Conference presentation
Fire in iron‐rich seasonal wetlands: thermal transformation of schwertmannite can alter arsenic speciation and enhance mobility
International Conference on the Biogeochemistry of Trace Elements (ICOBTE), 14th (Zurich, Switzerland, 16/07/2017 - 20/07/2017)
18/07/2017
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Abstract
Schwertmannite in the organic‐rich surface layerss of acid sulfate soil (ASS) wetlands can exert a major control on arsenic mobility. ASS wetlands in eastern Australia are subject to extreme seasonal climate fluctuations that drive oscillating soil redox conditions and can lead to drought‐induced fires. Fire can thermally transform poorly crystalline Fe(III) minerals, including schwertmannite, to more crystalline phases such as hematite and maghemite. Some transfer of electrons from organic matter to Fe(III) during burning is crucial to generate the Fe(II) required to form maghemite. 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. In addition, the likelihood of concurrent electron transfer to As(V) within schwertmannite has not been investigated. Here, we report on the results of two experiments involving thermal transformation of As(V)‐bearing schwertmannite. Firstly, we subject a synthetic As(V)‐coprecipitated schwertmannite to thermal transformation (200, 400, 600, 800 o C) followed by biotic reductive incubation (150 d) and examine aqueous and solid‐phase speciation of As, Fe and S. Heating to >400 o C caused transformation of schwertmannite to nano‐crystalline 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 by up to 60‐fold, thereby triggering enhanced As mobilisation during reductive incubation. Higher transformation temperatures also drove divergent pathways of Fe and S biomineralisation and led to more As(V) and SO4 reduction relative to Fe(III) reduction. In a second experiment, we subject a natural wetland soil (~5% organic C) mixed (4:1) with As(V)‐coprecipitated schwertmannite to heating at 200, 400, 600, 800 o C. Combustion of organic‐rich soil containing As(V)‐bearing schwertmannite generates both hematite and maghemite and we demonstrate that some As(V) is reduced to As(III). Electron transfer to As(V) was non‐linear with respect to combustion temperature, with maximum As(III) formation (~39%) at ~400 o C. Heating schwertmannite in the absence of soil organic matter did not cause electron transfer either to As(V) or Fe(III), nor form maghemite, thus highlighting the critical role of combustion‐facilitated organic matter oxidation. Combusted soil‐schwertmannite mixtures also displayed enhanced mobilization of As 3+ (aq) species upon rewetting with water, with the magnitude of mobilization and changes in As(aq) speciation also being non‐linear with respect to combustion temperature and achieving maximum As 3+ (aq) at 400‐600 o C. This study reveals that fires in iron‐rich seasonal wetlands have the potential to alter As speciation and substantially enhance As mobility following reflooding.
Details
- Title
- Fire in iron‐rich seasonal wetlands: thermal transformation of schwertmannite can alter arsenic speciation and enhance mobility
- Creators
- Scott G Johnston (Corresponding Author) - Southern Cross UniversityEdward D Burton (Author) - Southern Cross UniversityEllen Moon (Author) - Southern Cross UniversityWilliam Bennett (Author) - Griffith University
- Conference
- International Conference on the Biogeochemistry of Trace Elements (ICOBTE), 14th (Zurich, Switzerland, 16/07/2017 - 20/07/2017)
- Identifiers
- 991012895000002368
- Academic Unit
- School of Environment, Science and Engineering; Southern Cross GeoScience; National Centre for Flood Research; Faculty of Science and Engineering; Science
- Language
- English
- Resource Type
- Conference presentation