Assessment of the mode of occurrence and radiological impact of radionuclides in Nigerian coal and resultant post-combustion fly ash using scanning electron microscopy and gamma-ray spectroscopy

Natural radionuclide concentrations in coal and coal ash can occur at levels enough to raise potential health and environmental concerns when (re)suspended or disposed into the environment. To evaluate such concerns, this study characterised coal and simulant coal ash samples obtained from two Nigerian coal mines (Okaba and Omelewu) using high resolution gamma spectroscopy combined with scanning electron microscopy and energy dispersive spectroscopy. Discrete uraninite particles were observed dispersed within the coal ash samples, alongside U and Th containing mineral grains (monazite and zircon) with monazite the most abundant radioactive mineral particles. The pitted and cracked surface morphologies of these radioactive particles (with sizes between 10 μm and 80 μm) indicates their susceptibility for disintegration into more harmful and readily inhalable PM2.5 aerosol particles, with the potential to deliver localised dose and cause chronic respiratory diseases. The results of activity concentrations and radiological hazard indices for the coal ash samples from both mines were between 3 and 5 times higher than world average in soil, which imply that these coal ash materials should be suitably contained in slurry ponds to prevent hazard due to increased risk of prolonged indoor exposure to gamma radiation, radon gas and inhalation of liberated radioactive particles.