The formation of aluminium hydroxide-doped surface passivating

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Remediation and Control Technologies

The formation of aluminium hydroxide-doped surface passivating layers on pyrite for acid rock drainage control Yan Zhou, Rong Fan, Michael D. Short, Jun Li, Russell C. Schumann, Haolan Xu, Roger St.C. Smart, Andrea R Gerson, and Gujie Qian Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b04306 • Publication Date (Web): 19 Sep 2018 Downloaded from http://pubs.acs.org on September 21, 2018

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Environmental Science & Technology

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The formation of aluminium hydroxide-doped

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surface passivating layers on pyrite for acid rock

3

drainage control

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Yan Zhoua, Rong Fana, Michael D. Shorta,b, Jun Lia, Russell C. Schumannc, Haolan Xub,

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Roger St.C. Smarta,d, Andrea R. Gersond and Gujie Qiana,b,†*

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a

Natural and Built Environments Research Centre, School of Natural and Built Environments, University of South Australia, Mawson Lakes, SA 5095, Australia

9 b

10

Future Industries Institute, University of South Australia, Mawson Lakes, SA 5095, Australia c

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d

12

13

†

Blue Minerals Consultancy, Wattle Grove, TAS 7109, Australia

Present address: College of Science and Engineering, Flinders University, Bedford Park, SA 5042, Australia

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Levay & Co. Environmental Services, Edinburgh, SA 5111, Australia

*

Corresponding author: [email protected]; [email protected]; Tel:+61 404003470

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Postal address: College of Science and Engineering, Flinders University, GPO Box 2100, Adelaide,

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South Australia 5001

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1 ACS Paragon Plus Environment


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ABSTRACT

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The aim of this study was to test the performance of a novel method for acid rock drainage

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(ARD) control through the formation of Al(OH)3-doped passivating surface layers on pyrite. At

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pH 2.0 and 4.0, there was no obvious inhibition of the pyrite oxidation rate on addition of 20

23

mg L−1 Al3+ (added as AlCl3·6H2O). In comparison, the pyrite oxidation rate at circum-neutral

24

pH (7.4 ± 0.4) decreased with increasing added Al3+ with ≈98% reduction in long-term (282 days)

25

dissolution rates in the presence of 20 mg L−1 Al3+. Al3+ was added to the solution and allowed to

26

equilibrate prior to pyrite addition (2 g L−1). Consequently almost all Al3+ (> 99.9%) was initially

27

present as aluminium hydroxide precipitates at pH 7.4. X-ray photoelectron spectroscopy

28

analysis showed a significant concentration of Al3+ (20.3 at.%) on the pyrite surface reacted at pH

29

7.4 with 20 mg L−1 added Al3+, but no Al3+ on pyrite surfaces reacted at pH 2.0 and 4.0 with

30

added Al3+. Transmission electron microscopy and synchrotron X-ray absorption near edge

31

spectroscopy analyses indicated that compact surface layers containing both goethite and

32

amorphous or nano-crystalline Al(OH)3 formed in the presence of 20 mg L−1 Al3+ at circum-

33

neutral pH, in contrast to the porous goethite surface layers formed on pyrite dissolved in the

34

absence of Al3+ under otherwise identical conditions. This work demonstrates the potential for

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novel Al-based pyrite passivation of relevance to the mining industry where suitable Al-rich

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waste materials are available for ARD control interventions.

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Keywords: Acid and metalliferous drainage; Pyrite oxidation; Surface passivating layer;

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Transmission electron microscopy; X-ray absorption spectroscopy

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1.

INTRODUCTION

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Acid rock drainage (ARD), and associated release of metalliferous contaminants, is

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acknowledged as a major global environmental issue. Pyrite, the most abundant sulfide mineral

43

in the Earth’s crust, is the primary contributor to ARD and generates SO42−, Fe2+ and H+ when

44

oxidised on exposure to air, water, and commonly bacteria, due to natural weathering or mining

45

activities. Subsequent oxidation of aqueous Fe2+ and the formation of secondary Fe(III)

46

(oxy)hydroxides and hydroxysulfates, generally as precipitates, result in further release of acidity

47

(Eqs. 1–3) 1-3.

48

FeS2 + 3.5O2 + H2O → Fe2+ + 2SO42− + 2H+

(1)

49

Fe2+ + 0.25O2 + H+ → Fe3+ + 0.5H2O

(2)

50

Fe3+ + 2H2O → FeO(OH) + 3H+ (goethite precipitation)

(3)

51

ARD management is often focused on treating acid on release. In general, a better and more

52

environmental and economically sustainable ARD remediation approach is to reduce the release

53

of ARD at source 2, 4. Among various at-source approaches 4, surface passivation is an emerging

54

and appealing methodology for control of acid generation through reduction in pyrite oxidation

55

rate, due to reduced ingress of water, air, and/or oxidising bacteria to the pyrite surface 5, 6 or the

56

electrochemical activity of pyrite 7. The three most common types of surface coatings that have

57

been examined are organics

58

organics and phosphates can slow pyrite oxidation, their application in the field is hindered due to

59

the high concentrations of chemicals and harsh conditions required to develop organic coatings,

60

and possible secondary pollution from organics and eutrophication of freshwater environments

61

by phosphates.

5, 7-9

, phosphates

6, 10-12

and silicates


9, 11, 13, 14

. Although the use of


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Recently, we have described a viable mechanism for reduction of ARD through the formation of

63

a compact silicate-stabilised Fe (oxy)hydroxide surface layer on pyrite, limiting the access of

64

oxygen and/or water to the pyrite surface and thus reducing the pyrite oxidation rate 1, 13, 15. With

65

silicate addition, a smooth, continuous, coherent and apparently amorphous silicate-doped Fe

66

(oxy)hydroxide surface layer is observed, with pyrite dissolution rates consequently reduced by

67

more than 90% at circum-neutral pH 1. The formation of silicate/silica-based surface passivating

68

coating requires circum-neutral or alkaline pH conditions 9, 11, 13, 14. It has been proposed that this

69

is because these pH conditions favour the precipitation of Fe (oxy)hydroxides which then react

70

with silicate to form complex surface layers

71

commonly present due to the dissolution of reactive silicates

72

towards Fe (oxy)hydroxides

73

(AGR), neutralisation from reactive (alumino)silicate minerals may be used to maintain neutral

74

pH and the integrity of these layers for effective ARD management. This combination of reduced

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AGR and neutralisation offers cost-reduction in ARD management but has yet to be fully

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implemented in site trials or operation.

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Aluminosilicates such as K-feldspar, anorthite and chlorite, commonly found in pyritic waste

78

rocks and tailings, can provide not only aqueous silicate but also sources of aqueous Al3+ upon

79

dissolution. Incorporation of silicate has been found to hinder the transformation of semi-

80

amorphous Fe (oxy)hydroxides to more stable crystalline phases (e.g. goethite)

81

also observed a similar inhibitive role of aqueous silicate. In the presence of aqueous silicate, a

82

compact and dense amorphous-like Fe (oxy)hydroxide surface layer forms at circum-neural pH,

83

in contrast to a porous crystalline goethite surface layer formed in the absence of silicate 1. Al3+

17, 18

11

. In mine wastes, aqueous silicate species, 16

, have high adsorption affinities

. As a consequence of the greatly reduced acid generation rate


19-21

. We have

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84

species appear to have a similar effect to silicates in inhibiting the transformation of amorphous

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Fe (oxy)hydroxides to crystalline phases. Schwertmann et al.22 examined the effect of pH (4–7)

86

and co-precipitated versus mechanically added mixtures of Fe and Al ions and reported that the

87

incorporation of Al3+ into ferrihydrite inhibited the transformation to goethite and hematite at

88

25 °C. Cismasu et al.

89

with increasing precipitation rate and concentration of Al3+ doping at room temperature. Pinney

90

and Morgan

91

for transformation of Fe (oxy)hydroxides to hematite was reduced when doped with Al3+. In

92

comparison, the inhibitive role of Al3+ in the transformation of ferrihydrite to crystalline goethite

93

and hematite is less pronounced at elevated temperatures. For example, Lewis and Schwertmann

94

25

95

70 °C over 16 days in KOH solutions (0.001–1 M) with Al3+ (0.5–150 mM KAlO2), and that low

96

concentrations of Al3+ (e.g. < 10 mM Al in 0.1 M KOH) are favourable for the formation of Al3+-

97

doped goethite. It is noted that the transformation from ferrihydrite (molar volume of 34.5 cm3

98

mol–1) to goethite (20.3 cm3 mol–1) or hematite (30.4 cm3 mol–1) is associated with a decrease in

99

molar volume 26, possibly resulting in the generation of porosity.

24

23

also found that the crystallinity of Al-ferrihydrite precipitates decreased

found, using ab initio quantum simulation, that the thermodynamic driving force

found that ferrihydrite can be readily transformed into hematite and Al3+-doped goethite at

100

These studies suggest that inclusion of Al3+ into Fe (oxy)hydroxides on the pyrite surface could

101

potentially reduce both the rate and driving force for transformation from semi-amorphous Fe

102

(oxy)hydroxides to non-passivating crystalline Fe (oxy)hydroxides at moderate ambient

103

temperatures, thus stabilising the passivation layer in a way similar to silicates. However, the role

104

of Al3+ species in stabilising Fe (oxy)hydroxides surface layers on pyrite and the potential for

105

enhanced pyrite passivation and acid generation reduction has not been investigated. Therefore,



106

the aims of this study were to study the role of aqueous Al3+ species in pyrite oxidation under

107

various ARD-relevant pH conditions and explore the possible mechanism(s) for establishment of

108

Al3+-mediated surface passivation.

109

110

2.

METHODOLOGY

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2.1

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Pyrite (Geo Discoveries, NSW, Australia) was ground and wet-sieved to obtain a 38–75 µm

113

fraction prior to sonication for removal of fines. The pyrite particles were then washed with 3 M

114

HCl, followed by ethanol (99.9%), and dried under vacuum overnight at room temperature. These

115

dry particles were then immediately used for batch dissolution tests (Section 2.2). The pyrite was

116

bulk assayed and found to contain minor impurities (all in wt%) of 0.04 Si, 0.01 Al, 0.01 Ca, 0.01

117

Cu, 0.1 Pb, and 0.03 Zn. Powder X-ray diffraction analysis (Bruker D4 Endeavor diffractometer

118

with Co Kα radiation) indicated that the pyrite contained no other crystalline phases. The BET

119

surface area of the sized pyrite sample was determined to be 0.85 ± 0.01 m2 g−1.

120

2.2

121

Batch dissolution is generally carried out under well-stirred conditions. However, as the aim of

122

this study was to simulate ARD in rock or tailings storage where static conditions are more

123

commonly encountered, experiments were carried out under quiescent air-saturated conditions.

124

For each batch dissolution test, 2 g of pyrite was added to 1 L of solution (except for two

125

experiments where 6 g of pyrite was added, presented in Supporting Information, SI, S1) in a

126

wide-mouth Nalgene® HDPE bottle. KCl solution (0.01 M) was used for all batch dissolution

127

experiments to maintain similar ionic strength

Materials

Batch dissolution

27

. pH was controlled at 2.0 and 4.0 using dilute


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HCl or NaOH solution as necessary. pH was controlled at 7.0–7.8 using calcite-saturated solution,

129

as per our previous study 1. Batch dissolution experiments were run for 282 days at room

130

temperature (22 ± 2 °C). pH was monitored 3–4 times per week.

131

The total added Al3+ concentrations chosen are typical of acidic drainage waste waters

132

10 (0.37 mM) or 20 mg L−1 (0.74 mM) Al3+ (added as AlCl3·6H2O) referred to throughout this

133

study was the total added Al3+ concentration prior to equilibration and pyrite addition. Note: one

134

experiment at pH 7.4 was carried out with 20 mg L−1 Al3+ added after addition of pyrite (SI S1).

135

20 mg L−1 Al3+ was added at all pH conditions, whereas 10 mg L−1 Al3+ was added only at pH 7.4.

136

Solutions containing Al3+ were equilibrated overnight and initial pH was measured and adjusted,

137

if necessary, prior to addition of the pyrite. The actual concentration of aqueous Al3+ species after

138

equilibration is dependent on pH. At circum-neutral pH, no Al3+ was detectable by solution ICP-

139

MS at time zero, i.e. on addition of pyrite.

140

For pH measurement, collection of solid/solution samples and HNO3 extraction of Fe

141

(oxy)hydroxide precipitates on the container walls, please refer to SI S2.

28-30

. The

142 143

2.3

Instrumental analyses

144

The sized pyrite (38–75 µm) used in the batch dissolution tests was subjected to a 5-point, N2

145

Brunauer-Emmett-Teller (BET) surface area analysis, using a Micromeritics Gemini 2375

146

instrument, to determine specific surface area as per Nelsen and Eggertsen 31.

147

Solution ICP-MS (Agilent 7500ce) analysis was undertaken to obtain Al, Fe and S concentrations

148

in solutions, with detection limit of 0.1 ppm and around ± 10% analytical errors.



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149

Scanning electron microscopy (SEM; Quanta 450 Environmental SEM) equipped with energy

150

dispersive spectrometry (EDS) was used for examination of fresh pyrite and pyrite reacted for

151

282 days at various pH with and without Al3+ addition. The operating voltage was set to 15 kV.

152

Ultra-thin sections of pyrite were prepared using a dual-beam focused ion beam (FIB)/SEM (FEI

153

Helios Nanolab 600) with an in situ lift-out technique

154

applied on each pyrite sample prior to preparation of the FIB sections. Conditions used for

155

FIB/SEM were as per reference 1. These pyrite FIB sections were stored in a desiccator under

156

vacuum until transmission electron microscopy (TEM) analysis. TEM measurements including

157

imaging, elemental mapping and selected area electron diffraction (SAED) were carried out using

158

a JEOL TEM (JEM-2100HR) operated at 200 kV.

159

X-ray photoelectron spectroscopy (XPS) measurements were carried out using a monochromated

160

Al Kα (1486.6 eV) irradiation source operating at 225 W (Kratos AXIS Ultra DLD). A cold stage

161

(150 K) was used to minimise volatility of surface species such as elemental S. XPS survey, C 1s,

162

S 2p, Fe 2p and O 1s high-resolution spectra were recorded as per reference

163

resolution XPS spectra were aligned using the C 1s peak for adventitious carbon (284.8 eV 34).

164

X-ray absorption near edge structure (XANES) spectra at the Al K-edge were collected using the

165

soft X-ray spectroscopy beamline at the Australian Synchrotron, operating at 3 GeV with a beam

166

current at 200 mA. Al-containing reference samples − cancrinite, sodalite, zeolite, Al sulfate,

167

corundum, diaspore, boehmite, bayerite, kaolinite, nordstrandite, gibbsite and X-ray amorphous

168

Al(OH)3 − were obtained commerically or synthesised with phase purity examined using powder

32

. A protective Pt surface coating was


33

. All high-

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XRD analysis. For more details regarding the beamline, data collection/processing and reference

170

samples, please refer to SI S2 and Table S1.

171 172

3.

RESULTS AND DISCUSSION

173

3.1

174

Pyrite dissolution was calculated based on released S ([S]). Pyrite dissolution at pH 2.0 and 4.0

175

with and without added Al3+ and at pH 7.4 without added Al3+ followed parabolic behaviours

176

(Figure 1a–c). Both linear and non-linear pyrite dissolution trends have been observed previously

177

35-38

178

observed in our previous work

179

conditions were similar except for the addition of Al3+ species in this study as compared to the

180

addition of silicate species previously. It has been suggested that the presence of grain edges and

181

corners, defects, solid and fluid inclusions pits, cleavages and fractures within pyrite can all

182

influence its oxidation 35.

183

At pH 2.0 and 4.0, there was no significant difference in the pyrite dissolution in the absence or

184

presence of 20 mg L−1 Al3+ over 282 days (Figure 1a,b). In contrast, at pH 7.4 with 20 mg L−1

185

Al3+ addition the pyrite dissolution rate effectively plateaued after around 50 days and only 0.17

186

mmol·m−2 pyrite was dissolved over 282 days, as compared to 1.54 mmol·m−2 pyrite dissolved

187

without Al3+ over the same period of time, a reduction of ≈90% (Figure 1c). The addition of Al3+

188

subsequent to pyrite addition at pH 7.4 resulted in negligible differences in pyrite dissolution

189

kinetics (SI S1). For a clear presentation of the effect of Al3+ addition, the pyrite dissolved with

Dissolution Rates

. The contrasting parabolic dissolution trends in this study and the linear dissolution trends 1

are likely due to the different pyrite sources, as the dissolution



190

20 mg L−1 Al3+ minus that dissolved without Al3+, at each pH, is plotted as a function of time

191

(Figure 1d).

192

193 194

FIGURE 1. Surface area-normalised pyrite dissolution (calculated based on released S) and the

195

percentage of pyrite dissolved as a function of time at (a) pH 2.0, (b) pH 4.0 and (c) pH 7.4 with

196

and without added Al3+. (d) Pyrite dissolution for the system with 20 mg L−1 added Al3+ (total

197

amount added) minus pyrite dissolution with no added Al3+. Solid lines represent the best fits of

198

the data and are included in the figure as guides to the eyes. Errors for the experimental data were

199

estimated to be ±10% (solution ICP-MS analytical errors).

200 201

Both the initial (120 days) dissolution rates were calculated based on

202

linear fittings of the batch dissolution data normalised to the total initial pyrite surface area of


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203

1.7 m2 (Table 1). The greatest percentage of pyrite dissolved was ≈15% at 282 days (pH 7.4 with

204

no added Al3+; Figure 1c), with a maximum decrease in the surface area of ≈10% calculated

205

using a shrinking spherical particle model (Eq. 7 in reference 39). This suggests that variations in

206

pyrite surface areas during dissolution in this study were not significant and hence the initial

207

pyrite surface area was used for rate calculation. The rate of pyrite dissolution with 20 mg L−1

208

added Al3+ at pH 7.4 is initially ( 99.9%) precipitated after equilibration and prior to pyrite addition,

264

probably as Al(OH)3 (Figure 2c).

265

266 267

FIGURE 2. Pyrite dissolution without and with added 20 mg L–1 Al3+ (Pyrite diss, Pyrite diss +

268

Al); the concentration of Fe in the absence and presence of Al3+ ([Fe], [Fe] +Al); the amount of

269

all Fe (oxy)hydroxides precipitated (Fe precipitates on the pyrite surfaces and container walls)

270

(Fe precip, Fe precip +Al); and the precipitated Al3+ ([Al] precip) at (a) pH 2.0, (b) pH 4.0 and (c)

271

pH 7.4. The concentration of aqueous Al3+ is represented by the difference between the total

272

added amount of Al3+ (dashed line) and [Al] precip (black triangles). All data presented here are

273

normalised to the total initial pyrite surface area (1.7 m2). For clarity only approximately 50% of

274

the data are presented in these figures.


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275

3.3

Characterisation of the Passivating Surface Layer

276

3.3.1

X-ray photoelectron spectroscopy (XPS)

277

XPS survey spectral analyses of pyrite samples reacted at pH 7.4 over 282 days without added

278

Al3+ showed that the surface contained O, Fe, S and several other elements, but no Al (SI S4,

279

Table S2). The pyrite surface reacted at pH 7.4 with 20 mg L−1 added Al3+ comprised O, Al, Fe

280

(Al:Fe atomic ratio ≈15.6) and minor amounts of S and other elements, suggesting the formation

281

or accumulation of Al (hydro)oxides on the pyrite surface. All samples reacted at pH 2.0 and 4.0

282

with and without added Al3+ and unreacted pyrite had similar surface compositions (SI S4, Table

283

S2), with no Al observed.

284

Examination of Fe 2p data (SI S4) showed that Fe(III)-O/-OH species (Fe 2p3/2 ≈712 eV; 33, 39, 41,

285

42

286

No Fe 2p3/2 peak from unreacted pyrite (≈707 eV 43; see Fe 2p data for unreacted pyrite in SI S4)

287

was observed, indicating that the entire pyrite surface was covered with an oxidised layer

288

regardless of Al3+ addition, consistent with the thickness of the surface layers (≈250 nm)

289

observed using TEM (Section 3.3.2). Analysis of Fe 2p data for pyrite reacted at pH 2.0 and 4.0

290

with or without added 20 mg L−1 added Al3+ suggested that Fe(II)-S, Fe(III)-S and Fe(III)-O/-OH

291

species were present on the pyrite surfaces.

292

O 1s data (SI S4) showed that all pyrite samples gave rise to peaks centred in the range 531.2–

293

531.7 eV suggesting that surface O was predominantly in the form of hydroxide 44. Analysis of

294

S 2p data (SI S4) indicated that S on the pyrite surface reacted at pH 7.4 without Al3+ was in the

295

form of sulfate (S 2p3/2 binding energy ≈168.7 eV). No sulfate was observed for pyrite reacted at

) were present on the pyrite surfaces reacted with or without 20 mg L−1 added Al3+ at pH 7.4.



296

7.4 with added Al3+, due to the very low concentration of S on the pyrite surface (0.5 at%; SI S4,

297

Table S2) and the S 2p spectrum being not well-resolved. In comparison, dissolution at pH 2.0

298

and 4.0 with and without added Al3+ gave rise to various surface S species including disulfide

299

(162.3 eV), polysulfide (163.5 eV), elemental sulfur (164.5 eV), sulfite (166.5 eV) and, in some

300

cases, sulfate (168.7 eV) 44, 45.

301 302

3.3.2 Electron microscopy

303

SEM analysis showed that pyrite dissolved at pH 2.0 and 4.0 for 282 days, without and with

304

20 mg L−1 added Al3+, had largely similar surface morphology with numerous ‘islands’ (SI S5).

305

SEM-EDS analysis of these surfaces indicated the presence of Fe, S and a very minor

306

concentration of O (several at%), suggesting the presence of small concentrations of Fe

307

(oxy)hydroxides on the pyrite surfaces. No Al was found on the pyrite surfaces dissolved at pH

308

2.0 and 4.0 in the presence of added Al3+.

309

In contrast, a significant difference in the morphology was observed between the surface layers

310

formed on the pyrite after 282 days of dissolution without and with 20 mg L−1 added Al3+ at

311

circum-neutral pH. A coating with needle-like morphology was present in the absence of Al3+

312

(Figure 3a and c), similar to goethite (α-FeOOH) morphologies observed previously 1. In the

313

presence of added Al3+, the surface coating is distinctly different, with a more uniform and

314

smooth structure (Figure 3b and d). EDS analysis indicated that both surface layers contained

315

significant amounts of Fe, S and O, with a small amount of Al (≈ 2 at%) in the pyrite surface

316

layer reacted in the presence of added Al3+.


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317 318

FIGURE 3. SEM analysis of pyrite reacted at pH 7.4 for 282 days. (a) without added Al3+; (b)

319

with 20 mg L−1 added Al3+. (c) and (d): Enlarged SEM images of (a) and (b).

320 321

TEM imaging (Figure 4a) and electron diffraction analysis (SI S6, Figure S5a) of the FIB cross-

322

sections of the pyrite surface reacted without added Al3+ at pH 7.4 for 282 days showed a

323

crystalline goethite layer of varying densities, as indicated by the mottled colouring. In contrast,

324

the goethite surface layer (confirmed by TEM electron diffraction; SI S6, Figure S5c) on pyrite

325

reacted in the presence of Al3+ for 282 days was more uniform in density (Figure 4c).

326

As pyrite dissolved at a much faster rate (≈50-fold) in the absence of Al3+ than in the presence of

327

Al3+ at pH 7.4 and Fe in solution was below the detection limit of solution ICP-MS in both cases,

328

greater Fe-containing precipitation would be expected for pyrite dissolution in the absence of

329

Al3+ and the oxidation surface layer would be expected to be thicker. However, the thicknesses of



330

the surface layers were observed to be almost the same, regardless of Al3+ addition. This is due to

331

the large amount of Fe-containing precipitates on the reaction container for pyrite dissolution

332

without added Al3+, equivalent to ≈0.9 mmol m−2 dissolved pyrite (SI S2). In contrast, no

333

apparent Fe-containing precipitates were observed on the reaction container for pyrite dissolution

334

at pH 7.4 with added Al3+.

335 336

FIGURE 4. TEM images of cross sections of pyrite surface layers reacted at pH 7.4. (a) after

337

282 days of dissolution with no added Al3+, (b) after 7 days of dissolution with 20 mg L−1 added

338

Al3+, and (c) after 282 days of dissolution with 20 mg L−1 added Al3+ (c). In image (a), the Pt

339

coating was almost lost after the FIB-SEM sample preparation.

340


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341

The surface layers formed on pyrite reacted at pH 7.4 for 282 days in the presence of Al3+ (20 mg

342

L−1) comprised two distinct layers: an Al-rich layer

The formation of aluminium hydroxide-doped surface passivating

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