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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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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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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
