Date of Award
Master of Science (MS)
Environmental Engineering and Science
Understanding boron speciation is critical in determining how leaking coal ash basins could potentially affect nearby agricultural or drinking water sources due to its non-reactive and highly mobile nature. While boron is not typically considered a health risk driver at many coal ash basin sites, it is required for detection monitoring and generally indicates the leading edge of a plume when present downstream. Boron leaching rates from coal fly ash has been studied in many scenarios, but there is a lack of knowledge in the literature about bottom ash as a source of boron in basin pore waters. Additionally, most experiments conducted were on a time scale of hours to days while coal ash basins can sit idle for decades leaching constituents into the subsurface. In this work, long-term batch leaching studies were set up for industrial grade coal and coal ash sourced from the decommissioned coal fired power plant on Clemson University’s campus to determine differences in boron availability between fly ash and bottom ash. After 360 days, the fraction of boron leached from the bottom ash was the greatest at 0.89 followed by fly ash at 0.69 based only on the average of triplicate samples. However, one dataset from the bottom ash is likely an outlier due to its highly variable composition. Removing this sample gives an average of 0.50 with much lower variability and follows the expected result that fly ash leaches boron faster than bottom ash in the same conditions. The unburnt coal was then combusted in a box furnace at temperatures of 500, 600, 700, 800, and 900 °C to determine the extent of thermal fixation of boron into the structure of coal ash with increasing combustion temperature. A linear relationship was observed where increased furnace temperature decreased the fraction of boron leached after 180 days supporting the idea of thermal fixation in the ash matrix. An in- depth solid phase characterization of the materials showed that the texture of coal ash particles became more glass-like with increased combustion temperature. Boron content of the laboratory created coal ash generally decreased with depth into the solid matrix indicating an enrichment of boron on the surface of the particles. Peak fitting of the x-ray photoelectron spectroscopy spectra for the temperature dependent ash showed two species of boron present in the leachate likely borate bearing phases at a binding energy of 193 eV and boron salts at 191 eV. With the work of this study, the long-term leaching trends of boron were determined and two separate species of boron were found to exist in the coal ash pore water supporting the hypothesis that boron exists in coal ash either as soluble salts that condensed from volatilization during combustion or originated in unburnt coal as a mineralized form.
McLane, Allison, "Evaluation of Long-Term Boron Leaching Rates in Industrial Grade and Laboratory Created Coal Ash" (2021). All Theses. 3593.