Humic acids from oxidized coals I.: Elemental composition, titration curves, heavy metals in HA samples, nuclear magnetic resonance spectra of HAs and infrared spectroscopy

The formation of humic acids (HAs) from bituminous coal was verified by laboratory oxidation. In a relatively short time the oxidation by air at temperature above 150 degreesC led to the formation of HAs. These HAs were compared with those isolated from oxidized bituminous coal from the vicinity of red bed bodies, from weathered bituminous coal, oxihumolite and lignite. For this the organic and inorganic elemental composition of HAs, apparent dissociation constants, metal-binding capacities, nuclear magnetic resonance and infrared spectra were determined and evaluated. With increasing temperature of laboratory oxidation of bituminous coal the percentage of aromaticity of HAs increases. HAs prepared from coal oxidized at 150 degreesC are characterized by an aromaticity index 78% while for HAs prepared at 250 and 300 degreesC by 95% aromaticity. The same index for HAs isolated from naturally oxidized bituminous coals is of about 87% whereas it is of about 50% for oxihumolite and lignite. The apparent dissociation constants (K-app) are much higher in HAs isolated from oxihumolite and lignite (pK(app) from 3.35 to 3.80) than those from oxidized bituminous coal samples (pK(app) from 4.47 to 4.85). There is a good negative correlation between Pb-binding capacity and pKapp of all samples suggesting that metallic ions are bonded to acidic groups of HAs. Also contents of inorganic elements like Fe, Al, Si are much higher in HAs isolated from lignite and oxihumolite than those in HAs from oxidized bituminous coal. Thus, it seems that the temperatures below 150 degreesC and the long oxidation time are necessary conditions for the formation of HAs in oxidative altered bituminous coal deposited deeply under the earth surface. (C) 2003 Elsevier Ltd. All rights reserved.