The dissolution kinetics of humic acid particles and the need of modeling the solid humic acid/solution interface.

M. BRIGANTE; G. ZANINI; M. AVENA

Toulon, Francia

Workshop; 1st International workshop on organic matter modeling; 2005

Humic acids (HA) are very active in binding ions, organic molecules and solid surfaces, and may influence markedly the transport of pollutants in natural waters. They represent an important fraction of the organic matter in soils and sediments, where they are present either as dissolved molecules in the aqueous phase, adsorbed at the surface of minerals or forming relatively pure HA particles. HA molecules are mainly transported by water in the environment and thus it is very important to understand how HA particles dissolve and at which rate they dissolve. This work presents experimental data on the dissolution kinetics of HA particles. The effects of pH, temperature and presence of divalent cations are mainly analyzed. Experiments performed by UV-Vis spectrophotometry under well controlled conditions show that  the dissolution rate increases by around three orders of magnitude from pH 4 to pH 11. It is believed that the rate increases mainly because of the increased electrostatic repulsion between negatively charged molecules at the solid HA/solution interface. On the contrary, the presence of calcium ions in the solution reduces markedly the dissolution rate at a given pH, supposedly by blocking reactive surface groups and/or by decreasing the electrostatic repulsion between HA molecules. The need of developing models to understand the reactivity of the solid HA/solution interface and its application to estimate equilibrium constants between cations and HA surface groups will be also analyzed.