Browsing by Subject "Coal ash"

Coal fired power plants generate approximately 45% of the electricity produced in the United States every year, and each year, over 100 million tons of coal ash are produced as a by-product of electricity generation. Coal ash is a solid waste made up principally of bottom ash, fly ash, and flue gas desulfurization materials. The chemical composition of coal ash varies depending on the feed coal source, combustion parameters, and the presence and type of air pollution control devices that remove contaminants from the flue gas into the solid waste stream. Although a significant portion of coal ash waste is recycled, the majority of coal ash is disposed in landfills and holding ponds. Coal ash impoundments have a long history of environmental degradation, which includes: contaminant leaching into groundwater, the discharge of contaminant-laden effluent into surface waters, and catastrophic impoundment failures and ash spills. Despite these known problems, coal ash is not considered a hazardous waste, and thus is not subject to stringent disposal requirements. The current coal ash management system is based on risk assessments of coal ash that do not include environmental parameters that have a profound impact on coal ash contaminant mobility, particularly for the toxic elements such as mercury, arsenic, and selenium. This dissertation research focused on the biogeochemical transformations of mercury, arsenic, and selenium associated with coal ash materials in an effort to: (1) define the key environmental parameters controlling mercury, arsenic, and selenium fate during disposal and ash spills; and (2) delineate the relationship between coal ash characteristics, environmental parameters, and leaching potential.

The impact of coal ash on mercury transformations in anaerobic systems was assessed using anaerobic sediment-ash microcosms to mimic an ash spill into a benthic aquatic system. Anaerobic sediments are the primary zones for the microbial conversion of inorganic mercury to methyl mercury (MeHg), a process that is mediated by anaerobic bacteria, particularly sulfate reducing bacteria (SRB). MeHg is a potent neurotoxin that biomagnifies up the aquatic food chain, presenting a human health risk-- especially to children and pregnant women. The results of the sediment-ash microcosm experiments indicated negligible net production of MeHg in microcosms with no ash and in microcosms amended with the low-sulfate/low-Hg ash. In contrast, microcosms amended with sulfate and mercury-rich ash showed increases in MeHg concentrations that were two to three times greater than control microcosms without ash. The enhancement MeHg production in the microcosms was likely due to large quantities of leachable sulfate that stimulated the activity of methylating bacteria. Overall, these results highlight the importance of considering both the geochemical conditions of the receiving environment and the chemical composition of the coal ash in assessing the MeHg potential of coal ash.

The hypothesis that sulfate-rich coal ash can change sediment microbial communities, enhancing MeHg production, was tested by analyzing coal ash impacts on the SRB community in the sediment-ash microcosms using Terminal Restriction Fragment Length Polymorphism (T-RFLP), Quantitative Polymerase Chain Reaction (q-PCR), and Reverse Transcription-qPCR (RT-qPCR). Coal ash did not appear to cause significant changes to the structure of the overall bacterial community, though results showed that it may have caused a decrease in the evenness for species distribution for both SRB and the overall microbial community. During the five-day incubation experiment, the coal ash had a temporary significant effect on SRB abundance during the first one to two days of the experiment and a more sustained effect on SRB activity. This stimulation of SRB population growth and activity also corresponded with increasing net MeHg production. Overall, results indicate that coal ash amendments do not cause large shifts in the overall microbial community or the SRB community, but results indicate that there are connections between SRB abundance/activity and MeHg production. More research is needed to determine how coal ash directly impacts Hg methylating microorganisms, which include diverse array of microorganisms outside of SRB.

The effect of aerobic and anaerobic conditions on arsenic and selenium leaching from coal ash in an ash spill scenario was also assessed using sediment-ash microcosms. The fate of arsenic and selenium associated with coal ash is of particular concern due to the leachability of these elements at neutral pH and their tendency to bioaccumulate in aquatic organisms. Both the redox speciation of arsenic and selenium, and the pH of the aquatic system, are known to influence leaching into the environment, yet current environmental risk assessments of coal ash focus on pH alone as the primary driving force for arsenic and selenium leaching from coal ash and do not take into account the effects of anaerobic conditions and microbial activity. In this research, total dissolved concentrations of arsenic and selenium, dissolved speciation of arsenic, and solid phase speciation of selenium were monitored to determine the biogeochemical transformations and leaching of arsenic and selenium under differing redox conditions. The results from the sediment-ash microcosm studies showed that redox potential was the major determinant of arsenic and selenium mobility in the microcosm systems with greater arsenic leaching occurring in anaerobic microcosms and greater selenium leaching occurring in aerobic microcosms. Furthermore, the experiments provided clues to how coal ash influences the geochemistry of the benthic environment and how these influences affect the speciation and longer term solubility of arsenic and selenium.

Finally, experiments were conducted to determine how differing CaO, SO3, and Fe2O3 concentrations in coal ash affect the release of arsenic and selenium from sediment-ash mixtures in a simulated ash spill environment. Aerobic and anaerobic sediment-ash microcosms were constructed to mimic an ash spill into a benthic aquatic system, and a variety of coal ash materials were tested as amendments, including seven fly ashes, one lime-treated fly ash sample, and two FGD samples. Results showed that, in most cases, the sediment in the microcosm buffered the system at neutral, which counteracted leaching impacts of differing CaO and SO3 concentrations in the microcosms. Regardless of ash material, leaching of selenium was greater under aerobic conditions and was correlated with the total selenium content of the microcosm. Maximum leaching of arsenic occurred in anaerobic microcosms for some ash materials and in aerobic microcosms for other materials, suggesting that ash material chemistry played a significant role in controlling arsenic mobility. In both aerobic and anaerobic microcosms, dissolved arsenic concentration was correlated with total arsenic content of the ash material and in anaerobic microcosms, dissolved arsenic concentrations also correlated with the total iron content of the ash material. Overall, the results of these experiments showed that arsenic and selenium release under environmentally relevant conditions cannot be predicted by the CaO and SO3 content of the ash material. Rather, the total arsenic, total selenium content, and total iron content of the ash material are good predictors of the worst case environmental leaching scenario.

These investigations illuminated two major conclusions: (1) microbial activity and differing redox conditions are key in determining the impact of coal ash on the environment and in determining the mobility of coal ash contaminants, and (2) coal ash characteristics, such as sulfate and iron content, can change the redox chemistry and microbial activity of the surrounding environment, further influencing the fate of ash contaminants. This work will be useful in designing a framework that accurately predicts the leaching potential of ash contaminants under environmentally relevant conditions. The results will also be helpful in developing treatment technologies for ash impoundment effluent, guiding decisions on ash pond closure and remediation, and in designing long-term monitoring plans and remediation strategies for ash-impacted sites.

Coal fired power plants are ubiquitous in the United States and most developed countries around the world, providing affordable electricity to consumers. In the US, approximately six hundred power plants generate 136 million tons of Coal Combustion Residuals (CCRs) annually, encompassing fly ash, bottom ash, and flue gas desulfurization materials. The range and blends of CCRs varies substantially across coal-fired plants and depends on a unique set of circumstances for each plant and coal source. Current U.S. regulations mandate the installation of advanced capture technologies to reduce atmospheric pollution, but do not address the transfer and storage, or the potential impacts to water resources. Thus improved air quality is traded for significant enrichments of contaminants in the solid waste and effluent discharged from power plants.

This work examines the geochemical and isotopic characteristics of CCRs, as well as potential environmental impacts from CCRs. This investigation looks at several different aspects of CCR and environmental interactions from 1) the immediate impacts of the 2008 TVA coal ash spill in Kingston, TN, 2) the long-term (18-month) exposure of the spilled ash in the Emory and Clinch rivers, 3) impacts on waterways in North Carolina that receive CCR effluent from coal fired power plants, and 4) examination of boron and strontium isotopes of CCRs from leaching experiments and their application as tracers in the environment of the TVA spill and NC waterways. These investigations have illuminated several conclusions, including contact of surface water with CCRs leach high concentrations of leachable CCR contaminants, such as As, Se, B, Sr, Mo, and V in the surface waters; the dilution effect is critical in determining the concentration of contaminants from the CCRs in surface water (both at the spill and in waterways receiving CCR effluent); recycling of trace elements (such as As) through adsorption/desorption can impact water quality; and elevated boron and strontium concentrations, in addition to their isotopes, can trace CCR effluent in the environment. Combining the geochemical behavior and isotopic characteristics provides a novel tool for the identification CCR effluents in the environment.

Coal combustion residuals (CCRs or coal ash), phosphate rocks and fertilizers, and leaded gasoline and lead-based paint represent major anthropogenic sources of metal and metalloid (i.e., metal(loid)) contaminants released to the environment. This dissertation aims to characterize the compositions of trace elements and radiogenic isotopes (Pb, Sr) of these anthropogenic sources and further explore their individual applicability and/or conjunctive utility with radioactive isotopes (228Ra, 226Ra, 137Cs, and 210Pb) in tracing the origin, timing, and impacts of metal(loid) contamination at various scales and in multiple environmental settings.The trace element compositions and isotopic signatures of Pb and Sr in fly ash originating from coals of different coal basins in the United States were characterized. In addition, an extended database of the Pb isotope fingerprints of coal and coal ash from China and India – the world’s top two coal producers and consumers – was established, combining newly measured values of coal and coal ash samples in this dissertation and data compiled from the literature. The results showed that (1) the Pb isotope signature of coal fly ash is distinctive from the isotope compositions of both the legacy anthropogenic Pb sources (i.e., leaded gasoline and lead-based paint) as well as natural Pb, which can be used for detecting fly ash contamination in the environment; (2) the 87Sr/86Sr ratio of bulk coal fly ash is distinctive from that of water-soluble fraction, which reflects the heterogenous distribution of Sr in fly ash and indicates the different uses of 87Sr/86Sr ratio for tracing its contamination in different environmental settings (i.e., terrestrial versus aquatic); and (3) the integrative use of trace elements, Ra isotopes (228Ra/226Ra), and Pb isotopes can further improve the detection of trace levels of coal fly ash dispersed in the environment. Through the integration of geochemical and isotopic tools (i.e., trace element distribution and 87Sr/86Sr ratio) with morphological and magnetic observations, this dissertation revealed the decades of historical and current unmonitored releases of coal ash and associated metal(loid) contaminants from the inadequate coal ash disposal units to the adjacent freshwater lakes across North Carolina (NC). The temporal distribution and evolution of coal ash contamination in the lake sediments were constructed by 137Cs- and 210Pb-based chronology techniques. The contributions of coal fly ash to the total Pb accumulation in the sediments of these contaminated lakes were quantified using a Bayesian-based Pb isotope mixing model, and the results suggested that regionally Pb contamination from fly ash can significantly outweigh the Pb input of atmospheric deposition (i.e., leaded gasoline) in the environment. Furthermore, the Pb isotope compositions of coal fly ash from China, India, and the U.S. were constrained and the fluxes of Pb associated with coal fly ash disposal in the three countries were estimated, laying the groundwork for future research on the impacts of coal ash on the Pb biogeochemical cycles at larger scales. Additionally, this dissertation reported the first set of data on the Pb isotope compositions along with rare earth elements and yttrium (REY) of phosphate rocks and fertilizers sourced from different regions and origins around the world. The geological imprints reflected in the geochemical and Pb isotopic fingerprints of the phosphate rocks were discussed and their potential utilizations and limitations in tracing phosphate-associated metal(loid) contamination in the environment were evaluated. This lays the groundwork for future local and regional studies on tracing the impacts of metal(loid) contaminants from phosphate rock mining and phosphate fertilizer application. Furthermore, this dissertation showcased a holistic assessment of the legacy anthropogenic contamination of Pb and other metal(loid)s in urban soils from Durham, NC, whereby fallout radionuclides 137Cs and 210Pb were proposed as potential indicators of the extent of soil disturbances that can impact the mobilization and redistribution of metal(loid) contaminants. The imprints of distinctive Pb isotopic fingerprints of leaded gasoline and lead-based paint in the soils reflected the persistent presence of these legacy sources in the urban environment of today, and the potential bioavailability of toxic metal(loid)s in the contaminated soils upon oral ingestion was assessed via in vitro arrays.