A sequential extraction method was used to determine which dominant sedimentary mineral phase was involved in phosphorus retention in the sediments of Lake Rotorua and to verify the importance of iron phases in the role as a phosphorus sink. The observed influence of the experimental conditions upon the extent of phosphate adsorption to various iron phases shows a considerable quantity of phosphorus is present in the reducible phase and in the residual mineral phase. The phosphorus associated with iron(III) oxide phases was released into solution under reducing conditions when ferric iron oxide/oxyhydroxides, including amorphous and poorly crystalline Fe(III) phases, were solubilized. The residual primary and secondary mineral phases remained stable in the sediments until they were exposed to extremely acidic media analogous to strongly reducing conditions. Manganese is not involved in phosphorus retention to the same extent as iron. Aluminium phases present were released from surface complexes with relative ease and also from mineral structures under the prevailing conditions. The results show a strong agreement between aluminium and phosphorus suggesting it is associated with various aluminium phases to some extent. The sediments of Lake Rotorua are rich in organic-bound P which is released when organic material is oxidized under conditions analogous to anaerobic degradation. The degradation of refractory organic material represents a significant source of phosphorus for incorporation into diagenetic minerals forming in oxic and anoxic layers of the sediment. Heavy liquid separation of the sediments concentrated the small quantities of dense minerals into a separate fraction and the presence of iron sulfides could be verified. Three density fractions obtained by this method separated the diatoms (d less than 2.6 g cm-3), the silicates (d greater than 2.6 less than 3.7 g cm-3) and the heavy minerals (d greater than 3.7 g cm-3) present in the sediment sample. In the heavy mineral phase spherulitic framboidal pyrite and rhombohedrial siderite were observed by scanning electron microscopy (SEM). Energy dispersive x-ray fluorescence (XRF) analysis of the framboidal pyrite detected significant fluorescence's for sulphur and iron. The elemental analysis of siderite characterised it as an iron-rich, non-sulfidic particle with no phosphorus fluorescence. Particles were also observed that had a variable morphology to the framboidal pyrite minerals but similar ratio of Fe to S in the XRF spectrum. It is likely they are other stable forms of iron sulfides or pyrites in various stages of diagenetic dissolution. Digestion of the three density fractions shows the heavy mineral phase is significantly enriched in sulfur and in iron confirming the presence of sulfides. The sulfide-forming trace metals are concentrating in the heavy mineral phase but a progressive enrichment of trace metals down core is not found in the results. Many of the trace elements show maximum concentrations in the Tarawera tephra. There is a good agreement between iron and phosphorus in both treatments that implies iron phases are the predominant phosphorus fixers in the sediments of Lake Rotorua. However the identity of the phosphorus sink could not be confirmed by SEM or XRF analysis of the heavy minerals. The most likely explanations for the observed concentrations of iron and phosphorus and enrichment in the heavy mineral fraction are the persistence of the highly insoluble crystalline iron oxyhydroxides (goethite) in reducing sediments or the formation of the reduced iron mineral vivianite. Considering the density of vivianite it would have being taken into the heavy fraction by default which would account for the enrichment demonstrated by the solution analysis.

Once similarities between Earth and other terrestrial bodies were discovered, determining the conditions that contributed to the evolution of surface processes on these planets, particularly Mars, is of great interest. More importantly, such research and exploration can provide proof of previous existent life within these near-surface environments. As the past environmental conditions at Mars’ surface are mostly unknown, studies of comparable environments on Earth have been crucial toward deciphering the overall geological understanding of Mars. As the discovery of past conditions on Mars become more absolute, researchers can search for more constrained bio-signatures of life that may have been present. Using the geological similarities between Earth and Mars, analogues can be used to compare the conditions on Mars and Earth and how they evolved over time, further providing more precise understanding of our own environment as it relates to the future. In this study, acid mine drainage (AMD) systems, which are one of the most acidic environments on Earth, were compared to the surface of Mars as a potential analogue to the past conditions of the planet when such acid-impacted environments were widespread at the surface of the planet.

Formamide is a solvent of great interest to prebiotic chemists because it is liquid over a wide range, it is less volatile than either water or HCN, and it possesses a versatile reactivity. When formamide is heated in the presence of minerals or inorganic catalysts, a variety of products including purine nucleobases are generated. Irradiation of formamide reaction solutions with ultraviolet light increases the yield and diversity of products, and eliminates the need for a mineral catalyst. We have also performed formamide reactions in the presence of pyrite, a mineral which is likely to have been available on the primordial Earth, under a variety of atmospheric conditions. Our results indicate the greatest yield and diversity of products result from the combination of a pyrite mineral catalyst, heat, UV irradiation, and a carbon dioxide atmosphere. Purine nucleobases are simple to synthesize in model reactions and they stack well in aqueous solution; it has been hypothesized that the first nucleic acids were composed of only purine bases, and that water-soluble, cationic, aromatic molecules with large stacking surfaces (“”molecular midwives””) may have aided the assembly of the earliest nucleic acid analogs. We have characterized the interactions of various intercalators with a standard DNA duplex as well as with an antiparallel homopurine DNA duplex and have determined that molecules which possess four or more rings and a curved shape interact selectively with all-purine DNA; such molecules can serve as models for putative prebiotic midwives.

Les réactions d'oxydoréduction de l'antimoine (V) et (III) avec la mackinawite (FeS) et du plutonium (III) et (V) avec plusieurs minéraux à fer(II) et des oxydes de fer(III) ont été étudiées dans des conditions d'anoxie. La spectroscopie d'absorption des rayons X fut utilisée pour l'analyse de l'état d'oxydation et de la structure locale du Sb et Pu associés à la phase solide. Après réaction avec la mackinawite, la chukanovite et la magnétite, PuO2, Pu(III) ou des mixtures des deux états d'oxydation ont été observé. A la surface de la magnétite un complexe tridenté du Pu(III) a pu être mis en évidence à l'aide des spectres EXAFS couplé à une simulation de Monte-Carlo utilisant le code de calcul Feff. La quantité relative de Pu(III) est fonction de l'espèce minérale, du ratio solide/liquide, des valeurs pe et pH du système et, potentiellement, de la taille de particule et de la cristallinité de la phase solide de PuO2 en présence de laquelle le Pu(III) existe. Avec la mackinawite à pH 6,2 et à une occupation de surface de 67 nmol/m2 et avec la magnétite jusqu'à pH 8.4 et 56 nmol/m2 de Pu, uniquement du Pu(III) fut trouvé associé à la phase solide. Avec la maghémite contenant du fer(II) résiduel à pH6 Pu(III) et Pu(IV) était, probablement, présents dans des complexes de surfaces similaire à celui formé par le Pu(III) sur la magnétite. Dans les conditions expérimentales (couverture de surface ≤ 77 nmol/m2), aucune formation de PuO2 ne fut observé. Après réaction avec l'hématite et la goethite Pu(IV) était l'état d'oxydation prédominant associé à la phase solide. La sorption et la réduction du Sb(V) avec la mackinawite était fortement fonction du pH. A pH acide la sorption était rapide et Sb(V) fut complètement réduit en Sb(III), formant un complexe Sb(III)-S3 probablement associé à la surface de la mackinawite. La réduction du Sb(V) était couplée à l'oxydation de la mackinawite et la greigite (Fe3S4) fut détectée par XRD. A pH basique, la sorption du Sb(V) est lente et la réduction vers le Sb(III) n'était complète qu'à des ratios de Sb/FeS très bas. Pour des valeurs plus élevé de Sb/FeS la sorption de Sb se faisait en partie par la réduction envers le complexe de Sb(III)-S3 et en partie par une co-précipitation avec le Fe(III). Il a pu être démontré que les minéraux à fer(II) peuvent effectivement contribuer à la réduction et à l'immobilisation de l'antimoine et du plutonium qui sont des contaminants environnementaux d'importance croissante. / Redox reactions of Sb(V) and Sb(III) with mackinawite (FeS) and of aqueous Pu(III) and Pu(V) with various Fe(II)-bearing minerals and Fe(III)-oxides have been investigated under anoxic conditions. X-ray absorption spectroscopy was used to analyze oxidation state and local coordination environment of Sb and Pu associated with the solid phase. After reaction with mackinawite, chukanovite and magnetite, PuO2, Pu(III) or mixtures of the two oxidation states were observed. On magnetite, a tridentate Pu(III) surface complex could be identified from EXAFS combined with Feff-Monte-Carlo simulation. The relative amount of Pu(III) depends on the type of mineral, the solid/solution ratio, the system pe and pH, and, potentially, the particle size and crystallinity of the formed PuO2 solid phase. With mackinawite at pH 6.2 and a surface loading of 67 nmol/m2 and with magnetite up to pH 8.4 and a surface loading of 56 nmol/m2, only Pu(III) was identified associated with the solid phase. With maghemite containing residual Fe(II) at pH6, Pu(III) and Pu(IV) were present in, probably, inner-sphere surface complexes similar to the one formed by Pu(III) on magnetite. Under the given conditions (surface loadings ≤ 77 nmol/m2) formation of PuO2 was not observed. After reaction with hematite and goethite, Pu(IV) was the predominant oxidation state associated with the solid phase. Sorption and reduction of Sb(V) on mackinawite were strongly pH dependent. At acidic pH, sorption was fast and Sb(V) was completely reduced to an Sb(III)-sulfide complex associated with the solid phase. Reduction of Sb(V) was coupled to oxidation of mackinawite and formation of a greigite (Fe3S4) phase could be observed by XRD. At basic pH, Sb(V) was slowly removed from solution and reduction to Sb(III) was complete only at very small Sb/FeS ratios. At higher Sb/FeS, Sb(V) removal occurred partly through reduction to solid phase associated Sb(III)-S3 and partly through co-precipitation with Fe(III). In conclusion, it could be shown that Fe(II) bearing minerals can effectively contribute to the reduction and immobilization of antimony and plutonium, two contaminants of growing environmental importance.

The concomitant occurrence and reported discrepant behavior of uranium and arsenic in water bodies is a major health and environmental concern. This study combined batch and column experiments, hydrogeochemical simulations and XAFS spectroscopy to uncover the exchange mechanisms governing uranium fate between water and scrap metallic iron, minerals and Shewanella putrefaciens surfaces in the presence of arsenic. The main results suggest that both water chemistry and the solid phase composition influence uranium fate in the presence of arsenic. The importance of uranyl-arsenate species as a major control of uranium behavior in the presence of arsenic is shown. The toxicity of arsenic and the presence of nitrate are interpreted as limiting factors of the enzymatic reduction of both toxins. Besides, XANES fingerprinting and EXAFS modeling have confirmed precipitation/co-precipitation of uranyl-arsenates as a major mechanism controlling uranium behavior in the presence of arsenic.