Reductive degradation of aquatic humic acid for structural characterization

The structure of aquatic humic acid was investigated using reductive degradation by sodium metal. Solvents and cosolvents, proton donors and the form and amount of reductant were examined. Sodium amalgam in alkaline solution with diethyl ether as cosolvent provided a 4 to 5-fold increase in chromatographable yield over other methods evaluated. Most important in attaining the optimized method were the use of alkaline solution as the primary solvent and the use of diethyl ether as cosolvent. Proton donors were not a limiting factor.; Sodium reduction caused about 30% of the humic acid (based on total organic carbon measurements) to become soluble in acid but the alteration was attributable to solvolysis. About 7% of the original humic acid weight was extractable with diethyl ether and 5% was chromatographable, similar to other mild degradation methods. The difference between the latter two values is not significant due to method variability. The variety of the products identified in the chromatographable yield indicated that the reduction was not limited by substrate selectivity.; The chromatographable yield consisted of more than 200 compounds many of which were tentatively identified by matching their low resolution, electron impact mass spectra with library spectra. The compounds produced included noncarboxylated aliphatics, monocarboxylated aliphatics (saturated and unsaturated), dicarboxylated aliphatics (saturated and unsaturated), noncarboxylated aromatics, monocarboxylated aromatics, dicarboxylated aromatics, furans, cyclic alkenes and others. Aliphatic diacids were the most abundant component of the aquatic humic acid based on molar concentrations (48% of the identified compounds) followed by aliphatic monoacids (25%). Most aliphatic compounds contained three to six carbons. Most aromatic compounds were monocarboxylated and represented 13% of the identified products. There were small amounts of aromatic nonacids and aromatic diacids (4% and 2%, respectively). The prevalence of the aliphatic material was also demonstrated by the relative distributions of molar amounts of carbon, hydrogen and carboxyl carbon. A large majority of the carboxyl carbon (66%) was associated with aliphatic diacids. Reduction data indicated there was at least 3 times more carboxyl carbon associated with aliphatic compounds than was associated with aromatic compounds suggesting that aliphatic acidity is an important element in humic acid structure.