Assessment of Compost-Derived Humic Acids Structure from Ligno-Cellulose Waste by TMAH-Thermochemolysis

In this study, we used the TMAH-thermochemolysis to characterize more precisely the chemical structure of the humic acids (HAs) extracted from two composts a basically lignocellulosic substrate and to follow up their biotransformation during the composting process. The analysis of the pyrograms obtained for both composts [date palm waste alone (DPW) and date palm waste-couch grass clippings mixture (DPCG)] allowed to distinguish a dominance of aromatic compounds from lignin and from nonspecific origin (92.7 and 85.5% respectively for DPW and DPCG mixture). A few nitrogen-derived and polysaccharides compounds was recorded in our HAs. For both composts, the lignin structures were represented by guaiacyl (G) and syringyl (S) units. The lack of lignin derived from p-coumaric structures could be ascribed to their degradation by influence of the extraction. This support can be confirmed by the absence in our samples of compounds with the C3 alkyl side-chain of ferulic acid. The high level of lignin-derived compounds explains the slow biodegradation and humification observed especially during DPW composting. The lignocellulose nature of date palm substrate limits the microbial proliferation responsible of the degradation and transformation of OM which is partially ameliorated by couch grass addition. This is in correlation with temperature evolution observed for DPW compost. Some variations in the chemical composition of the macromolecular content were recorded for both composting. An increase of the abundance of the nitrogen products from 6.4 and 11.6% to 8.2 and 13.8% respectively for DPW and DPCG mixture was observed. This increase can attributed to the decomposition of certain protein structures and microbial metabolism with a possible incorporation into the new formed HAs. The strong increase of methylpyrrole (isomer 8) and the strong decrease of 1H-indole for DPCG mixture indicate that the degradation phenomena were more extensive for this mixture than for DPW compost. The decrease of the total abundance of aromatic compounds of non specific origin for DPW compost against an increase for DPCG mixture indicate that humification process was more advanced in this latter leading to the incorporation of some structures into the new formed HAs. The decrease of phenol (14) toward the end DPW composting suggests its degradation and transformation into others variety of phenolic derivatives, whereas its increase for DPCG mixture can be related to its incorporation d