438

(17) Mayo, J. T.; Yavuz, C.; Yean, S.; Cong, L.; Shipley, H.; Yu, W.; Falkner, J.; Kan, A.; Tomson,

439

M.; Colvin, V. L., The effect of nanocrystalline magnetite size on arsenic removal. Sci Technol

440 441 442

Adv Mat 2007, 8, (1-2), 71-75. (18) Yavuz, C. T.; Mayo, J. T.; Yu, W. W.; Prakash, A.; Falkner, J. C.; Yean, S.; Cong, L. L.; Shipley, H. J.; Kan, A.; Tomson, M.; Natelson, D.; Colvin, V. L., Low-field magnetic separation

443 444 445

of monodisperse Fe3O4 nanocrystals. Science 2006, 314, (5801), 964-967. (19) Chowdhury, S. R.; Yanful, E. K., Arsenic and chromium removal by mixed magnetite-maghemite nanoparticles and the effect of phosphate on removal. J Environ Manage

446 447 448

2010, 91, (11), 2238-2247. (20) Shipley, H. J.; Yean, S.; Kan, A. T.; Tomson, M. B., Adsorption of arsenic to magnetite nanoparticles: Effect of particle concentration, ph, ionic strength, and temperature. Environ

449 450 451

Toxicol Chem 2009, 28, (3), 509-515. (21) Yean, S.; Cong, L.; Yavuz, C. T.; Mayo, J. T.; Yu, W. W.; Kan, A. T.; Colvin, V. L.; Tomson, M. B., Effect of magnetite particle size on adsorption and desorption of arsenite and arsenate. J

452 453

Mater Res 2005, 20, (12), 3255-3264. (22) Jonsson, J.; Sherman, D. M., Sorption of As(III) and As(V) to siderite, green rust (fougerite)

454 455 456 457

and magnetite: Implications for arsenic release in anoxic groundwaters. Chem Geol 2008, 255, (1-2), 173-181. (23) Morin, G.; Wang, Y. H.; Ona-Nguema, G.; Juillot, F.; Calas, G.; Menguy, N.; Aubry, E.; Bargar, J. R.; Brown, G. E., EXAFS and HRTEM evidence for As(III)-containing surface

458 459 460 461 462

precipitates on nanocrystalline magnetite: implications for As sequestration. Langmuir 2009, 25, (16), 9119-9128. (24) Wang, Y. H.; Morin, G.; Ona-Nguema, G.; Menguy, N.; Juillot, F.; Aubry, E.; Guyot, F.; Calas, G.; Brown, G. E., Arsenite sorption at the magnetite-water interface during aqueous precipitation of magnetite: EXAFS evidence for a new arsenite surface complex. Geochim

463 464 465

Cosmochim Ac 2008, 72, (11), 2573-2586. (25) Wang, Y. H.; Morin, G.; Ona-Nguema, G.; Juillot, F.; Calas, G.; Brown, G. E., Distinctive arsenic(V) trapping modes by magnetite nanoparticles induced by different sorption processes.

466

Environ Sci Technol 2011, 45, (17), 7258-7266. 22

ACS Paragon Plus Environment

Page 23 of 32

Environmental Science & Technology

467

(26) Auffan, M.; Rose, J.; Proux, O.; Borschneck, D.; Masion, A.; Chaurand, P.; Hazemann, J.-L.;

468 469

Chaneac, C.; Jolivet, J.-P.; Wiesner, M. R.; Van Geen, A.; Bottero, J.-Y., Enhanced adsorption of arsenic onto maghemites nanoparticles:  As(III) as a probe of the surface structure and

470 471

heterogeneity. Langmuir 2008, 24, (7), 3215-3222. (27) Farquhar, M. L.; Charnock, J. M.; Livens, F. R.; Vaughan, D. J., Mechanisms of arsenic

472

pptake from aqueous solution by interaction with goethite, lepidocrocite, mackinawite, and

473 474

pyrite:  An X-ray absorption spectroscopy study. Environ Sci Technol 2002, 36, (8), 1757-1762. (28) Fendorf, S.; Eick, M. J.; Grossl, P.; Sparks, D. L., Arsenate and chromate retention

475 476

mechanisms on goethite. 1. Surface structure. Environ Sci Technol 1997, 31, (2), 315-320. (29) Guo, H. M.; Ren, Y.; Liu, Q.; Zhao, K.; Li, Y., Enhancement of arsenic adsorption during

477

mineral transformation from siderite to goethite: Mechanism and application. Environ Sci

478 479

Technol 2013, 47, (2), 1009-1016. (30) Manning, B. A.; Fendorf, S. E.; Goldberg, S., Surface structures and stability of arsenic(III)

480 481 482

on goethite:  spectroscopic evidence for inner-sphere complexes. Environ Sci Technol 1998, 32, (16), 2383-2388. (31) Manning, B. A.; Hunt, M. L.; Amrhein, C.; Yarmoff, J. A., Arsenic(III) and arsenic(V)

483 484 485 486

reactions with zerovalent iron corrosion products. Environ Sci Technol 2002, 36, (24), 5455-5461. (32) Morin, G.; Ona-Nguema, G.; Wang, Y.; Menguy, N.; Juillot, F.; Proux, O.; Guyot, F.; Calas, G.; Brown Jr, G. E., Extended X-ray absorption fine structure analysis of arsenite and arsenate

487 488 489

adsorption on maghemite. Environ Sci Technol 2008, 42, (7), 2361-2366. (33) Ona-Nguema, G.; Morin, G.; Juillot, F.; Calas, G.; Brown, G. E., EXAFS analysis of arsenite adsorption onto two-line ferrihydrite, hematite, goethite, and lepidocrocite. Environ Sci

490 491 492

Technol 2005, 39, (23), 9147-9155. (34) Sherman, D. M.; Randall, S. R., Surface complexation of arsenie(V) to iron(III) (hydr)oxides: Structural mechanism from ab initio molecular geometries and EXAFS

493 494 495

spectroscopy. Geochim Cosmochim Ac 2003, 67, (22), 4223-4230. (35) Wang, Y. H.; Morin, G.; Ona-Nguema, G.; Juillot, F.; Guyot, F.; Calas, G.; Brown, G. E., Evidence for different surface speciation of arsenite and arsenate on green rust: An EXAFS and

496 497 498

XANES study. Environ Sci Technol 2010, 44, (1), 109-115. (36) van Genuchten, C. M.; Addy, S. E.; Pena, J.; Gadgil, A. J., Removing arsenic from synthetic groundwater with iron electrocoagulation: an Fe and As K-edge EXAFS study. Environ Sci

499 500

Technol 2012, 46, (2), 986-94. (37) Waychunas, G. A.; Rea, B. A.; Fuller, C. C.; Davis, J. A., Surface-chemistry of ferrihydrite: 23

ACS Paragon Plus Environment

Environmental Science & Technology

Page 24 of 32

501

Part 1. EXAFS studies of the geometry of coprecipitated and adsorbed arsenate. Geochim

502 503

Cosmochim Ac 1993, 57, (10), 2251-2269. (38) Ona-Nguema, G.; Morin, G.; Wang, Y. H.; Foster, A. L.; Juillot, F.; Calas, G.; Brown, G. E.,

504

XANES evidence for rapid arsenic(III) oxidation at magnetite and ferrihydrite surfaces by

505 506

dissolved O-2 via Fe2+-mediated reactions. Environ Sci Technol 2010, 44, (14), 5416-5422. (39) Chowdhury, S. R.; Yanful, E. K.; Pratt, A. R., Arsenic removal from aqueous solutions by

507 508

mixed magnetite-maghemite nanoparticles. Environ Earth Sci 2011, 64, (2), 411-423. (40) Chen, C. P.; Gunawan, P.; Xu, R., Self-assembled Fe3O4-layered double hydroxide colloidal

509

nanohybrids with excellent performance for treatment of organic dyes in water. J Mater Chem

510 511

2011, 21, (4), 1218-1225. (41) Baalousha, M., Aggregation and disaggregation of iron oxide nanoparticles: Influence of

512 513 514 515 516

particle concentration, pH and natural organic matter. Sci Total Environ 2009, 407, (6), 2093-2101. (42) McBride, M. B., Environmental chemistry of soils. Oxford University Press: New York, 1994; p viii, 406 p. (43) Essington, M. E., Soil and water chemistry : an integrative approach. CRC Press: Boca

517 518 519

Raton, 2004; p 534 p. (44) Kalavathy, M. H.; Karthikeyan, T.; Rajgopal, S.; Miranda, L. R., Kinetic and isotherm studies of Cu(II) adsorption onto H3PO4-activated rubber wood sawdust. J Colloid Interf Sci

520 521 522

2005, 292, (2), 354-362. (45) Kundu, S.; Gupta, A. K., Investigations on the adsorption efficiency of iron oxide coated cement (IOCC) towards As(V)—kinetics, equilibrium and thermodynamic studies. Colloids and

523 524

Surfaces A: Physicochemical and Engineering Aspects 2006, 273, (1–3), 121-128. (46) Ranjan, D.; Talat, M.; Hasan, S. H., Biosorption of arsenic from aqueous solution using

525 526

agricultural residue ‘rice polish’. J Hazard Mater 2009, 166, (2–3), 1050-1059. (47) Sarı, A.; Tuzen, M., Biosorption of As(III) and As(V) from aqueous solution by

527 528 529

macrofungus (Inonotus hispidus) biomass: Equilibrium and kinetic studies. J Hazard Mater 2009, 164, (2–3), 1372-1378. (48) Milonjic, S. K., A consideration of the correct calculation of thermodynamic parameters of

530 531

adsorption. J Serb Chem Soc 2007, 72, (12), 1363-1367. (49) Li, H.; Teppen, B. J.; Johnston, C. T.; Boyd, S. A., Thermodynamics of nitroaromatic

532 533 534

compound adsorption from water by smectite clay. Environ Sci Technol 2004, 38, (20), 5433-5442. (50) Ravel, B.; Newville, M., ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray 24

ACS Paragon Plus Environment

Page 25 of 32

Environmental Science & Technology

535 536

absorption spectroscopy using IFEFFIT. J Synchrotron Radiat 2005, 12, 537-541. (51) Gimenez, J.; Martinez, M.; de Pablo, J.; Rovira, M.; Duro, L., Arsenic sorption onto natural

537 538 539

hematite, magnetite, and goethite. J Hazard Mater 2007, 141, (3), 575-580. (52) Velickovic, Z.; Vukovic, G. D.; Marinkovic, A. D.; Moldovan, M. S.; Peric-Grujic, A. A.; Uskokovic, P. S.; Ristic, M. D., Adsorption of arsenate on iron(III) oxide coated ethylenediamine

540 541 542

functionalized multiwall carbon nanotubes. Chem Eng J 2012, 181, 174-181. (53) Gupta, K.; Basu, T.; Ghosh, U. C., Sorption characteristics of arsenic(V) for removal from water using agglomerated nanostructure iron(III)-zirconium(IV) bimetal mixed oxide. J Chem

543 544

Eng Data 2009, 54, (8), 2222-2228. (54) Anderson, M. A.; Ferguson, J. F.; Gavis, J., Arsenate adsorption on amorphous aluminum

545 546

hydroxide. J Colloid Interf Sci 1976, 54, (3), 391-399. (55) Hingston, F. J.; Posner, A. M.; Quirk, J. P., Competitive adsorption of negatively charged

547 548 549 550

ligands on oxide surfaces. Discuss Faraday Soc 1971, (52), 334-&. (56) Sposito, G., The Surface Chemistry of Soils. New York; [Oxford Oxfordshire]: Oxford University Press, 1984. (57) Arai, Y.; Elzinga, E. J.; Sparks, D. L., X-ray absorption spectroscopic investigation of

551 552 553

arsenite and arsenate adsorption at the aluminum oxide-water interface. J Colloid Interf Sci 2001, 235, (1), 80-88. (58) Amstaetter, K.; Borch, T.; Larese-Casanova, P.; Kappler, A., Redox transformation of

554 555

arsenic by Fe(II)-activated goethite (alpha-FeOOH). Environ Sci Technol 2010, 44, (1), 102-108. (59) Hug, S. J.; Leupin, O., Iron-catalyzed oxidation of arsenic(III) by oxygen and by hydrogen

556 557 558

peroxide:  pH-dependent formation of oxidants in the Fenton reaction. Environ Sci Technol 2003, 37, (12), 2734-2742. (60) Jolivet, J.-P.; Tronc, E., Interfacial electron transfer in colloidal spinel iron oxide.

559 560

Conversion of Fe3O4-γFe2O3 in aqueous medium. J Colloid Interf Sci 1988, 125, (2), 688-701. (61) Rebodos, R. L.; Vikesland, P. J., Effects of oxidation on the magnetization of

561 562

nanoparticulate magnetite. Langmuir 2010, 26, (22), 16745-16753. (62) Korngold, E.; Belayev, N.; Aronov, L., Removal of arsenic from drinking water by anion

563

exchangers. Desalination 2001, 141, (1), 81-84.

564 565

25

ACS Paragon Plus Environment

Environmental Science & Technology

566

Table 1. Fitted Langmuir Isotherm Parameters and Thermodynamic Calculations Arsenic Temperature species (K) As(V)

As(III)

567 568

Page 26 of 32

a

Thermodynamics a

Langmuir parameters qmax

KL

R2

(mmol g−1) (L mmol−1)

∆G0

∆H0

∆S0

(kJ mol−1) (kJ mol−1) (kJ mol−1K−1)

283 298

0.195 0.214

15.5 27.8

0.979 0.995

−32.2 −35.3

313 328

0.225 0.246

32.8 36.1

0.996 0.995

−37.5 −39.6

283

0.212

6.59

0.971

−30.1

298 313

0.222 0.227

9.00 11.4

0.980 0.989

−32.5 −34.8

13.7

0.163

13.5

0.154

Calculated by the van′t Hoff plot.

26

ACS Paragon Plus Environment

Page 27 of 32

569 570 571 572

Environmental Science & Technology

Figure 1. Observed and fitted arsenic adsorption isotherms at different temperature (pH = 5, ionic strength = 0.01 M NaNO3, solid-liquid ratio = 1.0 g L−1) for (a) As(V) and (b) As(III).

27

ACS Paragon Plus Environment

Environmental Science & Technology

573 574 575

Page 28 of 32

Figure 2. Relationship of ln(qe/Ce) vs 1/T (inserts) and estimated enthalpy change (∆Hobs) as a function of adsorbed concentration (qe) for (a) As(V) and (b) As(III).

576

28

ACS Paragon Plus Environment

Page 29 of 32

Environmental Science & Technology

577

578 579 580 581 582 583 584

Figure 3. XAS spectra of As-adsorbed MNPs samples: (a) As K-edge XANES spectra, (b) k3-weighted k-space, (c) magnitude part of Fourier transformed R-space, and (d) real part of Fourier transformed R-space, without phase shift correction. The green and blue solid lines represent the component of As(III) and As(V) obtained from linear combination fitting of XANES spectra. The black solid lines and red open circles represent experimental and fitted data, respectively. The value after labels was the surface coverage.

585

29

ACS Paragon Plus Environment

Environmental Science & Technology

586 587 588 589

Page 30 of 32

Figure 4. Schematic of arsenic adsorption on magnetite with proposed surface complexes.

30

ACS Paragon Plus Environment

Page 31 of 32

Environmental Science & Technology

590 591 592 593 594

Figure 5. High-resolution XPS spectra in (a) Fe3p and (b) As3d regions of As-free MNPs, As(V)-MOD, As(III)-MOD samples. The relative peak area of component peaks was presented in percentage. The As3d spin orbit-split doublet peaks were fixed a ratio of 3:2 at 0.70 eV.

31

ACS Paragon Plus Environment

Environmental Science & Technology

595 596 597 598

Page 32 of 32

Figure 6. Schematic of proposed redox reactions on magnetite surface during adsorption (anoxic) and drying (oxic) processes.

32

ACS Paragon Plus Environment