ISSN 1064 2293, Eurasian Soil Science, 2010, Vol. 43, No. 11, pp. 1244–1254. © Pleiades Publishing, Ltd., 2010. Original Russian Text © Yu.N. Vodyanitskii, 2010, published in Pochvovedenie, 2010, No. 11, pp. 1341–1352. SOIL CHEMISTRY
Iron Hydroxides in Soils: A Review of Publications Yu. N. Vodyanitskii Dokuchaev Soil Science Institute, Russian Academy of Agricultural Sciences, per. Pyzhevskii 7, Moscow, 119017 Russia E mail: [email protected] Received December 15, 2008
Abstract—Iron hydroxides are subdivided into thermodynamically unstable (ferrihydrite, feroxyhyte, and lepidocrocite) and stable (goethite) minerals. Hydroxides are formed either from Fe3+ (as ferrihydrite) or Fe2+ (as feroxyhyte and lepidocrocite). The high amount of feroxyhyte in ferromanganic concretions is proved, which points to the leading role of variable redox conditions in the synthesis of hydroxides. The struc ture of iron hydroxides is stabilized by inorganic elements, i.e., ferrihydrite, by silicon; feroxyhyte, by man ganese; lepidocrocite, by phosphorus; and goethite, by aluminum. Ferrihydrite and feroxyhyte are formed with the participation of biota, whereas the abiotic formation of lepidocrocite and goethite is possible. The iron hydroxidogenesis is more pronounced in podzolic soils than in chernozems, and it is more pronounced in iron–manganic nodules than in the fine earth. Upon the dissolution of iron hydroxides, iron isotopes are fractioned with light weight 54Fe atoms being dissolved more readily. Unstable hydroxides are transformed into stable (hydr)oxides, i.e., feroxyhyte is spontaneously converted to goethite, and ferrihydrite, to hematite or goethite. DOI: 10.1134/S1064229310110074
INTRODUCTION Ferrihydrite Oxides and hydroxides predominate among the Properties. The chemical formula of ferrihydrite nonsilicate iron compounds in soils [5, 11, 24]. Min remained uncertain for a long time. The chemical erals of other classes, i.e., carbonates, sulfides, and composition of mineral samples depends to a great extent on the size of the domains composing them, sulfates, are met much more rarely. Iron hydroxides which range from 2 to 6 nm. Anions on the particles' form a sequence of minerals differing in their thermo – surface are represented by ОН groups and Н2О mole dynamic stability; they are ferrihydrite Fe2О3 ⋅ ⋅ δ cules bound to them, which changes the O : OH : Н2О 2FеООН 2.5Н2О, feroxyhyte FеООН, lepidocrocite ratio in them depending on the particles' volume. This γFеООН, and goethite αFеООН. All hydroxides are is the reason for the discrepancy in the chemical for classified as thermodynamically unstable, except for mulas of the mineral suggested by different authors. goethite, which possesses the minimal free Gibbs Russell [51], who was the first to discover the struc energy. The unstable iron hydroxides bear important tural hydroxyl groups, suggested the following formula soil information. First, these recently formed minerals for ferrihydrite: Fe2О3 ⋅ 2FеООН ⋅ 2.5Н2О.Next, Egg attest to the current iron oxidogenesis. With time, fer leton and Fitzpatrick [29] revealed that the alteration oxyhyte may be spontaneously transformed into goet in the total composition of ferrihydrite samples does hite, and ferrihydrite may be converted either into not influence the composition of an elementary cell of hematite or goethite. Second, their presence testifies the mineral, which is close to the composition of the to the activity of heterotrophic oxidizing microbes in cells in polymorphic FeOOH modifications. The later the soil [2]. These microorganisms are active upon a investigations performed by Drits and coauthors [13, low concentration of iron in the solution and upon a 45] showed that the actual ferrihydrite represents a low concentration of organic acids [14]. The chemical heterogenic mixture of three components, i.e., the precipitation of iron accompanied by goethite and structurally ordered and defect ferrihydrites, as well as lepidocrocite formation proceeds without the partici ultradispersed hematite with the size of the coherent pation of microorganisms and upon a high enough scattering domains being about 2 nm. The finest activity of iron [8]. Sometimes, goethite and lepi hematite particles operate as seeds or nuclei for a new docrocite particles are formed biogenically. phase. This explains the possible solid phase transfor mation of ferrihydrite to hematite under natural con The objective of this work is to systematize the evi ditions. Dehydration, which develops in soils and dence concerning the structure; properties; and the weathering crusts upon the temperature rise and arid conditions of formation, dissolution, and distribution ization, is the condition for the rearrangement of the of iron hydroxides in soils. anion ferrihydrite package into a hematite package.
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The value of the free energy of the ferrihydrite for Both anions and cations affect the ferrihydrite crys mation ΔG0 is equal to –695.8 kJ/mol. The standard tallization. The influence of anions is most compre redox potential of ferrihydrite is Ео = 1.06 V. The den hensively studied by the example of phosphates [33]. sity is equal to 3.96 g/cm3 [24, 56]. Crystals form The phosphate impact was estimated by the value of aggregates 100–300 nm in size. Ferrihydrite is well dis the P : Fe atomic ratio. With the growing P : Fe ratio, solved by acidic ammonium oxalate even in the dark. the degree of the ferrihydrite solubility by oxalate (i.e., All natural ferrihydrites produce an additional strip in the Feox : Fetot ratio) increases. That means that the the infrared spectra in the area of 1000–1100 cm–1 due ferrihydrite crystallization slows down even at a small to the Si–O bond. Fe–O–Fe bonds are partially share of phosphates in the system, i.e., upon the P : Fe destroyed in the hydroxide, and various Fe–O–Si bonds ratio <2%. The pH influence upon a low rate of phos are formed instead of them with Fe partially replacing Si phate was the same as in the absence of phosphate: the in a tetrahedral Si–ferrihydrite lattice [50]. growing pH increases the ferrihydrite crystallization The identification of ferrihydrite in soils with x ray degree. diffractometry is based on the maximal reflection Ferrihydrite fixes cations of many heavy metals. (110) with d = 0.252–0.256 nm. However, this reflec This process attracts the attention of soil scientists tion interval coincides with the maximal reflection involved in soil conservation. The codeposition of iron (100) produced by feroxyhyte δFеООН [11]. As a and heavy metals is believed to be the most significant matter of fact, ferrihydrite was earlier studied in the mechanism favoring the removal of heavy metals from soils by x ray diffractometry. As a result, the reflection the solution. This codeposition lowers the toxicity and with d = 0.252–0.256 nm was unambiguously assigned biological availability of the heavy metals in the soil. to ferrihydrite. The huge volume of X ray investiga Being codeposited with iron, Сd2+, Cu2+, Pb2+, and tions performed under the guidance of Schwertmann Zn2+ enter the x ray amorphous particles of iron showed that this reflection was very often met in the hydroxides [46]. With time, the heavy metals are fixed diffraction patterns of clay soil fractions [24, 56]. This as a result of the transformation of low crystallized fact was interpreted as the wide spreading of ferrihy hydroxide to its well crystallized form. The participa drite in soils. However, now it appears invalid to treat tion of Cu2+ in the lattice of crystallized hydroxide is this reflection as sure evidence of ferrihydrite’s pres proved. At pH 7, the codeposited Cd and Pb become ence in soils. Consequently, transmitting electron less soluble after the ferrihydrite transformation into microscopy with electron microdiffraction permitting goethite. In this case, Cd enters the hydroxide lattice us to reliably distinguish ferrihydrite and feroxyhyte is and, therefore, is fixed more firmly than Pb upon the the best method of ferrihydrite identification in soils. hydroxide ageing. Upon applying this method, the adopted rules of elec Cations of heavy metals slow down the transforma tron microdiffraction should be followed, in particu tion of ferrihydrite at a high atomic ratio of Me : Fe. lar, the analysis of no less than 20–30 particles. The The codeposition with iron of such heavy metals as best results may be obtained from the analysis of the Cu, Co, or Mn brakes the ferrihydrite transformation hydroxide composition in iron–manganic concre into better crystallized phases. The effect of slowing tions, in which the amount of these minerals reaches down the transformation is presented by the following whole percents and seven tens of percents sometimes. succession: Cu ӷ Co ӷ Mn [46]. Formation. Ferrihydrite originates from the hydrol Transformations. Ferrihydrite is a predecessor of ysis of chloride and nitrate Fe(III) salts, and this dif other more stable iron (hydr)oxides, above all, hema fers it from other thermodynamically unstable iron tite and goethite. Numerous experiments were per hydroxides (lepidocrocite and feroxyhyte) that are formed concerning ferrihydrite crystallization in labo formed from Fe(II) salts. As a result of this genetic dif ratories. The origin of the final products is found to be ference, the prevalence of either ferrihydrite or controlled by such factors as the pH, the temperature, hydroxides produced from Fе(ОН)2 in soils may point and the quantity and composition of the foreign ions. to different redox and pH conditions in the period of The influence of the pH and temperature is rather their synthesis. well studied. At pH 6, ferrihydrite mainly produces Ferrihydrite is formed both upon the organic and hematite upon lacking protons, and it produces goet inorganic catalysis of Fe(II) oxidation. In the latter hite at pH 5. As was shown recently by Schwertmann case, the role of silicic acid in the hydrolytic polymer and coauthors [55], a temperature increase from 4° to ization of Fe(III) is important, as it prevents the for 25°С favors goethite formation from ferrihydrite. mation of better crystallized goethite [59]. Taking into Despite the similarity of the ferrihydrite and hematite account the great significance of the Fe(III) + Si sys structures, the transition of ferrihydrite (via the disso tem, the latter constantly attracts the attention of sci lution phase) into goethite instead of hematite is pos entists, while being studied lately with the application sible, especially under a variable redox regime. Fe2+ is of synchrotron X ray techniques [50]. the driving force of this transformation in soils with a Biota participates in ferrihydrite synthesis upon the stagnant moisture regime, which operates as a catalyst utilization of organic ligands in Fe(III)–organic com of the ferrihydrite dissolution with the subsequent for plexes. mation of hydroxides.
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A reducing agent in the form of Fe2+ is used in the ments in brooks are represented by goethite and lepi more complicated model experiments with ferrihy docrocite rather than ferrihydrite [52]. drite. It radically changes the course of the ferrihydrite In addition to inorganic crystallization inhibitors, transformation. The redox potential falls under the organic inhibitors also are important in soils. The effect of Fe2+, which leads to the dissolution of the inhibiting effect of organic substances was revealed in low ordered ferrihydrite and the subsequent forma the illuvial horizons of podzols and in the pseudofibres tion of much better ordered hydroxide particles, i.e., of podzols and gleyed soils. Cemented layers are lepidocrocite and goethite [20, 36, 37, 48, 49, 62]. formed in sandy soils of Germany, Belgium, and Great This effect is of actual significance for the iron’s fate in Britain. This layer is only 2–10 mm thick, and it is col moderately acidic gleyed soils, where ferrihydrite is ored red brownish or black due to the iron and/or transformed into better crystallized goethite under the manganese (hydr)oxides or ferro organic complexes impact of Fе2+ ions. Thus, upon the decreasing redox cementing the quartz grains. The analysis of the potential (ЕН) in a hydromorphic soil, a part of the fer pseudofibres revealed ferrihydrite; goethite; and, more rihydrite is first biologically reduced to Fe2+ and, next, rarely, lepidocrocite in them. Ferrihydrite prevails in the newly formed Fe2+ favors the synthesis of a better the case when the amount of organic carbon is high crystallized hydroxide, i.e., goethite. All these com (Corg > 5%). In the case when Corg < 2–3%, goethite plex processes of the transformation of Fe compounds and lepidocrocite predominate in these layers. may be investigated with electron transmitting micros According to Schwertmann [52], ferrihydrite prevails copy combined with electron microdiffraction. upon a high content of organic substance and a large Ferrihydrite’s behavior in podzolic soils appears to amount of entering iron. be an important item. This very unstable ephemeral Sometimes, the role of the organic substances in hydroxide may be transformed into more stable forms, ferrihydrite synthesis is disguised as the acidity’s influ i.e., goethite or hematite. We determine the particular ence. Let us cite some evidence for this fact. Using form that ferrihydrite is transformed into in podzolic electron microdiffraction, we found manganic ferri soils according to the number of ferrihydrite + goethite hydrite in the humus horizon of a brown forest auto and ferrihydrite + hematite associations met. The ferri morphic soil on an upland (Lithuania) [5]. Its devel hydrite + goethite association turns out to be six times opment may be related to the acidification in the more abundant in these soils than the ferrihydrite + humus horizons, where the pH in the salt extract hematite association [5]. This indicates that the ferri decreases to 6.4–6.8 versus 7.2–7.6 in the underlying hydrite in podzolic soils may convert mainly into goet horizons without ferrihydrite. At the same test plot, at hite rather than hematite. This evolution route agrees a distance of 300 m, ferrihydrite is absent in the gleyed with the formerly known data. Three important fac brown soil in a depression, because this mineral can tors favor ferrihydrite’s transformation into goethite, not exist under the reductive conditions. As is known, i.e., a low temperature, a humid climate, and an acid this mineral is dissolved with stable goethite being pH ranging from 4 to 6 [24]. All three factors are met formed from the dissolved iron upon a decrease in the in the podzolic soils of the Russian Plain. Probably, Eh value and the penetration of the Fе2+ ions into fer due to the transformation of ferrihydrite into goethite, rihydrite lattice. It is goethite that is present in the the latter is wide spread in the podzolic soils of the brown forest gleyed soil. moderate climate. Ferihydrite is found in an iron–humus podzol in northern Quebec in Canada [40]. The hydroxide Distribution. Inorganic and organic crystallization occurs in the Bfc horizon at a depth of 15–21 cm. The inhibitors favor the ferrihydrite preservation as a low soil reaction is acid (the pHwater is equal to 5.1). The soil ordered hydroxide. contains a lot of dithionite and oxalate soluble iron, i.e., Silica plays an important part among the natural 9.6 and 10.3%, respectively. The complete solubility by inorganic inhibitors. Ferrihydrite often occurs in soils the acid ammonium oxalate (Feox : Fedcb ~ 1) is typical of where either the groundwater is rich in silicon or the ferrihydrite as a low ordered iron hydroxide. soils are enriched in available silicon. The electron The electron microdiffraction method allowed us microscopy permitted revealing the ferrihydrite parti to discover ferrihydrite in soddy forest soils in the cles in three soil samples taken from the Bw, Bs, and Oka–Meshchera Poles’e in Vladimir oblast [5]. In one Bs2 illuvial horizons of Al–Fe–humus podzols in the profile, ferrihydrite was formed together with goethite state of New York in the United States [35]. Poorly in the B1f horizon at a depth of 17–34 cm. The order crystallized aluminosilicates of the imogolite type are degree of the goethite particles is different there, being concentrated in these horizons, and the clay fraction sometimes very low, close to the x ray amorphous contains a lot of mobile oxalate soluble Si (>1.5%). state. The development of goethite pseudomorphs The abundance of silicon favors the synthesis of over ferrihydrite particles is proved. In the other pro weakly crystallized ferrihydrite in the illuvial horizons file, ferrihydrite was formed in the BC horizon at a of podzolic soils. On the contrary, in the case when the depth of 103–176 cm. It is represented by shapeless ferruginous water contains little silicon (e.g., the desil scraps and rounded disordered fibrous bodies closely icated Oxisols in central Brazil), the ocherous sedi associated with phyllosilicates.
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Ferrihydrite formation is typical of young soils on may vary within very wide limits in it, and, sometimes, granite eluvium on the Karelian Isthmus. Ferrihydrite it appears to be comparable to the iron amount. is found in the strongly acid soils. For instance, Al fer In the manganic feroxyhyte structure, О2– and rihydrite was developed in a podzolized podbur in the ОН– compose hexagonal packing with half of the Bhfe illuvial horizon at a depth of 30–40 cm, where octahedral positions being occupied by Mn4+ and Fе3+ 4+ 3+ the pHsalt is equal to 3.5. Ferrihydrite is formed in the ions. The Mn and Fе are distributed in a different raw humus burozem in the Bfe illuvial horizon at a way in this lattice forming clusters of different chemi depth of 40–50 cm, where the pHsalt is equal to 2.9 [6]. cal compositions within one and the same anion pack Ferrihydrite is more widespread in the forest soils ing. The manganic feroxyhyte structure consists of on the Russian Plain than in the steppe soils [3]. It is parallel fragments of δFеООН feroxyhyte and εMnO2 formed of inorganic and humus Fe(III) hydroxocom akhtenskite. plexes. These complexes were revealed by Degtyareva Carlson and Schwertmann [23] scrutinized natural in the water of the Yakhroma floodplain [12]. and synthetic feroxyhyte samples. They synthesized Ferrihydrite is typical of acidic soils (horizons) in feroxyhyte by oxidizing an FeCl2 solution with Н2О2 humid landscapes [3, 6]. In the brown forest soil, it is and treating it with NH4(ОН) or NaOH bases. The formed in the humus horizon. However, in podburs, deficit of protons in the synthesis (increasing pH from burozems, soddy–podzolic, and forest soddy soils, 5 to 8) results in the crystallization of feroxyhyte parti ferrihydrite is more often formed in the illuvial hori cles, the reduction of its specific surface, and an zons. increase in the resistance to acid ammonium oxalate action (in the dark). A threefold treatment of one of the natural samples Feroxyhyte with oxalate in the dark led to the 100% dissolution of Properties. The chemical formula of feroxyhyte is the ferrihydrite particles, 85% dissolution of the ferox δFеООН. The feroxyhyte structure is composed of yhyte, 50% of the lepidocrocite, and 10% of the goet О2– and ОН– ions arranged in closest packing with the hite particles [23]. According to the solubility by Fе3+ ions being statistically distributed in half of the oxalate, iron hydroxides form the following series: fer octahedral positions. Note that the feroxyhyte struc rihydrite > feroxyhyte > lepidocrocite > goethite. ε ture is identical to that of akhtenskite МnО2, promot An insignificant substitution of manganese for iron ing the formation of manganic feroxyhyte in the soils. (~5%) considerably increases the stability of manganic The parameters of the feroxyhyte hexagonal cell are as feroxyhyte particles. We revealed this fact upon the follows: а = 0.293 nm and с = 0.460 nm. The parame analysis of the Tamm reagent’s effect in daylight on a ter а fits the octahedral base side, and the parameter с soddy–podzolic soil developed on red colored Per corresponds to the thickness of two layers in the hex mian deposits [4]. The soil in the arable horizon con agonal closest packing of О2– and ОН– [11]. The local tains manganic feroxyhyte (5% Mn) of clearly bio structure (i.e., the cluster composition) of feroxyhyte genic nature. Its particles represent thin films and dis is close to the structure of hematite [45]. ordered fibrous aggregates that include bacterial The free energy of the fine dispersed feroxyhyte remnants. Despite its biogenic origin, manganic fer formation ΔG0 = –483.9 kJ/mol [44]. This value, oxyhyte turns out to be very resistant to Tamm’s being higher than that of lepidocrocite and, the more reagent. This stability of manganic feroxyhyte is the so, goethite, reflects the low thermodynamic stability more so remarkable taking into account the fact that of feroxyhyte. In an experiment, feroxyhyte com the hematite present in the same soils was almost com pletely converted to goethite in the course of 6 hours at pletely dissolved by Tamm’s reagent. The high chemi a temperature of 60°С. It was partially transformed to cal stability of feroxyhyte is surprising. It may only be hematite at 80°С. explained by either the stabilizing effect of a certain Feroxyhyte is a ferrimagnetic with a low Curie tem Mn amount in the feroxyhyte lattice or by the large perature (155°С). The widely varying order degree size of the particles. leads to the formation of both magnetic and nonmag Formation. The abundance of feroxyhyte in forest netic varieties. The specific magnetic susceptibility soils somehow contradicts the data obtained earlier reaches 5000 × 10–6 cm3/g and only 400 × 10–6 cm3/g from differential x ray diffractometry. The study of the in the studied coarse crystalline and fine crystalline illuvial horizons of podzols and the pseudofibres in samples of synthetic feroxyhyte, respectively. The gleyed forest soils in Germany, Belgium, and Great magnetic susceptibility is still lower in the finer crys Britain by x ray diffractometry revealed ferrihydrite; tals. Dispersed feroxyhyte crystals with low magnetic goethite; and, more rarely, lepidocrocite, but no ferox susceptibility are met predominantly in soils [5]. yhyte [52, 56]. The reason for the discrepancy in the As proceeds from our investigations using the data obtained from the electron transmitting micros methods of electron microdiffraction and energy dis copy and the x ray diffractometry is probably the fol persion analysis, pure ferruginous feroxyhyte lowing. The ferrihydrite identification in soils by x ray δFеООН rarely occurs in soils. Manganic feroxyhyte diffractometry is based on the maximal reflection is more abundant [3, 7, 9, 10]. The manganese content (110) fitting d = 0.252–0.256 nm. However, the max
EURASIAN SOIL SCIENCE Vol. 43 No. 11 2010 1248 VODYANITSKII imal reflection (100) of feroxyhyte also fits this interval ment, while the chemical oxidation of Fe(II) proceeds [9, 15]. upon a neutral pH and favors goethite formation. The The previous common opinion that feroxyhyte is biogenic feroxyhyte may be developed in acidic formed in soils only abiogenically at pH > 7 did not soddy–podzolic and brown forest soils. favor the discovery of this mineral in soils [11]. How ever, its wide distribution in moderately acidic soils was proved later, and it turned out that feroxyhyte par Lepidocrocite ticles include remnants of the same iron oxidizing Properties. The chemical formula of this hydroxide bacteria as ferrihydrite particles do. Thus, the state is γFеООН. According to Lindsay [42], the free energy ment that the ferrihydrite formation is related to the of its formation ΔG0 = –483.9 kJ/mol. This value is activity of ferrobacteria and the formation of feroxy higher than the energy of goethite formation; hence, hyte is related to their absence was proved to be false. lepidocrocite is less thermodynamically stable. The The other factors (the Е values and organic ligands) Н standard reduction–oxidation potential is Ео = 0.86 V. influence it to a greater extent. High values of the The hydroxide density is 4.09 g/cm3 [56]. redox potential and an abundance of organic ligands are favorable for ferrihydrite formation, while variable This hydroxide has an orthorhombic crystalline lattice similar to that of goethite. The twin bands of ЕН values and an organic substance deficit promote feroxyhyte formation. Fe octahedrons alternate with twin bands of empty hepta top polyhedrons, each of them representing a Distribution. The electron microdiffraction method combination of a trigonal prism and a pyramid. The revealed that ferroxyhyte is more abundant in soils crystallographic cell is described by the following than was believed before [3, 5, 7, 9, 10]. We found parameters: а = 0.388, b = 1.284, and с = 0.307 nm. many particles of this mineral in ferruginous concre The bands are weakly bound to each other, providing a tions in the eluvial horizon (with the pH salt 5.2) in a lamellar structure of the hydroxide and a platy habitus soddy–podzolic soil developed on mantle loam (Mos of the crystals. Highly ordered lepidocrocite is suc cow oblast). A significant amount of manganic ferox cessfully identified in soils using x ray diffractometry yhyte was discovered in the ferruginous concretions of according to the 0.626–0.630 nm reflection. the arable horizon in a soddy–podzolic soil developed on red colored Permian deposits (Perm krai). Its con Formation. According to our data [5], lepidocrocite tent was lower in the fine earth. Soddy–podzolic soils rarely occurs in forest and steppe soils (3–5%). This is on varved clay (Novgorod oblast) manifested the same probably explained by the high activity of the iron oxi feroxyhyte distribution. A lot of feroxyhyte was found dizing bacteria, which favor the synthesis of ferrihy in the concretions, and much less of it occurred in the drite and feroxyhyte rather than lepidocrocite. At the fine earth from the soil’s eluvial horizon. same time, lepidocrocite is developed in cooler tundra soils. This fact is explained by the termination of the Feroxyhyte is revealed in sandy soil forming ° deposits. Carlson and Schwertmann [23] registered bacterial synthesis at Т < 4–8 С, which favors lepi docrocite synthesis. Lepidocrocite is often formed in feroxyhyte containing samples in light textured sedi ° ments, including gravel, at two sites in southwestern sediments in the cold season at Т = 0–5 С [11]. Finland. Discussing the point of the feroxyhyte gene The formation of lepidocrocite in the soil is related sis, the authors regarded the conditions existing in to the variable redox conditions. In the reduction highly porous sandy deposits at a substantial depth period, Fe(III) is reduced to Fe(II), whereas Fe(II) is from the surface as favorable for the mineral’s forma oxidized and hydrolyzed to γFеООН in the oxidation tion with both Fe(III) and Fe(II) being contained in period. Lepidocrocite develops through a phase of the water there. Note that, in light textured soils, green colored products called green rust, which is an Fe(II) and Mn(II) are worse adsorbed by phyllosili unstable salt containing Fe(II) and Fe(III) hydroxide cates, which promotes iron and manganese sedimen with its charge being neutralized by the interlayer tation and the synthesis of feroxyhyte and manganic anions, more often, chloride, sulfate, or carbonate. feroxyhyte. In this case, the synthesis of vernadite This compound was produced in a laboratory from МnО2 is also facilitated, which catalyses Fe(II) oxida Fe(II) salts, and it was also found in nature in hydro tion. This is the reason for the more frequent synthesis morphic soils. Green rust is observed in soils under of manganic feroxyhyte in light textured soils and sed rice. At the same time, lepidocrocite is formed in soils iments as compared to heavy textured soils. with predominating oxidizing conditions, i.e., in cher The soils in which feroxyhyte is formed also often nozemic and chestnut soils in Transbaikal [1]. contain goethite. However, these hydroxides are found Lepidocrocite may evidently be formed in several to be confined to different acid–base conditions in ways. One of them (repeatedly described) proceeds via forest and steppe soils. Half of the feroxyhyte–con the green rust phase upon a variable redox potential taining samples are specified by pH > 6, whereas all the (ЕН). The other is of an ambiguous mechanism (if we soils with goethite but with no feroxyhyte manifest rule out the possibility of a short term decrease in the pH > 6. This difference attests to the activity of ferro ЕН in the spring) resulting in lepidocrocite formation bacteria synthesizing feroxyhyte in an acidic environ in the steppe soils of the Transbaikal region. The latter
EURASIAN SOIL SCIENCE Vol. 43 No. 11 2010 IRON HYDROXIDES IN SOILS: A REVIEW OF PUBLICATIONS 1249 poorly studied way of lepidocrocite formation deserves Citrates and phenols represent the principal com the most attentive attitude in the future. ponents of plant root exudates. Both are capable of The crystallization degree of lepidocrocite particles reducing Fe(III) to Fe(II) in (hydr)oxides. Further formed from the chemical oxidation of FeCl2 increase Fe(II) oxidation and hydrolysis results in lepidocrocite with an increase in the pH. At pH 4.5, low ordered formation. Thus, it becomes clear why this mineral lepidocrocite particles are formed, whereas highly predominates near plant roots [32]. ordered particles originate at pH 7. The stabilizing effect Among other organic acids, soil scientists pay of the OH groups in the solution on the hydroxide struc much attention to oxalic acid salts, i.e., oxalates, ture is seen under these conditions [52]. The crystalliza which are widespread in forest soils. Due to the forma tion degree of the lepidocrocite particles also depends on tion of stable complexes, oxalate facilitates an increase in the Fe(II) concentration in the solution: the higher the the Fe concentration in the solution. The maximal con concentration upon the chemogenic synthesis, the centration of oxalate is registered in the rhizosphere, higher the crystallization degree [56]. which makes Fe more available to plants. In podzolic Silicon appears to be a powerful inhibitor, as the soils, oxalate as the predominating organic anion is able amount of lepidocrocite in the forest soils in Great to significantly influence the iron mineralogy. Britain decreased with the increasing content of Highly dispersed lepidocrocite with a crystallite Si(ОН)4 in the soil solution [39]. The lepidocrocite size of 20 nm is completely soluble by ammonium content correlates inversely with the atomic Si : Fe oxalate [26]. A high solubility and a large specific sur ratio in the citrate–dithionite extract. Lepidocrocite is face are typical of the lepidocrocite particles formed in not formed in the gley soils of Bangladesh, which contain the presence of phosphates. This explains the vivianite a lot of Si(ОН)4 in the soil solution (0.7–1.0 mM). The efficiency of Fe3(РО4)2 ⋅ 8Н2О as an iron containing iron hydroxides remain in the amorphous form in the fertilizer against chlorosis in carbonate soils. As a soils with the maximal amount of dithionite soluble result of the oxidation and dissolution of vivianite, silicon (Sidit). Aluminum is another strong inhibitor of nanoparticles of lepidocrocite are formed, which are lepidocrocite synthesis. An increase in its amount in unstable to the impact of oxalate ions in the rhizo the FeCl2 solution in the model experiment decreased sphere. the share of lepidocrocite significantly and increased the share of goethite. Poorly crystallized lepidocrocite is often met in Phosphates exert a favorable effect on the lepi humus horizons, where it is preserved while being pro docrocite formation. In the experimental oxidation of tected by organic acids. This is proved by an experi Fe(II) sulfate, the share of lepidocrocite in relation to ment with the organic substances oxidation, in which goethite rose with the increasing atomic P : Fe ratio the soil treatment with Н2О2 resulted in the lepi [26]. Goethite instead of lepidocrocite was formed at docrocite transformation into goethite [56]. P : Fe < 0.5% with the pH ranging within 5–8.5. The In heavy textured soils, lepidocrocite is usually situation changes radically with the P : Fe ratio > 2%, well crystallized. This is explained by the high local with only lepidocrocite being registered among the activity of Fe(II) in the fine pores in clay soils. oxidation products. Phosphate promotes lepidocroc Distribution. Lepidocrocite is found in the clay ite formation by maintaining the disorder in green rust fraction of brown forest soils in the west of mid Wales (a lepidocrocite precursor). in Great Britain [16]. The iron hydroxides were ana Among the organic ligands, the influence of citrate lyzed using x ray diffractometry. In the humus hori on the crystallization of iron (hydro)oxides in an zons, only the x ray amorphous particles of iron Fe(ClO4)2 solution was studied in detail [41]. Citrate, hydroxides are present. This agrees with the recog being present in an acid solution (pH 6) in an amount nized inhibiting role of simple organic acids (such as of 10–5 M, inhibits goethite formation. In a neutral oxalic acid) preventing the crystallization of hydroxide solution with pH 7.5, citrate (in an amount of 10–4 M) particles. In the illuvial horizons of these brown forest inhibits the development of maghemite γFе2О3. Upon soils, where the content of organic acids is lower, the both pHs, citrate initiates the formation of lepidocroc hydroxide particles were better ordered in the clay frac ite. Citrate is considered [41] to increase the share of tion, which permitted their identification with x rays. In lepidocrocite at the expense of reducing the share of the automorphic soils, lepidocrocite prevails in the the competing metals, i.e., goethite and magnetite. In transitional A/B horizon with the pHwater equal to 5.7– this case, the catalyzing effect is pronounced due to 6.5. The lepidocrocite content constitutes 2.9–3.5% the fact that even a negligibly small content of citrate of the clay fraction’s mass in this horizon. In the illu induces lepidocrocite synthesis. It may be citrate that vial horizons of the hydromorphic varieties of these favors the lepidocrocite synthesis in gleyed soils with a soils, the share of lepidocrocite is still higher, reaching high amount of organic substances. However, too high 3.5–7.3% of the clay fraction’s mass. The illuvial hori a concentration of citrate in the solution prevents the zons are more acid (pHwater 4.3–4.7). The results crystallization of any iron hydroxides. Upon the cit obtained testify to the lepidocrocite synthesis from rate excess, only hydromorphic particles of iron Fe(II) upon its oxidation in hydromorphic rather than hydroxides are formed. automorphic soils.
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Lepidocrocirte is confined to certain neoforma tions. In the case when goethite contains excessive (as tions in gleyed soils. When studying them, Schwert compared to monohydrate) water, it is called hydrogo mann and Fitzpatrick [54] subdivided all the ferrugi ethite (αFеООН ⋅ nH2О). The extra water content nous neoformations from South African soils (i.e., may fluctuate from 1–2 to 25–30 mol %. Hydrogoet concretions, ocherous mottles, rohrensteins, etc.) into hites may be subdivided into two groups according to two groups, i.e., hard and soft neoformations. Hard the influence that the extra water exerts on the mineral neoformations are distinguished by their low content properties: those containing water in an amount less of clay minerals, while the soft ones show their high than and above 9 mol % Н2О [5]. The weakly hydrated content. The composition of the iron hydroxides was particles are of distinct texture, fibrous acicular fabric, substantially different. Goethite prevails in hard low and high anisotropy with their density constituting clay neoformations, with lepidocrocite being found in 3.9 g/cm3. Highly hydrated goethite, on the contrary, a small amount there (10 rel % in 70% of the samples). is usually isotropic with a smaller crystal size and a The pattern is different in soft high clay neoforma lower density of 3.6 g/cm3. tions. They contain little goethite, up to 10 rel %. The discovery of goethite in soils does not give Much more lepidocrocite is found there: the maxi much information to a researcher, as this iron hydrox mum of the statistical distribution is registered in the ide is the most widespread in soil. To judge the soil group of samples containing 20–30 rel % lepidocroc conditions that affected the goethite formation, we ite. Thus, lepidocrocite is evidently confined to the need more comprehensive information, e.g., on the neoformations with a high content of clay minerals. degree of the particle hydration, the type of the iron Both constantly high and constantly low oxidation replacement in the lattice, and the scale of this potentials similarly prevent lepidocrocite formation. replacement. The degree of goethite hydration is The Fe(II) deficit is a limiting factor in steppe and for impossible to determine in soils as yet. However, we est automorphic soils. We found lepidocrocite neither are already able to determine the type of iron replace in Voronezh chernozems nor in the gray forest soil in ment and the scale of the replacement. Vladimir opolie using thermomagnetic analysis. How The iron substitution in the goethite lattice is ana ever, a permanently low oxidation potential, in combi lyzed using X ray diffractometry, Mässbauer spectros nation with a very high content of organic acids, ham copy, and transmission electron microscopy. The shift pers lepidocrocite synthesis. According to our data, in the elementary cell parameters observed in the X ray this mineral is absent in the peat soils of Novgorod, diffraction patterns, as well as the alteration of the Moscow, and Kirov oblasts [4]. Mässbauer spectral parameters, is usually related to The principal factors favoring lepidocrocite syn the aluminum substitution for iron. Note, however, thesis are the following: (a) a fluctuating redox poten that the value of the c parameter in the goethite cell tial providing for both the formation of reactive Fe(II) may be reduced not only due to the entrance of alumi and its hydrolysis with the following oxidation to lepi num into the cell but also of manganese and other docrocite, although, in some regions, lepidocrocite is metals. The data of the Mässbauer spectroscopy are formed upon predominating oxidizing conditions similarly ambiguous. Electron microscopy (including (Transbaikal region); (b) a low content of Al and Si in energy dispersion analysis) is an exception, as this the soil solution; (c) a heavy texture facilitating mois method provides clearer information on the type of ture stagnation; (d) a weakly acid soil reaction; and (d) iron replacement in the goethite lattice. Nevertheless, a low summer temperature restraining the activity of the changing parameters of the goethite lattice and the iron oxidizing bacteria. Mässbauer characteristics in a number of soils, for example, Oxisols, are in fact caused by aluminum entering the goethite lattice. In recent years, the appli Goethite cation of highly local analytical methods, including Properties. The chemical formula of this hydroxide electron microscopy, has unambiguously proved the is αFеООН. The free energy of goethite formation presence of aluminogoethite in soils. ΔG = –492.1 kJ/mol [27]. The standard oxidation– The isomorphic replacement of iron for aluminum reduction potential is Ео = 0.71 V. The density is equal reaches 35 mol % in goethite. This replacement to 4.37 g/cm3 [56]. Goethite has an orthorhombic strongly influences the properties of the goethite par structure. Fе3+ ions fill half of the free octahedral posi ticles. Above all, it reduces the size of the goethite par tions formed by oxygen ions in the hexagonal packing. ticles and changes their habitus. Instead of the acicular Each О2– ion has half unsaturated valences, which are particles typical of goethite, aluminogoethite is repre compensated for by the entrance of Н+ with the for sented in soils by aggregates fine particles of irregular mation of a strong H–O–H bond. The elementary cell shape stuck together [5]. parameters are as follows: а = 0.460, b = 1.00, and с = Aluminum raises the chemical stability of alumino 0.302 nm. The large value of the b parameter controls goethite. As a result, aluminogoethite is much less sol the acicular form of the crystals. uble by dithionite containing reagents as compared to The natural diversity of goethite is pronounced in goethite. For instance, the goethite treatment with the different degrees of the hydration and substitu DCB in two replicates was enough for the dissolution
EURASIAN SOIL SCIENCE Vol. 43 No. 11 2010 IRON HYDROXIDES IN SOILS: A REVIEW OF PUBLICATIONS 1251 of goethite in the lateritic soils of Western Australia of iron with manganese. The maximal atomic ratio is and Tasmania; however, either three of four treatments Mn : Fe = 0.37. were required to dissolve the substantially replaced It is important to note that the formation and com aluminogeothite [5]. position of manganic goethite strongly depends on the Formation. Goethites and aluminogoethites are pH value. At pH 4, the Mn : Fe ratio does not reach formed in different geochemical environments. Sig 0.07 in goethite. However, it rises sharply to 0.37 with nificant results have been obtained concerning the the pH growing up to 6. With the further pH growth to connection between the degree of the aluminum sub 8–10 and with a high concentration of manganese in stitution for iron and the conditions of the goethite the solution, the situation changes even more radi formation in soils. The X ray diffractometry of the cally. In this case, manganese forms its own iron con goethite composition in ferruginous soil neoforma taining oxides (hausmannite) instead of entering the tions on the east coast of South Africa attested to the goethite lattice. In other words, manganese is worse low degree of iron replacement for aluminum in the deposited in an acid environment than iron and it pre goethite (0–15 mol % Al) in the neoformations of cipitates better than iron in an alkaline environment. hydromorphic moderately acid and calcareous soils; This agrees with the high mobility of manganese in an however, it reached a high level (15–32 mol % Al) in acid medium known to soil scientists. the goethite formed in the automorphic highly weath Dissolution. Iron (hydr)oxides are known to be dis ered and strongly acid soils. This difference is solved in soils in three different ways [24]: protona explained, above all, by the high activity of aluminum tion–deprotonation, complexing, and reduction. in an acid environment, which raises its ability for Protonation–deprotonation is the least efficient pro cosedimentation with iron to form aluminogoethite in cess. The participation of organic ligands significantly acid automorphic soils. On the contrary, in hydromor increases the rate of the (hydr)oxide dissolution. How phic soils with a neutral pH, aluminum is less mobile, ever, reduction appears to be the most efficient disso and the goethite formation proceeds in an iron contain lution mechanism. ing solution less contaminated with aluminum [31]. Lately, much attention is paid to the mechanism’s The question about the soil gleying’s influence on specifics in the dissolution of iron (hydr)oxides. For the degree of iron substitution in aluminogoethites this purpose, a new analytic method, i.e., mass spec appears to be important. The share of aluminum in trometry with inductively coupled plasma is used. newly formed goethite apparently depends in many Starting from the mid 1990s, this method has been respects on the activity of the Al3+ ions in the soil solu used for the analysis of the composition of stable iron, tion. With the gleying development, the Al3+ activity lead isotopes, and others in soils. The high precision may either rise or fall, and the share of aluminum in data about the isotopic composition of the transitional the goethite may either increase or decrease, respec metals in soils and sediments gave birth to a new tively. quickly developing area in geochemistry, i.e., the A reducing share of aluminum in goethite in gleyed geochemistry of isotopes [17, 18, 30, 34, 35, 38, 47, soils appears to be a widespread phenomenon. A low 61]. Iron isotopes are fractioned as a result of biologi content of aluminum in the goethite (< 5–10 mol % Al) cal and abiotic processes, including bacterial reduc is typical of a number of gleyed soils in Western Aus tion [19]. For example, the biological reduction of tralia and Central Europe [52]. The acidity’s neutral iron hydroxide (ferrihydrite) with Shewanella algae bacteria in newly formed Fe2+ causes a decrease in the ization in the gleyed forest soils is the probable reason. 56 54 Fe(III) reduction consumes Н+ and raises the pH of share of the Fe heavy isotope in favor of the light Fe acid gleyed soils according to the following equation: isotope as compared to the initial ferrihydrite. The following has been learnt about the abiotic + – 2+ Fe(OH)3 + 3H + e = Fe + 3H2O. reduction of iron (hydr)oxides. Significant fraction ation takes place in the course of hornblende’s disso As a result, the solubility of the aluminum contain lution in the presence of various organic ligands ing minerals falls in the neutral environment, which 3+ including oxalate [21, 22]. The solution was enriched decreases the activity of Al ions in the soil solution in light weight iron isotopes with the fractionation and prevents the formation of aluminogoethites. degree correlating with the constant of the bond Another situation is also possible. In the course of strength between the organic ligands and the Fe3+ ion. gleying, the clay mineral particles loose the protective Upon the complexing and goethite reduction, the iron film of iron hydroxides to become more soluble. As a isotopes fractionate; i.e., the 54Fe light atoms are more result, the activity of Al3+ ions in the acid solution rises readily dissolved at the first stage than the heavy 56Fe and so does the probability of aluminogoethite synthesis. [60], which disturbs the initial 54Fe : 56Fe ratio. The replacement of iron with manganese in the Due to the dissolution of the (hydr)oxides, the pro goethite structure is less common in soils. The substi portion between the iron isotopes varies considerably tution for manganese develops upon the synthesis in a in the profile of ferruginated soils [61]. The light laboratory [28]. The decreasing cell parameters in the weight isotopes are preferably transported upon pod synthetic goethite point to the substantial replacement zol formation in acid soils with dissolving iron
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(hydr)oxides. The light weight isotopes are accumu lower. Iron reducing bacteria are less active upon a lated in the semihydromorphic soils with a fluctuating high content of organic substance [11]. Since goethite redox regime under stagnant conditions, upon which (unlike feroxyhyte and ferrihydrite) is easily produced the dissolution of iron hydroxides is controlled by by chemical colloidal precipitation, a high content of reduction [61], and fractionation develops at the iron and Corg in the solution favors goethite accumula expense of Fe2+ formation in the solution rich in light tion in gleyed soil concretions. We observed this pat isotopes [19, 25]. The discovered phenomenon of iron tern in the concretions formed in soils on varved clay. isotope fractionation provides researchers with a new Feroxyhyte and vernadite prevail in the concretions in powerful tool for the study of the biogeochemical cycle ungleyed soil, whereas goethite and aluminogoethite of Fe in soils. predominate in the concretions of gleyic soils with a The replacement of iron with aluminum in the high amount of Fe and Corg. goethite lattice may explain some specifics of the iron Goethite synthesis is possible upon a high concen mineral dissolution in soils. For instance, in Brazilian tration of Fe(III) in the soil water; therefore, it natu Oxisols, microbial reduction is pronounced in the dis rally predominates in strongly ferruginated soils. solution of hematite rather than goethite [43]. In prin Goethite and aluminogoethite are inherited lithogenic ciple, this fact fits the thermodynamic data about the minerals in many tropical soils. Highly ferruginous stronger resistance of goethite to protolysis than neoformations, i.e., ironpans in sandy semihydromor hematite [42]. However, the difference in the equilib phic forest soils, are almost completely composed of 0 goethite [5]. rium constant values is insignificant: logKeq = 0.09 and –0.02 for hematite and goethite, respectively, in The acid–base conditions also affect the goethite the following reactions: synthesis. For example, in soils of Southern Brazil [52], the growth in the pH from 4.6 to 5.8 resulted in a + 3+ 1/2Fe2O3 + 3H = Fe + 3/2H2O. decrease in the goethite share among the iron oxides FeOОH + 3H+ + = Fe3+ + 2H O. from 90 to 39% with a corresponding increase in the 2 hematite share. In the natural minerals, this difference may be Aluminogoethite is a typical mineral in Oxisols smoothed by various unconsidered factors. In Brazil formed by tropical weathering. In that environment, it ian Oxisols, hematite is markedly distinguished from manifests a high degree of saturation with aluminum goethite according to the solubility, because goethite is reaching 13–25 mol %. It is specific that aluminogoet specified by a high amount of aluminum (34 mol %). hite occurs together with gibbsite and aluminohema Since aluminogoethite is much more resistant to tite, which cement the plinthite horizons. reduction as compared to pure goethite, it is clear why only hematite (but no aluminogoethite) particles are Although the laboratory experiments performed reduced. upon a high concentration of manganese in the solu However, another result may also be achieved: tion show the possibility of manganic goethite synthe goethite is dissolved, and hematite is preserved [58]. sis, this hydroxide is formed very seldom in soils. This This is explained by the difference in the size of the is not only because of the manganese deficit in the soil mineral particles: large hematite particles produce solution but also because the conditions favorable for superfine splitting of the Mässbauer spectra at room the biogenic precipitation of manganese do not coin temperature, whereas fine goethite particles do not cide with the conditions favorable for the chemogenic produce a corresponding sextet. Hence, upon predict synthesis of iron from goethite. ing the mineral’s behavior for an alternating redox regime in the soil, the particle size in the hematite– CONCLUSIONS goethite pair appears to be important. Distribution. Goethite is the most widespread iron According to the value of the alteration of the stan hydroxide, in particular, in the soils of humid and dard free energy of the formation reaction, iron semihumid areas. The organic substances in the soil hydroxides are subdivided into thermodynamically surely facilitate the goethite synthesis. unstable (ferrihydrite, feroxyhyte, and lepidocrocite) Soil scientists have registered goethite in many and stable goethite. nodules using Mässbauer spectroscopy. However, now, Thermodynamically unstable hydroxides are it is already a mistake to consider goethite to be a formed either from Fe3+ (ferrihydrite) or from Fe2+ unique iron hydroxide in nodules as was thought ear (feroxyhyte and lepidocrocite). Lately, the application lier. In the last years, the presence of other iron of electron transmission microscopy proved the ferox hydroxides in nodules has been proved by electron yhyte formation in ferromanganic concretions, which sounding microanalysis and other methods [5]. Goet points to the leading role of alternating redox condi hite prevails in concretions in some gleyed soils tions in the synthesis of hydroxides. formed upon a high content of iron and humus acids. The important role of inorganic stabilizers of the In these soils, the probability of chemogenic goethite structure of iron hydroxides is established. Each synthesis is higher, and that of biogenic synthesis is hydroxide has its specific stabilizing chemical ele
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EURASIAN SOIL SCIENCE Vol. 43 No. 11 2010