Characterisation of Schwertmannite - Geochemical Interactions with Arsenate and Chromate and Significance in Sediments of Lignite Opencast Lakes
Fe(III) oxyhydroxysulfate schwertmannite [idealised formula: Fe8O8(OH)6SO4], frequently precipitates as a product of sulphide weathering in acidic, sulfate-rich waters. In order to investigate the environmental importance of schwertmannite, geochemical and mineralogical methods were applied in field- and laboratory-experiments. Thereby this thesis focussed on two consequential questions: The first (1) was if schwertmannite could enrich toxic compounds and therefore act as a sink in natural systems. By means of the anions arsenate and chromate, the uptake capacity by adsorption or substitution and consequential changes in crystal structure were characterised. Additionally, a possible re-mobilisation of these compounds was investigated because the metastable schwertmannite easily dissolves or transforms into other minerals. Predominantly, experiments with synthetically produced schwertmannite took place. The other main focus (2) comprised examinations of schwertmannite formation in the chemical environment of acidic lignite mining opencast lakes. Thereby its importance as pH-buffer, its formation probability and its variation by ageing were to be characterised. (1) Schwertmannite, as contained in precipitates of former ore mines, featured high amounts of As (up to 6700 ppm) and Cr (up to 800 ppm). Since there was hardly any detection of these elements in the corresponding drainage waters, the hypothesis arose that schwertmannite acts as a scavenger for these compounds. Assuming that in the redox environment of schwertmannite formation, As and Cr are mostly present in their highest oxidation-level (as arsenate and chromate) these anions were used for coprecipitation, adsorption- and stability-investigations with schwertmannite, to characterise its geochemical interactions. Synthesis (coprecipitation) experiments proved that schwertmannite, normally containing 12 to 14 wt.-% sulfate (both, structurally and adsorptively bound), incorporates up to 10.4 wt.- % arsenate and up to 15.4 wt.-% chromate. While the complete substitution of sulfate by chromate was possible without substantial variation of the crystal structure, the incorporation of arsenate only took place in presence of sulfate or chromate. Oxyanion uptake resulted in an advanced stability of schwertmannite as confirmed in long-term experiments at constant pH. This means that the transformation (due to ageing) to the mineral goethite, as well as mineral dissolution as a consequence of acid addition, was decelerated and toxic compounds were released in lower concentrations compared to sulfate. (2) The chemical environment in opencast lakes of lignite mining frequently shows optimal requirements for schwertmannite formation. Hydrochemistry of surface waters, as well as the sediment composition and partly the colloids were examined in 18 acidic mining lakes (AML) located in Germany. To predict the formation of solid phases in the lakes, chemical processes were modelled by equilibrium calculations. Surface-water composition served as input for calculations of chemical reactions (“PhreeqC”). It was shown that this mineral is the Fe(III)-controlling phase which is in a redox equilibrium with most surface waters. Geochemical analysis of sediment proved that schwertmannite is the primary mineral forming in the AML. It was supposed that a steady supply of Fe(II) into the O2-rich lake provokes schwertmannite precipitation as a consequence of Fe(II) oxidation. The associated release of protons debases the pH of lake water to a value of ~ 3. Further acidification results in mineral dissolution, a reaction which is associated with a release of hydroxide. Therefore, the cycle of precipitation and mineral dissolution adjusts the pH in the AML to a constant value and schwertmannite acts as pH-buffer. The orange-coloured layer at the sediment-water interface of AML, mostly consists of schwertmannite and goethite. It was shown (in ML 77) that with increasing depth the proportion of schwertmannite to goethite decreases. Due to the thermodynamic instability of schwertmannite with respect to goethite, schwertmannite transforms by time (or sediment-depth). Enhanced ageing can be achieved by increasing pH as demonstrated in a stability experiment (1 year) with schwertmannite between pH 3 and 7.
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