Arsenic Mobilization Is Enhanced by Thermal Transformation of

Jul 12, 2016 - This omission raises several important questions directly relevant to water quality in seasonal ASS wetlands. For example, does thermal...
0 downloads 8 Views 2MB Size
Subscriber access provided by UNIV LAVAL

Article

Arsenic mobilization is enhanced by thermal transformation of schwertmannite Scott G Johnston, Edward D. Burton, and Ellen M. Moon Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02618 • Publication Date (Web): 12 Jul 2016 Downloaded from http://pubs.acs.org on July 13, 2016

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

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

Page 1 of 32

Environmental Science & Technology

1

Arsenic mobilization is enhanced by thermal transformation

2

of schwertmannite

3 4

Scott G. JohnstonA*, Edward D. BurtonA, Ellen M. MoonA

5

*Corresponding author (Scott G. Johnston: [email protected])

6 7 8 9 10

A

Southern Cross Geoscience

Southern Cross University, Lismore, NSW 2480, Australia

11 12

Graphical abstract

13

1

ACS Paragon Plus Environment

Environmental Science & Technology

Page 2 of 32

14

ABSTRACT

15

Fires in iron-rich seasonal wetlands can thermally transform Fe(III) minerals and alter their

16

crystallinity. However, the fate of As associated with thermally transformed Fe(III) minerals

17

is unclear, as are the consequences for As mobilization during subsequent reflooding and

18

reductive cycles. Here, we subject As(V)-coprecipitated schwertmannite to thermal

19

transformation (200 C, 400 C, 600 C, 800 C) followed by biotic reductive incubation (150 d)

20

and examine aqueous and solid-phase speciation of As, Fe and S. Heating to >400 C caused

21

transformation of schwertmannite to a nano-crystalline hematite with greater surface area and

22

smaller particle size. Higher temperatures also caused the initially structurally-incorporated

23

As to become progressively more exchangeable, increasing surface-complexed As (AsEx) by

24

up to 60-fold, thereby triggering enhanced As mobilisation during incubation (~70-fold in the

25

800 C treatment). Although more As was mobilized in biotic treatments than controls (~3-

26

20x), in both cases it was directly proportional to initial AsEx and mainly due to abiotic

27

desorption. Higher transformation temperatures also drove divergent pathways of Fe and S

28

biomineralisation and led to more As(V) and SO4 reduction relative to Fe(III) reduction. This

29

study reveals thermal transformation of schwertmannite can greatly increase As mobility and

30

has major consequences for As/Fe/S speciation under reducing conditions. Further research is

31

warranted to unravel the wider implications for water quality in natural wetlands.

32

KEYWORDS: Arsenic; Wetland; Sulfur; Iron; Hematite; Schwertmannite; Acid sulfate soil

o

o

o

o

o

o

33

2

ACS Paragon Plus Environment

Page 3 of 32

Environmental Science & Technology

34

INTRODUCTION

35

Arsenic behavior in aquatic and sedimentary environments is closely linked to the redox

36

cycling of various iron minerals.1-4 Poorly-crystalline Fe(III) minerals, such as

37

schwertmannite (Fe8O8(OH)6SO4), can be important sinks for both As(V) and As(III),

38

particularly in acid mine drainage settings and acid sulfate soils (ASS).4-9 There are millions

39

of hectares of ASS globally10 and they typically contain an abundant and diverse array of Fe

40

minerals.11,12 In the surface sediments of ASS wetlands, schwertmannite can exert a major

41

control on aqueous arsenic mobility.5, 13-16

42

Australian ASS wetlands are highly prone to extreme oscillations in water levels and redox

43

conditions due to seasonal climate fluctuations.17,18 During wet episodes, the schwertmannite-

44

rich surface sediments in ASS wetlands can be subject to Fe(III)- and SO4-reducing

45

conditions17, which can enhance As mobilization in both surface and porewaters.13,15,19

46

However, during prolonged drought conditions, large wild-fires can also occur in ASS

47

wetlands.20 Fires in ASS wetlands may burn organic-rich and schwertmannite-rich surface

48

sediments21, potentially causing spatially-extensive thermal transformation of Fe(III)

49

minerals.22

50

Thermal transformation of iron oxyhydroxides drives dehydroxylation and increases iron

51

oxide crystallinity.23-25 When temperatures exceed ~600 C, sulfur in schwertmannite

52

volatilizes from the crystal structure and schwertmannite transforms to hematite (αFe2O3).21

53

Increasing iron oxide crystallinity has major geochemical consequences for wetland sediment

54

during reductive cycles26,27 and can lead to SO4 reduction becoming thermodynamically

55

favored over Fe(III) reduction as a dominant pathway of anaerobic carbon metabolism.28

56

Therefore, a partial or complete transformation of schwertmannite to hematite via fire-

57

induced thermal transformation is likely to have profound consequences for Fe and S

o

3

ACS Paragon Plus Environment

Environmental Science & Technology

Page 4 of 32

58

biomineralisation pathways in ASS wetlands, especially during subsequent wet periods.

59

Thermal transformation of iron oxyhydroxides may also have consequences for the

60

partitioning and availability of coprecipitated trace metals or metalloids.25, 29-32 For example,

61

thermal transformation of naturally occurring goethite-rich material can increase trace metal

62

availability by causing preferential migration of some trace metals to the surface of neo-

63

formed hematite.24,25,33

64

However, the consequences of thermally transforming schwertmannite for the partitioning,

65

availability and subsequent mobilization of structurally incorporated As during Fe(III)- and

66

SO4-reducing conditions are essentially unknown and, to our knowledge, unstudied. This

67

omission raises several important questions directly relevant to water quality in seasonal ASS

68

wetlands. For example, does thermal transformation of schwertmannite to more crystalline

69

neo-hematite effectively retard As mobilization through incorporating As in a mineral phase

70

that is comparatively resistant to reductive dissolution? Or alternatively, does As re-partition

71

to a surface complex during thermal transformation and thus become more likely to

72

participate in surface exchange reactions?

73

This study directly addresses these questions, whereby we subject As(V)-coprecipitated

74

schwertmannite to a thermal transformation series (200 C, 400 C, 600 C and 800 C) and

75

examine the consequences for As mobilisation during subsequent long-term (150 d) biotic

76

reductive incubations. We investigate corresponding changes in mineralogy and Fe, S and As

77

species via aqueous phase analysis, selective extracts of solid-phase material, X-ray

78

diffraction (XRD), X-ray absorption spectroscopy (XAS), scanning electron microscopy

79

(SEM) and small angle neutron scattering (SANS). The aim is to explore how thermal

80

transformation of As-bearing schwertmannite, and thus fires, may influence As mobility and

81

the redox cycling of Fe and S in seasonal ASS wetlands.

o

ACS Paragon Plus Environment

o

o

o

4

Page 5 of 32

Environmental Science & Technology

82

EXPERIMENTAL SECTION

83

Synthesis of As(V)-coprecipitated schwertmannite and thermal transformation

84

As(V)-coprecipitated schwertmannite was synthesized by dissolving 1.5 kg of FeSO4·7H2O

85

in 50 L of water and then adding 800 mL of 30% H2O2.34 Na2HAsO4·7H2O (~9 g) was

86

dissolved in the initial solution (prior to the addition of H2O2)13 to generate an As(V) content

87

in the synthetic schwertmannite of ~2500 mg kg-1. While this concentration is higher than

88

typical ASS wetland sediment,19 it well within the range observed for natural

89

schwertmannite-rich precipitates formed in AMD settings.9 The resulting suspension was

90

rinsed 5 times with deionized water and subsequently dried at 50°C. Dried material was

91

finely ground and mineralogy verified by X-ray diffractometry. The initial schwertmannite

92

had a composition of 10.9 ±0.38 mmol g−1 Fe(III), 2.02 ±0.26 mmol g−1 SO42−, and 32.4

93

±0.59 µmol g−1 As. Dried schwertmannite was subject to thermal transformation by heating

94

in a ceramic crucible for 2 h at 200 C, 400 C, 600 C and 800 C in a muffle furnace in air.

95

This temperature range was selected to bracket the likely range of soil temperatures that can

96

occur during fire in organic-rich wetland sediments.20

97

Long-term biotic incubation experiment

98

The experiment involved batch incubations of synthetic As(V)-coprecipitated schwertmannite

99

and the thermally transformed products at room temperature (20 ± 1 °C) under anoxic

100

conditions for a maximum period of 150 d, with sampling intervals at 1, 2, 4, 7, 14, 22, 29,

101

37, 43, 57, 70, 90, 112 and 150 d. In brief, approximately 0.5 g of each mineral powder was

102

weighed into a series of 50 mL centrifuge tubes. Powder weights were varied slightly to

103

account for minor differences in total iron contents of each treatment and were between 103-

104

109 mmol L-1 of total solid-phase Fe equivalent (see Supporting Information Table SI1 for

105

mass loadings). Each centrifuge tube received 49.5 mL of deoxygenated artificial

106

groundwater with a composition comparable to that of ASS wetlands19,35 (ie. 2.5 mM CaCl2;

o

o

o

ACS Paragon Plus Environment

o

5

Environmental Science & Technology

107

5 mM KCl; 5 mM MgCl2; 0.05 mM KH2PO4; 1 mM Na4SiO4; 20 mM NaSO4; 1 mL L−1

108

Wolfe’s mineral solution; 1 g L−1 yeast extract; 0.1 g L−1 Aldrich humic acid; 6 g L−1

109

glucose; pH adjusted to 4.0 with HCl).13 Each mineral-groundwater suspension was

110

inoculated with 0.5 mL of a 1:20 soil/water suspension prepared from freshly collected

111

surface soil from a local ASS wetland. Suspensions were transferred to an anaerobic chamber

112

containing an O2-free atmosphere of 97−98% N2 and 2−3% H2 and the headspace allowed to

113

equilibrate for 16 h prior to closing the gas-tight screw caps of each centrifuge tube. To

114

resupply microbially consumed organic C, an additional 2 mL of an anoxic 100 g L−1 glucose

115

solution was added to remaining vials at day 22. A series of control incubations were also

116

prepared as described above, except they were not inoculated with 1:20 soil/water suspension

117

and lacked a source of electron donors (i.e. no glucose, yeast extract or humic acid), and the

118

additional 2 mL of anoxic solution added at day 22 also lacked a source of C.

119

Aqueous-phase analysis

120

Triplicate vials were sacrificed at each sampling time for analysis of aqueous- and solid-

121

phase properties. After centrifugation (4000 rpm, 5 min), the supernatant solution was filtered

122

to 4). Scattering from the 200°C, 400°C and 600°C samples had contributions from two

196

different phases; a phase similar to the schwertmannite internal structure (Rg1), and a smaller,

197

emergent phase (Rg2) (Table 1). The Porod exponent and dimensionality of the smaller,

198

emergent phase correspond to ellipsoidal particles with smooth surfaces. Applying the radii

199

of gyration and the relationship for spherical objects of Rg = R(3/5)½, the diameter of these

200

smaller particles is estimated to increase from ~5 nm at 200°C to ~24 nm at 600⁰C and ~75

201

nm at 800°C, while their volume fraction also increases markedly with temperature. The

202

SANS derived estimate of particle diameter at 800°C is broadly consistent with SEM

203

observations. At 800°C there is no scattering contribution from the schwertmannite lathe-like 9

ACS Paragon Plus Environment

Environmental Science & Technology

204

structure, only from the smaller, ellipsoidal, hematite phase.

205

Aqueous-phase dynamics

206

Figure 4 shows the evolution of aqueous As, pH, pE, Fe2+ and SO42- over time during

207

incubation of all treatments. During the first ~2-3 weeks of biotic incubation, the pH

208

increased to 4-5 in most treatments and pE decreased relative to controls (Figure 4). There

209

were also large increases in Fe2+ (~20-30 mM) during the first few weeks in all biotic

210

incubations relative to controls (which displayed no Fe2+), except for the 800°C treatment

211

where Fe2+ increases were modest (mostly