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Recrystallization of Manganite (#-MnOOH) and Implications for Trace Element Cycling Tobias Hens, Joël Brugger, Susan Cumberland, Barbara Etschmann, and Andrew J. Frierdich Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05710 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018
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Environmental Science & Technology
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Recrystallization of Manganite (γγ-MnOOH) and Implications for
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Trace Element Cycling
3 Tobias Hens1, Joël Brugger1, Susan A. Cumberland1,2, Barbara Etschmann1, Andrew J. Frierdich1,*
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
1
School of Earth, Atmosphere & Environment, Monash University, Clayton, VIC 3800, Australia 2 Australian Synchrotron, Clayton, VIC 3168, Australia
*Corresponding author: Tel.: (+61) 03 9905 4899; Fax: (+61) 03 9905 4903; E-mail:
[email protected] Word Count (excluding abstract, references, figure captions): 4828 Number of Figures: 5 Number of Tables: 2
Submitted to Environmental Science and Technology November, 2017 Revised January, 2018
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ABSTRACT
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The recrystallization of Mn(III,IV) oxides is catalyzed by aqueous Mn(II) (Mn(II)aq)
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during (bio)geochemical Mn redox cycling. It is poorly understood how trace metals associated
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with Mn oxides (e.g., Ni) are cycled during such recrystallization. Here, we use X-ray absorption
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spectroscopy (XAS) to examine the speciation of Ni associated with manganite (γ-Mn(III)OOH)
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suspensions in the presence or absence of Mn(II)aq under variable pH conditions (pH 5.5 and 7.5).
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In a second set of experiments, we used a 62Ni isotope tracer to quantify the amount of dissolved
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Ni that exchanges with Ni incorporated in the manganite crystal structure during reactions in
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1 mM Mn(II)aq and in Mn(II)-free solutions. XAS spectra show that Ni is initially sorbed on the
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manganite mineral surface and is progressively incorporated into the mineral structure over time
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(13% after 51 days) even in the absence of dissolved Mn(II). The amount of Ni incorporation
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significantly increases to about 40% over a period of 51 days when Mn(II)aq is present in
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solution. Similarly, Mn(II)aq promotes Ni exchange between Ni-substituted manganite and
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dissolved Ni(II), with around 30% of Ni exchanged at pH 7.5 over the duration of the experiment.
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No new mineral phases are detected following recrystallization as determined by X-ray
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diffraction and XAS. Our results reveal that Mn(II)-catalyzed mineral recrystallization partitions
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Ni between Mn oxides and aqueous fluids and can therefore affect Ni speciation and mobility in
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the environment.
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Environmental Science & Technology
INTRODUCTION
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Over 30 (oxyhydr)oxide minerals of manganese (hereon Mn oxides) occur in soils,
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sediments, and rocks.1 These ubiquitous minerals are powerful oxidants and among the strongest
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metal scavengers in the environment.2-4 This leads to Mn oxides being host phases of economic
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quantities of critical metals, for instance, in marine ferromanganese nodules and polymetallic
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crusts.5 They also often serve as electron acceptors during microbial respiration,6, 7 which leads
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to the reductive dissolution of solid-phase Mn(III,IV) and release of dissolved Mn(II) into
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groundwater and sediment pore water. Biogenic processes can oxidize aqueous Mn(II)
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(Mn(II)aq)8, 9 to form δ-MnO2-like phyllomanganates.6, 7, 10-12 Consequently, Mn(II)aq may coexist
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and abiotically back-react with Mn oxides at redox interfaces during biogeochemical manganese
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cycling.
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When the oxidation rate of Mn(II)aq is slow, or when newly formed phyllomanganates are
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in contact with low concentrations of Mn(II)aq (e.g., Mn(II)aq to solid-phase Mn(IV) ratios < 1),
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the mineral phase products have increasing crystallinity and decreasing Mn average oxidation
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states.13-20 This likely results from abiotic comproportionation reactions between Mn(II) and
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Mn(IV) to generate Mn(III) within and above/below octahedral sheet vacancies of
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phyllomanganates.21, 22 The filling or blocking of phyllomanganate sheet vacancies by Mn(III)
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limits trace element uptake by these phases.23-25
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In cases where phyllomanganates are subjected to high concentrations of Mn(II)aq, net
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Mn(II) oxidation and Mn(IV) reduction causes a reductive phase transformation to Mn(III)-
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bearing minerals (e.g., manganite [γ-MnOOH]).26-29 These chemical and mineralogical changes
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result in nearly complete exchange between dissolved and solid-phase Mn.27 Elzinga30 later
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demonstrated that even in the absence of a phase transformation, Mn(II)aq actively catalyzes
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phyllomanganate recrystallization through coupled comproportionation (Mn(II) + Mn(IV) →
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2Mn(III)) and disproportionation (2Mn(III) → Mn(II) + Mn(IV)) of Mn species. We have
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recently shown that Mn(II)aq enhances exchange between structural oxygen in manganite with
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water,31 suggesting that Mn(II)aq undergoes electron transfer and atom exchange with structural
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Mn(III) in stable Mn(III) oxides similar to what has been reported for aqueous Fe(II) and some
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Fe(III) minerals.32, 33 Based on these findings, we hypothesize that structurally compatible trace
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elements and contaminants associated with Mn oxides may undergo mineral-fluid repartitioning
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during reactions with Mn(II)-bearing fluids, as described for Fe oxides in the presence of
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aqueous Fe(II).34
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Here, we evaluate Ni(II) cycling during Mn(II)-catalyzed recrystallization of manganite
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in two sets of complementing experiments. First, the speciation of Ni in manganite suspensions
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over time was examined in the presence or absence of dissolved Mn(II) using wet-chemistry
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measurements and synchrotron-based X-ray absorption spectroscopy (XAS). Secondly,
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manganite with Ni preincorporated in the structure was reacted with a
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solution to quantify the labile solid-phase Ni pool with and without Mn(II)aq. In both cases we
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explored the effects of the concentration of Mn(II)aq, pH, and reaction time.
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Ni isotope tracer in
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METHODS
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Synthesis and Characterization of Manganite. Manganite was synthesized following a
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procedure similar to our prior work.31 Briefly, 210 mL of 0.2 M NH4OH was added to 700 mL of
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0.06 M MnSO4 over 1 min during vigorous stirring to yield a suspension of Mn(OH)2. This
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precipitate was oxidized by drop-wise addition of 21 mL of 30% H2O2 over 3 min. The resulting
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solid was then triply washed by centrifugation of the suspension (~10,000 g), decantation of the
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supernatant, and resuspension of the solid in ≥18.2 MΩ·cm water (hereon water). The washed
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solid was then dried overnight at 70 °C, ground to a fine powder using a mortar and pestle, and
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passed through a 90 µm sieve. Ni-substituted manganite was synthesized identically to
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manganite, except 0.084 mL of 1 M NiSO4·xH2O was added to the MnSO4 solution
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(Ni/Mn=0.002) to co-precipitate Mn(II) and Ni(II) during the addition of NH4OH. This mixed
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metal hydroxide was then oxidized by 30% H2O2 as noted above.
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Mineral phase identity of solid samples was determined by X-ray diffraction (XRD) on a
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Bruker D8 Advance diffractometer at the Monash X-ray Platform using Cu Kα radiation (see
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Supporting Information (SI) for further details). Iodometric titration was performed to determine
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the Mn average oxidation state (Mn AOS) of the synthetic manganite starting materials (SI
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Section S2). The mole percent of Ni (Ni mol% = [moles Ni / (moles Ni + moles Mn)] x 100) in
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Ni-substituted MnOOH was determined by inductively coupled plasma mass spectrometry (ICP-
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MS) using a Thermo Scientific iCAP-Q following dissolution of the solid in 5 mL of 5 M HCl.
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Analysis of the final Ni-substituted MnOOH product revealed a nickel content of 0.18 mol %.
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Reaction of Manganite with Ni(II)- and Mn(II)-bearing Solutions. All experiments
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were conducted in an anaerobic chamber (Coy Laboratory Products, Inc., 1.5-3.0% H2
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atmosphere with N2 balance) with oxygen levels maintained at less than 1 ppm by continual
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circulation of the atmosphere over Pd catalysts. All labware and reagents were equilibrated with
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the chamber atmosphere for ≥48 hours prior to use. Water was pre-sparged with N2 for 30 min
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outside the chamber in a closed vessel and further deoxygenated for 30 min by bubbling with the
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chamber atmosphere, which was first passed through an aqueous solution containing 15%
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pyrogallol and 50% KOH to remove trace O2 and CO2.
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Duplicate reactors were set up by adding 8.8 mL water to 15 mL tubes containing
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20 ± 0.2 mg of manganite. Solution pH of 7.5 ± 0.05 or 5.5 ± 0.05 was buffered by adding 1 mL
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of a 0.1 M MOPS (3-(1-morpholino)propanesulfonic acid)/NaCl solution or 1 mL of a 0.1 M
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MES (2-(4-morpholino)ethanesulfonic acid)/NaCl solution, respectively. Reactions were then
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initiated by addition of a constant volume of 0.2 mL of 10 mM NiCl2 solution ([Ni(II)]aq = 0.2
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mM) and either 0.02 mL or 0.1 mL of 100 mM MnCl2 solution to yield a final Mn(II)
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concentration of either 0.2 or 1 mM in the reactors. Small volumes (
