Study of Humic Acids during Composting of Ligno-Cellulose Waste by Infra-Red Spectroscopic and Thermogravimetric/Thermal Differential Analysis

Infra-red spectroscopy (FTIR) and thermogravimetric and differential thermal analysis (TGA/DTA) were used to characterize the chemical structure of the humic acids (HA) extracted from two composts of date palm waste: in one the date palm waste was alone (DPW) and in the other, it was mixed with couch-grass clippings (DPCG). FTIR showed that the HA of both origins were rich in aromatic and phenolic structures indicating the presence of lignin degradation products. Both aliphatic and aromatic components were found in the HA. The persistence of certain aliphatic structures resistant to degradation was equally observed. For the DPCG mixture, the HA formed had a structure richer in peptides and carboxylic compounds than DPW compost, which was richer in aliphatic compounds. We can deduce that the DPCG mixture reached greater maturity than the DPW compost. The IR absorption ratios did not show the same patterns of change during the gradual degradation of substrate occurring on composting. Underlying the variations specific to each compost was a different degree of humification. It was higher for the DPCG mixture. FTIR results confirmed those of TDA/TGA thermal analysis, again showing the aliphatic and aromatic character of HA. The R-TDA ratio stresses that functionalization of organic matter was more marked than its aromatization.