Structures and Properties of As(OH)3 Adsorption Complexes on

Subscriber access provided by UB + Fachbibliothek Chemie | (FU-Bibliothekssystem)

Article

Structures and properties of As(OH)3 adsorption complexes on hydrated mackinawite (FeS) surfaces: A DFT-D2 study Nelson Y. Dzade, Alberto Roldan, and Nora H. de Leeuw Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00107 • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on February 28, 2017

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 33

Environmental Science & Technology

2

Structures and properties of As(OH)3 adsorption complexes on hydrated mackinawite (FeS) surfaces: A DFT-D2 study

3

Dr. Nelson Y. Dzade1*, Dr. Alberto Roldan2and Prof. Nora H. de Leeuw1, 2

4 5 6 7

1

8

E-mail: [email protected] (N.Y.D); [email protected] (N.H.dL)

9

ABSTRACT

1

Department of Earth Sciences, Utrecht University, Princetonplein 9, 3584 CC, Utrecht, The Netherlands 2

School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 1DF, UK

10

Reactive mineral–water interfaces exert control on the bioavailability of contaminant arsenic

11

species in natural aqueous systems. However, the ability to accurately predict As surface

12

complexation is limited by the lack of molecular-level understanding of As−water−mineral

13

interactions. In the present study, we report the structures and properties of the adsorption

14

complexes of arsenous acid (As(OH)3) on hydrated mackinawite (FeS) surfaces, obtained

15

from density functional theory (DFT) calculations. The fundamental aspects of the

16

adsorption, including the registries of the adsorption complexes, adsorption energies, and

17

structural parameters are presented. The FeS surfaces are shown to be stabilized by hydration,

18

as is perhaps to be expected because the adsorbed water molecules stabilize the low-

19

coordinated surface atoms. As(OH)3 adsorbs weakly at the water−FeS(001) interface through

20

a network of hydrogen-bonded interactions with water molecules on the surface, with the

21

lowest-energy structure calculated to be an As−up outer-sphere complex. Compared to the

22

water−FeS(001) interface, stronger adsorption was calculated for As(OH)3 on the

23

water−FeS(011) and water−FeS(111) interfaces, characterized by strong hybridization

24

between the S-p and O-p states of As(OH)3 and the surface Fe-d states. The As(OH)3

25

molecule displayed a variety of chemisorption geometries on the water−FeS(011) and

26

water−FeS(111) interfaces, where the most stable configuration at the water−FeS(011)

27

interface is a bidentate Fe−AsO−Fe complex, but on the water−FeS(111) interface, a

28

monodentate Fe−O−Fe complex was found. Detailed information regarding the adsorption

29

mechanisms has been obtained via projected density of states (PDOS) and electron density

30

difference iso-surface analyses and vibrational frequency assignments of the adsorbed

31

As(OH)3 molecule. 1

ACS Paragon Plus Environment


32

Page 2 of 33

TOC/ABSTRACT GRAPHICS

33 34

1. INTRODUCTION

35

Arsenic is recognized as one of the most serious inorganic contaminants in soil and

36

groundwater worldwide, with significant public health implications. Arsenic often makes its

37

way into soil and water courses by the natural processes of weathering and dissolution of

38

minerals such as arsenian pyrite, Fe(As,S)2, and arsenopyrite, FeAsS.1 Anthropogenic

39

activities, particularly mineral extraction and processing can also introduce arsenic-rich

40

effluents into the environment if not carefully monitored and controlled.2 The effects of

41

arsenic on human health are highly detrimental, with arsenic poisoning being linked to

42

neurological disorders, dermatological and gastrointestinal problems, and it is also a known

43

carcinogen.3, 4

44

Arsenic can exist in a range of oxidation states from –3 to +5, although aqueous

45

solutions it is most commonly found as As(III) or As(V) oxyacids. As(III) is both more toxic

46

(20–65 times) and more mobile (being able to travel five to six times faster) than As(V) and

47

is one of the main toxic species in natural waters.5−7 Analyses of hydrothermal fluids show

48

that As is transported mainly as As(III),8 and the uptake of As(III) from aqueous solutions is

49

reported to occur via neutral molecules, which suggest that arsenous acid (As(OH)3) or

50

related species could be the common form of arsenic in contaminated waters.9, 2


10

An

Page 3 of 33


51

understanding of the geochemistry of As(OH)3 in low temperature anoxic sedimentary

52

environments is therefore crucial to the development of safe drinking water and food supplies

53

in many countries.11, 12 Of the processes influencing arsenite mobility, reactive mineral–water

54

interfaces exert control on the bioavailability of contaminant arsenic species in natural

55

aqueous systems. The adsorption of arsenic species onto mineral surfaces strongly affects

56

their concentrations in aqueous environments.13

57

In recent years, iron sulfide mackinawite (FeS), has attracted significant interests for

58

environmental remediation due to its natural abundance and high treatment efficiency in

59

anoxic environments.14−26 FeS is a layered iron sulfide mineral that crystallises in the

60

tetragonal structure shown in Figure 1, 27, 28 and it is known to be the first crystalline ferrous

61

sulfide phase to form under sulfate reducing conditions.29, 30 FeS is a non-toxic mineral and a

62

precursor to other stable iron sulfide minerals, such as greigite and pyrite.29, 30 Like other 2D

63

layered materials, for example, MoS2, FeS possesses a high specific surface area and reactive

64

surfaces that are ideal for the uptake of aqueous contaminants. Furthermore, FeS

65

nanoparticles can be synthesized easily,31−35 which makes it a promising candidate for the

66

treatment of groundwater and soil contaminated with arsenic,14−18 selenium,19, 20 and heavy

67

metals, including mercury,21−23 and chromium.24−26

68

Owing to its unique structure and surface chemical properties, mackinawite has been

69

reported to be very effective in immobilizing divalent metals such as Mg2+, Ca2+, Mn2+, Ni2+,

70

Cd2+, and Hg2+ from aqueous solutions.36−39 FeS has also been shown to have a high removal

71

capacity for inorganic oxyanions, including As under anoxic conditions.14−20 It has been

72

reported that mackinawite suspensions and synthetic nanoparticulate mackinawite can

73

effectively remove As(III) at a pH range of 5−10.14, 40 A comparative study of the removal

74

capacity of As(III) and As(V) in aqueous solutions by goethite, lepidocrocite, mackinawite,

75

and pyrite, by Farquhar et al.17 has shown that mackinawite was more efficient than iron3



76

oxide phases or pyrite. Their results suggested that the arsenic uptake by freshly prepared

77

mackinawite was due to outer-sphere complexation,17 but fundamental aspects of this

78

process, including the registries of the adsorption complexes, adsorption energies, and

79

structural parameters remain unclear. Such information cannot be obtained directly from

80

experimental work and the underlying physical driving forces that control the reactivity of

81

arsenic species with the FeS surfaces remain poorly understood. The diverse interactions and

82

reactions occurring at the mineral–water interfaces often create complex situations that are

83

difficult to interpret. However, molecular simulations provide an alternative way to gain

84

fundamental insight into these processes.41−44 Calculations based on the density functional

85

theory (DFT) have become indispensable in unravelling the interactions of organic and

86

inorganic molecules with solid surfaces as they are capable of accurately predicting lowest-

87

energy adsorption geometries and identifying charge transfer and other electronic effects.45−47

88

For example, DFT-based studies have been instrumental in elucidating the complex

89

adsorption processes of arsenic and arsenate on iron oxide mineral surfaces.42, 43 Goffinet and

90

Mason employed spin-polarized DFT calculations to study the inner-sphere As(III)

91

complexes on hydrated α-Fe2O3(0001) surface models.42 Blanchard and co-workers have

92

modelled arsenate adsorption on the hydrated (1−12) hematite surface, investigating charged

93

inner- and outer-sphere complexes using DFT calculations.43 To date, no systematic

94

theoretical study has been conducted to investigate the detailed adsorption mechanism of

95

arsenous acid at the water−FeS interface, which makes this investigation timely.

96

In this study, the structures and properties of the adsorption complexes of As(OH)3 on

97

hydrated mackinawite (FeS) surfaces was studied using dispersion-corrected density

98

functional theory calculations (DFT-D2). The energetically preferred As(OH)3 surface

99

complexes on the hydrated (001), (011), and (111) surfaces of mackinawite have been

100

identified. Detailed structural analysis of the adsorption complexes and insight into the nature 4


Page 4 of 33

Page 5 of 33


101

of adsorption on the different surfaces was determined via analysis of projected density of

102

states and differential charge density iso-surfaces. Vibrational frequency assignment of the

103

different identified adsorption complexes of As(OH)3 was carried out, which will be useful

104

for comparison with any future experimental studies.

105

2. COMPUTATIONAL DETAILS

106

The calculations were carried out using the VASP code,48, 49 which employs a basis

107

set of plane-waves to solve the Kohn-Sham (KS) equations of the density functional theory

108

(DFT) in a periodic system. Long-range dispersion forces were accounted for in our

109

calculations using the Grimme DFT-D2 method,50 which is essential for the accurate

110

description of the FeS interlayer interactions,51−54 as well as the interactions between the

111

As(OH)3 molecule and the water−FeS surfaces. The D2 method was used in this study to

112

remain consistent with previous work and to ensure that direct comparison could be made

113

with our earlier studies. However, we have carried out a number of test calculations using the

114

DFT-D3 method, as mentioned in the text where relevant, but no significant differences

115

between the two methods were observed.

116

The generalized gradient approximation (GGA), with the PW91 functional55 was used

117

to calculate the total free energies. The interactions between the valence electrons and the

118

cores were described with the projected augmented wave (PAW) method56 in the

119

implementation of Kresse and Joubert.57 The on-site potential, GGA+U, was not employed

120

for these calculations as previous studies on FeS using VASP have shown that the extra

121

localization of the d-electrons through the inclusion of +U correction term provides

122

inadequate structural optimizations.54 An energy cut-off of 400 eV for the plane-wave basis

123

set was tested to be sufficient to converge the total energy of mackinawite to within 0.0001

124

eV and the Brillouin zone was sampled using 11 x 11 x 7 and 5 x 5 x 1 Monkhorst-Pack58 K5



Page 6 of 33

125

points mesh for bulk and surface calculations, respectively, which ensures electronic and

126

ionic convergence. Geometry optimizations were performed using the conjugate gradient

127

minimization algorithm until the magnitude of the residual Hellman−Feynman force on each

128

relaxed atom reached 0.001 eV/Å.

129

The bulk FeS was modelled in the tetragonal structure (Figure 1). From a full geometry

130

optimization, the equilibrium lattice parameters were predicted to be a = 3.587 Å, c = 4.908

131

Å, and c/a = 1.368,44, 51−54 which agree well with those measured experimentally (a = 3.674

132

Å, c = 5.033 Å, and c/a = 1.370).27,

133

scheme, which predicted the lattice parameters to be a = 3.590 Å, c = 4.992 Å, and c/a =

134

1.390. From the fully relaxed bulk structure, we created the (001), (011), and (111) surfaces

135

of FeS, which are the commonly observed facets in mackinawite nanoparticles.44,

136

surfaces were created using the METADISE code,60 which generates non-polar supercells,

137

avoiding dipole moments perpendicular to the surface plane, as is required for reliable and

138

realistic surface calculations.61

28

Similar results were obtained within the DFT-D3

59

The

139

For each surface, a minimum slab thickness of 10 Å was used in each simulation cell,

140

and a vacuum region of 15 Å was tested to be sufficient to avoid interactions between

141

periodic slabs. The converged slab thickness used to model the (001), (011), and (111)

142

surfaces were constructed of 6, 9, and 12 atomic layers, respectively. Because the processes

143

take place in an aqueous environment, the FeS surfaces were hydrated through associative

144

adsorption of a monolayer of water, to provide a realistic picture of the As(OH)3

145

complexation in natural aqueous systems at the mackinawite–water interface. In an earlier

146

study, we showed that the dissociative water adsorption did not occur spontaneously at FeS

147

surfaces.62 We considered that a monolayer of water was obtained when all surface

148

cations/anions had been terminated by water. The hydrated (001), (011), and (111) surfaces

149

are modelled by large slabs constructed as (3 x 3)−9water, (4 x 2)−8water, and 6


Page 7 of 33


150

(3 x 2)−6water supercells, respectively. These simulation supercells are large enough to

151

minimize lateral interaction between the As(OH)3 molecules in neighbouring image cells.

152

Different binding modes of the As(OH)3 molecule were considered, for example,

153

monodentate or bidentate adsorption configurations, in order to obtain the lowest-energy

154

adsorption complexes. The adsorption energy (Eads) of the As(OH)3 on the hydrated FeS

155

surfaces was calculated as follows:

156

Eads = Ewater −surf + As (OH )3 − ( Ewater −surf + E As (OH )3 )

(1)

157

where E water − surf + As (OH )3 represents the total energy of the adsorbate-substrate system, E water − surf

158

represents the total energy of the relevant hydrated FeS substrate, and E As (OH )3 is the energy

159

of the free As(OH)3 molecule. Differences in the adsorption energies reflect trends in surface

160

reactivity, thus Eads is useful for characterizing activity trends and relative energetics. A

161

Bader population analysis was carried out for all the As(OH)3−water−FeS complexes, using

162

the code developed by Henkelman and co-workers63 in order to quantify any charge transfer

163

between the substrate surfaces and the adsorbate molecule. Vibrational frequency assignment

164

of the As−O and O−H bond stretching modes were performed within the framework of the

165

self-consistent density functional perturbation theory.64

166 167

3. RESULTS AND DISCUSSIONS

168

3.1 Hydrated FeS (001), (011), and (111) surface models

169

Prior to studying the adsorption and surface reactions of As(OH)3, we have

170

characterised the interaction of water with the (001), (011), and (111) surfaces of FeS and

171

how hydration affects their relative stabilities. Shown in Figure 2 are the optimized structures

172

of the hydrated (001) and (011), and (111) surfaces. The relaxed surface energies ( γ r ) of the

173

pure symmetric stoichiometric slabs were calculated using the equation: 7



γr =

174

relaxed E unrelaxed − nEbulk Eslab − nEbulk − γ u ; γ u = slab A 2A

Page 8 of 33

(2)

175

relaxed unrelaxed where E slab and Eslab are the energies of the relaxed and unrelaxed slabs, respectively,

176

nEbulk is the energy of an equal number (n) of bulk FeS units, and A is the area of one side of

177

the slab. Considering that the adsorption of water on the FeS surfaces may affect their

178

stability, we have also calculated the surface energies of the surfaces after water adsorption

179

using Equations 3.

180

γ water =

relaxed E slab + n ( water ) − nE water − nEbulk

A

−γu

(3)

181

relaxed where E slab + water is the energy of the surface with adsorbed water molecules and nEwater is the

182

energy of an equivalent number of free water molecules.

183

The calculated surface energies of different surfaces (pristine and hydrated) as listed

184

in Table 1, show that the order of increasing surface energies, and therefore decreasing

185

stability, before and after hydration is (001) < (011) < (111). All the FeS surfaces were

186

stabilized through hydration, as is perhaps to be expected because the adsorbed water

187

molecules stabilize the low-coordinated surface atoms. At the FeS(001) surface, we found

188

that the water molecules were only physisorbed with the hydrogen atoms pointing towards

189

the terminating surface sulfur ions (Figure 2a), similar to results obtained from previous

190

DFT,53, 62 and molecular dynamics (MD) simulations65 of the structure and dynamics of water

191

at the FeS(001) surface. The shortest H−S distance is calculated at 2.319 Å, which is larger

192

than the typical hydrogen-bond length in water of 1.97 Å,66 and therefore suggests that

193

dispersion forces may play an important role in stabilizing the water molecule on the

194

FeS(001) surface. In a previous study, we showed that the dispersion interactions contribute

195

approximately 87% of the total adsorption energy of water on the FeS(001).62 The average 8


Page 9 of 33


196

hydrogen to oxygen (H---O) interatomic distance between the water molecules on the (001)

197

surface is calculated at 1.824 Å.

198

Compared to the (001) surface, the water molecules on the (011) surface are oriented

199

in such a way that now the O atoms are closest to the surface Fe sites (average Fe−O =2.253

200

Å) as shown in Figure 2b. The hydrogen atoms are oriented towards the sulfur ions in the

201

next FeS layer at an average distance of 2.703 Å, which is larger than the average Fe−O bond

202

length of 2.253 Å and therefore suggests that the major interactions between the adsorbing

203

water molecules and the (011) surface is through the interaction of their oxygen ions with

204

surface Fe ions. In the case of the water−FeS(111) complex (Figure 2c), the water molecules

205

are located above the bridge sites between adjacent Fe ions (average Fe−O = 2.205 Å). The

206

hydrogen atoms are oriented towards the sulfur ions in the next FeS layer at an average

207

distance of 2.043 Å, compared to 2.703 Å at the FeS(011) surface, which indicates stronger

208

hydrogen-bonding at the FeS(111) surface than at the FeS(011). Generally, the FeS surfaces

209

were found to undergo modest relaxations relative to the bulk interlayer spacings upon

210

hydration, where the topmost three percentage relaxations of the interlayer spacings are

211

calculated to be +6.5 %, +3.3 %, and −3.4 % for the (001), −24.1 %, +10.9 %, and −2.3 % for

212

the (011), and +29.3 %, +12.1 %, and −6.6 % for the (111). The multilayer relaxations for the

213

hydrated surfaces were calculated as the percentage difference in the surface interlayer

214

spacing, dij-hydrated, from the layer spacing of the same orientation in the geometry of the

215

unrelaxed surface structure, dij-unrelaxed, created from the equilibrium bulk material. In these

216

simulations, since the models are constructed from the optimized bulk structure, the required

217

surface layer spacing is given by the spacing of the unrelaxed bulk-terminated slab structure.

∆d ij =

(d

ij − hydrated

− d ij −unrelaxed ) d ij −unrelaxed

218

× 100

9


(1)


Page 10 of 33

219

Within this definition, negative values correspond to inward relaxation (contraction) and

220

positive values denote outward relaxation (dilation) of the interlayer spacings.

221

3.2 As(OH)3 structural conformations

222

Arsenous acid (As(OH)3) exists in two conformations in the gas phase with either C1 or C3

223

symmetry. The optimized geometries of the C1 and C3 conformations are shown in Figure 3

224

and the calculated interatomic bond distances and bond angles along with earlier theoretical

225

results41, 67 are listed in Table 2. From our geometry optimization calculations, we found that

226

the C1 symmetry is 0.03 eV more stable than the C3 symmetry, in agreement with earlier

227

theoretical results of Ramírez-Solís et al.67 and Tossell et al.68 We show from climbing-image

228

nudged elastic band (cNEB) calculations that the C1 conformation has to overcome an

229

activation barrier of 0.34 eV to transform to the higher-energy C3 conformation. The three

230

As−O bond distances of the C1 and the C3 conformers do not differ significantly, calculated

231

to be 1.798, 1.801, and 1.811 Å for the C1 symmetry and 1.810, 1.811 and 1.813 Å for the C3

232

symmetry. Our calculated bond distances (As‒O and O‒H) and angles (O‒As‒O and

233

As−O−H) show good agreement with earlier theoretical results41,

234

obtained from X-ray absorption and EXAFS analysis.69, 70 In our study, we have explored

235

several possible adsorption structures including monodentate or bidentate binding geometries

236

on the different hydrated FeS surfaces.

237

3.3 As(OH)3 adsorption complexes at water-FeS(001) interface

238

Several possible modes of adsorption sites and configurations were studied for As(OH)3

239

adsorption at the water-FeS(001) interfaces but only the lowest-energy structure (denoted

240

As–up–outer) is shown in Figure 4a (the remaining conformations and calculated binding

241

energies are given in the Supporting Information (Figure S1 and Table S1, respectively)). In

242

the lowest-energy As–up–outer complex, the As(OH)3 is adsorbed outside the water layer 10


67, 68

and with those

Page 11 of 33


243

with the As atom pointing upwards, while the hydroxyl groups form hydrogen-bonded

244

interactions with the surface-bound water molecules. The adsorption energy of this structure

245

is −1.14 eV, which is 0.2 eV more favourable than the As–up inner-sphere complex (Figure

246

S1b), in which the As(OH)3 molecule is adsorbed within the water layer by displacing some

247

of the water molecules during the adsorption process. In the case of As–down configurations,

248

the inner-sphere complex (Figure S1c) is found to be energetically more favourable than the

249

outer-sphere complex (Figure S1d) by 0.23 eV. In all adsorption geometries, we observe

250

only small elongations in the As−O and O−H bonds (Table 3 and Table S1) compared to the

251

structural data of the free As(OH)3 molecule (Table 2), which may be attributed to the

252

hydrogen-bonded interactions with the surface water molecules. In the lowest-energy outer-

253

sphere As–up complex, the three hydrogen atoms of the As(OH)3 molecule interact with three

254

different surface water molecules at Hmol‒Owat distances of 1.702, 1.747, and 1.960 Å. We

255

also observe hydrogen-bonded interactions between hydrogen atoms of two water molecules

256

and O atoms of As(OH)3 at Hwat‒Omol distances of 1.639 and 1.783 Å. The Hmol‒Owat and

257

Hwat‒Omol bond lengths calculated in the present study compare closely with the typical

258

hydrogen-bond length in water of 1.97 Å,66 which therefore suggests that hydrogen-bonded

259

interactions contribute significantly to the stabilization of As(OH)3 at the water−FeS(001)

260

interface.

261


262

As with the water−FeS(001) surface, we have considered different possible adsorption

263

structures for As(OH)3 on the water−FeS(011) surface. During the adsorption, some of the

264

water molecules were displaced from the surface by the As(OH)3, enabling direct stronger

265

interactions with the surface cations sites. Shown in Figure 4b is the lowest-energy

266

adsorption configuration identified (the remaining conformations are given in the Supporting 11



267

Information (Figure S2), whereas the calculated adsorption energies and optimized structural

268

parameters are reported in Table 3 and Table S2. The lowest-energy adsorption structure of

269

As(OH)3 at the water−FeS(011) interface is calculated to be a bidentate Fe−AsO−Fe complex

270

(Figure 4b), wherein the As(OH)3 molecule interacts with the surface Fe atoms via the As

271

and one O atom. The adsorption energy of this structure is calculated at −1.82 eV, compared

272

to the adsorption energy of −1.43 eV for the monodentate Fe−O complex (Figure S2b),

273

wherein the As(OH)3 molecule interacts with the surface Fe atoms via only one O atom,

274

−1.06 eV for the monodentate Fe−As complex (Figure S2c), wherein the As(OH)3 molecule

275

interacts with the surface Fe atoms via the As atom, and −0.89 eV for the As−bridge complex

276

(Figure S2d), wherein the As(OH)3 is adsorbed in a bridging position between the FeS layers

277

and stabilized through hydrogen-bonded interactions with the surface water molecules. The

278

As−S interatomic distances are calculated in the range of 2.960−4.147 Å, whereas the As−Fe

279

are calculated in the range of 2.269−3.787 Å (Table 3 and Table S2). Similar interatomic

280

distances were reported from spectroscopic and extended X-ray absorption fine structure

281

(EXAFS) data fitting of As(III) sorbed on mackinawite (As−S =3.1 Å and As−Fe =3.4−3.5

282

Å) in aqueous solution.17

283


284

Again, we have explored several possible sites and modes of adsorption of As(OH)3 on the

285

water−FeS(111) surface. Similar to the water−FeS(011) surface, some of the water molecules

286

were displaced by As(OH)3 during the adsorption process, which allows for the formation of

287

direct interactions with the surface cation sites. Displayed in Figure 4c is the lowest-energy

288

adsorption complex identified (the remaining conformations are given in the Supporting

289

Information (Figure S3).The lowest-energy As(OH)3 adsorption configuration at the

290

water−FeS(111) interface was calculated to be the Fe−O−Fe complex (Figure 4c), wherein 12


Page 12 of 33

Page 13 of 33


291

the As(OH)3 molecule adsorbs at the bridge site between adjacent surface Fe atoms via one O

292

atom. The adsorption energy of this structure (Fe−O−Fe complex) is calculated at −1.76 eV,

293

whereas the energies of the other stable adsorption configurations are calculated at −1.57 eV

294

for the Fe−AsO−Fe complex (Figure S3b), −1.17 eV for the Fe−As complex (Figure S3c),

295

and −0.86 eV for the Hwat−OH−Ssurf complex (Figure S3d). In the lowest-energy Fe−O−Fe

296

complex, the bridging O−Fe distances were calculated at 2.159 Å and 2.138 Å, and the

297

average values are reported in Table 3. The As−S and As−Fe interatomic distances

298

are converged at 3.675 Å and 3.365 Å, respectively. Similar interatomic distances

299

were calculated for the As atom interacting with the surface S and Fe ions in the

300

other adsorption configurations (Table S3). At all three water−FeS interfaces, we

301

have observed elongations in the As−O bonds in all adsorption complexes (1.765−1.946 Å),

302

especially in the complexes in which the O atom interacts directly with the surface Fe ions.

303

O−H bond elongations were also observed (0.976−1.046 Å), which can be attributed to the

304

presence of hydrogen-bonded interactions between the hydrogen atom of As(OH)3 and the O

305

atom of the surface water molecules as reported in Table 3 and supporting information

306

Tables S1−S3.

307

3.6. Electronic structures

308

To gain insight into the nature of the interactions between the As(OH)3 molecule and the

309

different hydrated FeS surfaces, we have carried out an atom-by-atom projected density of

310

states (PDOS) analysis of the free molecule and compared it to those of the adsorbed states.

311

The PDOS for the free As(OH)3 molecule is shown in Figure 5a1, whereas those for the

312

lowest-energy adsorption configurations at the water−FeS (001), (011), and (111) interfaces

313

are shown in Figures 5b1, 5c1, and 5d1, respectively. In the free As(OH)3 PDOS, we note

314

that the states around the Fermi level are dominated by p-states of As and O, which are 13



315

associated with the lone pair electron density of the As and O atoms as shown in the highest

316

occupied molecular orbital (HOMO) in Figure 5a2. These orbitals are therefore expected to

317

interact strongly with the orbitals of the surface species during sorption processes at the

318

mineral surfaces. Indeed, we found that at the water−FeS (011) and (111) interfaces where

319

the As(OH)3 interacts directly with the surface Fe ions, we observe disappearance or

320

reduction of the As-p and O-p states of As(OH)3 around the Fermi level, due to their strong

321

hybridization with the interacting surface Fe-d states (Figures 5c1 and 5d1). At the

322

water−FeS(001) interface, however, we only observe a shift towards lower energy levels

323

(Figure 5b1), which signifies stabilization of the As(OH)3 via physisorption. The PDOS for

324

the interacting surface Fe d-states before and after the adsorption of As(OH)3 at the

325

water−FeS(111) and water−FeS(011) interface, and for the interacting surface S p-states at

326

the water−FeS(001) interface are shown in Figure 6. We found that the electronic properties

327

of the surfaces were essentially preserved after the adsorption of As(OH)3, with only small

328

shifts in the peak positions and heights, which indicates adsorption induced changes due to

329

the interactions between the As(OH)3 specie and the water-FeS interfaces. The electron

330

density redistributions within the adsorbate-substrate systems were determined through

331

analyses of the iso-surface plots of the differential charge density, which is obtained by

332

subtracting from the charge density of the total adsorbate-substrate complex, the sum of the

333

charge densities of the As(OH)3 molecule and the hydrated FeS surface. The atomic positions

334

of the water−FeS surface and of the As(OH)3 molecule are kept the same as those of the total

335

adsorbate-substrate system. In this way, the presentation highlights local electron density

336

rearrangement and bond formation in the As(OH)3−water−FeS complexes. Shown in Figures

337

5b2, 5c2, and 5d2 are the isosurfaces of the electron density differences due to As(OH)3

338

adsorption at the water−FeS (001), (011), and (111) interfaces, respectively. An inspection of

339

the iso-surfaces reveals electron density accumulation within the bonding regions between 14


Page 14 of 33

Page 15 of 33


340

As(OH)3 and the water−FeS (011) and (111) interacting surface Fe ions, which is consistent

341

with the formation of new bonds. In the case of the As(OH)3−water−FeS (001) complex, we

342

see electron density accumulation between the hydrogen and O atoms indicative of hydrogen-

343

bonded interactions. Despite the strong electron density redistribution within the

344

As(OH)3−water−FeS complexes, only little charge transfer occurs from the interacting

345

surface species to the adsorbed As(OH)3 molecule, as revealed from our Bader charge

346

population analyses (Tables 3 and Tables S1−S3). The charge gained by the As(OH)3 in the

347

different adsorption complexes is calculated to be in the range of 0.01−0.04 e− at the

348

water−FeS(001) surface, 0.08−0.30 e− at the water−FeS(011) surface, and 0.01−0.28 e− at the

349

water−FeS(111) surface (Tables 3 and Tables S1−S3).

350

3.7. Vibrational properties

351

In order to propose an assignment for the As–O and O–H stretching vibrational modes

352

of the adsorbed As(OH)3, which can serve as a guide for future experimental identification of

353

the different adsorption complexes of As(OH)3 at the water−FeS interfaces, we have

354

computed the wavenumbers of the normal modes of all the stable adsorption complexes at the

355

different water−FeS interfaces (Table 4 and Table S4). Our calculated As–O and O–H

356

stretching vibrational modes for the free As(OH)3 molecule compare closely with

357

experimental data,71 as shown in Table 4, which ensures the reliability and accuracy of our

358

approximate assignments. The three As−O stretching vibrational modes for the free As(OH)3

359

molecule were calculated at 700.8, 639.1, and 638.3, which compares with the experimental

360

values of 710.0, 655.0, and 655.0 cm-1.71 The O–H stretching vibrational modes are

361

calculated at 3743.5, 3715.3 and 3674.6 cm-1 which are similar to the O–H stretching modes

362

of water.72 Compared to the free As(OH)3 molecule, we observe a reduction in the stretching

363

vibrational modes of the As−O bonds upon As(OH)3 adsorption, indicative of weakening of 15



364

these bonds, in agreement with the elongated As−O bonds calculated for the As(OH)3

365

adsorption complexes at the different water−FeS surfaces (Table 4). For example, the three

366

stretching As−O bands of As(OH)3 adsorbed in the lowest-energy configuration at the

367

water−FeS(011) and water−FeS(111) surfaces can be assigned at 580.2, 501.5, 488.9 cm-1

368

and 673.5, 616.5, 456.1 cm-1, respectively, which are lower than the gas phase stretching

369

As−O band assigned at 700.8, 639.1, and 632.3 cm-1. We have also observed reductions in

370

the stretching O–H bands of the adsorbed As(OH)3 compared to the free unbound state

371

(Table 4), which can be attributed to the formations of hydrogen-bonded interactions with the

372

oxygen ions of the surface water molecules.

373

The unique information provided by our atomic-level investigations provide

374

fundamental insights into the structure–property relationships of FeS–water−As(OH)3

375

interfaces. Our simulations show that As(OH)3 adsorbs weakly onto the water−FeS(001)

376

interface through a network of hydrogen-bond interactions with water molecules at the

377

surface. Stronger interaction is however, calculated for As(OH)3 adsorption on the

378

water−FeS(011) and water−FeS (111) interfaces, which is characterized by hybridization

379

between the S-p and O-p states of As(OH)3 and the surface Fe-d states. Our calculated As‒Fe

380

and As‒S interatomic distances in the lowest-energy adsorption complexes at the various

381

water-mackinawite interfaces (As‒Fe = 2.269−3.369 Å and As‒S = 3.382−3.675 Å) show

382

good agreement with those obtained from K-edge EXAFS and XANES spectroscopic data

383

(As‒Fe = 3.4‒3.5 Å and As‒S = 3.1 Å).17 The long distances obtained from experiment

384

clearly suggest As interactions via outer sphere complexes at the FeS surface. However, from

385

our simulation results, the short As‒Fe distances (2.217−2.530 Å) calculated for the

386

Fe−AsO−Fe and Fe−As adsorption complexes at the water-FeS (011) and (111) interfaces

387

indicate that, depending on the surface structure and composition, inner-sphere complexation

388

with respect to the As atom is also possible at the water-mackinawite interface. Future 16


Page 16 of 33

Page 17 of 33


389

investigations will expand the work presented here to include classical MD simulations which

390

will provide a complete description of the dynamic processes occurring at the As(OH)3–

391

water−FeS interfaces. The calculated interatomic distances and adsorption energies from this

392

work will be useful in the derivation of forcefields to be employed in the classical MD

393

simulations to simulate more complex systems, including single and multiple As(OH)3

394

species adsorption from an explicit 3-dimensional aqueous environment.

395

396

ASSOCIATED CONTENT

397 398

*S Supporting Information

399

Figures of all other As(OH)3 adsorption conformations, and Tables of adsorption energies,

400

structural parameters and vibrational frequencies of As(OH)3 adsorbed on water-FeS (001),

401

(011), and (111) interfaces. It contains three Figures and four Tables.

402

403

AUTHOR INFORMATION

404

Corresponding Author

405

*Telephone: +31-6-8523-9288 (N.Y.D); +44-29-2087-0658 (N.H.dL)

406

Fax: +31-30-253-5096 (N.Y.D); +44-29-2087-4030 (N.H.dL)

407

E-mail: [email protected] (N.Y.D); [email protected] (N.H.dL)

408

409

ACKNOWLEDGMENTS

410

We acknowledge the Netherlands Foundation for Fundamental Research on Matter (FOM)

411

for funding (Grant No. 13CO26-2). This work made use of the facilities of ARCHER

17



412

(http://www.archer.ac.uk), the UK’s national supercomputing service via our membership of

413

the UK's HEC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202).

414

REFERNCES

415 416

1.

Welch, A.H.; Westjohn, D.B.; Helsel, D.R., Wanty, R. B. Arsenic in ground water of the United States: Occurrence and geochemistry. Ground Water, 2000, 38, 589−604.

417 418

2.

Nordstrom, D.K. Worldwide occurrences of arsenic in ground water. Science, 2000, 296, 2143−2144.

419 420

3.

Hughes, M.F. Arsenic toxicity and potential mechanism of action. Toxicology Letters, 2002, 133, 1−16.

421 422

4.

U. S. Environmental Protection Agency. Special reports on ingested inorganic arsenic skin cancer: Nutritional essentiality. Report EPA/625 3-87-13, 1999.

423 424

5.

Ferguson, J.F.; Gavis, J. A review of the arsenic cycle in natural waters. Water research, 1972, 6, 1259−1274.

425 426 427

6.

Amin, N.; Kaneco, S.; Kitagawa, T.; Begum, A.; Katsumata, H.; Suzuki T.; Ohta, K. Removal of arsenic from aqueous solutions by adsorption onto water rice husk. Industrial & Engineering Chemistry Research, 2006, 45, 8105−8110.

428 429 430

7. Gulens, J.; Champs, D.R.; Jackson, R.E. Influence of redox environments on the mobility of arsenic in groundwater. In: E.A. Jenne (Ed.), Chemical Modelling of Aqueous systems. ACS Symposium Series, 1979, 93, 81−95.

431 432

8.

Ballantine, J.M., Moore, J.N. Arsenic geochemistry in geothermal systems. Geochim. Cosmochim. Acta, 1988, 52, 475−483.

433 434 435

9.

Liu, Z.; Shen, J.; Carbrey, J.M.; Mukhopadhyay, R.; Agre P.; Rosen, B.P. Arsenite transport by mammalian aquaglyceroporins AQP7 and AQP9. Proc. Natl. Acad. Sci. U.S.A., 2002, 99, 6053−6058.

436 437 438

10. Sanders, O.I.; Rensing, C.; Kuroda, M.; Mitra, B.; Rosen, B.P. Antimonite Is Accumulated by the Glycerol Facilitator GlpF in Escherichia coli. J. Bacteriol., 1997, 179, 3365−3367.

439 440

11. Smedley, P.L.; Kinniburgh, D.G. A review of the source, behaviour and distribution of arsenic in natural waters. Appl. Geochem. 2002, 17, 517−568.

441 442 443

12. Williams, P.M.; Price, A.H.; Raab A.; Hossain, S.A.; Feldmann, J.; Meharg, A.A. Variation in arsenic speciation and concentration in paddy rice related to dietary exposure. Environ. Sci. Technol., 2005, 39, 5531−5540. 18


Page 18 of 33

Page 19 of 33


444 445 446

13. Gallegos-Garcia, M.; Ramírez-Muñiz, K.; Song, S. Arsenic Removal from Water by Adsorption Using Iron Oxide Minerals as Adsorbents: A Review. Mineral Processing & Extractive Metall. Rev., 2012, 33, 301−315.

447

14. Han, Y.; Jeong, H.Y.; Demond, A.H.; Hayes, K.F. X-ray absorption and photoelectron

448

spectroscopic study of the association of As(III) with nanoparticulate FeS and FeS-coated

449

sand. Water Res. 2011, 45, 5727−5735.

450

15. Han, Y.; Gallegos, T.J.; Demond, A.H.; Hayes, K.F. FeS-coated sand for removal of

451

arsenic (III) under anaerobic conditions in permeable reactive barriers. Water Res. 2011,

452

45, 593−604.

453

16. Wolthers, M.; Charlet, L.; van Der Weijden, C.H.; van der Linde, P.R.; Rickard, D.

454

Arsenic mobility in the ambient sulfidic environment: sorption of arsenic (V) and arsenic

455

(III) onto disordered mackinawite. Geochim. Cosmochim. Acta 2005, 69, 3483−3492.

456

17. Farquhar, M.L.; Charnock, J.M.; Livens, F.R.; Vaughan, D.J. Mechanisms of arsenic

457

uptake from aqueous solution by interaction with goethite, lepidocrocite, mackinawite,

458

and pyrite: an X-ray absorption spectroscopy study. Environ. Sci. Technol. 2002, 36,

459

1757−1762.

460 461 462

18. Jeong, H.Y.; Han, Y.S.; Hayes, K.F. X-ray absorption and X-ray photoelectron spectroscopic study of arsenic mobilization during mackinawite (FeS) oxidation. Environ. Sci. Technol. 2010, 44, 955−961.

463

19. Han, D.S.; Batchelor, B.; Abdel-Wahab, A. Sorption of selenium (IV) and selenium(VI)

464

to mackinawite (FeS): effect of contact time, extent of removal, sorption envelopes. J.

465

Hazard. Mater. 2011, 186, 451−457.

466

20. Breynaert, E.; Bruggeman, C.; Maes, A. XANES-EXAFS analysis of Se solid phase

467

reaction products formed upon contacting Se(IV) with FeS2 and FeS. Environ. Sci.

468

Technol. 2008, 42, 3595−3601.

469 470

21. Jeong, H.Y.; Klaue, B.; Blum, J.D.; Hayes, K.F. Sorption of mercuric ion by synthetic nanocrystalline mackinawite (FeS).Environ. Sci. Technol. 2007, 41, 7699−7705.

471 472 473

22. Skyllberg, U.; Drott, A. Competition between disordered iron sulfide and natural organic matter associated thiols for mercury (II)-An EXAFS Study. Environ. Sci. Technol. 2010, 44, 1254−1259.

474

23. Liu J.; Valsaraj K. T.; Devai I.; DeLaune R. D.; Immobilization of aqueous Hg(II)by

475

mackinawite (FeS). J. Hazard. Mater. 2008, 157, 432−440.

19



476

24. Mullet, M.; Boursiquot, S.; Ehrhardt, J.J. Removal of hexavalent chromium from

477

solutions by mackinawite, tetragonal FeS. Colloids Surf. A Physicochem. Eng. Aspects

478

2004, 244, 77−85.

479 480 481 482

25. Patterson R. R.; Fendorf S.; Fendorf M.; Reduction of hexavalent chromium by amorphous iron sulfide. Environ. Sci. Technol. 1997, 31, 2039−2044. 26. Boursiquot, S.; Mullet, M.; Ehrhardt, J.J. XPS study of the reaction of chromium (VI) with mackinawite (FeS). Surf. Interface Anal. 2002, 34, 293−297.

483 484 485

27. Lennie, A.R.; Redfern, S.A.T.; Schofield, P.F.; Vaughan, D.J. Synthesis and Rietveld crystal structure refinement of mackinawite, tetragonal FeS. Mineral. Mag. 1995, 59, 677−683.

486

28. Berner, R.A. Tetragonal Ion sulfide. Science, 1962, 137, 669.

487 488 489 490 491 492

29. Rickard, D.; Luther, G. W. Chemistry of Iron Sulfides. Chem. Rev., 2007, 107, 514–562.

493 494 495

31. Jeong, H. Y.; Lee, J. H.; Hayes, K. F. Characterization of synthetic nanocrystalline mackinawite: crystal structure, particle size, and specific surface area. Geochim Cosmochim Acta, 2008, 72, 493−505.

496 497

32. Wolthers, M.; van der Gaast, S. J.; Rickard, D. The structure of disordered mackinawite.

498 499 500

33. Liu, Y.; Xiao, W.; Wang, J.; Mirza, Z. A.; Wang, T. Optimized Synthesis of FeS

501

34. Kim, E.J.; Kim, J.H.; Azad, A.M.; Chang, Y.S. Facile synthesis and characterization of

502

Fe/FeS nanoparticles for environmental applications. ACS Appl. Mater. Interfaces, 2011,

503

3, 1457−1462.

30. Lennie, A. R.; Redfern, S.A.T.; Champness, P. E.; Stoddart, C. P.; Schofield, P. F.; Vaughan, D. J. Transformation of mackinawite to greigite: An in situ X-ray powder diffraction and transmission electron microscope study. American Mineralogist, 1997, 82, 302−309.

Am. Mineral. 2003, 88, 2007−2015.

Nanoparticles with a High Cr(VI) Removal Capability. Journal of Nanomaterials, 2016, 2016, 1−9.

504

35. Feng, H.; SiEmail, P.-Z.; Xiao, X.-F.; Jin, C.-H.; Yu, S.-J.; Li, Z.-F.; Ge, H.-L. Large

505

scale synthesis of FeS coated Fe nanoparticles as reusable magnetic photocatalysts. Front.

506

Mater. Sci. 2013, 7, 308–311

507 508

36. Morse, J.W.; Arakaki, T. Adsorption and coprecipitation of divalent metals with mackinawite (FeS). Geochim. Cosmochim. Acta 1993, 57, 3635−3640.

20


Page 20 of 33

Page 21 of 33


509

37. Arakaki, T.; Morse, J. W. Coprecipitation and adsorption of Mn(II) with mackinawite

510

(FeS) under conditions similar to those found in anoxic sediments. Geochim. Cosmochim.

511

Acta 1993, 57, 9−14.

512

38. Kornicker W. A. Interactions of Divalent Cations with Pyrite and Mackinawite in

513

Seawater and Sodium–Chloride Solutions. Ph.D. Thesis, Texas A&M University, 1988

514

39. Wharton, M.J.; Atkins, B.; Charnock, J.M.; Livens, F.R.; Pattrick, R.A.D.; Collison, D.

515

An X-ray absorption spectroscopy study of the coprecipitation of Tc and Re with

516

mackinawite (FeS).Appl. Geochem. 2000, 15, 347−354.

517

40. Gallegos, T.J.; Han, Y.S.; Hayes, K.F. Model predictions of realgar precipitation by

518

reaction of As(III) with synthetic mackinawite under anoxic conditions. Environ. Sci.

519

Technol. 2008, 42, 9338−9343.

520 521 522

41. Blanchard, M.; Wright, K.; Gale, J.D.; Catlow, C.R.A. Adsorption of As(OH)3 on the (001) Surface of FeS2 Pyrite: A Quantum-mechanical DFT Study. J. Phys. Chem. C, 2007, 111, 11390−11396.

523

42. Goffinet, C. J.; Mason, S. E. Comparative DFT study of inner-sphere As(III) complexes

524

on hydrated α-Fe2O3(0001) surface models. J. Environ. Monit. 2012, 14, 1860–1871.

525 526 527

43. Blanchard, M.; Morin, M.; Lazzeri, M.; Balan, E.; Dabo, I. First-principles simulation of arsenate adsorption on the (1-12) surface of hematite. Geochimica et Cosmochimica Acta, 2012, 86 182–195.

528

44. Dzade, N. Y.; Roldan, A.; de Leeuw, N. H. Surface and shape modification of

529

mackinawite (FeS) nanocrystals by cysteine adsorption: a first-principles DFT-D2 study.

530

Phys. Chem. Chem. Phys., 2016,18, 32007−32020.

531

45. Groß, A. Theoretical Surface Science, Springer, Berlin, Germany, 2003.

532 533

46. Nilsson, A.; Pettersson, L. G. M. Chemical bonding on surfaces probed by X-ray emission spectroscopy and density functional theory. Surf. Sci. Rep. 2004, 55, 49–167.

534 535

47. Bligaard, T.; Nørskov, J. K. In Chemical Bonding at Surfaces and Interfaces, eds Nilsson, A.; Petterson, L. G.M.; Nørskov, J. K. (Elsevier, Amsterdam), 1st Ed. (2008).

536 537

48. Kresse, G.; Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. J. Comput. Mat. Sci. 1996, 6, 15−50.

538 539

49. Kresse, G.; Hafner, J. Ab initio molecular dynamics for liquid metals. J. Phys. Rev. B. 1993, 47, 558.

540 541

50. Grimme, S. Semiempirical GGA-Type Density Functional Constructed with a LongRange Dispersion Correction. J Comput Chem. 2006, 27, 1787. 21



542 543

51. Dzade, N.Y.; Roldan, A.; de Leeuw, N.H. The surface chemistry of NOx on mackinawite (FeS) surfaces: A DFT-D2 study. Phys. Chem. Chem. Phys. 2014, 16, 15444−15456.

544 545

52. Dzade, N.Y.; Roldan, A.; de Leeuw, N.H. Adsorption of methylamine on mackinawite (FeS) surfaces: A density functional theory study. J. Chem. Phys. 2013, 139, 124708.

546 547 548

53. Dzade, N.Y.; Roldan, A.; de Leeuw, N.H. DFT-D2 study of the adsorption and dissociation of water on clean and oxygen-covered {001} and {011} surfaces of mackinawite (FeS). J. Phys. Chem. C, 2016, 120, 21441–21450.

549 550 551

54. Devey, A.J.; Grau-Crespo, R.; de Leeuw, N.H. Combined density functional theory and interatomic potential study of the bulk and surface structures and properties of the iron sulfide mackinawite (FeS). J. Phys, Chem C, 2008, 112, 10960−10967.

552 553 554 555

55. Perdew, J.P.; Chvary, J.A.; Vosko, S.H.; Jackson, K.A.; Pederson, M.R.; Singh, D.J.; Fiolhais, C. Atoms, molecules, solids, and surfaces: Applications of the generalized gradient approximation for exchange and correlation. Phys. Rev. B. 1992, 46, 6671−6687.

556

56. Blöchl, P.E. Projector augmented-wave method. Phys. Rev. B. 1994, 50, 17953.

557 558

57. Kresse, G.; Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B. 1999, 59, 1758−1775.

559 560

58. Monkhorst, H.J.; Pack, J.D. Special points for Brillouin-zone integrations. Phys. Rev. B. 1976, 13, 5188−5192.

561 562

59. Ohfuji, H.; Rickard, D. High resolution transmission electron microscopic study of synthetic nanocrystalline mackinawite. Earth Planet. Sci. Lett. 2006, 241, 227−233.

563 564 565

60. Watson, G.W.; Kelsey, E.T.; de Leeuw, N.H.; Harris, D.J.; Parker, S.C. Atomistic simulation of dislocations, surfaces and interfaces in MgO. J. Chem. Soc., Faraday Trans. 1996, 92, 433−438.

566 567

61. Tasker, P.W. The stability of ionic crystal surfaces. J. Phys. C: Solid State Physics, 1979, 12, 4977−4984.

568

62. Dzade, N.Y.; Roldan, A.; DFT-D2 simulations of water adsorption and dissociation on

569

the low-index surfaces of mackinawite (FeS), J. Chem. Phys. 2016, 144, 174704

570 571

63. Henkelman, G.; Arnaldsson, A.; Jónsson, H. A fast and robust algorithm for Bader decomposition of charge density. Comput. Mater. Sci. 2006, 36, 354−360.

572

64. Baroni, S.; de Gironcoli, S.; Dal Corso, A.; Giannozzi, P. Phonons and related crystal

573

properties from density-functional perturbation theory. Rev. Mod. Phys. 2001, 73, 515–

574

562. 22


Page 22 of 33

Page 23 of 33


575 576

65. Terranova, U.; de Leeuw, N. H. Structure and dynamics of water at the mackinawite (001) surface. J. Chem. Phys. 2016, 144, 094706

577 578

66. Yang, M.; Stipp, S. L. S.; Harding, J. Biological Control on Calcite Crystallization by Polysaccharides. Cryst. Growth Des. 2008, 8, 4066−4074

579 580 581 582

67. Ramírez-Solís, A.; Hernandez-Cobos, J.; Vargas, C. A New Nonsymmetric As(OH)3 Species. Comparison with the Known C3 Species and Themochemistry at the HF, DFT(B3LYP), MP2, MP4, and CCSD(T) Levels of Theory. J. Phys. Chem. A. 2006, 110, 7637−7641.

583 584

68. Tossell, J.A. Theoretical studies on arsenic oxide and hydroxide species in minerals and in aqueous solution. Geochim. Cosmochim. Acta, 1997, 61, 1613−1623.

585 586 587

69. Ramírez-Solís, A.; Mukopadhyay, R.; Rosen, B.P.; Stemmler, T.L. Experimental and theoretical characterization of arsenite in water: insights into the coordination environment of As-O. Inorg. Chem. 2004, 43, 2954−2959.

588 589 590 591

70. Testemale, D.; Hazemann, J. L.; Pokrovski, G. S.; Joly, Y.; Roux, J.; Argoud, R.; Geaymond, O. Structural and electronic evolution of the As(OH ) 3 molecule in high temperature aqueous solutions: An x-ray absorption investigation. J Chem Phys. 2004, 121, 8973.

592 593

71. Loehr, T.M.; Plane, R.A. Raman spectra and structures of arsenious acid and arsenites in aqueous solution. Inorg. Chem. 1968, 7, 1708−1714.

594 595 596

72. Shimanouchi, T. Tables of Molecular Vibrational Frequencies, Consolidated Volume II, NSRDS NBS-39, J. Phys. Chem. 1977, 6, 3.

597 598 599 600 601 602 603 604 605 606 23



Page 24 of 33

607 608

LIST OF TABLES

609 610

Table 1: Calculated surface energies of pristine (γr) and hydrated (γhydrated) FeS. The corresponding percentage relaxation after hydration is denoted as % Relaxation.

611

Surface

γr (J m−2)

γhydrated (J m−2)

% Relaxation

(001)

0.19

0.14

26.31

(011)

0.95

0.71

25.26

(111)

1.51

1.21

19.87

612 613 614 615

Table 2: Structural data (interatomic bond distance and angles) of As(OH)3. The experimental As‒O bond length is 1.77−1.82 Å.69, 70 C1 Symmetry

C3 Symmetry

Parameter

This Work

PBE41

B3LYP67

This work

PBE41

B3LYP67

d(As‒O) /Å

1.798

1.811

1.796

1.810

1.829

1.813

1.801

1.818

1.800

1.811

1.829

1.813

1.811

1.841

1.826

1.813

1.829

1.813

0.975

0.977

0.967

0.975

0.982

0.970

0.978

0.980

0.969

0.978

0.982

0.970

0.978

0.983

0.970

0.978

0.982

0.970

90.17

88.79

90.86

97.25

96.92

97.34

99.84

99.75

99.61

97.48

96.94

97.36

100.94

103.22

100.89

97.57

96.99

97.37

108.6

105.33

110.16

108.6

104.87

109.93

110.9

109.83

112.68

110.8

104.98

109.94

111.2

111.77

112.78

111.0

105.02

109.93

d(O‒H) /Å

α(O−As−O) /°

α(As−O−H) /°

616

24


Page 25 of 33


Table 3: Adsorption energies, variation of the total Bader charge, representative geometrical parameters, and interatomic distances of the lowestenergy As(OH)3 adsorption complexes at water−FeS (001), (011), and (111) interfaces. The DFT-D3 Eads are shown in parenthesis. Surface

FeS(001)

FeS(011)

FeS(111)

Configuration

As−up−outer

Fe−AsO−Fe

Fe−O−Fe

Eads /eV

‒1.14 (‒1.06)

‒1.82 (‒1.73)

‒1.76 (‒1.68)

∑q /e−

0.04

0.30

0.28

d(As‒O) /Å

1.834

1.889

1.946

1.835

1.838

1.810

1.781

1.877

1.765

0.988

1.024

1.018

1.003

0.977

1.004

1.005

0.980

0.977

d(Hmol‒Owat) /Å

1.702, 1.747, 1.960

1.645

1.817

d(Hwat‒Omol) /Å

1.639, 1.783

1.803

3.240

d(Hwat‒S) /Å

2.301

−−−

2.034

d(As‒S) /Å

−−−

3.382

3.675

d(As‒Fe) /Å

−−−

2.269

3.365

d(O‒Fe) /Å

−−−

2.133

2.149

d(O‒H) /Å

25



Page 26 of 33

Table 4: Molecular vibrational frequencies (in cm-1) of adsorbed As(OH)3 at water−FeS interfaces.

ν(As−O) Surface

Configuration Free As(OH)3

ν(O−H)

As−O1 700.8 (710)71

As−O2 639.1 (655)71

As−O3 638.3 (655)71

O1−H

O2−H

O2−H

3738.1

3711.5

3674.7

FeS(001)

As−up−outer

695.1

620.7

585.8

3465.9

3182.9

3140.5

FeS(011)

Fe−AsO−Fe

580.2

501.5

488.9

3715.1

3670.9

2829.1

FeS(111)

Fe−O−Fe

673.5

616.5

456.1

3731.2

3204.2

2884.3

26


Page 27 of 33


LIST OF FIGURES Figure 1: The layered structure of mackinawite, with the tetragonal unit cell highlighted by dash lines. (Colour scheme: Fe = grey, S = yellow).

27



Figure 2: Side view of the geometry-optimized structures of hydrated FeS (a) (001), (b) (011), and (111) surfaces. (Colour scheme: Fe = grey, S = yellow, O = red, and H = white).

28


Page 28 of 33

Page 29 of 33


Figure 3: Optimized structures and energetics of C1 and C3 conformations of As(OH)3. (Colour scheme: As = green, O = red and H = white).

29



Page 30 of 33

Figure 4: Lowest-energy adsorption complexes of As(OH)3 at the (a) (001), (b) (011), and (c) (111) water−FeS interfaces, in side (top) and top (bottom) views. (Colour scheme: Fe = grey, S = yellow, As = pink, O = red and H = white).

30


Page 31 of 33


Figure 5: (Right) PDOS for As(OH)3 in the (a) free state and adsorbed in the lowest-energy geometry at the water−FeS interfaces (b−d). (Left) the corresponding isosurfaces of the differential charge density, where the purple and orange contours indicate electron density increase and decrease by 0.02 e/Å3, respectively.

31



Figure 6: PDOS for the interacting surface Fe d-states before and after the adsorption of As(OH)3 at the (a) water−FeS(111) and (b) water−FeS(011) interface, and (c) for the interacting surface S p-states at the water−FeS(001) interface.

32


Page 32 of 33

Page 33 of 33


Adsorption complex of As(OH)3 at water-FeS(001) interface. 79x39mm (300 x 300 DPI)


Structures and Properties of As(OH)3 Adsorption Complexes on

Recommend Documents