Environ. Sci. Technol. 2003, 37, 4709-4716
Critical Evaluation of the Ability of Sequential Extraction Procedures To Quantify Discrete Forms of Selenium in Sediments and Soils MICHAEL T. WRIGHT, DAVID R. PARKER,* AND CHRISTOPHER AMRHEIN Soil and Water Sciences Section, Department of Environmental Sciences, University of California, Riverside, California 92521
Sequential extraction procedures (SEPs) have been widely used to characterize the mobility, bioavailibility, and potential toxicity of trace elements in soils and sediments. Although oft-criticized, these methods may perform best with redox-labile elements (As, Hg, Se) for which more discrete biogeochemical phases may arise from variations in oxidation number. We critically evaluated two published SEPs for Se for their specificity and precision by applying them to four discrete components in an inert silica matrix: soluble Se(VI) (selenate), Se(IV) (selenite) adsorbed onto goethite, elemental Se, and a metal selenide (FeSe; achavalite). These were extracted both individually and in a mixed model sediment. The more selective of the two procedures was modified to further improve its selectivity (SEP 2M). Both SEP 1 and SEP 2M quantitatively recovered soluble selenate but yielded incomplete recoveries of adsorbed selenite (64% and 81%, respectively). SEP 1 utilizes 0.1 M K2S2O8 to target “organically associated” Se, but this extractant also solubilized most of the elemental (64%) and iron selenide (91%) components of the model sediment. In SEP 2M, the Na2SO3 used in step III is effective in extracting elemental Se but also extracted 17% of the Se from the iron selenide, such that the elemental fraction would be overestimated should both forms coexist. Application of SEP 2M to eight wetland sediments further suggested that the Na2SO3 in step III extracts some organically associated Se, so a NaOH extraction was inserted beforehand to yield a further modification, SEP 2OH. Results using this fivestep procedure suggested that the four-step SEP 2M could overestimate elemental Se by as much as 43% due to solubilization of organic Se. Although still imperfect in its selectivity, SEP 2OH may be the most suitable procedure for routine, accurate fractionation of Se in soils and sediments. However, the strong oxidant (NaOCl) used in the final step cannot distinguish between refractory organic forms of Se and pyritic Se that might form under sulfurreducing conditions.
Introduction During the early 1980s, selenium (Se) was implicated in deaths and deformities of migratory waterfowl at the Kesterson National Wildlife Refuge in the San Joaquin Valley, CA, leading * Corresponding author e-mail:
[email protected]; phone: (909)787-5126; fax: (909)787-3993. 10.1021/es0342650 CCC: $25.00 Published on Web 08/28/2003
2003 American Chemical Society
to a surge of interest in the environmental behavior of this trace element (1). Much of the emphasis has been on irrigation projects in seleniferous areas of the western United States, where Se-enriched drainage waters are often discharged into topographically closed impoundments (2), but ecotoxicological concerns about Se have extended to other environments as well. Understanding the risks associated with Se in sediments or soils as well as possible remedies requires a detailed understanding of its biogeochemistry and thus its speciation. Synchrotron-source X-ray absorption spectroscopy has shown promise for unambiguous identification of solid-phase Se species in sediments (3-5) but has not enjoyed widespread use, in part due to the lack of sensitivity needed for many Se-enriched sediments (which often contain
