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- (6—36%),
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.
Water Resources Publications List
Water Resources Data System Library |
Water Resources Data System Homepage
