CHARACTERIZATION OF PHYLLOMANGANATES FROM AN ACID MINE DRAINAGE TREATMENT SYSTEM IN CENTRAL PENNSYLVANIA

High Mn levels in water outflow from abandoned coal mines in central Pennsylvania remain an environmental challenge. The primary remediation effort uses passive Mn removal beds consisting of a series of open-air dolostone lined ponds that promote oxidation and precipitation of Mn(IV/III) oxides. We investigated the structures and properties of the Mn oxides and the roles of microbial versus nonbiogenic Mn oxidation at a Mn removal site near Glasgow, PA. Powder X-ray diffraction for Mn oxides from the remediation ponds revealed buserite-like phyllomanganates that exhibit strong basal reflections, but otherwise broad, low-intensity peaks. Raman spectroscopy confirmed the layer structure of the Mn oxides, but also revealed that samples from different areas in the ponds had triclinic or hexagonal structures, respectively. Energy dispersive X-ray spectroscopy showed that the interlayer cations are Ca and Mg, with higher concentrations in triclinic relative to hexagonal samples. Additionally, concentrations of Co, Ni, and Cu correlated with Al in the triclinic samples, suggesting the possibility that they are accommodated in lithiophorite-like layers interstratified with the buserite-like layers. TEM revealed that the buserite occurs as nanoplate clusters that measure microns in diameter. SAED patterns revealed four rings of uniform intensities, consistent with randomly oriented nanoplates. HRTEM images reveal regions within curled nanoplates that are intergrowths of amorphous and crystalline material.

Back-scattered electron images of the Mn oxide samples from different areas in the Mn removal beds looked similar with the Mn oxide predominantly occurring as coatings on what appear to be microbial structures. Polished cross-sections commonly show hollow cores surrounded by layers of Mn oxide up to tens of microns thick. Images also show that associated dolomite fragments and diatoms are free of Mn oxide, indicating that the Mn oxide did not simply precipitate as a uniform coating onto particles, microbes, etc. The observations suggest a mechanism by which microbes initiate oxidation of aqueous Mn²⁺ to Mn⁴⁺, thereby forming a surrounding sheath of Mn oxide. Once formed, the Mn oxide surface continues to oxidize Mn²⁺, and deposit Mn oxide, via autocatalysis.