Open Access Kent State (OAKS)

The release of colloid-bound trace metals from abandoned coal mine spoils can potentially be a significant source of contamination during weathering. We examined the size-dependent enrichment of trace metals in mine spoil samples using centrifugation and acid extraction to compare metal loading in the bulk and colloid fractions. A combination of X-ray diffraction (XRD), scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), and focused ion beam (FIB) sectioning of selected colloids for transmission electron microscopy (TEM) analyses was used to determine the morphology and elemental and mineral composition at the micro- and nanoscales. In contrast to bulk soils, primary Fe-sulfides (up to 11%) and secondary Fe(III)-bearing phases (up to 5%) were a significant portion of the colloid mineralogy. Secondary Fe-(hydro)oxides and (hydroxy)sulfates were enriched with Mn, Ni, Cu, and Zn, and these metals showed stronger correlations with Fe in the colloid fraction (R² of 0.58, 0.77, 0.94, and 0.81, respectively) than in the bulk fraction (R² of 0.40, 0.09, 0.84, and 0.62, respectively), indicating that Fe-bearing colloids are likely major trace-metal-bearing phases. The results from this study will help to design better remediation projects for abandoned mine spoils to better account for a potentially underappreciated mode of contaminant transport.