IRON, SULFUR, AND TRACE METAL TRANSPORT DYNAMICS DURING WEATHERING OF HISTORIC COAL MINE SPOIL

Weathering of coal shale mine waste and oxidative dissolution of pyrite can generate a variety of Fe(III)-bearing phases including discrete Fe(III)-(oxy)hydroxide and sulfates, fine grained aggregates forming as secondary mineral surface coatings, and colloids. We have recently shown that the detachment of pyrite and other metal sulfide-bearing submicron particles from mine spoil can also occur, which are subsequently released and transported through soil pore water. Soil pore water was collected using suction lysimeters installed in soils developing on historic coal mine spoil, and colloids (<10μm) were separated using centrifugation to study the trace metal transport and elemental and mineralogical composition of the colloids. Trace metals showed higher colloidal contribution (Zn, Mn, Fe, and Cu showed 54%, 43%, 23% and 14% colloidal contribution, respectively) compared to base metals (Na, Ca, Mg, K, Si, and K, which all showed less than 10% colloidal contribution). The morphology, and composition of colloids, determined using a scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) and X-ray diffraction (XRD), indicated that the colloids were dominated by phyllosilicates (biotite, muscovite, and kaolinite) with minor amounts (< 10%) of Fe oxides (hematite, goethite) and metal sulfides (arsenopyrite, and chalcopyrite). Further SEM-EDS analyses of these soils has shown that the oxidative dissolution of pyrite and other metal sulfides results in the deposition of secondary Fe(III)-bearing phases which could be broadly categorized into the following categories based on morphology, texture, and composition: (1) un-weathered to partially weathered pyrite, (2) Fe(III)-(oxy)hydroxides shells formed via pseudomorphic replacement of pyrite, (3) Fe-oxides deposited as coatings on mineral grains, and (4) discrete Fe-oxide particles. Preliminary results indicate that the formation of these phases, based on local Fe oxidation dynamics, controls trace element (e.g. Cu, Mn, and Zn) partitioning during pyrite oxidative dissolution. On-going synchrotron micro-XRF data analyses aim to provide a better understanding of the impact of Fe and S speciation on trace metal partitioning to more clearly elucidate the fate and transport of metals in this system.