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Investigating aggregation in Suwannee River, USA, dissolved organic matter using diffusion-ordered nuclear magnetic resonance spectroscopy

Lam, Buuan, Simpson, André J.
Environmental toxicology and chemistry 2009 v.28 no.5 pp. 931-939
humic substances, nuclear magnetic resonance spectroscopy, colloids, pollution, freshwater, spectral analysis, molecular weight, temperature, diffusion, chemical composition, lignin
Aggregation of the Suwannee River (USA) dissolved organic matter (SRDOM) is studied using nuclear magnetic resonance spectroscopy. Diffusion-ordered spectroscopy identifies two distinct bands, one corresponding to smaller components and the other to larger components. At lower concentrations (500 ppm), the dissolved organic matter (DOM) is present mainly as small components; however, with increasing concentration, the larger components become more pronounced as a result of aggregation. Calibrations indicate the small materials behave in a manner similar to that of maltodextrins of approximately 180 to 950 Da or proteins of 100 to 1,000 Da. The aggregated species show behavior similar to that of maltodextrins of approximately 1,000 to 21,000 Da or proteins of 1,050 to 70,000 Da. The mean diffusivity of the aggregated components in SRDOM is consistent with that of maltodextrins of approximately 4,500 Da and proteins of approximately 8,000 Da at the highest concentration measured. At the lowest concentration (closest to environmental concentrations), little to no aggregation is observed. Diffusion profiles show an increase in large-molecular-weight material, with a simultaneous decrease in small-molecular-weight components with increasing DOM concentration. This suggests aggregates in SRDOM may be weak dispersive associations of low-molecular-weight material. Additionally, with a decrease in temperature, aggregates show faster diffusion, suggesting a tighter, more condensed arrangement. Further evidence supports DOM aggregation as conglomerations of numerous components in DOM rather than a more organized self-association. Carboxyl-rich alicyclic molecules (CRAM) play a prominent role in aggregation of SRDOM and, to a lesser extent, material-derived from linear terpenoids (MDLT). The role of lignin and carbohydrates is less clear, although at least some of the lignin is present as macromolecules and tends to show interactions with both the MDLT and CRAM components.