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Iron and carbon dynamics during aging and reductive transformation of biogenic ferrihydrite A. Cristina Cismasu, Kenneth H. Williams, and Peter S. Nico Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b03021 • Publication Date (Web): 25 Nov 2015 Downloaded from http://pubs.acs.org on November 29, 2015
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Iron and carbon dynamics during aging and
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reductive transformation of biogenic ferrihydrite
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A. Cristina Cismasu1*, Kenneth H. Williams1 and Peter S. Nico1 1
Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., Berkeley, CA, USA
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*correspondence
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Lawrence Berkeley National Laboratory
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1 Cyclotron Rd, Berkeley, CA 94720, USA
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MS 47R316C
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[email protected] 16
650-644-8588
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Abstract
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Natural organic matter is often associated with Fe(III) oxyhydroxides, and may be
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stabilized as a result of co-precipitation or sorption to their surfaces. However, the
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significance of this association in relation to Fe and C dynamics and biogeochemical cycling,
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and the mechanisms responsible for organic matter stabilization as a result of interaction with
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minerals under various environmental conditions (e.g., pH, Eh, etc.) are not entirely
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understood. The preservation of mineral-bound OM may be affected by OM structure and
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mineral identity, and bond types between OM and minerals may be central to influencing the
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stability, transformation and composition of both organic and mineral components under
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changing environmental conditions. Here we use bulk and submicron-scale spectroscopic
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synchrotron methods to examine in situ transformation of OM-bearing, biogenic ferrihydrite
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stalks (Gallionella ferruginea-like), which formed following injection of oxygenated
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groundwater into a saturated alluvial aquifer at the Rifle, CO field site. A progression from
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oxidizing to reducing conditions during an eight-month period triggered the aging and
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reductive transformation of Gallionella-like ferrihydrite stalks to Fe (hydroxy)carbonates and
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Fe sulfides, as well as alteration of the composition and amount of OM. Spectromicroscopic
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measurements showed a gradual decrease in reduced carbon forms (aromatic/alkene, aliphatic
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C), a relative increase in amide/carboxyl functional groups and a significant increase in
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carbonate in the stalk structures, and the appearance of organic globules not associated with
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stalk structures. Biogenic stalks lost ~30% of their initial organic carbon content. Conversely,
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a significant increase in bulk organic matter accompanied these transformations. The
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character of bulk OM changed in parallel with mineralogical transformations, showing an
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increase in aliphatic, aromatic and amide functional groups. These changes likely occurred as
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a result of an increase in microbial activity, or biomass production under anoxic conditions.
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By the end of this experiment, a substantial fraction of organic matter remained in identifiable
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Fe containing stalks, but carbon was also present in additional pools e.g., organic matter
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globules and iron carbonate minerals.
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Introduction
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The association of organic matter (OM) with ferric iron oxyhydroxides is recognized
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as a key process in the stabilization of OM in surface environments [1-5]. Owing to the ubiquity
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of iron and organic carbon in soils, surface waters, and aquifers, and to the high specific
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surface area and reactivity of Fe(III) oxyhydroxides, this association may have a substantial
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impact on OM stabilization. Occlusion of organic matter within mineral co-precipitates, as
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well as OM chemical structure and bond strength between OM and minerals may impact its
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preservation [1,2,6,7]. Alternatively, Fe(III) oxyhydroxide sorbent types may also play key roles
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in OM preservation
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understanding of the processes involved in the formation and stabilization of Fe(III)
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oxyhydroxide-OM associations is currently lacking, particularly regarding sorbed OM
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structure, localization, conformation
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environmental drivers that govern both C and Fe biogeochemical cycles (changes in pH,
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redox conditions, microbial community composition, etc.).
[1]
due to their variable surface areas and reactivities. A mechanistic
[1]
, and potential stages of OM degradation as a result of
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Ferrihydrite is among the most widespread Fe(III) oxyhydroxides at the earth’s
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surface, often forming at redox boundaries as a result of rapid Fe(II) oxidation. It is known for
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its nanoparticulate nature, poor crystallinity, high surface area and reactivity. It is commonly
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associated with organic carbon in soils, sediments
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hydrothermal settings
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as a result of biomineralization by Fe(II) oxidizing microorganisms
[10,11]
[8]
, as well as freshwater, marine
[5,9]
and
, with frequent organic matter-ferrihydrite associations occurring
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. The exposure of
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ferrihydrite to fluctuating redox conditions triggers recrystallization or mineralogical
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transformations that impact the chemical speciation, mobility and bioavailability of associated
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major or trace elements
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the types of secondary minerals depend on phase purity, e.g., sorption/incorporation of Al, Si,
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P, or organic matter. [11, 18-23]. It is anticipated that mineral transformations will also affect the
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speciation of ferrihydrite-bound natural organic matter; OM can exchange or shuttle electrons
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[23, 24]
[17]
. Ferrihydrite aging and (a)biotic transformation rates, as well as
but it may also oxidize or desorb selectively.
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The objective of this work is to present an example of Fe and C dynamics during the
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aging and reductive transformation of a biogenic ferrihydrite precipitated in situ within the
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saturated aquifer at the U.S. Department of Energy (DOE) Rifle, CO test site [23]. OM-bearing
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precipitates dominated by Gallionella-like ferrihydrite stalks formed during a forced
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oxidation event (injection of oxygenated water into the subsurface); the aging and reductive
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transformation of the precipitate was monitored for a period of eight months (between August
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2012 and April 2013) as groundwater reverted to reducing conditions. Iron speciation and
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mineralogy, organic matter character, as well as the likelihood for release or preservation of
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mineral-bound organic matter were evaluated using evidence derived from X-ray diffraction,
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submicron scale elemental mapping and X-ray absorption spectroscopy.
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Materials and Methods
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Field site and experimental design
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The DOE Rifle field site is located in northwestern Colorado on a floodplain adjacent
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to the Colorado River at the site of a former uranium and vanadium mill that operated
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between 1924 and 1958 (see Williams et al., 2011
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comprehensive descriptions of the site). Currently, the site supports research activities 4
[25]
; Yabusaki et al., 2007
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for more
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associated with the Lawrence Berkeley National Laboratory Sustainable Systems Scientific
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Focus Area (SFA), whose central aim is to develop a predictive understanding of climate-
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induced impacts on biogeochemical functioning and carbon cycling at the watershed scale.
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Mimicking the short-lived but significant increases in dissolved oxygen (DO) that accompany seasonal excursions in groundwater elevation at the Rifle site
[27]
, injection of
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oxygenated groundwater into an otherwise anoxic portion of the aquifer was designed to
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examine the impact of fluctuating redox conditions on biogeochemical pathways relevant to
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metal and carbon cycling. Aerated groundwater was injected into a formerly biostimulated
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well field [28] over a period of 125 days from August to December 2012; see Figure SI3.1 for
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a sketch of the injection setup. Prior to injection, groundwater in the well field was suboxic
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(DO concentration 3-7µM; ORP from -132 to -196mV) with a neutral pH (7.2-7.5) and
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elevated ferrous iron and dissolved sulfide concentrations (53-63µM and 0.3-3µM,
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respectively). The injectate consisted of groundwater pumped from an upgradient, previously
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un-impacted well having low DO and Fe(II) (
