COMPOSITION, MORPHOLOGY AND TRACE ELEMENT LEACHING FROM US COAL FLY ASH

The leaching of trace elements from coal ash has been studied extensively in the past few decades. However, the majority of these studies have been conducted by varying pH and liquid-to-solid ratios in oxic environments. Little is known concerning the fate of coal ash and its various trace element constituents in complex aquatic systems, where parameters such as redox potential and dissolved organic matter (DOM) can control trace element cycling. In this study, coal ash samples collected from coal-fired power plants located in Kentucky, USA were exposed to a buffered solution (50 mM MOPS, pH ~7) with DOM (20 mg/L) and varying redox potential (using 0.5 mM sodium sulfide as a chemical reductant) to examine trace element leaching. The experimental data suggested that redox potential affected the leached concentrations of iron, arsenic, selenium, thallium, uranium, tungsten, antimony, cobalt, chromium and vanadium, while concentrations of elements such as boron, molybdenum, lead, calcium, sodium and barium remained unaffected. However, for the DOM experiments, increased ionic strength due to leaching of major cations from the coal ash (e.g., calcium, magnesium, barium, sodium) appeared to affect the conformation of the DOM, resulting in the precipitation of the majority of the DOM out of solution. Coal ash was also characterized using Transmission Electron Microscopy coupled with Energy Dispersive Spectroscopy (TEM-EDS) and X-ray Diffraction (XRD) to examine elemental composition and particle morphology. TEM-EDS mapping indicated that the spatial occurrence of trace elements in coal ash varied, with certain elements widely distributed (e.g., arsenic) while other were located in hotspots at the particle surface (e.g., lead, chromium). Future work includes repeating the DOM experiments at lower ionic strength to prevent precipitation of DOM, as well as further TEM-EDS and synchrotron-based micro-X-ray absorption spectroscopy mapping for bulk elemental composition analysis.