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Green Rust and Related Iron Containing Compounds: Structure, Redox Properties and Environmental Applications: Part I

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Green Rust and Related Iron Containing Compounds: Structure, Redox Properties and Environmental Applications: Part I 152 Current Inorganic Chemistry, 2015, Vol. 5, No. 3 Editorial Editorial Green Rust and Related Iron Containing Compounds: Structure, Redox Properties and Environmental Applications: Part I 1. HISTORICAL BACKGROUND 1.1. Formation of Green Rust Green rust (GR) was first observed as a corrosion product in 1935 in the early work “Sur la consti- tution de la rouille” of Girard and Chaudron [1]. This compound was called by different names such as “hydromagnetite”, “ferrosic hydroxide” or “ferroso ferric hydroxide” since it was thought to obey to II III the general formula Fe Fe 2(OH)8. In 1950, Arden studied the solubility product of Fe(OH)2 in titra- tion experiments in the presence of small quantity of FeIII species in a sulfated aqueous solutions [2]. He observed the formation of a green “ferrosic hydroxide”, but mentioned that the length of the vari- II III ous pH titration plateau was not in agreement with the previously proposed Fe Fe 2(OH)8 formula. The same year, Feitknecht and Keller observed that GR was an oxidation product of ferrous hydroxide Fe(OH)2 in a chlorinated aqueous medium [3]. They proposed that GR should not only contain hydroxylated ferrous (FeII) and ferric (FeIII) species in its structure but also chloride ions. In 1959, Bernal et al. did a review concerning the structural inter-relationships of the oxides and hydroxides of iron [4]. They observed that the structure of GR was dependent from the nature of the incorporated anion and identified two types of GR structure called “Green rust I” (GRI) and “Green rust II” (GRII). The GRI structure was observed when either chloride or bromide ions were inserted, while GRII formed when sulfate ions were present in the interlayer. The crystallographical cells of GRI and GRII were shown to be rhombohedral and hexagonal, respectively, with quite different in- terplanar distances of the stacking sequence along the c axis (d0 7.6 - 8 Å for GRI and d0 10.9 Å for GRII). GR incorporat- 2- ing CO3 anions was identified later in 1969 by Stampfl [5] as a corrosion product in a water pipe. Stampfl observed that 2- II III GR(CO3 ) had a GRI structure with a similar X-ray diffractogram than pyroaurite (Mg 6Fe 2(OH)16 CO3 • 4H2O), the struc- tural properties of which having been determined one year before by Allmann [6]. The belonging of the GR compounds to the layered double hydroxides (LDH) family was then proposed in several other studies performed between 1970 and 1976 [7-9]. During the same period, the Belgium research group of CEBELCOR (Belgian center for corrosion study) studied the thermo- 2- dynamical properties of GR and the first Eh-pH Pourbaix diagram including GR(SO4 ) was drawn [10]. In the 1984s, Reginald Taylor proposed to use a simple preparation method to synthesize GR by inducing the hydrolysis of Fe(II) species in contact with ferric oxyhydroxides [11]. This method presented some analogy with the “coprecipitation method” of soluble MII and MIII metallic species already used earlier by Feitknecht [12] and Miyata [13] for the synthesis of other types of LDH. A few years later, R. Taylor did a collaboration work with the Mössbauer spectroscopist Enver Murad and published the first 57Fe Möss- bauer spectrum of carbonated GR [14]. A direct determination of the ferric molar fraction x = FeIII / (FeIII + FeII) present in the 2- II III solid structure of GR(CO3 ) was performed and the x values were shown to be situated between 0.25 (Fe : Fe = 3 : 1) and 0.33 (FeII : FeIII = 2 : 1). Between the mid-1980s and the 1990s, studying the physicochemical properties of GR with Möss- bauer spectroscopy became more systematically used [15-18]. The x values measured for GR incorporating other anions, e.g. - 2- Cl or SO4 , were again always situated in the same range, i.e. between x 0.25 and x 0.33. A study concerning the air oxida- 2+ tion of {Fe(OH)2, Fe } mixture allowed also the determination of an accurate value of the standard chemical potential of 2- -1 2- GR(SO4 ) in its anhydrous form, i.e. 0 = -3790 kJ ± 10 kJ mol [19]. In the 2000s, the crystallographic structures of GR(SO4 2- ) and GR(CO3 ) were determined by using Rietveld refinements of high resolution powder X-Ray diffractograms [20-22]. The 2- higher value of the interplanar distance d0 observed for GR(SO4 ) was explained to be due to an interlayer constituted by a bi- layer of sulfate anions and water molecules. In some specific oxidation experiments, i.e. when GR(Cl-) was oxidized rapidly by 2- using a strong oxidant such as H2O2 [23] or when GR(CO3 ) were air oxidized in aqueous solution in the presence of phosphate [24], a fully oxidized GR compound (x = 1) called “ferric GR” [23] was synthesized. Ruby et al. [25, 26] proposed a systematic approach for studying the flexibility of the x ratio of GR based on the use of a mass balance diagram and pH titration curves 2- measured during both coprecipitation and controlled oxidation experiments. GR(CO3 ) was oxidized in situ within the solid with a continuous range of x values situated between 0.33 and 1 in an aqueous medium in the presence of dissolved oxygen and phosphate or in contact with a strong oxidant such as H2O2. The formation of “ferric GR” from GR was shown to be due to a partial deprotonation of the hydroxyl groups by X-Ray photoelectron spectroscopy [27]. More recently, interlayer distances d0 as high as 4.4 nm were observed for GR intercalating linear aliphatic monocarboxylic acids [28]. 1.2. Studying the Chemical Reactivity of Green Rust in Aqueous Solution 1.2.1. Green Rust Synthesized by an Abiotic Pathway As the other members of the LDH family, GR may react with molecules in aqueous solution via several mechanisms such as adsorption, anion intercalation, redox reaction or dissolution-reprecipitation. In 1994, Hansen et al. was the first to study the reactivity of GR towards a series of molecules in anoxic condition. The first work was devoted to study the kinetics of reaction 2- II of GR(SO4 ) towards nitrite and nitrate anions [29, 30]. Due to the presence of both the Fe species and the hydrated interlayer, - + II GR was able to reduce NO3 into ammonium NH4 relatively quickly in comparison with other Fe bearing minerals, e.g. mag- Editorial Current Inorganic Chemistry, 2015, Vol. 5, No. 3 153 2- netite or siderite. Later, GR(SO4 ) was shown to be able to reduce carbon tetrachloride [31] and was transformed into vivianite Fe3(PO4)2•8(H2O) in the presence of phosphate [32]. In the 2000s, the reactivity of GR with a series of pollutants was the sub- ject of several other studies; let us cite here only some of the pioneer works concerning the reduction of pollutants such as se- lenite [33], chromate [34] and uranyl [35]. A review of the various studies performed before 2001 concerning the reactivity of GR in contact with different pollutants can be found elsewhere [36]. Soluble metals such as AgI, AuIII, CuII, and HgII were also 2- 0 0 0 0 0 easily reduced by GR(SO4 ) into Ag , Au , Cu , and Hg , respectively [37]. The metallic Cu aggregates formed at the surface of GR(Cl-) were shown to enhance significantly the reduction rate of organochloride species [38]. Finally, GR accelerated the Fenton oxidation reaction of organic molecules such as phenol [39]. 1.2.2. Formation and Reactivity of Biogenerated Green Rust 2- In 2002, G. Ona-Nguema et al. [40] were able to produce a single phase of GR(CO3 ) by the bioreduction of synthetic lepi- docrocite -FeOOH in contact with dissimilatory iron reducing bacteria (DIRB), i.e. Shewanella putrefaciens. Then, the influ- ence of silicate and phosphate species on the bioreduction of ferrihydrite was studied [41]. It was shown that phosphate induced 2- the strongest effect on the bioreduction rate and on the nature of the mineralization products. GR(SO4 ) as a single phase was shown to be formed in a sulfated aqueous medium devoid of organic compound by using H2 as an electron donor [42]. Lepido- crocite -FeOOH amended with arsenate, citrate, molybdate, phosphate, silicate, tungstate led also to the formation of biogen- erated GR (Bio-GR) [43-44]. Bioreduction of As(III)-adsorbed -FeOOH led to the formation of Bio-GR prior to the formation II of a ferrous-carbonate hydroxide Fe 2(OH)2CO3 [43]. The presence of sorbed species on the surface of -FeOOH slowed down significantly the transformation [43, 44]. In contrast to GR synthesized abiotically, the reactivity of biogenerated GR (Bio-GR) towards molecules such as nitrite, ni- trate, mercury and methyl red was studied only very recently [45-47]. Bio-GR reacted relatively quickly with nitrite ions and, - + interestingly, NO2 species were not reduced into NH4 ions. Bio-GR was oxidized into a poorly crystallized ferric oxyhydrox- ides and such final oxidation products were easily transformed by DIRB into Bio-GR in order to regenerate the initial reactive - material. Measurements with chemical force microscopy showed that Bio-GR reacted in aqueous solution containing NO3 spe- cies much more slowly than chemically formed GR and the quasi-absence of reactivity of Bio-GR was attributed to the pres- ence of bacterial biopolymers [46]. On the contrary to what was observed for nitrate species, Bio-GR and chemically formed GR reacted with quite comparable kinetics in contact with mercury and methyl red [47]. 1.2.3. Natural Occurrences of Green Rust The mineralogist Reginald Taylor was the first to propose that the bluish-green color observed in hydromorphic gleysol could be due to the presence of GR.
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