DISTRIBUTION OF STRUCTURAL UNITS IN HUMIC SUBSTANCES AS REVEALED BY MULTISTEP SELECTIVE DEGRADATIONS AND C-13-NMR OF SUCCESSIVE RESIDUES

A quantitative assessment of structural constituents in humic substanc es has been achieved on the basis of C-13-NMR spectra of the residues obtained after multi-step chemical degradations. The sequence used (ul trasonic disruption in methylene chloride, followed by boron-trifluori de-methanol transesterification and hydroiodic acid treatment) was des igned to progressively remove structures predominantly associated by p hysical interactions, ester bonds and ether bonds, leading to a residu e presumably consisting of a C-C linked skeletal network. The substanc es analyzed were humic acids (HAs) from four widely different soil typ es in addition to the corresponding humins isolated by two procedures based, respectively, on flotation in a dense liquid after ultrasonic d isaggregation and on partition in water-methylisobutylketone, About 30 % of the carbon in HAs and humins was transformed into soluble materia ls by the transesterification treatment, whereas a large variability i n the amount of the structures removed by the ether-cleaving treatment was observed, the greatest values corresponding to humins. In both ca ses, 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 p olymethylene material in humic substances. (C) 1998 Elsevier Science L td. All rights reserved.