Assessment of isotope exchange methodology to determine the sorption coefficient and isotopically exchangeable concentration of selenium in soils and sediments

Isotope exchange methodology is invaluable to determine the solution-solid-phase distribution (K-d) and isotopically exchangeable concentration (E value) of elements in soils and sediments. This work examined the use of species-specific stable isotope exchange techniques to determine the K-d and E value of selenium (Se), as selenite (SeO3) and selenate (SeO4), in nine soils and sediments varying in concentration and source of Se. High-performance liquid chromatography-inductively coupled plasma-mass spectrometry (HPLC-ICP-MS) was used to quantify the isotope (e.g., Se-76, Se-78, Se-80, and Se-82) concentrations of the soluble Se oxyanions. The two Se oxyanions were detected in the solution phase of all of the soils and sediments. However, upon spiking the suspensions with stable isotope-labeled (SeO3)-Se-78 and (SeO4)-Se-76, it was observed that isotope self-exchange was insignificant to the derivation of Se oxyanion K-d and E values during 24 h (and up to 120 h in four of the samples). These results demonstrate that valid determinations of the E value of Se necessitate that the Se oxyanions are speciated in solution. This is clearly evident for these soils and sediments where it was observed that the E values of SeO3 and SeO4 represented, respectively, 5-97% and 3-95% of the total Se E value.