Speciation of Selenium(IV) and Selenium(VI) using Coupled Ion Chromatography—Hydride Generation Atomic Absorption Spectrometry

A simple method was developed to speciate inorganic Se in the mg L range using coupled ion chromatography-hydride generation atomic absorption spectrometry. Because of the differences in toxicity and adsorption behavior, determination of the redox states selenite, Se(IV), and selenate, Se(VI), is important. We used anion exchange chromatography to separate Se(IV) and Se(VI) based on differences in retention times. Samples were then mixed with concentrated HCl and passed through a 130 C sand bath to reduce Se(VI) to Se(IV) for Se determination as the hydride. Detection limits were 0.68 mg L for Se(IV) and 0.55 mg L for Se(VI). Spiking of actual sample solutions with Se(IV) and Se(VI) showed the procedure to be accurate for solutions with Se(IV)/Se(VI) ratios ranging from 1:4 to 4:1. Average recovery was 93.1% for Se(IV) and 108% for Se(VI). The technique was used to determine Se(IV) and Se(VI) in deionized water and actual and synthetic irrigation waters. SELENIUM is an essential element in animal nutrition that is toxic at elevated concentration (Girling, 1984). Elevated solution Se concentrations can occur as a result of mining operations and drainage of seleniferous soils (Sharma and Singh, 1984). The range between Se deficiency and toxicity is narrow (Gupta and Watkinson, 1985). The major inorganic Se species found in soil solution are selenite, Se(IV), and selenate, Se(VI) (Adriano, 1986). Selenium toxicity is dependent on its oxidation state (Harr, 1978). Selenite is generally considered to be more toxic than selenate (Cobo Fernandez et al., 1993). The transformation rates of selenite to selenate and selenate to selenite are slow so that both redox states can coexist in soil solution (Masscheleyn et al., 1990). Adsorption reactions with soil mineral surfaces attenuate solution Se concentrations. Selenite adsorbs strongly on soil surfaces while selenate generally adsorbs weakly or not at all and is readily leached (Neal and Sposito, 1989). Because of the differences in toxicity and adsorption behavior, speciation of inorganic Se is important. Understanding the speciation of Se in agricultural drainage waters is extremely important as these waters can impact drinking water quality and wildlife habitat (Schuler et al., 1990). The method of choice for Se analysis in aqueous solution is usually hydride generation atomic absorption spectrometry (HGAAS) because of its relative sensitivity and the general availability of atomic absorption spectrometers in most laboratories (Huang and Fujii, 1996). While inductively coupled plasma mass spectrometry (ICP–MS) exhibits superior sensitivity, it suffers from interferences between the dominant isotope of Se and the argon dimer, Ar2 (both mass 80), unless reaction cell technology is used, which is not yet standard in most laboratories. In Se speciation analysis by HGAAS, two separate analyses are performed (Huang and Fujii, 1996). The Se(IV) redox state is determined directly. Total inorganic Se is determined after quantitative reduction of selenate to selenite by digestion withHCl at elevated temperature. The Se(VI) redox state is subsequently determined by difference. This procedure is necessary because chemical species must be fully protonated to form hydrides (Howard, 1997). This is the case for selenite, pKa15 2.5, but not for selenate whose pKa1 value is approximately 23. An inherent disadvantage in this difference method is that any error in either the measurement of selenite or of total inorganic Se will automatically compromise the accuracy of the selenate analysis. Direct simultaneous analysis of Se(IV) and Se(VI) redox states requires separation of the Se species. The preferred species separation method has been ion chromatography (e.g., Kölbl et al., 1993; Lei and Marshall, 1995; Vassileva et al., 2001). The Se species are separated by differences in retention time since the higher charged selenate anion is retained longer on the chromatography column than the selenite anion. Subsequent to separation, the Se redox states are detected using nonsuppressed conductivity (Mehra and Frankenberger, 1988), flame atomic absorption spectroscopy (FAAS) (Lei and Marshall, 1995), graphite furnace atomic absorption spectroscopy (GFAAS) (Kölbl et al., 1993), or ICP–MS (Jackson andMiller, 1999;Wallschläger and Roehl, 2001). To avoid the interference of the argon dimer, Ar2, Jackson andMiller (1999) and (Wallschläger and Roehl, 2001) measured the less abundant Se isotope. Although easy to employ, ion chromatography with conductivity detection is usually not the method of choice for analyzing agricultural drainage waters because of the high level of anions such as SO4 22 and NO3 , which can elute near selenite and selenate, respectively, masking the proportionally smaller Se peaks. As such, conductivity detection is prone to interferences, while FAAS lacks the sensitivity of HGAAS and ICP–MS, and GFAAS is not S. Goldberg and H.S. Forster, USDA-ARS, George E. Brown Jr., Salinity Lab., 450 W. Big Springs Rd., Riverside, CA 92507; D.A. Martens, USDA-ARS Southwest Watershed Research Center, Tucson, AZ 85719; M.J. Herbel, Dep. of Environmental Sciences, Univ. of California, Riverside, CA 92521. Contribution from the George E. Brown Jr., Salinity Lab. Received 3 May 2005. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 70:41–47 (2006). Soil Chemistry doi:10.2136/sssaj2005.0141 a Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: FAAS, flame atomic absorption spectroscopy; HGAAS, hydride generation atomic absorption spectrometry; ICPMS, inductively coupled plasma mass spectrometry; SRM, standard reference material. R e p ro d u c e d fr o m S o il S c ie n c e S o c ie ty o f A m e ri c a J o u rn a l. P u b lis h e d b y S o il S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d .