Synthesis and properties of sulphate, carbonate and chloride forms of Green Rust Synthèse de la forme chlorurée de la rouille verte

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1 Registration n : 1907 Symposium n : 22 Presentation : poster Synthesis and properties of sulphate, carbonate and chloride forms of Green Rust Synthèse de la forme chlorurée de la rouille verte BENDER KOCH, Christian and HANSEN, Hans Christian Bruun Chemistry Department, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark. (cbk@kvl.dk). Abstract A reproducible method for the synthesis of the crystalline chloride form of green rust (GR Cl ) has been developed comprising the slow aerial oxidation of 25 mm FeCl 2 /0.5 M NaCl solutions at ph 7.5 and 25 o C. Mössbauer spectroscopy provides an Fe(II):Fe(III) ratio of the GR Cl of 3:1. When GR Cl is diluted with oxygen-free water it dissolves with formation of Fe(OH) 2. GR Cl is less stable than the corresponding carbonate and sulphate forms of GR and is not likely to form in anoxic terrestrial environments. Introduction Green rusts (GRs) comprise mixed valence, greenish to blue coloured Fe(II, III) hydroxides structurally belonging to the pyroaurite group of double metal hydroxides (Allmann, 1970; Brindley & Bish, 1976). The alternating layers of positively charged trioctahedral Fe(II, III) hydroxides and interlayers of n-valent anions (A n- ) and water molecules is represented by the general formula [Fe(II) 6-x Fe(III) x (OH) 12 ] x+ [A x/n yh 2 O] x-. The specific bounds of x and y are not known precisely. For synthetic GRs the anion form during synthesis apparently determines the value of x (e.g. 1.5, 2) with only minor variation found for a particular anion form. Natural GRs may form on anoxic conditions in soils and sediments (Trolard et al., 1997; Koch & Mørup, 1991) or corrosion of iron (Kassim et al., 1982; Stampfl, 1969). GRs are intensively colored compounds and geochemically they are strong reductants which can reduce nitrate at considerable rates (Hansen & Koch, 1998; Koch & Hansen, 1997; Hansen et al., 1996). GRs are also reductants of xenobiotics such as chloro- and nitrosubstituted aliphatics and aromatics. GRs have not been found pure in a natural environment, hence most of our knowledge of their properties has been obtained from studies of synthetic samples. GRs can be synthesized in a number of ways. (i) Air oxidation of soluble Fe(II) at constant ph (Koch & Hansen, 1997; Lewis, 1997; Vins et al., 1987; Tamaura et al., 1984b), (ii) Partial air oxidation of Fe(OH) 2 (Drissi et al., 1994; Refait & Genin, 1993) (iii) Oxidation or electrolysis of Fe(0) (McGill et al., 1976; Bernal et al., 1959) and (iv) 1

2 Reductive dissolution of Fe(III)(hydr)oxides by soluble Fe(II) (Tamaura et al., 1984a; Taylor & McKenzie, 1980). It is important that robust methods for the synthesis of GRs are established to enable comparisons of the physical-chemical properties of synthetic GRs from different laboratories. In particular for kinetic experiments it is very important that crystallite size variation can be minimised between batches (Hansen & Koch, 1998). Here we describe a reproducible synthesis method for the chloride form of GR (GR Cl ) resulting in a pure and a well crystalline product. Materials and methods GR Cl s were synthesized by air oxidation of FeCl 2 solutions at constant ph. All solutions and suspensions were kept in flasks sealed with rubber septa. All in- and outlets to/from the flasks were made of 1 mm stainless steel needles or 1 mm teflon tubing which could easily penetrate the rubber septa (Fig. 1). All flasks were wrapped in Al foil to restrict photochemical side reactions. A stock solution of FeCl 2 was prepared in a 100 cm 3 flask by reacting 65 mmol of iron powder (Merck 3819, particle size 10µm) with 100 cm 3 of 1.00 M HCl, which beforehand had been Ar-bubbled ( % Ar; flow 50 cm 3 min -1 ) for periods 1 hour. The solutions were magnetically stirred and heated to approximately 80 o C until H 2 evolution ceased (approx. 1½ h). The solutions were stored under a slight overpressure of H 2 at 5 o C to minimize oxidation. A 300 cm 3 flask was filled with 190 cm 3 of water (± 0.5 M NaCl) which was Ar-bubbled for 1 hour. The flask was thermosttated at 25±0.1 o C. Now, the original rubber septum was exchanged with another septum containing inlets for a ph combination electrode (Methrohm ), base-titrant, Ar and air, and outlets for gas and a sampling valve (Fig. 1). Approximately 10 cm 3 of the FeCl 2 stock solution was transferred into the synthesis flask via an overpressure of Ar in the FeCl 2 stock solution flask, and ph adjusted to the desired value of the synthesis ph (7 7.5) after which the ph-stat control was switched on. The rate of air oxidation was controlled by the flow of air via a peristaltic pump ( cm 3 min -1 ); the air was passed through a wash bottle containg 2 M NaOH to remove CO 2. During synthesis the Ar inlet was raised from the bottom of the flask to a position where it flushed only the upper 0.5 cm of the suspension. The suspensions were magnetically stirred (250 rpm) and ph maintained by the automatic addition of NaOH ( M) previously flushed by Ar. Oxidation was stopped when approximately 60 % of the initially Fe(II) had been incorporated into precipitated solids. After settling of the greenish-blue precipitate, the synthesis flask was transferred into a glove box flushed by Ar. The flask was opened, the supernatant discarded and the remaing slurry filtered on a suction filter (pore diameter µm) and washed with 4 x 10 cm 3 of Ar-bubbled water. Now the GR Cl was ready for analysis or redispersion. In the latter case the material was transferred into 80 cm 3 of Ar-bubbled water in 100 cm 3 crimp seal vials, the vials sealed, taken outside the glove box, and placed on a shaking table (50 strokes min -1 ). Examination by X-ray diffraction (XRD) and Mössbauer spectroscopy (MS) of suspension samples withdrawn from the synthesis suspension or samples of the separated and washed products were carried out according to Koch & Hansen (1997). 2

3 Results and Discussion The concentration of the NaOH-titrant, the rate of oxidation, the synthesis-ph and the concentration of chloride containing salt was found to greatly affect the nature of the products formed (Table 1). The use of very high concentration of the titrant caused fluctuations in ph. No GR was formed at ph 7. At higher ph Fe(OH) 2 precipitated and ph 7.5 was found the best compromise. The crystals became larger by applying lower oxidation rates. Specifically the addition of NaCl to the synthesis suspensions strongly increased the thickness of crystallites as evidenced by the narrow basal reflections (W HH in Table 1). Apparently an increase in the Cl - concentration stimulates the assembly of more Fe(II, III) hydroxide layers. This may be due to a combined effect of the low affinity of Cl - for the interlayer and OH - /Cl - competition. The following optimum synthesis parameters were obtained: Oxidation rate corresponding to an OH consumption of mol min -1 (air flow rate of 0.3 cm 3 min -1 ), ph 7.5, and a NaCl concentration of 0.5 M. Under these conditions a typical synthesis takes about 24 h. XRD traces of products obtained at different synthesis conditions are shown in Fig. 2. Basal reflections (003, 006) are very intense compared with non-basal reflections, where only 102, 105, 108, 110 and 113 reflections can be discerned at best. The low intensity of non-basal reflection probably is due to preferred orientation of platy crystals and turbostratic disorder. The recorded d-spacings (Fig. 2) are in good agreement with those reported by Bernal et al. (1959), Vins et al. (1987), Lewis (1997) and Refait & Genin (1993). The 003 basal spacing of the GR Cl differs by 0.4 nm from that of the carbonate form of GR (d(003) = nm) (Hansen, 1989) and thus distinction between these two interlayer forms of GR by means of XRD should be possible when the anions occurs separately in the interlayer of the crystals. Mössbauer spectra reveal a Fe(II):Fe(III) ratio of 3 in agreement with Refait & Genin (1993) and Lewis (1997). The ratio is significantly different from the ratio of 2 found for carbonate and sulphate forms (Koch,1998). This indicates different local cation ordering in the different GR forms, which may cause significant differences in the thermodynamic properties and reactivity. An interesting observation was made on prolonged equilibration of GR Cl s redispersed in water in the crimp seal vials. In XRDs a reflection at nm attributed to Fe(OH) 2 appeared after a few hours and after 1500 h the intensity of this reflection was about three times more intensive than the 003 reflection of GR Cl (Fig. 3). During the 60 day period the 003 spacing of the GR Cl decreased from nm to nm, indicating an increase in layer charge and higher relative content of Fe(III). The ph in the suspensions determined from the measured concentration of Fe(II) in solution (1.5 mm) and assuming equilibrium with Fe(OH) 2 (log K sp = ) was This is close to the ph which triggered the formation of GR Cl. Other authors also have observed dissolution of GR Cl on dilution, but with the formation of lepidocrocite (Lewis, 1997) or magnetite (Vins et al., 1987). It can be concluded that GR Cl show poor stability compared with the sulphate and carbonate forms of GR, which can form and are stable at ph 7 at similar Fe(II) concentrations (Hansen, 1989; Koch & Hansen, 1997). Due to its low stability the here reported GR Cl can not be expected to form in anoxic terrestrial environments. 3

4 References Allmann, R. (1970) Doppelschichtstrukturen mit brucitähnlichen Schichtionen [Me(II) 1- xme(iii) x (OH) 2 ] x+. Chimia 24, Bernal, J.D., Dasgupta, D.R., and Mackay, A.L. (1959) The oxides and hydroxides of iron and their structural inter-relationships. Clay Miner. Bull., Brindley, G.W. & Bish, D.L. (1976) GR. a pyroaurite type structure. Nature 263, 353. Drissi, H., Refait, P. & Genin, J.-M.R. (1994) The oxidation of Fe(OH) 2 in the presence of carbonate ions: Structure of carbonate green rust one. Hyperf. Interact. 90, Hansen, H.C.B. (1989) Composition, stabilization, and light absorption of Fe(II)Fe(III) hydroxy carbonate (Green Rust). Clay Mineral. 24, Hansen, H.C.B., Koch, C.B., Nancke-Krogh, H., Borggaard, O.K. & Sørensen, J. (1996) Abiotic nitrate reduction to ammonium: Key role of green rust. Env. Sci. Technol. 30, Hansen, H.C.B. & Koch, C.B. (1998) Reduction of nitrate to ammonium by sulphate green rust: Activation energy and reaction mechanism. Clay Miner. (in press). Kassim, J.; Baird, T. & Fryer, J.R. (1982) Electron microscope studies of iron corrosion products in water at room temperature. Corr. Sci. 22, Koch, C.B. & Hansen, H.C.B. (1997) Reduction of nitrate to ammonium by sulphate green rust. Adv. GeoEcol. 30, Koch, C.B. (1998) Structure and properties of anionic clays. Hyperf. Interact. (in press). Koch, C.B. & Mørup, S. (1991) Identification of GR in an ochre sludge. Clay Mineral. 26, Lewis, D.G. (1997) Factors influencing the stability and properties of green rusts. Adv. GeoEcol. 30, McGill, J.R., McEnaney, B. & Smith, D.C. (1976) Crystal structure of green rust formed by corrosion of cast iron. Nature 259, Refait, P. & Genin, J.M.R. (1993) The oxidation of ferrous hydroxide in chloridecontaining aqueous media and pourbaix diagrams of green rust one. Corr.Sci. 34, Stampfl, P.P. (1969) Ein basisches eisen-ii-iii-karbonat in Rost. Corros. Sci. 9, Tamaura, Y., Saturno, M., Yamada, K. & Katsura, T. (1984a) The transformation of γ- FeO(OH) to Fe 3 O 4 and green rust II in an aqueous solution. Bull. Chem. Soc. Jpn. 57, Tamaura, Y., Yoshida, T. & Katsura, T. (1984b) The synthesis of green rust II (Fe III 1- Fe II 2) and its spontaneous transformation into Fe 3 O 4. Bull. Chem. Soc. Jpn. 57, Taylor, R.M. & McKenzie, R.M. (1980) The influence of aluminium on iron oxides. VI. The formation of Fe(II)-Al(III) hydroxy-chlorides, -sulfates, and -carbonates as new members of the pyroaurite group and their significance in soils. Clay Clay Miner. 28, Trolard, F., Genin, J. M. R., Abdelmoula, M., Bourrie, G., Humbert, B. & Herbillon, A. (1997) Identification of a green rust mineral in a reductomorphic soil by Mössbauer and Raman spectroscopies. Geochim Cosmochim Acta, 61, Vins, J., Subrt, J., Zapletal, V. & Hanousek, F. (1987). Preparation and properties of green rust type substances. Collect. Czech. Chem. Commun., 52,

5 Key words: Reductomorphic soils, green rust, crystal chemistry, Mössbauer spectroscopy, cation ordering. Mots clés : rouille verte, hydroxydes, Fe (II), Fe (III), double couche Legends to figures Fig. 1 Schematic diagram of the laboratory set-up used for synthesis of GR Cl. Fig. 2 XRD traces of products obtained at different synthesis conditions according to Table 1, and listing of observed d-spacings. Indices refer to a hexagonal cell. Fig. 3 Transformation of GR Cl to Fe(OH) 2 with time for suspensions of approximately 0.2 mmol GR Cl in 80 cm 3 of water. (M: Magnetite). 5

6 Table 1. Examples of products obtained under different synthesis conditions. Sample ph NaOH a Ox. rate b NaCl c W HH (003) d Colour Remarks (M) (10-6 mol min -1 ) (M) GRCl Brown No GR GRCl Dark brown No GR GRCl Greenish GR, impossible to flocculate GRCl Green GR, difficult to flocculate GRCl Bluish-green GR, difficult to flocculate GRCl Bluish-green GR, easy flocculation/washing GRCl // - - // - GRCl // - - // - GRCl // - - // - a. Concentration of titrant used in ph-stat syntheses. b. Rate of oxidation represented by the rate of consumption of OH - per min. c. Concentration of NaCl in the synthesis suspension. d. Width at half height of the 003 diffraction peak (in o 2θ). 6

7 Fig. 1 7

8 Fig. 2 Intensity (hkl) d (nm) GRCl-8 GRCl-5 GRCl Degrees 2 θ Co K α 8

9 Fig. 3 Fe(OH) 2 GR Cl M GR Cl 1500 h Intensity 168 h 2 h 0 h Degrees 2 θ Co K α 9

Green rust articles (key and from the consortium marked with *)

Green rust articles (key and from the consortium marked with *) Stability, structure, formation and transformation Hansen et (1989) Composition, stabilization, and light absorption of Fe(II)Fe(III) hydroxy-carbonate