A comparative chemical-structural study of fossil humic acids and those extracted from urban wastes

Abstract

Chemical-structural features of commercial humic acids (HAs) from leonardite or lignite were studied and the data obtained were compared with those of humic acids extracted from composted urban wastes. The greatest differences showed by the elemental analysis between the three HAs were in N and H contents, both of which diminished with the oxidation degree of the starting materials. Unlike the HA from the more oxidized material HAL1 showed the highest content of oxygen-containing functional groups. FTIR spectra did not show differences between HAs from evolved materials such as leonardite or lignite. However, differences were found between these HAs and those extracted from composts of urban wastes, which showed a greater aliphatic character and a more pronounced peak in the absorption band attributed to secondary amides and in that of carbohydrates. The 13C-NMR spectra were similar for both HAs from leonardite regardless of their oxidation degree. The percentage of aromaticity of these HAs was 45%. HALi showed a low percentage of aromaticity, probably due to the addition of non-humified carbon to this HA during its industrial extraction, which would cause an enlargement of the peak appearing at 30 ppm, thus decreasing the peaks indicative of aromaticity. The spectra of compost HAs showed a low aromaticity degree for these HAs as a consequence of the pronounced peak appearing at 73 ppm corresponding to carbon of carbohydrates and/or polyalcohols and aminoacids. Py-GC revealed a high content of benzene and toluene in all the commercial HAs. The values of these fragments as well as those of the ratios between pyrolitic fragments, used as humification index for soils, were the highest in the HA extracted from the more oxidized leonardite.