USE OF BORON ISOTOPES TO TRACK THE INTERACTION OF COAL UTILIZATION BYPRODUCTS WITH WATER IN THE ENVIRONMENT

Coal-fired power plants produce a number of byproducts that are returned to the environment through disposal or industrial and environmental applications, such as mine-void filling, cement mixing, as an agricultural amendment, or in wallboard gypsum. With coal burning on the rise, there is new interest in finding more applications for the growing quantity of these coal utilization byproducts (CUB).  As more CUB are introduced into the environment, new techniques to understand the mobility of potentially toxic metals associated with these materials will become increasingly important.  Recent work has demonstrated that coal is enriched in the light isotope of boron (B) relative to most natural waters.  Boron has also been shown to be concentrated in coal byproducts, particularly ash, and is one of the most soluble trace elements in these materials.  If the relative ¹⁰B enrichment of coals is retained by CUB, B isotopes could serve as a powerful tool for tracking the interaction of these materials with groundwaters and precipitation.  We have initiated a study of B isotopic composition of various CUB leachates using N-TIMS to investigate the potential use of this tracer.

           

Two samples of coal fly ash with different chemical compositions and one sample of flue-gas desulfurization gypsum (FGD) were leached at room temperature in ultrapure water for 40 minutes and then filtered through a 0.20 µm cellulose acetate filter.  The pH of leachates varied between 8.9 and 10.  The amount of B leached from the CUB ranged from 5 to 95 micrograms per gram of starting solid material, with the FGD being the lowest.  The fly ash leachates both had d¹¹B of about -16‰ to -17‰ relative the NIST boric acid standard SRM951.  The FGD leachate had a d¹¹B value of about -13‰.  These data are consistent with other measurements of CUB, and indicate that coal mineral matter retains its ¹⁰B enrichment even through the combustion process.  Given the significant differences between the negative d¹¹B values of these CUB leachates and the positive d¹¹B values of most natural waters measured so far, B isotopes show considerable promise as an environmental tracer for water-CUB interaction.