Thermogravimetric properties of aquatic humic substances

Humic substances from open ocean, coastal, estuarine and fluvial environments were characterized and compared on the basis of their thermogravimetric properties. Thermogravimetric analysis revealed between two and four degradation rate peaks that were used to quantify the different thermal behavior of three fractions of humic substances (humic acids, fulvic acids and XAD4 fractions). Humic acids are the most thermo-resistant (degradation temperature>300°C) while fulvic acids and the XAD4 fraction have some components that decompose at lower temperatures, and the XAD4 fraction has a high percentage of components that degrade at temperatures below 350°C. Thermogravimetric properties of humic substances appear to be related to their structural characteristics. In addition, the differences in shape of the thermogravimetric curves and their derivatives can give important clues about the origin of aquatic humic substances, which can be seen in marine fulvic acids that show a characteristic degradation rate peak at 150–170°C and in estuarine humic acids that show only two degradation peaks.