Solid-phase Fe Speciation along the Vertical Redox Gradients in Floodplains using XAS. and Mössbauer Spectroscopies

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1 Supporting Information Solid-phase Fe Speciation along the Vertical Redox Gradients in Floodplains using XAS and Mössbauer Spectroscopies Chunmei Chen 1*, Ravi K. Kukkadapu 2, Olesya Lazareva 1, and Donald L Sparks 1 1. Department of Plant and Soil Sciences Delaware Environmental Institute University of Delaware, Newark, DE, USA Environmental Molecular Sciences Laboratory Pacific Northwest National Laboratory, Richland, WA, USA Number of Pages: 43 Number of Figures: 18 Number of Tables: 11 Corresponding Author *Phone: (302) Fax: (302) cmchen@udel.edu S1

2 Eh measurements In-situ redox sensors (Paleo Terra, Amsterdam, Netherlands) were paired with the sediment sampling locations including the near-surface, water-saturated subsurface (intermediate) and gravel sandy aquifer of both the eastern and western floodplains. The sensors were built from fiber glass-epoxy probes with an external diameter of 8 mm by 30 cm length, calibrated in the laboratory, installed horizontally, and connected to Hypnos dataloggers (Amsterdam, Netherlands). Eh was monitored in the field for ~ 6 months and averaged for comparisons among samples. General Characterization Particle-size distribution was determined by wet sieving and sedimentation using the pipette sampling technique. Total C content was analyzed by dry combustion with a vario cube CNS analyzer (Elementar, Germany). Total element contents were analyzed by ICP-AES in extracts obtained by microwave digestion. Oxalate-extractable Fe was determined by using the method of Schwertmann (1964) and dithionite-extractable Fe determined according to Mehra and Jackson (1960). Fe(II) was measured by a ferrozine method (Stookey, 1970) following 0.5 M HCl extraction for 2 hrs at a 1:10 dry soil : solution ratio. X-ray diffractograms were recorded with a Rigaku Miniflex X-ray diffractometer equipped with CuKα radiation generated at 35 kv and 10 ma. The XRD patterns were recorded from 2 to 35 2θ at a scanning speed of 0.5 2θ min 1. XAS data collection X-ray adsorption spectroscopy (XAS) S2

3 The speciation and structural environment of Fe were determined using XAS. Fe K-edge XAS spectra were collected on beamline 4-1 at the Stanford Synchrotron Radiation Laboratory (SSRL) and beamline 14W at the Shanghai Synchrontron Radiation Facility (SSRL), running under dedicated conditions. Energy selection was maintained by a Si(220) monochromator calibrated to ev for Fe foil. Higher-order harmonic frequencies were rejected by detuning 50 % from the maximum incident intensity. Four to six scans of each sample were collected either in transmission mode using an ion chamber or fluorescence mode using a Lytle detector (Lytle et al., 1984). XAS data analysis All Fe spectra were energy calibrated, averaged, pre-edge subtracted, and post-edge normalized using the Sixpack program (webb, 2005). Fe XANES spectra were used to survey the oxidation state and mineral class of the Fe-bearing phases. Linear combination fitting (LCF) was performed on the k 3 -weighted EXAFS spectra over the k-range 2 11 Å 1 using the same E 0 for all the samples to quantify the proportions of various Fe phases in the samples using a set of reference compounds, including 2-line ferrihydrite (Chen et al., 2015a), goethite (α-feooh) (Chen et al., 2015a), lepidocrocite (γ-feooh) (Chen et al., 2015a), hematite (α-fe 2 O 3 ) (Hansel et al., 2003), siderite (FeCO 3 ) (Hansel et al., 2003), iron sulfide (FeS) (Hansel et al., 2003), green rust Fe II 6 xfe III x[oh] 12 [(SO 4 ) x/2 3H 2 O]) (Hansel et al., 2003),, vivianite (Fe 3 [PO 4 ] 2 nh 2 O) (Hansel et al., 2003), magnetite (Fe 3 O 4 ) (Hansel et al., 2003) and ilmenite (FeTiO 3 ) (Chen et al., 2014a). In addition, our reference library includes Fe XAS data for natural phyllosilicate model compounds: Vermiculite (VTx-1), Illinite (IMt-1), Chlorite (CCa-2) and Biotite. Biotite was from the Stanford mineral collection. The clay minerals VTx-1, IMt-1, CCa- 2 were obtained from the Source Clays Repository of the Clay Minerals Society (West Lafayette, S3

4 U.S.A.). In the dioctahedral clay minerals, VTx-1 and IMt-1, Fe(II) accounts for 7% and 12% of total Fe and Fe represents 12 and 21% of the cations in the octahedral sheets, respectively. In the trioctahedral CCa-2 and biotite reference, Fe(II) accounts for 91% and 87% of total Fe and Fe accounts for 52% and 71% of the octahedral cations, respectively. With respect to phyllosilicate- Fe, the references used for LCF indicate the average oxidation state and coordination of phyllosilicate-fe rather than the presence of distinct phyllosilicate minerals. An organic matter (OM)-Fe(III) model compound, formed by precipitating Fe(III) with dissolved organic matter extracted from field -fresh samples of a forest floor layer at ph ~2 (Chen et al., 2014b), was also used as a reference representing organically-complexed Fe(III). Fe(II) oxalate (FeC 2 O 4 ) was used as a model compound for organically-complexed Fe(II). No energy shift was included in the LCF and the sum of the fitted fractions was not constrained (LCF data are presented normalized to 100%). The goodness of fit was established by minimization of the R-factor (=Σ(data fit) 2 /Σdata 2 ). Because vermiculite and illite were observed in all samples by XRD, the subsequent LCF analysis for all the floodplain sediment samples was performed using these two components (vermiculite and illite) in combination with other suitable references. Considering that the nearsurface and gravel sediments contained a large proportion of Fe(III)-oxides according to the selective chemical extractions and Mössbauer analysis, LCF for the near-surface and gravel sediment samples was then performed using the first 2 components (vermiculite and illite), with addition of Fe(III)-oxides (ferrihydrite, goethite, lepidocrocite and hematite). Among these Fe(III)-oxides, adding a ferrihydrite or goethite component improved the goodness of fit (Rfactor in SI Table S3) by 21-68% and 42-63%, respectively. a lepidocrocite component did not significantly improve the goodness of fit (SI Table S3). Although addition of S4

5 hematite improved the quality of the fits by 10-19%, hematite was not included in the final fits because (1) only a minor fraction (<3%) of hematite was detected by Mössbauer analysis, and (2) observations of fits of EXAFS spectra revealed that better fits were obtained with goethite than hematite (SI Figure S9). a OM-Fe(III) component did not improve the quality of the fits (Table S3). Both ferrihydrite and goethite components were included in the LCF analysis of the near-surface and gravel-sediment samples. LCF was then performed using four components (vermiculite, illite, ferrihydrite and goethite) with addition of a Fe(II)-rich component. Addition of a chlorite component largely improved the goodness of fit by 31-36% except for the gravel sediment sample from the eastern floodplain at cm (Table S4). While addition of an ilmenite component significantly improved the goodness of fit by 26-36%, adding biotite, magnetite, green rust, vivianite or Fe(II)-oxalate did not improve the quality of the fits. Therefore, the final components that were able to produce the best reasonable fits for the near-surface and gravel sediment samples were vermiculite, illite, ferrihydrite, goethite, chlorite (eliminated for the gravel sediment sample from the eastern floodplain at cm), and ilmenite. Examination of first-derivative XANES spectra indicates that the reduced sediment samples from the intermediate horizon displayed strong peak at ~7121 ev, which suggests a large occurrence of Fe(II), since Fe(III)-bearing phases lack a peak at ~7121 ev (Figure 2 and SI Figure S4 and S5). Fe(II) sulfide and carbonate should be minor in these reduced sediment samples, because first-derivative XANES spectra of Fe(II) sulfide (pyrite) and carbonate (siderite) showed no peak at ~7121 ev (SI Figure S4), and the XANES spectrum of pyrite is totally different from that of the samples (Figure 2 and SI Figure S4). In addition, carbonate was not detectable based on carbon K-edge XANES spectra (Chen et al., 2015b). Therefore, Fe(II) sulfide and carbonate were excluded from the final fits. We started LCF analysis for the reduced S5

6 sediment samples from the intermediate horizon with vermiculite and illite, and assessed the fit improvements by adding an additional Fe(II)-rich component. chlorite or biotite significantly improved the goodness of fit by 47-63%, whereas addition of magnetite, green rust and Fe(II)-oxalate did not improve the quality of the fits (SI Table S5). Although the goodness of fit was improved by 17-24% by adding a vivianite component (SI Table S5), we excluded vivianite from the final fits, because (1) no vivianite was detected by Mössbauer analysis, and (2) the total P/Fe ratio of the reduced sediments was below 0.04 (Chen et al., unpublished). Adding an ilmenite component improved the fits by 31-45%. Therefore LCF of the reduced sediment samples was then performed using vermiculite, illite, chlorite, biotite and ilmenite (5 component), with adding an additional Fe(III)-oxide or OM-Fe(III) component. For the reduced samples from the intermediate horizon of the western floodplain at cm, the goodness of fit was not improved by adding ferrihydrite, goethite, lepidocrocite, or hematite, whereas addition of ferrihydrite or goethite improved the quality of the fits for the reduced intermediate sediments at cm of the eastern floodplain by 31-47% (SI Table S6). In addition, we compared two LCF approaches for the reduced intermediate sediments from both floodplains: fits with only PS-Fe (vermiculite, illite, chlorite & biotite) and ilmenite versus fits with PS-Fe (vermiculite, illite, chlorite & biotite), ilmenite and Fe(III)-oxides (ferrihydrite & goethite) (Table S7 and Figure S6). For the intermediate sediments from the western floodplain, adding ferrihydrite and goethite did not improve the goodness of fit (Table S7 and Figure S6). In addition, LCF fits with ferrihydrite and goethite showed ~4% of ferrihydrite and goethite respectively in the reduced intermediate sediments of the western floodplain, which was below the detection limit of LCF fit (Table S7). In contrast, for the intermediate sediments from the eastern floodplain, addition of ferrihydrite and goethite largely improved the fit by ~70% (Table S7 and Figure S6), and LCF fits with S6

7 ferrihydrite and goethite implied the presence of significant amounts of goethite (~10% of total Fe) and ferrihydrite (~17% of total Fe). OM-Fe(III) did not improve the goodness of fit for both reduced intermediate samples (SI Table S6). 57 Fe-Mössbauer Spectroscopy To adequately characterize Fe-mineral suite of the sediments in terms of: a) the type of Fe(II)/(III)-(oxyhdr)oxide (hereafter labeled as Fe-oxide), b) nature of the Fe-oxide: Changes in particle size, the extent of metal substitution (e.g., Al for Fe), and/or the magnitude of organic matter (OM)/metal coatings were shown to significantly affect the (bio)reducibility of Fe-oxide (e.g., Roden et al., 1996; Kukkadapu et al., 2001; Ekstrom et al., 2012; Eusterhues et al., 2008; Chen et al., 2015b). Pedogenic ferrihydrite and goethite are often Al-substituted, up to 20 and 30 mole%, respectively (Cornell and Schwertmann, 2003), c) the extent of PS-Fe (bio)reduction (hereafter referred as clay): Relatively higher amounts of PS-Fe is reduced by DCB reagent (dithionite-citrate-bicarbonate, ph 7) than the microbes (Ribeiro et al., 2009), d) the presence of adsorbed Fe(II) (Fox et al., 2013), and e) the nature of secondary Fe(II) minerals (Peretyazhko et al., 2012), 57 Fe-Mössbauer spectroscopy measurements was carried out at various temperatures. Spectra at various temperatures, RT and below RT, is essential to resolve Fe-phase features that considerably overlap at certain temperature but display unique signatures at lower temperatures due to variation in their magnetic behavior with temperature. Some examples of the temperature effect on Mössbauer spectral features, relevant to the present study are: a) particle-size of goethite: pure <8-nm is doublet at room temperature (RT) and sextet at 77 K (Yamashita et. al., 2000), unlike its large particle counterpart that is sextet at RT (Murad and Cashion, 2004), b) Algoethite s: large particle Al-goethite s with Al/Fe ratio of >0.16 is doublet at RT and sextet at 77 K (Fysh and Clark, 1982a), c) nature of ferrihydrite-like mineral phases in <77 K spectra: e.g., S7

8 Mössbauer blocking temperatures of Si sorbed ferrihydrite and OM-ferrihydrite decrease with increase in sorbed Si content and OM/Fe ratio of the coprecipitate, respectively (Zhao et al., 1996; Eusterhues et al., 2008; Chen et al., 2015b), e) magnitude of the PS-Fe(III) reduction: In <12 K spectra highly reduced clays display a Fe(II) magnetically-ordered octet feature (Ribeiro et al., 2009), and f) nature of the secondary Fe(II): siderite, PS, and adsorbed Fe(II) contributions can be resolved adequately from each other by obtaining spectra at various temperatures (Peretyazhko et al., 2012; Fox et al., 2013). Earlier studies from our group indicated that from a comparison of spectra obtained at RT, 77 K, 12 K and 5 K, the select temperatures used in the study, it is possible to model contributions from various Fe-species, albeit ambiguous at times, in soils that are primarily composed of Fe-oxides and Fe-clays (Peretyazhko et al., 2012). Data Collection Mössbauer spectroscopic data of the samples was collected using a WissEl Elektronik (Germany) instrument that included a closed-cycle cryostat SHI-850 obtained from Janis Research Co., Inc. (Wilmington, MA), a Sumitomo CKW-21 He compressor unit, and an Ar-Kr proportional counter detector (LND, Inc. NY). A 57 Co/Rh source (50-mCi to 75-mCi, initial strength) was used as the gamma energy source. The transmitted counts were stored in a multichannel scalar (MCS) as a function of the energy (transducer velocity) using a channel analyzer. The raw data were folded to 512 channels to provide a flat background and a zero-velocity position corresponding to the center shift (CS) of a metal Fe foil at room temperature (RT). Calibration spectra were obtained with a 20-μm-thick Fe foil placed in the same position as the samples to minimize any geometry errors. The Mössbauer spectroscopy data was modeled with Recoil software (University of Ottawa, Canada) using a Voigt-based structural S8

9 fitting routine (Rancourt and Ping, 1991) The sample preparation (type of the sample holder, etc.) was identical to the procedures reported in Peretyazhko et al,. (2012). Mössbauer Data Fitting at RT The Mössbauer spectra of the floodplain samples at RT was dominated by a Fe(III) doublet with secondary peaks due to Fe(II) (SI Figure S13-18 a). The Fe(II) doublet with center shift (CS) = mm/s and quadrupole shift (QS)= mm/s, was due to PS-Fe(II) (Murad and Cashion, 2004; Smyth, 1997). Information regarding the type and relative distribution of various phyllosilicates, however, is not straightforward from the Mössbauer spectral data. The fact that at least a 2 Gaussian component quadrupole splitting distribution (QSD) model is needed to yield an optimum fit, on the other hand, clearly suggests that the PS- Fe(II) exists in at least two different environments (either in the same phyllosilicates or two phyllosilicates clays) (Rancourt and Ping, 1991). The Fe(III) doublet represents both Fe(III) in PS and Fe-oxides that display doublet features at RT (Murad and Cashion, 2004; van der Zee et al., 2003). There was a small sextet component in the RT-mössbauer spectrum of the reduced intermediate sediments at cm in the eastern floodplain (SI Figure S17a). The magnetic hyperfine field distribution (HFD) revealed Bhf value to be 38.2 and the QS was mm/s (SI Table S11), indicating a small portion (~12%) of large-particle goethite (>12 nm)( Murad and Cashion, 2004; Thompson et al, 2011; van der Zee et al., 2003) is present in this reduced sediment sample from the eastern floodplain. Absence of well-defined sextets in their RT Mössbauer spectra for all the other samples except the reduced sediment at cm, unambiguously suggests that those samples contain little or no pure, large-particle Fe-oxides, S9

10 such as goethite, hematite, and magnetite/maghemite (Murad and Cashion, 2004). RT Mössbauer spectral analysis suggests that a small fraction of the total Fe could be present as ilmenite. 77K Cooling the samples to 77K generated considerable sextet structure for all the samples except the reduced sediments from the western floodplain at cm (SI Figure S13-18 b). The dominant sextets (~17-43%) were at Bhf= and QS= mm/s. Transformation of Fe(III) doublet to a sextet at 77K could be attributed to small particle goethite (<8 nm), Al substituted goethite with Al/Fe ratio of >0.16 and less-disordered pure ferrihydrite, because these Fe-oxides display doublets at RT and sextet at 77K (Yamashita et. al., 2000; Fysh and Clark, 1982a; Murad and Cashion,2004; van der Zee et al., 2003; Mitsunobu et al., 2008). The Fe(II) doublets (CS= mm/s; QS= mm/s) representing PS-Fe(II) has the same intensity as at RT. 12K The 12 K spectrum of the reduced sediment from the western floodplain at cm (Figure 3b) unambiguously suggests that the sediment Fe(II) is mainly confined to PS without contributions from Fe(OH) 2 nor siderite and magnetite, which display octet feature(s) at this temperature (e.g., Larese-Casanova and Scherer, 2007; Peretyazhko et al., 2012). Partial magnetic ordering of PS-Fe(II) occurred at 5 K, therefore 5K spectra was not modeled (SI Figure S14d). Partial magnetic ordering of the PS-Fe(II) in the 5 K spectrum further implies substantial PS-Fe reduction without major contribution of adsorbed Fe(II) (Ribeiro et al., 2009; Fox et al., 2013). However, 0.5 M HCl, which was thought to desorbs the majority of adsorbed Fe(II) (Lovely et al., 2004; Thompson et al., 2011), extracted ~11% of total Fe (SI Table S1). This HCl-extractable Fe(II) might be largely due to the acid dissolution of the reduced clay domains, S10

11 as shown by Kukkadapu et al. (2006). Absence of sextets in 12 K Mössbauer spectrum of the reduced sediment from the western floodplain at cm (Figure 3b) clearly indicates that the sample is essentially free of Fe-oxides (Fysh and Clark, 1982a; Fysh and Clark, 1982b; Zhao et al., 1996; Kukkadapu et al., 2001; Eusterhues et al., 2008; Chen et al., 2015b). Seven spectral components could be identified in all the other spectra at 12K (Figure 3a and b-f). Three components exhibited a quadrupole splitting and were fitted as quadrupole doublets with Lorentzian lines. The quadrupole doublet with the smaller splitting and isomer shift represents paramagnetic PS-Fe(III) (CS= 0.24 to 0.25 mm/s, QS = 0.59 to 0.66 mm/s) agreed well with illite/vermiculite (Murad and Cashion, 2004). The Fe(II) doublets(cs = 1.02 to 1.04 mm/s, QS = 2.86 to 2.93 mm/s) (Table S10 and S11) has the same intensity as at RT and 77K. The gradual increase in the sextet spectral contribution with simultaneous decrease in the central Fe(III) doublet content with decrease in measurement temperature from RT to 12 K further indicates that the RT-Fe(III) doublet in both these sediments is a mix of at least 3 different Fe(III)-oxide phases/domains, labeled as A, B, and C (Figure 3 and SI Figure S13c and S15-18 c). A minor fraction of hematite with Bhf ~52 T (labeled as A1, <3%) was also present in sediments (Table S10 and S11). The spectral parameters of sextet A (Bhf =~48 T and QS=~-0.1 mm/s) approach those reported for goethite standards (van der Zee et al., 2003). Oxide-B and C could be due to ferrihydrite-like mineral phases with different crystallinity or OM/metal contents, as evidenced by the increased sextets from 77K to 12K. Oxide-B, which displays a wide sextet feature with QS close to 0, is more crystalline or contains lower amounts of OM/metal than Oxide C, which was modeled using a collapsed sextet and has blocking temperatures near 12K (Zhao et al., 1996; Eusterhues et al., 2008; Chen et al., 2015b). S11

12 Sediment General Properties In the western floodplain, highest C content was observed in the reduced intermediate sediments, whereas in the eastern floodplain, soil C content generally decreased with depth (SI Table S1). In both western and eastern floodplain, particle size analysis showed the near-surface and intermediate sediments were composed of wt.% sand, wt.% silt, and wt.% clay, while the gravel were dominated by sand particles (>85% wt.%) (SI Table S2). The presence of Fe-oxides was below detection by XRD (SI Figure S3). References 1. Chen, C.; Dynes, J.; Wang, J.; Karunakaran, C.;Sparks, D. L. Soft x-ray spectromicroscopy study of mineral-organic matter associations in pasture soil clay fractions. Environ. Sci. Technol. 2014a, 48 (12), Chen, C.; Dynes, J.; Wang, J.; Sparks, D. L. Properties of Fe-organic matter associations via coprecipitation versus adsorption. Environ. Sci. Technol. 2014b, 48 (23), Chen, C.; Kukkadapu, R. L.;Sparks, D. L. Influence of coprecipitated organic matter on Fe 2+ (aq)- catalyzed transformation of ferrihydrite: implications of carbon dynamics. Environ. Sci. Technol. 2015a, 49(8), Chen, C.; Sparks, D. L. Multi-elemental scanning transmission X-ray microscopy near edge X- ray absorption fine structure spectroscopy assessment of organo mineral associations in soils from reduced environments. Envir. Chem. 2015b, 12(1), Cornell, R. M.; Schwertmann, U. The Iron Oxides: Structure, Properties, Reactions, Occurrences, and Uses. Wiley-VCH, Ekstrom, E. B.; Learman, D. R.; Madden, A. S.; Hansel, C. M. Contrasting effects of Al substitution on microbial reduction of Fe(III) oxides. Geochim. Cosmochim. Acta 2010, 74, Eusterhues, K.; Wagner, F. E.; Häusler, W.; Hanzlik, M.; Knicker, H.; Totsche, K. U.; Kögel- Knabner, I. Schwertmann, U. Charcterization of ferrihydrite-soil organic matter coprecipitates by X-ray diffraction and Mössbauer spectrosocpy. Environ. Sci. Technol. 2008, 42, Fox, P. M.; Davis, J. A.; Kukkadapu, R. K.; Singer, D. M.; Bargar, J.; Williams, K. H. Abiotic U(VI) reduction by sorbed Fe(II) on natural sediments. Geochim. Cosmochim. Acta 2013, 117, Fysh, S. A.; Clark, P. E. Aluminous goethite: A Mössbaur study. Phys. Chem. Minerals 1982a, 8, Fysh, S. A.; Clark, P. E. Aluminous hematite: A Mössbaur study. Phys. Chem. Minerals. 1982b, 8, Hansel, C. M.; Benner, S. G.; Neiss, J.; Dohnalkova, A.; Kukkadapu, R. K.; Fendorf, S. Secondary mineralization pathways induced by dissimilatory iron reduction of ferrihydrite under advective flow. Geochim. Cosmochim. Acta 2003, 67, S12

13 12. Larese-Casanova, P.; Scherer, M. Fe(II) sopriton on hematite: New insights based on spectroscopic measurements.. Environ. Sci. Technol. 2007, 41(2), Lovely, D. R.; Holmes, D. E.; Nevin, K. P. Dissimilatory Fe(III) and Mn(IV) reduction. Adv. Microb. Physiol. 2004, 49, Mitsunobu. S.; Sakai,Y.; Takahashi, Y. Characterization of Fe(III) (hydr)oxides in arsenic contaminated soil under various redox conditions by XAFS and Mössbauer spectroscopies. Appl. Geochem. 2008, 23, Murad, E.; Cashion, J. Mössbauer spectroscopy of environmental materials and their industrial utilization. Kluwer Academic Publishers, Norwell, MA, Kukkadapu, R. K.; Zachara, J. M.; Fredrickson, J. K.; McKinley, J. P.; Kennedy, D. W.; Smith, S. C.; Dong, H. Reductive biotransformation of Fe in shale-limestone saprolite containing Fe(III) oxides and Fe(II)/Fe(III) phyllosilicates. Geochim. Cosmochim. Acta 2006, 70, Kukkadapu, R. K.; Zachara, J. M.; Smith, S. C.; Fredrickson, J. K.; Liu, C. Dissimilatory bacterial reduction of Al-substituted goethite in subsurface sediments. Geochim. Cosmochim. Acta 2001, 65, Peretyazhko, T.; Zachara, J. M.; Kukkadapu, R. K.; Heald, S. M.; Kutnyakov, I. V.; Resch, C. T.; Arey, B. W.; Wang, C. M.; Kovarik, L.; Phillips, J. L.; Moore, D. A. Pertechnetate (TcO 4 ) reduction by reactive ferrous iron forms in naturally anoxic, redox transition zone sediments from the Hanford Site, USA. Geochim. Cosmochim. Acta 2012, 92, Rancourt, D.; Ping, J. Y. Voigt-based methods for arbitrary-shape static hyperfine parameter distributions in Mössbauer spectroscopy. Nucl. Instrum. Methods Phys. Res. B 1991, 58, Ribeiro, F. R.; Fabris, J. D.; Kostka, J. E.; Komadel, P.; Stucki, J. W. Comparisons of structural iron reduction in smectites by bacteria and dithionite: II. A variable-temperature Mössbauer spectroscopic study of Garfield nontronite. Pure Appl. Chem. 2009, 81(8), Roden, E. E.; Zachara, J. M. Microbial reduction of crystalline Fe(III) oxides: Influence of oxide surface area and potential for cell growth. Environ. Sci. Technol. 1996, 30, Thompson, A.; Chadwick, O. A.; Rancourt, D. G.; Chorover, J. Iron solid-phase differentiation along a redox gradient in basaltic soils. Geochim. Cosmochim. Acta 2011, 75(1), Van Der Zee, C.; Roberts, D. R.; Rancourt, D. G.; Slomp, C. P. Nanogoethite is the dominant reactive oxyhydroxide phase in lake and marine sediments. Geology 2006, 31, Yamashita, M.; Misawa, T.; Oh, S. J.; Balasubramanian, R.; Cook, D. C. Mössbauer spectroscopic study of X-ray amorphous substance in rust layer of weathering steel subjected to long-term exposure in North America. Corrosion Engineering 2000, 49, Zhao, J.; Huggins, F.; Feng, Z.; Huffman, G. P. Surface-induced superparamagnetic relaxation in nanoscale ferrihydrite particles. Phys.Rev. B 1996, 54(5), S13

14 Table S1. Fe concentrations from selective extractions, Eh and total carbon (TC) concentrations of the eastern and western floodplain samples. Horizon Depth (cm) Eastern Floodplain Eh (mv) Total Fe (Fe t ) (g kg -1 soil) Fe from selective extractions (g kg -1 soil) HCl (Fe II ) Oxalate (Fe o ) Dithionite (Fe d ) Fe o / Fe d Fe d / Fe t TC (g kg -1 ) B Bg Gravel Western Floodplain B Agb Gravel S14

15 Table S2. ph, cation exchange capacity, and soil texture of the eastern and western floodplain samples. Horizon Depth (cm) ph CEC Sand (%) Silt (%) Clay (%) Eastern Floodplain B Bg Gravel Western Floodplain B Agb Gravel S15

16 Table S3. R-factor improvement of LCF for the near-surface and gravel sediments performed with the first 2 components (vermiculite and illite), with addition of an Fe(III)-oxide (ferrihydrite, goethite, lepidocrocite, and hematite) or OM-Fe(III) component. sample With first 2 compounds ferrihydrite goethite hematite lepidocrocite OM-Fe(III) R-factor R-factor % 1 R-factor % 1 R-factor % 1 R-factor % 1 R-factor % 1 Western floodplain cm cm Eastern floodplain cm cm Improvement of fit R-factor with the 2 components by % S16

17 Table S4. R-factor improvement of LCF for the near-surface and gravel sediments performed with the 4 components (vermiculite, illite, ferrihydrite and goethite), with addition of an Fe(II)-rich component including chlorite, biotite, magnetite, green rust, vivianite, ilmenite, and Fe(II)-oxalate. sample With 4 compounds chlorite biotite magnetite green rust vivianite ilmenite Fe(II)-oxalate R-factor R-factor % 1 R-factor % 1 R-factor % 1 R-factor % 1 R-factor % 1 R-factor % 1 R-factor % 1 Western floodplain 40-80cm cm Eastern floodplain cm cm Improvement of fit R-factor with the 4 components by % S17

18 Table S5. R-factor improvement of LCF for the reduced intermediate sediments performed with the first 2 components (vermiculite and illite), with addition of an Fe(II)-rich component including chlorite, biotite, magnetite, green rust, vivianite, Fe(II)-oxalate, or ilmenite. sample With first 2 compounds chlorite biotite magnetite green rust vivianite ilmenite Fe(II) oxalate R-factor R-factor % 1 R-factor % 1 R-factor % 1 R-factor % 1 R-factor % 1 R-factor % 1 R-factor % 1 Western floodplain cm Eastern floodplain cm Improvement of fit R-factor with the 2 components by % Table S6. R-factor improvement of LCF for the reduced intermediate sediments performed with the 5 components (vermiculite, illite, chlorite, biotite, and ilmenite), with addition of an Fe(III)-oxide (ferrihydrite, goethite, lepidocrocite, and hematite) or OM-Fe(III) component. sample Western floodplain Western floodplain With 5 compounds ferrihydrite goethite 1 Improvement of fit R-factor with the 5 components by % hematite lepidocrocite OM-Fe(III) R-factor R-factor % 1 R-factor % 1 R-factor % 1 R-factor % 1 R-factor % cm Eastern floodplain cm S18

19 Table S7. Two Linear Combination Fitting (LCF) solutions of the Fe EXAFS spectra for the reduced intermediate sediment samples. The LCF analyses include vermiculite, illite, chlorite, biotite, and ilmenite, as model reference spectra with or without additional ferrihydrite and goethite. The experimental k 3 -weighted EXAFS functions and LCF Fits of Fe K-edge EXAFS spectra were shown in Figure S6. Sample Vermiculite Illite Chlorite Biotite Ilmenite Ferrihydrite Goethite R-factor Without ferrihydrite and goethite Western floodplain cm Eastern Floodplain cm With ferrihydrite and goethite Western floodplain cm Eastern Floodplain cm S19

20 Table S8. Analysis of Fe EXAFS spectra of eastern and western floodplain samples by Linear Combination Fitting (LCF) a. Horizon Depth (cm) Phyllosilicates (PS) (%) Fe(III)-oxides (%) Vermiculite Illite Chlorite Biotite Ferrihydrite Goethite Ilmenite (%) R-factor Eastern Floodplain B Bg Gravel Western Floodplain B Agb Gravel a Fits were conducted over k-range 2-11 Å -1 based on spectra of vermiculite, illite, chlorite, biotite, ferrihydrite, goethite, and ilmenite. Individual fractions were normalized to a sum of 100%. S20

21 Table S9. PS-Fe(II) and PS-Fe(III) fractions (% of total Fe in samples) calculated based on EXAFS LCF estimates shown in Table S7. PS-Fe(II) and PS-Fe(III) were calculated based on Fe(II) contents of 7% in vermiculite, 12% in illite, 91% in chlorite and 87% in biotite. For example, the contribution of PS-Fe(II) to total Fe in the reduced sediments (55-80 cm) of the eastern floodplain is calculated as: (26%*7%+15%*12%+12%* %*0.87)*100. Horizon Depth (cm) PS-Fe(II) (% of total Fe) PS-Fe(III) (% of total Fe) PS-Fe(II)/Fe(III) ratio Eastern floodplain B Bg Gravel Western floodplain B Agb Gravel S21

22 Table S10. Fitting parameters of mössbauer spectra of the western floodplain samples at RT, 77K and 12K. S22

23 S23

24 Table S11. Fitting parameters of mössbauer spectra of the eastern floodplain samples at RT, 77K and 12K S24

25 S25

26 Figure S1. Map and cross section of transect location in White Clay Creek, Christina River Basin Critical Zone Observatory (Southeastern Pennsylvania, USA). (left) Elevation map showing transect A-A'. The western floodplain is broader than thee eastern floodplain. (right) Locations of floodplain sampling ports. S26

27 Figure S2. Photographs of floodplain profiles. S27

28 Q Q East bank, cm V/C I/B K/C I/B K F East bank, cm East bank, cm West bank, cm West bank, cm West bank, cm Degree 2 Figure S3. XRD of of the eastern and western floodplain samples. Abbreviations are as follows: V, vermiculite; C, Chlorite; I, illite; B, biotite; K, Kaolinite; Q, quartz; F, feldspar. S28

29 Fe(II), ~7121 ev Reduced sediment western floodplain Pyrite First Derivative Absorbance Siderite Vivianite Green rust Magnetite Fe(II)-oxalate Energy (ev) Figure S4. Fe K-edge first-derivative XANES spectra of the reduced sediment sample ( cm) from the western floodplain, as well as Fe(II)-rich reference compounds. S29

30 Reducedd sediment westernn floodplain Figure S5. Fe K-edgee first-derivative XANES spectra of the reducedd sediment sample ( cm) from the western floodplain, OM-Fe(III), and Fe(III)-oxide reference compounds. S30

31 Western floodplain Eastern floodplain With ferrihydrite and goethite Without ferrihydrite and goethite Figure S6. Two Linear Combination Fitting (LCF) solutions of the Fe EXAFS spectra for the reduced intermediate sediment samples from both the western (left) and eastern (right) floodplains. The LCF analyses include vermiculite, illite, chlorite, biotite, and ilmenite, as model reference spectra with or without additional ferrihydrite and goethite. Experimental k 3 -weighted EXAFS functions (black lines) and LCF Fits (purple lines) of Fe K-edge EXAFS spectra of the reduced sediment samples. S31

32 Figure S7. Fe K-edgee EXAFS spectra of the Fe references compounds that were included in the final LCF analysis. S32

33 Figure S8. Experimental k 3 -weighted EXAFS functions (black lines) ) and LCF Fits (purple lines) of Fe K-edge EXAFS spectra of the bulk sedimentt samples. S33

34 Figure S Figure S9. Two Linear Combination Fitting (LCF) solutions of the Fe EXAFS spectra of the floodplain sediment samples. The LCF analyses include vermiculite,, illite, chlorite, biotite (only for the sample from the eastern floodplain at cm), ferrihydrite and ilmenite, as model referencee spectra with additional goethite (the left figure) ) or hematitee (the right figure). Experimental k 3 -weighted EXAFS functions (black lines) and LCF Fits (purple lines) of Fe K- edge EXAFS spectra of the bulk sediment samples. S34

35 Figure S10. Two Linear Combination Fitting (LCF) solutions of the Fe EXAFS spectra of the floodplain sediment samples. The LCF analyses include vermiculite,, illite, chlorite, biotite (only for the sample from the eastern floodplain at cm), ferrihydrite and ilmenite, as model referencee spectra with (the left figure) or without (the right figure) additional goethite. Experimental k 3 -weighted EXAFS functions (black lines) and LCF Fits (purple lines) of Fe K- edge EXAFS spectra of the bulk sediment samples. S35

36 Figure S11. Two Linear Combination Fitting (LCF) solutions of the Fe EXAFS spectra of the floodplain sediment samples. The LCF analyses include vermiculite,, illite, chlorite, biotite (only for the samples from the eastern floodplain at cm and from the western floodplain at cm), ferrihydrite (eliminated for the sample from the western floodplain at cm), goethite (eliminated for the sample from the western floodplain at cm) and ilmenite, as model reference spectra with (the left figure) or without (the right figure) additional chlorite. Experimental k 3 -weighted EXAFS functions (black lines) and LCF Fits (purple lines) of Fe K- edge EXAFS spectra of the bulk sediment samples. S36

37 Figure S12. Two Linear Combination Fitting (LCF) solutions of the Fe EXAFS spectra of the floodplain sediment samples. The LCF analyses include vermiculite,, illite, chlorite, biotite (only for the sample from the eastern floodplain at cm), ferrihydrite,, and goethite, as model referencee spectra with (the left figure) or without (the right figure) additional ilmenite. Experimental k 3 -weighted EXAFS functions (black lines) and LCF Fits (purple lines) of Fe K- edge EXAFS spectra of the bulk sediment samples. S37

38 Figure S Fe Mössbauer spectra of the near-surface sediments at cm of the western floodplain at (a)rt, (b)77k, (c) 12K, and (d) 5K. The black solid line is the total calculated fit, through the discrete data points (circles). S38

39 Figure S Fe Mössbauer spectra of the reduced intermediate sediments at cm of the western floodplain at (a) RT, (b)77k, (c) 12K, and (d) 5K. The black solid line is the total calculated fit, through the discrete data points (circles). S39

40 Figure S Fe Mössbauer spectra of the suboxic gravel aquifer sediments at cm of the western floodplain at (a) RT, (b)77k, (c) 12K, and (d) 5K. The black solid line is the total calculated fit, through the discrete data points (circles). S40

41 Figure S Fe Mössbauer spectra of the oxic near-surface sediments at cm of the eastern floodplain at (a) RT, (b)77k, (c) 12K, and (d) 5K. The black solid line is the total calculated fit, through the discrete data points (circles). S41

42 Figure S Fe Mössbauer spectra of the reduced intermediate sediments at cm of the eastern floodplain at (a) RT, (b)77k, (c) 12K, and (d) 5K. The black solid line is the total calculated fit, through the discrete data points (circles). S42

43 Figure S Fe Mössbauer spectra of the oxic gravel aquifer sediments at cm of the eastern floodplain at (a) RT, (b)77k, (c) 12K, and (d) 5K. The black solid line is the total calculated fit, through the discrete data points (circles). S43