Iron mineralogy across the oxycline of a lignite mine lake

Abstract

International audienceIron-rich pelagic aggregates of microbial origin named “iron snow” are formed in the water column of some acidic lignite mine lakes. We investigated the evolution of Fe mineralogy across the oxycline of the Lusatian lake 77, Germany at two sampling sites differing by their pH and mixing profiles. The central basin (CB) of this lake shows a dimictic water regime with a non-permanent anoxic deep layer and a homogeneous acidic pH all over the water column (pH 3). In contrast, the northern basin (NB) is meromictic with a permanently anoxic bottom layer and a pH increase from pH 3 in the mixolimnion (superficial part of the lake) to pH 5.5 in the monimolimnion (anoxic bottom layer). Fe minerals above and below the oxycline were identified using X-ray Absorption Spectroscopy (XAS) at the Fe K-edge and further characterized down to the atomic scale by High Resolution Transmission Electron Microscopy (HRTEM) and Scanning Transmission Electron Microscopy (STEM) coupled to Energy Dispersive X-ray Spectroscopy (EDXS). We explored local Fe redox state and C speciation using Scanning Transmission X-ray Microscopy (STXM) at the Fe L2,3-edges and C K-edge. Schwertmannite [Fe8O8(OH)8-2x(SO4)x] identified as the sole Fe mineral in CB, was polycrystalline, consisting in the aggregation of nanodomains of 2–3 nm each one exhibiting the crystal structure of schwertmannite. In contrast, schwertmannite was partly (40%) converted to aluminous ferrihydrite when reaching the oxycline in NB. This mineralogical transformation was most probably due to a combination of abiotic and microbial anaerobic processes promoting pH increase and release of Fe(II) (e.g. via heterotrophic Fe(III) reduction) that induce the catalytic hydrolysis of schwertmannite to ferrihydrite. Mineral products were stabilized in the monimolimnion by the adsorption of aluminum, silicate and organic matter. Noteworthy, local Fe redox state heterogeneities were observed, with a few areas enriched in Fe(II) as evidenced by STXM analyses at the Fe L2,3-edges. These local redox heterogeneities could arise from microbial activity (e.g. Fe(III) and/or sulfate reduction). All these results provide an in-depth mineralogical overview of iron phases forming in lake 77 as a basis for future investigations of microbial vs. abiotic parameters controlling their stability and transformation

Arsenic-Imposed Effects on Schwertmannite and Jarosite Formation in Acid Mine Drainage and Coupled Impacts on Arsenic Mobility

“…Schwertmannite was the main phase in ochreous precipitates formed at the adit entrance and in pools downstream of the sediment retention pond and was a major phase in all other samples downstream of zone A at the study site. Schwertmannite is a nanocrystalline Fe­(III) oxyhydroxy-sulfate mineral that is perhaps the most common direct precipitate of Fe­(III) from acid-sulfate waters in the pH range of 2–4. , As such, it has been widely documented in acid-sulfate environments, including metalliferous mine drainage, ,,,,, coal mine drainage, ,− mine pit lakes, − and acid-sulfate soils. − …”

Biologically Assisted One-Step Synthesis of Electrode Materials for Li-Ion Batteries

“…XAS spectra were merged and normalized and extended x-ray absorption fine structure (EXAFS) data were extracted using the Athena software [ 42 ]. X-ray absorption near edge structure (XANES) and k 3 -weighted EXAFS data were analyzed using the linear combination fit (LCF) procedure in Athena and a custom-built software based on the Levenberg–Marquardt minimization algorithm, respectively, as described in [ 43 , 44 ].…”

Elucidating heterogeneous iron biomineralization patterns in a denitrifying As(iii)-oxidizing bacterium: implications for arsenic immobilization

“…[27][28][29][30] Additionally, NDFO bacteria can precipitate these Fe minerals at different cell sites, including the periplasm and the cell surface. [31][32][33][34][35][36][37] The periplasm is a crucial cellular compartment for energy generation and the transit of nutrients and waste, therefore, its encrustation with minerals is thought to be lethal to the cells. 38 Furthermore, it has been proposed that Fe(II) oxidation in anaerobic denitrifying conditions is a detoxification mechanism rather than an energy conserving reaction, [38][39][40] where the Fe(II) concentration in these incubations is typically higher than in environmental conditions and could be the reason for this cell mineralization.…”