WWRC 95-10
Role of Natural Organic Solutes on the Sorption of Selenium by Coal Mine Backfill-Core Samples from the Powder River Basin, Wyoming
Abstract
Concentrations of dissolved selenium (Se) as large as 330 ug L-1 have been detected in water samples
from backfill aquifers in the southern Powder River Basin (PRB) of northeastern Wyoming. During surface coal
mining, rock material overlying the coal is redistributed in relation to its original stratigraphic position; exposure of
the previously buried material to surface oxidizing conditions decreases the stability of sulfides and organic matter
containing Se, increasing Se concentrations in associated ground waters. Ground-water and backfill-core samples
were collected from three sites located at two active mines in the PRB. Dissolved organic carbon (DOC) in the
ground-water samples was dominated by the acid fraction (sum of hydrophobic and hydrophilic acids) at 38 to 84%
of the DOC; the neutral fraction (sum of hydrophobic and hydrophilic neutrals) ranged from 10 to 55% and the base
fraction (sum of hydrophobic and hydrophilic bases) was generally less than 10%. Three sorption experiments were
conducted to investigate selenite sorption on backfill-core samples. Selenite concentrations in these experiments
ranged from 0 to 11 mg kg-1 in backfill-core samples. The first experiment determined the selenite sorption capacity
of the backfill-core samples with selenite in distilled-deionized water as a background solution. In the second
experiment, selenite in 0.1 M CaCl2 was used as a background solution. In the third experiment, hydrophobic and
hydrophilic acid fractions, isolated and concentrated from ground-water samples, were added to the selenite
background solutions. Results of the three experiments indicated that the backfill-core samples have a high sorption
capacity for Se (generally, greater than 90% of the added Se) and can sorb as much as 99% of Se added as selenite.
Selenium sorption increased in most samples when 0.1 M CaCl2, was used as a background solution compared to
distilled-deionized water background solution. The increase in selenite removal might have been caused by
precipitation reactions or by an increase in positively charged surfaces caused by calcium sorption, or both. A
substantial decrease in selenite sorption rates occurred in the presence of individual organic-solute fractions.
Hydrophilic acids caused the largest decrease in Se sorption by the backfill-core samples.
Additional Key Words: Selenium Geochemistry; Coal Mines; Sorption/Competition; Dissolved Organic Carbon;
Mobility; Powder River Basin, Wyoming.
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