Current Inorganic Chemistry (v.5, #3)

Meet Our Editorial Board Member by Henryk Kozlowski (151-151).

Reaction of Uranium(VI) with Green Rusts: Effect of Interlayer Anion by Drew E. Latta, Maxim I. Boyanov, Kenneth M. Kemner, Edward J. O'Loughlin, Michelle Scherer (156-168).
Green rusts are widely recognized as an important metastable intermediate phase in Fe biogeochemical cycling and Fe metal corrosion and are strong reductants capable of reducing a widerange of contaminants. Here we investigate the effect of interlayer anion on the reaction of green rusts with hexavalent uranium (U(VI)). We react three synthetic green rusts, including carbonate, sulfate, and chloride green rust, as well as pyroaurite, a redox-inactive Mg(II)-Fe(III) structural analog of carbonate green rust with U(VI). The majority of U(VI) (> 80%) was removed from solution in about an hour at pH 8.0 in 0.1 M N-Tris(hydroxymethyl)methyl-3-aminopropanesulfonic acid (TAPS) buffer. Similar kinetics of U(VI) uptake on green rusts and pyroaurite suggest that the observed uptake kinetics reflect an adsorption step rather than reduction of U(VI) by structural Fe(II) in the green rusts. X-ray absorption spectroscopy (XAS) of the green rust solids indicates significant reduction of U(VI) to U(IV) for all three green rusts, with complete reduction observed for sulfate and chloride green rust and varied extents of reduction (34 to 100%) observed for carbonate green rust depending on the solution conditions. No reduction of U(VI) was observed in the presence of pyroaurite, consistent with the absence of Fe(II) in the pyroaurite structure. The decreased extent of U(VI) reduction observed with carbonate green rust in TAPS buffer may be due to modification of the redox reactivity of U(VI) or green rust due to interaction with the TAPS buffer molecules. XAS results indicate that U(VI) was reduced to U(IV) and was present as a monomeric-type U(IV) species in the presence of TAPS buffer. In deionized water, however, carbonate green rust reduced U(VI) to nanoparticulate UO2. Green rusts may be an important phase in the fate and transport of U(VI) in the contaminated subsurface, or under conditions where it forms on corroding U-bearing waste containers.

The ease of preparation as well as the fine tuning of their chemical composition makes layered double hydroxides (LDH) very attractive for a large variety of applications. The aim of this feature article is to provide the keys that permit to finely tune the structure of the materials in controlling simple parameters involved in the synthesis procedures. In a first part, the influence of the synthesis parameters on the crystallinity and morphology of MII-FeIII LDH (MII = NiII, MgII and CoII) is studied. In a second part, further insight into the variability of composition of the layer is proposed to explain the interdependence between the cationic nature of the layer and its layer charge flexibility. The third part is devoted to the anion exchange property of LDH. We showed that a previously proposed method for anionic exchange can be successfully applied to all couple of cations in their range of composition. Finally, a molecular description of the interlayer organisation is given.

Crystal Chemistry of Iron Containing Cementitious AFm Layered Hydrates by Guillaume Renaudin, Adel Mesbah, Belay Zeleke Dilnesa, Michel Francois, Barbara Lothenbach (184-193).
The crystal structure of the three main Fe-containing AFm phases (Al2O3-Fe2O3-mono: family of lamellar calcium alumina-ferrite hydrates) encountered in cement hydration process are characterized and compared with their Al-analogs. This includes AFm phases containing sulfate (which is present in Portland cement to regulate the hydration kinetic), carbonate (which is present in Portland cements, or originates from atmospheric carbon oxide) and chloride (either from the water used or from the environment). The results show that Fe-AFm and Al-AFm compounds are not (or rarely) isostructural. Iron in AFm phases does not simply substitute aluminium. Fe-carbonate has a rhombohedral symmetry whereas Al-carbonate has a triclinic symmetry, with carbonate anions located in different crystallographic sites in both compounds. Fe-Friedel's salt corresponds to a 3R polytype whereas Al-Friedel's salt corresponds to a 6R polytype. Both compounds have a temperature dependent transition with two different HT- and two different LT-polymorphs descriptions (HT: high-temperature, LT: low-temperature). Only Fesulfate and Al-sulfate are isostructural. Despite this isostructural feature, only limited solid solutions have been observed between both sulfate end-members. In a general way, this system (when considering sulfate, carbonate and chloride with aluminium and iron) leads to extremely complicated subsystems with limited solid solutions. The crystallographic studies and comparisons developed here have been fully completed by thermodynamic characterisations in order to make possible thermodynamic modelling of the hydrates assemblage during the hydration process and the aging of Portland concrete.

Current Trends in Iron Complexes Intercalated Layered Double Hydroxides by Claude Forano, Mustapha Abdelmoula, El Mouloudi Sabbar, Vanessa Prevot, Christine Taviot-Gueho (194-207).
In this review, we comprehensively summarize the synthetic approach that can be developed for the design and the preparation of iron complex intercalated LDH structure which mainly involves oxalate, citrate, ferrocyanide, ferrocene, N,N'-ethylenebis(salycylideneiminato) (salen), porphyrine and proteins. Then, we propose a detailed overview of the characterization tools allowing a deep insight into the iron complex intercalated LDH. Finally, the potential applications reported in the literature are also considered highlighting the huge potential of such kind of iron based nanostructured layered materials.

Hydroxyl-chloride green rust (GR(Cl-)) was mainly transformed to lepidocrocite (γ- FeOOH) due to the oxidation reaction via aqueous solution containing different solid substances such as metallic Zn, metallic Sn and natural sand. The particle sizes of γ-FeOOH obtained from suspensions containing Sn and natural sand were close to that obtained from the suspension without coexisting solid substances. However, particles of γ-FeOOH obtained from the suspension containing Zn were smaller than those obtained from the suspension without coexisting solid substances. In addition, the crystallite size of the former was also smaller than that of the latter. These results are probably related to the adsorption of Zn ions dissolved from the coexisting metallic Zn.

The activity of microorganisms is a key component of the biogeochemical cycle of Fe in natural systems, where green rusts are often observed as products of microbially driven redox processes. To better define the factors that control green rust formation during microbial Fe(III) reduction, we examined the effects of the presence of an electron shuttle [9,10-anthraquinone-2,6-disulfonate (AQDS)] and phosphate on akaganeite (β-FeOOH) bioreduction by the iron(III)-reducing bacterium (IRB) Shewanella putrefaciens CN32. Framboidal magnetite was the principal secondary mineral formed during akaganeite bioreduction in the absence of phosphate; this is the first time framboidal magnetite has been reported as a product of microbial Fe(III) oxide reduction. Framboidal magnetite was less crystalline when formed in the presence of AQDS than without AQDS and over time was further reduced to chukanovite. Carbonate green rust was the primary secondary mineral observed from akaganeite bioreduction in the presence of phosphate, with and without AQDS; however, siderite was also observed in the presence of AQDS. This first report of green rust as a product of akaganeite bioreduction expands the range of Fe(III) oxides that can be transformed to green rust by IRB, suggesting that the reduction of Fe(III) oxides such as ferrihydrite, lepidocrocite, and akaganeite by IRB is a key process leading to the formation of green rusts in aquatic and terrestrial environments.

Environmental Conditions Affecting Iron Sulfides Reactivity and Transformation of Iron Oxyhydroxide Compounds by Rene H. Lara, Martine Mallet, Miguel A. Escobedo, Hugo Ramirez-Aldaba (225-232).
Oxidation of iron sulfide minerals (mainly pyrite and pyrrhotite) occurs in mine waste and other mining environments which is a natural process associated to weathering reactions and produced by waste spillage due to eolic or pluvial dispersion. Iron sulfides environmental weathering results in secondary sulfur and iron (III) oxyhydroxides (IOH's) formation whose mineralogical transformation affects iron sulfides reactivity inducing sulfides passivation and/or enhanced mineral reactivity. Review reports considering IOH's dynamic behavior are sparse and barely found. In this review work, authors up-date iron sulfides weathering and IOH's relationship and also include some original and experimental data, aiming to establish main IOH's occurrence and stability during iron sulfides weathering under environmental conditions in mining environments.