Arsenic Mobilization from Historically Contaminated Mining Soils in a

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Arsenic mobilization from historically contaminated mining soils in a continuously operated bioreactor: Implications for risk assessment Liwia Rajpert, Boris A. Kolvenbach, Erik M Ammann, Kerstin Hockmann, Maarten Nachtegaal, Elisabeth Eiche, Andreas Schaeffer, Philippe F.-X. Corvini, Aleksandra Sklodowska, and Markus Lenz Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02037 • Publication Date (Web): 25 Jul 2016 Downloaded from http://pubs.acs.org on July 31, 2016

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Environmental Science & Technology

A

B

C

Figure 1. Mobilization of As (A), Fe (B) and Mn (C) in terms of elemental mobilization rate (circles, primary Y-axis) and total element mobilized (triangles, secondary Y-axis).

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A

B

Figure 2. XRF elemental mapping (1 × 1.5 mm) of the initial (A) and reduced (B) soil with arsenic hotspots (As ≥ 0.3% wt) in overlay (dashed lines).


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normalized absorbance

intial soil

reduced soil

As(V)

As(III) FeAsS

11860 11870 11880 11890 energy [eV]

Figure 3. Normalized As K-edge XANES spectra of initial and reduced soil. Vertical lines mark main edge crest energies of reduced (FeAsS, arsenopyrite) and oxidized (As[III], NaAsO2; As[V], Na2HAsO4 × 7H2O) species.



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Figure 4. Arsenite (open circles), arsenate (solid circles) effluent concentration and number of copies of arrA normalized to 16S rRNA genes (solid squares, secondary Y axis).


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200

0.4

DOC [mg L-1]

0.0 100

-0.2 -0.4

50 -0.6 0 0

500

1000 1500 Time [hours]

2000

Redox potential [V]

0.2 150

-0.8 2500

Figure 5. Dissolved Organic Carbon (DOC) concentrations in reactor effluent (circles, primary Y axis) and normalized redox potential (squares, secondary y axis).



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Table 1. Linear regression analysis using multiple independent variables (top) and Mn as sole independent variable (bottom). Coefficients Standard Error t Stat

P-value

Intercept

1913.1

169.14

11.31

2.53 × 10-11*

Mn [µg L-1]

0.578

0.06

8.94

2.92 × 10-09*

Fe[II] [µg L-1]

1.998

1.41

1.42

0.1683

DOC [µg L-1]

-7.488

3.09

-2.42

0.0231

Redox [mV]

1076.246

493.59

2.18

0.0388

R² = 0.835; adjusted R²= 0.809; ANOVA F = 31.7; p-value: 1.86 × 10-09

Intercept

1621.8

118.09

13.73

5.80 × 10-14*

Mn [µg L-1]

0.499

0.05

9.68

1.96 × 10-10*

R² = 0.770; adjusted R²= 0.762; ANOVA F = 93.7 p-value: 1.96 × 10-10

* highly significant (< 0.001)


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Table 2. Elemental composition of spots containing elevated concentrations of As (≥ 0.3% wt) alone or in conjunction with elevated Mn (≥ 0.14% wt) and/or Fe (≥ 8% wt) concentrations. Initial soil

Reduced soil

Hotspot total area composition

No. of spots

total area No. of spots

[%]

[%]

As

1

0.34

7

1.43

As+Mn

3

0.21

1

0.28

As+Fe

2

0.52

1

0.40

As+Mn+Fe

10

3.34

1

0.56

Sum

16

4.41

10

2.67



mobilization

from

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1

Arsenic

historically

2

contaminated mining soils in a continuously

3

operated bioreactor: Implications for risk

4

assessment

5

Liwia Rajpert†, Boris A. Kolvenbach†, Erik M. Ammann†, Kerstin Hockmann‡, Maarten

6

Nachtegaal§, Elisabeth Eiche#, Andreas Schäffer¥, Philippe F.-X. Corvini†, Aleksandra

7

Skłodowska˟, Markus Lenz†,ǁ,*

8 9

†

Institute for Ecopreneurship, School of Life Sciences, University of Applied Sciences and

10

Arts Northwestern Switzerland, Gründenstrasse 40, 4132 Muttenz, Switzerland

11

‡

12

Zürich, Universitätstrasse 16, 8092 Zürich, Switzerland, current address: Southern Cross

13

GeoScience, Southern Cross University, 1 Military Road, Lismore 2480, Australia

14

§

Paul Scherrer Institute, 5232 Villigen – PSI, Switzerland

15

#

Institute of Applied Geosciences, Karlsruhe Institute of Technology (KIT), Adenauerring

16

20b, 76131 Karlsruhe, Germany

17

¥

18

Aachen, Germany

19

˟ Laboratory of Environmental Pollution Analysis, University of Warsaw, 02-096 Warsaw,

20

Poland

21

ǁ

22

Wageningen, The Netherlands

Institute of Terrestrial Ecosystems, Department of Environmental Systems Science, ETH

Institute for Environmental Research (Biology V), RWTH Aachen University, 52074

Sub-Department of Environmental Technology, Wageningen University, 6700 EV


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23 24

*

25

TOC ART

corresponding address: [email protected], (T) +41 614 674 791, (F) +41 614 674 290

26 27 28

Abstract: Concentrations of soil arsenic (As) in the vicinity of the former Złoty Stok gold

29

mine (Lower Silesia, southwest Poland) exceed 1000 µg g-1 in the area, posing an inherent

30

threat to neighbouring bodies of water. This study investigated continuous As mobilization

31

under reducing conditions for more than 3 months. In particular, the capacity of autochthonic

32

microflora that live on natural organic matter as the sole carbon/electron source for mobilizing

33

As was assessed. A bi-phasic mobilization of As was observed. In the first two months, As

34

mobilization was mainly conferred by Mn dissolution despite the prevalence of Fe (0.1% wt

35

vs. 5.4for Mn and Fe, resp.) as indicated by multiple regression analysis. Thereafter, the

36

sudden increase in aqueous As[III] (up to 2400 µg L-1) was attributed to an almost quintupling

37

of the autochthonic dissimilatory As-reducing community (quantitative polymerase chain

38

reaction). The aqueous speciation influenced by microbial activity led to a reduction of solid

39

phase As species (X-ray absorption fine structure spectroscopy) and a change in the elemental

40

composition of As hotspots (micro X-ray fluorescence mapping). The depletion of most

41

natural dissolved organic matter and the fact that an extensive mobilization of As[III]

42

occurred after two months raises concerns about the long term stability of historically As-

43

contaminated sites. 2 ACS Paragon Plus Environment


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44 45

Introduction

46

Arsenic (As) is an element with a low crustal average abundance (2–3 µg g-1)1 that is

47

unevenly distributed in the environment. Often, elevated As concentrations are associated

48

with mining activities because As is commonly found in copper, lead, or gold ores.2,3 The

49

Złoty Stok mine (Lower Silesia, southwest Poland) was founded in 1273 and was one of the

50

main gold suppliers for Europe from the 17th century until its closure in 1962.4–6 The basic

51

rock in the mine is composed of mica schists, mica-quartz schists, and quartzite schists7 while

52

the main As-bearing minerals are loellingite (FeAs2), scorodite (FeAsO4 × 2H2O), and

53

arsenopyrite (FeAsS).8 These mining activities generated a large amount of As-rich waste

54

material, which was disposed of in nearby valleys, including the Trująca Valley.5–7

55

Occasional seepage and dam overflow affected parts of the valley localized downstream from

56

the tailings disposal areas,9,4 which caused As soil contamination exceeding 1000 µg As g-1.5,4

57

Mobilization of As from its solid host phases into surface and groundwater caused increased

58

As concentrations ranging from 990 µg As L-1 (springs located on the deposit’s periphery) to

59

26160 µg As L-1 (water-draining audits).6 Poisoning from As in this region was reported as

60

early as the 19th century and described as “Reihensteiner Krankheit” (Złoty Stok Disease).2,10

61

Mobilization of As causing contamination of natural waters is understood to occur

62

primarily through four mechanisms: (I) via direct desorption under alkaline conditions; (ii) via

63

reductive dissolution of As bearing minerals; (iii) via oxidation of reduced As-S-minerals and

64

(iv) via geothermal waters. 11 Microbes may play a catalytic role in mobilization, for instance

65

through dissimilatory reduction of sorbed As[V] and subsequent release of As[III] in the

66

aqueous phase.12

67

For the Złoty Stok area, the main source of As-contaminated groundwater stems from an

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oxidative, microbe-catalysed dissolution of As-bearing minerals.13–15 Mobilization of As that

69

is driven by the reductive dissolution of Fe phases has received a great deal of attention in the 3 ACS Paragon Plus Environment

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context of the mass poisoning in southeast Asia,12 and was presumed to be dominant in

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different lake and river sediments,16–18 shallow and intermediate aquifers,19 and paddy

72

soils.20,21 In contrast to that, the reductive dissolution of Mn phases has been studied to a

73

lesser extent.22–26 Whereas the microbial diversity of mine biofilms and their impact on As

74

mineral dissolution has been studied well,15,27,28 mobilization of As by autochthonic soil

75

microbial communities has not been extensively investigated. In particular, studies are scarce

76

on As mobilization in soils impacted by old mines under reducing conditions. Such reducing

77

conditions may be induced locally through changes in soil use (compaction and resulting

78

water-logging) and at larger scale throughflooding events29. In addition, heterogenic soil

79

aggregates within the bulk soil may bear redox / chemical gradients, allowing for spatially

80

confined anoxic biogeochemical reactions30

81

Therefore, the present study investigated the mechanisms that govern As mobilization

82

from heavily contaminated soil in a continuously operated bioreactor over a period of more

83

than three months. A continuously operated bioreactor was used to distinguish the

84

contribution of solid phase reductive dissolution (Fe, Mn phases) and dissimilatory As

85

reduction on As mobilization in time, which is hard to accomplish in batch experiments. A

86

combination of speciation methods in the solid and liquid phases, i.e., liquid chromatography-

87

inductively coupled plasma mass spectrometry (LC-ICP-MS), X-ray absorption near edge

88

structure spectroscopy (XANES), elemental soil mapping (micro X-ray fluorescence, µXRF),

89

and molecular biology (quantitative polymerase chain reaction, qPCR) was applied. Emphasis

90

was given to elucidating the role on As mobilization of the autochthonic soil microflora that

91

feed only on natural organic matter.

92 93

Materials and Methods

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Soil sampling



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The soil originated from the gold mine in Złoty Stok (Lower Silesia, southwest

96

Poland). Samples (~20 kg) were collected from the surface layer (0-20 cm) from one point on

97

the bank of the Trująca River (“Poisonous Stream”), into which the former gold mine

98

drained.4,6 The soil was air-dried, sieved (mesh size 2 mm), and homogenized using the cone

99

and quarter technique.31 This soil is hereinafter referred to as “initial soil”, whereas the soil

100

obtained after a reactor operation is referred to as “reduced soil”.

101 102

Bioreactor operation

103

Mesophilic (21 ± 5 °C) completely stirred 1 L tank reactor (Multifors, Infors HT,

104

Bottmingen, Switzerland) was inoculated with soil (10% w/v) and operated for about 100

105

days at a hydraulic retention time of 48 h at a slightly basic pH (pH = 8.0 +/- 0.6, using

106

automated acid/base dosing). The medium contained no external organic carbon source (SI).

107

Anaerobic conditions were maintained by providing a continuous N2 flow in the reactor

108

headspace. The off gas was directed through two gas traps (1% H2O2,32 and 69% HNO333) to

109

capture potentially formed As-volatile species. Fresh medium was stored at 4 °C, filtrated

110

(0.22 µm filters, Merck Millipore, Molsheim, France) and continuously supplied to the reactor

111

by a peristaltic pump. The reactor effluent was pumped through a self-made, low-density

112

polyethylene filter to a gravitational settler (additional hydraulic retention time of 72 h) in

113

order to separate fine soil particles. Subsequently, the liquid was transported from the settler

114

to a waste container.

115 116

Liquid phase analysis

117

Samples (~20 mL) were collected from the reactor, centrifuged (4500 rcf, 10 min, 21

118

°C), and the resulting supernatant was syringe filtered (0.45 µm and 0.2 µm pore size,

119

Whatman, Hertogenbosch, The Netherlands). The totally dissolved element concentrations

120

were analysed on an Agilent 7500cx ICP-MS (SI).Samples for As speciation were preserved 5 ACS Paragon Plus Environment

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with EDTA/AcOH34, stored at 4 °C, and measured within 72 h after sampling. For detection

122

of As species, i.e., arsenite (As[III]), arsenate As[V], monomethylarsonic acid (MMA[V]),

123

and dimethylarsinic acid (DMA[V]), a LC system (SI) was coupled to the ICP-MS. Dissolved

124

iron (Fe[II]) was determined by a spectrophotometric method with 1.10-phenantroline35 using

125

0.5 M HCl to preserve the original speciation. Dissolved organic carbon (DOC) was detected

126

with a Liquid Chromatography-Organic Carbon Detection Model 8 (LC-OCD, DOC Labor

127

Huber, Karlsruhe, Germany)36

128 129

Solid phase characterization

130

Total element soil concentrations were determined using an energy dispersive X-ray

131

fluorescence (EDXRF) analysis (Ametek SpectroXRF Xepos III, Spectro, Kleve, Germany).

132

Major crystalline soil phases were characterized by X-ray powder diffraction (XRD) on

133

milled samples using a Bruker AXS D8 (Bruker AXS GmbH, Karlsruhe, Germany) Advance

134

system (CuKα1) equipped with a Lynxeye superspeed detector at a scanning speed of 0.009°

135

s-1. Elemental mapping in the energy range 2–20 keV was performed on a bench scale µ-XRF

136

instrument (µ-Eagle III, Röntgenanalytik GmbH, Taunusstein, Germany) equipped with a

137

Si(Li) detector. Excitation conditions in the rhodium (Rh) X-ray tube were set to 50 kV and

138

500 mA. An additional 50 µm-thick Rh filter was set into the path of the X-rays to eliminate

139

most of the Bremsstrahlung and to provide nearly monochromatic X-rays with the

140

characteristic RhKα and RhKβ radiation. The beam was focused by a polycapillary lens to a

141

spot size of about 30 µm in diameter. Areas of 1 × 1.5 mm2 were scanned with a 30 µm step

142

size with 100 s (initial) and 50 s (reduced) dwell time. PyMca software37 was used for the

143

analysis of the received X-ray intensity-energy spectra. Prior to the analysis, the soil samples

144

were pellet pressed (Manual Hydraulic Press 15–25 Ton, Specac Limited, River House,

145

England) and moulded with EpoMet®G hot mounting epoxy compound (SimpliMet 2



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Mounting Press, Buehler, Lake Bluff, USA). Subsequently, polished sections of each sample

147

were prepared using an optical polisher (Tegra-Pol 21, Struers A/S, Ballerup, Denmark).

148

The arsenic K-edge spectra of soil samples were recorded at the SuperXAS beamline

149

of the Swiss Light Source (SLS, Paul Scherrer Institut, Villigen, Switzerland). The

150

measurements and data processing is detailed in the supplementary information.

151 152

Metagenomic DNA isolation

153

Soil samples were collected into 50 mL falcon tubes (Greiner Bio-One GmbH,

154

Frickenhausen, Germany) that had been rinsed beforehand with a 10% (v/v) NaClO solution

155

in sterile, nanopure water to remove any DNA.38 Soil slurry aliquots were centrifuged (4500 ×

156

g, 15 min), the supernatants were discarded, and the pellets were stored at -20 °C until

157

analysis. Metagenomic DNA was isolated from the soil pellets according to Miller et al. 38 and

158

Sagova-Mareckova et al.,39 with minor modifications (SI). The purity of the extracted DNA

159

was measured using a UV-VIS spectrophotometer (SI, Table S1) (NanoDrop ND-1000

160

Spectrophotometer, Thermo Scientific, Wilmington, USA) and analysed by means of

161

electrophoresis (BioRad Sub-Cell®GT, BioRad Laboratories, Hercules, USA).

162 163

Quantification of the 16S rRNA genes and dissimilatory arsenate reduction (arrA) genes

164

Quantification of 16S rRNA (~180bp) genes was performed with the primers

165

described by Clifford et al.40 Each 25-µL reaction volume contained 12.5 µL of SYBR Green

166

qPCR Master Mix, 40 nM of each primer, 1 µL of DNA templates of known concentrations of

167

standards (10-1–10-8) or 1 µL of 100 × diluted DNA extracted from soil samples. The PCR

168

protocol was 15 min at 95 °C, with 45 cycles consisting of 10 s at 95 °C, 20 s at 58 °C, and 1

169

min at 72 °C, then 50 s at 72 °C. The amplified 16S rRNA gene (1466bp) from Chrysiogenes

170

arsenatis BAL-1 in dilutions of 10-1–10-8 was used for preparation of the standard curve.


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171

Quantification of arrA (~180bp) genes was performed with primers described by

172

Malasarn et al.41 Each 20 µL reaction contained 10 µL SYBR Green qPCR Master Mix, 1.25

173

mM MgCl2, 0.5 µM of each primer, and 1 µl of DNA templates of known concentrations of

174

standards (10-1–10-8) or 1 µl of 100 × diluted DNA extracted from soil samples. The

175

amplification cycles included an initial denaturation at 95 °C for 15 min followed by 40

176

cycles of denaturation at 95 °C for 30 s, annealing at 52 °C for 40 s, and extension at 72 °C

177

for 1 min. The amplified arrA gene (893bp) from Chrysiogenes arsenatis BAL-1 in dilutions

178

of 10-1–10-10 was used for preparation of the standard curve (SI).

179 180

Statistical Data Treatment

181

A multiple linear regression analysis was performed to determine possible correlations

182

between total As, Fe[II], Mn, and DOC-effluent concentrations as well as the redox values

183

using Microsoft Excel (Microsoft Corporation, Redmond, USA). These were done over the

184

entire reactor operation (SI, Figure S5, Table S2) and during the first 1400 h only (Table 1, SI,

185

Figures S6-7). Local polynomial regression fitting (LOESS) was employed on all replicate

186

measurements to smooth the As mobilization rate (SI, Figure S1).

187 188

Results

189

Continuous element mobilization

190

Mobilization of As followed two distinctive phases: 0–1400 h (phase I) and 1400–

191

2352 h (phase II). Their identification was based on the total As mobilization rate (Figure 1A,

192

SI, Figure S1), the increase in As[III] and arrA gene copies (Figure 4), redox potential, and

193

DOC concentration (Figure 5).

194

Phase I was characterized by an initially high As mobilization rate (max. 94 µg L-1 h-1

195

at 72 h) and a gradual decrease until ~1400 h, with a minimal mobilization rate of 25 µg L-1 h-

196

1

(912 h, Figure 1A). In this timespan, 685 µg g-1 of As (~35% of total) were mobilized from 8 ACS Paragon Plus Environment


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the soil. Phase II started at ~1400 h, with a maximal mobilization rate of 83 µg L-1 h-1. After

198

~1600 h, the As mobilization decreased considerably and reached a minimum mobilization

199

rate of 4 µg L-1 h-1 after 2088 h. (Figure 1A). Overall, 990 µg g-1 (corresponding to 49% of the

200

total As) were mobilized during reactor operation.

201

Initially, Fe was mobilized to an average of 28 µg L-1 h-1 (0-576 h). At ~700 h,

202

mobilization peaked and 120 µg L-1 h-1 were released for a short period of time (Figure 1B;

203

SI, Figures S2-3). Afterwards, Fe was mobilized at a considerable lower rate (0.16% Mn, Figure 2

248

middle).

249

The As K-edge spectra of the initial and reduced soil were clearly different in the

250

XANES region. In the initial soil dominated features at the energy characteristic for more

251

oxidized species (11873.8 eV for As[V]). Whereas in the sample taken upon termination of

252

the reactor operation appeared a distinct feature at the energy characteristic for more reduced

253

species (11870.0 eV for As[III]) (Figure 3). Accordingly, the best LCF that was obtained in

254

the initial sample yielded scorodite almost exclusively (11874.6 eV, 96%, SI, Figure S10),

255

which matches the mineralogy of the Złoty Stok region.8 In the reduced soil, the best LCF that

256

was obtained showed a ~60% contribution of the reduced species (the best fit yielded

257

arsenolite and As[Cys]3) and only a ~40% contribution of scorodite (SI, Figure S10, Table

258

S3). However, despite the large amount of possible reference compounds used, some residual

259

components could not be fitted. As a result, species structurally similar to scorodite may

260

contribute to the soil speciation as well, particularly because As K-edge XANES features can

261

be very similar for different model compounds43,44.

262 263

Development of a dissimilatory arsenate-reducing bacterial community

264

Under reducing conditions (0.0–0.4 V) occurring within the reactor, arsenite was the

265

main species detected (Figures 4-5). Mobilization of both arsenite and arsenate was initially

266

high (effluent concentrations up to ~3100 µg L-1 As[III] and ~2400 µg L-1 As[V]), and

267

decreased during phase I. After ~1000 h, the increase in the As mobilization rate (Figure 1A)

268

was mainly related to arsenite (max. As[III] concentration 2398 µg L-1 at ~1500 h). Within the

269

same time frame, an increase of arrA gene copies was detected. The latter is a reliable marker

270

for metabolically active Dissimilatory Arsenate Reducing Bacteria (DARBs).41 The highest

271

amount of arrA gene copies that had normalized to an overall number of 16S rRNA gene

272

copies was observed at 1536 h (max. arrA gene copy number 1.57-3). Subsequently, the 11 ACS Paragon Plus Environment

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273

considerable decline of arrA gene copies number was observed (min. arrA gene copy number

274

3.00-4 at1872 h, Figure 4). The increase of DARB activity was accompanied by a peak in

275

DOC concentration and an increase in redox potential (~1500 h, Figure 5).

276 277

Discussion

278

Continuous As mobilization from historically contaminated soil

279

Under reducing conditions, the indirect arsenate mobilization via dissolution of Fe[III]

280

(oxyhydroxides) and oxides is considered as one of the main routes of As release in sediments

281

and water-logged soils.12,45 This study gives evidence that, despite their considerably lower

282

soil concentrations, the reductive dissolution of Mn phases was the main factor controlling As

283

release for more than 2 months (phase I). When both Fe and Mn pools that were prone to

284

reductive dissolution were depleted, the mobilization of As was ultimately controlled by the

285

activity of autochthonous DARBs that were developing on natural organic matter.

286

Surprisingly, this occurred suddenly and after a 2-month reactor operation under constant

287

conditions. Due to historical As soil contamination, potential autochthonic DARBs were

288

believed to have developed over long periods of time2,4,6 and available As directly depleted.

289

The dynamics of As continuous mobilization may be related to a complex interplay between

290

solid As-bearing phases and bio-induced changes in speciation. In the initial soil, the elements

291

As, Fe, and Mn co-occurred as indicated by µXRF mappings (Figure 2AB, Table 2), although

292

no crystalline As/Fe/Mn mineral phases were detectable using XRD (SI, Figure S9). The

293

XANES analysis demonstrated a high (96%) contribution of oxidised As species in the initial

294

soil (Figure 3). The best LCF yielded mainly scorodite (FeAsO4 × 2H2O), which is one of the

295

most common natural arsenates in acidic, oxidizing environments (SI, Table S3). Furthermore

296

the presence of scorodite as an oxidation product of arsenopyrite or loellingite46–48 is in good

297

accordance with previous studies on the mineralogy of the region.4–6. The primary dissolution

298

phase was characterized by a high As mobilization rate (Figure 1A), which was accompanied 12 ACS Paragon Plus Environment


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299

by a rapid Fe mobilization within the first ~800 h (Figure 1B). However, multiple linear

300

regression analysis yielded dissolved Mn alone as a significant factor (p = 2.9×10-9)

301

influencing As concentrations until ~1400 hours of reactor run (Table 1; SI, Figure S7). In

302

contrast to other studies,18,49,50 neither dissolved Fe (SI, Figures S2-3) nor DOC (Figure 5)

303

impacted As mobilization from the soil (p = 0.17 and p = 0.02, respectively) (Table 1). The

304

fact that Mn mobilization steadily increased during the first ~3 days after the start suggests

305

that the dissolution is bio-mediated. Due to the similarity in As K-edge XANES features of

306

different model compounds43,44 species structurally similar to scorodite may well contribute to

307

the soil As speciation. Thus, we rather assign As being associated to Mn and/or mixed Fe-Mn

308

phases than pure scorodite, which is supported by µXRF mappings as well (Table 2).

309

Manganese-oxides have been reported to affect overall As mobility via adsorption and

310

oxidation of arsenite to arsenate,45 which may explain the occurrence of arsenate (~24% of

311

total released arsenic) in the liquid phase despite the reducing conditions (Figure 5) and

312

DARB activity (phase II, Figure 4). Those results are in good agreement with Lafferty et al.,51

313

who showed that As, once adsorbed to δ-MnO2, exists only in the form of arsenate. Though

314

the precise mechanism remains to be discovered, the use of a continuous bioreactor in contrast

315

to batch studies had the clear advantage of separating the underlying processes in time. In

316

batch experiments, steadily increasing element concentrations may lead to a misinterpretation

317

of the dominant processes.

318

During phase I, low redox potentials (-0.2–-0.4 V) and high concentrations of DOC

319

(Figure 5) constituted a readily available carbon source for developing autochthonic

320

microflora. In consequence, a sudden increase in As mobilization was observed after more

321

than 2 months of operation. Since As mobilization was mainly related to As[III] (Figure 4)

322

and an almost fivefold relative increase in the number of arrA gene copies was detected

323

(Figure 4), this could be attributed to the activity of indigenous DARBs. The higher redox


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324

potential measured during this period was consistent with the As[V]/As[III] couple as the

325

dominant redox reaction.52,53

326

Upon the end of the reactor operation (~1700 h), a small amount of As was mobilized,

327

which could be explained by a depletion of As-bearing Fe/Mn pools and/or formation of

328

secondary As-bearing phases. For instance, the arsenate produced in reaction with Mn-oxides

329

might be subsequently adsorbed to Fe[III]/Mn-(oxyhydr)oxides and/or precipitated with

330

microbially produced Fe[II] as As-sequestering ferrihydrite.45,51,54–56 Reductive dissolution of

331

scorodite was recently found to yield Fe-As phases (biogenic ferrous arsenite and

332

parasymplesite (Fe3[AsO4]2 × 8H2O).57 However, the µXRF mapping did not indicate an

333

increase of Fe/As rich hotspots in the soil after the reactor was run (Figure 2B, Table 2).

334

Whereas elemental maps showed mainly mixed Mn/Fe/As phases in the initial soil, the

335

reduced soil contained mostly hotspots consisting only of As (Table 2). XANES of the

336

reduced soil demonstrated a more reduced speciation of As in the solid phase (Figure 3),

337

though the exact identity remains unknown due to the inability of the XANES analysis to

338

clearly distinguish between structurally similar As phases.44, 43 Yamaguchi et al.

339

that the share of arsenite in the paddy soil solid phase after anaerobic incubation increased

340

considerably, while the proportion of arsenite in the solid phase in gamma-irradiated samples

341

did not change. This suggests that the presence of As[III] in the soil solid phase of this study

342

may have a biotic origin as well (Figure 3). The autochthonic microflora was either able to

343

reduce arsenate directly in the soil solid phase,21,58 or As[III] released by DARB was

344

secondarily immobilized. The formation of As[III]-NOM complexes might indeed explain

345

why no µXRF amendable element (Fe, Mn, S, Ca, etc.) was found to be associated with the

346

As hotspots in the reduced soil (Table 2, Figure 2).

20

reported

347 348

Implications for risk assessment of historically contaminated soils



Page 22 of 31

349

Contamination of the Złoty Stok region with As has been under investigation since the

350

1960s.59,60 The emerging role of As-hypertolerant, oxidizing/reducing bacteria that contribute

351

substantially to the release of As from minerals inside the gold mine was recently

352

described.15,27,28 This study adds to the identification of historically contaminated soil as a

353

latent and considerable source of As release. The fact that monitoring total element

354

concentration is an insufficient way to assess contaminant risk has become generally accepted

355

in recent years.61 Therefore, different simplified approaches such as sequential extraction

356

schemes, eluate tests, or in vitro bioaccessibility tests are applied to estimate site-specific

357

risks.62–66 This study questions the applicability of the aforementioned: (i) a substantial and

358

sudden mobilization of As occurred after times much longer than any accelerated test (~60

359

days); (ii) the mobilization was not related to the reductive dissolution of any major As-

360

bearing phase and is thus not accessible to chemical extraction targeting the latter; (iii) the

361

mobilization occurred due to the DARBs that inevitably developed on a refractory pool of As

362

(i.e., after easily accessible As was already leached). Furthermore, the DARB activity resulted

363

in the release of As[III] yet not in formation ofvolatile arsines (LOD = 1 µg g-1 of gas traps),

364

being of high toxicity in comparison to the oxyanions.

365

contaminated soils. 69–72

67,68

The latter are often emitted from

366

The fact that mining activities ceased more than 50 years ago would suggest that most

367

of the potentially easily available As would have already been leached. This study proves the

368

contrary, which is that an induction of reducing conditions, e.g., by changes in soil use

369

(compaction and resulting water logging; heavy irrigation), changes in hydrology and / or

370

seasonal flooding events, certainly poses a threat to potable water contamination of this

371

region. The As deposits in the area of the Złoty Stok gold mine are the largest in Poland and

372

are the main source of surface and groundwater contamination in the Klodzko Valley.73 In

373

spite of this, the lower part of the Trująca Valley is still in agricultural use as arable fields,

374

meadows, and pastures, which increases the risk of As exposure and the possible entry of As 15 ACS Paragon Plus Environment

Page 23 of 31


375

into the human food chain.4,5,74 Due to an increasing demand for potable water in the Złoty

376

Stok area, use of the surface water (i.e., the Gold Stream) has been considered.6 This study

377

underlines that this requires not only suitable water treatment, but also long-term monitoring

378

programs for historically contaminated soil as one of the main reservoirs of this carcinogenic

379

element.6

380 381

Associated Content

382

Supporting Information

383

Bulk X-ray fluorescence analysis, X-ray diffraction patterns, XANES reference compounds,

384

detailed information on extraction schemes, total element mobilization, molecular biology

385

techniques and statistical analysis can be found in the supporting information. This

386

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

387 388

Author Information

389

Corresponding Author

390

*Phone/fax: +41 614 674 791/ +41 614 674 290; e-mail: [email protected].

391

Notes

392

The authors declare no competing financial interest.

393 394

Acknowledgments

395

The authors thank Christoph Giger for his contribution to the bioreactor design. We thank the

396

participants of the " Cook and Look " course at PSI, during which the XAFS data were

397

recorded and Professor Yungmin Pan (University of Saskatchewan), who kindly provided

398

additional reference spectra. The support of the Rectors' Conference of the Swiss Universities

399

CRUS within the Sciex-NMSch scholarship (11.130) is gratefully acknowledged. Authors

400

thank Utz Kramar for his help during µXRF measurements at KIT. 16 ACS Paragon Plus Environment


Page 24 of 31

401 402

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