Distribution of structural units in humic substances as revealed by multi-step selective degradations and 13C-NMR of successive residues

A quantitative assessment of structural constituents in humic substances has been achieved on the basis of 13C-NMR spectra of the residues obtained after multi-step chemical degradations. The sequence used (ultrasonic disruption in methylene chloride, followed by boron–trifluoride–methanol transesterification and hydroiodic acid treatment) was designed to progressively remove structures predominantly associated by physical interactions, ester bonds and ether bonds, leading to a residue presumably consisting of a C–C linked skeletal network. The substances analyzed were humic acids (HAs) from four widely different soil types in addition to the corresponding humins isolated by two procedures based, respectively, on flotation in a dense liquid after ultrasonic disaggregation and on partition in water–methylisobutylketone. About 30% of the carbon in HAs and humins was transformed into soluble materials by the transesterification treatment, whereas a large variability in the amount of the structures removed by the ether-cleaving treatment was observed, the greatest values corresponding to humins. In both cases, the above treatments seem to lead to a kerogen-like residue. The results provide indirect evidence for the presence of non-hydrolyzable esters, heavily-altered glycolipid-like structures and recalcitrant polymethylene material in humic substances.