Sedimentary iron geochemistry in acidic waterways associated with coastal lowland acid sulfate soils

Edward D. Burton, Richard T. Bush, Leigh A. Sullivan

Research output: Contribution to journalArticle

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Abstract

We examined the solubility, mineralogy and geochemical transformations of sedimentary Fe in waterways associated with coastal lowland acid sulfate soils (CLASS). The waterways contained acidic (pH 3.26-3.54), FeIII-rich (27-138 μM) surface water with low molar Cl:SO4 ratios (0.086-5.73). The surficial benthic sediments had high concentrations of oxalate-extractable Fe(III) due to schwertmannite precipitation (kinetically favoured by 28-30% of aqueous surface water Fe being present as the FeIIISO4+ species). Subsurface sediments contained abundant pore-water HCO3 (6-20 mM) and were reducing (Eh < -100 mV) with pH 6.0-6.5. The development of reducing conditions caused reductive dissolution of buried schwertmannite and goethite (formed via in situ transformation of schwertmannite). As a consequence, pore-water FeII concentrations were high (>2 mM) and were constrained by precipitation-dissolution of siderite. The near-neutral, reducing conditions also promoted SO4-reduction and the formation of acid-volatile sulfide (AVS). The results show, for the first time for CLASS-associated waterways, that sedimentary AVS consisted mainly of disordered mackinawite. In the presence of abundant pore-water FeII, precipitation-dissolution of disordered mackinawite maintained very low (i.e. <0.1 μM) S-II concentrations. Such low concentrations of S-II caused slow rates for conversion of disordered mackinawite to pyrite, thereby resulting in relatively low concentrations of pyrite (<300 μmol g-1 as Fe) compared to disordered mackinawite (up to 590 μmol g-1 as Fe). This study shows that interactions between schwertmannite, goethite, siderite, disordered mackinawite and pyrite control the geochemical behaviour of sedimentary Fe in CLASS-associated waterways.

Original languageEnglish
Pages (from-to)5455-5468
Number of pages14
JournalGeochimica et Cosmochimica Acta
Volume70
Issue number22
DOIs
Publication statusPublished - 2006
Externally publishedYes

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mackinawite
acid sulfate soil
Geochemistry
Sulfates
Iron
geochemistry
Soils
iron
Acids
schwertmannite
pyrite
siderite
Sulfides
Surface waters
porewater
Dissolution
Surficial sediments
dissolution
sulfide
surface water

Cite this

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title = "Sedimentary iron geochemistry in acidic waterways associated with coastal lowland acid sulfate soils",
abstract = "We examined the solubility, mineralogy and geochemical transformations of sedimentary Fe in waterways associated with coastal lowland acid sulfate soils (CLASS). The waterways contained acidic (pH 3.26-3.54), FeIII-rich (27-138 μM) surface water with low molar Cl:SO4 ratios (0.086-5.73). The surficial benthic sediments had high concentrations of oxalate-extractable Fe(III) due to schwertmannite precipitation (kinetically favoured by 28-30{\%} of aqueous surface water Fe being present as the FeIIISO4+ species). Subsurface sediments contained abundant pore-water HCO3 (6-20 mM) and were reducing (Eh < -100 mV) with pH 6.0-6.5. The development of reducing conditions caused reductive dissolution of buried schwertmannite and goethite (formed via in situ transformation of schwertmannite). As a consequence, pore-water FeII concentrations were high (>2 mM) and were constrained by precipitation-dissolution of siderite. The near-neutral, reducing conditions also promoted SO4-reduction and the formation of acid-volatile sulfide (AVS). The results show, for the first time for CLASS-associated waterways, that sedimentary AVS consisted mainly of disordered mackinawite. In the presence of abundant pore-water FeII, precipitation-dissolution of disordered mackinawite maintained very low (i.e. <0.1 μM) S-II concentrations. Such low concentrations of S-II caused slow rates for conversion of disordered mackinawite to pyrite, thereby resulting in relatively low concentrations of pyrite (<300 μmol g-1 as Fe) compared to disordered mackinawite (up to 590 μmol g-1 as Fe). This study shows that interactions between schwertmannite, goethite, siderite, disordered mackinawite and pyrite control the geochemical behaviour of sedimentary Fe in CLASS-associated waterways.",
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Sedimentary iron geochemistry in acidic waterways associated with coastal lowland acid sulfate soils. / Burton, Edward D.; Bush, Richard T.; Sullivan, Leigh A.

In: Geochimica et Cosmochimica Acta, Vol. 70, No. 22, 2006, p. 5455-5468.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Sedimentary iron geochemistry in acidic waterways associated with coastal lowland acid sulfate soils

AU - Burton, Edward D.

AU - Bush, Richard T.

AU - Sullivan, Leigh A.

PY - 2006

Y1 - 2006

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AB - We examined the solubility, mineralogy and geochemical transformations of sedimentary Fe in waterways associated with coastal lowland acid sulfate soils (CLASS). The waterways contained acidic (pH 3.26-3.54), FeIII-rich (27-138 μM) surface water with low molar Cl:SO4 ratios (0.086-5.73). The surficial benthic sediments had high concentrations of oxalate-extractable Fe(III) due to schwertmannite precipitation (kinetically favoured by 28-30% of aqueous surface water Fe being present as the FeIIISO4+ species). Subsurface sediments contained abundant pore-water HCO3 (6-20 mM) and were reducing (Eh < -100 mV) with pH 6.0-6.5. The development of reducing conditions caused reductive dissolution of buried schwertmannite and goethite (formed via in situ transformation of schwertmannite). As a consequence, pore-water FeII concentrations were high (>2 mM) and were constrained by precipitation-dissolution of siderite. The near-neutral, reducing conditions also promoted SO4-reduction and the formation of acid-volatile sulfide (AVS). The results show, for the first time for CLASS-associated waterways, that sedimentary AVS consisted mainly of disordered mackinawite. In the presence of abundant pore-water FeII, precipitation-dissolution of disordered mackinawite maintained very low (i.e. <0.1 μM) S-II concentrations. Such low concentrations of S-II caused slow rates for conversion of disordered mackinawite to pyrite, thereby resulting in relatively low concentrations of pyrite (<300 μmol g-1 as Fe) compared to disordered mackinawite (up to 590 μmol g-1 as Fe). This study shows that interactions between schwertmannite, goethite, siderite, disordered mackinawite and pyrite control the geochemical behaviour of sedimentary Fe in CLASS-associated waterways.

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DO - 10.1016/j.gca.2006.08.016

M3 - Article

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EP - 5468

JO - Geochmica et Cosmochimica Acta

JF - Geochmica et Cosmochimica Acta

SN - 0016-7037

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