Chemical imaging analysis of environmental particles using the focused ion beam/scanning electron microscopy technique: microanalysis insights into atmospheric chemistry of fly ash

Airborne fly ash from coal combustion may represent a source of bioavailable iron (Fe) in the open ocean. However, few studies have focused on Fe speciation and distribution in coal fly ash. In this study, chemical imaging of fly ash has been performed using a dual-beam focused ion beam/scanning electron microscope (FIB/SEM) system for a better understanding of how simulated atmospheric processing can modify the morphology, chemical composition and element distribution within individual particles. A novel approach has been applied for cross-sectioning fly ash particles with the FIB in order to explore element distribution within the interior of individual particles. Our results indicate that simulated atmospheric processing can cause disintegration of aluminosilicate glass, a dominant material in fly ash particles. Fe present in the inner core of fly ash spheres within the aluminosilicate phase is more easily mobilized compared with Fe oxides present as surface aggregates on the exterior of fly ash spheres. Fe dissolution depends strongly on Fe speciation in fly ash particles. The approach for preparation of a cross-sectioned specimen described here opens up new opportunities for particle microanalysis, particularly with respect to inorganic refractive materials like fly ash and mineral dust.