ISSN 10642293, 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 Email: [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 lightweight 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 solidphase 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. 1244 IRON HYDROXIDES IN SOILS: A REVIEW OF PUBLICATIONS 1245 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 xray 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 xray 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 Xray investiga Being codeposited with iron, Сd2+, Cu2+, Pb2+, and tions performed under the guidance of Schwertmann Zn2+ enter the xray 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.