exhibit pronounced cation-exchange and adsorption properties [1â3] and are promising as ion-exchange materials, superionic conductors, cathode materials for.
ISSN 0012-5008, Doklady Chemistry, 2006, Vol. 409, Part 1, pp. 101–105. © Pleiades Publishing, Inc., 2006. Original Russian Text © S.V. Balakhonov, Yu.V. Kolen’ko, B.R. Churagulov, E.A. Gudilin, A.G. Veresov, Yu.D. Tret’yakov, 2006, published in Doklady Akademii Nauk, 2006, Vol. 409, No. 1, pp. 52–56.
CHEMISTRY
Morphological Features and Ion-Exchange Properties of the H-Form of Todorokite S. V. Balakhonov, Yu. V. Kolen’ko, B. R. Churagulov, E. A. Gudilin, A. G. Veresov, and Academician Yu. D. Tret’yakov Received March 14, 2006
DOI: 10.1134/S0012500806070019
Phases with framework, tunnel, and layered structures attract considerable interest because they can exhibit pronounced cation-exchange and adsorption properties [1–3] and are promising as ion-exchange materials, superionic conductors, cathode materials for lithium batteries, molecular sieves, and catalysts [4–7]. Amongst manganese oxides with tunnel structures, todorokite MgxMn4O8 · yH2O has a record tunnel size of ~10 Å, which predetermines its unique characteristics [8]. In this work, we studied the physicochemical properties of a todorokite-type manganese dioxide synthesized by the hydrothermal method and determined its sorption capacity for some toxic ions of heavy metals. The starting reagents for obtaining the initial Mgtodorokite were MnCl2 · 4H2O (analytical grade), NaOH (reagent grade), K2S2O8 (analytical grade), and MgSO4 · 7H2O (analytical grade). A 6 M NaOH solution was added dropwise over 15 min to a solution of MnCl2 · 4H2O in a small amount of water. Then, to the resulting yellow precipitate of manganese(II) hydroxide, the appropriate amounts of K2S2O8 and MgSO4 · 7H2O were added over 30 min to obtain Na-birnessite [8]. Na-birnessite was stirred with 200 mL of a 1.0 M solution of Mg(NO3)2 for 12 h. Mg-buserite formed was hydrothermally treated in a sealed Teflon container filled with distilled water to 75–85% of its volume. A steel autoclave with the container was placed in a furnace and kept at 160°ë for 24 h. Then, the autoclave was taken out of the furnace and cooled to room temperature for 8–12 h. The resulting crystalline precipitate of Mg-todorokite was repeatedly washed with distilled water until the chloride test was negative. H-todorokite was prepared by stirring Mg-todorokite in a 1.0 M HNO3 solution for 48 h with a magnetic stirrer. To carry out ion-exchange reactions with heavy
Moscow State University, Vorob’evy gory, Moscow, 119992 Russia
metal cations, Mg-todorokite was immersed in a 0.33 M solution of CsNO3, Pb(NO3)2, or Ba(NO3)2 (about 0.1 g per 30 ml of solution) and stirred for ~20 days at room temperature. The reactions of H-todorokite with solutions of the same salts were carried out under similar conditions for ~10 days. X-ray powder diffraction analysis was performed on a DRON–3M diffractometer in the range 5°–75° (ëuäα radiation; the step was 0.1°, and the counting time was 2 s). The phases were identified using the standard “WinXpow” JCPDS-PDF2 database and literature data. The microstructure of the samples was studied by transmission electron microscopy (a JEOL JEM2000FX(II) microscope with an accelerating voltage of 200 kV) and scanning electron microscopy (a LEO Supra 50VP digital electron microscope equipped with an Oxford Instruments INCA Energy+ X-ray microanalysis system for electron probe X-ray microanalysis (EPXMA)). EPXMA was carried out by scanning a sample surface area of ~20 µm2 at a counting time of ~1 min. Thermal analysis (TGA) of the samples was performed with the use of a Perkin–Elmer PYRIS Diamond TG-DTA thermoanalyzer at a heating rate of 10 K/min up to 1100°ë (a sample weight of ~10–50 mg). IR spectra of the samples were recorded on a Perkin-Elmer LLC Spectrum One spectrophotometer in the range 400–4000 cm–1 with a step of 4 cm–1. To do this, samples (0.1 wt %) were compacted with KBr (spectrally pure grade) into tablets of ~10 mm in diameter. According to the X-ray powder diffraction data, the first stage of the synthesis of Mg-todorokite yielded Na-birnessite, manganese oxide with a layered structure. In this structure, each sodium ion is coordinated by a water molecule in the interlayer space; the interlayer distance was about 7.3 Å. The reaction of the Na+/Mg2+ ion exchange was nearly completed in ~12 h of soaking Na-birnessite in a 1.0 M Mg(NO3)2 solution at room temperature. As a result of this exchange, the interlayer spacing increased to ~9.9 Å due to formation
101
102
BALAKHONOV et al.
Intensity
Absorbance, %
[002] 4.8 Å
34 32 30 28 26 24 22 20 18 16 14 4000
[001] 9.7 Å
10
920 Òm–1
1155 Òm–1 1642 Òm–1
758 Òm–1 3366 Òm–1
632 Òm–1 514 Òm–1 428 Òm–1
3000 [210]
2000 1000 Wave number, Òm–1
[–302]
30
20
40
50
60
70 2θ, deg
Fig. 1. X-ray powder diffraction pattern of the Mg-todorokite after hydrothermal synthesis. The IR spectrum of the sample is shown in the inset.
of Mg-buserite, which is in good agreement with the literature data [8]. Hydrothermal treatment transforms Mg-buserite into Mg-todorokite, whose X-ray diffraction pattern (Fig. 1) is completely indexed based on the data in [8]. The formation of the tunnel structure of todorokite from the layered precursor phase (buserite) under hydrothermal conditions is associated with migration of some manganese atoms into the interlayer space, after which the MnO6 octahedra link together 2+
sharing edges to form tunnels [9]. Ions Mg hydr hinder
the migration of manganese atoms; however, a high pressure in the reaction zone facilitates the transformation of the matrix to match the template chosen. In Mg-todorokite, the Mg2+ ions in the tunnels are coordinated by four H2O molecules [8]. In addition, the matrix itself adsorbs a small amount of water. The IR spectrum of Mg-todorokite (Fig. 1, inset) showed the presence of coordinated and adsorbed water molecules in its structure. The phase synthesized consists of flaky crystallites 1 µm in size with ~50-nm MgO cuboctahe-
Table 1. Thermal stability of H-todorokite and initial Mg-todorokite Temperatures of DTA peaks, °C
Weight loss, %
Phase composition
H-form
Mg-todorokite
H-form
Mg-todorokite
H-form
Mg-todorokite
317
249
~15
~6
H-todorokite
Mg-todorokite
560
332
~23
~12
Mn2O3
Mg-todorokite
945
640
~27
~20
Mn3O4
MgMn2O4
Table 2. Ion-exchange reactions of H-todorokite Ion
Initial pH
Final pH
Cs+
5.99
5.45
1
0.114
Pb2+
3.08
2.56
1
0.148
Ba2+
3.91
2.84
1
0.091
–
–
Initial Mg-todorokite
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Mn O
Mn
Mg
Mn
1 2 3
H2O H2O H2O H2O 1 µm
(a)
50 nm
Fig. 3. TEM image of Mg-todorokite. (1) Subgrains of the todorokite; (2) MgO insertions; (3) the interface between two grains. The inset shows the structure of todorokite.
about 18° and 37° in the X-ray powder pattern of H-todorokite decreased as compared to those for Mg-todorokite. EPXMA data indicated that Mg ions were nearly completely removed.
1 µm
(b)
Fig. 2. SEM images of todorokite samples before and after acid treatment: (a) initial Mg-todorokite and (b) H-todorokite. The EPXMA spectrum of the Mg-todorokite is shown in the inset.
dral inclusions (Figs. 2, 3). According to the EPXMA and TGA data (Table 1), the compound obtained had the composition MgMn4O8 · yH2O (y ~ 2.8). On keeping of Mg-todorokite in a 1.0 M HNO3 solution at room temperature for 48 h, H-todorokite was formed. The intensities of the reflections at 2θ values of DOKLADY CHEMISTRY
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The tunnel structure of H-todorokite obtained by the topotactic ion-exchange reaction was similar to that of Mg-todorokite; however, the tunnels accommodate hydrated H+ ions rather than Mg2+ ions [8, 10]. The morphology of the acid-treated H-todorokite did not change (Fig. 2). According to the TGA data (Table 1), the Mg-todorokite synthesized lost adsorbed and coordination water at ~249 and ~332°ë, respectively. Anhydrous Mgtodorokite retained its structure up to ~640°ë; stable MgMn2O4 spinel was formed above this temperature: MgMn4O8
MgMn2O4 + Mn2O3 + 1/2O2.
(1)
Heat treatment of H-todorokite led to water loss at 317°ë (Table 1, Fig. 4). The contents of the adsorbed and coordination water in H-todorokite (H2Mn4O8 · zH2O, z ~ 3.4) were somewhat higher than those in Mg-todorokite (y ~ 2.8). On further
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BALAKHONOV et al. DTA
TGA, % 100
40
95 20 90 0
85
–20
80 317°ë
945°ë 75
–40 560°ë –60
0
200
400 600 Temperature, °ë
70 800
Fig. 4. Thermal analysis data for H-todorokite: 317°ë: H2Mn4O8 · zH2O → H2Mn4O8 + zH2O (z = 3.4; ∆mexp = 15%), 560°ë: H2Mn4O8 → 2Mn2O3 + 1/2O2 + H2O (∆mexp = 8%; ∆mcalcd = 9%), 945°ë: 3Mn2O3 → 2Mn3O4 + 1/2O2 (∆mexp = 3.5%; ∆mcalcd = 4%).
heating, anhydrous H-todorokite decomposed at ~560°ë to bixbyite (Mn2O3) H2Mn4O8
2Mn2O3 + 1/2O2 + H2O
(2)
and then to hausmannite (Mn3O4) at ~945°ë. Phase transitions of H-todorokite on heating with two-stage oxygen loss at ~560 and ~945°ë are common for manganese oxide. For Mg-todorokite, no oxygen loss was observed at ~945°ë. The formation of the highly dispersed phase of todorokite with a tunnel structure allowed us to carry out experiments on the determination of the sorption capacity of the material obtained. The size of the tunnels (~10 Å) allows them to accommodate nearly any inorganic cations, including heavy metal cations and, potentially, components of radioactive wastes. A key factor for the ion exchange to occur was found to be the presence of hydrated H+ ions in the structure, obviously, due to their high mobility and the easy deformability of their hydrate shells. In addition, the capillary effect can promote the entering of ions into the tunnels; according to the TEM data for todorokite (Fig. 3), these tunnels are rather long and narrow. In particular, after soaking of the initial Mg-todorokite in the CsNO3, Pb(NO3)2, and Ba(NO3)2 solutions, the fraction of the replaced Mg2+ ions was negligible (not detectable by EPXMA). The extent to which the ionexchange reaction between H-todorokite and the salt solutions proceeded was considerable (Table 2). This is
confirmed also by a change in the pH values of the solutions. Thus, in this work, we obtained the Mg- and H-forms of todorokite and studied their thermal stabilities and ion-exchange reactions. It was found that Htodorokite can be used for sorption of heavy cations, which replace hydrated H+ ions in the tunnels of the structure. This suggests that H-todorokite is a promising material for solving many environmental problems. ACKNOWLEDGMENTS We are grateful to N.V. Lyskov for carrying out thermal analysis and to A.V. Knot’ko for the X-ray powder diffraction measurements and studies of the microstructures. This work was supported by the Federal Targeted Scientific and Technical Program “Research and Development in Priority Fields of Science and Technology” and the Russian Foundation for Basic Research, project nos. 04–03–32183a, 04–03–32827a, 04–03–32295a, and 04–03–08078-ofi-a. REFERENCES 1. Huo, Q., Margolese, D.I., Ciesla, U., Feng, P., Gier, T.E., Sieger, P., Leon, R., Petroff, P.M., Schuth, F., and Stucky, G.D., Nature, 1994, vol. 368, pp. 317–321. 2. Shen, Y.F., Zerger, R.P., DeGuzman, R.N., Suib, S.L., McCurdy, L., Potter, D.I., and O’Young, C.L., Science, 1993, vol. 260, pp. 511–517. DOKLADY CHEMISTRY
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MORPHOLOGICAL FEATURES AND ION-EXCHANGE PROPERTIES 3. Cao, H. and Suib, S.L., J. Am. Chem. Soc., 1994, vol. 116, pp. 5334–5343. 4. Tsuji, M. and Abe, M., Bull. Chem. Soc. Jpn., 1985, vol. 58, pp. 1109–1117. 5. Parant, J.-P., Olazcuaga, R., Devalette, M., Fouassier, C., and Hagenmuller, P., J. Solid State Chem., 1997, vol. 3, pp. 1–11. 6. Koksbang, R., Barker, J., Shi, H., and Saidi, M.Y., Solid State Ionics, 1996, vol. 84, pp. 1–9.
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7. Strobel, P. and Mouget, C., Mater. Res. Bull., 1993, vol. 28, pp. 93–101. 8. Ching, S., Krukowska, K.S., and Suib, S.L., Inorg. Chim. Acta, 1999, vol. 294, pp. 123–130. 9. Liu, Z.-H. and Ooi, K., Chem. Mater., 2003, vol. 15, pp. 3696–3709. 10. Feng, Q., Kanoh, H., and Ooi, K., J. Mater. Chem., 1999, vol. 9, pp. 319–326.
