Comparison study of sedimentary humic substances isolated from contrasting coastal marine environments by chemical and spectroscopic analysis

To gain a better understanding of the chemical structural composition and reactivity of sedimentary organic matter in coastal environments, the properties of sedimentary humic substances (HSs) isolated from contrasting coastal marine environments (i.e., salt marsh estuary, continental shelf, and continental slope) in China were investigated and compared. The complementary analytical approaches employed included elemental analysis, cross polarization magic angle spinning (CP/MAS)-¹³C nuclear magnetic resonance, pyrolysis-gas chromatography–mass spectrometry, and potentiometric titrations. The results indicate that HSs isolated from the salt marsh site in the Changjiang Estuary, which is strongly influenced by terrestrial river inputs, had the highest content of lignin-derived components, such as phenols and alkyl phenols, and acidic functional groups; HSs from the continental slope site in the South China Sea, which mainly originated from the inputs of marine algae and planktons, contained the highest level of aliphatic compounds, carbohydrates, and nitrogen-containing compounds; however, HSs from the continental shelf site in the East China Sea presented a mixture of both land and marine properties due to the influence of the two sources. Humic acids were found to contain comparatively more highly branched, long-chain aliphatic components but fewer oxygen-containing functional groups, such as carboxyl groups, and fewer sulfur-containing compounds than their fulvic acid counterparts. The total concentrations of proton binding sites and the structural characteristics of the studied HSs may provide insights into the migration and fate of HS-bound contaminants, such as heavy metals and organic pollutants, in coastal marine systems.