Arsenic Mobilization Is Enhanced by Thermal ... - ACS Publications

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Arsenic Mobilization Is Enhanced by Thermal Transformation of Schwertmannite Scott G. Johnston,* Edward D. Burton, and Ellen M. Moon Southern Cross Geoscience, Southern Cross University, Lismore, New South Wales 2480, Australia S Supporting Information *

ABSTRACT: Fires in iron-rich seasonal wetlands can thermally transform Fe(III) minerals and alter their crystallinity. However, the fate of As associated with thermally transformed Fe(III) minerals is unclear, as are the consequences for As mobilization during subsequent reflooding and reductive cycles. Here, we subject As(V)-coprecipitated schwertmannite to thermal transformation (200, 400, 600 and 800 °C) followed by biotic reductive incubation (150 d) and examine aqueousand solid-phase speciation of As, Fe and S. Heating to >400 °C caused transformation of schwertmannite to a nanocrystalline hematite with greater surface area and smaller particle size. Higher temperatures also caused the initially structurally incorporated As to become progressively more exchangeable, increasing surface-complexed As (AsEx) by up to 60fold, thereby triggering enhanced As mobilization during incubation (∼70-fold in the 800 °C treatment). Although more As was mobilized in biotic treatments than controls (∼3−20×), in both cases it was directly proportional to initial AsEx and mainly due to abiotic desorption. Higher transformation temperatures also drove divergent pathways of Fe and S biomineralization and led to more As(V) and SO4 reduction relative to Fe(III) reduction. This study reveals thermal transformation of schwertmannite can greatly increase As mobility and has major consequences for As/Fe/S speciation under reducing conditions. Further research is warranted to unravel the wider implications for water quality in natural wetlands.

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transforms to hematite (αFe2O3).21 Increasing iron oxide crystallinity has major geochemical consequences for wetland sediment during reductive cycles26,27 and can lead to SO4 reduction becoming thermodynamically favored over Fe(III) reduction as a dominant pathway of anaerobic carbon metabolism.28 Therefore, a partial or complete transformation of schwertmannite to hematite via fire-induced thermal transformation is likely to have profound consequences for Fe and S biomineralization pathways in ASS wetlands, especially during subsequent wet periods. Thermal transformation of iron oxyhydroxides may also have consequences for the partitioning and availability of coprecipitated trace metals or metalloids.25,29−32 For example, thermal transformation of naturally occurring goethite-rich material can increase trace metal availability by causing preferential migration of some trace metals to the surface of neo-formed hematite.24,25,33 However, the consequences of thermally transforming schwertmannite for the partitioning, availability and subsequent mobilization of structurally incorporated As during Fe(III)- and SO4-reducing conditions are essentially unknown and, to our

INTRODUCTION Arsenic behavior in aquatic and sedimentary environments is closely linked to the redox cycling of various iron minerals.1−4 Poorly crystalline Fe(III) minerals, such as schwertmannite (Fe8O8(OH)6SO4), can be important sinks for both As(V) and As(III), particularly in acid mine drainage settings and acid sulfate soils (ASS).4−9 There are millions of hectares of ASS globally10 and they typically contain an abundant and diverse array of Fe minerals.11,12 In the surface sediments of ASS wetlands, schwertmannite can exert a major control on aqueous arsenic mobility.5,13−16 Australian ASS wetlands are highly prone to extreme oscillations in water levels and redox conditions due to seasonal climate fluctuations.17,18 During wet episodes, the schwertmannite-rich surface sediments in ASS wetlands can be subject to Fe(III)- and SO4-reducing conditions,17 which can enhance As mobilization in both surface and porewaters.13,15,19 However, during prolonged drought conditions, large wild-fires can also occur in ASS wetlands.20 Fires in ASS wetlands may burn organic-rich and schwertmannite-rich surface sediments,21 potentially causing spatially extensive thermal transformation of Fe(III) minerals.22 Thermal transformation of iron oxyhydroxides drives dehydroxylation and increases iron oxide crystallinity.23−25 When temperatures exceed ∼600 °C, sulfur in schwertmannite volatilizes from the crystal structure and schwertmannite © 2016 American Chemical Society

Received: Revised: Accepted: Published: 8010

May 25, 2016 July 11, 2016 July 12, 2016 July 12, 2016 DOI: 10.1021/acs.est.6b02618 Environ. Sci. Technol. 2016, 50, 8010−8019

Article

Environmental Science & Technology

groundwater suspension was inoculated with 0.5 mL of a 1:20 soil/water suspension prepared from freshly collected surface soil from a local ASS wetland. Suspensions were transferred to an anaerobic chamber containing an O2-free atmosphere of 97−98% N2 and 2−3% H2 and the headspace allowed to equilibrate for 16 h prior to closing the gastight screw caps of each centrifuge tube. To resupply microbially consumed organic C, an additional 2 mL of an anoxic 100 g L−1 glucose solution was added to remaining vials at day 22. A series of control incubations were also prepared as described above, except they were not inoculated with 1:20 soil/water suspension and lacked a source of electron donors (i.e., no glucose, yeast extract or humic acid), and the additional 2 mL of anoxic solution added at day 22 also lacked a source of C. Aqueous-Phase Analysis. Triplicate vials were sacrificed at each sampling time for analysis of aqueous- and solid-phase properties. After centrifugation (4000 rpm, 5 min), the supernatant solution was filtered to Porod exponent (d) > 4). Scattering from the 200, 400 and 600 °C samples had contributions from two different phases; a phase similar to the schwertmannite internal structure (Rg1), and a smaller, emergent phase (Rg2) (Table 1). The Porod exponent and dimensionality of the smaller, emergent phase correspond to ellipsoidal particles with smooth surfaces. Applying the radii of gyration and the relationship for spherical objects of Rg = R(3/ 5)1/2, the diameter of these smaller particles is estimated to increase from ∼5 nm at 200 °C to ∼24 nm at 600 °C and ∼75 nm at 800 °C, whereas their volume fraction also increases markedly with temperature. The SANS derived estimate of particle diameter at 800 °C is broadly consistent with SEM observations. At 800 °C, there is no scattering contribution from the schwertmannite lathe-like structure, only from the smaller, ellipsoidal, hematite phase. Aqueous-Phase Dynamics. Figure 4 shows the evolution of aqueous As, pH, pE, Fe2+ and SO42− over time during incubation of all treatments. During the first ∼2−3 weeks of biotic incubation, the pH increased to 4−5 in most treatments and pE decreased relative to controls (Figure 4). There were also large increases in Fe2+ (∼20−30 mM) during the first few weeks in all biotic incubations relative to controls (which displayed no Fe2+), except for the 800 °C treatment where Fe2+ increases were modest (mostly