Microbial influence on fast-growing iron oxyhydroxide-based speleothems in the acidic environment of Sitarjevec mine, Slovenia

Abstract

This study investigates the mineralogical and morphological structure of iron oxide/hydroxide speleothems formed in the acid mine drainage (AMD) of the abandoned Sitarjevec mine near Litija in Slovenia (EU). These speleothems grow in the mine shafts under stable environmental conditions (complete darkness, a constant temperature of 10 °C and water with a pH value of 2.2–3.3), but show rapid growth of up to 5 cm/year. Using ion chromatography (IC) and carbon analysis, we detected phosphates, sulfates and carbon in the water dripping from the ceiling onto the stalagmite, indicating that bacteria can indeed colonize the surface of the stalagmite. SEM measurements provided evidence of the bacterial population and the morphology of the colored mineral (precipitate), closely resembling mineralogy of schwertmannite, which was formed in the mine water stored for several months under conditions normally found in the mine. Comparisons of the morphology, elemental composition and structure determined for different locations on the stalagmite (inner/outer part) with the morphological characteristics of the precipitate obtained by SEM/EDS measurements in combination with the analysis of XRD patterns and ATR-IR spectra showed the presence of bacteria and schwertmannite in the form of a hedge-hog particles. Analysis of the ATR-IR spectra of the outer part of the stalagmite and the mine water precipitate confirmed the presence of schwertmannite and a small amount of goethite. Goethite was predominantly present in the inner part of the stalagmite, but the ATR-IR spectra showed no evidence of schwertmannite or bacteria. From the XRD spectra, using the Rietveld method, it was confirmed that the outer part of the stalagmite contains mainly goethite, while the amount of schwertmannite did not exceed 17 wt%, which is also consistent with the results of the infrared spectra analysis. For the interior of the stalagmite, the Rietveld analysis showed no schwertmannite but only goethite.