Pipelines at Iron Mountain Mine (CA, USA) carrying acid mine drainage (AMD) from mine workings to water treatment plants develop an iron-rich precipitate, or “scale”, that causes clogging of the pipelines. The AMD water has low pH (2.5-3.2), elevated concentrations of iron and sulfate, and elevated concentrations of associated trace elements, including oxyanion-forming elements (As, Mo, Sb, Se), metals (Al, Cd, Co, Cu, Ga, Mn, Ni, Pb, Zn), and some rare earth elements. The mineralogy and microbial community (16S rRNA analysis of bacteria and archaea) of the scale was characterized. The bulk scale was primarily schwertmannite (nominally Fe8O8(OH)6SO4) with minor amounts of goethite (FeOOH) and jarosite (KFe3(SO4)2(OH)6), and contained laminations with higher abundance of goethite. The scale was host to a diverse group of acidophilic organisms, including several types of Fe(II)-oxidizing organisms, with variable composition as the water traveled from the mine water source down the pipelines. Laboratory experiments confirmed that the scale is a direct result of microbial Fe(II) oxidation, and in situ scale contains a &lt;0.5cm-thick biofilm driving the mineral formation. Trace elements accumulate in the scale, with higher concentrations in the layers with higher schwertmannite content. Although schwertmannite is a metastable phase, it appears to be stable in the pipeline for at least several years, providing a high surface area phase for metal adsorption. Aqueous metal-sulfate complexes affect the extent of adsorption; metal-sulfate complexes adsorb preferentially to positively charged metal ions in low-pH conditions. In addition, biosorption may provide a sink for trace elements in the scale. Deeper understanding of fundamental interactions between the microbial community, hydrogeochemical controls on scale formation and composition, and metal complexation/adsorption through the study of pipe scale may allow for utilization and optimization of microbial Fe(II) oxidation and mineralization in bioremediation of AMD.

Biologically-driven schwertmannite precipitation in acid mine drainage pipelines: Trace metal implications

253rd National Meeting (2017)

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