Speciation of some selected heavy metals in coal bottom ash from Okaba Coal, Ankpa, Nigeria

Abstract

Speciation of coal cotton ash from Okaba in Kogi State Nigeria was carried out using flame atomic absorption spectrometry. Results of this research showed that the elements studied (Cr, Fe, Cu, Ni, Pb, Cd and Zn) were all observed in at least three fractions of the sequential extraction procedure. Pb showed more presence in the exchangeable fraction (80±0.03 µg/g) which is a serious problem to plants and human due to weathering activities in the soil. Most of the elements with high concentration in the residual fractions were found to be thermally stable and of low volatility (Cu and Zn). Chromium occurred mostly as a reducible fraction (80±0.02 µg/g) while Fe occurred mostly as bound to carbonate species. Cadmium was generally low in all the fractions analysed. This trend may be attributed to high volatility of Cd species. Similarly, Ni had a distribution in all the fractions with the highest in the reducible fraction (39±0.01 µg/g) and lowest in the carbonate bound fraction (24 ±0.01 µg/g). The exchangeable fraction of the elements studied is consistent with previous studies of coal bottom ash speciation. The ash matrix may also have contributed to a significant rise in the concentration of Ni at the exchangeable fraction thereby leading to significant variations in metal concentration over the ash particle.

Keywords: Speciation, heavy metals, coal, Ankpa, bottom ash.