Dissolution and oriented aggregation: transformation from lepidorocite to goethite by the catalysis of aqueous Fe(II)

Abstract

At a low temperature, lepidocrocite-to-goethite transformation occurred in the presence of Fe(II) ions, whereas lepidocrocite was stable in the absence of Fe(II) ions. The results reveal that Fe(II) accelerates the dissolution of the lepidocrocite. The dissolved Fe(III) ions hydrolysed to form goethite. The mechanism of transformation included dissolution-reprecipitation and oriented aggregation.

---

1. Introduction

The iron oxide family consists of iron oxides, iron hydroxides and iron oxyhydroxides, all of which exist in nature. Iron oxides have 16 established polymorphs. Numerous transformations occur between the iron oxides and iron oxyhydroxides.^1–3 Among iron oxyhydroxides, goethite is the most stable phase, because of its lower Gibbs free energy.⁴ Other iron oxyhydroxides can transform to goethite under appropriate conditions. These transformations between iron oxyhydroxides and iron oxides occur in soil and iron corrosion and other natural phenomena.

Previous works by Liu et al.^2,5 on the transformation of ferrihydrite and feroxyhyte to hematite showed that the introduction of trace Fe(II) resulted in a rapid transformation from ferrihydrite and feroxyhyte to hematite at high temperatures. In phase transformation, the Fe(II) ions in solutions play an important catalytic role.^3,5–7 The contact of unreacted Fe(II) ions with Fe(III)-bearing minerals can lead to Fe(III) mineral transformation or change in of grain size through the conversion of an adsorbed Fe(II) into an Fe(III) atom (electron transfer).^8–10 Therefore, understanding the effect of ferrous ions and the mechanism of the transformation between iron oxyhydroxides and iron hydroxides is important to clarify iron cycling, rust layer formation and the applications of the varying properties of Fe(III) phases.

The previous work mainly researched phase transformation and the mechanisms from ferrihydrite to iron oxides or iron oxyhydroxides.³ The objectives of this study are therefore to investigate the transform between iron oxyhydroxides at room temperature and the mechanism of transformation.

2. Experimental

All reagents were used as received without further purification. Exactly 120 mL of 0.01 mol L⁻¹ EDTA-Na solution was poured into 1200 mL of 0.1 mol L⁻¹ FeSO₄ solution. Then, the solution was adjusted to within a pH range of 8.3–8.5 using 6.0 mol L⁻¹ NaOH solution. Air was pumped into the prepared solution at room temperature by stirring for 2 h. The orange-yellow deposit was centrifuged and repeatedly washed using distilled water.

The γ-FeOOH particles were again uniformly dispersed in distilled water with a concentration in 0.3 mol L⁻¹. An aqueous FeSO₄ solution was added into the γ-FeOOH suspension. The molar ratio of ferrous ions to the γ-FeOOH suspension (R = C_[Fe(II)]/C_[FeOOH]) ranged from 0 to 0.06. The suspension was adjusted to pH 7 by adding a diluted NaOH solution. The suspension was aged for different lengths of time at 30 °C. The experiments were carried out under nitrogen gas, and oxygen-free distilled water was used in all stages. The final products were centrifuged, repeatedly washed with distilled water, and dried at room temperature.

X-ray diffraction (XRD) data were collected from the dried samples on a Bruker D8 Advance diffractometer with Cu-Kα radiation. Infrared (IR) spectra were obtained over a range of 400–4000 cm ⁻¹ with a NICOLET IS 50 Fourier transform IR (FTIR) spectroscopy. The morphologies of the samples were characterised by transmission electron microscopy (TEM) using a Hitachi H-7650 microscope operated at 80 kV. X-ray photoelectron spectroscopy analyses were carried out by using Thermo Scientific Escalab 250Xi spectrometer with Al K-alpha X-rays and Theta Probe. High-resolution TEM HRTEM images were obtained with a JEM-2100 at 80–200 kV.

3. Results and discussion

3.1. Synthesis of original γ-FeOOH aged in the absence of Fe(II) ions

The XRD and IR patterns of the original iron oxides and the samples aged for 120 h with R = 0 are shown in Fig. 1a and b. Fig. 1a indicates that the synthesised original γ-FeOOH particles were pure lepidocrocite with poor crystallisation. The IR spectra (Fig. 1b) confirmed that no other iron oxyhydroxide phases were present in detectable amounts in the samples, as specifically shown in the O–H bending (1154–1160, 1021, 744–755 cm⁻¹)¹¹ modes of lepidocrocite. The XRD patterns and IR patterns in Fig. 1 indicate that no transformation into a new phase is observed. The TEM images in Fig. 1c and d indicate no obvious difference between the original iron oxides and the samples aged for 120 h with R = 0. Therefore, although lepidocrocite is a thermodynamically metastable phase of iron oxyhydroxides, it was stable in our experiment.

Fig. 1 (a) XRD patterns of the samples, (a1) the original iron oxides, and (a2) the sample aged for 120 h with R = 0; L: lepidocrocite; (b) IR spectra of the samples, (b1) the original iron oxides and (b2) the sample aged for 120 h with R = 0; (c) TEM image of the original iron oxides; (d) TEM image of the sample aged for 120 h with R = 0.

3.2. Synthesis of original γ-FeOOH aged in the presence of Fe(II) ions

The synthesis of the original γ-FeOOH aged in the presence of Fe(II) ions was characterised by XRD, IR spectroscopy and TEM (Fig. 2). As shown in Fig. 2a, with R = 0.004, no goethite formed when the samples were aged for 120 h. With R = 0.02, portions of the lepidocrocite particles transformed into goethite. The FTIR spectra (Fig. 2b) exhibit in-plane and out-of-plane bending mode vibrations of Fe_O_O_H at 791 and 890 cm⁻¹, respectively,¹² thereby confirming that goethite started to form in the samples. With R = 0.04 and R = 0.06, little magnetite formed (Fig. 2a). The TEM image obtained from the lepidocrocite particles aged for 120 h with R = 0.02 (Fig. 2c) indicates that an acicular and irregular-shaped morphology formed at the moment of aging for 120 h. The results reveal that an appropriate amount of Fe(II) ions has obvious accelerating action in the transformation of lepidocrocite even at room temperature.

Fig. 2 (a) XRD patterns of the samples aged for 120 h; (b) IR spectra of the samples aged for 120 h; (c) TEM image of the sample aged for 120 h with R = 0.02.

3.3. Mechanism of fast transformation in the presence of Fe(II)

The sample aged with R = 0.02 was monitored during the aging process to investigate the phase transformation from lepidocrocite to goethite. The synthesis of the original γ-FeOOH aged with R = 0.02 for different lengths time was characterised by XRD and TEM (Fig. S1 and S2†). In Fig. S1,† the diffraction peaks are assigned to lepidocrocite aged for 72 h. However, after aging for 73 h, peaks caused by goethite appeared. The acicular goethite particles formed after aging for 73 h, as shown in Fig. S2a.† The number of goethite particles increased with aging time (Fig. S2b and c†). The results indicate that the transformation occurred between 72 and 73 h of aging. The sample aged with R = 0.02 for different length time were characterised by XPS (Fig. 3). Fig. 3 displays the spectra of Fe 2p in original sample and the original sample aged for 73 h. The peaks at Binding Energy of 711.59 and 725.4 eV in the original sample are attributed to Fe 2p_3/2 and Fe 2p_1/2 respectively, which is in good agreement with γ-FeOOH.¹³ When γ-FeOOH aged for 73 h with R = 0.02, the α-FeOOH has a Fe (2p) spectrum similar, in many respects,¹³ so there were no obviously changes in Fe 2p between the samples before and after aged with Fe(II) ion for 73 h. Fe(III) compounds are always high-spin, leading to complex multiplet-split Fe 2p spectra. The satellite peaks around 719 eV of the XPS spectra of Fe 2p is clearly distinguishable. The satellite peaks have been ascribed to shake-up or charge transfer process.¹⁴ The satellite peaks pertain to Fe(III) 2p_3/2.¹⁴ The existed satellite in Fig. 3a indicates that there was not Fe(II) in it. To further explore the change of the sample before and after aged with Fe(II) ion, the O 1s region was also characterized (shown in Fig. 3b). As shown in Fig. 3b, the O 1s signal is multicomponent and was fitted with three curves¹⁵ centered at 529.95, 531.27 and 532.56 eV, which corresponds to O²⁻, OH⁻ and adsorbed H₂O. However, the O²⁻ peak, the OH⁻ peak and the adsorbed H₂O peak in the sample aged for 73 h shift 0.15, 0.23, 0.49 eV, respectively. Such a shift may suggest a change in chemical environment of the elements. Though there were no changes of element, lepidocrocite was built with FeO₄(OH)₂ octahedral and goethite was built with FeO₃(OH)₃ octahedra.¹⁶ The change of the binding energy indicated that the ratio of the chemical bond between Fe³⁺ and OH⁻ and chemical bond between Fe³⁺ and O²⁻ changed.¹⁷

Fig. 3 The high resolution XPS spectra of (a) Fe 2p (b) O 1s collected from the original sample and the sample aged in Fe(II) ion for 73 h.

The pH and C_[Fe(III)] values were monitored during the aging process for a more in-depth exploration of the phase transformation mechanism and the results are shown in Fig. S3.† Fig. S3a† show that, in the absence of Fe(II), the concentration of Fe(III) in the supernatant liquor of the suspension was almost zero, whereas in the presence of Fe(II) ions, the concentration of Fe(III) in the supernatant liquor of the suspension distinctly increased. These results indicates that adding Fe(II) accelerated the dissolution of lepidocrocite. The obvious increasing of Fe(III) ions in the supernatant liquor of suspension maybe caused by an electron transfer taking place within the lepidocrocite and Fe(II) ions.^18,19 Firstly, the added Fe(II) ion was adsorbed on the surface of the particles of lepidocrocite. Secondly, the adsorbed Fe(II) ion lost the electron to form a new Fe(III) ion, and it hydrolyzed to form new polymer.¹² Meanwhile, the Fe(III) ion in lepidocrocite captured the electron from the adsorbed Fe(II) ion and changed into a new-born Fe(II) ion. And the new-born Fe(II) ion could obtain electron from the adjacent Fe(III). The electron transfer process conducted continuously. Therefore, the concentration in the supernatant liquor of the suspension increased obviously. Due to the very low activation energy, electron transfer is very quick.^20,21 So the Fe(III) ion rate of hydrolysis is slower than electron transfer and the concentration of the Fe(III) ion in the supernatant liquor of the suspension increased rapidly. In Fig. S3b,† the pH value decreased from 7 to 6.14 with R = 0, whereas the value of pH decreased from 7 to 3.71 with R = 0.02 when lepidocrocite was aged for 120 h. The reasons for the decrease in the pH value are as follows: the dissolved Fe(III) ions hydrolysed to form a new polymer¹² and released protons. The HRTEM images (Fig. 4) were obtained to illustrate crystal growth. The morphology of the original lepidocrocite particles showed bullet-headed tails (Fig. 4a). Fig. 4a shows that the lattice fringe spacing of 0.209 nm corresponded to the (060) plane of lepidocrocite mineral and synthetic alike.²² The HRTEM images in Fig. 4b and c indicate that the lattice fringe spacings of 0.256 and 0.269 nm corresponded to the (021) and (130) planes of goethite, respectively. The HRTEM images in Fig. 4b and c show that some small particles formed acicular shapes. The lepidocrocite particles were assumed to have been initially dissolved by the catalysis of Fe(II) ions, and then Fe(III) ions hydrolysed to form Fe(OH)²⁺ and/or Fe(OH)²⁺ and/or Fe³⁺.¹² The absolute quantity of Fe(III) ions increased with aging time, and the primary nanoparticles produced by hydrolysed Fe(III) ions in number. Fig. 4b shows that the lattice fringe spacing of 0.256 nm corresponded to a (021) plane. The acicular shapes of goethite particles have a closing form of {021} at the end.²³ Goethite particles seemed to have nucleated first before a few primary nanoparticles approached each other with crystallographic alignment.²⁴ Burleson and Penn²⁵ also proved that the transformation of ferrihydrite to goethite preceded the formation of oriented aggregate. The isolated nanoparticles and relatively dense nanoclusters formed the primary nanoparticles. Then, the aggregates of primary nanoparticles increased separately. Long, thin assemblies composed of primary particles subsequently began to appear, with their number concentrations increasing with time.^24–26

Fig. 4 HRTEM images obtained from the rectangular area in each inset (a) original samples; (b, c) lepidocrocite aged for 73 h with R = 0.02.

4. Conclusions

Phase transformation from lepidocrocite to goethite in the presence of Fe(II) ions at a low temperature was investigated in this paper. Results show that the mechanism of the phase transformation from lepidocrocite to goethite with trace Fe(II) ions at room temperature is not only dissolution re-precipitation mechanism but also including oriented aggregation mechanism. The dissolution of lepidocrocite with Fe(II) ions is a precondition. Then, new Fe(III) ions hydrolysed into primary nanoparticles. The primary nanoparticles aggregated with each other to form mesocrystals. Finally, mesocrystals recrystallised to form goethite crystals.

Acknowledgements

This work is supported by the National Natural Science Foundation of China, Grant No. (21403052, 21477032, 21277040, 21206026, 11179029), Hebei Natural Science Foundation, Grant No. (B2015205227, B2012205041), Hebei Province Science and Technology Support Program, Grant No. 15211109D and Hebei Education Department, Grant No. Y2012028. And we thank Dr Jinming Zhou and Prof. Yanfeng Zhang for their contribution providing constructive advice and language polishing of the manuscript.

References

1. U. Schwertmann, J. Friedl and H. Stanjek, J. Colloid Interface Sci., 1999, 209, 215–223.
2. H. Liu, H. Guo, P. Li and Y. Wei, J. Phys. Chem. Solids, 2009, 70, 186–191.
3. H. D. Pedersen, D. Postma, R. Jakobsen and O. Larsen, Geochim. Cosmochim. Acta, 2005, 69, 3967–3977.
4. A. Navrotsky, L. Mazeina and J. Majzlan, Science, 2008, 21, 319.
5. H. Liu, M. R. Ma, M. Qin, L. J. Yang and Y. Wei, J. Solid State Chem., 2010, 183, 2045–2050.
6. K. M. Rosso, S. V. Yanina, C. A. Gorski, P. Larese-Casanova and M. M. Scherer, Environ. Sci. Technol., 2010, 44, 61–67.
7. D. E. Latta, C. A. Gorski and M. M. Scherer, Biochem. Soc. Trans., 2012, 40, 1191–1197.
8. R. Handler, B. Beard, C. J ohnson and M. M. Scherer, Environ. Sci. Technol., 2009, 43, 1102–1107.
9. M. Usman, M. Abdelmoula, K. Hanna, B. Grégoire, P. Faure and C. Ruby, J. Solid State Chem., 2012, 194, 328–335.
10. H. A. Crosby, C. M. Johnson, E. E. Roden and B. L. Beard, Environ. Sci. Technol., 2005, 39, 6698–6704.
11. J.-F. Boily and P. A. Kozin, Geochim. Cosmochim. Acta, 2014, 141, 567–578.
12. H. Liu, P. Li, M. Y. Zhu, Y. Wei and Y. H. Sun, J. Solid State Chem., 2007, 180, 2121–2128.
13. P. A. Kozin, A. Shchukarev and J.-F. Boily, Langmuir, 2013, 29, 12129–12137.
14. A. P. Grosvenor, B. A. Kobe, M. C. Biesinger and N. S. McIntyre, Surf. Interface Anal., 2004, 36, 1564–1574.
15. L. L. Hao, O. Y. Tong, L. M. Lai, Y. X. Liu, S. S. Chen, H. Y. Hu, C. T. Chang and J. J. Wang, RSC Adv., 2014, 4, 51984–51990.
16. Y. Cudennec and A. Lecerf, Solid State Sci., 2005, 7, 520–529.
17. L. F. Mei, L. B. Liao, Z. S. Wang and C. C. Xu, Adv. Mater. Sci. Eng., 2015, 2015, 1–10.
18. A. G. B. Williams and M. M. Scherer, Environ. Sci. Technol., 2004, 38, 4782–4790.
19. H. A. Crosby, C. Johnson, E. Roden and B. Beard, Environ. Sci. Technol., 2005, 39, 6698–6704.
20. E. Tronc, P. Belleville, J. P. Jolivet and J. Livage, Langmuir, 1992, 8, 313.
21. H. Liu, Y. Wei, P. Li, Y. F. Zhang and Y. H. Sun, Mater. Chem. Phys., 2007, 102, 1–6.
22. A. Sarkar, A. K. Dozier, U. M. Graham, G. Thomas, R. J. O'Brien and B. H. Davis, Appl. Catal., A, 2007, 326, 55–64.
23. H. B. Guo and A. S. Barnard, J. Mater. Chem., 2013, 1, 27–42.
24. V. M. Yuwono, N. D. Burrows, J. A. Soltis and R. Lee Penn, J. Am. Chem. Soc., 2010, 132, 2163–2165.
25. D. J. Burleson and R. L. Penn, Langmuir, 2006, 22, 402–409.
26. D. S. Li, M. H. Nielsen, J. R. I. Lee, C. Frandsen, J. F. Banfield and J. J. De Yoreo, Science, 2012, 336, 1014–1018.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra19787b

---