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Phosphate-imposed constraints on schwertmannite stability under reducing conditions Valerie Schoepfer, Edward D. Burton, Scott G Johnston, and Peter Kraal Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 2, 2017
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Environmental Science & Technology
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Phosphate-imposed constraints on schwertmannite
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stability under reducing conditions
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Valerie A. SchoepferA, Edward D. Burton*A, Scott G. JohnstonA, Peter KraalB.
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A
Southern Cross GeoScience, Southern Cross University, PO Box 157, Lismore, New South
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Wales 2480, Australia B
Department of Earth Sciences - Geochemistry, Faculty of GeoSciences, Utrecht University, PO Box 80021, 3508 TA Utrecht, The Netherlands.
*Corresponding author; email:
[email protected] 9
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ABSTRACT: Schwertmannite is a ferric
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oxyhydroxysulfate
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common in acid sulfate systems. Such
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systems contain varying concentrations of
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phosphate (PO43-) - an essential nutrient
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whose availability may be coupled to
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schwertmannite formation and fate. This study examines the effect of phosphate on
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schwertmannite stability under reducing conditions. Phosphate was added at 0, 80, 400 and
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800 µmoles g-1 (i.e. zero, low, medium and high loading) to schwertmannite suspensions
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which were inoculated with wetland sediment and suspended in N2-purged artificial
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groundwater. pH remained between 2.7 and 4.3 over the 41 day experiment duration. Fe(II)
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accumulated in solution due to dissimilatory Fe(III)-reduction, which was most pronounced
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at intermediate PO43- loadings (i.e. in the low PO43- treatment). Partial transformation of
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schwertmannite to goethite occurred in the zero and low PO43- treatments, with negligible
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transformation in higher PO43- treatments. Overall, the results suggest that intermediate PO43-
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loadings provide conditions which facilitate optimal reductive dissolution of schwertmannite.
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At zero PO43- loading, reductive dissolution appears to be constrained by the rapid
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transformation of schwertmannite to goethite, which thereby decreases the bioavailability of
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solid-phase Fe(III). Conversely, at high loadings, PO43- appears to stabilise the
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schwertmannite surface against dissolution; probably via the formation of strong surface
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complexes.
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KEYWORDS. Iron reduction, phosphate, schwertmannite,
mineral,
which
is
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INTRODUCTION
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Acid sulfate systems are found around the world in relation to mining operations, rock
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weathering and acid sulfate soils1. These systems are characterized by low pH due to the
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oxidation of iron sulfide minerals, such as pyrite1. One of the most common secondary
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minerals in acid sulfate systems is schwertmannite; a poorly crystalline iron(III)
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oxyhydroxysulfate1–4. Schwertmannite is metastable with regard to goethite, with previous
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research showing that schwertmannite is stabilized by low pH2,3,5,6, high SO42-
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concentrations3, and the presence of strongly sorbed species (e.g. Si and humic/fulvic acids)6–
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.
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The formation and stability of schwertmannite is of significant interest in acid-mine
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drainage (AMD) environments and in acid sulfate soils. This is because schwertmannite plays
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a major role in controlling water quality by influencing both the generation and consumption
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of acidity and by acting as a sorbent for contaminants and nutrients9–11. Although
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schwertmannite forms under oxidizing conditions, it may be subjected to prolonged reducing
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conditions in re-flooded acid sulfate soils, mine pit-lake sediments or AMD-treatment
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wetlands6,12. Under reducing conditions, schwertmannite may strongly influence both Fe and
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S biogeochemistry and microbial electron flow through microbially-mediated dissimilatory
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Fe(III)-reduction13,14 or abiotic reduction by H2S produced via dissimilatory SO42-
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reduction15.
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Schwertmannite is capable of sorbing large amounts of PO43-, an essential nutrient which
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can also act as a pollutant depending on the levels of P in the system16,17. Collins et al.6 also
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observed significant P-enrichment in natural schwertmannite collected from an acid sulfate
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soil environment. Although very little research has examined interactions between PO43- and
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schwertmannite, PO43- interactions have been extensively documented with respect to other
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iron(III) oxides, such as ferrihydrite, goethite and hematite18-22. Borch et al.18 found that PO43-
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sorption stabilizes ferrihydrite against reductive dissolution, and that it also alters secondary
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mineral transformation pathways. As schwertmannite and ferrihydrite are similar with respect
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to surface area, structural disorder and metastability10,23–25, it can be hypothesized that
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schwertmannite stability may also be enhanced by the sorption of PO43- to the mineral
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surface.
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The objective of this study was to examine the effect of PO43- sorption on schwertmannite
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stability under reducing conditions. To achieve this objective, we subjected schwertmannite,
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with varying loadings of added PO43- to microbially-mediated reducing conditions for 41
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days and observed the aqueous and mineralogical species over time.
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METHODS
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Schwertmannite synthesis. Schwertmannite was synthesized via the peroxide method
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described by Regenspurg et al.5.
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deionized water, dried at 50° C and then ground to a fine powder using a ring mill.
The precipitated mineral was rinsed five times with
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Phosphate sorption isotherm analysis. PO43- sorption to schwertmannite was
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characterized at pH 2.6 – 3.0 using a batch sorption isotherm approach, involving 0.5 g of
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schwertmannite suspended in 40 mL of artificial groundwater (Supporting Information, Table
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S1) with initial aqueous PO43- concentrations of 0, 0.5, 1, 2, 3, 4, 5, 6, 6.5, 7, 7.5, 8, 9, and 10
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mM, added as KH2PO4. After a 24-hr equilibration period, the aqueous phase was separated
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by filtration to
