Highlights• The total rare earth content of the studied samples is promising for recovery.• Critical rare earths comprised 30wt% of rare earth content within samples studied.• Micromanipulation method employed to isolate individual monazite particles.• Radial zonation observed in monazite particles, with rim rich in rare earths.• Ce oxidised and monazite structure amorphized, increasing chemical reactivity.

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Significant amounts of rare earth elements, REEs (14% to 31%) and critical REEs (16 wt% to 38 wt%) in the fly ash samples were recovered in the acid-soluble fraction of the sequential extraction procedure using ethanoic acid.

Natural radionuclide concentrations in coal and coal ash can occur at levels sufficient to raise potential health and environmental concerns when (re)suspended or disposed into the environment. To evaluate such concerns, this study characterized 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) indicate their susceptibility for disintegration into more harmful and readily inhalable PM2.5 aerosol particles, with the potential to deliver a localized 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 three and five times higher than world average in soil, which imply that these coal ash materials should be suitably contained in slurry ponds to prevent hazards due to increased risk of prolonged indoor exposure to gamma radiation, radon gas, and inhalation of liberated radioactive particles.