Changes in humic acid composition with molecular weight as detected by 13C-nuclear magnetic resonance spectroscopy

Abstract

Humic acid extracted from a grassland soil was fractionated with respect to molecular weight using gel chromatography. The resulting elution pattern consisted of a well-defined exclusion peak and a broad second peak. The eluate was divided into four subfractions corresponding to four features of the elution curve. These subfractions were further fractionated using a range of gels to give samples with a relatively narrow molecular weight range. 13C-NMR spectra were obtained for the whole humic acid and the four molecular size subfractions. The spectra showed well-resolved signals from carbon in carboxylic acids and carbonyl groups, CO groups in phenols or phenolic ethers, aromatic or other unsaturated structures, carbohydrates, methoxyl groups, and saturated aliphatic groups. The spectra showed substantial variation in chemical composition with changing molecular weight. With decreasing molecular weight, aliphatic carbon and aliphatic CO content decreased whereas aromatic carbon and CO content increased. The carboxyl group content determined independently by titration showed the same trends as the carboxylate content determined by 13C-NMR.