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WWRC 95-06
A Method for Detecting Selenium Speciation in Groundwater

Abstract

To better understand the potential toxicity of Se, it is necessary to know the concentration of different Se ionic species (e.g., SeO32- and SeO42-). The hydride generation atomic absorption spectrophotometry (HGAAS) method of Se analysis cannot separate Se into individual ionic species. Ion chromatography (IC) can determine SeO32- and SeO42- concentrations simultaneously; however, common anions, such as sulfate (SeO42-), in groundwater interferes with SeO32- and SeO42- speciation. The purpose of this study was to measure the concentration of ionic SeO32- and SeO42- species in groundwater, thereby determining the chemical speciation of dissolved Se. Three groundwater samples with high concentrations of Mg2+ and SO42- were used in this study. The ionic SeO32- and SeO42- species in groundwater samples were selectively adsorbed onto copper oxide (CuO) particles by lowering the pH to 5.5. These ionic species were desorbed from the surface of CuO particles by increasing the pH to 12.5. Subsequently, the concentrations of SeO32- and SeO42- ionic species in solutions were determined with HGAAS and IC. The effect of divalent cations (e.g., Mg2+) on the concentration of SeO42- in aqueous solutions was also evaluated. The dissolved Se concentration in three groundwater samples ranged from 22 to 151 µg/ L The CuO particles extracted 97% of SeO32- from groundwater samples, suggesting that Se(IV) concentrations were dominated by the SeO32- ion. However, CuO particles extracted 80% of SeO42- from groundwater samples. These results suggest that Se(VI) concentrations consisted of SeO42- and metal SeO42- solution species. The dissolved Mg2+ in groundwater samples formed a strong neutral ion pair with SeO42- (MgSeO40), which was not adsorbed by the CuO particles. Overall chemical speciation of dissolved Se, extracted with CuO particles, suggests that groundwater samples consisted of SeO32- (636%), SeO42- (32-65%), organic Se species (14-23%), and neutral ion pairs (9-16%). An important aspect of the proposed method is that CuO can be used in the field to extract both SeO32- and SeO42- ionic species from groundwater samples, and these species could be desorbed from CuO and measured using HGAAS or IC methods, depending upon the concentrations of these species.

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