ISSN 1064 2293, Eurasian Soil Science, 2015, Vol. 48, No. 3, pp. 240–249. © Pleiades Publishing, Ltd., 2015. Original Russian Text © Yu.N. Vodyanitskii, S.A. Shoba, 2015, published in Pochvovedenie, 2015, No. 3, pp. 277–287.
SOIL CHEMISTRY
Ephemeral Fe(II)/Fe(III) Layered Double Hydroxides in Hydromorphic Soils: A Review Yu. N. Vodyanitskii and S. A. Shoba Faculty of Soil Science, Moscow State University, Moscow, 119991 Russia e mail: [email protected] Received July 11, 2014
Abstract—Ephemeral green rust is formed seasonally in some hydromorphic soils. It consists of Fe(II)/Fe(III) layered double hydroxides with different types of interlayer anions and different oxidation degrees of iron (x). In synthetized stoichiometric green rust, x = 0.25–0.33; in soil fougerite, it may reach 0.50–0.66. The mineral sta bility is provided by the partial substitution of Mg2+ for Fe2+. The ephemeral properties of the green rust are manifested in the high sensitivity to the varying redox regime in hydromorphic soils. Green rust disappears dur ing oxidation stages, which complicates its diagnostics in soils. For green rust formation, excessively moist min eral soil needs organic matter as a source of energy for the vital activity of iron reducing bacteria. In a gleyed Cambisol France, where fougerite is formed in the winter, the index of hydrogen partial pressure rH2 is 7.0–8.2, which corresponds to highly reducing conditions; upon the development of oxidation, fougerite is transformed into lepidocrocite. In the mineral siderite horizon of peatbogs in Belarus, where green rust is formed in the sum mer, rH2 is 11–14, which corresponds to the lower boundary of reducing conditions (rH2 = 10–18); magnetite is formed in these soils in the winter season upon dehydration of the soil mass.
Keywords: green rust, fougerite, hydrogen partial pressure rH2, lepidocrocite, magnetite DOI: 10.1134/S106422931503014X
INTRODUCTION anions in the interlayer space and with different Fe(II)/Fe(III) ratios, i.e., with different oxidation Minerals containing Fe(II) are formed in hydro degrees of iron. Thus, fougerite is the name of a min morphic soils providing their cold colors [4, 5]. Fe(II) eral that was earlier referred to as green rust according minerals are studied to very different extents. More to its color. stable minerals (siderite, magnetite, vivianite) were well studied long ago, whereas ephemeral compounds, At present, a considerable amount of naturally rapidly oxidized in the air, are poorly investigated. occurring LDHs are known with most of them con taining magnesium, aluminum, and iron with the Starting with the works of Bernal, the ephemeral – – 2− Fe(II) compounds have been called “green rust” [2, main interlayer anions being OH , Cl , and CO3 . 22]. Green rusts are found in paddy soils, as well as in Many of them were officially recognized long ago, 2− ⋅ other soils with an unstable moisture regime [7, 31]. In e.g., hydrotalcite [Mg6Al2(OH)16] [CO3 4H2O], the , green rust 2− World Reference Base for Soil Resources pyroaurite [Mg Fe (OH) ][ ) ⋅ 4H O], iowaite is referred to in the description of gleyic properties 6 2 16 CO3 2 – ⋅ [37], and the different stability of green rusts is not [Mg4Fe2(OH)10][Cl ) 2H2O], meixnerite – mentioned. The gleyed horizon may be either resistant [Mg6Al2(OH)16][OН )2 ⋅ 4H2O] [9, 32]. to oxidation, with the green color being preserved in Unlike lamellar aluminosilicates charged nega the soil profile for a long time without getting brown tively and manifesting a high cation exchange capac ish, or nonresistant, with the dove colored horizon ity, LDHs are positively charged, and they possess a becoming brownish in less than an hour [15]. This high anion exchange capacity under neutral pH con points to different Fe(II) pigments. In this paper, we ditions. Sometimes, they are called anionic clays to consider only unstable ephemeral green rusts. emphasize their layered structure with a high anion The soil Fe(II)/Fe(III) mineral belonging to a large exchange capacity and specific electrochemical and group of layered double hydroxides (LDH), or ephem magnetic properties. eral green rust, was registered in 2004; it was called Fougerites with different anionic compositions fougerite after the town of Fougéres in Bretagne, have been studied in laboratories. X ray diffractome France, in the vicinity of which it was described in try, Mössbauer spectroscopy, and transmission elec detail [13, 32–34]. The ephemeral green rust is classi tron microscopy are the principle analytic methods for fied as an LDH with different types of interlayer model ephemeral green rusts (fougerites). However,
240 EPHEMERAL Fe(II)/Fe(III) LAYERED DOUBLE HYDROXIDES 241 the specimen shooting used upon routine analyses (a) appears to be inapplicable for ephemeral green rust. Structure of layers For example, the routine preparation of specimens on С a glass plate for X ray diffractometry leads to the quick c/3 loss of the green color in the green rust, which points Ba to its oxidation. Therefore, diffractometers are Interlayer space Bc equipped with special devices for shooting wet sus Hydroxide layer pended samples [24]. The same measures are under A taken when using Mössbauer spectroscopy [24]. These b Cl– methods permit studying the composition and chemi – cal formula of ephemeral green rusts. Transmission H2O OH Fe(III) Fe(II) electron microscopy reveals the unambiguously syn thesized green rust crystals according to their typical (b) hexagonal shape and their peculiar electron microdif Structure of layers fraction pattern. С c/3 The composition of the interlayer anions in natural Ba fougerite cannot be directly determined, because the Interlayer space Mössbauer spectroscopy focused on the study of iron Bc forms is unable to distinguish the anion type. At the Hydroxide layer same time, it provides data on the iron oxidation A b degree, which is very helpful, since the x parameter is OH– very sensitive to the seasonal variations in the pH–Eh Fe(II) OH– Fe(III) conditions in hydromorphic soils. Iron is known to be H2O Mg(II) the principle geochemical marker of mineral hydro morphic soils [4, 5]. Soil scientists are interested in (c) Fe(II)/Fe(III) LDHs as pigments responsible for the Structure of layers “cold” colors in gleyed soils, as well as the active inter A mediate phase controlling the trend of oxidogenesis c C development in hydromorphic soils. B The aim of this study was to systematize the data on the ephemeral Fe(II)/Fe(III) LDHs in soils with the variable moisture regime. A Interlayer space STRUCTURE, COMPOSITION, B Hydroxide layer AND PROPERTIES OF Fe(II)/Fe(III) LAYERED b DOUBLE HYDROXIDES S Diverse Fe(II) minerals occur in soils with variable H O – Fe(III) Fe(II) moisture regimes. Some Fe(II) minerals, in particu 2 O OH lar, ferruginated phyllosilicates (biotite, ferrous chlo rite, etc.), are inherited by soils from the parent rock Fig. 1. Crystalline lattice of green rust, the layer sequence, and, genetically, are not related to Fe(II) neoforma and the position of water molecules and anions in the tions (pedofeatures) in hydromorphic soils. Very interlayer space along the [001] direction: (A) hydroxyl chloride green rust of group I; (B) hydroxyl fougerite of diverse compounds are related to Fe(II) neoforma group I; (C) hydroxyl sulfate green rust of group II [32]. tions (figure), i.e., phosphates, carbonates, oxides, and hydroxides (including Fe(II)/Fe(III) layered double hydroxides). vivianite Fe3(РO4)2 ⋅ 8Н2О particles and ephemeral Thus, it is convenient to subdivide all Fe(II) miner green rust. Vivianite, i.e., bivalent iron phosphate, was als into two groups according to their resistance to oxi discovered nearly 200 years ago (in 1817), and it has dation. One group is formed by the stable Fe(II) com been well studied [9]. The unoxidized vivianite is pounds suitable for routine mineralogical analysis. uncolored; however, it becomes blue due to the quick 2+ They include yellow carbonate siderite FeCO3, which partial oxidation of Fe , which is registered in gleyed often forms massive light yellow accumulations beneath soils containing iron phosphate. Ephemeral green the peat layer [7], and black oxide magnetite Fe3O4, rust, i.e., layered double hydroxides Fe(II) and which is able to accumulate in Fe–Mn concretions Fe(III), is less studied than vivianite. [10]. Stable green rusts are also included in this group. All ephemeral green rusts crystallize in a trigonal Another group comprises the nonresistant to oxi system. They are subdivided depending on the chemi dation Fe(II) minerals requiring reducing conditions cal composition of the interlayer anions. Fougerite, as to be preserved during the analysis. These are disperse one of the LDH types, is a later mineralogical name of
EURASIAN SOIL SCIENCE Vol. 48 No. 3 2015 242 VODYANITSKII, SHOBA
Table 1. Structural parameters of green rust [32] Type Interlayer spacing, Mössbauer spectral parameters: isomeric shift δ of interlayer anion nm and quadrupole splitting ΔE Green rust I Carbonate D1: δ = 1.27 mm/s; ΔE= 2.86 mm/s Hydroxyl D2: δ = 1.25mm/s; ΔE = 2.48 mm/s Chloride D3: δ = 0.46 mm/s; ΔE = 0.48 mm/s D4: δ = 0.46 mm/s; ΔE = 0.97 mm/s
Carbonate d003 = 0.750 Hydroxyl d003 = 0.792 Chloride d003 = 0.797 Green rust II
Sulfate d001 = 1.10–1.16 D1: δ = 1.27 mm/s; ΔE = 2.83 mm/s; D3: δ = 0.47 mm/s; ΔE = 0.45 mm/s The Mössbauer spectra were obtained at 77 K. D1 and D2 show Fe2+ doublets; D3 and D4 show Fe3+ doublets. ephemeral green rust. At the same time, as the term For anion containing ephemeral sulfate green rust green rust is widely used [37], we also address it in this with two layers, the interlayer distance d001 = 0.110– way when citing appropriate sources. 0.116 nm. 2− The generalized chemical formula Fe(II)/Fe(III) The tetrahedral anions (SO4 ) form the second of layered double hydroxides (fougerite) is recorded as group of green rusts GR(II) with a different X ray dif follows [14]: fraction pattern. Sulfate green rust may be supposedly formed in gleyed sulfate soils. [ II III x+ ⋅ n – 1 ⋅ Fe()1 – x Fex (ОН)2] [A ] x/n nН2O, Any factor changing the availability and reactivity of any component necessary for the Fe(II)/Fe(III) layered where A is the interlayer anion with n valence, and x is doubled hydroxides formation influences the stability of the share of Fe(III) in fougerite standing for its oxida the minerals. They have been studied for a considerably tion degree. The x value varies from 0.25 to 0.33 in syn long time, and the conditions of their formation have thesized stoichiometric minerals. The oxidation been revealed. The most important parameters are the degree and types of green rusts are distinguished by the following [23]: (1) the oxidation rate of Fe(II) to Fe(III) x value on the basis of Mössbauer spectroscopy. with oxidized iron immediately hydrolyzing to low The data on the structure of the synthesized ordered Fe(III) hydroxide, which reacts quickly with Fe(II) in solution, together with excessive anions, to Fe(II)/Fe(III) LDH was obtained from X ray diffrac – tometry (figure). They contain octahedral layers with form green rust, and (2) the activity of ОН and other Fe(II) and Fe(III), with hydroxyl groups OH– located anions in the system. Mössbauer spectroscopy is one of at the tops of octahedrons. The interlayer space is the principle methods of fougerite analysis. The mea occupied by anions [32]. surement is performed at room temperature (273 K), as well as upon cooling the sample for achieving more pre Layered hydroxides containing anions of spherical cise results (at the temperature of liquid nitrogen or liq – – 2− uid helium of 77–78 K or 5–12 K, respectively).The or flat shapes (OH , Cl , or CO3 ) form the first group of green rusts GR(I) producing the same X ray dif structural parameters listed in Table 1 were obtained in a laboratory upon cooling the green rust to 77 K. In the fraction pattern (Table 1). The interlayer distance d003 varies depending on the anion origin: 0.75 nm for car field, it is impossible to cool the soil artificially, and the bonate, 0.792 nm for hydroxyl, and 0.797 nm for chlo analysis is performed at the temperature of 285–287 K ride green rust. [15]. The spectral parameters of the ephemeral green rusts obtained in the field differ from those obtained in Chloride green rust has not been found in soils as a laboratory upon cooling samples to the liquid nitrogen yet, probably due to its very low stability. This is favored temperature (77 K). The advantages of the Mössbauer by an increase in the crystalline lattice parameter in the spectra investigation in the field are obvious, as it per с direction, because the Cl– ion diameter (0.181 nm) is mits observations over the seasonal transformations of larger than the ОН– ion diameters (0.145 nm). In green green rust under natural conditions. This advantage is rusts of the first group, the lattice parameter along the compensated for by the not high analytical accuracy. с axis is also determined by the geometry and the size of Due to the absence of cooling, the Mössbauer data the interlayer anion: с = 0.2385 nm for the chloride obtained in the field are less precise than the laboratory green rust, and с = 0.2256 nm for carbonate green rust. data obtained upon artificial cooling of the sample.
EURASIAN SOIL SCIENCE Vol. 48 No. 3 2015 EPHEMERAL Fe(II)/Fe(III) LAYERED DOUBLE HYDROXIDES 243
Table 2. Chemical composition of the soil solution and solid phase in the soils containing green rust [6, 15]
Component, Solution Component, Solid phase mM/kg cambisol, France peatbog, BelarusmM/kg cambisol, France peatbog, Belarus pH 7.2–8.1 6.05–6.60 pH 4.1–5.2 log(Fe2+) –4.7…–3.2 –5.5…–3.2 Fe2+/Fe3+ 2595/2025 log(Ca2+)–3.0…–2.8ΣFe 170–640 4620 log(Mg2+) –4.0…–3.2 –3.5 Si 10500–13700 − –7.6…–6.4 –2.9…–2.3 Al 820–1730 log(HCO3 ) log(Cl–) –3.2…–2.9 –4.0…–3.6 Ca 950 2− –5.6…–5.1 –4.3…–3.3 Mg 240 log(SO4 )
Feder et al. [15] cites examples of Mössbauer spec sometimes oversaturated with respect to hydroxyl foug tra obtained from the long term field monitoring of erite, and, sometimes, it is unsaturated with respect to green rust near the town of Fougéres. The doublet val carbonate and sulfate fougerite [15]. The year round ues are the following: for D1, i.e., for Fe2+, isomeric monitoring proved that the soil solution composition shift δ≈ 1.06 mm/sec and quadruple splitting changes rapidly, which decreases the reliability of the ΔЕ ≈ 2.70 mm/sec, whereas, for D3, i.e., for Fe3+ δ ≈ calculations. Δ ≈ 0.37 mm/sec and Е 0.65 mm/sec. As is seen, these Cations in Fe(II)/Fe(III) LDH. The role of magne doublet values differ from those obtained for the green sium has been studied in detail in natural hydroxide rust in the cooled soil sample. The reason is that the fougerite, where Fe(II) cations are partially replaced structure of the sample becomes ordered on cooling, by magnesium (II). The specified fougerite formula is and, the stronger the cooling is, the more ordered the written as follows [25, 33]: structure becomes. Note also that, on insignificant oxi II III x+[ n – 1 x– dation, green rust is partially transformed to ferrihy [(Fe ,Mg)(1 – x)Fex (OH)2] Ax/n , nH2O] drite or lepidocrocite, which leads to the appearance of brown mottles on the gray green matrix of the gley where A is the interlayer anion, and x is the oxidation horizon. However, in the field, due to the impossibility degree, which may reach 0.5–0.66. The inclusion of of cooling the sample and, hence, the low sensitivity of magnesium stabilizes the fougerite lattice and provides the method, the Mössbauer spectroscopy fails to iden for its stability at a high value of x [33]. The complete tify the composition of brown/orange low ordered iron replacement of Fe(II) cations with Mg(II) and the hydroxides. An additional Fe(III) doublet produced by replacement of hydroxyl groups by carbonate trans low ordered hydroxides (either by ferrihydrite or lepi forms fougerite to another mineral, i.e., pyroaurite, docrocite) is only registered [12, 18]. which is magnesium Mg(II) and iron Fe(III) hydrox Anions in Fe(II)/Fe(III) layered double hydroxides. ylcarbonate. In nature, Fe(II) in the fougerite lattice is The anion type in the interlayer space of replaced by Mg(II) up to the ratio Mg : Fe = 2. As pro Fe(II)/Fe(III) LDHs are of great importance. Four ceeds from the XANES analysis, natural fougerite occupies an interim position between the theoretically – – 2− types of LDH with different anions (OH , Cl , CO3 , homogeneous hydroxyl green rust and pyroaurite 2− and SO4 . ) are well studied in laboratories. Ephemeral Mg6Fe2(ОН)18 [25, 34]. No Fe(II) is registered in the green rusts (fougerites) are called hydroxyl, chloride light brown pyroaurite. carbonate, or sulfate green rusts, respectively [34]. In some cases, the excess of х = 0.5 value points to The chemical composition of green rusts (fouger the oxidation of fougerite; a part of Fe(III) creates ites) is mainly studied in laboratories. Chloride foug O ⎯ Fe–O bonds to form ferrihydrite, which is regis erite is minimally oxidized, and the FeIII share is only tered by the changing color (from greenish to orange) х = 0.25 in it. The oxidation degree of carbonate and and by the appearing of a sextet in the Mössbauer sulfate fougerites are equal to х = 0.33 [29]. spectrum on strong cooling (to 4–12 K) [14]. The anion composition of green rusts is identified Redox conditions necessary for the formation of reliably only for synthetic samples. It is hard to reveal green rusts in soils. The development of redox reac the composition of the interlayer anions in natural green tions is characterized by the redox potential Eh. In rust with Mössbauer spectroscopy. Researchers usually thermodynamic calculations, the redox potential Eh is try to obtain it indirectly by thermodynamic calcula expressed through the activity of electrons pe. Its value tions proceeding from the soil solution composition. is found from the following equation [6]: This approach does not give reliable results (figure). For example, near the town of Fougéres, the soil solution is рe = Eh ⋅ F : [(ln10)RT],
EURASIAN SOIL SCIENCE Vol. 48 No. 3 2015 244 VODYANITSKII, SHOBA where F is the Faraday constant, R is the gas constant, sequence: for ferrous hydroxide [Fe(OH)2] ΔG = and T is the absolute temperature. For Т = 25°C and ⎯ 490; for chloride green rust, ΔG = –540; for hydroxyl Eh measured in mV, pe = Eh : 58. green rust, ΔG = –570; for carbonate green rust, ΔG = Δ The index of the hydrogen partial pressure rH2 is –600; and, for sulfate green rust, G = –640 kJ/mol. calculated as [37]: The role of organic matter and biota in green rust rH = 2 (ре + рН). formation in the soils. Green rust is formed upon the 2 partial oxidation of Fe(II). In turn, bivalent iron is Sposito [11] distinguishes four soil groups accord formed in solution upon oxidation of Fe(III) minerals, ing to the oxidation degree and the pe value of the soil above all, thermodynamically unstable hydroxides: solution at pH = 7: oxidized, moderately reduced, ferrihydrite Fe5HO8 ⋅ 4H2O, lepidocrocite γFeOOH reduced, and strongly reduced. Oxidized soils show (the role of these minerals is well known [36]), and +7 < pe < +13.5, which fits rH2 = 28–41 at pH 7, and possibly feroxyhyte δFeOOH. This is an endothermic such oxides as hematite and goethite predominate in process with the organic substance being the energy the soils. Moderately reduced soils show +2 < pe < source. This process is catalytically accelerated by Fe +7 fitting rH2 = 18–28 at pH 7. Reduced soils mani reducing bacteria [16, 21]. fest –2 < pe < +2, which fits rH2 = 10–18 at pH 7; Feder and coauthors [15] emphasize the depen under these conditions, the concentration of water dence of the fresh organic matter input to the soil, while soluble Fe2+ is controlled by Fe(II)/Fe(III) layered noting that, in Western France, fougerite starts forming double hydroxides, i.e., green rust [33]. According to in the fall after the rainy leaf fall. Both factors, i.e., the World Reference Base for Soil Resources, Gleysols strong soil moistening and its enrichment with organic are identified by rH2 values below 19 [37]. Finally, substances, trigger the lepidocrocite reduction, after strongly reduced soils show –6.8 < pe < –2, which fits which, with the partial oxidation of Fe(II), fougerite rH2 = 0.4–10 at pH = 7; iron sulfides precipitate in forms, which is preserved till the dry summer season. soils with a high S content. The reduction of Fe(III) (hydr)oxides is investi Let us assess the index of hydrogen partial pressure gated by microbiologists who are involved in the stud rH2 in hydromorphic soils when ephemeral green rust ies of anaerobic bacteria, above all, dissimilative iron is formed. In a gleyed Cambisol near Fougeres, rH2 = reducers [27, 28]. Among them, the effect of 7.0–8.2 in the period of fougerite formation [15]. The Shewanella putrefaciens bacteria on the bioreduction redox conditions correspond to a strongly reduced of iron hydroxides (most often, ferrihydrite and lepi environment, although there are no iron sulfides in the docrocite) is studied best of all [3, 16, 24]. The forma soil because of the low content of S in the solution. tion of carbonate green rust was investigated upon the Different redox conditions are registered in the peat biological transformation of ferrihydrite with the par bogs of Belarus: rH2 = 14–20; however, this value is ticipation of Shewanella putrefaciens in two variants: lower (only 11–14) in mineral siderite horizons, where without phosphorus, and with phosphorus in a sus green rust is formed in the summer [7]. The formation pension in the amount of 4 mM [21]. Under anaerobic of ephemeral green rust there fits the lower boundary conditions, these bacteria favored the ferrihydrite for the reduced soils (rH2 = 10–18). reduction to green rust. The ways of green rust trans The stability of Fe(II)/Fe(III) layered double hydrox formation upon the rising hydrogen partial pressure ides above all depends on the hydrogen partial pressure depended on the presence of phosphates in the envi rH2. In the region of Fougéres, LDHs are registered in ronment. In the absence of phosphates, green rust is the winter upon rH2 = 4.0–14.4, and, in the region of transformed to magnetite. Phosphate inhibits the for Camargue (southern France), they are registered at still mation of magnetite (as was proved in the earlier lower rH2 values (varying from 4 to 5.5) [33]. investigation [20]), and green rust is transformed to Let us discuss the effect of the anion type in the vivianite Fe3(PO4)2 ⋅ 8H2O. interlayer space. Fe(II)/Fe(III) LDH with nitrate The biological factor influences not only the anions are the least stable. The presence of halogens reduction of Fe(III) (hydr)oxides but also the further (Cl, Br, and I) somewhat increases the stability of oxidation processes, either favoring or preventing green rust. Although bicarbonate ions may predomi green rust formation. In a laboratory experiment, the nate in the soil solution at pH ~ 8, thus favoring the ways of lepidocrocite transformation were studied in formation of carbonate green rust, the latter is not the presence of an electron donor and with the par referred to the most stable varieties. Sulfate contain ticipation of Shewanella putrefaciens depending on ing green rust is the most stable with pH ~ 7 or a little the concentration of bacterial cells in the water sus higher being favorable for it [23]. pension [38]. Aggregates consisting of bacterial cells Theoretically, the stability of ephemeral Fe(II) and lepidocrocite particles are formed in the suspen compounds is estimated according to the Gibbs ΔG sion. The lepidocrocite transformation depends not potential reduced to one Fe atom [32]. The free energy only on the speed of the Fe reduction but also on the of ΔG of green rust formation depends on the nature of density of these aggregates. Upon a high ratio the atoms in the interlayer space. Their stability between the content of cells and the content of lepi increases with decreasing ΔG in the following docrocite (>1 × 107 cells/mmol), dense aggregates
EURASIAN SOIL SCIENCE Vol. 48 No. 3 2015 EPHEMERAL Fe(II)/Fe(III) LAYERED DOUBLE HYDROXIDES 245 are formed, which favor lepidocrocite reduction to ture parameters with the electrodes plunged in the soil, green rust, whereas, upon a low ratio, magnetite i.e., the pH, Eh, temperature, and chemical composi develops. These laboratory data may be interpreted as tion of the water. On the basis of these physicochemi follows: lepidocrocite reduction to green rust is more cal parameters, we may judge about the seasonal min active in the period of considerable biological activity eralogical transformations of iron compounds in the of soils, which is favored by high humidity, a suffi gleyed soil. cient amount of organic matter, and the optimal tem Direct observations of the seasonal variations in the perature. content and composition of fougerite remained impossible until the possibility appeared to use a por table field Mössbauer spectrometer. This equipment GEOCHEMISTRY OF EPHEMERAL GREEN permitted highly precise determinations of the chang RUSTS IN SOILS WITH AN ALTERNATING ing fougerite content in time and space (by the soil MOISTURE REGIME profile depth). A portable Mössbauer spectrometer In hydromorphic soils, iron plays an important role was used for the field monitoring of the unstable iron as the main geochemical marker identifying the min minerals in a gleyed soil near the town of Fougéres eralogical transformations depending on the moisture [15]. This Mössbauer spectrometer was developed for and organic matter content variations. The discovery a space expedition to Mars, and one copy of the device of ephemeral green rust resulted in the revision of the was adjusted for investigating the changes in Fe miner oxidogenesis and reductogenesis processes developing als in a gleyed Cambisol. The spectrometer was placed in hydromorphic soils with an unstable redox regime. into a polyvinyl chloride tube plunged into the soils. Let us consider two contrasting cases of ephemeral After two day long shooting of the reflected Möss green rust formation: (1) in the mineral soil, in which bauer spectrum at the given depth, the equipment was green rust is formed upon the partial oxidation of Fe2+ moved to another depth and the shooting was ions, which is converted to lepidocrocite in the sum repeated. The values of the pH and Eh in the soil solu mer on rising Eh, and, (2) in the peat soil, where green tion were measured every hour. Considerable varia rust is formed in the summer upon the moderate oxi tions in the Eh upon the quasi stable pH and vice versa dation of siderite, which converts to magnetite during were registered. the winter dehydration. The pattern of the fougerite composition variation Mineral soils. As an example, let us scrutinize a during 16 months was obtained for the Cg horizon at gleyed soil, i.e., a light loamy Cambisol France depths of 15–106 cm [15]. Fougerite was registered in (burozem, according to the Russian classification) in a the winter in the heavy rain season, when the soil was deciduous forest near Fougéres. In the dry summer water saturated, whereas, in the summer, upon the season, the soil profile is completely oxidized with iron soil desiccation, lepidocrocite was identified instead of in the hydroxide and lepidocrocite γFeООН [33]. In fougerite. The seasonal transformations of the Fe the rainy fall–winter season with excessive moisture minerals agree with the fluctuations of the groundwa and sufficient ingress of organic substances, iron ter level, providing the alternation of aerobic and hydroxides are reduced either totally or partially by anaerobic conditions in the soil. anaerobic iron reducers to enrich the soil solution with Almost the complete absence of silicate iron in the Fe2+. Affected by the dissolved oxygen coming with soils should be noted [15]. This appears to be an impor the water, Fe2+ is partially oxidized to Fe3+; together tant condition for green rust formation. On the con with Mg2+, it precipitates in the form of fougerite. trary, abundant Fe containing aluminosilicates lead to In the winter, the groundwater pH rises to 8.1 (ver the adsorption of Fe2+ ions instead of their partial oxi sus 7.2 in the summer); i.e., the water is enriched in dation and the precipitation of green rust particles. OH groups. The activity of Mg2+ in the Cambisol In the winter, despite the presence of bright orange water is relatively low: log(Mg2+) = –3.7 to –4.0. The mottles against the gray background in the partially soil solutions are impoverished in bicarbonates gleyed horizon (at a depth up to 50 cm), Mössbauer (log(HCO3) = –6.4 to –7.6) and sulfates (log(SO4) = spectroscopy failed to reveal any other phases except –5.1 to –5.6). Under these hydrochemical condi for fougerite [15]. This is probably due to the low accu tions, low oxidized fougerite (or hydroxyl green rust) racy of the measurements conducted in the field with is formed in the winter due to the cosedimentation of out the sample cooling. However, the laboratory anal Fe3+ with Fe2+ and Mg2+. ysis performed on strong cooling of the samples When the soil profile is cut open, fougerite is pointed to the presence of lepidocrocite. quickly (in the course of half an hour or an hour) oxi Similarly, the laboratory study of the hydromorphic dized, and the soil color changes from blue green to soil sampled from the Nasine location neighboring grey or brown [15]. The field investigation of fougerite Fougéres in the west of France explained the reason for excludes cutting the profiles, since the air oxygen oxi orange mottles against the gray background [18]. In a dizes the ephemeral compound quickly. The regime laboratory, upon the strong cooling to 17 K, the Möss studies of hydromorphic soils are restricted to the col bauer spectra show (in addition to fougerite doublets) lection of indirect parameters, mainly the soil mois the sextets of Fe(III) hydroxide, which are registered as
EURASIAN SOIL SCIENCE Vol. 48 No. 3 2015 246 VODYANITSKII, SHOBA ferrihydrite rather than lepidocrocite (as in Fougéres). precautionary measures, partially preserved Fe2+ is The superfine field value H = 491–503 kE for sextet S1, registered in this mixture in the laboratory. It was and H = 441 kE for sextet S2. Fougerite evidently pro impossible to separate the green rust and siderite par vides a cold gray color (5BG6/1 according to the Mun ticles. In the winter, green rust is dehydrated to mag sell color charts) for the soil matrix, whereas, within netite. orange mottles (7.5YR 6/8–7.5YR 5/8), the color is The role of green rust in iron oxidogenesis in the determined by orange ferrihydrite or lepidocrocite soils. Bivalent iron plays a key role in the geochemical manifesting a high pigment capacity. cycle of Fe, being a source of its new compounds. The Peat soils. As an example, let us consider the chemical composition of ephemeral green rusts varies Sinyukha peat deposit in the Zuika River floodplain substantially; therefore, their role in iron oxidogenesis in Minsk oblast in Belarus [7]. In Poles’e, on the differs with the maximal difference being registered periphery of the soddy–boggy ore field, the surface between mineral and peat soils. There are several rea ferrugination is manifested in the form of mottles and sons for this phenomenon, including the different bands of irregular shape covering the continuously composition of the ions in the soil solution, either the rather large areas; in a number of Belarus peatbogs, presence or absence of siderite, etc. The differences the surface red colored ocher covers thousands and are controlled by different redox processes in mineral tens of thousands of square m. For example, at the and peat soils. Polykovichy site in the vicinity of the city of Mogilev, In the above described hydromorphic and peat the bog ocher areas reach 20 ha. Mantle ocher is soils, an intermediate ephemeral mineral nearly always underlain by thick siderite FeCO3 bod Fe(II)/Fe(III) layered double hydroxide is formed on ies. In the profile, so called “root bulges” are formed the oxidation branch on the way from Fe2+ ions to the at the boundary of surface ocher and siderite massifs solid phase relatively stable compounds. In mineral consisting of a siderite and green rust mixture. In the soils, upon deviation from the optimal acidic–basic 1970s–1980s, the equipment did not permit research conditions, Fe(II)/Fe(III) layered double hydroxides ers to identify green rust directly. X ray patterns are oxidized quickly to relatively stable compounds, showed reflections ordinary for iron hydroxides. The i.e., lepidocrocite and ferrihydrite. Thus, in mineral authors identified the gray green mineral mass as semihydromorphic soils, fougerite acts as a transi green rust proceeding from indirect signs: (1) the tional metastable phase between Fe2+ ions to Fe(III) color; (2) the deep endothermic effect on DTA curves hydroxides. In peat soils, green rust as an intermediate in the interval of 140–160°C corresponding to the product is formed upon the oxidation of siderite in the unbound water loss in the amount of 34–45% of the period of Fe oxidogenesis. In peat soils, Fe2+ in solu initial mass; (3) the comparable total content of Fe2+ tion does not influence the formation of green rust as (10.0%) and Fe3+ (14.5%) [7]. the product of siderite oxidation but preserves the green The chemical composition of the groundwater in rust. During the oxidation period, the following reac Belarus bogs is substantially different from the Cam tions develop subsequently in the peat soil: siderite → bisol water in Bretagne (France). In the summer, in the green rust → magnetite. The process of transforming zone of oxidizing siderite in a bog of Minsk oblast, the green rust to magnetite has been thoroughly investi pH values are lower than in the Cambisol: 6.0–6.6 (the gated in laboratory [30]. Green rust is more stable in content of OH groups in the water is lower). The ionic peat soils; it is transformed to magnetite Fe3O4 on composition also differs. The magnesium activity is decreasing moisture. Kovalev revealed this phenome higher in the water of the Belarus peatbog: log(Mg2+) = non by the high magnetic susceptibility of black coat –3.5. As was pointed out, Mg2+ stabilizes the fougerite ings on siderite particles [7]. The seasonal variability of structure. The difference in anion activity is still more Fe minerals in peat soils is pronounced in the dehy important. The water of the Belarus peatbog manifests a dration of green rust to magnetite upon the loss of high concentration of bicarbonates: log(HCO3) = –2.9 water in the winter and the reduction of magnetite to to –2.3. The activity of sulfates is also high due to the Fe2+ to restore green rust in the spring and summer [8]. oxidation of sulfide admixtures in the Belarus peatbog No lepidocrocite is formed in peatbogs. water: log (SO4) = –3.3 to –4.3. The oxidation mechanisms have been studied in In these peatbogs, siderite FeCO3 in the initial con laboratories by the example of carbonate green rust in dition consists of a yellowish white plastic mass [7]. an NaHCO3 solution using various analytical meth However, already in 20–30 min, at the first stages of ods: electrochemical methods, IR analysis, and X ray oxidation in the air, its color changes to gray green, diffractometry, as well as scanning and transmitting and green rust is formed. This process also develops electron microscopy [22]. The first mechanism repre inside the peatbog deposits upon limited oxidation, for sents a complex three step process: green rust dissolu example, upon peat mineralization after drainage of tion → oxidation of Fe(II) to Fe(III) → precipitation excessive water. Green rust is formed under the ferrous of Fe(III) in the form of unstable hydroxides. This red colored ocher, being intermixed with siderite. mechanism is most probable in hydromorphic soils, Green rust coexisting with siderite is more resistant to where the products of green rust oxidation are repre oxidation by the air oxygen, and, even without taking sented by the thermodynamically unstable iron
EURASIAN SOIL SCIENCE Vol. 48 No. 3 2015 EPHEMERAL Fe(II)/Fe(III) LAYERED DOUBLE HYDROXIDES 247 hydroxides: ferrihydrite, lepidocrocite, and oxide time period. Hence, the following question arises: how (magnetite). Their nonresistance to reduction leads to to register and give an index to these unstable horizons? reversible seasonal transitions from reductogenesis to Upon the laboratory analysis, differences were oxidogenesis in hydromorphic soils. derived from the specific properties of this mineral. In addition to the discussed mechanisms—disso The adopted chemical diagnostics of iron compounds lution → oxidation → sedimentation—the mecha in the soil are based on using two extracting agents: nism of solid phase oxidation of green rust to “ferrite acidic ammonium oxalate (Tamm’s method) for learning the amount of amorphous and weakly crystal III ⋅ green rust” with the formula Fe6 O(2 + x)(OH)(12 – 2x) lized compounds and dithionite–citrate–bicarbonate ⋅ H2Ox CO3 is possible [22]. Although the mineral pre (the Mehra–Jackson method) for determination of serves the layered structure, bivalent iron is completely the amount of all the free nonsilicate iron compounds. absent in it, and its color is red brown. On ageing, the However, both reagents turned out to be inapplicable ferrite green rust converts to stable minerals, i.e., goet for the dissolution of green rusts. Tamm’s reagent is hite αFeOOH and hematite αFe2O3. The mechanism inapplicable, because oxalate penetrating to the inter of solid phase oxidation of green rust is probably layer space stabilizes the green rust lattice [26]. In the untypical for hydromorphic soils, as neither goethite Mehra–Jackson reagent, the reducer (sodium nor hematite are registered in soils with well studied dithionite) appears to be excessive, as Fe(II) predom green rust. The seasonal reversibility to iron inates in green rust. Therefore, a simplified recipe is (hydr)oxides appears to be the most important prop used, i.e., a citrate–bicarbonate solution [36]. Cit erty of green rust in hydromorphic soils, resulting from rate–bicarbonate dissolves not only green rust but also its ephemeral properties. The subsequent retransition Fe–organic complexes; therefore, a kinetic method is to green rust goes most quickly upon the reduction applied taking into consideration that Fe–organic dissolution of unstable iron (hydr)oxides, i.e., lepi complexes are dissolved first and, green rust, next. docrocite, ferrihydrite, and magnetite. As proceeds from laboratory tests, goethite and hematite reduction goes much more slowly than that of ferrihydrite [36]. CONCLUSIONS Therefore, the mechanism of solid phase oxidation of (1) Minerals containing Fe(II) provide cold colors green rust with the formation of hematite and goet of hydromorphic soils. Some of them are relatively sta hite, which could participate in recurrent redox reac ble (siderite, magnetite, and vivianite) and have been tions in hydromorphic soils, appears to be less proba well studied long ago. Other, ephemeral minerals, ble than the dissolution → oxidation → sedimentation which are quickly oxidized in the air, are poorly stud mechanism with the participation of unstable iron ied with green rust being among them. (hydr)oxides. Ephemeral green rusts are referred to the class of However, the solid phase oxidation of green rust layered double hydroxides with different interlayer may also develop under other conditions, i.e., in the anions in the interlayer space and different working zone of artificial geochemical barriers Fe(II)/Fe(III) ratios. Four types of green rusts with arranged for purifying soil and ground water from – – 2− different anions are well studied: ОН , Cl , CO3 , inorganic and organic pollutants. In many of these 2− barriers, zero valent iron powder is used as a reagent and SO4 , i.e., hydroxyl, chloride, carbonate, and with green rust being formed upon its corrosion [1, sulfate green rusts, respectively. The anions form the 19]. Goethite and hematite are formed in the working following row according to their influence on the sta – – 2− 2− zone of these barriers as the products of green rust age bility of green rust: Cl < ОН < CO3 < SO4 . The ing, and this zone acquires a brown color. layered hydroxides that contain spherical or flat – – 2− Problems in identifying hydromorphic soils with shaped anions (ОН , Cl , or CO3 ) form the first ephemeral green rust. Difficulties arise both in field group of green rust GR(I) with the common X ray identification and laboratory investigation of soils due diffraction pattern, while tetrahedral anions (SO4) to the ephemeral properties of green rust. Problems in form the second green rust group GR(II) with the field identification are caused by the seasonal dis another X ray diffraction pattern. The ephemeral appearance of green rust. For example, in western soil green rust, i.e., Fe(II)/Fe(III) layered double France, in the area of Fougéres, the Cambisol is col hydroxide, was named fougerite in 2004. In natural ored brown upon the low groundwater level, whereas fougerite, a part of the Fe(II) cations is replaced by reduction starts in the fall in the season of rains and the Mg(II) to stabilize the mineral lattice. The quick ingress of new organic substances with the falloff [33]. response to changing redox conditions in the hydro Similar variability was noted in the Amazon River morphic soils is a distinguishing feature of fougerite: basin, where soils were brown colored in the dry sea it is formed under reductive conditions, and it is son with a dove gray horizon appearing in the moist destroyed under oxidative conditions. season [17]. The depth of the reductomorphic horizon (2) In a gleyed Cambisol in western France, during may be different from year to year even in the same the dry summer season, the soil profile is completely
EURASIAN SOIL SCIENCE Vol. 48 No. 3 2015 248 VODYANITSKII, SHOBA oxidized with iron present in hydroxide form, i.e., lep 10. D. E. Pukhov, Candidate’s Dissertation in Biology idocrocite γFeООН. In the rainy fall–winter season, (Moscow, 2002). upon excessive moisture and the sufficient input of 11. G. Sposito, The Thermodynamics of Soil Solutions organic substances, lepidocrocite is reduced under the (Clarendon Press, Oxford, 1981). effect of iron reducers and enriches the soil solution 2+ 12. M. Abdelmoula, F. Trolard, G. Bourrie, and with Fe ions. With dissolved oxygen coming with J. M. R. Genin, “Evidence for the Fe(II) Fe(III) 2+ 3+ water, Fe is partially oxidized to Fe , and it copre green rust “fougerite” mineral occurrence in a hydro cipitates with Mg2+ as fougerite. morphic soil and its transformation with depth,” In this period, the index of hydrogen partial pres Hyperfine Interact. 12, 235–238 (1998). sure rH2 = 7.0–8.2 fitting strongly reductive condi 13. B. C. Christiansen, T. Balic Zunic, P. O. Petit, tions, although iron sulfides are absent in the soil C. Frandsen, et al., “Composition and structure of an because of the low content of S in the solution. iron bearing, layered double hydroxide (LDH) – green In Belarus peatbogs, green rust is formed in the rust sodium sulphate,” Geochim. Cosmochim. Acta 73, 3579–3592 (2009). summer, when rH2 = 14–20; however, these values are lower (equal to only 11–14) in the mineral siderite 14. D. G. Evans and R. C. T. Slade, “Structural aspects of horizons, where green rust is formed. The zone of layered double hydroxides,” Struct. Bond. 119, 1–87 green rust formation there agrees with the lower (2006). boundary of reduced soils (rH2 = 10–18). In the win 15. F. Feder, F. Trolard, G. Klingelhofer, and G. 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