The dissolution kinetics of humic acid particles

M. BRIGANTE; G. ZANINI; M. AVENA

Kyoto

Conferencia; 5th International Conference Interfaces Against Pollution; 2008

Humic acids (HA) are a very active fraction of the refractory organic matter in soils, sediments and natural waters. Their molecules can bind solid surfaces, metal ions and organic molecules such as pesticides or other contaminants. As a consequence of this binding HA can affect significantly the distribution and transport of contaminants in the environment. In fact, when HA molecules are in the dissolved state, they are rather mobile and can be transported by water flows (rivers, groundwaters), transporting with them the attached pollutants. On the contrary, when they are in the solid state forming part of the soil or sediment matrix, they are rather immobile, limiting the mobility of the attached pollutants. A study of the dissolution kinetics of solid HA particles is presented here, and the ability of HA to become solubilized in aqueous media by different substances and conditions is evaluated. Thus, the effects on the dissolution rate of pH, temperature, concentration of inorganic and organic cations, organic acids and pesticides are investigated. The results show that the dissolution rate is strongly dependent on the pH of the aqueous solution; there is a 400-fold increase in the dissolution rate by increasing the pH from 4 to 11. Cations, either organic or inorganic, usually decrease the dissolution rate whereas organic acids and anionic pesticides increase that rate at constant pH. Dissolution appears to be controlled by sorption-desorption processes at the solid HA-water interface. These sorption-desorption processes are believed to modify the attractive forces that hold HA molecules together in the solid, modifying their rate of detachment from the surface.