Elemental analysis, FTIR and 13C-NMR of humic acids from sewage sludge composting

The effect of composting on the chemical structure of humic acids extracted from a mixture of sewage sludge and straw at different times of treatment was investigated by elemental analysis, Fourier transform infrared and carbon 13-nuclear magnetic resonance. The starting material was composed of a heterogeneous mixture of etherified aromatic structures bearing peripheral long-chain lipidic, peptidic and carbohydrate structures. The results show a decrease in aliphatic compounds during composting, and an increase in aromatic structures - mainly those substituted by functional groups such as phenolic, methoxylic and carboxylic groups. These changes support the preferential oxidation of readily accessible moieties such as aliphatic side chains of lipidic and peptidic structures, leading to the production of highly functionalized aromatic macromolecules. The latter compounds are resistant to microbial degradation and are therefore preserved as part of the newly-formed humic acid structures.