Analysis and speciation of selenium ions in mine environments

Aqueous extracts of five mine soil samples and a set of selenite (SeO32-)-selenate (SeO42-) solutions (0.5, 1, 5, 10 and 25 mg/l) were speciated using atomic absorption spectroscopy with hydride generation (AAS-HG) and ion chromatography (IC) to compare these methods for Se speciation. In another experiment, a SeO32--SeO42--SO42- solution (25 mg/l) was reacted with CaO, MgO, MnO2, CuO, La2O3, and WO3 to evaluate the relative distribution of the Se species and SO42- in the SORB (sorbed ions that were desorbed by NaOH), SOLN (equilibrium concentrations), CMPX (irreversibly sorbed and neutral ion pair complexes) fractions. The AAS-HG method was capable of analyzing Se as low as 0.002 mg/l, which was below the detection limit of IC. High concentration of SO42- affected the chromatographic Se Speciation either by shifting or overlapping Se peaks, in which AAS-HG was more useful. However, IC was capable of speciating aqueous SeO32--SeO42- directly without any sample pretreatment, whereas AAS-HG measured SeO32-+SeO42-, and SeO32- in separate runs and SeO42- was calculated from the difference, i.e., spectrophotometric speciation was an indirect method. For both Se species, AAS-HG and IC data were comparable within detection standard deviations. Ratios of different Se species at measured soil pH and pe indicated that SeO32- or SeO42- would be the dominant Se species; the p(SeO42-/SeO32-) values further suggested SeO42- would be the major solution species. Except for CaO and MgO treatments, the %SeO32- in the SORB fraction was greater than or equal to%SeO42-. In the SOLN fraction %SeO42- was greater than or equal to%SeO32- for all oxides but CaO, whereas in the CMPX fraction this order was observed for only CaO and MnO2. The %SeO32- was highest in the SOLN fraction for all oxides but MgO and La2O3 for these two oxides SeO32- dominated in CMPX and SORB fractions, respectively. The SOLN fraction also contained the highest %SeO42- for all oxides but MgO which retained SeO42- primarily in the SORB fraction. The %SeO32- and %SeO42- in the CMPX fraction were highest for MgO, thus, suggesting a possible formation of MgSeO30 and MgSeO40. A similar trend of SORB %SO42- and SORB %SeO42- was attributed to the analogous adsorption mechanisms (outer-sphere complexation). For all oxides but MgO, %SO42- was <%SeO42- in the SOLN fraction and greater than or equal to%SeO42- in the CMPX fraction. Comparative ionic distributions provide an overall picture of the relative abundance of different Se species in various fractions associated with different oxides present in micro- and macrolevels in soils and geological materials.