Martine Mullet

Surface chemistry and structural properties of mackinawite prepared by reaction of sulfide ions with metallic iron

Tetragonal FeS1−x mackinawite, has been synthesized by reacting metallic iron with a sodium sulfide solution and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), transmission Mossbauer spectroscopy (TMS) and X-ray photoelectron spectroscopy (XPS). Based on XRD and TEM analyses, synthetic mackinawite exhibits crystallization and is identical to the natural mineral. Unit cell parameters derived from XRD data are a = b = 0.3670 nm and c = 0.5049 nm. The bulk Fe:S ratio derived from the quantitative dispersive energy analysis is practically 1. XPS analyses, however, showed that mackinawite surface is composed of both Fe(II) and Fe(III) species bound to monosulfide. Accordingly, monosulfide is the dominant S species observed at the surface with lesser amount of polysulfides and elemental sulfur. TMS analysis revealed the presence of both Fe(II) and Fe(III) in the mackinawite structure, thus supporting the XPS analysis. We propose that the iron monosulfide phase synthesized by reacting metallic iron and dissolved sulfide is composed of Fe(II) and S(-II) atoms with the presence of a weathered thin layer covering the bulk material that consists of both Fe(II) and Fe(III) bound to S(-II) atoms and in a less extent of polysulfide and elemental sulfur.

U(VI) sorption and reduction by Fe(II) sorbed on montmorillonite.

The influence of surface-bound Fe(II) on uranium oxidation state and speciation was studied as a function of time (6 min-72 h) and pH (6.1-8.5) in a U(VI)-Fe(II)-montmorillonite (Ca-montmorillonite, MONT) system under CO(2)-free, anoxic (O(2) <1 ppmv) conditions. The results show a rapid removal of U(VI) from the aqueous solution within 1 h under all pH conditions. U L(III)-edge X-ray absorption near-edge structure spectroscopy shows that 96% of the total sorbed U(VI) is reduced at pH 8.5. However, the extent of reduction significantly decreases at lower pH values as specifically sorbed Fe(II) concentration decreases. The reduction kinetics followed by X-ray photoelectron spectroscopy during 24 h at pH 7.5 demonstrates the presence of partially reduced surface species containing U(VI) and U(IV). Thermodynamically predicted mixed valence solids like U(3)O(8)/beta-U(3)O(7)/U(4)O(9) do not precipitate as verified by transmission electron microscopy and extended X-ray absorption fine-structure spectroscopy. This is also supported by the bicarbonate extraction results. The measured redox potentials of Fe(II)/Fe(III)-MONT suspensions are controlled by the Fe(II)/hydrous ferric oxide [HFO(s)] couple at pH 6.2 and by the Fe(II)/lepidocrocite [gamma-FeOOH(s)] couple at pH 7.5. The key finding of our study is the formation of a sorbed molecular form of U(IV) in abiotic reduction of U(VI) by sorbed Fe(II) at the surface of montmorillonite.

Galena weathering under simulated calcareous soil conditions.

Exploitation of polymetallic deposits from calcareous mining sites exposes galena and others sulfides to weathering factors. Galena weathering leads to the formation of lead phases (e.g., PbSO(4), PbCO(3)) with a higher bioaccessibility than galena, thus increasing the mobility and toxicity of lead. Despite the environmental impacts of these lead phases, the mechanisms of galena oxidation and the transformation of lead secondary phases, under neutral-alkaline carbonated conditions, have rarely been studied. In this work, an experimental approach, combining electrochemical and spectroscopic techniques, was developed to examine the interfacial processes involved in the galena weathering under simulated calcareous conditions. The results showed an initial oxidation stage with the formation of an anglesite-like phase leading to the partial mineral passivation. Under neutral-alkaline carbonated conditions, the stability of this phase was limited as it transformed into a cerussite-like one. Based on the surface characterization and the formation of secondary species, the weathering mechanisms of galena in calcareous soil and its environmental implications were suggested.

Adhesion of Campylobacter jejuni and Mycobacterium avium onto polyethylene terephtalate (PET) used for bottled waters

Adhesion of the bacteria Campylobacter jejuni and Mycobacterium avium onto polyethylene terephtalate (PET), a polymer widely used within the bottled water industry was measured in two different groundwater solutions. From this, it was found that whilst the percentage cell adhesion for a given strain did not change between groundwater types, substantial variation was obtained between the two bacterial species tested: M. avium (10-30% adhered cells) and C. jejuni (1-2%) and no major variations were measured as a function of groundwater composition for a given strain. To explain this, the interfacial electro-hydrodynamic properties of the bacteria were investigated by microelectrophoresis, with the resultant data analysed on the basis of electrokinetic theory for soft biocolloidal particles. The results obtained showed that M. avium carries a significant volume charge density and that its peripheral layer exhibits limited hydrodynamic flow permeation compared to that of C. jejuni. It was also demonstrated that steric hindrance to flow penetration and the degree of hydrophobicity within/of the outer bacterial interface are larger for M. avium cells. In line with this, the larger amount of M. avium cells deposited onto PET substrates as compared to that of C. jejuni can be explained by hydrophobic attraction and chemical binding between hydrophobic PET and outer soft surface layer of the bacteria. Hydrophobicity of PET was addressed by combining contact angle analyses and force spectroscopy using CH(3)-terminated AFM tip.

Electrochemical and Spectroscopic Analysis of the Arsenopyrite (FeAsS) Oxidation under Calcareous Soil Conditions

In Mexico, the U.S.A. and Australia there are important calcareous mining soil. The mineralogy of this soil is dominated by carbonates rich deposits containing sulfide minerals type skarn or vein associated with economic values (e.g., Au, Zn, Cu) (1, 2). Arsenopyrite is perhaps the most important source of arsenic in calcareous soil (1, 3). In some of these sites important concentrations of arsenic have been identified in water and soil systems, presumably associated with the arsenopyrite weathering process, spite of the semi-alkaline carbonated conditions in this kind of soil (3, 4). Additionally, it has been found the formation of ferric arsenate phases like scorodite, as well as ferric compounds with sorbed arsenic on arsenopyrite crystals (4). This fact shows the feasibility of the arsenopyrite weathering in this kind of soil; however, the mechanisms and interfacial processes implied during the arsenopyrite weathering are not clear. Therefore, the aim of this study was to evaluate the oxidation process of arsenopyrite, and to elucidate the oxidation products under similar weathering conditions than those found in calcareous soil. This goal was reached by using electrochemical, geochemical and spectroscopic studies of arsenopyrite before and after leaching with a carbonated solution. The devices for arsenopyrite weathering process of consisted of Buchner funnels (mini-cells) designed to promote the natural oxidation of sulfides by applying two humid cycles per week, with similar results as those obtained with humid cells proposed by ASTM (1999). A similar methodology was successfully employed for evaluate the reactivity in pyritic waste from Mexico and Canada (5). The geochemical study suggests important dissolution cycles of arsenic and iron from arsenopyrite (Fig. 1). In addition, these processes were accompanied by sulfates generation and consumption of total alkalinity, however, not any significant changes of pH was observed during these processes in system. These results could be indicative of the precipitation of iron (II) compounds, such as siderite (FeCO3). The voltammetric study of arsenopyrite pristine sample indicated the initial oxidation of arsenopyrite under calcareous conditions. It was found the formation of surficial metal-deficient layers (e.g., Fe1xAs1-yS) and, in agreement with geochemical study, probably the formation of ferrous carbonates. However, higher potentials brought the formation of ferric hydroxide, as indicated in positive-going sweeps in Fig. 2 (peak R1, without the addition of H2AsO4 in system). Additionally, if the electrochemical system is previously enrich with H2AsO4species, the formation of ferric hydroxide is suppressed and the interaction between As(V) and iron species seems to occur (Fig. 2). Figure 1. Chemical evolution of leachate for As (●) and iron (▲) as a function of the alteration time of arsenopyrite.

Mössbauer and X-ray diffraction study of Mackinawite FeS1−x air oxidation

The dry oxidation of mackinawite FeS1−x at room temperature and atmospheric pressure has been studied by X-ray diffraction (XRD), transmission electron microscopy (TEM) and transmission Mossbauer spectroscopy (TMS). After 7 days of air-oxidation, slow disappearance of mackinawite occurs with concomittant appearance of greigite Fe3S4, elemental sulphur S(0), as well as poorly crystallized iron (oxyhydr)oxides (magnetite Fe304 and probably goethite ±-FeOOH). Greigite, which is an intermediate reaction product, and mackinawite have disappeared after 6 months of oxidation, and the final oxidation products are elemental sulphur and iron (oxyhydr)oxides.