SPECIATION OF SELENIUM ASSOCIATED WITH COAL-COMBUSTION BY-PRODUCTS USING HPLC-ICPMS

Selenium found in coal combustion by-products, such as fly ash, can be released into the environment. Se is most commonly found as oxyanions, selenite (Se^IVO₃^-2) and selenate (Se^VIO₄^-2). Separate analysis of the species is crucial because Se toxicity and mobility are species dependent. A method was developed to speciate inorganic Se in coal fly ash extractants on a reverse-phase column using a Perkin Elmer HPLC-ICPMS system. The HPLC separates the Se species and the ICPMS provides low detection limits. A dynamic reaction cell eliminates interference from the argon gas dimer and allows all six isotopes of Se to be quantified. Reverse-phase chromatography is primarily used for organic separations in non-ionic solutions; this method allows for concurrent speciation of organic and inorganic in ionic solutions.

Separation of selenite and selenate is achieved within 2 minutes on a Brownlee C8 column using a 50µL injection loop and a flow rate of 1.0 mL/min at 25C. The mobile phase consists of an acetic acid-ammonium acetate buffer plus tetrabutylammonium hydroxide for the ion-pairing agent. The resulting pH is 5.4. At this pH, selenate is fully de-protonated (SeO₄^-2) but selenite is partially protonated (HSeO₃^-). This allows for separation by polarity, as well as size and weight. The addition of 5% methanol to the mobile phase acts as an organic modifier which creates cleaner peaks and shorter retention times. Variations of temperature, flow rates, methanol concentrations, pH, and mobile phase concentrations determine the optimal peak detections and retention times.

Separation was achieved for Se in spiked and un-spiked fly ash leachates. Initial results show that selenite is the dominant form of Se extracted from the fly ash with de-ionized water.