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Langmuir 2002, 18, 7804-7809
Si Incorporation into Hematite by Heating Si-Ferrihydrite A. S. Campbell,*,† U. Schwertmann,‡ H. Stanjek,‡ J. Friedl,‡ A. Kyek,§ and P. A. Campbell† Institut fu¨ r Bodenkunde, Technische Universita¨ t Mu¨ nchen, D-85350, Freising-Weihenstephan, Germany, Soil and Physical Sciences Group, Lincoln University, Canterbury 8150, New Zealand, and Physik Department E15, Technische Universita¨ t Mu¨ nchen, D-85747, Garching, Germany Received November 30, 2001. In Final Form: July 2, 2002 The presence of Si up to a Si/(Si + Fe) mole ratio (XSi) of 0.270, added during or after the preparation of a 2-line ferrihydrite, raised the temperature of its transformation to hematite (Tt), as determined by differential thermal analysis, from 340 °C without Si to 740 °C at XSi ) 0.270. Deformation of the hematite unit cell and the greater suppression of crystal growth along a as compared to along c as XSi increased from 0 to 0.0403 were probably caused by structural incorporation of Si. Rietveld fits of X-ray diffraction (XRD) data showed a consistent Fe deficit of up to 1 Fe atom (from the ideal of 12.0) in the hematite unit cells. The magnetic hyperfine field (Bhf) at room temperature of the hematites heated to 800 °C fell regularly from 51.65 to 51.16 T as XSi increased from 0 to 0.0679. No separate SiO2 phase was detected by XRD, and the Morin transition was suppressed even at 4.2 K. At XSi g 0.134, however, Bhf returned to 51.40 T, a value similar to that obtained at XSi ∼ 0.03. This increase indicates that part of the Si was ejected from the structure and, as seen by a weak Mo¨ssbauer doublet, formed a Si-Fe-O phase with a quadrupole split of 0.92 mm s-1. This phase gave a broad XRD feature centered at 0.36 nm.
Introduction The formation of iron (Fe) oxides in natural, near-surface environments rarely proceeds in conditions comparable to those used in laboratory syntheses. In the natural world, “impurities” such as soluble silicate (probably the most frequent), organics, and phosphate are invariably present. As silicate (Si) has a high affinity for the Fe-OH functional group of Fe oxides, it is likely that Si interferes with the growth of Fe oxide crystals. For example, synthesis experiments have shown that the formation of lepidocrocite by oxidation of FeII is strongly hampered by soluble Si.1,2 This effect is probably connected to the formation of inner sphere complexes at the surfaces of the iron oxides, as is shown by an infrared Si-O band shift from that of polymeric SiO2 (1080 cm-1) toward lower wavenumbers.2 The rather high Si content of up to 100 g SiO2 kg-1 in many natural ferrihydrites3-5 may also contribute to the formation and poor crystallinity of this mineral and hinder the subsequent rearrangement of Fe octahedra to form hematite.6 On the basis of X-ray photoelectron spectroscopy evidence,7 Soma et al. suggest that Si tends to bridge the surfaces of crystalline domains within ferrihydrite particles and link particles within aggregates, whereas it appears8 that phosphate may be able to enter the hematite structure.
The number of X-ray diffraction (XRD) peaks of natural ferrihydrites decreases with increasing Si concentration.3 Furthermore, the temperature of transformation for synthetic, Si-containing ferrihydrite to hematite increased from 330 to 780 °C as the Si/(Si + Fe) ratio increased from 0 to 0.15. In a similar way, the impeded formation of nanohematite from ferrihydrite, in small pores of SiO2, may be explained by an Fe-Si interaction in these pores.9 Nano-hematite, with strong anisotropic XRD line broadening and a lowered magnetic hyperfine field, has been detected in hydrothermal sediment from the Atlantis Deep.10 EDXRA showed Si to be in close association with this hematite and possibly responsible for its low crystallinity. From this brief summary, it becomes obvious that although Si has a marked effect on the transformation of ferrihydrite to hematite by heating, details about the mechanism are still lacking. The aim of this investigation is, therefore, to study how the presence of Si affects the transformation of a 2-line ferrihydrite to hematite and to characterize the hematite-Si system formed from it. Of particular interest is the extent to which Si may be incorporated into the structure of hematite.
* Corresponding author. Tel: 64-3-325-2811. Fax: 64-3-3253607. E-mail:
[email protected]. † Soil and Physical Sciences Group, Lincoln University. ‡ Institut fu ¨ r Bodenkunde, Technische Universita¨t Mu¨nchen. § Physik Department E15, Technische Universita ¨ t Mu¨nchen.
Ten 2-line ferrihydrites, containing coprecipitated Si concentrations from XSi ) Si/(Si + Fe) ) 0 to 0.270 on a molar basis, were prepared by dissolving 80 g of Fe(NO3)3‚9H2O in 1 L of distilled water, adding 660 mL (free of Si) or 700 mL (containing the required amount of Si standard) of 1.0 mol L-1 NaOH to bring the pH to 7-8, and then stirring vigorously for 15 min.11 The usual dialysis step in this method was replaced by washing with water and centrifuging until the supernatant was free of electrolytes. XRD showed that washing and dialysis yielded
(1) Karim, Z. Clays Clay Miner. 1984, 32, 181. (2) Schwertmann, U.; Thalmann, H. Clay Miner. 1976, 11, 189. (3) Carlson, L.; Schwertmann, U. Geochim. Cosmochim. Acta 1981, 45, 421. (4) Childs, C. W. Z. Pflanzenerna¨ hr. Bodenkd. 1992, 155, 441. (5) Childs, C. W.; Kanasaki, N.; Yoshinaga, N. Clay Sci. 1994, 9, 65. (6) Glasauer, S. M.; Hug, P.; Weidler, P. G.; Gehring, A. U. Clays Clay Miner. 2000, 48, 51. (7) Soma, M.; Seyama, H.; Yoshinaga, N.; Theng, B. K. G.; Childs, C. W. Clay Sci. 1996, 9, 385. (8) Ga´lvez, N.; Barron, V.; Torrent, J. Clays Clay Miner. 1999, 47, 375.
Materials and Methods
(9) Morris, R. V.; Agresti, D. G.; Lauer, H. V. J.; Newcomb, J. A.; Shelfer, T. D.; Murali, A. V. J. Geophys. Res. 1989, 94, 2760. (10) Schwertmann, U.; Friedl, J.; Stanjek, H.; Murad, E.; Bender Koch, C. Eur. J. Mineral. 1998, 10, 953. (11) Schwertmann, U.; Cornell, R. M. Iron oxides in the laboratory, 2nd ed.; Wiley-VCH Verlagsgesellschaft: Weinheim, 2000; Chapter 8.
10.1021/la011741w CCC: $22.00 © 2002 American Chemical Society Published on Web 09/21/2002
Si Incorporation into Hematite similar products. After the final wash, samples were dispersed using an ultrasonic probe for 2 min, at 50 J s-1, before being freeze-dried. After dissolution in a sufficient volume of HCl (2 mol L-1), ferrihydrite Fe was measured by flame-AAS (atomic absorption spectroscopy) and Si using a Leeman Laboratories ICP spectrophotometer. For comparison, a Si-free ferrihydrite sample was prepared, as above, but after its precipitation sufficient Si was then added to give a XSi ) 0.0512. The mixture was then stirred for a further 15 min. Samples weighing between 100 and 130 mg were heated at 10 °C min-1 in static air, from room temperature (RT) to 995 °C, using a Linseis L81 thermal analysis (simultaneous DTA/TGA (differential thermal analysis/thermogravimetric analysis)) instrument. Two temperatures above the ferrihydrite f hematite transformation (Tt), namely, 672 ( 5 and 995 ( 5 °C, were used to check if higher temperatures caused changes in the hematiteSi system following its initial formation. The former temperature was chosen as a constant temperature value midway between the lowest observed Tt and the upper temperature limit reached by the furnace (995 °C), and because a small exothermic peak was observed at this temperature in some samples. The equipment enabled heating to be stopped immediately before, at
