Antoine Géhin

Electrochemical formation of a new Fe(II)Fe(III) hydroxy-carbonate green rust: characterisation and morphology

Abstract Electrochemical behaviour of iron in deaerated 0.2 M carbonate/bicarbonate solution pH 9.6 and T =25°C was investigated. Oxidation of iron leads to the formation of green rust as intermediate product and ferrihydrite as ultimate product. Characterisation of green rust was done through FTIR, TEM, XRD and Mossbauer spectroscopy. Results point out differences in the structure and morphology, such as size of hexagonal crystal lattice, parameter c and Fe(II)/Fe(III) ratio, between our electrochemically formed GR and GR1(CO 3 2− ) obtained by oxidation of Fe(OH) 2 . Co-precipitation of Fe(II) and Fe(III) species during the electrochemical procedure leads to a new GR. We propose the following chemical formula for this GR, [Fe (II) 2 Fe (III) 2 (OH) 8 ] 2+ ·[CO 3 ] 2− . However, the possibility that incorporation of both CO 3 2− and Cl − ions could occur is not ruled out.

Coprecipitation of Fe(II) and Fe(III) cations in sulphated aqueous medium and formation of hydroxysulphate green rust

Fe(II)–Fe(III) solutions having a variable Fe(III) molar fraction were titrated with a NaOH solution. Three plateau regions separated by two equivalent points characterise the pH titration curves. The nature and relative abundance of the phases that form during the first and second plateaux are predicted by using the positions of equivalent points in an iron-compound mass-balance diagram. A slowing down of the pH increase is observed at a point of titration curve called E∗ for solutions that have a Fe(III) molar ratio xFe(III)=[nFe(III)/(nFe(II)+nFe(III))] lower than ∼0.27=(2/7). It corresponds to the addition of ∼7±0.5 moles of OH− per mole of Fe(III). Mossbauer spectroscopy shows that the complete formation of green rust is achieved when exactly 7 moles of OH− per mole of Fe(III) are added. This quantity would be larger than expected to form [FeII4FeIII2(OH)12]2+·[SO4·mH2O]2−, if the {Fe(II)/Fe(III)} initial ratio is conserved. This excess is attributed to the adsorption of {[FeII4(OH)8]SO4}2− layers on the ferric oxyhydroxide surface that dissolves it and thus forms the hydroxysulphate; this replaces Misawas ad hoc green complex {FeIImFeIIInOp[OH]q}(2m+3n−2p−q) prior precipitation.

Nanocomposite pyrite-greigite reactivity toward Se(IV)/Se(VI).

A nanopyrite/greigite composite was synthesized by reacting FeCl(3) and NaHS in a ratio of 1:2 (Wei et al. 1996). Following this procedure, the obtained solid phases consisted of 30-50 nm sized particles containing 28% of greigite (Fe(2+)Fe(3+)(2)S(4)) and 72% pyrite (FeS(2)). Batch reactor experiments were performed with selenite or selenate by equilibrating suspensions containing the nanosized pyrite-greigite solid phase at different pH-values and with or without the addition of extra Fe(2+). XANES-EXAFS spectroscopic techniques revealed, for the first time, the formation of ferroselite (FeSe(2)) as the predominant reaction product, along with elemental Se. In the present experimental conditions, at pH 6 and in equilibrium with Se(0), the solution is oversaturated with respect to ferrosilite. Furthermore, thermodynamic computations show that reaction kinetics likely played a significant role in our experimental system.

Microbial Reduction of Lepidocrocite γ-FeOOH by Shewanella putrefaciens; The Formation of Green Rust

Dissimilatory iron-reducing bacteria (DIRB) couple the oxidation of organic matter or H2 to the reduction of iron oxides. The bacterial reduction of a most common well-crystallised ferric oxyhydroxide, γ-FeOOH was investigated using DIRB Shewanella putrefaciens, strain CIP 8040. Experiments were conducted in the presence of neither organic buffer nor phosphate, with formate as electron donor, bicarbonate, and anthraquinone-2,6-disulfonate (AQDS, a humic acid analogue) that influenced the extent of ferric oxide bioreduction. The production of Fe2+ was followed with time. The solid phases obtained after bacterial iron reduction were analysed by transmission Mossbauer spectroscopy (TMS) and X-ray diffraction (XRD). Biogenic formation of green rust 1 compound, which contains carbonate anions, [FeII2FeIII2(OH)8]2+⋅[CO32−]2− was observed. TMS was used to follow the evolution of the green rust abundance during the bacterial culture.

Synthesis by Coprecipitation of Al-Substituted Hydroxysulphate Green Rust FeII4FeIII(2−y)AlIIIy(OH)12SO4,nH2O

Al-substituted hydroxysulphate green rust (Al-GR{SO4}) were synthesised by the co-precipitation of FeII, FeIII and AlIII cations. The Al-GR{SO4} crystals (~50 nm) are significantly smaller than the hydroxysulphate green rust GR{SO4} crystals (~500 nm). The Mossbauer spectrum of Al-GR{SO4} was adjusted with two ferrous doublets D1 and D3 and one ferric doublet D2. Doublet D3 is attributed to FeII ions that have AlIII ions as a first neighbour.