Determination of Selenium Species in Coal Fly Ash Extracts Nancy D. Nlss,' John F. Schabron, and Terry H. Brown Western Research Institute, P.O. Box 3395, Laramie, Wyoming 82071
An ion chromatography method was developed to determine selenite and selenate in the presence of other common anions in extracts from coal fly ash samples. The anions were separated using a Dionex AG5 guard column and an AS4A separator column and then quantified using conductivity detection. The eluant consisted of 2.0 mM sodium carbonate and 1.0 mM sodium hydroxide. This method separates selenite and selenate without interference problems from nitrate, phosphate, and sulfate. The detection limits for selenite and selenate were 0.05 mg/L using a 50-pL sample injection loop and a detector setting of 1pS full scale. The selenium concentrations detected using ion chromatography were compared to total selenium values obtained on the same leachates using inductively coupled plasma spectroscopy (ICP) analysis. All the leachates analyzed displayed good agreement between IC and ICP values.
Introduction The leaching and transport of trace elements from coal fly ash material is an area of environmental concern because of the proposed use of coal combustion byproducts in roadbed stabilization, construction materials, and other applications. Selenium is generally an enriched element in coal, and under alkaline conditions that are present in many soils of the western United States, selenium species display anionic behavior, increasing their solubility and mobility (I). Methods for the determination of the selenium species extracted from solid matrices are needed to provide important information on the parameters that control selenium speciation in aqueous systems. The chemical form of selenium solubilized from solids is dictated by various parameters including pH, dissociation constants, and oxidation/reduction potentials (2). To determine what forms are present in a solid/solution environment, a leaching method is necessary to extract selenium species while preserving their chemical form. Most digestion procedures do not preserve the chemical forms of selenium present due to the oxidative nature of the procedures. Cutter and Bruland developed a leaching procedure using 1 M sodium hydroxide to extract selenite and selenate from biogenic particles. They performed studies using 75Se-labeled selenite and selenate and nonradiogenic 0013-936X/93/0927-0827$04.00/0
0 1993 Amerlcan Chemical Society
selenium standards and showed that no speciation changes occurred during the 4-h leaching procedure (3). Their work provided a starting point for the development of a method to extract selenium species from coal fly ash samples in this study. Once a method has been found to solubilize species from solid matrices, a method must be developed to determine the major species present. Various non-ion chromatographic methods are available to determine selenite and selenate using separate analyses. These methods are both time consuming and plagued with interference problems. Ion chromatographic methods offer the advantages of rapid analysis time, increased sensitivity over wet chemical methods, simultaneous analysis of selenite, selenate, and other common anions, and freedom from interference problems. Karlson and Frankenberger have developed singlecolumn ion chromatography (SCIC) methods to determine selenite and selenate in aqueous solutions. These methods use two different runs with different eluants to determine selenite and selenate (4, 5). Mehra and Frankenberger determined selenite and selenate in aqueous soil extracts with a single SCIC run. No interferences were present in their work, but high levels of nitrate and sulfate could pose serious problems (6). A procedure was developed to separate selenite and selenate using suppresed ion chromatography with ultraviolet (UV) absorption detection. The procedure does not suffer from sulfate interferences since sulfate does not absorb above 190 nm, but nitrate does present a problem. In addition, the technique lacks sensitivity, as the limits of detection are about 15 mg/L (7). An objective of this project was to develop a leaching method to extract inorganic selenium species from coal fly ash samples without changing their chemical form. Another objective was to develop a suppressed ion chromatography method to separate inorganic selenium species and eliminate interference problems with selenite and selenate from nitrate, sulfate, and phosphate. Experimental Section
Samples. The samples used in this study included the NIST 1633a fly ash standard reference material (SRM) from the National Institute for Standardsand Technology (NIST) in Gaithersburg, MD, with a certified value of 10.3mg/kg Se, and three fly ash samples from coal-powered electrical generation facilities. Fly ash A was generated Envlron. Sci. Technol., Vol. 27, No. 5, 1993 827
Table I. Extraction of Selenium Over Time with 0.5 M Sodium Hydroxide fly ash
NIST 1633a
FA-A
FA-B
FA-C
extractn time, h
[SeOPI, mgikg (as Se)
[SeOi*-], mgikg (as Se)
4 6 8 24 4 6 8 24 4 6 8 24 4 6 8
11.2 8.70 8.60 8.30 7.70 6.60 5.50 5.50 8.10 6.30 8.00 5.90
