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International audienceHumic substances (HS) perform a fundamental role in aquatic environments, exhibiting different levels of reactivity in retaining metal ions and organic pollutants. Also, they control the primary production of these ecosystems and act in the carbon sequestering process. In order to improve our understanding vis-à-vis the structural and functional features of HS from aquatic systems, this study aimed to chemically and spectroscopically characterize humic acids (HA) isolated from bottom sediment samples of a stream in a Brazilian subtropical microbasin by elemental analysis, and infrared (FT-IR), ultraviolet and visible (UV-Vis) and solid-state 13C nuclear magnetic resonance (CP-MAS 13C NMR) spectroscopies, thermogravimetry (TG), and scanning electron microscopy (SEM). Although all samples originated from the same environment, the data showed that the HA have distinct chemical and spectroscopic properties, and that the location and characteristics of the sampling points from which the sediments were collected played an important role in the differences observed. Furthermore, vascular plant matter is probably the main contributor to these samples

The acid decomposition of some substituted methyldithiocarbamates was studied in water at 25 °C in the range of H o −5 and pH 5. The pH−rate profiles showed a bell-shaped curve from which were calculated the acid dissociation constants of the free and conjugate acid species and the specific acid catalysis rate constants k H. The Brønsted plot of k H vs pK N, the dissociation constant of the conjugate acid of the parent amine, suggests that the acid cleavage occurs through two mechanisms that depend on the pK N. The plot presents a convex upward curve with a maximum at pK N 9.2, which is consistent with the cleavage of the dithiocarbamate anion through a zwitterion intermediate and two transition states. For pK N < 9.2, the N-protonation is slower than the C−N bond breakdown. Inverse SIE showed that the zwitterion is formed through a late transition state. At pK N > 9.2, the C−N bond breakdown is the slowest step, and according to the inverse SIE, the transition state changes rapidly with the increase of pK N to a late transition state. The plot shows a minimum at pK N ∼10, indicating that a new mechanism emerges at higher values, and it is postulated that it represents a path of intramolecular S to N proton-transfer concerted with the C−N bond breakdown. The thiocarbonyl group acts as a powerful electron-withdrawing group, decreasing the basicity of the nitrogen of the parent amine by 14.1 pK units.

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