The characterization of humic substances in seawater

Abstract

Humic substances were isolated in gram quantities from seawater by a newly developed procedure of adsorption on a crosslinked polystyrene-divinylbenzene resin. The chemical and physical characteristics of both humic acid and fulvic acid fractions were studied. The elemental composition, the acidimetric titration characteristics, the l3C: l2C ratios, and the ultraviolet-visible, fluorescence, and infrared spectra were determined. Molecular weight distributions of coastal and Sargasso Sea fulvic acids were measured by gel permeation chromatography. Structural features were further investigated by both proton and carbon-13 nuclear magnetic resonance spectroscopy. In addition, the fulvic acids and their derivatives were analyzed by low and high resolution mass spectrometry and combined gas chromatography-mass spectrometry (GC-MS). Amino acids and organic solvent soluble products in acid hydrolyzates were investigated. An array of biogenic hydrocarbons produced from fulvic acid by a new reduction scheme were characterized by GC-MS. The structural features of seawater humic substances are complex. They are highly aliphatic, polyfunctional materials containing both polar and nonpolar moieties. Hydrolyzable amino acids constitute a low percentage of the nitrogen. Fatty acids and other lipids are important structural components. Seawater humic substances have significant structural differences from those of terrestrial origin; this seems to result mainly from the relatively low input of lignin to the marine environment and the differences between the physical environment of the soil and the sea. A mechanism is proposed for the formation of seawater humic substances from amino acids, sugars and lipids. The effects and fate of humic substances in the sea are discussed.