Selenium Adsorption onto Iron Oxide Layers beneath Coal-Mine Overburden Spoil

Abstract

A field experimental study to determine the feasibility of sequestering dissolved selenium (Se) leached from coal-mine waste rock used an iron (Fe)-oxide amendment obtained from a mine-drainage treatment wetland. Thirty lysimeters (4.9 × 7.3 m), each containing 57.7 t (1.2-1.8 m thickness) of mine-run carbonaceous shale overburden, were installed at the Hobet mine in southeastern West Virginia. The fine-grained Fe-oxide was determined to be primarily metal oxides (91.5% ferric and 4.37% aluminous), with minor (<3%) SO and Ca, perhaps as gypsum. The mineralogy of the Fe was goethite, although residual ferrihydrite may have been present. Various thicknesses of this amendment (0.0064, 0.057, 0.229, and 0.457 m, plus a zero-amendment control) were used, ranging from 0 to 2.2% weight percent of the spoil. The control and each treatment were replicated six times to estimate uncertainty due to compositional and hydrological variation. Infiltration of rainfall created leachate that drained to individual batch-collection tanks that were sampled 46 times at approximately 2-wk intervals from 2010 to 2012. Basal Fe-oxide layers in the three highest amendment categories removed up to 76.1% selenium (in comparison to unamended piles) from leachate by adsorption. Only lysimeters with very thin Fe-oxide layers showed no significant reduction compared with unamended piles. Reproducibility of replicates was within acceptable limits for amended and unamended lysimeters. Results indicate that in situ amendment using Fe-oxide obtained from treatment of mine water can sequester Se by adsorption on surfaces of goethite and possibly also ferrihydrite. This process is demonstrated to substantially reduce dissolved Se in leachate and improve compliance with regulatory discharge limits.

Reservoirs of Selenium in Coal Waste Rock: Elk Valley, British Columbia, Canada

“…The total Se mass release is consistent with the lack of measurable loss of Se from the piles based on Se D data, which, in turn, is consistent with the results of a fieldscale trial on coal waste rock in West Virginia conducted by Donovan and Ziemkiewicz. 28 They observed no depletion in Se over 2 years and acknowledged that the variability in Se concentrations in the waste rock could have made the depletion rate difficult to detect. In contrast to the above, samples from FRO differed from those from LCO and the Mist Mountain Formation (see Table S3 of the Supporting Information).…”

Regional scale selenium loading associated with surface coal mining, Elk Valley, British Columbia, Canada

“…This discrepancy was attributed to differences in leaching rate caused by differences in temperature, water throughput, and surface area. A similar field scale experiment in West Virginia over two years did not provide any evidence of depletion of Se from waste rock, though natural variability would have made a depletion rate difficult to detect over such a short time frame (Donovan and Ziemkiewicz, 2013). Several studies have found a strong relationship between sulfate and Se concentrations in surface coal mining watersheds (Palmer et al, 2010;Lindberg et al, 2011;Day et al, 2012;Kennedy et al, 2012), due to similar geochemical processes releasing both sulfate and Se (Diehl et al, 2012).…”

Liberation of selenium from alteration of the Bowland Shale Formation: evidence from the Mam Tor landslide

“…A combination of fine particles, high surface area and the affinity of iron oxides for Se and other trace elements makes ochres a natural trap for groundwater solutes. Ochre has even been deployed to extract Se from waters draining through coal waste (Donovan & Ziemkiewicz 2013). The situation at Mam Tor is very similar, where ochres are precipitated from, but also extracting Se from, the waters draining through the landslipped shale.…”