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Solid-phase Fe Speciation along the Vertical Redox Gradients in Floodplains using XAS and Mössbauer Spectroscopies Chunmei Chen, Ravi K. Kukkadapu, Olesya Lazareva, and Donald L. Sparks Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 15 Jun 2017 Downloaded from http://pubs.acs.org on June 16, 2017

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Environmental Science & Technology

To Environ. Sci. & Technol.

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Solid-phase Fe Speciation along the Vertical Redox Gradients in Floodplains using XAS

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and Mössbauer Spectroscopies

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Chunmei Chen1*, Ravi K. Kukkadapu2, Olesya Lazareva1, and Donald L Sparks1

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1. Department of Plant and Soil Sciences

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Delaware Environmental Institute

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University of Delaware, Newark, DE, USA 19711

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2. Environmental Molecular Sciences Laboratory

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Pacific Northwest National Laboratory, Richland, WA, USA 99354

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Corresponding Author

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*Phone: (302)8318345. Fax: (302)8310605. E-mail: [email protected]

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Abstract

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Properties of Fe minerals are poorly understood in natural soils and sediments with

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variable redox conditions. In this study, we combined 57Fe Mössbauer and Fe K-edge X-ray

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absorption spectroscopic (XAS) techniques to assess solid-phase Fe speciation along the vertical

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redox gradients of floodplains, which exhibited a succession of oxic, anoxic and suboxic-oxic

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zones with increasing depth along the vertical profiles. The incised stream channel is bounded on

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the east by a narrow floodplain and a steep hillslope, and on the west by a broad floodplain. In

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the eastern floodplain, the anoxic conditions at the intermediate horizon (55-80 cm) coincided

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with lower Fe(III)-oxides (particularly ferrihydrite), in concurrence with a greater reduction of

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phyllosilicates(PS)-Fe(III) to PS-Fe(II), relative to the oxic near-surface and sandy gravel layers.

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In addition, the anoxic conditions in the eastern floodplain coincided with increased crystallinity

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of goethite, relative to the oxic layers. In the most reduced intermediate sediments at 80-120cm

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of the western floodplain, no Fe(III)-oxides were detected, concurrent with the greatest PS-Fe(III)

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reduction (PS-Fe(II)/Fe(III) ratio≈1.2 (Mössbauer) or 0.8 (XAS)). In both oxic near-surface

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horizon and oxic-suboxic gravel aquifers beneath the soil horizons, Fe(III)-oxides were mainly

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present as ferrihydrite with a much less amount of goethite, which preferentially occurred as

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nanogoethite or Al/Si-substituted goethite. Ferrihydrite with varying crystallinity or impurities

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such as organic matter, Al or Si, persisted under suboxic-oxic conditions in the floodplain. This

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study indicates that vertical redox gradients exert a major control on the quantity and speciation

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of Fe(III) oxides as well as the oxidation state of structural Fe in PS, which could significantly

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affect nutrient cycling and carbon (de)stabilization.

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Introduction Iron (Fe) redox cycling between oxidized and reduced forms regulates a number of

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environmental processes, such as nutrient cycling1, contaminant migration and retention2, and

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organic matter (OM) preservation and mineralization3, 4. Multiple Fe phases can be present in

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sediments, including poorly and highly crystalline Fe(III) oxides, hydroxides, and oxyhydroxides

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(hereafter referred collectively as Fe-oxides), as well as Fe(II)/(III) phyllosilicate (PS) clay

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minerals. The reactivity of Fe-oxide phases depends on their crystallinity (i.e., crystal size and

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bonding environment) 5, surface area and coverage6, and solubility7. Generally, short-range

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ordered and small-particle-size Fe oxides are more reactive than the larger bulk Fe oxides.7-9

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The trajectory of Fe mineral differentiation is primarily controlled by environmental

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redox conditions. In well drained soils, Fe(II) released during weathering is rapidly oxidized and

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precipitated as poorly crystalline Fe (oxyhydr)oxide minerals such as ferrihydrite10. During soil

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development, these poorly crystalline minerals transform into more crystalline oxides such as

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goethite and hematite.11 Under anoxic soil conditions, even well-crystalline Fe oxides become

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subjected to reductive dissolution.12 Various forms of biogenic Fe(II) may exist—aqueous Fe(II),

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adsorbed Fe(II) or mineral precipitates 13-15, and structural Fe(II) in PS 16, 17. The vertical

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translocation of Fe(II) into the oxic soil zone could lead to re-oxidization and precipitation as Fe

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oxides. Depending on geochemical conditions, such as Eh, pH, O2, Fe2+ concentrations and

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supply rate, as well as the presence of other ions (e.g., Al and Si) and OM, different Fe mineral

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phases form during redox reactions 10. The primary phase forming upon rapid re-oxidation of

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Fe2+ is weakly crystalline ferrihydrite, which then slowly transforms to thermodynamically stable

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Fe oxides such as goethite 10. Van der Zee et al. (2003) showed that nano-crystalline goethite

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instead of ferrihydrite is the predominant Fe oxide phase forming at oxic–anoxic interfaces of



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lacustrine and marine sediments.18 The degree of Fe oxide crystallinity increased in soil during

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redox cycles in closed redox-state reactors 19, while under in situ field conditions, the variable

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supply of OM (topsoils) and of Si and Ti (subsoils), inhibits the aging of Fe oxides 20. Therefore,

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the formation and stability of Fe oxides in response to alternating redox conditions still remains

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unclear.

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Virtually all 2:1 type PS clay minerals, including the classes of smectite, chlorite,

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vermiculite, illite and mica, contain ferric Fe in various amounts.21 Microbial reduction of

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structural Fe(III) in PS has been well documented in the laboratory (Dong et al., 2009 and

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references therein)22. The redox state of structural Fe in PS strongly affects the physical and

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chemical properties of PS. For instance, as a result of reduction of PS-Fe(III) to PS-Fe(II),

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specific surface area of PS decreases24, which may affect its capacity to retain OM, nutrients or

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pollutants. In one of the few field studies, Favre et al. (2002)25 observed increases in both the

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cation exchange capacity and structural Fe(II) in smectite upon reduction in paddy soils.

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Structural Fe(II) in PS resulting from reduction, can reduce a variety of pollutants such as

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nitroaromatic explosives26 and metals27. Despite the importance of Fe redox cycling in PS, there

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was little information on the changes in the structural Fe of PS as a function of redox potentials

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(Eh) in the natural field.

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The objective of the present work, therefore, was to examine Fe solid-phase speciation

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along the vertical redox gradients of floodplain soil profiles, which exhibited a succession of

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oxic, anoxic and suboxic-oxic zones with increasing depth along the vertical profiles (SI Figure

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S1 and S2, Lazareva et al., 201228; Sawyer et al., 2014 29). The permanently-saturated

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intermediate sediments are generally reducing and the overlying near-surface sediments are

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generally oxidizing. The saturated sandy gravel aquifers beneath the soil horizons are under oxic


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to suboxic conditions, as a result of percolation of oxygenated vadose water into saturated

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floodplain gravels through preferential flow paths (macropores).29 To determine the Fe mineral

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type and characteristics, X-ray absorption spectroscopy (XAS) and Mössbauer spectroscopic

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analyses were employed to directly obtain information on the Fe mineral species, oxidation state

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and structural order.

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Materials and Methods

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Field sampling

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The floodplain transect is located in a third-order reach of White Clay Creek within the

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Christina River Basin Critical Zone Observatory in Southeastern Pennsylvania. Locally, White

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Clay Creek is a shallow, cobble and sand-bedded piedmont stream with contiguous riparian

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forest. The incised channel is bounded on the west by a broad floodplain and on the east by a

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narrow floodplain and steep hillslope (SI Fig.S1).

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The eastern floodplain typically includes a sandy gravel base with dark to yellowish

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brown Fe oxide precipitates, overlain by a water-saturated subsurface soil horizon consisting of

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light gray to yellowish brown loamy sediments (12 nm;

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12%) in the reduced sediments at 55-80cm in the eastern floodplain (SI Table S10), because of

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the presence of sextet features at RT (SI Figure S17a). In contrast, goethite was present mainly as

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small-particle or metal-substituted populations in the oxic near-surface and gravel sediments,

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based on the absence of sextets at RT (SI Figure S16a and S18a). Mössbauer fitting showed that

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9-26% total Fe was in less-disordered (higher crystallinity) ferrihydrite (Fe-oxide B), while very

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low-crystalline ferrihydrite accounted for 17-23% of total Fe in the eastern floodplain sediments

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(Fe-oxide C) (Figure 4b).

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Discussion

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Mössbauer Analysis The Fe(II) doublets representing PS-Fe(II) have the almost identical intensity at RT, 77K

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and 12K (SI Figure S13-18; Table S10 and S11). The 12 K spectrum of the reduced sediment

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from the western floodplain at 80-120 cm (Figure 3b) unambiguously suggests that the sediment

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Fe(II) is mainly confined to PS without contributions from Fe(OH)2 nor siderite and magnetite,

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which display octet feature(s) at this temperature e.g. 27, 39. Partial magnetic ordering of PS-Fe(II)

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occurred at 5 K (SI Figure S14d), further implying substantial PS-Fe reduction without major

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contribution of adsorbed Fe(II) 40, 41. However, a considerable amount of Fe (II) (~11% of total

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Fe) was extracted by 0.5 M HCl, which was thought to desorb the majority of adsorbed Fe(II) 20,

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domains, as shown by Kukkadapu et al. (2006)43.

.This HCl-extractable Fe(II) might be largely due to the acid dissolution of the reduced clay



There was a small sextet component in the RT-Mössbauer spectrum of the reduced

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intermediate sediments at 55-80 cm in the eastern floodplain (SI Figure 17a). The magnetic

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hyperfine field distribution (HFD) revealed a Bhf value of 38.2 and the QS was -0.12 mm/s (SI

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Table S5), indicating a small portion (~12%) of large-particle goethite (>12 nm)18, 20, 44 is present

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in this reduced sediment sample from the eastern floodplain. The absence of well-defined sextets

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was found in the RT Mössbauer spectra of the near-surface and gravel sediments from both

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western and eastern floodplains (SI Figure 13a, 15a, 16a and 18a), suggesting that these

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sediments contained little or no pure, large-particle Fe-oxides, such as goethite, hematite, and

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magnetite/maghemite44. Cooling the samples to 77K generated considerable sextet structure for

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all the samples except the reduced sediments from the western floodplain at 80-120cm (SI Figure

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S13-18 b). The dominant sextets (17-43%) were at Bhf=36.6-38.7 and QS=-0.09-0.12 mm/s (SI

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Table S10 and S11). Transformation of Fe(III) doublet to a sextet at 77K could be attributed to

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small particle goethite (0.16 and less-

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disordered pure ferrihydrite, because these Fe-oxides display doublets at RT and sextet at 77K18,

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44-47

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. The absence of sextets in the 12 K Mössbauer spectrum of the reduced sediment from the

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western floodplain at 80-120 cm (Figure 3b) clearly indicates that the sample is essentially free

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of Fe-oxides 13, 46, 48-51. The gradual increase in the sextet spectral contribution with simultaneous

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decrease in the central Fe(III) doublet content with decrease in measurement temperature from

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RT to 12 K further, indicates that the RT-Fe(III) doublet in all the samples except the reduced

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intermediate sediments from the western floodplain, is a mix of at least 3 different Fe(III)-oxide

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phases/domains, labeled as A, B, and C (Figure 3 and SI Figure S13-18 b). A minor fraction of

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hematite with Bhf ~52 T (labeled as A1, 12nm) is likely

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present in the reduced intermediate sediment at 55-80 cm, whereas the surface and gravel

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sediments mainly contain small-particle goethite ( 0.16; (b) Similarly, ferrihydrite with varying crystallinity or OM/Al/Si

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contents exists in the floodplain sediments, with “Fe oxide-C” (Figure 3 and 4 and SI Table S9

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and S10) less crystalline or containing larger amounts of OM/Al/Si than “Fe oxide-B”.

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However it is difficult to use Mössbauer to distinguish OM-Fe(III) complexes from PS-

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Fe(III), since both Fe phases display doublets at as low as 4K (Chen et al., unpublished data).

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Therefore OM-Fe(III) may contribute to the Fe(III) central doublet at 12K, which was

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completely attributed to PS-Fe(III). For EXAFS, the spectrum of OM-Fe(III) differs dramatically

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from that of Fe(III)-PS (e.g., vermiculite, illite) 58, therefore EXAFS is better suited to

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distinguish between PS-Fe(III) and OM-Fe(III) than Mössbauer. Addition of OM-Fe(III) (formed

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by precipitating Fe(III) cation with dissolved OM extracted from a forest floor layer at pH ~2)58


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for EXAFS LCF analysis, did not improve the goodness of fit (SI Table S3 and S6), suggesting

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that OM-Fe(III) is below detection in all the samples by EXAFS LCF. In the reduced sediment

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sample at 80-120 cm of the western floodplain, dithionite and oxalate extracted ~14% of total Fe

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(SI Table S1), which could be partly from the reduced clay due to the presence of Al and Si in

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the extracts (data not shown).

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PS-Fe redox state in the sediments

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Our spectroscopic analyses demonstrated that PS-Fe in the anoxic intermediate horizon

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of both eastern and western floodplains was more reduced, relative to that in the near-surface and

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gravel sediments. In addition, the degree of PS-Fe reduction, as evidenced by the PS-

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Fe(II)/Fe(III) ratio is linearly and negatively correlated with redox potentials (Figure 5b). It is

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known that PS-Fe is subjected to microbial reduction under anoxic conditions (Dong et al.,

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200922 and references therein). Anoxic incubation of sediments in the laboratory resulted in

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substantial microbial reduction of structural Fe(III) in PS.43, 59 Although less frequently reported,

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the abiotic reduction of PS-Fe in the sediments could not be ruled out. For example, it was

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shown that Fe(II) can abiotically reduce the model PS mineral nontronite 60, 61.

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Reduction and transformation of Fe-oxides in the reduced subsurface zone

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Chemical extractions (oxalate and dithionite) and Mössbauer and EXAFS spectroscopy

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demonstrated that the reduced intermediate sediments had lower Fe-oxide contents than the near-

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surface and underlying gravel sediments, probably due to the reductive dissolution of Fe(III)

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oxides under anoxic conditions (Figure 1, 4 and 5a). For the reduced intermediate sediments of

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the western floodplain at 80-120 cm, no Fe oxides were detected by Mössbauer and EXAFS LCF

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analysis, suggesting anoxic conditions may have resulted in a complete dissolution of Fe(III)

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oxides. Fe reduction is potentially constrained by C availability and the characteristics of Fe



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oxides.62, 63 This reduced sediment sample has a high C content (~4.6 %) (SI Table S1), which

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might have served as sources to fuel dissimilatory reducing bacteria. Similarly, the degree of

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crystallinity, surface area and solubility are major controls, with minerals of poor crystallinity

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and high surface area and solubility being most reducible (e.g. the iron oxide ferrihydrite; see

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Roben 2003)63. Although we do not know the starting Fe mineralogy, the sediments may be

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mainly composed of small-particle goethite and ferrihydrite since these minerals are mainly

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dominated in this floodplain. The high C availability, in combination with the presence of small-

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particle goethite/ferrihydrite with potentially high reactivity drives the complete reductive

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dissolution of Fe oxides.

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In addition, Mössbauer analysis indicates that reducing conditions may lead to increasing

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crystallinity of the Fe oxides in the eastern floodplain, because large-particle goethite, below

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detection in the oxic near-surface and gravel aquifer sediments, are present in the reduced

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intermediate sediments at 55-80 cm. The formation of large-particle goethite under reducing

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conditions might be attributed to Fe(II)-catalyzed recrystallization of Fe-oxides. Under stable

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oxic conditions goethite is forming via dissolution/ re-precipitation from ferrihydrite10 and/or

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lepidocrocite 64. Under reducing conditions, Fe(II), produced from Fe(III) reduction, can induce a

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catalytic reaction, resulting in the recrystallization and/or secondary transformation of Fe oxides

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to more stable Fe-oxides as a result of electron transfer between Fe(II) and Fe(III) oxides. 50, 65-67

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Small concentrations of Fe(II) can cause conversion of ferrihydrite to goethite.37, 65 Thompson et

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al. (2006)19 demonstrated in batch experiments that redox oscillations can lead to increasing

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crystallinity of goethite due to Fe(II)-catalysed solid state transformation of Fe(III) oxides. Since

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the more crystalline goethite is less prone to reductive dissolution than ferrihydrite 52, it can,

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once formed, accumulate in the sediments.


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Precipitation of Fe-oxides in the suboxic to oxic sandy gravel aquifer In both the western and eastern floodplain, the sandy gravel aquifer is mainly composed

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of ferrihydrite with a less proportion of goethite. Reducing conditions in the overlying

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intermediate sediments cause reductive dissolution of Fe(III)-oxides to Fe(II). Vertical

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infiltration along preferential flow paths promoted the delivery of dissolved Fe from the anoxic

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layers to the underlying floodplain sandy aquifer 29, where the mobilized Fe(II) may re-oxidize

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and subsequently precipitates as Fe(III)-oxides. It is assumed that ferrihydrite is often the first

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phase to form in soils, especially at rapid oxidative precipitation.10 It slowly transforms into

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thermodynamically more stable Fe oxides like goethite (half-life of ~100 days at pH 7 and

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25 °C).10 Adsorption of Fe(II) to ferrihydrite has been shown to mediate a far more rapid

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transformation to more crystalline forms.50, 65, 67 The presence of a large proportion of

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ferrihydrite, with less goethite in the sandy gravel aquifer, suggests that the rate of ferrihydrite

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precipitation is much faster than the rate of ferrihydrite transformation to goethite. Only a short

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exposure of sandy gravel aquifer to Fe(II) by oxidation under oxidizing conditions, may impede

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the more crystalline phases. In addition, the presence of constituents such as Al, silicate and OM

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may suppress the transformation. Retardation of Fe solid-phase transformation is well known to

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occur as a result of surface coating of OM, Si and Al constituents.50, 68-70 Thompson et al (2011)

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suggested that the preservation of short-range-ordered Fe oxides is due to leaching of Fe(II) and

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the prevalence of organic matter in the surface horizons and Si and Ti in the subsurface horizons

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that retard the re-crystallization process.20

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Environmental Implications



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Riparian zones with strong redox gradients are widely considered ‘‘hot spots’’ of

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enhanced biogeochemical cycling within watersheds.71,72 In this study, we directly compared

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XAS and Mössbauer spectroscopic analysis of solid-phase Fe speciation in redox-affected soils

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for the first time. The results were in reasonably good agreement. This study showed that Fe

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solid-phase speciation varied strongly with the vertical redox stratification in floodplains, which

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may subsequently affect the cycling of carbon and nutrients in the riparian zones. In the reducing,

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intermediate layers, Fe-oxides were low (east) or non-detectable (west) and PS-Fe was more

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reduced, suggesting that the reducing conditions may promote the reduction of both Fe-oxides

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and PS-Fe. The reductive dissolution of Fe-oxides can drive the associated OM release to pore

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water.73, 74 Similarly, as a result of PS-Fe reduction, specific surface area of PS decreases 23, 24

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and therefore this may also lead to increased OM mobilization to pore water (although rarely

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reported in the literature). Taken together, these redox-sensitive reactions may explain the

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observed high dissolved OM and Fe in the reducing subsurface in a previous study on the same

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floodplain transect 29. In addition, this study also raises the question as to what is the relative

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importance of Fe(III)-oxides vs. PS-Fe in C cycling in natural environments.

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Vertical infiltration drives the mobilization of dissolved Fe and OM from the reducing

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zones to the underlying oxic-suboxic sandy aquifer 29, and may result in Fe-oxide precipitation.

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In the sandy aquifer, Fe oxides are hosted by reactive ferrihydrite with less amounts of small-

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particle goethite. These reactive Fe phases at the gravel sandy aquifer can potentially act as a

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barrier for dissolved OM and phosphate through coprecipitation or adsorption75, since dissolved

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OM and phosphate has a large affinity for the reactive Fe phases. This may explain the previous

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findings on the same floodplain transect, that dissolved OM and Fe in streams and deep ground-

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water is very low 29. Because redox interfaces are ubiquitous in the environment (e.g., in the pore


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waters of lakes, estuaries, tidal flats, peat lands, paddy fields, and marine sediments), the coupled

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cycling of Fe, C and phosphate at anoxic/oxic transitions may strongly control the fate of C and

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phosphate in the natural environment.

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Supporting Information

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The Supporting Information is available. Details on XAS and Mössbauer data analysis. Figures

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showing XRD data, additional XAS spectra, and Mössbauer data.

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Acknowledgements

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We thank Dr. Anthony Aufdenkampe from the Stroud Water Research Center for his help

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with field selection. The research is a part of the Christina River Basin Critical Zone Observatory

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(CRB-CZO) project that was supported by the National Science Foundation (EAR 0724971). We

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are grateful to NSF for their financial support (EAR 0724971). Mössbauer spectroscopic

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analyses were performed at the Environmental Molecular Sciences Laboratory (EMSL), a

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national scientific user facility sponsored by the Department of Energy’s Office of Biological

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and Environmental Research, at Pacific Northwest National Laboratory (PNNL). PNNL is a

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multiprogram national laboratory operated for the US Department of Energy by the Battelle

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Memorial Institute under Contract DE-AC06-76RLO 1830. XAS analysis was carried out at the

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Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC National Accelerator

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Laboratory and an Office of Science User Facility operated for the U.S. Department of Energy

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Office of Science by Stanford University. We are also grateful to the Shanghai Synchrotron

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Radiation Facility (SSRF) for use of the synchrotron radiation facilities at beamline 14W.

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(b) Eastern Floodplain

120-140 cm

55-80 cm

80-120 cm

20-55 cm

40-80 cm

(a) Western Floodplain

0

10

20

80-100 cm

total Fe 0.5 M HCl Oxalate Dithionite

0

30

10

20

30 -1

Fe (g kg )

-1

Fe (g kg ) Fig.1 Fe extractability as a function of soil depth in the (a) western and (b) eastern floodplain.


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7121.2 eV 7129.6 eV

Ferrihydrite

First Derivative Absorbance

Goethite Illite Vermiculite Biotite Chlorite

Eastern floodplain 20-55 cm 55-80 cm 80-100 cm Western Floodplain 40-80 cm 80-120 cm 120-140 cm

7100

7110

7120

7130

7140

7150

7160

Energy (eV) Fig.2. Fe first-derivative XANES spectra of the references, and the western and eastern floodplain samples at varying depths. The vertical dash lines indicate features at 7121.2 eV and 7129.6 eV, respectively.



Fig.3 Fitted 57Fe Mössbauer spectra of the (a) western and (b) eastern floodplain at 12K. The black solid line is the total calculated fit, through the discrete data points (circles). The resolved spectral components at 12 K are: (1) phyllosilicates(PS)-Fe(II) (green line), (2) PS-Fe(III) (gray line), (3) ilmentite (cyan line), (4) hematite (red line), (5) Fe-oxide A (goethite) (purple line), (6) Fe-oxide B (ferrihydrite) (dark yellow line), and (7) Fe-oxide C (ferrihydrite) (blue line).


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Fig.4 Fe speciation derived from Mössbauer spectra fitting at 12K and Linear-combination-fitting (LCF) results of EXAFS spectra, respectively for the (a) western and (b) eastern floodplain. Detailed LCF fitting results are provided in the Supporting Information Table S8 and S9. Detailed Mössbauer spectra fitting results are shown in the Supporting Information Table S10 and S11. 27 ACS Paragon Plus Environment


Goethite from Mossbauer Goethite from EXAFS Ferrihydrite from Mossbauer Ferrihydrite from EXAFS

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Mossbauer EXAFS

50 1.4

Pyllosilicates-Fe(II)/Fe(III) ratio

Proportions of Fe-oxides (%)

(a) 40

30

20

10

(b)

1.2 1.0 0.8 0.6 0.4 0.2

0 0.0 -200

0

200

400

600

-200

800

0

200

400

600

800

Eh (mV)

Eh (mV)

Fig.5 Soil Fe-oxide content (a) and phyllosilicates-Fe(II)/Fe(III) ratio (b), derived from Mössbauer spectra fitting at 12K and Linear-combination-fitting (LCF) results of EXAFS, as a function of redox potentials (Eh).


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