Rapid arsenic(V)-reduction by fire in schwertmannite-rich soil enhances arsenic mobilisation

Rapid arsenic(V)-reduction by fire in schwertmannite-rich soil enhances arsenic mobilisation Arsenic in acid sulfate soil (ASS) landscapes commonly associates with schwertmannite, a poorly crystalline Fe(III) mineral. Fires in ASS landscapes can thermally transform Fe(III) minerals to more crystalline phases, such as maghemite (γFe2O3). Although thermal genesis of maghemite requires electron transfer via organic matter pyrolysis, the possibility of fire causing concurrent transfer of electrons to schwertmannite-bound As(V) remains unexplored. Here, we subject an organic-rich soil with variable carbon content (∼9–44% organic C) mixed (4:1) with As(V)-bearing schwertmannite (total As of 4.7–5.4 μmol g−1), to various temperatures (200–800 °C) and heating durations (5–120 min). We explore the consequences for As and Fe via X-ray absorption spectroscopy, X-ray diffraction, 57Fe Mössbauer spectroscopy and selective extracts. Heating transforms schwertmannite to mainly maghemite and hematite at temperatures above 300–400 °C, with some transitory formation of magnetite, and electrons are readily transferred to both Fe(III) and As(V). As(V) reduction to As(III) is influenced by a combination of temperature, heating duration and carbon content and is significantly (P < 0.05) positively correlated with Fe(II) formation. During 2 h heating, higher carbon content favours greater As(III) and Fe(II) formation, while peak As(III) formation (∼44–70%) occurs at relatively modest temperatures (300 °C) and diminishes at higher temperatures. Kinetic heating experiments reveal fast maximum As(III) formation (∼90%) within 5–10 min at 400–600 °C, followed by partial re-oxidation to As(V) thereafter. In contrast, heating As(V)-schwertmannite in the absence of soil-organic matter did not cause reduction of As(V) or Fe(III), nor form maghemite; thus highlighting the critical role of organic matter as an electron donor. Importantly, combusted organic soil-schwertmannite mixtures display greatly enhanced mobilisation of As(III)aq species within 1 h of re-wetting with water. The magnitude of As(III)aq mobilisation is positively correlated with solid-phase As(III) formation. Overall, the results suggest that moderate fires in ASS landscapes, even of short duration, may generate considerable labile As(III) species and cause a pulse of As(III)aq mobilisation following initial re-wetting. Further research is warranted to examine if analogous As(III) formation occurs during combustion of organic-rich soil containing common As-bearing Fe(III) minerals such as ferrihydrite and goethite. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Geochimica et Cosmochimica Acta Elsevier

Rapid arsenic(V)-reduction by fire in schwertmannite-rich soil enhances arsenic mobilisation

Loading next page...
 
/lp/elsevier/rapid-arsenic-v-reduction-by-fire-in-schwertmannite-rich-soil-enhances-nuauC6ee8R
Publisher
Elsevier
Copyright
Copyright © 2018 Elsevier Ltd
ISSN
0016-7037
eISSN
1872-9533
D.O.I.
10.1016/j.gca.2018.01.031
Publisher site
See Article on Publisher Site

Abstract

Arsenic in acid sulfate soil (ASS) landscapes commonly associates with schwertmannite, a poorly crystalline Fe(III) mineral. Fires in ASS landscapes can thermally transform Fe(III) minerals to more crystalline phases, such as maghemite (γFe2O3). Although thermal genesis of maghemite requires electron transfer via organic matter pyrolysis, the possibility of fire causing concurrent transfer of electrons to schwertmannite-bound As(V) remains unexplored. Here, we subject an organic-rich soil with variable carbon content (∼9–44% organic C) mixed (4:1) with As(V)-bearing schwertmannite (total As of 4.7–5.4 μmol g−1), to various temperatures (200–800 °C) and heating durations (5–120 min). We explore the consequences for As and Fe via X-ray absorption spectroscopy, X-ray diffraction, 57Fe Mössbauer spectroscopy and selective extracts. Heating transforms schwertmannite to mainly maghemite and hematite at temperatures above 300–400 °C, with some transitory formation of magnetite, and electrons are readily transferred to both Fe(III) and As(V). As(V) reduction to As(III) is influenced by a combination of temperature, heating duration and carbon content and is significantly (P < 0.05) positively correlated with Fe(II) formation. During 2 h heating, higher carbon content favours greater As(III) and Fe(II) formation, while peak As(III) formation (∼44–70%) occurs at relatively modest temperatures (300 °C) and diminishes at higher temperatures. Kinetic heating experiments reveal fast maximum As(III) formation (∼90%) within 5–10 min at 400–600 °C, followed by partial re-oxidation to As(V) thereafter. In contrast, heating As(V)-schwertmannite in the absence of soil-organic matter did not cause reduction of As(V) or Fe(III), nor form maghemite; thus highlighting the critical role of organic matter as an electron donor. Importantly, combusted organic soil-schwertmannite mixtures display greatly enhanced mobilisation of As(III)aq species within 1 h of re-wetting with water. The magnitude of As(III)aq mobilisation is positively correlated with solid-phase As(III) formation. Overall, the results suggest that moderate fires in ASS landscapes, even of short duration, may generate considerable labile As(III) species and cause a pulse of As(III)aq mobilisation following initial re-wetting. Further research is warranted to examine if analogous As(III) formation occurs during combustion of organic-rich soil containing common As-bearing Fe(III) minerals such as ferrihydrite and goethite.

Journal

Geochimica et Cosmochimica ActaElsevier

Published: Apr 15, 2018

References

You’re reading a free preview. Subscribe to read the entire article.


DeepDyve is your
personal research library

It’s your single place to instantly
discover and read the research
that matters to you.

Enjoy affordable access to
over 18 million articles from more than
15,000 peer-reviewed journals.

All for just $49/month

Explore the DeepDyve Library

Search

Query the DeepDyve database, plus search all of PubMed and Google Scholar seamlessly

Organize

Save any article or search result from DeepDyve, PubMed, and Google Scholar... all in one place.

Access

Get unlimited, online access to over 18 million full-text articles from more than 15,000 scientific journals.

Your journals are on DeepDyve

Read from thousands of the leading scholarly journals from SpringerNature, Elsevier, Wiley-Blackwell, Oxford University Press and more.

All the latest content is available, no embargo periods.

See the journals in your area

DeepDyve

Freelancer

DeepDyve

Pro

Price

FREE

$49/month
$360/year

Save searches from
Google Scholar,
PubMed

Create lists to
organize your research

Export lists, citations

Read DeepDyve articles

Abstract access only

Unlimited access to over
18 million full-text articles

Print

20 pages / month

PDF Discount

20% off