Trace Element and Contaminant Fate during Fe(II)-Catalyzed Iron Oxide Surface

Transformations

Jeffrey G. Catalano

Goldschmidt 2013

Acknowledgements

Earth and Planetary Sciences • Washington University

Financial SupportUS NSF Geobiology and Low-Temperature GeochemistryUS NSF CAREERACS Petroleum Research FundWashington University in St. LouisArgonne National LaboratoryUS DOE/BES Geosciences Research Program

ContributorsAndrew Frierdich (U. Wisconsin)Katherine Becker (Wash U)Margaret Anne Gray Hinkle (Wash U)Yun Luo (GIA)Bamidele Otemuyiwa (Wash U)Frédéric Moynier (Wash U)

Biogeochemical Iron Cycling: Alteration of Iron Oxidation States

■ Microorganisms drive the cycling of iron between Fe(III) and Fe(II)

■ This redox cycling is never instantaneous and is often incomplete

■ Fe(II) and Fe(III) often coexist in many natural systems and may react

■ These secondary abiotic reactions constitute a “hidden component” of iron cycling

Earth and Planetary Sciences • Washington UniversityFigure from: Weber et al. (2007) Nature Rev. Micro. 4, 753-764

Mineral Recrystallization during Fe Cycling

Earth and Planetary Sciences • Washington University

Oxidative Fe(II) AdsorptionWilliams and Scherer (2004)

Electron ConductionYanina and Rosso (2008)

Atom ExchangeHandler et al. (2009)

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Effect of Fe(II)-Promoted Iron Oxide Recrystallization on Trace Element Fate

■ Trace element adsorption onto goethite is enhanced in the presence of Fe(II)– Fe(II) also causes a fraction of adsorbed Ni and Co to be bound

irreversibly, suggesting entrapment in the mineral structure■ Microbial reductive dissolution of Co-substituted goethite

shows excess, nonstoichiometric Co releaseFrom: Coughlin and Stone (1995) ES&T 29, 2445-2455; Zachara et al. (2001) GCA 65, 75-93

Excess Co Release during Fe ReductionFe(II)-Enhances TE Adsorption to Goethite

Earth and Planetary Sciences • Washington University

Effect of Fe(II)-Fe(III) Reactions on Trace

Element Fate is Uncertain

■ How does recrystallization affect adsorbates?■ What happens to trace elements substituting in iron

oxides during recrystallization?

■ Trace elements often incorporate into or adsorb on iron oxides

■ These elements are contaminants, micronutrients, or geochemical proxies

Effect of Fe(II) on As(V) Surface Speciation

■ EXAFS spectroscopy shows that As(V) surface complex structure is unchanged in the presence of Fe(II)

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pH 7

Catalano et al. (2011) ES&T 45, 8826–8833

Earth and Planetary Sciences • Washington University

Fe(II)-Induced Metal Incorporation into Iron Oxides

■ Fe(II) induces substantial changes in XANES and EXAFS spectra of Ni adsorbed onto hematite and goethite

■ Comparison with the spectra of Ni incorporated into and adsorbed onto hematite and goethite show clear isosbestic points– Indicates two-component mixing, i.e., adsorbed and incorporated Ni– Up to 50% of the adsorbed Ni becomes incorporated

4 g/L solid, 0.2 mM Ni(II), ±1 mM Fe(II), pH 7.5, 80 days

Frierdich et al. (2011) Geology 39, 1083-1086

Controls on and Possible Signatures of

Metal Incorporation

■ Substantial Zn incorporation occurs after 5 days of reaction

■ Longer reaction times and higher pH promote incorporation

■ Distinct Zn stable isotope fractionations produced by adsorption on goethite and hematite

■ Possible fractionation from incorporation into goethite

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Zn Incorporation into Goethite

Zn Isotope Fractionation

Green Rust Forms

Fe(II)-Catalyzed Metal Release from

Substituted Iron Oxides■ Metal-substituted iron oxides are stable

under circumneutral conditions– Little release occurs in electrolyte solutions

■ Fe(II) induces the release of nearly 10% of incorporated Ni or Zn from hematite or goethite after 2 weeks of reaction– Release rates orders of magnitude slower

than Fe(II) adsorption but comparable to Fe(II)-iron oxide isotope exchange

– Release varies among metals and minerals• Goethite: Ni release greater than Zn

• Hematite: Zn release greater than Ni

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1 g/L solid, pH 7, 1 mM MOPS, 0.01 M NaCl

Goethite

Hematite

Frierdich et al. (2011) Geology 39, 1083-1086Frierdich and Catalano (2012) ES&T 46, 1519-1526

Earth and Planetary Sciences • Washington University

Fe(II) Activates Trace Element Cycling Through Iron Oxides

■ Promotes equilibration among the mineral, mineral surface, and fluid– Metal incorporation in regions of Fe(II) oxidative adsorption and growth– Metal release in regions of Fe(III) reductive dissolution

Frierdich et al. (2011) Geology 39, 1083-1086

Copper, Cobalt, and Manganese Release Behaviors Suggest Reduction by Fe(II)

■ Limited, transient release of copper suggests reduction of Cu(II) to Cu(I)

■ Substantial Co and Mn release observed from iron oxides containing insoluble Co(III) and Mn(III/IV)

Earth and Planetary Sciences • Washington UniversityFrierdich and Catalano (2012) ES&T 46, 11070-11077

Evidence for Trace Element Reduction: Excess Fe(II) Consumption

■ More Fe(II) is removed from solution when iron oxides contain reducible trace elements

Earth and Planetary Sciences • Washington UniversityFrierdich and Catalano (2012) ES&T 46, 11070-11077

Spectroscopic Confirmation of Trace Element Reduction

■ Cu(II) is reduced to Cu(I); form is unknown■ Co(III) is reduced to Co(II), some released to solution but

substantial remains in mineral■ Mn in solid is reduced only to Mn(III); all Mn(II) produced

is released to solution

Earth and Planetary Sciences • Washington UniversityFrierdich and Catalano (2012) ES&T 46, 11070-11077

Effect of Fe(II)-induced Recrystallization on Trace Element Fate Varies with Structural Compatibility

■ Elements that cannot substitute into iron oxides are little affected by Fe(II)-induced recrystallization

■ Compatible elements cycle through iron oxides– Cycling of elements through iron oxides competes with (e.g., Cu)

or is aided by (e.g., Co, Mn) redox transformations■ Fe cycling substantially alters the fate and bioavailability

of trace element contaminants and micronutrients

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Incompatible Element (e.g., As, Se) Compatible Element (e.g., Mn, Co, Ni, Cu, Zn)

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Iron Oxide Recrystallization without Fe(II)?

■ Ni incorporation into goethite observed in absence of Fe(II)■ Ni release from iron oxides observed at acidic pH without Fe(II),

suggesting proton-promoted recrystallization■ Recrystallization may be rate-limited by [Fe]TOT in solution

– Processes that enhance [Fe]TOT may stimulate metal repartitioning

Data from: Frierdich and Catalano (2012) ES&T 46, 1519-1526

No Fe(II)80 d Rxn

No Fe(II)