In situ Measurements of X-ray Absorption Spectra during Transformation of Green Rust to Ferric Oxyhydroxide Via Aqueous Solution

Abstract

In situ measurements of X-ray absorption spectra at an Fe K absorption edge were carried out during the transformation of chloride-containing green rust (GR(Cl^–)) to oxyhydroxide (that is, lepidocrocite (γ-FeOOH)), by oxidation via aqueous solution. Results showed that the Fe K absorption edge in X-ray absorption near-edge structure (XANES) spectrum shifted toward the higher energy side with increasing oxidation time. The factor analysis of the XANES spectra, using reference data, implied that dissolution of GR(Cl^–) and precipitation of γ-FeOOH occur simultaneously. The fraction of γ-FeOOH in the suspension increased continuously with increasing oxidation time, in response to the continuous decrease of the fraction of GR(Cl^–). The results lead us to conclude that GR(Cl^–) transforms to γ-FeOOH during oxidation under the present experimental conditions.

1. Introduction

Green rusts (GRs) are classified as iron hydroxides containing ferric (Fe(III)) and ferrous (Fe(II)) ions. In the crystal structure of GRs, Fe(II) and Fe(III) ions are bonded octahedrally to six hydroxide ions.¹⁾ The edge-shared Fe(OH)₆ octahedral units form the brucite-like iron hydroxide layers, which are alternating with interlayers containing anions and water molecules. Several species of anions, such as Cl^–, SO₄^2–, and CO₃^2–, are present in the interlayers.^1,2,3,4,5) It has been reported that GRs are formed as intermediate products when corrosion products are precipitated on metallic Fe in aqueous solution.^6,7) Hence, the formation of GRs is related to corrosion processes of metallic Fe in aqueous solution, although the corrosion rate of iron is strongly influenced by a number of variables, such as humidity and temperature cycles, and covering layer of corrosion products. Several species of GR suspensions have been synthesized by a chemical route, and oxidation experiments have been performed on these suspensions.^{8,9,10,11,12)} It has been demonstrated that GRs are transformed to oxidation products such as goethite (α-FeOOH), akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH), and magnetite (Fe₃O₄) through the injection of oxygen gas into their suspensions. The species of oxidation products and their morphology have been found to be sensitive to various oxidation conditions, such as temperature, oxygen partial pressure, and other coexisting ion species. To elucidate the mechanism of transformation of GRs to oxidation products, many efforts have been made to characterize GRs during oxidation.^13,14,15)

In this study, we focused on chloride-containing green rust (GR(Cl^–)) with the chemical formula Fe(II)₃Fe(III)(OH)₈Cl· 2H₂O,¹⁶⁾ because the corrosion of metallic Fe and steel due to chloride ions is common in various environments. In previous laboratory experiments on the oxidation of GR(Cl^–) suspension,^{12,17,18,19,20,21,22)} the in situ measurements of electrochemical properties such as pH, dissolved oxygen (DO), and oxidation-reduction potential (ORP) have been performed for the suspension during oxidation.¹⁷⁾ The correlation between the species of the oxidation products and the solution conditions in the suspension during oxidation has been compared with the electrochemical potential (E)–pH equilibrium diagram, including GR(Cl^–).^19,21) For representative example, the ORP-pH curve of the GR(Cl^–) suspension during the controlled oxidation using 21% oxygen gas at 298 K (Ref. 17) is overwrote in an experimentally obtained E–pH equilibrium diagram for iron in chloride-containing aqueous solution,²⁾ as shown in Fig. 1. In the figure, the values of ORP and pH for the suspension before oxidation are indicated by the closed circle and those for the suspension after complete oxidation are marked by the open square. Though the values of ORP and pH in the GR(Cl^–) suspension before oxidation are very close to the conditions, in which GR(Cl^–) is stable, these values are changed to the conditions, in which γ-FeOOH is stable, by the oxidation. As a result, γ-FeOOH is preferentially formed in the suspension as a oxidation product.¹⁷⁾ Thus, the species of the oxidation products are strongly affected by the solution conditions in the suspension during oxidation.^{17,18,19,20,21,22)} To investigate the transformation of GR(Cl^–) during oxidation, X-ray diffraction pattern of solid particles extracted from the suspension at different oxidation times was also measured.^{17,18,19,20,21,22)} However, the oxidation process of GR(Cl^–) suspension has not been fully understood. For example, the relative amounts of GR(Cl^–) and oxidation products in the suspension during oxidation have not been determined, because it is difficult to estimate the fractions of reaction products from X-ray diffraction data.

Fig. 1.

The oxidation-reduction potential (ORP)-pH curve of the GR(Cl^–) suspension during the controlled oxidation using 21% oxygen gas at 298 K,¹⁷⁾ together with an experimentally obtained electrochemical potential (E)–pH equilibrium diagram for iron in chloride-containing aqueous solution.²⁾

It is known that in situ measurements of X-ray absorption spectroscopy (XAS) are effective in non-destructively characterizing both the chemical state of Fe and the local structure around Fe in iron-containing specimens. Spectra for X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) provide the chemical state of Fe and the local structure around Fe in α-FeOOH, β-FeOOH, γ-FeOOH, Fe₃O₄, and GR(Cl^–).²³⁾ Therefore, the relative amounts of GR(Cl^–) and the oxidation products in the suspension during oxidation can be analyzed by in situ XAS measurements. To obtain a detailed understanding of the transformation process of GR(Cl^–) during oxidation, we applied the quick X-ray absorption fine structure (QXAFS) technique in a synchrotron radiation facility.²⁴⁾ A better transformation pathway of GR(Cl^–) to oxidation products during oxidation was discussed on the basis of analytical results.

2. Experimental

The GR(Cl^–) suspension was synthesized from ferric chloride (Fe(III)Cl₃) and ferrous chloride (Fe(II)Cl₂) hydrates and aqueous sodium hydroxide (NaOH). First, deaerated water, which was prepared by bubbling of Ar gas, was used to prepare an iron chloride solution with an [Fe(II)]/[Fe(III)] ratio of 4.5, in which the total iron concentration was 0.2 mol/L. Subsequently, an aqueous NaOH solution was added to the iron chloride solution in a reaction vessel with continuous bubbling of Ar gas at 278 K. The addition of aqueous NaOH solution was continued until the [OH^–]/{[Fe(II)]+[Fe(III)]} ratio equaled 1.5.

For in situ XAS measurements, we prepared a specimen holder consisting of cellulose fabric and polyethylene films. In order to maintain a homogeneous dispersion of GR(Cl^–) during the in situ measurements, a cellulose fabric was used. Such fabric was allowed to absorb 0.4 ml of the as-synthesized GR(Cl^–) suspension. The fabric with suspension was packed by 0.02-mm-thick polyethylene films. Atmospheric oxygen can permeate through the polyethylene films. Thus, in order to avoid oxidation of GR(Cl^–) suspension prior to the in situ XAS measurements, procedures mentioned above were performed in a glove box, in which the oxygen content was controlled below 0.1%. Then, the specimen holders containing the GR(Cl^–) suspensions were kept in a desiccant box immediately prior to the in situ XAS measurements. After the specimen holders were exposed to air, atmospheric oxygen permeated through the polyethylene films. Consequently, the GR(Cl^–) suspension in the specimen holder was oxidized and transformed to ferric oxyhydroxides.

In situ XAS measurements were performed at an Fe K absorption edge (7112 eV) in the transmission mode using the synchrotron radiation facility at the beam line station BL14B2 at SPring-8, Japan Synchrotron Radiation Research Institute, Japan. The specimen was irradiated with a beam of size 1 mm × 5 mm, and the intensities of transmitted X-rays were measured using an ion chamber. X-ray absorption spectra at the Fe K absorption edge were collected for approximately 4 min. The measurements were periodically repeated during the oxidation, up to a duration of about 94 min. The measured data were analyzed using the software REX2000 (Rigaku Corporation).

After in situ XAS measurements, fully oxidized suspension was dried, and hence the cellulose fabric containing the final oxidation products was obtained. XRD measurement of such dried specimen was performed using a Rigaku RINT-2200 diffractometer with Mo Kα radiation (0.071073 nm). The morphologies of the final oxidation products were observed using a transmission electron microscope (TEM, HITACHI H-7650).

3. Result and Discussion

Figure 2 shows the XANES spectra at the Fe K absorption edge for the GR(Cl^–) suspension during oxidation, in which the normalized absorbance is plotted against the energy of the incident X-ray. The position of the Fe K absorption edge in XANES spectra shifts toward the higher energy side with increasing oxidation time t_ox, implying that GR(Cl^–) is oxidized and transformed to other iron oxides. It is known that XANES spectra at the Fe K absorption edge are sensitive to the chemical state of compounds containing iron.^25,26,27) For instance, the position of the Fe K absorption edge of Fe₃O₄, composed of both Fe(II) and Fe(III) ions, has been reported to be located at a lower energy in comparison with that of maghemite (γ-Fe₂O₃), composed of Fe(III) ion.⁹⁾ Therefore, it is possible to estimate the fraction of the oxidation products, composed mainly of Fe(III) ion, in the suspension as a function of t_ox, as discussed later.

Fig. 2.

X-ray absorption near-edge structure (XANES) spectra at the Fe K absorption edge for the GR(Cl^–) suspension during oxidation. The abscissa and ordinate are the energy of the incident X-ray and the normalized absorbance, respectively.

The EXAFS spectra at the Fe K absorption edge for the GR(Cl^–) suspension during oxidation are given in Fig. 3, in which the k³-weighted value of χ is plotted as a function of the wave number k of the incident X-ray. The results show that the amplitude and periodicity in EXAFS spectra gradually change with increasing t_ox. As the amplitude and periodicity in EXAFS spectra are closely related to the atomic distance and the coordination number, that is, the local structure,²⁸⁾ the oxidation of GR(Cl^–) is accompanied not only by a change in the chemical state of Fe but also by a change in its local structure. It has been reported that oxidation products such as α-FeOOH, β-FeOOH, γ-FeOOH, and Fe₃O₄ are formed as final oxidation products of GR(Cl^–), depending on the oxidation conditions.^{12,17,18,19,20,21,22)} α-FeOOH, β-FeOOH, and γ-FeOOH consist of Fe(III) ion, and their crystal structures are composed of FeO₆ octahedral units. The linkages of these units are edge shared, corner shared, or combinations of the two.²³⁾ Although Fe₃O₄ consists of both Fe(II) and Fe(III) ions, the number of Fe(II) ion in Fe₃O₄ is smaller than that of Fe(III) ion. In addition, Fe₃O₄ is composed of both FeO₆ octahedral units and FeO₄ tetrahedral units. Hence, the chemical state of Fe and the local structure around Fe of these oxidation products are different from those of GR(Cl^–). Therefore, the results presented in Figs. 2 and 3 suggest that GR(Cl^–) was transformed to final oxidation products via aqueous solution.

Fig. 3.

Extended X-ray Absorption Fine Structure (EXAFS) spectra at the Fe K absorption edge for the GR(Cl^–) suspension during oxidation. The abscissa and ordinate are the wave number k of the incident X-ray and the k³-weighted value of χ, respectively.

Figure 4 shows the radial structure function (RSF) of the suspension oxidized for t_ox = 90 min, which was obtained by Fourier transform of the EXAFS spectrum shown in Fig. 3. The RSFs of α-FeOOH, β-FeOOH, γ-FeOOH and Fe₃O₄ are also presented for comparison purposes. The imaginary parts of the Fourier transform are represented by broken lines. Correlation peaks are observed at about R = 0.16 nm and R = 0.27 nm in the specimen oxidized for t_ox = 90 min. These peaks correspond to the Fe–O correlation and the Fe–Fe correlation, respectively. Though such RSF is different from those of α-FeOOH, β-FeOOH and Fe₃O₄, it is very similar to that of γ-FeOOH. Thus, the local structure around Fe in the final oxidation products are similar to that of γ-FeOOH.

Fig. 4.

Radial structure function of Fe atom obtained by Fourier transform of the EXAFS spectrum of the suspension oxidized for t_ox = 90 min (shown in Fig. 3), together with those of α-FeOOH, β-FeOOH, γ-FeOOH and Fe₃O₄. The imaginary parts of the Fourier transform are represented by broken lines.

X-ray diffraction measurement is useful to identify the species of final oxidation products. However, it was difficult to obtain the information of the final oxidation products from X-ray diffraction measurement of the fully oxidized suspension because of large background. To avoid the background due to solution in the suspension, dried specimen composed of the cellulose fabric containing the oxidation products was prepared. Figure 5 presents X-ray diffraction pattern of such dried specimen. The reference diffraction patterns for α-FeOOH, β-FeOOH, γ-FeOOH and Fe₃O₄ provided in the International Center for Diffraction Data (ICDD) database are also shown in Fig. 5. Diffraction peaks are assigned to γ-FeOOH, cellulose, and NaCl. The diffraction peaks corresponding to cellulose are caused by the sample holder, and the diffraction peaks corresponding to NaCl results from residual ions in the suspension. Thus, these peaks are irrelevant to the final oxidation products obtained from the GR(Cl^–) suspension.

Fig. 5.

X-ray diffraction pattern of final oxidation products of the GR(Cl^–) suspension. Reference diffraction patterns for α-FeOOH, β-FeOOH, γ-FeOOH and Fe₃O₄ provided in the International Center for Diffraction Data (ICDD) database are also given. The arrows in the figure indicate the diffraction peaks corresponding to cellulose and NaCl.

Figure 6 indicates the TEM image of the final oxidation products of the GR(Cl^–) suspension. Particles with a thin plate-like shape are mainly observed in images. It has been described in many cases that α-FeOOH, γ-FeOOH, and Fe₃O₄ have spindle-like, plate-like and spherical shapes, respectively.²³⁾ Hence, the characteristic morphology of particles shown in Fig. 6 is consistent with that of γ-FeOOH. The presence of α-FeOOH and Fe₃O₄ is not observed in the X-ray diffraction pattern and the TEM images obtained under the present experimental conditions as seen in Figs. 5 and 6. Accordingly, it is clear that γ-FeOOH is mainly formed as the final oxidation product, under the present conditions.

Fig. 6.

Transmission electron micrograph of final oxidation products of GR(Cl^–).

A linear combination analysis of the XANES spectra for the GR(Cl^–) suspension during oxidation was performed to elucidate the oxidation process. In this analysis, we used the XANES spectrum of the GR(Cl^–) suspension before oxidation, ${\mu }_{{\text{GR(Cl}}^{-}\text{)}}$ , and that of γ-FeOOH obtained from the controlled oxidation of the GR(Cl^–) suspension, μ_γ-FeOOH. Hence, the normalized absorbance for the GR(Cl^–) suspension during oxidation, μ, can be estimated using the following relation,   

$$\mu =f_{{\text{GR(Cl}}^{-}\text{)}} {\mu }_{{\text{GR(Cl}}^{-}\text{)}}+f_{\gamma -\text{FeOOH}} {\mu }_{\gamma -\text{FeOOH}}$$(1),

where $f_{{\text{GR(Cl}}^{-}\text{)}}$ and f_γ-FeOOH indicate weighting factors for GR(Cl^–) and γ-FeOOH, respectively. The values of $f_{{\text{GR(Cl}}^{-}\text{)}}$ and f_γ-FeOOH were refined under the constraint that the sum of $f_{{\text{GR(Cl}}^{-}\text{)}}$ and f_γ-FeOOH be equal to one. As representative examples, the analysis results of XANS spectra at t_ox = 18 min and t_ox = 86 min are presented in Figs. 7(a) and 7(b), respectively. From the difference between the observed and calculated spectra, the reliability factor R was evaluated. A good fit is obtained by using the parameters given in Table 1. The values of $f_{{\text{GR(Cl}}^{-}\text{)}}$ = 0.67 and f_γ-FeOOH = 0.33, obtained for the suspension oxidized for a relatively short period (t_ox = 18 min), mean that the GR(Cl^–), composed of both Fe(II) and Fe(III) ions, is the major component in this case. On the other hand, the major component in the suspension oxidized for a relatively long period (t_ox = 81 min) is γ-FeOOH. Thus, the shape change of the XANES spectrum at Fe K absorption edge in Fig. 2 is considered to correspond to the fractional changes of GR(Cl^–) and γ-FeOOH.

Fig. 7.

Results of the linear combination analysis according to Eq. (1), together with the experimental XANES spectra of the suspension oxidized for (a) t_ox = 18 min and (b) t_ox = 81 min.

Table 1. The weighting factors $f_{{\text{GR(Cl}}^{-}\text{)}}$ , f_γ-FeOOH, and the reliability factor R obtained from the linear combination analysis of XANES spectra measured in situ for t_ox = 18 min and t_ox = 81 min.

tox (min)	$f_{{\text{GR(Cl}}^{-}\text{)}}$	fγ-FeOOH	R
18	0.66	0.34	0.003
81	0.13	0.87	0.006

The values of $f_{{\text{GR(Cl}}^{-}\text{)}}$ and f_γ-FeOOH versus t_ox are estimated from the analysis of a series of XANES spectra, as summarized in Fig. 8. The value of f_γ-FeOOH increases with increasing t_ox, in response to the decrease of $f_{{\text{GR(Cl}}^{-}\text{)}}$ . Such changes of f_γ-FeOOH and $f_{{\text{GR(Cl}}^{-}\text{)}}$ become smaller with increasing t_ox. It has been suggested that intermediate phases are formed during the oxidation process of the carbonate-containing green rust GR(CO₃^2–), depending on the oxidation conditions.^29,30,31) For example, GR(CO₃^2–) in a HCO₃^– and CO₃^2– solution has been reported to transform to the amorphous active FeOOH phase at first, and then to α-FeOOH and γ-FeOOH.²⁹⁾ It has also been proposed that the intermediate phase of GR(CO₃^2–) is either ferrihydrite (Ref. 30) or ferric oxyhydroxycarbonate.³¹⁾ Note that the values of $f_{{\text{GR(Cl}}^{-}\text{)}}$ and f_γ-FeOOH shown in Fig. 8 change continuously. In other words, the presence of the intermediate phase proposed by such studies does not seem to be observed in the present investigation. Consequently, it is probable that the dissolution of GR(Cl^–) and precipitation of γ-FeOOH occur simultaneously via solution during oxidation. Although details of the oxidation or transformation kinetics of GR(Cl^–) to γ-FeOOH via aqueous solution should be investigated further, the present results suggest that the transformation of GR(Cl^–) in the present conditions are comprised by various processes, such as the oxygen permeation through the film, the dissolution of oxygen into the solution, and the reaction of dissolved oxygen with GR(Cl^–) in the solution.

Fig. 8.

Fractions of GR(Cl^–), $f_{{\text{GR(Cl}}^{-}\text{)}}$ , and γ-FeOOH, f_γ-FeOOH, estimated from the linear combination analysis of the XANES spectra versus t_ox.

4. Conclusion

Oxidation of the chloride-containing green rust (GR(Cl^–)) suspension consisting of ferric (Fe(III)) and ferrous (Fe(II)) ions was investigated by in situ measurements of X-ray absorption spectroscopy (XAS). The Fe K absorption edge in X-ray absorption near-edge structure (XANES) spectra was systematically shifted toward the higher energy side with increasing oxidation time. In addition, the extended X-ray absorption fine structure (EXAFS) spectra were gradually changed over the oxidation time. Therefore, the oxidation of GR(Cl^–) suspension results not only in a change of the chemical state of Fe, but also in a change of the local structure around Fe. The final oxidation product obtained under the present oxidation conditions was identified as lepidocrocite (γ-FeOOH) consisting of Fe(III) ion. It was demonstrated that the XANES spectra of the specimens during oxidation were reproduced by the linear combination of the spectra of GR(Cl^–) and γ-FeOOH. From the change of XANES spectra during oxidation, the fractional changes of GR(Cl^–) and γ-FeOOH were estimated. The estimated results show that the fraction of γ-FeOOH in the suspension increases with increasing oxidation time while that of GR(Cl^–) decreases. It is concluded that the dissolution of GR(Cl^–) and precipitation of γ-FeOOH occur simultaneously via solution during oxidation under the present conditions. In addition, this study indicates that in situ XAS measurements are useful to investigate the oxidation process of reactive species like GRs in aqueous solution.

Acknowledgements

This research was partially supported by a Grant-in-Aid for Scientific Research Fund from the Japan Society for Promotion of Science (No. 23360276 and No. 23760620). This work was performed with the approval of the Japan Synchrotron Radiation Research Institute (Proposal No. 2012A1403).

References

• 1)   J. M. R.  Génin and  C.  Ruby: Solid State Sci., 6 (2004), 705.
• 2)   J. M. R.  Génin,  G.  Bourrié,  F.  Trolard,  M.  Abdelmoula,  A.  Jaffrezic,  P.  Refait,  V.  Maitre,  B.  Humbert and  A.  Herbillon: Environ. Sci. Technol., 32 (1998), 1058.
• 3)   L.  Simon,  M.  François,  P.  Refait,  G.  Renaudin,  M.  Lelaurain and  J. M. R.  Génin: Solid State Sci., 5 (2003), 327.
• 4)   J. M. R.  Génin,  C.  Ruby and  C.  Upadhyay: Solid State Sci., 8 (2006), 1330.
• 5)   P.  Refait,  M.  Abdelmoula and  J. M. R.  Génin: Corros. Sci., 40 (1998), 1547.
• 6)   S.  Suzuki,  E.  Matsubara,  T.  Komatsu,  Y.  Okamoto,  K.  Kanie,  A.  Muramatsu,  H.  Konishi,  J.  Mizuki and  Y.  Waseda: Corros. Sci., 49 (2007), 1081.
• 7)   S.  Suzuki,  S.  Suzuki,  K.  Inoue,  E.  Matsubara and  Y.  Waseda: Surf. Interface Anal., 40 (2008), 307.
• 8)   H. C. B.  Hansen,  S.  Guldberg,  M.  Erbs and  C. B.  Koch: Appl. Clay Sci., 18 (2001), 81.
• 9)   K.  Inoue,  K.  Shinoda,  S.  Suzuki and  Y.  Waseda: Corros. Sci., 52 (2010), 1421.
• 10)   K.  Inoue,  S. K.  Kwon,  K.  Kimijima,  K.  Kanie,  A.  Muramatsu,  K.  Shinoda,  S.  Suzuki and  Y.  Waseda: ISIJ Int., 47 (2007), 453.
• 11)   S.  Suzuki,  K.  Shinoda,  M.  Sato,  S.  Fujimoto,  M.  Yamashita,  H.  Konishi,  T.  Doi,  T.  Kamimura,  K.  Inoue and  Y.  Waseda: Corros. Sci., 50 (2008), 1761.
• 12)   P.  Refait,  O.  Benali,  M.  Abdelmoula and  J. M. R.  Génin: Corros. Sci., 45 (2003), 2435.
• 13)   S.  Naille,  M.  Abdelmoula,  D.  Louber,  J. P.  Moulin and  C.  Ruby: J. Phys.: Conf. Series, 217 (2010), 012084.
• 14)   J. M. R.  Génin,  A.  Renard,  C.  Ruby and  M.  Abdelmoula: J. Phys.: Conf. Series, 217 (2010), 012095.
• 15)   A.  Sumoondur,  S.  Shaw,  I.  Ahmed and  L. G.  Benning: Mineral. Mag., 72 (2008), 201.
• 16)   P.  Refait and  J. M. R.  Génin: Corros. Sci., 34 (1993), 797.
• 17)   F.  Nagata,  K.  Inoue,  K.  Shinoda and  S.  Suzuki: ISIJ Int., 49 (2009), 1730.
• 18)   F.  Nagata,  K.  Inoue,  K.  Shinoda,  I.  Bergmann,  V.  Šepelák and  S.  Suzuki: Mater. Trans., 50 (2009), 2557.
• 19)   F.  Nagata,  K.  Inoue,  S.  Fujieda,  K.  Shinoda,  S.  Suzuki and  S.  Tsuri: Mater. Corros., 62 (2011), 1100.
• 20)   G.  Sahoo,  S.  Fujieda,  K.  Shinoda and  S.  Suzuki: Corros. Sci., 53 (2011), 2446.
• 21)   G.  Sahoo,  S.  Fujieda,  K.  Shinoda,  S.  Yamaguchi,  M.  Korosaki and  S.  Suzuki: Corros. Sci., 53 (2011), 4001.
• 22)   K.  Shinoda,  S.  Fujieda,  S.  Tsuri and  S.  Suzuki: Mater. Corros., 63 (2012), 343.
• 23)   R. M.  Cornell and  U.  Schwertmann: The Iron Oxides, 2nd ed., Wiley-VCH, Wemheim, (2003).
• 24)   R.  Frahm: Nucl. Instr. Meth., A270 (1988), 578.
• 25)   S.  Bajt,  S. R.  Sutton and  J. S.  Delaney: Geochim. Cosmochim. Acta, 58 (1994), 5209.
• 26)   J.  Prietzel,  J.  Thieme,  K.  Eusterhues and  D.  Eichert: Eur. J. Soil Sci., 58 (2007), 1027.
• 27)   M.  Giorgetti,  M.  Berrettoni,  S.  Scaccia and  S.  Passerini: Inorg. Chem., 45 (2006), 2750.
• 28)   J. J.  Rehr and  R. C.  Albers: Rev. Mod. Phys., 72 (2000), 621.
• 29)   S. H.  Drissi,  P.  Refait,  M.  Abdelmoula and  J. M. R.  Génin: Corros. Sci., 37 (1995), 2025.
• 30)   O.  Benali,  M.  Abdelmoula,  P.  Refait and  J. M. R.  Génin: Geochim. Cosmochim. Acta, 65 (2001), 1715.
• 31)   L.  Legrand,  L.  Mazerolles and  A.  Chaussé: Geochim. Cosmochim. Acta, 68 (2004), 3497.