Geochemical factors affecting the transport and reactivity of metal and pyrite colloids in soils developing on coal mine spoil

The generation of Acid Mine Drainage (AMD) from abandoned coal mines has been long recognized as a source of legacy contamination. Oxidative dissolution of pyrite in mine spoil can act as a non-point source of AMD. Colloidal pyrite present in the soils developing on mine spoil can be transported with soil pore water and be oxidized to generate AMD. The traditional AMD treatments focus on point sources of AMD overlooking the potential AMD leaching from mine spoil and potential colloidal pyrite transport; which leads to a persistent AMD generation in abandoned mine sites even after the treatment. In this study, the geochemical factors affecting the transport of pyrite and metal-bearing colloids through pore water in soils developing on abandoned mine spoil were studied. The soil pore water samples (acidified and filtered; unacidified and unfiltered) were collected using lysimeters with a pore size of approximately 1.3 μm. The unacidified and unfiltered pore water samples were ultra centrifuged to separate the colloidal and aqueous fractions. The acidified and filtered samples were used to determine the total metal concentrations. The colloidal metal concentrations were determined from the difference between total and the aqueous phase metal concentrations. The metal concentrations (Na, Ca, Mg, K, Al, Fe, and Mn) were analyzed using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The morphology, elemental, and mineral composition of colloids were determined by a scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS) and X-ray diffraction (XRD). Preliminary results suggest a significant amount of Fe, Mn, and Al transport in colloidal phase (20+/-36%, 20+/-33%, and 20+/-23% respectively) whereas Na, Ca, Mg and K were present only in the aqueous phase. The X-ray diffraction analyses of the colloids have confirmed the presence of pyrite in the colloidal phase. The SEM-EDS analyses of the colloids have also suggested the presence of framboidal pyrite in colloids and colloidal aggregates. The elemental compositions of the colloids were consistent with pyrite composition (Fe and S). The results from this study will help design more effective AMD treatment system by addressing the leaching of AMD and mobilization of colloidal pyrite from soils developing on mine spoil.