Role of Natural Organic Solutes on the Sorption of Selenium by Coal Mine Backfill- Core Samples from the Powder River Basin, Wyoming

Concentrations of dissolved selenium (Se) as large as 330 pg L-' have been detected in water samples &om bacW aquifers in the southern Powder River Basin (PRB) of northeastern Wyoming. During d a c e coal mining, rock mstwial overiymg the coal is redistributed in relation to its original stratigraphic position; exposure of the prewiously buried material to surface oxidizing conditions decreases the stability of sulfides and organic matter contammg Se, increasing Se concentrations in associated ground waters. Ground-water and backfill-core samples were collected fkom three sites located at two active mines in the PRB. Dissolved organic carbon (DOC) in the ground-water samples was dominated by the acid fkt ion (sum of hydrophobic and hydrophilic acids) at 38 to 84% of the DOC; the neutral W o n (sum of hydrophobic and hyc!rophilic neutrals) ranged fiom 10 to 55% and the base W o n (sum ofhydrophobic and hyhphdic bases) was generally less than 10%. Three sorption experiments were conducted to investigate selenite sorption on baca-core samples. Selenite concentrations in these experiments ranged h m 0 to 11 mg kg-' in backtill-core samples. The fitst experiment detexmined the seleniie sorption capacity of the bacicfill-core samples with selenite in distiileddeionized water as a background solution. In the second expaimtent, selenite in 0.1 M CaCIz was used as a background solution. In the third experiment, hydrophobic and hydrophilic acid hctions, isolated and concentrated fkom ground-water samples, were added to the selenite background solutions. Results of the three experiments indicated that the b a c r n a r e samples have a high sorption capacity for Se (generauy, 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 CaCl, 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 seienite sorption rates o c c ~ e d in the presence of individual organic-solute kactions. Hydrophilic acids caused the largest decrease in Se sorption by the backfill-core samples. . . Additional