PII S0016-7037(99)00279-3 Selenium isotope ratios as indicators of selenium sources and oxyanion reduction THOMAS M. JOHNSON, 1, *MITCHELL J. HERBEL, 1,2 THOMAS D. BULLEN, 1 and PETER T. ZAWISLANSKI 3 1 Geology Department, 245 Natural History Bldg., MC-102, University of Illinois at Urbana-Champaign, Urbana, IL 61820, USA 2 Water Resources Division, U.S. Geological Survey, Menlo Park, CA 94025, USA 3 Earth Sciences Division, E. O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA (Received February 10, 1999; accepted in revised form July 20, 1999) Abstract—Selenium stable isotope ratio measurements should serve as indicators of sources and biogeo- chemical transformations of Se. We report measurements of Se isotope fractionation during selenate reduction, selenite sorption, oxidation of reduced Se in soils, and Se volatilization by algae and soil samples. These results, combined with previous work with Se isotopes, indicate that reduction of soluble oxyanions is the dominant cause of Se isotope fractionation. Accordingly, Se isotope ratios should be useful as indicators of oxyanion reduction, which can transform mobile species to forms that are less mobile and less bioavailable. Additional investigations of Se isotope fractionation are needed to confirm this preliminary assessment. We have developed a new method for measurement of natural Se isotope ratio variation which requires less than 500 ng Se per analysis and yields 0.2‰ precision on 80 Se/ 76 Se. A double isotope spike technique corrects for isotopic fractionation during sample preparation and mass spectrometry. The small minimum sample size is important, as Se concentrations are often below 1 ppm in solids and 1 g/L in fluids. The Se purification process is rapid and compatible with various sample matrices, including acidic rock or sediment digests. Copyright © 1999 Elsevier Science Ltd 1. INTRODUCTION Selenium is an essential nutrient at low concentrations and a toxin at higher concentrations (e.g., Cooper and Glover, 1974; Ganther, 1974; Skorupa, 1998). Chemically similar to sulfur, Se can be found in +6, +4, 0, and -2 valences, and in a variety of organic compounds, in natural settings (e.g., El- rashidi et al., 1987; McNeal and Balistrieri, 1989). The Se 6+ and Se 4+ valence states form the oxyanions selenate and se- lenite/biselenite, respectively. The Se oxyanions are highly soluble, but Se 4+ adsorbs to solids more strongly (Neal and Sposito, 1989). Se 0 readily precipitates as elemental Se, and more reduced forms of Se readily form precipitates such as ferroselite, FeSe 2 , or clausthalite, PbSe, or may be incorporated into proteins and other organic molecules. Because of differ- ences in chemical behavior between the different oxidation states, the mobility and bioavailability of Se are strongly de- pendent on redox transformations. Documenting the occurrence of reduction or oxidation in nature is thus a key goal in studies of Se biogeochemistry. Selenium redox transformations, especially reduction of selenate, are generally sluggish at environmental temperatures, and in Se-contaminated soils and sediments, selenate, selenite, elemental Se, and organically bound Se often coexist (e.g., Tokunaga et al., 1991; Zawislanski and McGrath, 1998; Zhang and Moore, 1996). Certain bacteria can respire with selenate or selenite as a terminal electron acceptor (e.g., Blum et al., 1998; Dungan and Frankenberger, 1998; Oremland et al., 1989), or oxidize elemental Se (Dowdle and Oremland, 1998), and it is likely that most of the important redox transformations are microbially mediated. Algae and other organisms are known to bioconcentrate Se and/or emit significant masses of Se as volatile alkylselenides (Fan and Higashi, 1998; Frankenberger and Karlson, 1994). The geochemistry of Se is thus strongly influenced by biology, and consideration of a variety of pro- cesses is necessary in any natural system. Shales rich in organic matter are often rich in selenium, and Se-rich soils may develop on seleniferous shales in arid cli- mates (e.g., Presser, 1994a; 1994b; Seiler, 1998). Irrigation practices designed to flush salts from the soils can produce Se-rich effluent. In the San Joaquin Valley of California, dis- posal of this water in evaporation ponds led to deformities and deaths of waterfowl at Kesterson reservoir (Ohlendorf and Santolo, 1994; Presser, 1994b), and the wastewater disposal problem has not yet been resolved. Many other areas in the western U.S. have similar problems (Seiler, 1998). Se stable isotope ratios should be useful as indicators of biogeochemical processes and environmental sources, as are nitrogen and sulfur isotope ratios. Nitrogen isotope ratios indi- cate nitrogen sources (Kendall et al., 1995; Komor and Ander- son, 1993) and are used to detect reduction of nitrate in ground- water, because this process fractionates the isotopes (Boettcher et al., 1990; Kohl and Shearer, 1978). Sulfur isotope ratios are used as indicators of sulfur sources and sulfate reduction (e.g., Dowuona et al., 1993; Kaplan et al., 1963; Strebel et al., 1990). Se isotope fractionation appears to be similar to sulfur isotope fractionation, according to previous research discussed below. Accordingly, isotopic shifts observed in nature should be useful as evidence of reduction processes. This is particularly valuable with Se because reduction of soluble oxyanions to insoluble Se 0 decreases the mobility and bioavailability of Se (e.g., Elrashidi et al., 1987; McNeal and Balistrieri, 1989; Tokunaga et al., 1994). Selenium concentrations in most natural waters and soils are below 1 g/L and 1 g/g, respectively, and a mass spectrom- etry technique requiring less than 1 g of Se per analysis is thus highly desirable. Gas-source mass spectrometry methods used * Address reprint requests to Thomas M. Johnson, (tmjohnsn@uiuc. edu). Pergamon Geochimica et Cosmochimica Acta, Vol. 63, No. 18, pp. 2775–2783, 1999 Copyright © 1999 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/99 $20.00 + .00 2775