SELENIUM ADSORPTION ONTO IRON-OXIDE LAYERS BENEATH COAL-MINE SPOIL

A field-scale experimental study was performed to determine feasibility of sequestering dissolved selenium (Se) leached from coal-mine waste rock with an iron oxide amendment. Sequestration is required because concentrations of Se in early leachate approach 0.40 mg/L, well above the 0.005 mg/L discharge limit for ecological toxicity. Dried ferric sludge was obtained from a >30-year-old limestone-treated mine-drainage wetland. The fine-grained iron oxide was determined to be 91.5% ferric oxide with minor Al, SO₄ , and Ca. The mineralogy of the iron is goethite, although residual ferrihydrite may also be present.

Thirty rectangular lysimeters (4.9 x 7.3 m), each containing 55 metric tons of homogenous fresh mine-run Pennsylvanian-age carbonaceous shale overburden, were installed at the Hobet mine, West Virginia. Various thicknesses (0.0064, 0.057, 0.229, and 0.457 m, plus an unamended control) of the amendment, in a single basal layer, were employed, ranging from 0 to 2.2% mass percent of the spoil. The control and each treatment were replicated 6 times, to examine uncertainty due to compositional/hydrological variations. Infiltration of rainfall created leachate that drained to individual batch-collection tanks, sampled 46 times at approximate 2 week intervals from 2010-12. Basal iron oxide layers in the highest amendment category removed an average of 76.1% Se (in comparison to unamended piles) from leachate by adsorption. Lysimeters with <0.2 m iron oxide layers showed no significant reduction from unamended piles. Reproducibility of replicates was within acceptable limits for both amended and unamended lysimeters. Results indicate that iron oxide amendment from treatment of mine water can sequester a significant fraction of Se leached from organic-rich black shale overburden. The dominance of goethite over ferrihydrite suggests that adsorption capacity may be limited and large volumes of iron oxide could be required. Both leachate and Se evolution are markedly seasonal, with highest fluxes (and flux reduction) of Se in late winter and spring months.