WWRC 94-15
Geochemical Processes Affecting the Mobility of Dissolved Selenium in Surface Coal Mine Backfill Environments
Abstract
During surface coal mining, rock material overlying the coal is redistributed with
respect to its original stratigraphic position. Oxidation of the redistributed backfill material may
release dissolved selenium (Se) to groundwater in backfill systems. This study was initiated to
examine the chemistry of Se in the groundwater backfill environment. Both backfill core material and
groundwater samples were collected from three sites in Wyoming. Dissolved organic carbon (DOC)
from groundwater samples was fractionated into six categories (hydrophobic acids, bases, and
neutrals; and hydrophilic acids, bases, and neutrals). The DOC was comprised primarily of
hydrophobic and hydrophilic acids which were isolated and used in competitive sorption/desorption
studies with Se. Core samples were also subjected to precipitation/dissolution studies to determine
potential solid phases controlling dissolved Se concentration. Results suggested that coal mine backfill
material have a very high sorption capacity for Se. Hydrophilic acids reduced Se sorption by coal
mine backfill materials. DOC showed very little effect on Se speciation and potential solid phases
controlling dissolved Se. Water extracts from coal mine backfill materials were dominated by
MgSeO30 ion pair followed by CaSeO30, and SeO32- species. Dissolved Se concentrations in coal mine
backfill environments appeared to be controlled by a Cu2Se solid phase.
KEY TERMS: Selenium; sorption/desorption; chemical modeling; precipitation/dissolution.
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