Trace Metal And Metalloid Behavior In The Submicron Scale Components Of Coal Fly Ashes: Insights From Combined Microscopic And Microbiological Studies

For over a century, coal has been used worldwide to meet ever-growing energy demands. Coal combustion accounts for over 30% of electricity generation in the United States. In the U.S., coal combustion produces annually over 100 million tons of coal combustion products (CCPs) of which only 30% are used in beneficial and economic ways. One of the primary CCPs from coal-burning power plants is fly ash, typically fine-sized and enriched in many trace elements (e.g., arsenic, chromium, lead, and vanadium). When disposed in the environment, fly ash may release significant concentrations of trace elements into surrounding ecosystems and induce toxicity in relevant biotas. In this study, focus has been given to the distribution and potential mobility of a range of trace elements in the submicron components of fly ash obtained from various sources. The specific goals of this study are to illuminate i) what trace elements tend to accumulate in the fine (i.e., nanoscale) fractions of fly ash and in what patterns, ii) what are the release rates of trace elements from the fine fractions as a result of aqueous dissolution, and iii) what role does microorganisms play in affecting the mobilization of trace elements from the fine fractions of fly ash. Overall, this work has direct implication for understanding the fate of CCPs in the environment. The fine fractions of CCPs such as economizer fly ash (EFA) and, fly ash (FA) collected from two coal-fired power plants in Ohio and New Mexico were characterized by electron microprobe (EMP), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and inductively coupled plasma mass spectrometer (ICP-MS), respectively, with a focus on the trace element behavior during combustion and disposal. Previous analyses of the fly ash samples using X-ray diffraction (XRD) found that they are dominantly amorphous glass material (~70%) with minor mineral phases (~30%). Specifically, the fly ash consists of glass-like alumino-silicate spheres, sized smaller than 50 µm, as observed by EMP, SEM, and TEM. To focus on the (sub) micron fractions, the fly ash was further separated using a cyclone apparatus with a particle size cutoff of ~2 µm. Examination of the submicron fractions using SEM and TEM revealed the presence of nano-scale core and rim structures where metals like iron are prone to forming an oxide coating layer on the particles; in comparison, metals like zinc are mainly retained in the core of the fine particle. Also, calcium and phosphorus were found to grow solely on the particle surfaces, suggesting that these elements deposit later during particle formation within a power plant. A series of microbiological experiments were conducted on the fine sieved fly ash (\u3c150 µm) involving a model bacterial strain Pseudomonas aeruginosa to assess the influence of biological activities on the metal release from CCPs to an aqueous environment, and reversely, on the impact of the presence of CCPs on the viability of the bacterial cells. The presence of P. aeruginosa cells lowered the release of trace elements in the solution by 14.72% to 84.1% depending on the element. In particular, the samples with microbes exhibited significant decrease of iron, aluminum, manganese, vanadium, chromium, and arsenic. Coupling electron microscopy characterization with microbiological experiments provides a useful tool and perspective for studying coal combustion products environmental interactions. Scanning electron microscope and transmission electron microscope techniques are powerful in illuminating the overall distribution patterns of trace metals within coal fly ashes as well as pinpointing the elements of special interest. Additionally, our microbiological study shows that the ubiquitous biofilm-forming Pseudomonas aeruginosa has a strong preference to remove some leached trace elements from coal fly ash in solution, likely through biosorption onto their cellular components. This observation has significant implication for the application of similar microorganisms in mitigating the potential hazards of coal ash deposits and in bioremediation of coal ash spills