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Geochimica et Cosmochimica Acta 74 (2010) 30–40

Coordination chemistry and hydrolysis of Fe(III) in a peathumic acid studied by X-ray absorption spectroscopy

Torbjorn Karlsson *, Per Persson

Department of Chemistry, Umea University, SE-901 87 Umea, Sweden

Received 17 April 2009; accepted in revised form 15 September 2009; available online 22 September 2009

Abstract

The speciation of iron (Fe) in soils, sediments and surface waters is highly dependent on chemical interactions with naturalorganic matter (NOM). However, the molecular structure and hydrolysis of the Fe species formed in association with NOM isstill poorly described. In this study extended X-ray absorption fine structure (EXAFS) spectroscopy was used to determine thecoordination chemistry and hydrolysis of Fe(III) in solution of a peat humic acid (5010–49,200 lg Fe g�1 dry weight, pH 3.0–7.2). Data were analyzed by both conventional EXAFS data fitting and by wavelet transforms in order to facilitate the iden-tification of the nature of backscattering atoms. Our results show that Fe occurs predominantly in the oxidized form as ferricions and that the speciation varies with pH and Fe concentration. At low Fe concentrations (5010–9920 lg g�1; pH 3.0–7.2)mononuclear Fe(III)–NOM complexes completely dominates the speciation. The determined bond distances for the Fe(III)–NOM complexes are similar to distances obtained for Fe(III) complexed by desferrioxamine B and oxalate indicating the for-mation of a five-membered chelate ring structure. At higher Fe concentrations (49,200 lg g�1; pH 4.2–6.9) we detect a mixtureof mononuclear Fe(III)–NOM complexes and polymeric Fe(III) (hydr)oxides with an increasing amount of Fe(III) (hydr)o-xides at higher pH. However, even at pH 6.9 and a Fe concentration of 49,200 lg g�1 our data indicates that a substantialamount of the total Fe (>50%) is in the form of organic complexes. Thus, in environments with significant amounts of organicmatter organic Fe complexes will be of great importance for the geochemistry of Fe. Furthermore, the formation of five-mem-bered chelate ring structures is in line with the strong complexation and limited hydrolytic polymerization of Fe(III) in oursamples and also agrees with EXAFS derived structures of Fe(III) in organic soils.� 2009 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

Iron (Fe) is one of the most abundant metals in soils,sediments and surface waters where it may occur in a widerange of chemical forms (species) dependent on the environ-mental conditions. The occurrence of these different specieshas a large influence on the overall mobility and availabilityof Fe as well as on the behavior of other nutrients andpotentially toxic trace elements. Undoubtedly the geochem-istry of Fe is important in aquatic and terrestrial environ-ments but there are significant gaps in knowledge within

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doi:10.1016/j.gca.2009.09.023

* Corresponding author. Tel.: +46 (0) 90 786 6328; fax: +46 (0)90 13 6310.

E-mail address: Torbjorn.Karlsson@chem.umu.se (T. Karls-son).

this area, one important example being the interactions be-tween Fe and natural organic matter (NOM). Typically,NOM is a complex mixture of organic substances contain-ing a variety of functional groups, such as carboxyls, phe-nols, thiols and amines many of which interact stronglywith Fe(II) and/or Fe(III). These interactions influenceamong other things redox, hydrolysis and solubility of Feand therefore it is of great interest to determine the struc-tures and reactivities of Fe associated with NOM.

Extended X-ray absorption fine structure (EXAFS)spectroscopy is one of the most powerful methods tostudy the local structure and bonding of metal com-plexes/compounds and in a few cases it has been usedto characterize Fe species present in NOM. Key parame-ter in these analyses are the identity, coordination num-bers and distances of the atoms present in the second

Table 1Chemical composition (% w/w) and content of carboxylic andphenolic functional groups in the Pahokee peat humic acid(PPHA), as provided by the IHSS.

Element/compound % (w/w) meq g�1 C

H2O 10.4a

Ash 1.72b

C 56.8c

H 3.60c

O 36.6c

N 3.74c

S 0.70c

P 0.03c

Fe 0.01d

Carboxylic 8.87e

Phenolic 2.05f

a H2O in the air equilibrated sample.b Inorganic residue in the dry sample.c Element composition in the dry ash free sample.d Fe content in the dry sample (Heitmann and Blodau, 2006),

corresponds to 100 lg Fe g�1.e The charge density (meq g�1) at pH 8.0.f Two times the charge density (meq g�1) between pH 8.0 and

10.0.

Coordination chemistry of Fe(III) in a humic acid studied by XAS 31

coordination shell as this will reveal the distribution be-tween organic Fe complexes and polymeric Fe (hydr)o-xides (the latter expression is used herein to describeboth polymeric Fe hydroxo complexes and precipitatedFe (hydr)oxides). Furthermore, the second-shell Fe–Cand Fe–Fe distances may indicate the coordination modesof the organic Fe complexes and the polymeric Fe (hy-dr)oxides, respectively. Gustafsson et al. (2007) reportedthat Fe(III) in organic soils (pH 4) occurred either asFe (hydr)oxides or organically complexed likely as a mix-ture of di- and trinuclear (O5Fe)2O and (O5Fe)3O com-plexes where each Fe atom was surrounded by fivesecond-shell C atoms at a distance of 3.0–3.02 A. In con-trast, Karlsson et al. (2008) showed that Fe(III) occurredas mononuclear Fe–NOM complexes and polymeric Fe(hydr)oxides in the solid phase of organic soils at pH4.6–6.0. Based on Fe–C distances of 2.78–2.86 A they alsosuggested the formation of a predominant five-memberedchelate ring structure of the mononuclear Fe–NOM com-plexes, which is in accordance with the structures ofFe(III) desferrioxamine B and Fe(III)-oxalate solutioncomplexes (Edwards and Myneni, 2005; Persson andAxe, 2005). Rose et al. (1998) and Vilge-Ritter et al.(1999) determined the speciation of Fe in NOM from nat-ural freshwaters (pH 5.5–7.5) and found that Fe waspoorly polymerized due to complexation with NOM andreported Fe–C distances in the range 2.76–3.0 A. In astudy of the reaction between Fe and fulvic acids at pH4 van Schaik et al. (2008) found that Fe(III) formedmononuclear complexes with fulvic acids in solution withFe–C distances in the range 2.82–2.98 A. For humic acidsand ligneous material only first shell contributions havebeen reported, showing that Fe(III) is coordinated bysix O atoms at a distance of approximately 2.0 A (Davieset al., 1997; Guillon et al., 2003). Obviously there is a rel-atively large variation in reported Fe–C distances (2.76–3.02 A), but whether this indicates actual differences incoordination environments is not entirely clear. Further-more, the role of NOM in Fe hydrolysis and in the for-mation of various hydrolyzed species is still very muchunknown.

As already mentioned, a key in EXAFS studies of Feassociated with NOM is a proper analysis of the secondcoordination shell. However, in conventional analysis ofEXAFS spectra it may prove difficult to separate contri-butions from different backscattering atoms in highercoordination shells present at similar bonding distancesfrom the central iron atom. In such cases analysis basedon wavelet transforms (WT) can provide valuable comple-mentary information (Munoz et al., 2003; Funke et al.,2005; Persson et al., 2006; Karlsson et al., 2008). In thisstudy we have used wavelet transforms in combinationwith conventional EXAFS data fitting to determine thecoordination chemistry and hydrolysis of Fe(III) in a peathumic acid, at concentrations of 5010–49,200 lg Fe g�1 inthe pH range 3.0–7.2. The main objective of the study hasbeen to characterize the Fe–NOM complexes and theirresistance towards hydrolysis and concurrent polymeriza-tion whereas less emphasis has been put on detailed anal-ysis of the polymeric Fe(III) hydroxide species.

2. MATERIAL AND METHODS

2.1. Sample preparation

The Pahokee peat reference humic acid, 1R103H(PPHA) used in this study was purchased from the Interna-tional Humic Substance Society (IHSS), and selected chem-ical data is presented in Table 1. The Pahokee peat is atypical agricultural peat soil of the Everglades formed in or-ganic deposits from a freshwater marsh. Samples for theEXAFS analysis were prepared by weighing 100 mg offreeze-dried humic acid into 2 mL Eppendorf tubes fol-lowed by addition of Fe(NO3)3 � 9H2O dissolved inMilli-Q water (pH < 1.5). This yielded Fe concentrationsin the range 5010–49,200 lg g�1 on a dry mass basis, whichcorresponds to 0.018–0.175 mol Fe per mol of carboxylicfunctional groups. The suspensions were equilibrated for10 min before NaOH (2 M) was added to adjust pH tothe target value between 3.0 and 7.2. The total solution vol-umes in the samples were adjusted to ca. 0.4 mL by additionof Milli-Q water. In order to dissolve the freeze-dried humicacid the suspensions were vigorously shaken (Vortex-genie2, Scientific industries) and subsequently equilibrated on arotary mixer for 7 h at room temperature. The samples werestored in a refrigerator at 8 �C for 8–9 days prior to EXAFSanalysis. The ionic strength in the samples was in the range0.06–0.75 M depending on the Fe concentration. Calcula-tions were made to investigate if any sample was oversatu-rated with respect to formation of the solid phase,Fe(OH)3(s). Stability constants (at I = 0, 25 �C) fromStumm and Morgan (1996) were used in the calculations.Data for the prepared samples are presented in Table 2.

Aqueous solutions of iron(III) desferrioxamine B(Fe(III)–DFO-B) and trisoxalatoiron(III) (FeðC2O4Þ33�)(Persson and Axe, 2005) together with synthetic goethite

Table 2Fe concentrations, additions of NaOH (2 M) and pH for thePahokee peat humic acid (PPHA) samples prepared for EXAFSanalysis.

Samplea [Fe]b

(lg g�1)[Fe](lmol)

Volumec

(mL)NaOH(lmol)

pH

PPHA1 5010 8.97 0.41 84 3.0PPHA2 5010 8.97 0.46 200 4.3PPHA3 5090 9.11 0.45 320 5.5PPHA4 5090 9.11 0.44 400 7.2PPHA5 9920 17.8 0.43 344 5.6PPHA6 49,200 88.1 0.36 312 3.0PPHA7 49,200 88.1 0.35 400 4.2PPHA8 49,200 88.1 0.43 520 5.6PPHA9 49,200 88.1 0.45 600 6.9

a 100 mg PPHA in each sample. The concentration of carboxylicfunctional groups is 5042 lmol g�1.

b Based on a dry mass basis.c The total solution volume in the samples.

32 T. Karlsson, P. Persson / Geochimica et Cosmochimica Acta 74 (2010) 30–40

and mixtures of synthetic goethite and ammonium trisoxa-latoiron(III) trihydrate [(NH4)3[Fe(C2O4)3] � 3H2O] (Kar-lsson et al., 2008), were selected as reference samples forcomparison with the PPHA samples. The Fe(III)–DFO-Bsample was prepared by mixing equal amounts of 80 mMDFO-B and Fe(NO3)3 solutions and pH was adjusted to1.8 by addition of NaOH (2 M). Mixtures of goethite/tris-oxalatoiron(III) were prepared at 50/50 and 25/75 basedon the percentage of Fe(III). The goethite and goethite/tris-oxalatoiron(III) mixtures were ground in a mortar and di-luted 1:40 with boron nitride. Before EXAFS analysis thePPHA and reference samples were transferred to Teflonsample holders and sealed with Kapton tape (CHR-Furon).

2.2. EXAFS data collection and analysis

Iron K-edge (7.112 keV) spectra were collected at thesuperconducting beam line I811, at MAX-lab (Lund Uni-versity, Sweden), with 1.5 GeV beam energy and 100–200 mA electron current. The energy resolution of thisbeamline at the Fe K-edge is 1 eV. The beamline wasequipped with three consecutive ion chambers to monitorthe transmitted beam and one Lytle detector for fluores-cence measurements. The former detectors were filled withN2 while the latter detector contained Ar. The beam wasmonochromatized with a double crystal monochromator(Si [1 1 1]). Energy calibration was performed by simulta-neously measuring the transmission spectra of a Fe foil dur-ing all scans. All spectra of the PPHA and referencesamples were collected in fluorescence mode with the Lytledetector and a Mn filter was placed between the detectorand the sample to reduce unwanted scattering and fluores-cence contributions. Samples were positioned at 45� to theincident beam, and the monochromator was detuned 50%to reduce higher order harmonics. Data (If/I0 against en-ergy) were collected in the energy range from 150 eV beforethe edge to 650 eV after the edge, with 5 eV steps from150 eV to 55 eV before the edge, 0.5 eV steps from 50 eVbefore the edge to 59.5 eV after edge and with k steps(0.07 A�1) from 60 eV to 650 eV after the edge. The collec-

tion time of each scan was 45 min and each spectrum repre-sents the average of 1–4 scans. We did not observe spectralchanges during data collection in any of the samples.

EXAFS spectra were quantitatively analyzed using theprograms WinXAS v3.1 (Ressler, 1998) and Viper (Kle-mentev, 2001a). From each averaged and energy calibrated(7.112 keV) spectrum a first-order polynomial pre-edgefunction was subtracted and the data were normalized.Above the absorption edge a cubic spline fit was used to re-move the background and the data were k3-weighted to en-hance the higher k-values. The k3-weighted spectra wereFourier transformed (FT) over the k-range 3.1–12.0 A�1

using a Bessel window function. For all samples, fits weremade to the resulting FT in a non-linear least-squaresrefinement procedure with theoretical phase and amplitudefunctions calculated by the ab initio code FEFF7 (Zabinskyet al., 1995). The input structures used in the FEFF calcu-lations were those of goethite and the Fe(III) desferriox-amine B complex (Szytula et al., 1968; Dhungana et al.,2001). During the fitting procedure the edge energy (DE0)was varied but correlated to be the same for all shells in asample. In all refinements the amplitude reduction factor(S0

2) was set to 0.96. This was based on the value obtainedfrom fitting the spectrum of the Fe(III)–DFO-B complex,while fixing the coordination number in the first coordina-tion shell to 6.0.

A qualitative analysis of the nature of backscatteringatoms in higher coordination shells was conducted usingthe wavelet transform (WT) method as implemented inthe Igor Pro script developed by Funke et al. (2005). Thismethod complements the conventional FT analysis and re-veals the energies where backscattering takes place that giverise to the FT peaks. The WT results are typically visualizedas contour plots in three dimensions: the wave vector (k),the interatomic distance uncorrected for phase shifts (R)and the WT modulus. Contributions from different back-scattering atoms at specific regions in k and R are visualizedin these plots as ridges, and the location of the ridges helpsto differentiate between light and heavy backscatteringatoms. The WT modulus of the PPHA samples were com-pared with the WT for the reference samples with contribu-tion from C/N and/or Fe backscattering in the secondcoordination shell. All spectra were analyzed by means ofthe Morlet-wavelet, and different g and r values were usedin the overview and high-resolution wavelet plots, respec-tively (see below). Since the resolution of the plots criticallydepends on the selection of these Morlet parameters (Funkeet al., 2005) different values were tested in order to optimizethe resolution in both k and R space for our samples.

3. RESULTS AND DISCUSSION

3.1. EXAFS results for the first coordination shell

The WT analysis showed similar first shell contributionsfor all Pahokee peat humic acid (PPHA) samples, with abackscattering maxima around 5 A�1, which is in accor-dance with the Fe–O scattering in the reference compounds(Fig. 1). The conventional EXAFS data fitting also showedthat Fe in the PPHA samples was coordinated by approxi-

Coordination chemistry of Fe(III) in a humic acid studied by XAS 33

mately six O/N atoms (O and N atoms cannot be distin-guished by means of EXAFS) at distances in agreementwith an approximate octahedral configuration (Table 3).There were no signs of reduction of Fe(III) to Fe(II). Thiswas apparent in the first derivatives of the X-ray absorptionnear edge structure (XANES) region (Fig. 2) where thePPHA samples were similar to the reference compoundscontaining only Fe(III); the main edge of compounds con-taining Fe(II) would appear at lower energies (La Forceand Fendorf, 2000; O’Day et al., 2004). Furthermore, thebond distances (1.99–2.02 A) also shows that Fe(III) isdominating, as the bond distance for Fe(II) is substantiallylonger (2.10 A; D’Angelo and Benfatto, 2004). The distancefound in the first shell for the PPHA samples are in accor-dance with Fe–O distances reported for Fe(III) (hydr)o-xides, and Fe(III) complexed to organic soils, humic acidsand small organic molecules (Davies et al., 1997; O’Dayet al., 2004; Persson and Axe, 2005; Karlsson et al., 2008).

a

0

1

2

3

4

5

6

R (Å

)

FT Magnitude

3 5 7 9 11

3 5 7 9 11

k (Å -1)

k3-w

eigh

ted

EXA

FS

0

1

2

3

4

5

6

R (Å

)

k (Å -1)

k3 -w

eigh

ted

EXA

FS

c

FT Magnitude

Fig. 1. Overview of the WT modulus displaying the first, second and thirdFe(III)–DFO-B, and (d) PPHA1 plotted as a function of k (A�1) on the

3.2. Wavelet analysis of higher coordination shells

Both the overview and the high-resolution wavelets(Figs. 1a and 3a) of the goethite EXAFS spectrum reveala strong feature corresponding to the second peak in theFT. This feature appears at significantly higher energy(ca. 8 A�1) as compared to the WT modulus of the firstshell discussed above, indicating the presence of heavierbackscatterers in accordance with the second nearest Feneighbors of the goethite structure (Szytula et al., 1968).This Fe–Fe feature is weaker, but still clearly present, inthe goethite/oxalate mixtures (Fig. 3b and c) and as ex-pected not present in the WT plot for the Fe(III)–DFO-Bcomplex (Figs. 1c and 3d). The decrease in Fe backscatter-ing in these samples is accompanied by new features at dis-tances of 2.0–2.5 A and 3.1–3.7 A. These features appearsat lower energies (3–4 A�1) indicating that they are causedby lighter atoms, which is in agreement with single and mul-

b

0

1

2

3

4

5

6R

(Å)

k (Å -1)

k3-w

eigh

ted

EXA

FS

0

1

2

3

4

5

6

R (Å

)

k (Å -1)

k3 -w

eigh

ted

EXA

FS

d

FT Magnitude

FT Magnitude

3 5 7 9 11

3 5 7 9 11

coordination shells (g = 10, r = 1) for (a) goethite, (b) PPHA9, (c)x-axis and R (A) on the y-axis.

Table 3Fits to Fourier transformed EXAFS data over the k-range 3.1–12.0 A�1, using a Bessel window function, for the Fe(III) desferrioxamine Bcomplex (Fe(III)–DFO-B) and the Pahokee peat humic acid (PPHA) samples.

Sample Fe–O (SS) Fe–C (SS)b Fe–C (SS)c Fe–C–C (MS)d Fe–Fe (SS)e Fe–Fe (SS)f DE0 Fig

CN R (A) r2 (A2) CN R (A) CN R (A) CN R (A) CN R (A) CN R (A) (eV) (%)

Fe(III)–DFO-Ba 6.0 2.01 0.0046 6.0 2.84 6 4.25 12 4.33 2.6 1.5PPHA1 6.4 2.00 0.0065 2.4h 2.87 2.4h 3.77 4.8h 4.20 1.0 2.1PPHA2 6.4 2.00 0.0073 1.8h 2.84 1.8h 3.79 3.6h 4.15 -0.4 2.2PPHA3 5.7 2.02 0.0074 1.7h 2.85 1.7h 3.82 3.4h 4.22 1.9 3.5PPHA4 5.8 2.01 0.0074 1.5h 2.87 1.5h 3.83 3.0h 4.19 0.1 3.0PPHA5 5.8 2.00 0.0073 1.7h 2.89 1.7h 3.80 3.4h 4.18 0.5 2.8PPHA6 5.2 2.01 0.0067 1.9h 2.94 1.9h 3.85 3.8h 4.26 3.0 3.3PPHA7 5.5 2.00 0.0073 1.5h 2.91 1.5h 3.86 3.0h 4.27 0.2i 3.05 0.4i 3.48 2.1 4.0PPHA8 6.0 1.99 0.0101 1.4h 2.92 1.4h 3.93 2.8h 4.28 0.6i 3.05 1.2i 3.48 0.3 3.7PPHA9 5.2 1.99 0.0087 1.1h 2.91 1.1h 3.91 2.2h 4.28 0.7i 3.06 1.4i 3.49 1.0 4.6

The following abbreviations are used: single scattering (SS), multiple scattering (MS), coordination number (CN), bond distance (R), Debye–Waller factor (r2) and edge energy (DE0). S0

2 was set to 0.96 (see text). Uncertainties in R (A) are estimated to be 0.02 in the first shell and 0.04in higher coordination shells. Uncertainties in CNs are estimated to at least 20%. These uncertainties are based on a previous study of Fe(III)complexes with oxalate and malonate using the same fitting procedure (Persson and Axe, 2005).

a CN in all shells were fixed and the r2’s for the short Fe–C (SS), the long Fe–C (SS), and the Fe–C/N–C (MS) paths were determined to0.0026, 0.002 and 0.004, respectively. Furthermore, MS paths for Fe–O–C (CN = 12, R = 3.15 and r2 = 0.005) and Fe–O–O (CN = 12,R = 4.06 and r2 = 0.0098) were also included in the fit. CN = 12 is a consequence of the double degeneracy of these paths.

b For PPHA1 r2 was determined to be 0.0036 and this value was used for PPHA1–5. For PPHA6 r2 was determined to be 0.0044 and thisvalue was used for PPHA6–9.

c r2 of PPHA1 was determined to 0.0042 and this value was used for PPHA1–5. r2 of PPHA6 was determined to 0.002 and this value wasused for PPHA6–9.

d r2 of PPHA1 was determined to 0.0025 and this value was used for PPHA1–5. r2 of PPHA6 was determined to 0.0044 and this value wasused for PPHA6–9.

e r2 = 0.0037, which is a literature average for goethite and ferrihydrite (O’Day et al., 2004).f r2 = 0.0083, which is a literature average for goethite and ferrihydrite (O’Day et al., 2004).g Fi is defined as ((

P(k3vexp � k3vfit)

2)/(P

(k3vexp)2)) � 100, k3vexp and k3vfit represents experimental and fitted data points, respectively.h CNs of these paths were correlated as follows: CN (short Fe–C (SS)) = CN (long Fe–C (SS)) = 1/2 CN (Fe–C–C (MS)). This is in

accordance with the Fe(III)-oxalate and Fe(III)–DFO-B structures, and CN of the Fe–C–C (MS) is due to the double degeneracy of this path.i CN for the long Fe–Fe path was correlated to 2 � CN of the short Fe–Fe path in agreement with the goethite and ferrihydrite structures.

7,10 7,11 7,12 7,13 7,14 7,15Energy (keV)

Firs

t Der

ivat

ive

Abs

orba

nce

a

e

d

c

b

f

Energy (keV)

Firs

t Der

ivat

ive

Abs

orba

nce

a

e

d

c

b

f

A B

7,10 7,11 7,12 7,13 7,14 7,15

Fig. 2. First derivatives of the XANES region for (A) Pahokee peat humic acid (PPHA) samples at 49,200 lg Fe g�1 (a) goethite, (b) pH 6.9,(c) pH 5.6, (d) pH 4.2, (e) pH 3.0 and (f) Fe(III)–DFO-B. (B) PPHA samples at 5010–5090 lg Fe g�1 (a) goethite, (b) pH 7.2, (c) pH 5.5, (d)pH 4.3, (e) pH 3.0 and (f) Fe(III)–DFO-B. The vertical broken line indicates the maximum of the first derivative for Fe(III)–DFO-B.

34 T. Karlsson, P. Persson / Geochimica et Cosmochimica Acta 74 (2010) 30–40

Fig. 3. High-resolution WT modulus displaying the second and third coordination shells (g = 8, r = 1) for (a) goethite, (b) goethite/trisoxalatoiron(III) mixture 50/50, (c) goethite/trisoxalatoiron(III) mixture 25/75, (d) Fe(III)–DFO-B, (e) PPHA1, (f) PPHA2, (g) PPHA3, (h)PPHA4, (i) PPHA6, (j) PPHA7, (k) PPHA8, and (l) PPHA9, plotted as a function of k (A�1) on the x-axis and R (A) on the y-axis in the range2.0–4.0 (A).

Coordination chemistry of Fe(III) in a humic acid studied by XAS 35

tiple backscattering from C/N/O in the second and thirdcoordination shells of the Fe(III)–DFO-B and trisoxalato-iron(III) complexes.

The WT plots of the PPHA samples display a distincttrend indicating increasing second-shell Fe–Fe contributionwith increasing pH and Fe concentrations (Fig. 3). The pHeffect is obvious at 49,200 lg Fe g�1 where the WT plotsshow a relatively strong Fe contribution at pH 5.6 and6.9, a weak contribution at pH 4.2 and finally no contribu-tion at pH 3.0. At 5010–5090 lg Fe g�1, however, there isno indication of Fe in the second-shell irrespective of pHwhich emphasizes also the strong effect of total Fe concen-tration. These samples show features in agreement with theFe(III)–DFO-B and trisoxalatoiron(III) complexes(Fig. 3b–h) that suggest no or very little hydrolysis and thatmononuclear organic complexes is the predominating Fespecies. A contribution from a polymeric Fe(III) (hydr)o-xide phase in these samples must be relatively small(<25%) as a significant Fe–Fe contribution shows up inthe WT plot of the (25/75) goethite/oxalate mixture(Fig. 3c). An important observation is that the WT plotsfor all PPHA samples to some extent show features thatare in accordance with backscattering from an organicstructure (Fig. 3). Thus, in summary the WT results indi-cate that the second coordination shell of the PPHA sam-ples consists of either a mixture of C/N and Fe atoms orC/N atoms alone, depending on Fe concentration and pH.

These qualitative results are corroborated by the firstderivative XANES spectra (Fig. 2). As shown, the XANESfeatures of the model compounds are distinctly differentwhere mononuclear Fe(III)–DFO-B is characterized byone peak at 7.1240 keV whereas goethite displays two peaksat 7.1210 and 7.1240 keV. The XANES spectra of thePPHA samples prepared at low Fe concentrations (5010–5090 lg Fe g�1) show one peak only (perhaps with a slightbroadening at the highest pH), which is in agreement withFe(III)–DFO-B and indicating predominately mononuclearcomplexes. At 49,200 lg Fe g�1 we observe a more appar-ent pH-trend, and with increasing pH the XANES spectrabecome more similar to the double-peak characteristics ofgoethite. This suggests the formation of polymeric Fe(III)(hydr)oxides. Thus, both the pH and Fe concentrationdependence of the Fe speciation detected by the WT analy-sis are also observed in the XANES spectra.

3.3. Quantitative results from the EXAFS data fitting

The information gained from the WT analysis was usedas basis for the quantitative modeling of EXAFS data.Thus, scattering paths for C/N, C/O and Fe in the secondand third coordination shells were included in the models,and the paths originating from the organic structure weresimilar to the dominating scattering paths in the Fe(III)–DFO-B and trisoxalatoiron(III) complexes. Note that we

36 T. Karlsson, P. Persson / Geochimica et Cosmochimica Acta 74 (2010) 30–40

have chosen to use single scattering Fe–O and Fe–C andmultiple scattering Fe–C–C paths in the models, but againit should be stressed that we cannot explicitly distinguishbetween C, N and O. The Debye–Waller factors (r2) ofthe Fe–C and Fe–C–C paths were determined from fittingspectra of two low-pH samples (PPHA1 and PPHA6; Table2); that is samples with no second-shell Fe contribution(Fig. 3). The first two FT peaks of these samples were fittedwith a Fe–O and a Fe–C single scattering path. Subse-quently, the obtained coordination numbers (CN), De-bye–Waller factors and bond distances were fixed and theDebye–Waller factors of the third shell contributions weredetermined by fitting the complete FT spectrum. These sec-ond and third shell Debye–Waller factors were then used asfixed parameters during the refinement of the remainingEXAFS spectra (Table 3). The PPHA1 and PPHA6 fitparameters were applied to the 5010–9920 lg Fe g�1 andthe 49,200 lg Fe g�1 samples, respectively. In order to fur-ther constrain the fits, literature values of Fe–Fe Debye–Waller factors were used as fixed parameters (Table 3). Fur-thermore, the CNs of the Fe–Fe paths were correlated inagreement with the goethite and ferrihydrite structures (Ta-ble 4). This relatively constrained model with several fixedparameters was used in order to facilitate comparisons be-tween the samples and to reveal contingent pH and Fedependent trends. However, the EXAFS spectra for the hu-mic acid samples, consisting of a very complex andheterogeneous matrix, most likely contain scattering contri-

Table 4Bond distances in the second coordination shell for different Fe(III) com

Compound Interaction CN

Trisoxalatoirona,b Fe–C 5.9Fe–Odistal 5.9Fe–C–O (MS) 11.8

Fe(III)–DFO-Ba,b Fe–C 5.4–6.2Trismalonatoirona,c Fe–C 6.1

Fe–Odistal 6.1Fe–C–O (MS) 12.2

Fe(acetylacetonate)3c,d Fe–C 6.0

Goethitea Fe–Fe 2Fe–Fe 2Fe–Fe 4

Ferrihydritea Fe–Fe 2Fe–Fe 2Fe–Fe 4

Hematitee Fe–Fe 1Fe–Fe 3Fe–Fe 3Fe–Fe 6

Akaganeitee Fe–Fe 2Fe–Fe 2Fe–Fe 4

Coordination number (CN), bond distance (R), Debye–Waller factor (r2)(2005), (2) Edwards and Myneni (2005), (3) Duckworth et al. (2008), (4) W(1993).

a Structures determined by EXAFS.b Mononuclear five-membered chelate ring structure.c Mononuclear six-membered chelate ring structure.d Distance calculated from EXAFS derived interatomic distances and be Fe–Fe distances calculated after crystal structure refinements.

butions from a much larger number of paths than includedin the model. Thus, this fitting approach may result in sec-ond and third shell fit parameters for the Fe–C contribu-tions that are not entirely physically logical given themodel structure for the organic complexes. As a result ofthese constraints, the maximum number of free variablesin the fitting procedure (11) never exceeded the number ofindependent points (Nind = 19) as calculated by the Nyquisttheorem (Nind = 2 � Dk � DR/p, where Dk is the k-range ofthe EXAFS spectrum and DR is the R-range of the FT con-sidered). In agreement with the WT analysis, the overall re-sults from quantitative EXAFS analysis show that theaverage composition of the second-shell varies with pHand total Fe concentration (Table 3). This trend is also seenin the unfiltered k-space data and in the FTs where sampleswith low Fe concentration and pH show similarities withthe Fe(III)–DFO-B complex, while samples at higher Feconcentration and pH show features more in agreementwith goethite (Figs. 4 and 5).

EXAFS spectra of samples containing low Fe concen-trations (5010–9920 lg g�1) were successfully modeledincluding second and third shell backscattering from organ-ic structures only (Table 3 and Figs. 4 and 5). The contribu-tion from the Fe–C–C multiple scattering path wascorroborated by the F-test (Klementev, 2001b) and forthese samples a model including the Fe–C–C path is betterthan one without within at least 95% probability. The shortsingle scattering Fe–C and multiple scattering Fe–C–C dis-

pounds.

R (A) r2 (A2) Reference

2.82 0.0056 14.03 0.00294.04 0.00612.84 0.002–0.004 2, 32.97 0.0077 14.18 0.00954.22 0.00842.98 43.04 0.0033 53.28 0.01043.43 0.00743.03 0.0040 53.24 0.01793.42 0.00922.89 62.973.373.703.03 63.343.51

and, desferrioxamine B (DFO-B). References: (1) Persson and Axeestre et al. (1995), (5) O’Day et al. (2004), (6) Manceau and Drits

ond angles.

2 4 6 8 10 12

k (Å-1)k3 χ(

k)

a

b

f

e

d

c

2 4 6 8 10 12

k (Å-1)

k3 χ(k)

a

f

e

d

c

b

A B

Fig. 4. k3-weighted EXAFS spectra (solid lines) and fit results (broken lines) for (A) Pahokee peat humic acid (PPHA) samples at49,200 lg Fe g�1 (a) goethite, (b) pH 6.9, (c) pH 5.6, (d) pH 4.2, (e) pH 3.0 and (f) Fe(III)–DFO-B. (B) PPHA samples at 5010–5090 lg Fe g�1

(a) goethite, (b) pH 7.2, (c) pH 5.5, (d) pH 4.3, (e) pH 3.0 and (f) Fe(III)–DFO-B.

0 1 2 3 4 5 6R (Å)

Four

ier T

rans

form

Mag

nitu

de

e

b

d

c

a

f

g

0 1 2 3 4 5 6R (Å)

Four

ier T

rans

form

Mag

nitu

de

e

b

d

c

a

f

g

A B

Fig. 5. Fourier transforms (solid lines), not corrected for phase shift, and fit results (broken lines) for (A) Pahokee peat humic acid (PPHA)samples at 49,200 lg Fe g�1 (a) goethite, (b) pH 6.9, (c) pH 5.6, (d) pH 4.2, (e) pH 3.0, (f) Fe(III)–DFO-B and (g) trisoxalatoiron(III) complexin solution. (B) PPHA samples at 5010–5090 lg Fe g�1 (a) goethite, (b) pH 7.2, (c) pH 5.5, (d) pH 4.3 and (e) pH 3.0, (f) Fe(III)–DFO-B and(g) trisoxalatoiron(III) complex in solution. Vertical broken lines shows the peak positions of the second-shell for Fe(III)–DFO-B/trisoxalatoiron(III) and goethite and the third shell for Fe(III)–DFO-B/trisoxalatoiron(III).

Coordination chemistry of Fe(III) in a humic acid studied by XAS 37

tances are similar to distances in the Fe(III)–DFO-B andtrisoxalatoiron(III) complexes (Tables 3 and 4), indicatingsimilar coordination environments, and hence the forma-tion of five-membered chelate rings in the Fe(III)–PPHA

complexes. This is in agreement with Fe(III) complexes inorganic soils (Karlsson et al., 2008) that were characterizedby Fe–C distances of 2.78–2.86 A indicative of five-mem-bered chelate ring structures. The longer single scattering

38 T. Karlsson, P. Persson / Geochimica et Cosmochimica Acta 74 (2010) 30–40

Fe–C distance at ca. 3.8 A differs from the Fe–C distance inFe(III)–DFO-B and the Fe–Odistal distance in trisoxalato-iron(III) (Table 3 and 4). This could be an effect of the com-plex matrix in our samples and the occurrence of a numberof different scattering paths which are accounted for in themodel by this single Fe–C path. At 5010–5090 lg Fe g�1 theobtained Fe–C and Fe–C–C distances are practically invari-ant with pH (Table 3), suggesting that small polynuclearFe(III)–NOM complexes do not form to any greater extent.However, the possible existence of small polymeric Fe hy-droxo species in these samples is a question that needs tobe further investigated, and to address this EXAFS spec-troscopy needs to be complemented with other techniquessuch as small-angle X-ray scattering (SAXS), high-resolu-tion transmission electron microscopy (HRTEM) andRaman spectroscopy. Indeed, Vilge-Ritter et al. (1999) usedSAXS and showed the initial predominance of trimericcomplexes when Fe(III) is hydrolyzed in the presence ofNOM.

At the higher Fe concentrations (49,200 lg g�1), and pHvalues above 4.2, features characteristic of both C and Febackscattering appear in the EXAFS spectra (Figs. 3–5and Table 3). The modeled Fe–Fe distances at 3.05–3.06 A and 3.48–3.49 A correspond to octahedral edge-sharing and double corner geometries (Manceau and Drits,1993). Comparison with distances in Fe minerals and com-plexes does not unambiguously identify the Fe phase, butthey are in fair agreement with Fe–Fe distances in goethite,ferrihydrite and akaganeite (Table 4). However, comparedto these minerals an Fe–Fe path at 3.24–3.34 A is appar-ently missing in the PPHA samples. Attempts were madeto include also this path in the models but the improve-ments of the fits were statistically insignificant as indicatedby F-tests. This may be a consequence of a comparativelyhigh Debye–Waller factor and hence a minor contributionto the overall EXAFS spectra in agreement with values re-ported for this path (Table 4). An important result is thedetection of edge-sharing Fe–Fe at ca. 3.0 A. In l-oxo com-plexes such as (O5Fe)2O and (O5Fe)3O the Fe–Fe distancesare substantially longer (Kurtz, 1990; Anson et al., 1997;Gustafsson et al., 2007), indicating no or very minor contri-butions from such structures in the PPHA samples. Fur-thermore, at 49,200 lg Fe g�1 the Fe–C and Fe–C–Cdistances are practically invariant with pH (Table 3), whichsuggests that the formation of other types of small polynu-clear Fe(III)–NOM complexes (i.e. complexes with rela-tively high second-shell C:Fe ratios) at higher pH is notprominent as these are expected to cause distortions ofbackscattering from the organic structures. However, basedon the results presented herein we cannot determine theaverage size of the polymeric Fe(III) (hydr)oxide species.

We observe a statistically significant difference betweenthe Fe–C distances at 49,200 lg Fe g�1 (2.91–2.94 A) andthose at 5010–5090 lg Fe g�1 (2.84–2.87 A), indicating adifference in the average structure of the Fe(III)–NOMcomplexes. The shorter Fe–C distances are in agreementwith the five-membered ring structures of Fe(III)–DFO-Band trisoxalatoiron(III) complexes (2.82–2.84 A) whereasthe longer ones are approaching those of six-membered che-late rings for example trismalonatoiron and ferric acetylace-

tonate (2.97–2.98 A; Table 4). Thus, our hypothesis is thatPPHA functional groups forming very stable five-mem-bered chelate complexes become saturated at the high Feconcentration and under these conditions additional weak-er complexes involving six-membered chelate rings areformed. Furthermore, the elongation of Fe–C may also re-flect the involvement of different functional groups.Although we observe a pH and concentration dependentvariation in the Fe–C distance (2.84–2.94 A) it is fairlysmall compared to the variation reported in literature forFe(III)–NOM complexes; 2.76–3.02 A (Rose et al., 1998;Vilge-Ritter et al., 1999; Gustafsson et al., 2007; van Schaiket al., 2008). This variation may reflect differences in thechemical compositions and structures of the NOM studied,but may also be caused by the diversity of fitting ap-proaches applied in the EXAFS analysis. For example,the use of fixed parameters and the inclusion/exclusion ofcertain paths can influence the obtained Fe–C distances.In this respect the complementary information providedby the WT analysis is very valuable.

Although the CNs determined herein are subjected torelatively large errors (20%) they may be used to estimatethe distribution of Fe(III). As the CNs of Fe–C at ca.2.88 A is on average 1.8 in the samples completely domi-nated by Fe(III)–NOM complexes (PPHA1–6; Table 3), itis reasonable to assume that the Fe(III)–NOM complexeshave a second-shell that includes two C atoms in accor-dance with a mononuclear chelate structure. Furthermore,the polymeric Fe(III) (hydr)oxide phase seems to have a lo-cal structure similar to ferrihydrite, goethite or akaganeitethat is a CN of 2 for Fe–Fe at ca. 3.0 A. Based on theseassumptions, the second-shell CNs suggest that at49,200 lg Fe g�1 PPHA the contribution from Fe(III) (hy-dr)oxides increases from 10% at pH 4.2 to 35% at pH 6.9,and this is accompanied with a decrease of mononuclearFe(III)–NOM from 75% to 55%. Although these are veryapproximate values the results suggest that a substantialfraction of Fe(III) is in the form of mononuclear complexeseven at neutral pH and a high Fe concentration. Accord-ingly, the hydrolysis and polymerization of Fe(III) seemsto be strongly suppressed, indicating that the organic com-plexes formed are very stable which is in line with the for-mation of five-membered chelate ring structures. This isalso supported by the fact that without the PPHA presentall samples, even at the lowest pH and Fe concentration,were oversaturated with respect to the formation of the so-lid phase Fe(OH)3(s). An interesting similarity is found be-tween NOM and aqueous silica, a major component ofnatural waters, which also significantly inhibits Fe polymer-ization and solid-phase formation (Pokrovski et al., 2003).

4. CONCLUSIONS

The WT and XANES analyses together with the quanti-tative EXAFS data fitting unanimously corroborate theexistence of two predominant classes of Fe species: mono-nuclear Fe(III)–NOM complexes and polymeric Fe(III)(hydr)oxides. This is in accordance with EXAFS results re-ported by Karlsson et al. (2008) for organic soils (�2000–

Coordination chemistry of Fe(III) in a humic acid studied by XAS 39

20,000 lg Fe g�1; pH 4.6–6.0), showing that Fe(III) oc-curred either as mononuclear Fe–NOM complexes or as amixture between these complexes and Fe (hydr)oxides.They also agree with results reported by van Schaik et al.(2008) where mononuclear Fe(III)-fulvic acid complexeswere detected at pH 4. Our data covering a muchlarger pH (3.0–7.2) and concentration range (5010–49,200 lg Fe g�1) show that the formation of five-mem-bered chelate rings make the Fe(III)-humic acid complexesvery stable, and that these stable complexes strongly sup-press the hydrolysis and polymerization of Fe(III). Thisindicates that in oxic environments with significantamounts of organic matter the fate of Fe will to a large ex-tent be controlled by the properties of the organic Fe(III)complexes. These complexes most certainly have differentreactivities as compared to Fe(III) in (hydr)oxide phasesor other Fe(III)-bearing minerals. Hence, the stableFe(III)–NOM complexes will have important implicationsfor the geochemistry of other elements, such as phosphorus,that are known to be strongly associated with Fe(III).

ACKNOWLEDGMENTS

All EXAFS measurements were carried out at beamline I811,MAX-lab synchrotron radiation source, Lund University, Sweden.Funding for the beamline I811 project was kindly provided by TheSwedish Research Council and The Knut och Alice WallenbergsStiftelse. Dr. Stefan Carlson, Dr. Katarina Noren and the rest ofthe staff at beam line I811 are gratefully acknowledged for theirhelp and advice. We also thank the associate editor, Prof. LianeBenning, and three reviewers for constructive suggestions whichsignificantly improved the manuscript. Funding for this projectwas provided by The Swedish Research Council.

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Associate editor: Liane Benning