Characterization of humic material formed by composting of domestic and industrial biowastes : Part 2 spectroscopic evaluation of humic acid structures

Humic acids (HAs) were extracted from domestic and industrial biowaste composts in three different phases of the composting process during a two-year period. The effects of humification on molecular structures, functionalities, and the resistance of compost HAs to acid hydrolysis were evaluated by elemental and functional group analysis and by FT-IR, fluorescence, EST and UV-VIS spectroscopies. As the composting proceeded, a tendency towards increasing carbon-carbon double bond structures was observed in the HAs of domestic biowaste and pulp and paper industry biowastes, and their hydrolysis residues. The older HAs of domestic biowaste appeared to be more resistant to hydrolysis; FT-IR study of these after acid hydrolysis showed them to contain aliphatic carbohydrate-like structures as stable, covalently bound components. The active functionalities of HAs were found to form complexes with paramagnetic metal cations (Fe(III), Cu(II)) even in the early phases of the process. The composition and concentration of the HA-metal complexes varied during composting. Fe(III) resonances decreased in intensity with compost maturity and disappeared upon acid hydrolysis, whereas Cu(II) resonances were recorded even from hydrolysed HAs.