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

Abstract

Concentrations of dissolved selenium (Se) as large as 330 µg 1-1 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. Groundwater and backfill-core samples were collected from three sites located at two active mines in the PRB. Dissolved organic carbon (DOC) in the groundwater samples was domimrtoo 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 IO to 55% and the base fraction (smn 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 rangoo from O to 11 mg kl in backfill-core samples. The first experiment determined the selenite sorption capacity of the backfill-core samples with selenite in distilloo-deionized water as a background solution. In the second experiment, selenite in 0.1 M CaCl 2 was used as a background solution. In the third experiment, hydrophobic and hydrophilic acid fractions, isolated and concentrated from groundwater 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 CaC1 2 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.

Kinetic Model Evaluation of Arsenic and Selenium Sources and Mobility in Waste Rock of the Powder River Basin, USA

“…Yet, back ill aquifers in this region have shown As and Se concentrations exceeding water quality criteria (See et al 1995). Dissolution of salts that were previously unavailable to groundwater in the overburden and the oxidation of contaminant-bearing minerals associated with the coal have been hypothesized as the likely sources of the As and Se (Reed and Singh 1986;Dreher and Finkelman 1992;Reddy et al 1995;Vance et al 1998;Hoy et al 2003). This perception of potentially no impact on groundwater quality from back ill aquifers and multiple hypothesized sources of As and Se is the basis of this study's question regarding new mineral surface availability, generation of contaminant-containing nanomaterials, and the need to identify such sources for prediction of future water quality.…”

Assessment of Hydration Process And Mechanical Properties of Cemented Paste Backfill By Rebound Method

“…Uniaxial compressive strength (UCS) is the most intuitive and important index to measure the mechanical properties of CPB. At present, most scholars study the CPB by making the standard cylindrical specimen of the backfilling body 7 in the laboratory or coring from backfill stopes at construction site [8][9][10] , then carrying out various mechanical tests on it. Uniaxial compression test is usually used to measure the UCS of backfilling specimens.…”

A detailed sequential extraction study of selenium in coal and coal-associated strata from a coal mine in West Virginia

“…During surface coal mining, rock materials overlying the coal are redistributed from their original stratigraphic position and are often placed in oxygenated environments. The stability of Se-containing sulfides and organic matter can be substantially decreased by exposing buried horizons to surface oxidizing conditions which in turn can increase the potential solubility of Se species (Vance et al, 1995).…”