Characterization of Mn oxides precipitated by passive treatment of Appalachian coal mine wastes

Coal mine drainage sites across Pennsylvania often contain high concentrations of Mn, along with contaminants such as Zn, Ni, Al, Co, and Fe. Passive treatment systems consisting of cobbled limestone/dolostone beds have been used to promote Mn oxidative precipitation, which can also remove other metals via incorporation into or adsorption onto the resulting Mn oxide minerals. Here, we examine Mn oxide precipitates and aqueous geochemistry of a dolostone-lined passive coal mine drainage treatment system in Glasgow near Bellwood, PA. We focus on characterizing the precipitated Mn oxide phases with exceptional detail to bridge our understanding of synthetic Mn oxides from the laboratory into the field. Mn oxides were collected and analyzed using X-ray diffraction, scanning electron microscopy/energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, transmission electron microscopy, X-ray absorption fine structure analysis, and Raman spectroscopy. Water samples were also collected and analyzed for pH and for metal concentrations using inductively coupled plasma-optical emission spectroscopy (ICP-OES). Analyses of the coatings on the cobbles revealed mixtures of tectomanganates and phyllomanganates, including todorokite and both triclinic and hexagonal birnessite/buserite-like structures. Raman mapping showed that Mn oxide phases are intermixed at the micron-scale. Differences in the compositions of triclinic and hexagonal birnessite-like phases were observed in the samples, with triclinic birnessites containing higher Ca, Mg, Al, Co, and Ni concentrations than hexagonal birnessites. Water analyses indicated that the treatment system was effective at removing Mn, Zn, Ni, and Fe as water infiltrated the beds of cobbled dolostone.