Occurrence and distribution of metals and porphyrins in Nigerian coal minerals

The metal contents of Nigerian coal minerals were analyzed using an atomic absorption spectrophotometer. Calcium, Na and Fe occurred as major elements with concentrations ranging from 9782 μg/g for Ca to 432 μg/g for Na, whereas K, Mg, Mn, Ni, Cr, Zn, Pb and Cu, which occurred at trace levels ranged from 673.73 μg/g for Mg to 2.97 μg/g for Mn. Results of quantitative analysis of porphyrins extracted from the coal minerals showed that Onyeama coal has the highest amount of porphyrins (ca 0.96 μg/g) while Okpara has the lowest (ca 0.30 μg/g). The porphyrins were qualitatively characterized by a combination of thin layer chromatography (TLC), infrared and ultraviolet-visible spectrophotometers. The results of the mid infrared analysis (MIR) showed the presence of absorption bands at 3440–3450 and 1640–1680 cm−1 which are owing to stretching vibrations of N–H and C=C of aromatics, with C–H out of plane (oop) bending vibrations at wavenumbers less than 900 cm−1, all of which are characteristic absorptions of porphyrin free base. The ultraviolet-visible data showed prominent peaks at 400 nm (Soret band) and at wavelength ranges of 535–550 nm (β–band) and 565–600 nm (α–band) for the coal porphyrins analyzed. The geochemical significance of the metals and porphyrins in coal minerals are discussed.