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Schwertmannite, a ferric oxyhydroxysulfate mineral typically found under acidic, high sulfate and iron aqueous conditions, such as acid mine drainage environments, was studied using analytical high resolution transmission electron microscopy (HRTEM). HRTEM offers advantages over bulk techniques such as powder x-ray diffraction and pair distribution function (PDF) analysis of synchrotron data, in its ability to discern multiple phases within poorly crystalline nanominerals. Based on extensive HRTEM observations of both natural and synthetic schwertmannite samples, the authors suggest that schwertmannite should not be described as a single phase mineral with a repeating unit cell, but as a polyphasic nanomineral with crystalline areas spanning less than a few nanometers within an amorphous matrix. The few visible lattice fringes observable in both natural and synthetic schwertmannite agree well with d-spacings of goethite (and jarosite in natural samples) implying that the transformation from schwertmannite to these phases occurs as a gradual structural reordering at the nanoscale. In the synthetic study, the complete transformation from schwertmannite to goethite nanorods and nanoparticles within 24 hours at 75°C was observed, indicating a low energetic barrier to schwertmannite's phase transformation. We also found that amorphous silica can be intimately entrained within natural schwertmannite, and that high concentrations of arsenic can be held in close association of nanocrystalline regions of the mineral. / Ph. D.

New types of laboratory reactors that are highly customizable, low-cost and easy to produce are needed to investigate low-temperature geochemical processes. We recently showed that desktop 3D printing stereolithography (SLA) can be used to efficiently fabricate a mixed flow reactor (MFR) with high dimensional accuracy comparable to traditional machining methods (Michel et al., 2018). We also showed that the SLA method allowed for the addition of complex features that are often beyond the capabilities of traditional methods. However, the stability of 3D printed parts at low-temperature geochemical conditions has not been fully evaluated. The objectives of this work were twofold: 1) to provide a framework for assessing the stability and compatibility of SLA printed materials at geochemically relevant conditions, and 2) to show how 3D printed specialty devices can enable new laboratory geochemical experiments. Part 1 of this Master's thesis presents findings for enhancing mechanical and solvent resistance properties of a commercial 3D printing material (Formlabs Clear) by UV post-curing procedures and also provide data showing its stability in aqueous solutions at pH 0, 5.7, and 12 for periods of up to 18 days. Thermal degradation patterns, mechanical analysis, and leachable fraction data are provided. Part 2 shows experiments coupling 3D printed reactors and flow devices for in situ small-angle x-ray scattering (SAXS). Schwertmannite (pH 2.7) and goethite (6.2) are precipitated from solution using various setups and observed differences in growth rates are discussed. The data show the potential of 3D printing for enabling novel laboratory geochemical experiments. / MS

Iron is often present in leach liquors produced in chemical and hydrometallurgical processes. It is known that voluminous iron precipitates with high impurity values are formed if the conditions during its formation are not controlled well. These products are also often difficult to treat in downstream processes. This study therefore focused on the determination of product quality parameters for the production of good quality iron precipitates from zinc-rich solutions. Special attention was given to the quality of metastable phases such as ferrihydrite and schwertmannite formed at elevated temperatures and in the pH range 1.5 to 3.5 in a continuous crystallizer. These phases are produced over a range of supersaturation levels with the best quality products formed at lower supersaturation. It was shown that most industrial processes are operated well above the metastability limit at relatively high supersaturation. However, stagewise precipitation of iron, even above the metastability limit, yielded better quality products. It was also shown that localized supersaturation levels could be controlled through changes in the micro and macromixing environments. The three-zone model approach was used to improve the quality of ferrihydrite and schwertmannite precipitates. Changes in the reactor design and the position of reagent feed points also impacted on the quality of the precipitates. Control over the localized supersaturation not only ensures the production of good quality nuclei, but also impacts on particle growth, which is required to make downstream processing of precipitates possible. In precipitation processes, growth mainly takes place through agglomeration as the rate of molecular growth is generally low. The final quality of iron precipitates is greatly influenced by the quality of the agglomerates formed during iron precipitation. A Hadamard matrix was used to indicate the relative importance of the most relevant operating parameters for the formation of good quality iron precipitates. / Thesis (PhD (Metallugical Engineering))--University of Pretoria, 2007. / Materials Science and Metallurgical Engineering / unrestricted

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Natural nanominerals are abundant in Earth's critical zone and important in innumerable environmental processes that affect water quality. The chemical behavior of many natural nanominerals is related to their extreme small size (<10 nm) and high surface area. Atomic structural and chemical heterogeneity are also important factors affecting nanoparticle reactivity, and are a consequence of the mechanisms and complex (natural) conditions by which they form. The relationships between these factors remain poorly understood and limit our ability to predict the formation, transformation, and chemical behavior of natural nanominerals in the environment. We are using a poorly crystalline ferric hydroxysulfate nanomineral, schwertmannite, as a model system to understand the effect of formation conditions, specifically solution chemistry, on its physico-chemical characteristics. Previous studies indicate schwertmannite has highly variable bulk sulfate (Fe/S molar from 3-15) and water contents (Caraballo et al., 2013). In addition, both natural and synthetic schwertmannites have recently been described as "polyphasic" (i.e., consisting of sulfate-poor, goethite-like ordered domains embedded in a sulfate-rich, amorphous material) from observations using transmission electron microscopy (French et al., 2012). We hypothesize that solution chemistry at the time of schwertmannite formation directly affect its composition and structure. Using a factorial experiment design, we investigated the effects of increasing solution sulfate concentration ([SO4]/[Fe] at 1, 2, 3 and 5) and pH (2.4-5.6) on the crystallinity and composition of the products. Ferric hydroxide and hydroxysulfate solids were precipitated in batches by the rapid oxidation of Fe(II) by hydrogen peroxide, similar to what is seen in natural environmental systems. Sulfate and hydroxide concentrations were varied by addition of NaSO4 and NaOH, respectively. Solids were characterized using synchrotron X-ray diffraction (XRD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), inductively coupled plasma-mass spectrometry (ICP-MS), scanning electron microscopy (SEM), and high resolution- transmission electron microscopy (HR-TEM). Our results show that schwertmannite is the only precipitate formed at low pH and that goethite rapidly becomes dominant at pH > 3.5. High-resolution TEM showed our synthetic schwertmannite samples consist of poorly crystalline goethite-like nanodomains within an amorphous solid, similarly seen in previous results. ICP-MS results reveal a narrow Fe/S molar ratio of 4.5 ±0.1 for our synthetic schwertmannite, which suggests that schwertmannite chemical composition does not depend strongly on pH or initial solution sulfate concentration. Increasing pH from 2.4 to 3.2 also has little effect on the crystallinity, bulk Fe/S ratio and water contents of schwertmannite. Increasing solution [SO4]/[Fe] also has little to no impact on crystallinity, water content or the amount of sulfate incorporated in schwertmannite. Thus, schwertmannite crystallinity and composition is not affected by initial solution sulfate and concentration under our experimental conditions. Thermal analysis allows us to independently measure OH and SO4 content in synthetic schwertmannite. In doing so, we propose a more accurate chemical formula (Fe8Oz(OH)24-2z-2x(SO4)x). The average stoichiometry based on thermal analysis of schwertmannite precipitated at [SO4]/[Fe] = 1 and pH ranging from ~2.4 2.9 is Fe8O6.51(OH)8.4(SO4)1.28. Interestingly, the calculated number of moles of oxygen is less than 8, which suggests that the standard formula Fe8O8(OH)8-2x(SO4)x is incorrect. These results for synthetic samples provide important constraints for future studies aimed at better understanding the formation, compositional variability and chemical behavior of natural schwertmannite. / Master of Science

Les eaux et les plantes des zones volcaniques sont souvent très pauvres en sélénium, alors que les teneurs totales observées dans les sols sont normales. Ceci est très spécifique aux zones volcaniques et semble dû aux argiles des sols qui ne sont pas des phylloaluminosilicates comme dans la plupart des autres régions du globe, mais des aluminosilicates tubulaires, les imogolites. Ces minéraux sont dotés d'une très grande surface spécifique (400-1000 m2 g-1 selon la méthode) et réagissent avec les anions de sélénium en formant des complexes de sphère interne, liées par des liaisons covalentes, qui réduisent la mobilité du sélénium en affectant sa biodisponibilité. D'un autre cote, l'interaction de la surface externe de ces nanotubes d'imogolite, similaire a la surface (001) de la gibbsite, avec l'eau a été étudié par des simulations de dynamique moléculaire. Les simulations décrivent une surface plus hydrophobique que celle de la gibbsite, étant l'hydrophobicite induite par la courbure de la structure. Ce résultat a des importantes implications environnementales, car il peut expliquer la formation de complexes organo-minerales entre les aluminosilicates nanotubulaires ou nanoparticulés. Comme dernier résultat, une structure pour la schwertmannite, un oxyhydroxy-sulphate de fer nanoparticulé, a été décrite a partir de données de diffraction de rayons X de haute énergie et des simulations ab-initio.

The mining of sulphidic ore to extract metals such as zinc and copper produces huge quantities of waste material. The weathering and oxidation of the waste produces what is commonly known as Acid Mine Drainage (AMD), a dilute sulphuric acid rich in Fe(II) and heavy metals. This thesis serves to summarise five papers reporting how the precipitation of Fe(III) phases can attenuate the contamination of heavy metals by adsorption processes. Schwertmannite (Fe8O8(OH)6SO4) is a common Fe(III) mineral precipitating in AMD environments at pH 3-4. The stability and surface chemistry of this mineral was investigated. It was shown that the stability depended strongly on pH and temperature, an increase in either promoted transformation to goethite (α-FeOOH). Two pH dependent surface species of SO42- were detected with infrared (ATR-FTIR) spectroscopy. The adsorption of Cu(II), Pb(II) and Zn(II) to schwertmannite occurred at lower pH than to goethite, whereas Cd(II) adsorption occurred in a similar pH range on both schwertmannite and goethite. Extended x-ray absorption fine structure (EXAFS) spectroscopy suggests two surface species for Cu(II) and Cd(II) at the schwertmannite surface. Cu(II) adsorbs monodentately and Cd(II) bridging bidentately to adsorbed SO42-. Both metal ions also adsorb in a bridging bidentate mode to the surface hydroxyl groups. At pH 7.5 up to 2.7 μmol Cd(II) m-2 could be adsorbed to schwertmannite, indicating a large adsorption capacity for this mineral. The acid-base properties of two NOM samples were characterised and could be well described as diprotic acids below pH 6. The adsorption of NOM to schwertmannite and goethite was very similar and adsorption occured in a very wide pH range. High concentrations of NOM increased the adsorption of Cu(II) to goethite at low pH whereas a slight decrease was noted at low concentrations of NOM. No effect was detected in the schwertmannite system. The formation of Fe(III) phases from precipitation of AMD was shown to be very pH dependent. At pH 5.5 a mixture of minerals, including schwertmannite, formed whereas at pH 7 only lepidocrocite (γ-FeOOH) formed. The concentration of Zn(II) in AMD could by adsorption/coprecipitation be reduced to environmentally acceptable levels.

<p>The mining of sulphidic ore to extract metals such as zinc and copper produces huge quantities of waste material. The weathering and oxidation of the waste produces what is commonly known as Acid Mine Drainage (AMD), a dilute sulphuric acid rich in Fe(II) and heavy metals. This thesis serves to summarise five papers reporting how the precipitation of Fe(III) phases can attenuate the contamination of heavy metals by adsorption processes. </p><p>Schwertmannite (Fe8O8(OH)6SO4) is a common Fe(III) mineral precipitating in AMD environments at pH 3-4. The stability and surface chemistry of this mineral was investigated. It was shown that the stability depended strongly on pH and temperature, an increase in either promoted transformation to goethite (α-FeOOH). Two pH dependent surface species of SO42- were detected with infrared (ATR-FTIR) spectroscopy.</p><p>The adsorption of Cu(II), Pb(II) and Zn(II) to schwertmannite occurred at lower pH than to goethite, whereas Cd(II) adsorption occurred in a similar pH range on both schwertmannite and goethite. Extended x-ray absorption fine structure (EXAFS) spectroscopy suggests two surface species for Cu(II) and Cd(II) at the schwertmannite surface. Cu(II) adsorbs monodentately and Cd(II) bridging bidentately to adsorbed SO42-. Both metal ions also adsorb in a bridging bidentate mode to the surface hydroxyl groups. At pH 7.5 up to 2.7 μmol Cd(II) m-2 could be adsorbed to schwertmannite, indicating a large adsorption capacity for this mineral.</p><p>The acid-base properties of two NOM samples were characterised and could be well described as diprotic acids below pH 6. The adsorption of NOM to schwertmannite and goethite was very similar and adsorption occured in a very wide pH range.</p><p>High concentrations of NOM increased the adsorption of Cu(II) to goethite at low pH whereas a slight decrease was noted at low concentrations of NOM. No effect was detected in the schwertmannite system. </p><p>The formation of Fe(III) phases from precipitation of AMD was shown to be very pH dependent. At pH 5.5 a mixture of minerals, including schwertmannite, formed whereas at pH 7 only lepidocrocite (γ-FeOOH) formed. The concentration of Zn(II) in AMD could by adsorption/coprecipitation be reduced to environmentally acceptable levels.</p>

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