Coupled Redox Transformation of Chromate and Arsenite on

Subscriber access provided by SELCUK UNIV

Article

Coupled Redox Transformation of Chromate and Arsenite on Ferrihydrite Elizabeth B. Cerkez, Narayan Bhandari, Richard J Reeder, and Daniel R. Strongin Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es505666w • Publication Date (Web): 06 Feb 2015 Downloaded from http://pubs.acs.org on February 8, 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 35

Environmental Science & Technology

1 2 3

Coupled Redox Transformation of Chromate and Arsenite on

4

Ferrihydrite

5 6

Elizabeth B. Cerkez1, Narayan Bhandari1, Richard J. Reeder2 and Daniel R. Strongin1,*

7

1

Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122

8

2

Department of Geosciences, Stony Brook University, Stony Brook, New York 11794

9 10 11 12 13 14 15 16 17 18 19 20

*

To whom correspondence should be addressed; [email protected]

21 22

1 ACS Paragon Plus Environment


23

Abstract

24

The redox chemistry of chromate (Cr(VI)) and arsenite (As(III)) on the iron

25

oxyhydroxide, ferrihydrite (Fh), was investigated. Attenuated total reflectance Fourier

26

transform infrared spectroscopy (ATR-FTIR), X-ray absorption spectroscopy (XAS) and

27

X-ray photoelectron spectroscopy (XPS) were used to determine the composition of the

28

adsorbed layer on Fh during and after exposure to solution phase Cr(VI) and As(III). The

29

individual exposure of Cr(VI) or As(III) on Fh resulted in the adsorption of the respective

30

species and there was no change in the oxidation state of either species. In contrast,

31

exposure of Fh simultaneously to Cr(VI) and As(III) led to an adsorbed layer that was

32

primarily Cr(III) and As(V). This redox transformation occurred over various

33

experimental conditions, at pH 3, 5 and 7 and in the presence or absence of O2, as

34

demonstrated by in-situ ATR-FTIR results. A similar complete redox transformation was

35

not observed at a solution of pH 9, due to minimal Cr(VI) adsorption. Post-reaction XPS

36

showed that the majority of adsorbed arsenic existed as As(V) at pH 3, 5 and 7, while

37

As(III) was the main species detected at pH 9. At pH 3 the redox chemistry between

38

Cr(VI) and As(III) led to a As(V) product surface loading of ~600 mmol/kg.

39

Experiments performed in the absence of dissolved O2 resulted in less As(V) on the

40

surface compared to experiments in which O2 was present for equivalent reaction times.

41 42 43 44 45


Page 2 of 35

Page 3 of 35

46


1. Introduction

47

The interaction of toxic inorganic contaminants in the environment is becoming

48

an increasingly important area of study as abandoned industrial sites are reclaimed for

49

new uses. The toxicity, mobility and fate of various inorganic contaminants are often

50

influenced by their redox speciation, and hence a microscopic understanding of the

51

critical redox chemical processes would be expected to help in the development of

52

effective remediation strategies.1

53

Two inorganic contaminants relevant to the present contribution are chromium

54

and arsenic. In general, chromium in the environment is typically in the form of trivalent

55

[Cr(III)] and hexavalent [Cr(VI)] species. The toxic hexavalent species occur mainly as

56

the oxyanions chromate, CrO42-, and dichromate, Cr2O72-. Cr(III) forms a sparingly

57

soluble precipitate (Cr(OH)3) in the pH range of ~6-11.5; only below pH 3 does it exist as

58

the Cr+3 ion. At pH values below 9, inorganic arsenite, As(III), mainly occurs as a

59

neutral species (H3AsO3), while arsenate, As(V), generally exists as the oxyanions

60

H2AsO4– (pH 2-6.8) and HAsO4–2 (pH 6.8-12). Unlike Cr compounds, all As species are

61

generally considered toxic to human beings;2, 3 albeit As(III) is more toxic and mobile

62

than As(V).4 In the environment Cr(VI) and As(III) are found in a variety of settings that

63

include soil, groundwater, and industrial wastewater. An analysis of Cr and As speciation

64

at particular acid mine drainage (AMD) sites, for example, has shown significant

65

amounts of Cr(VI) and As(III) in the aqueous phase.5 Additionally, the US

66

Environmental Protection Agency estimates that Cr and As are two major heavy

67

metal(loid)s present in most superfund sites.



Page 4 of 35

68

Significant prior research has investigated the individual redox chemistries of

69

Cr(VI) and As(III) in solution,6-9 as well as the individual adsorption behaviors of Cr(VI)

70

and As(III) on environmentally relevant surfaces that include iron (oxy)hydroxides.10-14

71

Research presented in this contribution builds on prior surface related studies and

72

addresses the redox chemistry of Cr(VI) and As(III) on the environmentally relevant iron

73

oxyhydroxide, ferrihydrite (Fh). The motivation for the current study is two-fold. First, a

74

study of the coadsorption of Cr(VI) and As(III) on environmentally relevant surfaces has

75

potential relevance to the understanding of redox chemistry occurring in environmental

76

settings that contain both contaminants.5,

77

results presented in this contribution shed light on how a substrate can facilitate electron

78

transfer between two redox active species. Composite redox reactions between Cr(VI)

79

and As(III) can be written as

15-17

Second, on a more fundamental level,

80 81

2HCrO4- + 3H3AsO3 + 5H+ → 2CrIII + 3H2AsO4- + 5H2O

(1)

82

2CrO4-2 + 3H3AsO3 + 4H+ → 2CrIII + 3HAsO4-2 + 5H2O

(2)

83 84

The above reactions reflect the changes in species protonation as a function of pH; below

85

pH 6.8 eq. 1 predominantly applies while above pH 6.8 we refer to eq. 2.

86

Research has generally shown that redox chemistry between aqueous Cr(VI) and

87

As(III) in a variety of aqueous environments does not result in the formation of As(V)

88

and Cr(III).18 Recent contributions illustrate that the redox chemistry between the two

89

species in solution can be facilitated in certain circumstances. For example, studies have

90

shown that redox chemistry between aqueous Cr(VI) and As(III) occurs if hydrogen


Page 5 of 35


91

peroxide is present19 or if aqueous Cr(VI) and As(III) are irradiated with UV light.14

92

Other research has shown that the concurrent reduction of Cr(VI) and oxidation of As(III)

93

does occur when the two species are contained within an ice matrix

94

of specific micro-organisms.20 Results from the former study suggested that the

95

concentration of electron donor (i.e., As(III)) and protons in the grain boundaries of ice

96

led to the efficient reduction of Cr(VI).18

18

or in the presence

97

The hypothesis tested in the current research is that co-exposure to a solid surface

98

will facilitate redox chemistry between Cr(VI) and As(III). To test the hypothesis we

99

exposed Fh to a solution containing both aqueous Cr(VI) and As(III). The surface would

100

concentrate, or co-localize, Cr(VI) and As(III) complexes through the adsorption process

101

that would then facilitate electron and/or proton transfer. Oxidation states of the adsorbed

102

species before, during, and after exposure to Cr(VI) and As(III) were determined with in

103

situ attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), X-

104

ray absorption spectroscopy (XAS), and X-ray photoelectron spectroscopy (XPS). We

105

analyzed the effect of the order of As(III) and Cr(VI) exposure to the Fh surface and the

106

effect of O2 on the system to help elucidate the mechanism by which the coupled

107

oxidation-reduction occurs. Our experimental observations showed that the exposure of

108

Fh to an aqueous mixture of Cr(VI) and As(III) resulted in the reduction of Cr(VI) to

109

Cr(III) and the oxidation of As(III) to As(V) in both anoxic and oxic environments on the

110

Fh surface.

111 112

2. Experimental Section



113

2.1 Synthesis and characterization of materials: 2-line Fh was prepared using a

114

modified

115

area measurements of synthesized Fh yielded an average value of 330 ± 20 m2/g, which

116

agrees well with the literature.24 Transmission electron microscopy (TEM) analysis of the

117

sample was also consistent with properties expected for 2-line Fh [see supporting

118

information (SI), Fig. S1].24, 25

21, 22

version of a method developed by Cornell and Schwertmann.23 Surface

119

Crystalline aluminum hydroxide (bayerite) used in this study was prepared by

120

neutralizing an Al(III)-bearing solution (resulting from AlCl3 addition) with NaOH26

121

Briefly, 1 M NaOH was added step-wise to a 0.1 M solution of AlCl3 (initial pH of ~3)

122

with constant stirring until the pH reached ~7, at which point a white particle suspension

123

was observed. The suspension was dialyzed with de-ionized water (18 MΩ cm-1) for 4-5

124

days to remove counter ions, then centrifuged and air-dried. XRD characterization

125

showed the material to be primarily bayerite (95%) and a minority amount of the gibbsite

126

phase26. The surface area of the material was determined to be 120 ± 1 m2/g.

127 128

2.2 Batch studies: The methodology by which the batch reactions were carried out in this

129

study was similar to that used in prior publications from our laboratory that investigated

130

the redox chemistry of As(III) on iron oxyhydroxides.27, 28 In brief, 50 mg of dry solid

131

powder, Fh, was suspended in 199 mL of 2.5 mM NaCl solution and then sonicated for 5-

132

10 minutes to disperse the solid phase into solution. An appropriate amount of As(III)

133

and Cr(VI) solutions (from individual 50 mM stock solutions prepared from NaAsO2 and

134

K2Cr2O7) were added to the suspension and the pH was maintained at a desired value

135

using an autotitrator (718 STAT Titrino, Metrohm). Experiments were performed in


Page 6 of 35

Page 7 of 35


136

solutions that were exposed to the ambient atmosphere and hence contained dissolved O2

137

and CO2, unless otherwise noted. Other experiments were performed in a glove bag with

138

an O2 level below 0.10 mg/L, additionally the suspensions were purged with argon gas

139

for 1 hr to exclude dissolved O2..

140 141

2.3 Solution Analysis: Collected samples were centrifuged and filtered through a

142

Millipore filter (0.2 µm). Aqueous phase Cr(VI) and As(V) were analyzed using an ion

143

chromatograph (IC, Dionex ICS-1000) with a detection limit for each species of ~5 µM

144

and ~2 µM, respectively. The IC was equipped with a Dionex IonPac®AS22 (4 mm × 250

145

mm) analytical column and a conductivity detector. Details regarding the analytical

146

methodology for As(III) and As(V) species determination were described previously.27

147

All the chemicals used in this study, including sodium arsenite (NaAsO2), ferric chloride

148

(FeCl3), sodium hydroxide (NaOH), hydrochloric acid (HCl), aluminum chloride (AlCl3),

149

and potassium dichromate (K2Cr2O7), were obtained from Sigma Aldrich (analytical

150

grade). All batch reactions were conducted in triplicate and calculated standard errors of

151

estimation were within 2-7 %.

152

2.4 X-ray absorption spectroscopy (XAS):

153

adsorbed on Fh were determined using As and Cr K-edge X-ray absorption near-edge

154

structure (XANES) spectroscopy. As discussed in a previous contribution29, prior

155

XANES analyses of As-reacted iron (oxy)hydroxides revealed that partial oxidation of

156

As(III) was possible upon exposure to the synchrotron beam over the time scale needed

157

to collect a conventional near-edge spectrum. To avoid this experimental artifact we have

158

taken advantage of rapid data collection using quick-scanning X-ray absorption

Oxidation states of As and Cr species



159

spectroscopy (Q-XAS) as implemented at beamline X18B at the National Synchrotron

160

Light Source (Brookhaven National Laboratory). The Q-XAS technique developed at

161

X18B has been described previously in detail.30, 31 The monochromator was calibrated by

162

assigning the first peak in the derivative spectrum of the aqueous As(III) reference

163

sample an energy value of 11867 eV.

164

Cr K-edge XANES data, which were found not to be subject to such artifact, were

165

collected by conventional edge-scan methods. Monochromator calibration was achieved

166

by assigning the first peak in the derivative spectrum of a Cr metal foil to 5989 eV. At

167

least two scans were averaged to improve signal/noise. Spectra from sorption samples

168

(As and Cr) were collected in fluorescence mode using a partially implanted planar

169

silicon detector. Spectra for reference samples were collected before and after the

170

sorption samples to confirm no change in monochromator calibration. A linear pre-edge

171

background was subtracted, and the XANES spectra were normalized using a single post-

172

edge point (11915 eV for As; 6075 eV for Cr) as described in a previous study. 27

173

Sorption samples were prepared for analysis by centrifuging the reacted

174

suspension. The collected solid was washed once with corresponding pH deionized

175

water, dried in air, loaded into Lucite sample holders, and sealed with Kapton tape.

176 177

2.5 X-ray photoelectron spectroscopy (XPS): Oxidation states of As and Cr species

178

adsorbed on Fh were also determined using a VG Scientific XPS, with a Mg Kα X-ray

179

source operating at 280 W, 15 kV x 25 mA. Pass energy of 30 eV was used for all

180

spectra. The Fe 2p and 3p peak positions were used to eliminate static charge effects and

181

all spectra were analyzed using CasaXPS software. Elements present were identified by


Page 8 of 35

Page 9 of 35


182

their strongest binding energy peak, Fe 2p (707 eV), As 3d (42 eV), and Cr 2p (572eV).

183

XPS normalized peak intensities were calibrated to the surface loading of Cr and As

184

species, determined by measuring the amount of Cr(VI) or As(III) from solution for a

185

given mass of Fh (known surface area). The raw As and Cr peak areas were normalized

186

against the Fe 3p peak area to account for changes in sample position and/or the amount

187

of material loaded into the spectrometer. Furthermore, As 3d spectra were processed by

188

subtracting XPS spectra for clean Fh (no adsorbate), obtained for the same binding

189

energy window.

190 191

2.6 Attenuated total reflectance Fourier transform infrared (ATR-FTIR): The

192

experimental apparatus and methodology were the same as described in prior

193

publications.27, 28 Briefly, a thin film of Fh ( .05 mg) was prepared on a diamond ATR

194

element by depositing a suspension of Fh in H2O and then dried under an argon

195

environment. The sample was then enclosed by a Teflon flow cell which allowed the

196

passage of a specified solution over the sample. Films were first exposed to H2O at the

197

desired pH to equilibrate the film. Most experiments then involved flowing a solution

198

containing 0.10 mM Cr(VI) and 0.15 mM As(III) at the selected pH over the sample at a

199

rate of 1 mL/min. For experiments where O2 was excluded, the Fh film was dried under

200

Ar and sealed to outside air within an Ar purged glove bag. The solutions introduced to

201

the flow cell were purged with Ar for 1 hr to remove dissolved O2.

202

Some ATR-FTIR spectra were fitted to determine the relative amount of different

203

adsorbed species as function of time and pH. Spectral fitting was carried out using Origin

204

Lab software. Individually adsorbed Cr(VI), As(III) and As(V) on Fh were first obtained 9 ACS Paragon Plus Environment


Page 10 of 35

205

as a function of pH. The spectra associated with the individually adsorbed species were

206

fitted to achieve a R2 of 0.99 at each pH, and our spectra agree well with vibrational data

207

reported for the same three adsorbates in prior literature.13,

208

selected peak fits of Cr(VI), As(III) and As(V) at pH 5.] These individual fits were then

209

used together to fit experimental data where one or all three species were present on Fh.

210

When fitting experimental data associated with the exposure of Fh to As(III) and Cr(VI),

211

only the peak areas of the individual synthesized spectra associated with Cr(VI), As(V),

212

and As(III) were allowed to vary (energy positions were fixed). Typical fitted

213

experimental spectra containing contributions from multiple adsorbed species had a R2 of

214

0.99.

32, 33

[see SI, Fig. S2 for

215 216

3. Results and Discussion

217

3.1 ATR-FTIR studies of As(III) and Cr(VI) on Fh

218

3.1.1 Effect of pH with Simultaneous Exposure

219

Figure 1 displays ATR-FTIR data obtained when Fh was exposed to a flowing

220

solution containing 0.10 mM Cr(VI) and 0.15 mM As(III) at pH 3, 5, 7 and 9 (A, B, C

221

and D, respectively) as a function of time. Each data set includes a representative

222

spectrum that has been fitted with spectral components for Cr(VI), As(V), and As(III)

223

(see section 2.6 for method). Data obtained for pH 3, 5, and 7 show similar behavior

224

where the vibrational modes associated with Cr(VI) grow for a period of time, reach a

225

maximum value (depending on the pH) and then show a continuous decrease with time (a

226

complete set of curves for pH 3 showing an isosbestic point is provided in SI, Fig. S3).

227

We attribute the increase in the intensity of the Cr(VI) associated modes at early reaction


Page 11 of 35


228

times to its adsorption rate34 on Fh being faster than its consumption via redox reactions

229

with As(III). In contrast to Cr(VI), the vibrational mode intensities associated with As(V)

230

show a monotonic rise with time. Vibrational modes attributable to adsorbed As(III) are

231

absent in the pH 3 data set, but are present (albeit weak) in the pH 5 and 7 data sets. To

232

support these statements, selected time points are fitted with Cr(VI), As(III), and As(V)

233

for pH 3, 5 and 7, are shown in SI Fig. S4 and furthermore plots of fitted peak areas (as a

234

function of time) for the individual Cr(VI), As(III) and As(V) contributions to each

235

experimental spectrum, at each pH, are shown in SI Fig. S5.

236

ATR-FTIR spectra associated with the simultaneous exposure of Fh to As(III) and

237

Cr(VI) at pH 9 also show an increasing Cr(VI) vibrational mode intensity, weak

238

compared to other pH conditions, at early reaction times and a loss of these modes at later

239

reaction times. Unlike the lower pH data, the dominant As-derived modes are due to the

240

presence of As(III) and only weak spectral weight is present that can be assigned to

241

As(V) product. A comparison of the vibrational mode absorbance associated with

242

adsorbed Cr(VI) at early reaction times (i.e., 10 minutes: spectra c in each pH data set in

243

Fig. 1) for the different pH data sets shows a decreasing Cr(VI) intensity/absorbance as

244

the solution pH was raised from 3 to 9. This experimental observation is consistent with

245

expected increased electrostatic repulsion between HCrO4-/CrO42- and Fh (pzc of 8.2)35 as

246

the pH is raised from 3 to 9. Based on the fitted vibrational spectra, the binding

247

geometries of the three adsorbates, Cr(VI) (bidentate at low pH, monodentate at high

248

pH13), As(III) (monodentate and outersphere33), and As(V) (primarily bidentate

249

bridging36), are similar whether adsorbed independently or coadsorbed.



250

ATR-FTIR data to this point show that As(V) is produced on the surface when

251

Cr(VI) and As(III) are exposed to Fh. Also, the results so far suggest that there is an

252

increasing amount of As(III) oxidation with decreasing pH based on the amount of

253

spectral weight attributable to As(V). We expect that both the increasing Cr(VI)

254

adsorbate concentration and increasing proton concentration with decreasing pH would

255

facilitate the redox reaction (see eqn. 1 and 2).

256

oxidation occurs we note two other experimental observations: 1) the individual

257

adsorption of As(III) on Fh does not result in As(V) formation,27 and 2) the homogeneous

258

solution reaction (in the absence of Fh) between Cr(VI) and As(III) does not result in

259

As(V) formation at any of the solution pH conditions used in this study (see SI, Fig. S6).

260

Therefore, the concomitant reduction of Cr(VI) to Cr(III) would be consistent with the

261

loss of vibrational modes attributable to Cr(VI) at the later reaction times and hence the

262

cause of As(III) oxidation. Due to lack of Cr(III) active vibrational modes, in the

263

vibrational region investigated, the reduction cannot be confirmed by ATR-FTIR. The

264

oxidation state of the Cr product is characterized with XANES and XPS analysis below.

265 266

In order to help determine how As(V)

. 3.1.2 Sequential Exposure of Fh to As(III) and Cr(VI)

267

Further insight into the redox chemistry was obtained by investigating how the

268

production of As(V) was affected by the sequence of exposure of Fh to As(III) and

269

Cr(VI). Experiments were carried out that individually exposed Fh to flowing 0.15 mM

270

As(III) solution or 0.10 mM Cr(VI) solution for 3 h at pH 5. After this initial 3 h

271

exposure, the As(III) or Cr(VI) solution was flushed from the reaction cell with pH 5

272

water. The As(III)/Fh and Cr(VI)/Fh systems prepared in this way were then exposed to


Page 12 of 35

Page 13 of 35


273

flowing 0.10 mM Cr(VI) and 0.15 mM As(III) solution, respectively. ATR-FTIR data

274

were collected for both these exposure scenarios (Fig. 2A and 2B). Figure 2A shows that

275

the exposure of As(III)/Fh to Cr(VI) led to increases in both Cr(VI) and As(V) vibrational

276

modes. Data presented in Fig. 2B show that the exposure of Cr(VI)/Fh to a flowing

277

solution of As(III) led to a decrease in modes attributable to Cr(VI) and an increase in

278

modes associated with As(V) product. Although some Cr(VI) loss may be due to

279

desorption, the increase of As(V) modes suggests that the electron reduction of Cr(VI)

280

contributes to its loss in surface concentration. Comparing the two exposure scenarios,

281

the Cr(VI)/Fh case produces more As(V) on the surface compared to As(III)/Fh (See SI,

282

Fig. S7). We note that in both exposure scenarios, at the end of reaction, indicated by no

283

further change in mode intensity, we observe that Cr(VI), As(III), and As(V) all coexist

284

on the surface. This result suggests that only a fraction of Cr(VI) and As(III) on Fh react

285

to form As(V).

286 287

3.1.3 Effect of O2 with Simultaneous Exposure

288

The effect of O2 on the redox chemistry between As(III) and Cr(VI) was

289

investigated at pH 5 (Fig. 3). In the anoxic case we observe the increase of Cr(VI)

290

vibrational modes reaching a maximum, followed by a continuous decrease in intensity,

291

similar to that observed in the oxic case (Fig. 1, B). Also comparable is the observation of

292

the continuous increase of As(V) vibrational modes. A comparison of the As(V) peak

293

area derived from fitted spectra (Fig. 3 inset), however, shows that less As(V) product

294

forms after 3 hours of reaction in the absence of dissolved O2 (i.e., anoxic case). This



295

particular analysis suggests that the presence of dissolved O2 leads to more As(III)

296

oxidation in the presence of Cr(VI) and Fh than in the absence of dissolved O2.

297 298

3.2 Characterization of the adsorbed layer on Fh with XANES and XPS

299

3.2.1 Surface Analysis of Batch Studies (XANES and XPS)

300

XANES spectroscopy was carried out to determine the oxidation states of Cr and

301

As on Fh that had been exposed to a solution containing 0.10 mM Cr(VI) and 0.15 mM

302

As(III) at pH 5. Based on the decrease in aqueous concentrations of Cr(VI) and As(III)

303

the loading of Cr and As on Fh was 310 and 610 mmol/kg, respectively, and we note that

304

in all batch experiments completed that no As(V) was detected in the aqueous phase

305

before or after reaction. Figure 4(a) compares Cr K-edge XANES spectra obtained for

306

this sample to Fh that was individually exposed to aqueous Cr(III) and Cr(VI). A

307

prominent pre-edge feature at ~5993 eV is present in the XANES data for Fh that had

308

been exposed to Cr(VI). This pre-edge feature is indicative of the presence of

309

tetrahedrally coordinated Cr(VI) and consistent with chromate adsorbed on the surface.37

310

The feature is absent in the spectrum associated with Fh that had been exposed to

311

solution containing both Cr(VI) and As(III), and the spectrum is instead similar to the

312

Cr(III)/Fh reference spectrum. These data indicate that Cr(VI) is reduced to Cr(III) on Fh

313

in the presence of As(III), as suggested by ATR-FTIR experimentation. Figure 4(b)

314

exhibits complementary As K-edge XANES data collected for Fh that was exposed to a

315

mixture of 0.10 mM Cr(VI) and 0.15 mM As(III) and for individual controls (either

316

As(III) or As(V) adsorbed individually on Fh). XANES data associated with all of the Fh

317

samples exhibit well-resolved As(III) or As(V) edge structures.38 A least-squares linear


Page 14 of 35

Page 15 of 35


318

combination fit of the data, using reference spectra for As(III) and As(V) adsorbed

319

individually on Fh, shows that As(III) and As(V) were both present on the Fh sample that

320

was simultaneously exposed to Cr(VI) and As(III), with the majority As species being

321

As(V) (80 ± 5 %) at pH 5. While the surface As complex is not exclusively As(V), the

322

results do support our interpretation of the ATR-FTIR data that the exposure of Fh to

323

both Cr(VI) and As(III) results in the conversion of As(III) to As(V). The XANES data

324

also show that this redox chemistry occurs together with the conversion of Cr(VI) to

325

Cr(III).

326

While our XANES results are limited to the pH 5 oxic experiment, we

327

investigated with XPS the oxidation state of adsorbed Cr and As after Fh was exposed to

328

solution phase As(III) and Cr(VI) at pH 3, 5, 7, and 9. We provide these spectra in the

329

supporting information (SI, Fig S8, Fig S9) and make oxidation state assignments based

330

on comparison to standards adsorbed on Fh. With regard to the oxidation state of

331

adsorbed As, the fitted XPS spectra in general show that there is an increase in the

332

relative proportion of As(V) as the pH of the solution decreases. The fitted pH 9 spectrum

333

shows almost entirely As(III) (>90%) while at pH 3 the adsorbed As exists primarily as

334

As(V). This particular result is consistent with our ATR-FTIR results (Figure 1A) at this

335

pH that show no significant As(III) mode intensity. XPS also suggests that Cr(III) is

336

present on the surface of Fh after exposure to Cr(VI) and As(III) at pH 3, 5, and 7, but the

337

data also contain spectral weight that is attributable to Cr(VI) (SI, Fig. S9). We do not

338

detect Cr(III) with XPS in the pH 9 reaction product, but this is likely due to the

339

concentration of adsorbed Cr being below our detection limit. The loading of Cr on Fh at

340

pH 9 was calculated to be 5 mmol/kg based on an analysis of Cr(VI) loss from solution.



341

As mentioned before, the ATR-FTIR experiments carried out at pH 9 (Figure 1D) also

342

suggest that the amount of Cr(VI) adsorption is significantly less than at lower pH

343

conditions investigated in this study.

344

XPS data also allow us to compare the relative proportion of As(V) and As(III) on

345

Fh after exposure to 0.10 mM Cr(VI) and 0.15 mM As(III) at pH 5 under both oxic and

346

anoxic conditions. In both these cases the loading of Cr(VI) and As(III) was 310 and 610

347

mmol/kg, respectively. An analysis of the data shows that in the anoxic circumstance ~52

348

% of the adsorbed As on Fh is As(V), while the relative proportion of As(V) is ∼80%

349

(similar to the XANES result) under oxic conditions. These results also support our

350

conclusions from in situ ATR-FTIR results that suggest more As(V) product is formed on

351

Fh in the oxic circumstance (Fig. 3). Finally, we mention that pH 3 XPS data shows

352

exclusively As(V) on Fh after reaction (consistent with ATR-FTIR). The As-loading on

353

the particles was 610 mmol/kg, based on the amount of As(III) that was removed from

354

solution in the presence of Fh. Hence, we estimate that in the pH 3 circumstance the

355

As(V) loading after reaction is also 610 mmol/kg, since we do not detect any aqueous

356

As(V) product (detection limit of 5µM).

357 358

3.2.2 Characterization of Cr(III) product

359

We carried out studies to characterize the nature of the Cr(III) product formed

360

during the Cr(VI) and As(III) redox chemistry on Fh. At no point was aqueous phase

361

Cr(III) experimentally observed during any of the experiments. This experimental

362

observation suggests that the majority of Cr(III) product was present on the iron

363

oxyhydroxide surface, possibly as a surface complex or as a Cr(III), Cr(III)-Fe or Cr(III)-


Page 16 of 35

Page 17 of 35


364

As bearing secondary phase. To evaluate these possibilities, XRD was carried out on

365

post-reaction samples (see SI, Fig. S10). The X-ray diffraction data showed no

366

experimentally resolvable differences between pure Fh and the reaction product after

367

exposure to Cr(VI) and As(III). To further understand the nature of the surface Cr(III) we

368

compared Cr XANES data from our reaction products to selected Cr(III) reference

369

phases, including chromite (FeCr2O4), a Cr/Fh co-precipitated sample, and amorphous

370

chromium hydroxide [Cr(III)(OH)3]. The latter two spectra were taken from the study by

371

Tang et al. 37 By comparison of the first derivatives of the relevant XANES spectra (See

372

SI, Fig. S11) the Fh co-exposed to Cr(VI) and As(III) is found to be nearly identical to

373

that of amorphous Cr(III) hydroxide, and slightly different than Cr(III) coprecipitated

374

with Fh.37 Finally, analysis of the full EXAFS data did not support the presence of a

375

Cr(III)-arsenate phase, although we cannot entirely rule out its possible formation.

376 377

3.3 Reaction mechanism

378

3.3.1 General Reaction Mechanism

379

Our experimental observations show that Cr(III) and As(V) product form when

380

Fh is exposed to a solution containing both Cr(VI) and As(III) or if one reactant species is

381

adsorbed (Cr(VI) or As(III)) and then exposed to a solution containing only the other

382

reactant as an aqueous species (i.e., As(III) or Cr(VI), respectively). These experimental

383

observations suggest that the surface is providing an energetically favorable pathway for

384

electron transfer between Cr(VI) and As(III) that cannot be achieved when both species

385

interact in the aqueous phase (in the absence of the solid substrate). Fh likely acts to

386

concentrate one or both of the reactants (and intermediate species) in binding geometries



Page 18 of 35

387

that make the multi-electron transfer process between Cr(VI) and As(III) energetically

388

favorable. We emphasize, however, that our experimental results do not allow us to

389

determine whether both Cr(VI) and As(III) need to be adsorbed to produce As(V)

390

product. It is perhaps useful to point out that prior aqueous-based studies have suggested

391

that the reaction of As(III), which is considered to be a two-electron reductant, and

392

Cr(VI) to form As(V) and Cr(III) product would involve the generation of Cr(IV)

393

intermediate species. Comproportionation of Cr(IV) species would form Cr(V) and the

394

reduction of this latter species by As(III) would lead to Cr(III) product: 39, 40

395 396

Cr(VI) + As(III) → Cr(IV) + As(V)

(3)

397

Cr(VI) + Cr(IV) → 2Cr(V)

(4)

398

2Cr(IV) → Cr(III) + Cr(V)

(5)

399

Cr(V) + As(III) → Cr(III) + As(V)

(6)

400 401

Such a mechanism might be expected to be facilitated by the co-localization of Cr and As

402

species, presumably through their close proximity on a solid surface. Our experimental

403

observations also show that dissolved oxygen, when present, increases the amount of

404

As(V) product. At this time it is not possible to unequivocally determine the role of O2.

405

However, based on prior studies, a possible enhancement in As(V) production could

406

conceivably be due to the interaction of dissolved O2 with intermediate Cr-41 and/or As-

407

19, 33

valence state species.

408

The surface facilitated redox chemistry may be also attributable to potential

409

changes in the redox potentials of Cr(VI) (and intermediate species) and/or As(III) upon


Page 19 of 35


410

adsorption (compared to the respective Cr(VI) and As(III) aqueous phase species). While

411

we cannot test this possibility in the present research, results from prior studies may be

412

taken to be consistent with such a possibility. With regard to Cr(VI), prior research has

413

shown that adsorbed Cr(VI) on Fe, Al, and Ti oxides demonstrates a much more facile

414

reduction to Cr(III) by organic reductants than does aqueous Cr(VI).42, 43 In a different

415

study, reduction of U(IV) by Fe(II), which was only observed when the species were

416

adsorbed on a hematite surface, was attributed to the change in redox potentials of

417

adsorbed species.44 While the relevance of these prior studies to ours is limited, they do

418

suggest that adsorbed oxidizing species (in our case Cr(VI)) can be more effective

419

oxidizers than their solution phase counterparts in environmental redox reactions.

420 421

3.3.2 The Role of the Electronic Properties of the Solid Substrate

422

Prior research has suggested that small band gap semiconductors, such as Fh, can

423

act as conduits for electron transfer between a donor and acceptor.45, 46 To test whether

424

electron transfer between Cr(VI) and As(III) on Fh requires bulk conduction, we

425

conducted additional experiments where we exposed dielectric nano-dimensioned

426

Al(OH)3 (mixture of bayerite and gibbsite) to Cr(VI) and As(III) under similar conditions

427

to those described here. Post analysis of the surface composition after reaction by

428

XANES (SI, Fig. S12) shows that Cr(III) and As(V) are present, confirming that Cr(VI)

429

is reduced to Cr(III) concomitant with As(III) oxidation to As(V), analogous to our

430

findings for Fh. Hence, exposure of As(III) and Cr(VI) to Al(OH)3 is thought to be the

431

primary factor allowing the experimentally observed redox chemistry between Cr(VI)

432

and As(III). These results suggest that the electrical properties, i.e., positions of



433

conduction and valence bands, do not have a primary effect on the ability of the surface

434

to aide in the coupled redox of Cr(VI) and As(III) under our experimental conditions.

435

However, future kinetic studies are planned that more carefully compare the rate of

436

reaction on well-defined semiconductor and dielectric surfaces to determine whether

437

electron transport in the substrate bulk/surface plays a significant role.

438

In closing, our results demonstrate that the exposure of high surface area Fh to

439

Cr(VI) and As(III) results in the formation of the more immobile Cr(III) and As(V)

440

products. Hence, it is conceivable that the addition of high surface area metal

441

(oxy)hydroxide to contaminated water environments might be a useful remediation

442

strategy, resulting in contaminant immobilization by adsorption, as well as through redox

443

chemistry. Perhaps more importantly, the results and interpretations presented in this

444

contribution add to our understanding of the fundamental principles that control redox

445

chemistry at the mineral-water interface in environmental settings outside of the

446

remediation framework.

447 448

5.0 Acknowledgements

449

This work was supported by a National Science Foundation (NSF) Collaborative

450

Research in chemistry grant (CHE-0714121). We also thank Syed Khalid and Hyuck

451

Hur for assistance with XAS data collection. Use of the National Synchrotron Light

452

Source, Brookhaven National Laboratory, was supported by the U.S. Department of

453

Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-

454

AC02-98CH10886.

455


Page 20 of 35

Page 21 of 35


456 457 458 459

Supporting Information

460

The following is included in the Supporting Information. TEM image of synthetic Fh,

461

ATR-FTIR of Cr(VI), As(V) and As(III) individually adsorbed on Fh at pH 5, ATR-FTIR

462

Cr(VI) and As(III) adsorbed on Fh at pH 3, ATR-FTIR fitted spectra of selected time

463

points for pH 3, 5 and 7, ATR-FTIR peak areas of Cr(VI), As(V) and As(III) at pH 3, 5,

464

and 7 versus time, Cr(VI) and As(III) concentration data of aqueous reaction without

465

ferrihydrite, ATR-FTIR peak areas of Cr(VI), As(V) and As(III) at pH 5 versus time

466

comparing order of exposure, XPS of As 3d region for batch reactions, XPS of Cr 2p

467

region for batch reactions, XRD patterns of reaction products, 1st derivative Cr XANES

468

data, XANES of Cr and As from pH 5 batch reaction with Al(OH)3, and XPS of As 3d

469

and Cr 2p regions for As(III)/Fh reacted with aqueous Cr(VI). This material is available

470

free of charge via the Internet at http://pubs.acs.org.

471 472 473 474 475 476 477 478 21 ACS Paragon Plus Environment


479

6.0 References

480

1.

481

Voegelin, A.; Campbell, K., Biogeochemical redox processes and their impact on

482

contaminant dynamics. Environ. Sci. Technol. 2010, 44, (1), 15-23.

483

2.

484

2002, 296, (5576), 2143, 2145.

485

3.

486

distribution of arsenic in natural waters. Appl. Geochem. 2002, 17, (5), 517-568.

487

4.

488

Environ. Control 1991, 21, (1), 1-39.

489

5.

490

C.; Potin-Gautier, M.; Leblanc, M.; Elbaz-Poulichet, F.; Donard, O. F. X.,

491

Biogeochemical cycle and speciation of As and Cr in an acid mine environment: the case

492

of Carnoules Creek, France. Journal de Physique IV: Proceedings 2003, 107, (XIIth

493

International Conference on Heavy Metals in the Environment, 2003, Volume 2), 735-

494

738.

495

6.

496

Sci. Technol. 1996, 30, (5), 1614-17.

497

7.

498

hydrogen peroxide: pH-dependent formation of oxidants in the fenton reaction. Environ.

499

Sci. Technol. 2003, 37, (12), 2734-2742.

Borch, T.; Kretzschmar, R.; Kappler, A.; Van Cappellen, P.; Ginder-Vogel, M.;

Nordstrom, D. K., Worldwide occurrences of arsenic in ground water. Science

Smedley, P. L.; Kinniburgh, D. G., A review of the source, behaviour and

Korte, N. E.; Fernando, Q., A review of arsenic (III) in groundwater. Crit. Rev.

Le Hecho, I.; Pecheyran, C.; Charles, S.; Monperrus, M.; Pavageau, M. P.; Casiot,

Fendorf, S. E.; Li, G., Kinetics of Chromate Reduction by Ferrous Iron. Environ.

Hug, S. J.; Leupin, O., Iron-catalyzed oxidation of arsenic(III) by oxygen and by


Page 22 of 35

Page 23 of 35


500

8.

Owlad, M.; Aroua, M. K.; Daud, W. A. W.; Baroutian, S., Removal of hexavalent

501

chromium-contaminated water and wastewater: A review. Water, Air, Soil Pollut. 2009,

502

200, (1-4), 59-77.

503

9.

504

electrochemical and biological methods for aqueous Cr(VI) reduction. J. Hazard. Mater.

505

2012, 223-224, 1-12.

506

10.

507

ferrihydrite and crystalline FeOOH: re-evaluation of EXAFS results and topological

508

factors in predicting sorbate geometry, and evidence for monodentate complexes.

509

Geochim. Cosmochim. Acta 1995, 59, (17), 3655-61.

510

11.

511

retention mechanisms on goethite. 1. surface structure. Environ. Sci. Technol. 1997, 31,

512

(2), 315-320.

513

12.

514

oxides evaluated using macroscopic measurements, vibrational spectroscopy, and surface

515

complexation modeling. J. Colloid Interface Sci. 2001, 234, (1), 204-216.

516

13.

517

ferrihydrite by in situ ATR-FTIR spectroscopy and theoretical frequency calculations.

518

Environ. Sci. Technol. 2012, 46, (11), 5851-5858.

519

14.

520

Equilibria, kinetics, and spectroscopic analyses on the uptake of aqueous arsenite by two-

521

line ferrihydrite. Environmental Technology 2014, 35, (3), 251-261.

Barrera-Diaz, C. E.; Lugo-Lugo, V.; Bilyeu, B., A review of chemical,

Waychunas, G. A.; Davis, J. A.; Fuller, C. C., Geometry of sorbed arsenate on

Fendorf, S.; Eick, M. J.; Grossl, P.; Sparks, D. L., Arsenate and chromate

Goldberg, S.; Johnston, C. T., Mechanisms of arsenic adsorption on amorphous

Johnston, C. P.; Chrysochoou, M., Investigation of chromate coordination on

Kim, S.-O.; Chun Lee, W.; Goo Cho, H.; Lee, B.-T.; Lee, P.-K.; Hee Choi, S.,



Regenspurg,

S.;

Peiffer,

S.,

Arsenate and

chromate incorporation

Page 24 of 35

522

15.

in

523

schwertmannite. Appl. Geochem. 2005, 20, (6), 1226-1239.

524

16.

525

activated carbon. J. Hazard. Mater. 2008, 159, (2-3), 380-384.

526

17.

527

contaminated with wood preservation chemicals. Soil Sediment Contam. 2010, 19, (2),

528

142-159.

529

18.

530

ice. Environ. Sci. Technol. 2011, 45, (6), 2202-2208.

531

19.

532

of Chromium(VI) and Arsenic(III) under Acidic Conditions. Environmental Science &

533

Technology 2013, 47, (12), 6486-6492.

534

20.

535

reduction of Cr(VI) and oxidation of As(III) by Bacillus firmus TE7 isolated from

536

tannery effluent. Chemosphere 2013, 90, (8), 2273-2278.

537

21.

538

R., Ferrihydrite reactivity toward carbon dioxide. J. Colloid Interface Sci. 2009, 337, (2),

539

492-500.

540

22.

541

Chupas, P. J.; Middlemiss, D. S.; Grey, C. P.; Parise, J. B., Investigation of surface

542

structures by powder diffraction: a differential pair distribution function study on arsenate

543

sorption on ferrihydrite. Inorg. Chem. 2010, 49, (1), 325-330.

Wu, Y.; Ma, X.; Feng, M.; Liu, M., Behavior of chromium and arsenic on

Amofah, L. R.; Maurice, C.; Bhattacharya, P., Extraction of arsenic from soils

Kim, K.-T.; Choi, W.-Y., Enhanced redox conversion of chromate and arsenite in

Wang, Z.; Bush, R. T.; Sullivan, L. A.; Liu, J., Simultaneous Redox Conversion

Bachate, S. P.; Nandre, V. S.; Ghatpande, N. S.; Kodam, K. M., Simultaneous

Hausner, D. B.; Bhandari, N.; Pierre-Louis, A.-M.; Kubicki, J. D.; Strongin, D.

Harrington, R.; Hausner, D. B.; Bhandari, N.; Strongin, D. R.; Chapman, K. W.;


Page 25 of 35


544

23.

Cornell, R. M.; Schwertmann, U., The iron oxides: structure, properties, reactions,

545

occurerences and uses, 2nd ed. Wiley-VCH: Weinheim 2003.

546

24.

547

D. R.; Schoonen, M. A. A.; Phillips, B. L.; Parise, J. B., The structure of ferrihydrite, a

548

nanocrystalline material. Science 2007, 316, (5832), 1726-1729.

549

25.

550

E.; Strongin, D. R.; Parise, J. B., Neutron Pair Distribution Function study of two-line

551

ferrihydrite. Environ. Sci. Technol. 2011, 45, (23), 9883-9890.

552

26.

553

D.; Strongin, D., Adsorption of carbon dioxide on Al/Fe oxyhydroxide. Journal of

554

Colloid and Interface Science 2013, 400, (0), 1-10.

555

27.

556

arsenate on ferrihydrite. Environ. Sci. Technol. 2011, 45, (7), 2783-2789.

557

28.

558

Photodissolution of ferrihydrite in the presence of oxalic acid: An in situ ATR-FTIR/DFT

559

study. Langmuir 2010, 26, (21), 16246-53.

560

29.

561

arsenate in the presence of goethite. Environ. Sci. Technol. 2012, 46, (15), 8044-8051.

562

30.

563

Wang, Q.; Frenkel, A. I.; Marinkovic, N.; Hould, N.; Ginder-Vogel, M.; Landrot, G. L.;

564

Sparks, D. L.; Ganjoo, A., Quick extended x-ray absorption fine structure instrument with

565

millisecond time scale, optimized for in situ applications. Rev. Sci. Instrum. 2010, 81, (1),

566

015105/1-015105/7.

Michel, F. M.; Ehm, L.; Antao, S. M.; Lee, P. L.; Chupas, P. J.; Liu, G.; Strongin,

Harrington, R.; Hausner, D. B.; Xu, W.; Bhandari, N.; Michel, F. M.; Brown, G.

Pierre-Louis, A.-M.; Hausner, D. B.; Bhandari, N.; Li, W.; Kim, J.; Kubicki, J.

Bhandari, N.; Reeder, R. J.; Strongin, D. R., Photoinduced oxidation of arsenite to

Bhandari, N.; Hausner Douglas, B.; Kubicki James, D.; Strongin Daniel, R.,

Bhandari, N.; Reeder, R. J.; Strongin, D. R., Photoinduced oxidation of arsenite to

Khalid, S.; Caliebe, W.; Siddons, P.; So, I.; Clay, B.; Lenhard, T.; Hanson, J.;



567

31.

Ginder-Vogel, M.; Landrot, G.; Fischel, J. S.; Sparks, D. L., Quantification of

568

rapid environmental redox processes with quick-scanning x-ray absorption spectroscopy

569

(Q-XAS). Proc. Natl. Acad. Sci. U. S. A. 2009, 106, (38), 16124-16128.

570

32.

571

evidence for specific adsorption of chromate on hydrous iron oxide. Chemosphere 1993,

572

26, (10), 1897-904.

573

33.

574

on the surface of ferrihydrite: an in situ ATR-FTIR study. Environ. Sci. Technol. 2003,

575

37, (5), 972-978.

576

34.

577

and Chromate Retention Mechanisms on Goethite. 2. Kinetic Evaluation Using a

578

Pressure-Jump Relaxation Technique. Environmental Science & Technology 1997, 31,

579

(2), 321-326.

580

35.

581

Cosmochimica Acta 2013, 105, (0), 316-325.

582

36.

583

ferrihydrite: Part 1. EXAFS studies of the geometry of coprecipitated and adsorbed

584

arsenate. Geochim. Cosmochim. Acta 1993, 57, (10), 2251-69.

585

37.

586

Structural properties of the Cr(III)-Fe(III) (oxy)hydroxide compositional series: insights

587

for a nanomaterial "solid solution". Chem. Mater. 2010, 22, (12), 3589-3598.

588

38.

589

Brown, G. E., Jr., XANES evidence for rapid arsenic(III) oxidation at magnetite and

Hsia, T. H.; Lo, S. L.; Lin, C. F.; Lee, D. Y., Chemical and spectroscopic

Voegelin, A.; Hug, S. J., Catalyzed oxidation of arsenic(III) by hydrogen peroxide

Grossl, P. R.; Eick, M.; Sparks, D. L.; Goldberg, S.; Ainsworth, C. C., Arsenate

Hiemstra, T., Surface and mineral structure of ferrihydrite. Geochimica et

Waychunas, G. A.; Rea, B. A.; Fuller, C. C.; Davis, J. A., Surface chemistry of

Tang, Y.; Michel, F. M.; Zhang, L.; Harrington, R.; Parise, J. B.; Reeder, R. J.,

Ona-Nguema, G.; Morin, G.; Wang, Y.; Foster, A. L.; Juillot, F.; Calas, G.;


Page 26 of 35

Page 27 of 35


590

ferrihydrite surfaces by dissolved O2 via Fe2+-mediated reactions. Environ. Sci. Technol.

591

2010, 44, (14), 5416-5422.

592

39.

593

chromate-induced oxidative DNA damage and cancer. J. Environ. Pathol., Toxicol.

594

Oncol. 2000, 19, (3), 215-230.

595

40.

596

Reviews 1949, 45, (3), 419-451.

597

41.

598

and mechanism of oxidation of apple pectin by CrVI in aqueous acid medium. Journal of

599

Physical Organic Chemistry 2008, 21, (12), 1059-1067.

600

42.

601

TiO2−CrVI−Mandelic Acid System. Environmental Science & Technology 1996, 30, (2),

602

463-472.

603

43.

604

Comparisons of Different Organic Reductants and Different Oxide Surfaces.

605

Environmental Science & Technology 1996, 30, (8), 2484-2494.

606

44.

607

reduction by iron(II). Geochim. Cosmochim. Acta 1999, 63, (19/20), 2939-2955.

608

45.

609

semiconducting mineral surfaces: a new aspect of mineral surface reactivity and surface

610

complexation theory? Geochim. Cosmochim. Acta 2001, 65, (16), 2641-2649.

Sugden, K. D.; Stearns, D. M., The role of chromium(V) in the mechanism of

Westheimer, F. H., The Mechanisms of Chromic Acid Oxidations. Chemical

Bellú, S. E.; González, J. C.; García, S. I.; Signorella, S. R.; Sala, L. F., Kinetics

Deng, B.; Stone, A. T., Surface-Catalyzed Chromium(VI) Reduction: The

Deng, B.; Stone, A. T., Surface-Catalyzed Chromium(VI) Reduction: Reactivity

Liger, E.; Charlet, L.; Van Cappellen, P., Surface catalysis of uranium (VI)

Becker, U.; Rosso, K. M.; Hochella, M. F., The proximity effect on



611

46.

Rosso, K. M.; Becker, U., Proximity effects on semiconducting mineral surfaces

612

II: Distance dependence of indirect interactions. Geochim. Cosmochim. Acta 2003, 67,

613

(5), 941-953.

614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633


Page 28 of 35

Page 29 of 35


634

Figure captions:

635

Figure 1: ATR-FTIR of Fh that was exposed to a solution containing both 0.10 mM

636

Cr(VI) and 0.15 mM As(III) at pH 3, 5, 7 and 9, (A, B, C, and D, respectively) for (a) 0

637

min, (b) 5 min, (c) 10 min , (d) 30 min, (e) 60 min, and (f) 120 min, (g) 180 min, (h) 240

638

min, and (i) 300 min. The spectra are offset from one another for clarity. Plot (e) also

639

shows the fitting of modes for Cr(VI), As(V) and As(III). All plots were fitted in the

640

same manner but omitted for clarity.

641 642

Figure 2: (A) ATR-FTIR of Fh (a) in pH 5 H2O, (b) after exposure to a flowing solution

643

of 0.15 mM As(III) for 120 min, (c) after flushing with pH 5 H2O for 15 min, and after

644

exposure to flowing 0.10 mM Cr(VI) solution for (d) 10 min, (e) 30 min, (f) 60 min, and

645

(g) 120 min. (B) ATR-FTIR of Fh (a) in pH 5 water, (b) after exposure to a flowing

646

solution of 0.10 mM Cr(VI) for 120 min, (c) after flushing with pH 5 H2O for 15 min,

647

and after exposure to flowing 0.15 mM As(III) solution for (d) 10 min, (e) 30 min, (f) 60

648

min, (g) 120 min., and 180 min. All As(III) and Cr(VI) solutions were at pH 5.

649 650

Figure 3: ATR-FTIR of Fh that was exposed to a flowing solution of 0.10 mM Cr(VI)

651

and 0.15 mM As(III) at pH 5 for (a) 0 min, (b) 5 min, (c) 10 min , (d) 30 min, (e) 60 min,

652

(f) 120 min, (g) 180 min, (h) 240 min, and (i) 300 min under anoxic conditions. Inset

653

shows comparison of As(V) peak area (arb. units) at pH 5 anoxic (squares) and pH 5 oxic

654

(circles) conditions. Anoxic case shows similar reaction to oxic case, growth and then

655

decrease in mode attributed to Cr(VI) coupled with the constant growth of As(V)

656

vibrational modes, but less As(V) production.



657

Figure 4: (a) Normalized Cr K-edge XANES spectra of Fh that was individually exposed

658

to Cr(VI), Cr(III), and for Fh that was exposed to a solution containing both Cr(VI) and

659

As(III). (b) Normalized As K-edge XANES spectra of Fh that was individually exposed

660

to As(III), As(V), and for Fh that was exposed to a solution containing both Cr(VI) and

661

As(III). All exposure times were 12 h and the solution pH was 5 in all cases.

662


Page 30 of 35

Page 31 of 35

663


Figures:

664 665

As(V)

A A

As(III)

As(V)

B

.004

.004

As(III)

666

668 669

Cr(VI)

Absorbance

667

Absorbance

Cr(VI)

i h g f e

i h g f e

670 d c b a

d c b a

671 672

950

900

850

800

750

950

700

-1

C

850

As(V) As(III)

678 679

D

.002

As(III)

Absorbance

Absorbance

Cr(VI) i h g f e

h g f e d

c b a

c b a

950

682

Cr(VI)

i

d

680 681

700

As(V)

675

677

750

Wavenumbers (cm )

674

676

800

-1

Wavenumbers (cm )

673

900

900

850

800

750

700

-1

950

900

850

800

750 -1

Wavenumbers (cm )

Wavenumbers (cm )

Figure 1

683 684 685 31 ACS Paragon Plus Environment

.001


686

A

As(V)

As(III)

Page 32 of 35

B

.002

As(V)

Cr(VI)

687

As(III)

.004

Cr(VI)

688

691

Absorbance

690

Absorbance

689

h

g

g f e

692 693

f e d c b a

d c b a

694 950

695

900

850

800

750

700

950

Wavenumbers (cm-1)

850

800

750

Wavenumbers (cm-1)

696 697

900

Figure 2

698 699 700 701 702


700

Page 33 of 35


Peak Area

1.5 Oxic Anoxic

.002

0.5

As(V) 0

100

200

As(III)

300

Absorbance

Time (min)

Cr(VI)

i h g f e d c b a 950

703 704

900

850

800

750

Wavenumbers (cm-1) Figure 3


700


Normalized Absorbance

2.0

Page 34 of 35

(a)

1.5

1.0

0.5

Cr(VI)/Fh Cr(III)/Fh

0.0

Cr(VI)/As(III)/Fh

5.96

5.98

6.00

6.02

6.04

6.06

11.89

11.90

Photon Energy (keV)

Normalized Absorbance

5.0

(b)

4.0

3.0

2.0 As(III)/Fh

1.0

As(V)/Fh

As(III)/Cr(VI)/Fh

0.0 11.85

11.86

11.87

11.88

705

Photon Energy (keV)

706

Figure 4

707 708 709 710 711 712 713 34 ACS Paragon Plus Environment

Page 35 of 35

714


TOC

715 716 717 718 719 720 721 722 723 724 725 726 727 728 729


Coupled Redox Transformation of Chromate and Arsenite on

Recommend Documents