Arsenic Mobilization Is Enhanced by Thermal Transformation of

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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

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Environmental Science & Technology

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Arsenic mobilization is enhanced by thermal transformation

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of schwertmannite

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

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

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A

Southern Cross Geoscience

Southern Cross University, Lismore, NSW 2480, Australia

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Graphical abstract

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ACS Paragon Plus Environment


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ABSTRACT

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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INTRODUCTION

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

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

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

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

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

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

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control on aqueous arsenic mobility.5, 13-16

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

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

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

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

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

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

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

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minerals.22

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Thermal transformation of iron oxyhydroxides drives dehydroxylation and increases iron

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

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

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Increasing iron oxide crystallinity has major geochemical consequences for wetland sediment

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

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

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

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

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

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Thermal transformation of iron oxyhydroxides may also have consequences for the

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

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

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

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formed hematite.24,25,33

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

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

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

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

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

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

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

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

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participate in surface exchange reactions?

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

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

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

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

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

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

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

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

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

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EXPERIMENTAL SECTION

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

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

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

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

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

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

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

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

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

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

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

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

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

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occur during fire in organic-rich wetland sediments.20

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Long-term biotic incubation experiment

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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Aqueous-phase analysis

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

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

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

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

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

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

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

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

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

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

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



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

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Aqueous-phase dynamics

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

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

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

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

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

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where Fe2+ increases were modest (mostly

Arsenic Mobilization Is Enhanced by Thermal Transformation of

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