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WWRC 95-11
Selenite and Selenate Determination in Surface Coal Mine Backfill Ground Water


A majority of Wyoming's public water supplies are based on ground water resources. It is necessary, therefore, to protect these resources from contamination by pollutants (e.g., selenium), which can be harmful to human health. Dissolved selenium (Se) in ground water may contain different species, including selenite (SeO32-), selenate (SeO42-), organic Se, and Se ion pairs (e.g., MgSeO4-, CaSeO4-). For an understanding of the toxicity potential of Se, it is necessary to know the concentrations of SeO32- and SeO42-, as well as total Se content in ground water. The hydride geneiation atomic absorption spectrometry (HGAAS) method of Se analysis requires oxidation of organic matter, which destroys the natural distribution of Se. Ion chromatography (IC) can determine SeO32- and SeO42- concentrations, but other common anions (e.g.,SeO42-)will interfere with SeO32- and SeO42- measurements. The objective of this study was to determine SeO32- and SeO42- concentrations to evaluate the chemical speciation of Se in ground water. Both SeO32- and SeO42- from ground water samples were adsorbed onto copper oxide (CuO) at pH 5.5 and subsequently desorbed with NaOH solution at pH 12.5. Extracted SeO32- and SeO42- were determined using HGAAS and IC. From the SeO32- and SeO42- measurements, Se speciation in ground water was calculated. Results showed that dissolved Se concentrations in ground water ranged from 22 to 151 µg/L. Selenium speciation suggested that ground water consisted primarily of SeO42- ion (33 to 66%), SeO32- ion (6 to 38%), organic Se (14 to 23%), and Se ion pairs (9 to 18%) (e.g., MgSeO4-, CaSeO4-, Na2SeO4-). These results also show that mobility of dissolved Se will depend on the chemical form in which Se is present in the ground water.

Additional Key Words: dissolved selenium, selenite, selenate, ionic species, toxicity, ion pairs, mobility.

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