Chemical weathering of pyrite via oxidative dissolution can generate Fe(III)-bearing colloids at acid mine drainage (AMD) sites; however, the potential for physical weathering of pyrite-bearing materials in mine spoil and subsequent release and transport of colloidal pyrite and associated trace metals has not been studied. We monitored colloidal metal transport in soil developing on abandoned coal mine spoil to systematically study the elemental and mineralogical composition of colloids and determine their contribution to base and trace metal transport. Soil pore water samples were collected using lysimeters with a pore size of ~1.3μm, and centrifugation was used to separate the colloids (&lt;10μm) from the solution. Metal concentrations of Na, Ca, Mg, K, Si, Al, Mn, Fe, Cu, and Zn were analyzed. Results show a higher colloidal contribution relative to total metal concentrations for the transport of Zn (54%), Mn (43%), Fe (23%), and Cu (14%) in the porewater.  In contrast, all base metals were primarily present in the aqueous phase (&lt;10% in colloidal fraction). 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). XRD analyses indicated that colloids were dominated by phyllosilicates (biotite, muscovite, and kaolinite) and contained between 1 to 10% hematite, goethite, arsenopyrite, and chalcopyrite, and SEM-EDS analyses identified the phases with composition and morphology consistent with framboidal pyrite. Our study indicates that the physical weathering of pyrite-bearing coal shale can generate colloidal pyrite which is mobilized and transported by soil porewater. Further SEM-EDS analyses of colloids present in the soils revealed that trace element (Cu, Mn, and Zn) partitioning during pyrite oxidative dissolution is controlled in part by whether subsequent Fe(III) precipitation occurs immediately (e.g. pseudomorphic replacement of pyrite) or after transport in pore water (e.g., Fe(III) coatings on primary grains and/or formation of discrete Fe(III)-bearing phases. On-going Transmission Electron Microscopic analyses of colloidal pyrite and secondary Fe mineral surface coatings prepared by a Focused Ion Beam (FIB) instrument aims to provide a better understanding of the morphology, mineralogical, and composition of these phases to more clearly elucidate the fate and transport of metals in this system.

Colloidal metal transport and Fe oxidation dynamics in soils developing on historic coal mine spoil

ACS Fall 2021

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