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., SeO 3 2- and SeO 4 2- ). The hydride generation atomic absorption spectrophotometry (HGAAS) method of Se analysis cannot separate Se into individual ionic species. Ion chromatography (IC) can determine SeO 3 2- and SeO 4 2- concentrations simultaneously ; however, common anions, such as sulfate (S0 4 2- ), in groundwater interferes with SeO 3 2- and SeO 4 2- speciation. The purpose of this study was to measure the concentration of ionic SeO 3 2- and SeO 4 2- species in groundwater, thereby determining the chemical speciation of dissolved Se. Three groundwater samples with high concentrations of Mg 2+ and SO 4 2- were used in this study. The ionic SeO 3 2- and SeO 4 2- species in groundwatersamples 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 SeO 3 2- and SeO 4 2- ionic species in solutions were determined with HGAAS and IC. The effect of divalent cations (e.g., Mg 2+ ) on the concentration of SeO 4 2- 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 SeO 3 2- from groundwater samples, suggesting that Se(lV) concentrations were dominated by the SeO 3 2- ion. However, CuO particles extracted 80% of SeO 4 2- from groundwater samples. These results suggest that Se(Vl) concentrations consisted of SeO 4 2- and metal SeO 4 2- solution species. The dissolved Mg 2+ in groundwater samples formed a strong neutral ion pair with SeO 4 2- (MgSeO 4 0 ), which was not adsorbed by the CuO particles. Overall chemical speciation of dissolved Se, extracted with CuO particles, suggests that groundwater samples consisted of SeO 3 2- (6-36%), SeO 4 2- (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 SeO 3 2- and SeO 4 2- 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.