Simultaneous incorporation of Mn and Al in the goethite structure

Geochimica et Cosmochimica Acta 71 (2007) 1009–1020 www.elsevier.com/locate/gca

Simultaneous incorporation of Mn and Al in the goethite structure Mariana Alvarez a, Elsa H. Rueda b

a,*

, Elsa E. Sileo

b

a Departamento de Quı´mica, Universidad Nacional del Sur, Av. Alem 1253, B8000CPB, Bahı´a Blanca, Argentina INQUIMAE, Dpto. de Quı´mica Inorga´nica, Analı´tica y Quı´mica Fı´sica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabello´n II, Ciudad Universitaria, C1428EHA, Buenos Aires, Argentina

Received 10 July 2006; accepted in revised form 7 November 2006

Abstract Two series of (Al,Mn)-substituted goethites were synthesized from ferrihydrite made in alkaline media, with different Al/Mn mole ratios ([Al + Mn]/Fe molar ratio up to 0.12). Powder X-ray diffraction and extended X-ray absorption fine structure (EXAFS) techniques were used to assess the structural characteristics of the simultaneous substitution in goethite. XRD patterns revealed that all the obtained solids remain in a goethite-like structure. Rietveld refinement of X-ray diffraction data indicates that the increasing Mn substitution and consequent decrease of Al substitution causes an increase in the unit cell volume. This change is accompanied by the increment of the various Me–Me distances. XANES spectra at the Al and Mn K-edge confirm the octahedral coordination of Al and the trivalent oxidation state of the Mn ion in all the synthesized samples. EXAFS spectra at the Fe K-edge indicate that the local order around the Fe atom remains practically constant upon (Mn,Al) substitution. Measurements in the Mn K-edge show that distances Mn–Me suffer different changes with the increase in Mn substitution: a marked decrease in E and a slight decrease in E0 , while DC remains constant. E and E0 values correspond to the distance between one Mn and one neighboring Me (Fe, Mn, Al) atom, both situated in two polyhedra linked by an edge. These polyhedra belong to the same double row of the goethite structure. DC value corresponds to the distance between one Mn and one Me (Fe, Mn, Al) atom, situated in two octahedral linked by one corner and belonging to two adjacent double chains. All the intermetallic distances are minor than the corresponding singly substituted goethites, this fact is attributed to the structure contraction due to the presence of Al(III) which restrains the axial distortion of Mn. Dissolution–time curves, resulting from exposure to 6 M HCl at 318 K, show that the dissolution rate slows with increasing Al substitution and consequent decrease of Mn substitution, and the shape of the curve becomes increasingly sigmoidal for mixed goethite with large Al content and Al-goethite. Dissolution kinetics of most samples are well described by the Kabai equation. Al dissolves almost congruently with respect to Fe, implying that it is homogeneously distributed in the structure. However, the convex vMn:vFe curve indicates that Mn tends to be concentrated in the outer layers of the goethite particles.  2006 Elsevier Inc. All rights reserved.

1. Introduction Iron oxides and oxi-hydroxides, such as hematite and goethite, are commonly encountered weathering products at the surface of the earth. They have a high potential to absorb or incorporate an important number of cations isovalent or heterovalent to Fe(III) (Schwertmann and Taylor, 1989; Cornell and Schwertmann, 1996). Owing to the

*

Corresponding author. Fax: +291 4595160. E-mail address: [email protected] (E.H. Rueda).

0016-7037/$ - see front matter  2006 Elsevier Inc. All rights reserved. doi:10.1016/j.gca.2006.11.012

habitual presence of goethite in the environment, the geochemical uptake of trace elements in this mineral is important in terms of mass balance, control of the concentration and migration of metals in natural waters, and availability of nutrients and mobility of toxic elements to organisms. The extent of incorporation of metal ions into the structure of iron oxides and oxi-hydroxides is of interest because they alter the nucleation and the crystal grow process, the unit cell dimensions, and dimensions of the crystal habit; the latter of which can affect their reactivity. Changes in transition and heavy metal partitioning during hydrous iron oxide aging could be attributed to the structural

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M. Alvarez et al. 71 (2007) 1009–1020

incorporation of these metals into the goethite or hematite structure (Ford et al., 1997). Naturally occurring goethite is unlikely to exist as aFeOOH; it can be associated with foreign elements, which can be incorporated by isomorphous substitution. Among these elements, Al(III) is the most widely studied and has been shown to replace up to 33 mol% of the Fe in goethite. Its effect on the crystallographic and physico-chemical properties is used as an indicator of pedogenic processes (Norrish and Taylor, 1961; Fitzpatrick and Schwertmann, 1982; Schulze, 1984; Stucki et al., 1988; Schwertmann and Carlson, 1994; Carlson, 1995; Cornell and Schwertmann, 1996). In natural goethites other cations besides aluminum are generally present in trace concentrations (Manceau et al., 2000; Carvalho-e-Silva et al., 2003). The isomorphous substitution of divalent cations Ni, Zn, Cu and Cd; trivalent Cr, V, Mn, Ga, Co and Sc; and tetravalent Pb, Ge and Si also have been reported for natural and synthetic goethites (Stiers and Schwertmann, 1985; Vandenberghe et al., 1986; Cornell and Giovanoli, 1987, 1988; Ebinger and Schulze, 1989; Fazey et al., 1991; Singh and Gilkes, 1992; Vempati et al., 1995; Gasser et al., 1999; Manceau et al., 2000; Scheinost et al., 2001; Sileo et al., 2001, 2003, 2004; Singh et al., 2002; Sudakar et al., 2004; Triverdi et al., 2004; Alvarez et al., 2005). EXAFS spectroscopy has been used by Singh et al. (2002) in order to investigate the local coordination environment of Cr(III), Mn(III) and Ni(II) in the structure of synthetic-substituted goethites. The doping cation content in the samples was up to 8, 15 and 5 mol mol1%, respectively. It was found that the next nearest-neighbor coordination environment changed with composition, and suggested that the doping cation provokes perturbations in the structural lattice that limit the degree of substitution. In a series of synthetic samples aged during 153 days, Sileo et al. (2004) reported a maximum Cr-for-Fe substitution of 12.2–12.4 mol mol1%, and using the EXAFS technique they showed that the Cr electronic nearing-neighbor environment remains unchanged in the whole series. The effect of Cu-substitution in goethite is less known (Cornell and Giovanoli, 1988). Huynh et al. (2002) found that Cu(II) was substituted for iron at a maximum of 3 mol mol1% within goethite. They also observed a decrease in crystallinity and significant changes in the shape of the goethite crystals as a result of copper incorporation. Huynh et al. (2003) studied the structural and physical effects of partially substituting Cd-for-Fe in goethite. The limit of substitution (up to 9.5 mol mol1%) was attributed to both the build up of negative charge, due to the replacement of Fe(III) for Cd(II), and the weakening of covalent bonds since cadmium incorporation led to increasing all the unit cell parameters. Sileo et al. (2003) reported the same trend in the unit cell dimensions of Cd-goethites, and found a maximum of 5.90 mol mol1% of Cd-for-Fe substitution in the goethite structure. Cadmium-substituted hematite nucleation was detected at a preparative ratio of about 7 mol mol1%.

The dissolution behavior of ferrihydrite is also affected by the extent of isomorphous substitution. If the dissolution of Me-ferrihydrite is congruent during its transformation to goethite, the Me is homogeneously up taken by the goethite structure, this occurs with Mn (Giovanoli and Cornell, 1992). Some cations as Co(II), Ni(II) and Zn(II) are released from ferrihydrite more slowly than Fe(III) during dissolution. In this case the obtained goethite presents an enrichment of the non-Fe(III) cations on the surface layers (Cornell and Schwertmann, 1996). Although natural samples of Fe oxides often contain several foreign cations (Ku¨hnel et al., 1978; Fonseca and Martin, 1986; Trolard et al., 1989; Singh and Gilkes, 1992), the simultaneous incorporation of several ions into the synthetic goethite structure has been scarcely studied. Cornell (1991) reported the simultaneous substitution of Mn(II), Ni(II) and Co(II) into the structure of goethite formed from ferrihydrite. The total metal uptake of 12 mol mol1%, and the partial degree of substitution was found to depend on the extent to which the ion coordinates with the functional groups of ferrihydrite. Kaur et al. (2006) also studied di-metals systems in synthetic goethites using the Fe(III) hydrolysis pathway via the formation of ferrihydrite. Knowledge of the mechanisms by which trace metals associate with goethite in multi-element systems is essential to assess their bioavailability and to manipulate the role of goethite in controlling these processes under natural conditions. Although alternative pathways of goethite formation in natural systems are possible, in this work we study the synthesis of goethite formed from ferrihydrite in the presence of two foreign metal cations, Al and Mn. Both cations are associated with iron oxides in soils and sediments and the individual effect of each cation on the kinetics and products of the crystallization of ferrihydrite have already been fully investigated. The object of this study is to establish the maximum uptake of each cation when they are simultaneously incorporated and the consequent changes in the goethite structure. The dissolution behavior of the doubly substituted oxi-hydroxides are also presented and compared with that of singly substituted goethites. 2. Experimental Two series of (Mn,Al)-goethites were synthesized by the following procedures adapted from Bousserrhine et al. (1999). 2.1. Goethite preparation 2.1.1. Series I The total metal content, vMe in the series was kept constant at 12 mol mol1% (vMe = Me · 100/(Me + Fe)). Higher Me concentrations were not used in order to avoid the formation of spinel phases, occurring at vMn > 12, and hematite favored by rich Al-ferrihydrites.

Al and Mn substitution in goethite

Al-goethite was prepared by mixing solution A (Fe(NO3)3, 1 M), 25.0 mL, solution B (Al(NO3)3 0.5 M, 62.5 mL + KOH 5 M, 37.5 mL, ratio [OH]/[Al] = 6), 29.6 mL and KOH 5 M, 45 mL. Mn-goethite was obtained by mixing solution A, 25.0 mL, solution C (Mn(NO3)2, 0.5 M), 6.8 mL, and KOH 5 M, 45.0 mL. Al,Mn-goethites with different ratio Al:Mn (9:3, 6:6 and 3:9 mol mol1%) were obtained by mixing appropriate volumes of solutions B, C, and A, followed by KOH, 5 M. In all cases Teflon bottles were used, and bidistilled water was added to reach a final KOH concentration of 0.3 M. The obtained suspensions were aged for 15 days at 60 C. Bottles were opened daily, recapped, and shaken by hand end-over-end for 5 s. 2.1.2. Series II The Al,Mn-goethites were prepared as described above, but in all the preparations vAl was kept constant at 12; and vMn varied from 3 to 12. The reaction products were washed, dried at 313 K and gently crushed in an agate mortar. In order to remove any non-incorporated metal or poorly crystalline material from the surface, samples were treated with acid oxalate solution at room temperature for 4 h in the dark (Schwertmann, 1964). 2.2. Chemical analysis The level of Mn, Al and Fe contents, were calculated from total dissolution of 70 mg of the oxalate-extracted samples in HCl, 6 M, at 353 K, using a GBC, Model B-932 atomic absorption spectrometer. 2.3. X-Ray diffraction (XRD) X-Ray diffraction patterns were recorded in a Siemens D5000 diffractometer using a CuKa radiation. Generator settings were 40 kV, 35 mA. Divergence, scattered and receiving slits were 1, 1 and 0.2 mm, respectively. A curved graphite monochromator was used. Data were collected in the 2h range: 18.5–130.5, with scanning step of 0.025 and a counting time of 15 s per point. The step width assured a minimum of about 12 intensity points for the narrower peaks in order to obtain good fitting results. The data were analyzed using the GSAS (Larson and Von Dreele, 1994) system. Starting unit-cell parameters and atomic coordinates for goethite were taken from the literature (Szytula et al., 1968). Peak profiles were fitted using the Thompson-Cox-Hastings pseudo-Voigt function (Thompson et al., 1987). 2.4. Soft and hard X-ray spectroscopy data collection and analysis The X-ray absorption measurements were performed at the D04A-SXS (Abbate et al., 1999) and D04B-XAS beam

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lines of the Laborato´rio Nacional de Luz Sı´ncrotron (LNLS, Campinas, Brazil) (Tolentino et al., 2001). The electron storage ring was operated at 1.37 GeV with nominal current of 130 mA. Al-K XANES spectra were collected using two quartz monochromator crystals. The powdered samples were mounted directly on copper slides after dispersion in acetone. The spectra were collected at 108 torr over a photon energy range of 1530–1600 eV. The step size and counting time were 0.2–0.5 eV and 3 s. The X-ray beam of 2.0 · 3.0 mm2 was focused on the sample using a toroidal mirror. Data were collected in the total electron yield mode by recording the drain current while the incoming flux was monitored by an ion chamber. The spectra were calibrated with an Al metallic foil at the inflexion point of the Al Kedge (1559 eV). The data were linearly background fitted in the region before to the pre-edge. The XAS spectra were obtained in transmission mode at the Mn and Fe K-edges. The data were collected at room temperature, and at least three individual spectra were recorded and averaged. Incident and transmitted X-ray fluxes were measured with an air-filled ionization chambers, and the beam was monochromatized with a ‘‘channel cut’’ Si(111) monochromator. XANES spectra at the MnK edge were collected in the range 6480–6750 eV. The energy step and counting time were 0.5–1 eV and 5 s. The energy was calibrated at the inflexion point of edge jump in the XANES spectrum of metallic foil (6540 eV). The spectra were background corrected using a lineal fit in the region before the pre-edge and normalized close to 6540 eV. The EXAFS spectra were recorded from 6430 to 7050 eV for the Mn K-edge and from 7010 to 7920 eV for the Fe K-edge. The scanning steps varied between 1– 2 and 1–3 eV, respectively. Counting time was 5 s. The data were analysed using WinXAS 2.2 code (Ressler, 1997). Spectra were normalized by fitting a second-degree polynomial function to the pre-edge and the XAFS region. E0 was set to the first of two inflexion points in the main edge for Mn K-edge. The XAFS oscillations were separated using six spline sections. The spectra were weighted by k3. Radial structure functions (RSF) were calculated using a Bessel window. The positions of the peaks in ŒF(r)Œ are shifted compared to the true distances. These shifts are related to an additional phase accounting for the influence of the potential of the absorbing atom and the scattering atoms on the electron wave. The contribution of each shell was then back-transformed to the k-space. Theoretical paths were calculated with ATOMS and FEFF 7.02 code (Rehr et al., 1991), using the structures obtained from the open literature for goethite (Szytula et al., 1968) and groutite (Dent-Glasser and Ingram, 1968). The ˚ was used as FEFF input to obfull structure out to 6.0 A tain correct potentials. The short-range structure around Mn and Fe were modeled by multishells fits. The amplitude reduction factors S20 in FEFF calculations were set to 1 for both Me–O and Me–Me shells. Parameters allowed to float in the fits at the Fe- and Mn-edges were interatomic

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distances (RMe-O, RMe-(Fe,Mn)), Debye-Waller factors or disorder parameters (r2MeO , r2MeMe Þ and the difference between the threshold energy (E0) relative to FEFF phase shift function (DE). 2.5. Chemical dissolution Dissolution experiments were performed under stirring, on 70 mg of sample, sealed in a cylindrical Pyrex vessel containing HCL 6 M, 100 mL. The experiments were carried out at 318 K. The kinetic behavior was recorded from the amount of Fe released by taking, at regular intervals, 1mL aliquots of the suspension that were filtered through a 0.22 lm membrane. The amounts of total Fe dissolved were determined by the o-phenantroline method, as Fe2+ (Vogel, 1960). 3. Results and discussion 3.1. Chemical and physical analysis Table 1 shows the chemical analyses for Mn and Al in the two series prepared. The level of metal incorporation was calculated from the chemical analyses of samples oxalate-extracted. In Series I the degree of incorporation of each ion is closely related with their initial concentration in solution. Samples of Series II were synthesized to determine the maximum level of Mn incorporation in the presence of a fixed and high amount of Al. Here, the amount of Al incorporated to the solid phase decreases with the increment of Mn(II) in the initial solution. The results of the chemical analysis of series II stress the dominant effect of Mn over Al in the simultaneous incorporation of both ions. Mn appears to inhibit the substitution of Fe by Al. For example, starting from a nominal content of 12 mol% of both ions (sample SII-Mn12Al12), the chemical analysis indicated 15.5 and 4.9 mol% of Mn and Al substitution, respectively. The higher solubility of Mn-ferrihydrite relative to Al-ferrihydrite could be partially responsible for this behav-

ior. Another factor that limits the Al incorporation could be the significant structural strain caused by the Mn(III) substitution, a d4 ion that presents Jahn-Teller distortion. The color on the dry oxides was determined using the Munsell Soil Color Charts. As Mn content increases, goethites become darker than the unsubstituted or the Al-substituted goethites, the colors ranging from yellow to very dark grayish brown (Table 1). XRD patterns were carried out for both series of samples, but only in the series I structural studies concerning the short and long-range were performed. Samples of series II were synthesized with the only aim to determine the upper limit on the extent of Mn substitution in the presence of a high and fixed initial vAl, and the crystalline structures present in each sample. According to XRD, samples of series II consisted of a majority phase goethite and additional phases such as hematite and jacobsite in SII-Mn3Al12 and SII-Mn12Al12 samples, respectively. From now on, studies about morphology, short- and long-range behavior and reactivity will only be discussed for samples in series I. XRD patterns are presented in Fig. 1 3.2. Morphology Fig. 2 shows the SEM images of samples §I-Mn0Al0, SIMn0Al12, SI-Mn6Al6 and SI-Mn12Al0. Particles of Al-goethite (SI-Mn0Al12) are markedly shorter than those of pure goethite (Fig. 2b). Mn-containing particles enlarge with the increase in vMn (Fig. 2d). The polydispersion observed in sample with simultaneous Aland Mn-substitution (Fig. 2c) could be indicative of irregular distribution of both ions in the goethite structure. 3.3. Rietveld refinement of XRD data Table 2 collects the parameters that describe the goodness of the fitting for series I. Reliability factors are in the range Rwp = 8.58–10.46; RB = 3.15–4.46, G of F values between 1.11 and 1.27 are reasonably adequate. Lattice parameters and phases composition, as obtained in the Rietveld refinement, are shown in Table 3.

Table 1 Chemical composition for the samples Sample

vMn (Initial)

vAl (Inicial)

Series I SI-Mn0Al0 SI-Mn0Al12 SI-Mn3Al9 SI-Mn6Al6 SI-Mn9Al3 SI-Mn12Al0

0.0 0.0 3.0 6.0 9.0 12.0

0.0 12.0 9.0 6.0 3.0 0.0

Series II SII-Mn3Al12 SII-Mn6Al12 SII-Mn9Al12 SII-Mn12Al12

3.0 6.0 9.0 12.0

12.0 12.0 12.0 12.0

vMn (Incorporated)

vAl (Incorporated)

Munsell color designation

0.0 0.0 2.4 4.7 7.2 8.6

0.0 11.6 10.0 5.9 4.0 0.0

2.5 Y 7/8 10 YR 5/8 2.5 Y 5/6 2.5 Y 5/2 2.5 Y 4/2 5 Y 3/2

4.0 7.8 11.9 15.5

8.6 7.4 6.7 4.9

2.5 Y 4/4 2.5 Y 4.4 2.5 Y 3/2 5 Y 2.5/1

vMe expressed as Me · 100/(Me + Fe) (mol mol1). Subscripts indicate the nominal content of Mn and Al in each sample.

Al and Mn substitution in goethite

Fig. 1. XRD patterns of all samples of the Series I.

Fig. 3 shows the changes in unit cell dimensions in the series. All cell-parameters increase with the increase of vMn and the decrease of vAl. The trend is different to that found by Sileo et al. (2001) that reported that the increase of Mn-for-Fe substitution causes the increase of the b-parameter and the decrease in the a- and c-parameters. The observed trend agrees with the increase of Al-for-Fe substitution that provokes a decrease in all unit cell dimensions (Fey and Dixon, 1981; Schulze, 1984). Clearly the inclusion of Al determines the cell-distortion, and only in samples with low Al-content the observed parameters are closer to that reported for Mn-goethite. The data indicate that the structural framework is more sensible to the presence of Al, even at low levels of aluminum substitution.

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The structure of goethite may be described in terms of Fe octahedra (FeO3(OH)3) linked in pairs by an edge, through two hydroxide groups. These pairs form double chains that run parallel to [001] (space group Pbnm). Along the chain, the octahedra are joined by edges through one oxide and one hydroxide ligands. Oxide groups connect the double rows. This arrangement leads to double chains separated by vacant double rows. The arrangement determines the presence of three different Me–Me distances. In each double row the polyhedra joined by the edges determine the Me– Me distances E and E0 . The connection between double rows determines a different Me–Me distance, named a DC distance. These features are shown in Fig. 4. Table 2 also shows the refined (x, y) atomic parameters for Me (Fe, Mn, Al), O1 (oxide oxygen) and O2 (hydroxide oxygen) and the calculated distances E, E0 and DC. All Me–Me distances decrease from the pure goethite (SI-Mn0Al0) to the Al-substituted goethite (SI-Mn0Al12), and increase with vMn, with the consequent decrease of the Al content. 3.4. XAS analysis 3.4.1. Local order around Al(III) In order to determine the local Al-environment in the synthesized goethites, the XANES spectra of samples SI-Mn0Al12 to SI-M9Al3 were measured and compared with

Fig. 2. SEM images of samples (a) SI-Mn0Al0, (b) SI-Mn0Al12, (c) SI-Mn6Al6 and (d) SI-Mn12Al0 (20,000·).

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M. Alvarez et al. 71 (2007) 1009–1020

Table 2 Atomic parameters, calculated Me–Me distances and agreement factors of the refinements for (Al,Mn)-inserted goethites SI-Mn0Al0

SI-Mn0Al12

SI-Mn3Al9

SI-Mn6Al6

SI-Mn9Al3

SI-Mn12Al0

Atom parameters x(Me) y(Me) x(O1) y(O1) x(O2) y(O2)

0.0471(2) 0.1460(1) 0.2934(7) 0.1989(3) 0.2010(7) 0.0541(3)

0.0487(1) 0.1460(1) 0.2925(1) 0.1996(1) 0.2057(1) 0.0533(1)

0.0481(1) 0.1459(1) 0.2919(1) 0.1989(1) 0.2059(1) 0.0544(1)

0.0477(2) 0.1458(1) 0.2922(6) 0.1991(3) 0.2024(5) 0.0547(3)

0.0478(2) 0.1457(1) 0.2926(1) 0.1999(1) 0.2030(1) 0.0539(1)

0.0476(1) 0.1456(1) 0.2901(1) 0.1984(1) 0.1987(1) 0.0544(1)

Me–Me distances E0 E DC

3.024(1) 3.306(2) 3.449(1)

3.001(1) 3.287(1) 3.431(1)

3.009(1) 3.290(1) 3.438(1)

3.012(1) 3.296(1) 3.442(1)

3.014(1) 3.299(2) 3.447(1)

3.018(1) 3.304(1) 3.454(1)

8.58 6.43 3.73 1.16

9.01 6.81 4.46 1.11

9.37 7.02 3.15 1.23

Crystallographic factors Rwp 10.05 Rp 7.29 RB 4.14 G of F 1.15

10.42 7.82 4.22 1.23

10.46 7.96 4.06 1.27

Atom parameter z = 0.250 in all cases. Rp: 100 RŒIo  IcŒ/RIo Rwp: 100 [Rwi (Io  Ic)2/Rwi I 2o 0:5 . RB: 100 RŒIko  Ikc Œ/RIko G of F = Rwi (Io  Ic)2/(N  P). Io and Ic: observed and calculated intensities. wi: weight assigned to each step intensity. Iko and Ikc: observed and calculated intensities for Bragg k-reflection. N and P: number of data points in the pattern and number of parameters refined.

Table 3 Unit cell parameters and phases composition obtained from the Rietveld refinement Sample

SI-Mn0Al0 SI-Mn0Al12 SI-Mn3Al9 SI-Mn6Al6 SI-Mn9Al3 SI-Mn12Al0

Wt% Gt

Ht

Js

Goethite ˚] A [A

100.00 97.82(2) 100.00 100.00 100.00 99.68(4)

0.00 2.18(2) 0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00 0.00 0.32(4)

4.6116(2) 4.5966(2) 4.5999(2) 4.6002(2) 4.6029(3) 4.6075(3)

Hematite

Jacobsite

˚] b [A

˚] C [A

˚ 3] Volume [A

a=b

c

a=b=c

9.9581(3) 9.8936(3) 9.9150(3) 9.9391(3) 9.9558(3) 9.9794(4)

3.0238(1) 3.0051(1) 3.0088(1) 3.0118(1) 3.0135(1) 3.0182(1)

138.860(9) 136.663(9) 137.223(9) 137.707(9) 138.095(9) 138.784(15)

— 5.0329(4) — — — —

— 13.7548(18) — — — —

— — — — — 8.4867(11)

Gt: goethite; Ht: hematite; Js: jacobsite.

reported data (Ildefonse et al., 1998). The spectra are displayed in Fig. 5. According to Ildefonse et al. (1998) the XANES spectra of diaspore (a-AlOOH), the Al isomorphous compound to goethite, presents two main peaks (A and B) at 1567.4 and 1571.2 eV, separated by a small plateau. In the case of Al-goethites, the positions and the relative amplitude of these two peaks change significantly with the Al content. At low Al concentrations the low energy peak is shifted to larger values, and the amplitude is less intense, meanwhile the B peak is displaced to smaller energies. When the Al content increases, this separation between the peaks increases, together with the relative amplitudes. As can be seen in Fig. 5, the positions of peaks A and B change in the series, peak A displaces from 1567.8 to 1567.4 eV and peak B is shifted from 1570.4 to 1571.2 eV. The relative amplitude is also changed. These data confirms the isostructural substitution of Al-for-Fe.

3.4.2. Local order around Fe(III) The EXAFS spectra of SI-Mn0Al12, SI-Mn3Al9, and SIMn12Al0 samples at the Fe K-edge are similar, indicating small changes in the coordination shells of the absorbing atom. The Fourier Transformations of the k3-weighted v(k) for these samples are presented in Fig. 6. The first peak corresponds to the oxygen shell surrounding the absorber (Fe–O); the second and third peaks correspond to the sum of the three different Fe–Me contributions: two Fe– MeI, two Fe–MeII and four Fe–MeIII interactions at distances E0 , E and DC, respectively (Charlet and Manceau, 1991; Manceau and Drits, 1993). The second peak in the spectra is mainly attributable to the E0 interaction, whereas the location of the maximum of the third peak suggests that it results from the average of the unresolved E and DC contributions. To analyze the variation of the local environment around Fe shown in Fig. 6, the three peaks in the ranges ˚ and 2.2–4.0 A ˚ were isolated, backtransformed 0.7–2.0 A

Al and Mn substitution in goethite

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Fig. 5. XANES spectra of samples SI-Mn0Al12 to SI-Mn9Al3 at the Al K-edge.

Fig. 6. Fourier transform at the Fe K-edge for samples SI-Mn0Al12, SI-Mn3Al9 and SI-Mn12Al0.

Fig. 3. Variation of (a) a-, (b) b-, and (c) c- unit cell parameters with simultaneous substitution of Mn and Al in the goethite structure, and comparison with the parameters of Mn-goethite.

and analyzed in k3-space in the range 3.5 < k < 12. The fits indicate that the Fe–O and the Fe–Me distances remain practically constant indicating no significant variations of the local structure of Fe upon substitution. Because of the agreement in the values of all samples only the fitting for sample SI-Mn3Al9, is presented in Table 4.

The average distances to O2 and OH ions remain con˚ ) and the values agree within ±0.02 A ˚ stant (1.94 and 2.09 A with the average distances Fe–O and Fe–OH obtained by the Rietveld simulation procedure (calculated average ˚ and Fe–OH = 2.111 A ˚ ). distances Fe–O = 1.945 A 3.4.3. Local order around Mn(III) The XANES spectra of the samples were measured in all (Mn,Al)-goethites. The technique examines the variation of the absorption of X-rays with wavelength in the vicinity of an absorption edge, and the energy position of the edge is correlated with the valence state of the atom in the sample (Wong et al., 1984). XANES spectra of samples SI-Mn3Al9

Fig. 4. Goethite structure showing the polyhedra linkage: (a) (100) and (b) (001) directions.

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Table 4 Summary of EXAFS data fitting Sample

Oequat.

˚ 2) r2 (A

Oaxial

˚ 2) r2 (A

DE0

Residual

Me–O distances Fe K-edge SI-Mn3Al9

3 · 1.94(2)

0.002

3 · 2.09(2)

0.003

1.0

13.1

Mn K-edge SI-Mn3Al9 SI-Mn6Al6 SI-Mn9Al3 SI-Mn12Al0

4 · 1.92(2) 4 · 1.93(2) 4 · 1.92(2) 4 · 1.93(2)

0.003 0.008 0.005 0.009

2 · 2.47(2) 2 · 2.48(2) 2 · 2.48(2) 2 · 2.52(2)

0.004 0.016 0.004 0.012

0.5 0.9 0.9 0.9

18.4 19.5 19.9 18.6

Sample

˚) 1st shell (A

˚ 2) r2 (A

˚) 2nd shell (A

˚ 2) r2 (A

˚) 3rd shell (A

˚ 2) r2 (A

DE0

Residual

Me–Me distances Fe K-edge SI-Mn3Al9

2 · 3.01(2)

0.002

2 · 3.13(2)

0.014

4 · 3.40(3)

0.009

0.4

15.1

Mn K-edge SI-Mn3Al9 SI-Mn6Al6 SI-Mn9Al3 SI-Mn12Al0

2 · 2.98(2) 2 · 2.97(2) 2 · 2.96(2) 2 · 2.96(2)

0.003 0.002 0.003 0.003

2 · 3.16(2) 2 · 3.16(2) 2 · 3.13(2) 2 · 3.11(2)

0.005 0.003 0.008 0.009

4 · 3.37(3) 4 · 3.37(3) 4 · 3.36(3) 4 · 3.37(3)

0.011 0.013 0.019 0.025

1.2 1.0 0.9 1.0

21.3 23.7 23.4 23.0

CN, coordination number; R, interatomic distances; r2, Debye-Waller factor; DE, difference between the threshold energy (E0) relative to FEFF phase ˚ for shells one to three, uncertainty for shell four is estimated to be ±0.03 A ˚ . Fits were performed shift function. Uncertainties are estimated to be ±0.02 A by fixing the CN to their crystallographic values.

to SI-Mn12Al0, and MnSO4, LaMnO3 and CaMnO3 used as reference compounds are presented in Fig. 7. The first prominent signal of the prepared goethites appears at 6557.6 eV, and the energy position coincides with that of the Mn3+ reference (LaMnO3), confirming that Mn is incorporated in its trivalent state. An overlay of the EXAFS spectra is displayed in Fig. 8; k3-weighted signals are shown to enhance possible differences. The signals are slightly different. To analyze the variation of the local environment ˚ and around Mn, the three peaks in the ranges 0.7–2.1 A ˚ 2.2–3.9 A were isolated, backtransformed and analyzed in k3-space in the range 3.5 < k < 11. The FT in the Mn K-edge shows changes in the Mn–O and Mn–Me shells (Fig. 9). The fitting indicates an enlargement of the axial Mn–O distances with the increment in vMn. The data show that

the presence of Al inhibits the Jahn-Teller distortion in the Mn–O coordination sphere. Distances Mn-(Fe, Al, Mn) also change. In going from SI-Mn3Al9 to SI-Mn12Al0, a marked decrease is found in E. E0 slightly decreases, and DC remains unchanged (see Table 4). These results are in disagreement with those observed by Scheinost et al. (2001) in Mngoethites, they observed an increment in E distance, a slight decrease in the DC distance and no changes in E0 distance. In spite of that, in our Mn,Al-substituted samples the average distances Mn–O increase with the Mn content (which indicates an axial distortion of the polyhedra), all the calculated Me–Me distances are minor than the corresponding Mn-goethites. This fact could be attributed to the structure contraction due to the presence of Al(III), which restrains the axial distortion of Mn.

Fig. 7. XANES at the Mn K-edge for samples SI-Mn3Al9 to SI-Mn12Al0, and reference compounds.

Fig. 8. k3-weighted EXAFS spectra for samples SI-Mn3Al9 to SI-Mn12Al0 at the Mn K-edge.

Al and Mn substitution in goethite

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3.5. Acid dissolution

Fig. 9. Fourier transforms at the Mn K-edge for samples SI-Mn3Al9 to SIMn12Al0.

Fig. 10. Comparison of FT[k2v(k)] at the Fe K-edge and at the Mn Kedge for sample SI-Mn3Al9.

3.4.4. RDF Comparison of EXAFS spectra at the Fe K- and Mn K-edges Fig. 10 shows the k2 weighted FT [k2.v(k)] for SIMn3Al9 at the Fe K- and Mn K-edges. The traces confirm the difference between both coordination environments. The Mn–O shell is shifted towards lower values than the corresponding Fe–O shell. The refinement presented in Table 2 can explain the differences. In the goethite structure, ˚ and three O Fe(III) presents three O atoms at 1.94 A ˚ , while Mn presents four equatorial O atoms at 2.09 A ˚ and two axial O at 2.47 A ˚ . Although the averat 1.93 A age distances around Mn are larger than around Fe, the coherency of the first shell around Mn dominates the spectra and the signal appears closer to the edge, as shown in Fig. 10. The second peak corresponding to the 1st Me–Me shell (E0 distance) is slightly shifted towards the Mn atom, this is ˚ for Fe and coincident with the refinement data (3.01(2) A ˚ 2.98(2) A for Mn). The corresponding signal of the E and DC distances appear overlapped although the greater E ˚ ) shifts the value in the Mn K-edge (3.16(2) vs. 3.13(2) A whole signal to greater distances.

3.5.1. Dissolution curves The representative plots of % Fe dissolved at 318 K in HCl, 6 M vs. time for samples in series I are shown in Fig. 11. Dissolution curves of samples with major Al content (SI-Mn0Al12 and SI-Mn3Al9) show sigmoidal shapes. In the other samples, dissolution followed a decelerating trend to completion of the reaction. The time period required to reach complete dissolution was about 85 min for SI-Mn12Al0 and 1300 min for SI-Mn0Al12. This fact is a clear evidence of differences in the dissolution behavior of Mn and Al-goethites. The initial dissolution rates for the samples, calculated at t = 0 from the first derivative of the dissolution curves, are presented in Table 5. The data indicates that the initial dissolution rate diminishes as the Al content increases, being the more reactive sample the Mn-goethite. This is in line with the behavior observed in single Al- and Mnsubstituted goethites (Lim-Nun˜ez and Gilkes, 1987). 3.5.2. Dissolution rate constant (k) – coefficients obtained from data fitted to the Kabai equation In order to fit the kinetic profiles % Fe vs. t several kinetic adjustments were carried out, such as the three- and twodimensional contracting geometry kinetic laws (Brown, 1980), the Avrami-Erofe’ev equation, based on the assumption that dissolution is surface controlled with random initiation of dissolution sites (Cornell and Giovanoli, 1993), and the Kabai equation (Kabai, 1973). The dissolution data for most samples were better h fitted i by the Kabai equation in its lineal form, ln ln 1v1 Fe ¼ ln K þ a ln t. Thus, only Kabai fitting results are shown and the obtained coefficients are listed in Table 5. The dissolution data for SI-Mn0Al12 sample were not well described by the Kabai equation and the plot approximately conforms to two straight-line components. Schwertmann and Latham (1986) indicated that dissolution curves for natural samples containing only goethite give one straight line whereas those containing a mixture of goe-

Fig. 11. Representative dissolution curves expressed as % Fe dissolved in HCl, 6 M at 318 K vs. time, as affected by Me (Mn,Al) substitution.

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M. Alvarez et al. 71 (2007) 1009–1020

Table 5 Initial dissolution rate and coefficients for the Kabai equation for samples dissolved at 318 K Sample

Initial rate (min1)

Linear part

N

a

k (min1)

R2

SI-Mn0A10 SI-Mn0Al12

1.838 0.083

SI-Mn3Al9 SI-Mn6Al6 SI-Mn9Al3 SI-Mn12Al0

0.187 0.684 1.367 3.740

— I II — — — —

6 3 6 11 9 8 4

1.511 0.541 1.772 1.319 1.059 1.070 0.846

0.0262 2.28 · 104 0.0015 0.0033 0.0080 0.0171 0.0423

0.994 0.992 0.993 0.987 0.996 0.992 0.999

thite and hematite show two straight lines (part I, II) when fitted to the Kabai relationship. The two straight-line fitting in SI-Mn0Al12 is in line with the coexistence of a small percentage of hematite present in the sample, together with the majority phase goethite, as it was showed in the Rietveld refinement (Table 3). The break between the two lines occurs at 20% dissolution and may represent the point at which most of the hematite has dissolved together with the smaller goethite particles, so that continuing slower dissolution of the remaining goethite determines the slope of the second line.

tion of synthetic goethites and their dissolution behavior does not allow us the comparison of our results. Other measurements such as high resolution TEM could clarify the distribution of both Al and Mn ions into the goethite structure. In spite of Mn prevails over Al in the simultaneous isomorphous substitution, the dissolution behavior and the variations in the unit cell parameters indicate that Al, even in little amounts impart the proper qualities of the single Al-substituted goethites to these series of samples. 4. Conclusions

3.5.3. Congruency of dissolution of Fe, Mn and Al Analysis of sequential solution extracts during the dissolution of a (Al,Mn)-substituted goethite can provide information regarding the homogeneity of Al(III) and Mn(III) incorporated within crystals of goethite. Dissolution data for Fe, Mn and Al for SI-Mn6Al6 sample are shown in Fig. 12 as a plot of vMn and vAl dissolved vs. vFe dissolved. As can be seen, in the earlier stage of the dissolution, a slight preferential leaching of Al is observed, suggesting the fast dissolution of smaller goethite grains with higher Al content. After this stage Al dissolves almost congruently with Fe and accelerates the release of Mn. The convex up vMn:vFe curve indicates that Mn tends to be concentrated in the outer layers of the goethite particles. This behavior of Mn in the presence of Al is different from the congruence observed in the acid dissolution of single Mn-substituted goethites (Lim-Nun˜ez and Gilkes, 1987; Alvarez et al., 2005, 2006). The lack of similar reports concerning to the simultaneous substitu-

Fig. 12. Plot of vMnand vAldissolved vs. vFe dissolved in HCl, 6 M for SI-Mn6Al6.

In the simultaneous incorporation of Mn and Al in the goethite structure, Mn is the prevailing substituting ion. Unit cell parameters of (Al,Mn)-substituted goethites increase with the increase of vMn and the decrease of vAl. No other phases, besides goethite, are detected. XANES studies confirm the incorporation of Al in octahedral coordination, and Mn incorporation as Mn(III). Average Fe–O and Fe–Me distances do not change significantly upon substitution, and although average Mn–O distances increase, no significant changes in Mn–Me distances are observed with the increase of Mn incorporation, because of the Al presence. Only the shortening of the E0 distance is observed. Even though long-range changes observed in the substituted samples must be traced to local changes in the coordination polyhedron of the individual metal atoms, complex changes in the various interatomic distances may result in deceivingly simple long-range trends. Acid dissolution behavior of mixed samples follows the same trend of singly Al- and Mn-substituted goethites, that is, Mn-goethite is the most reactive sample, while increasing Al substitution diminishes the initial rate values. The kinetic profiles are better described by the Kabai equation. While Al and Fe dissolve almost congruently, Mn suffers a diversion of this tendency. This indicates that Mn is concentrated nearer the surface of the goethite crystals, when both Al and Mn ions are simultaneously incorporated. Although Mn inhibits Al incorporation, the mixed (Al,Mn)-goethites present a general behavior closer to the single-substituted Al-goethites. This behavior brings out the importance of the simultaneous substitution of Mn and Al in natural iron oxides and their consequent effect in the environment processes.

Al and Mn substitution in goethite

Acknowledgments This work was partially supported by LNLS (proposals XAS 2804/04 and SXS 2351/04), and by Grants UBACYT X800 from Universidad de Buenos Aires and SGCyT 24/ Q005 from Universidad Nacional del Sur. The authors thank the reviewers for their insightful comments. Associate editor: Laura J. Crossey

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